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BIOLOGICAL PROCESSES 
IN TROPICAL SOILS 



A. STEVEN CORBET 



^CMILLAN LIBRARY 
STORAGE ITEM 

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BIOLOGICAL PROCESSES 
IN TROPICAL SOILS 



LONDON AGENTS - . 
SIMPKIN MARSHALL LTD. 




PRIMEVAL FORES! \i GINTING SIMPAH, 



BIOLOGICAL PROCESSES 
IN TROPICAL SOILS 

with special reference to Malaysia 



By 
A. STEVEN CORBET, 

B.Sc, Ph.D. (London), F.I.C. 
(Sometime Bacteriologist, Rubber Research Institute of Malaya) 



CAMBRIDGE 
W. HEFFER & SONS LTD. 



First Published in 1935 



PRINTED AND BOUND IN GREAT BRITAIN 

AT THE WORKS OF 

W. HEFFER a SONS LTD.. CAMBRIDGE 



To My Parents 



The prosperity of the human race depends, in the last 
analysis, upon the soil. We ought to know just what is 
occurring in the various soils of the world. — 

Professor E. M. East. 

I could not help thinking that if rubber planters were 
not so anxious to get a large area cleared at once, it would 
probably pay better in the end to leave all the poorer 
patches of land in forest as a shelter to the rubber (Hevea). — 

H. J. Elwes. 



PREFACE 

It is, perhaps, not sufficiently recognized that sweeping 
generalizations cannot be made upon so wide a theme as 
tropical agriculture. Within the tropics is included such a 
great diversity of climatic conditions that the problems and 
their solutions affecting one region may have no bearing 
upon those of another. The author, therefore, has applied 
the observations in this book to the special geographical 
sub-region of which he has the most intimate knowledge. 
Climatic conditions throughout the hot, wet equatorial 
belt, however, are so similar that the reader in other such 
regions should find much that is applicable to his own 
problems, provided that he makes due allowance for the 
variations in flora and fauna which are such vital factors 
in determining and in solving the problems of the soil 
biologist. 

The following statements are borne out by such a mass 
of practical evidence that there seems to be little doubt of 
their validity. 

(a) A re-statement of Jenny's law to the effect that 
the nitrogen and organic matter content of the soil 
varies inversely with the soil temperature and the 
amount of solar radiation received. 

(b) At temperatures below 25 C. there is an accumulation 
of organic matter in the soil but, at temperatures 
above this, humus decomposition outpaces its 
formation. 1 

When these tenets are accepted the whole process of forma- 
tion and decomposition of soil organic matter in the humid 
tropics becomes clear. A practical agriculturist would rind 
confirmation of these views on every hand yet, although 
the writer has shown in the laboratory that, under certain 
conditions, soils can lose nitrogen as a result of irradiation 
by ultraviolet light, the difficulty arises that, as far as can 

1 It is, of course, evident that, to a certain extent, (a) is a consequence 
of (b) : see Fig. 8 on page ioo. 



Vlll PREFACE 

be tested in the laboratory, ultraviolet light does not pene- 
trate far below the soil surface. It may be concluded that 
the photochemical processes in soils are purely surface 
phenomena but many established facts are at variance 
with such a view. 

After filtration by the earth's atmosphere, the solar 
spectrum comprises light of wave lengths between 30,000 
and 2900 Angstrom units 1 : the ultraviolet region consists 
of light of wave length below 4000 A. and it is the radiations 
between 3100 and 2900 A. which effect the greatest chemical 
activity. 

At present, the practical course seems to be to accept the 
statements (a) and (b) given above as a working hypothesis, 
awaiting results of further research by those in a position 
to carry out such work. 

A considerable volume of research work on the chemistry 
and microbiology of soils in temperate regions has been 
carried out during the past fifty years and among the 
institutions responsible for the advances made may be 
mentioned particularly the Rothamsted Experimental 
Station in England, under the direction of Sir E. John Russell, 
and the New Jersey Agricultural Experiment Stations with 
Drs. J. G. Lipman and S. A. Waksman. Important work 
has been done also in Germany, France, Holland, Hungary 
and other European countries. On the other hand, little 
progress has been made towards elucidating the problems 
peculiar to tropical soils; in fact, it is perhaps only within 
the last decade that any fundamental distinction has been 
recognized between the processes of temperate and those 
of tropical soils. The experimental work and field observa- 
tions responsible for many of the conclusions elaborated 
in this book were carried out at the Rubber Research 
Institute of Malaya and are described in a series of papers 
entitled "Studies on Tropical Soil Microbiology," appearing 
in Soil Science. 

This book was written primarily for agricultural chemists 
working in tropical countries, but it is hoped that planters, 
foresters and others may find it intelligible and useful. 

1 An Angstrdm unit (A.) = i/ioo.ooo.ooo cm. = io — 8 cm. 



PREFACE IX 



It is, perhaps, as well to point out that the author has, of 
necessity, viewed the subject largely from the standpoint 
of the cultivation of a perennial crop, namely rubber. It 
may be suggested that some of the matter included is to be 
found in standard text-books: this is so because many 
well known works of reference are not always to be found 
in tropical laboratories. For this reason it has been thought 
advisable to include standard methods of soil analysis in 
the appendix. 



August, 1934. 



ACKNOWLEDGMENTS 



I am greatly indebted to Mr. H. Gunnery, not only for 
taking the photographs contained in this book, but for 
much practical help in Malaya in matters botanical and 
mycological. Thanks are due also to Mr. H. M. 
Pendlebury, Dr. L. A. Allen and Mr. P. E. Turner for 
valuable help and suggestions. To my wife I am deeply 
grateful for continuous help during the preparation of this 
book both in Malaya and in England. 

A. S. C. 



CONTENTS 

PAGE 

Preface --------- v ii 

List of Illustrations ------ x iii 

CHAPTER 

I. Malaysia ------- j_ 

II. The Plant Life of Malaysia 25 

III. The Soil Fauna ----- 35 

IV. Soil Micro-Organisms 46 
V. The Bacterial Growth Curve - - 62 

VI. The Soil Organic Matter - - - 70 

VII. The Nitrogen Cycle 83 

VVIII. Jenny's Law ------ 97 

IX. Some Practical Considerations - 108 

Appendix : — Standard methods employed for the ex- 
amination of soils — Classification of bacteria — 
Classification of fungi — Conversion factors — 

Bibliography - - - - - - *33 

Author Index ------- 149 

Subject Index ------- ^ 

Index to Animals ------- 154 

Index to Plants ------- 155 



LIST OF ILLUSTRATIONS 

TEXT-FIGURES 

FIGURE PAGE 

i. Table of Geological Ages - 2 

2. Malaysia ________ 4 

3. The Growth of a Population of Yeast Cells - 64 

4. The Nitrogen Cycle in Soil under Primeval 

Forest --------85 

5. The Nitrogen Cycle in Soil cleared of Forest 

and exposed to the sun 85 

6. The Nitrogen Cycle in Soil under Secondary 

Growth --------86 

7. The Transformation of Nitrite to Nitrate at 

varying pH Values ----- 90 

8. The Relation between Temperature and Humus 

Formation and Decomposition - 100 

9. Changes in the Nitrogen Content of the Soil 

when Primary Forest is cleared and then 
subsequently allowed to revert to Jungle 106 

10. Distribution Frequencies of the Logarithms of 

"Bacterial Counts" made in Malaya - - 140 

PLATES 
plate 

I. Primeval Forest at Ginting Simpah Frontispiece 

Facing page 
II. Limestone Outcrop in Selangor - - 1 

III. Nipa fruticans - - - - - - 11 

IV. Terracing on a Laterite Slope on the Road- 

side -------- 21 

V. Epiphytes on Enterolobium saman - - 25 

VI. Rhizophora Tree in Mangrove Forest - - 27 

VII. Lalang Grass in a Coconut Plantation - 31 

xiii B 



Xiv LIST OF ILLUSTRATIONS 

plate Facing page 

VIII. Preparation of Land for Padi Planting - 51 

IX. Secondary Growth following drastic Clear- 
ing of Forest ------ 83 

X. Mimosa pudica, the Sensitive Plant 93 

XI. Clearing Forest for Rubber Cultivation - 103 

XII. Newly-planted Rubber Seedlings - - in 

XIII. Rubber with a Cover of Calapogonium mucun- 

oides -------- 117 

XIV. Stag-horn Moss (Lycopodium cernuum) - 122 
XV. Rubber with a mixed Natural Cover - - 124 

XVI. Tapping a Rubber Tree - 130 





***w. 



z 



I. MALAYSIA 

The countless ages that have elapsed since the earth reached 
its present form have witnessed many profound changes, both 
geographical and climatic. Malaysia has not always been 
the forest-covered group of tropical islands that we know 
to-day, and, in order to form a proper appreciation of 
present conditions, it is necessary to give at least a cursory 
glance at some of the outstanding events which have taken 
place in the past. However, although the geological record 
of western Europe is known in some considerable detail, 
much of the former history of south-east Asia is shrouded in 
obscurity. 

Geological time is divided into a number of eras (Fig. i) : 
although life dawned in the earliest of these, the Eozoic era, 
the formation of the earth antedates the first appearance of 
organic life by millions of years. As far as the record of 
the rocks shows, the oldest fossils are sponges but other 
organisms also existed in pre-Cambrian times. 

During the Primary era the lands which now constitute 
south-east Asia were at the bottom of the ocean but events 
of considerable importance were taking place in other parts 
of the world. It was in the Cambrian age that living 
organisms first occurred in numbers, although at this time 
they were entirely marine; the crustacean-like trilobites, 
whose nearest living allies are the king-crabs (Xiphosura), 
were characteristic of this period. By the Devonian age 
the trilobites had been eclipsed and fishes represented the 
highest achievement of creation; vegetation had appeared 
on the land, but the flora was largely composed of conifers, 
tree ferns and club mosses. A ghostly silence must have 
pervaded the land for few terrestrial creatures existed, no 
birds, and few insects save cockroaches, for still almost the 
entire animal world was marine in habitat. Insects began 
to appear in numbers at the close of the Primary era but few 
of them would appear familiar to an entomologist to-day. 

It appears that Malaysia had its beginnings in the 
Secondary era, probably as a consequence of the intense 



2 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

volcanic activity in some of the eastern parts of the continent 
of Gondwanaland, which stretched across the Indian Ocean 
to Africa at least. It was during the Jurassic age that the 
monstrous reptiles, such as the flying Pterodactyl, the 



EOZOIC ERA 




PRIMARY ERA 


Cambrian Age 
Ordovician Age 
Silurian Age 
Devonian Age 
Carboniferous Age 
Permian Age 


SECONDARY ERA 


Triassic Age 
Jurassic Age 
Cretaceous Age 


TERTIARY ERA 


Eocene Age 
Oligocene Age 
Miocene Age 
Pliocene Age 


QUATERNARY ERA 


Pleistocene Age 
Holocene Age 



Fig. i. Table of Geological Ages. 

In a general way, it may be said that existing species first appeared in 
Pliocene times, existing genera appeared in the Miocene age, existing 
families were found in Oligocene times, while existing orders date from the 
Eocene age. Man first appeared in late Pliocene times. 

predacious Tyrannosaurus and the herbivorous Atlanto- 
saurus (over seventy feet in length), were masters of the 
earth. These creatures occurred in Britain, and in fact 
all over the world, but they probably became extinct before 
Malaysia was in a habitable condition. 

By the time of the Eocene age, which ushered in the 
Tertiary era, the forms of life prevailing were not vastly 
different from those existing to-day. Mammals first ap- 
peared in the Secondary era but they made little progress 



MALAYSIA 3 

in development or numbers until Eocene times; birds 
were represented by an ever-increasing number of species, 
but the age of giant reptiles was deep in the past, and such 
creatures as the Iguanodon now slept in a grave of chalk. 
Then, and during the successive Oligocene and Miocene 
ages, the climate of western Europe was subtropical and 
supported a rich flora and fauna ; there was still a tremendous 
amount of volcanic activity in south-east Asia and, some- 
where about this time, the mighty Himalayas came into 
being. 

At the close of the Pliocene age many animals, such as 
antelopes, apes and elephants which still exist in tropical 
regions of the world at the present time, roamed western 
Europe and man had already appeared and walked erect. 
In these times, comparatively little remote from the present, 
where to-day in south-east Asia exists a number of islands, 
was a unified land mass known as Sundaland. This 
land lay between the parallels of io° N. and 13 S. and 
between longitudes 94 and 120 E. : the general trend of 
the mountain ranges was east and west; many of the 
rivers were probably long and characterized by wide mouths, 
the climate was uniformly warm and humid and the plains 
and mountain slopes were covered with dense forest. In 
fact, except for the difference in the area of land there is 
no reason for supposing that, at the close of the Tertiary 
period, Sundaland differed in any remarkable way from 
this region to-day. The plants and animals found on 
Sundaland largely comprised those occurring in the Malay 
Peninsula, Sumatra, Borneo and Java at present. 

But in western Europe the Pliocene age was marked by 
a progressive cooling until most of Britain was covered with 
a permanent mantle of snow and the Ice age set in ; the flora 
and fauna characteristic of tropical and subtropical regions 
disappeared and were replaced by organisms adapted to 
colder climates. It is probable that at this time there was 
a lowering of temperature over the whole of the earth's 
surface although it is unlikely that any appreciable cooling 
took place in south-east Asia where there was still much 
volcanic disturbance. But, contemporaneously, geological 



4 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

movements were taking place in Sundaland which cul- 
minated in its disintegration into a number of islands. 
Geological evidence indicates that there was a number of 
upheavals and subsidences, not necessarily of a violent 
character, which effected alterations in the level of the land : 
most of the changes known to have taken place can be 
accounted for by the gradual sinking of the land level by 




Fig. 2. Malaysia. 



a few feet. Of what are termed the Large Sunda Islands, 1 
it is evident that Java was the first to be cut off, then later, 
probably much later, occurred the isolation of Borneo, 
followed closely by the separation of the present Malay 
Peninsula from Sumatra. 

The wide plains of the ancient continent of Sundaland 
now form a continental shelf varying from 50 to 100 fathoms 
in depth below sea level : in fact, a steamer can anchor almost 
anywhere on the Sunda shelf. What were foothills or elevated 
plateaus are now the green coastal plains of the Malay Pen- 
insula and the Sunda islands: the mountains of Sundaland 

1 The superscript numerals refer to references at the end of the chapter. 



MALAYSIA 5 

are still the forest-clad heights which sweep in a wide 
curve through the whole region. 

What remains of Sundaland above sea level to-day is 
termed Malaysia and this includes the Malay Peninsula 
south of latitude io° N., the Large Sunda Islands (Sumatra, 
Borneo and Java), Bali, almost certainly Palawan and 
Balabac, and innumerable lesser islands situated in the 
vicinity. The deep sea islands off the west coast of Sumatra 
(Engano and the Mentawei Islands), Christmas and Cocos- 
Keeling Islands, and possibly the Nicobar Islands also, 
are to be regarded as part of Malaysia, although it is certain 
that Christmas and Cocos-Keeling Islands were never 
united with the mainland. 2 The Malay Archipelago is 
usually regarded as comprising the Malay Peninsula and 
all the islands forming a chain from Sumatra to New Guinea. 
The term Malaya is now restricted to that part of the 
Peninsula under British control or protection and, although 
there may be certain objections to its use, it is now pretty 
generally employed. Strictly speaking, the Malay Peninsula 
extends from the Isthmus of Kra to Cape Roumania in 
Johore. 

Beyond the Sunda shelf considerable depths are soon 
reached, particularly in an easterly direction where the 
ocean bed drops to some 3000 fathoms. 

The history of the land connection between Sundaland 
and the mainland of Asia is of interest. Biological evidence 
(p. 27) suggests that the Sunda mountains were linked with 
the Himalayan range at some remote date; but it is clear 
that at no very distant time the present Malay Peninsula 
was cut off from the mainland of Asia by the sea. The 
flora and fauna show that Malaya was separated from Siam 
and south Burma in south Kedah, the boundary being 
the Kedah river : as far as the geological evidence goes, it 
supports this conclusion, which also finds confirmation in 
early tradition. 3 

It is of particular interest to note that the remains of 
one of the earliest known men, Pithecanthropus credits, 
were found in Pleistocene deposits in Java in 1891-92. 
It is possible that the negrito peoples of Malaysia (Semang 



6 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

of the Malay Peninsula, Kubus of Sumatra, Aita of Palawan 
and possibly the Andaman Islanders) occupied Sundaland 
while it was still one large land mass. 

Sumatra and Borneo are intersected by the equator, 
Java lies to the south of it and the Malay Peninsula to the 
north. In all these countries the geological formation, 
climate, flora and fauna are similar, and, in the natural 
state, the land is covered with dense primeval forest. 
With the exception of Java, all these lands still retain most 
of the original jungle and on this account they are of 
particular interest to the biologist and bacteriologist. 
The former has the opportunity of investigating the flora 
and fauna of a natural region as yet unspoilt by civilization 
while the latter has the advantage of studying the activities 
of micro-organisms with two important factors, temperature 
and humidity, maintained at a constant level. Other reasons 
that render Malaysia of peculiar interest to the soil micro- 
biologist will become apparent later on. 

THE MALAY PENINSULA 

The Malay Peninsula is usually regarded as extending 
from approximately i° N. to the Isthmus of Kra, io° N. 
but British Malaya does not reach y° N. and the total area 
under British control or protection is about 51,000 square 
miles. The peninsula is formed of strata of Primary and 
Secondary origin and the geology has been investigated 
in some detail. 4 The greater proportion of the rocks 
consists of granite and, to a lesser extent, quartzite but 
there are numerous outcrops of other formations such as 
gneiss, schist and marble. Sedentary deposits include slate, 
limestone and sandstone: coal occurs at Rawang and in 
one or two other small areas of Tertiary rocks. On the 
west coast there are several limestone bluffs, particularly 
in the neighbourhood of Kuala Lumpur, and in Perak, 
Kedah and Pedis. 

Running through the centre of the Peninsula is a range 
of granite mountains: this range is more or less continuous 
from north Johore to Peninsular Siam although it is in- 
terrupted at the point where, it is believed, the Peninsula 



MALAYSIA 7 

was formerly separated from the mainland of Asia. In 
addition to the main range are a few smaller parallel chains, 
of which the most important are the Larut Hills in Perak 
and two or three ranges in Pahang. In general the main 
range attains a height of some 4000 feet, although it is 
broken by numerous peaks of which the highest is Gunong 
Kerbau (7160 feet) : the highest mountain in the Peninsula 
is Gunong Tahan (7186 feet) on the Pahang- Kelantan 
border. There are no extensive plateaus in the mountains 
such as occur in Sumatra but a large raised valley near the 
Perak-Pahang border, known as Cameron Highlands, is 
being developed as a hill station. 

Although Malaya possesses numerous rivers few of them 
are suitable for navigation by sizeable craft for any con- 
siderable portion of their length. In the upper part of 
their courses, all the rivers are swift and usually characterized 
by numerous cascades and rapids. The longest river, the 
Sungei Pahang, flowing into the China Sea on the east coast, 
is navigable for native craft for some 250 miles; the Sungei 
Kelantan, entering the China Sea further to the north, can 
be used for nearly the same distance. Of the rivers flowing 
into the Straits of Malacca on the west coast the most 
important are the Sungei Perak, the Sungei Bernam and 
the Sungei Muar: although shallow, the first-named is 
navigable by native craft for over 200 miles. 

The coast line is fairly regular but, at the river mouths, 
the water is shallow. On the west coast, mangrove swamps 
persist for a depth of several miles, except in a few spots 
where sandy beaches occur. On the east coast mangrove 
forests are to be found only at the tidal estuaries, sandy 
beaches fringed with coconut palms and Casuarina trees 
being the rule. The formation of mangrove forests occurs 
in sheltered areas where the sea is shallow and quantities 
of silt are deposited in the neighbourhood of river mouths: 
the absence of such forests on the east coast of Malaya is 
attributable to the force of the north-east monsoon which 
blows from November to February. 

Several islands lie off the coasts of the IVnmsula; the 
largest being Pulau Langkawi in the group of Langkawi 



8 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Islands west of Kedah. Penang and Pangkor are other 
large islands off the west coast and Singapore Island is 
the chief member of the Rhio Archipelago. On the east 
coast, off Pahang and Johore are Pulau Tioman and Pulau 
Aor with some smaller islands, while the Perhentian Islands 
lie farther north. 

In its primitive state the whole of the Malay Peninsula 
was covered with dense primeval forest and even to-day 
over seventy per cent, is forested. The only cultivated 
crops of any importance are rubber, coconuts and rice, and 
of these the first-named is by far the most outstanding. 
Some thirty per cent, of the world's tin is obtained from the 
Peninsula where it occurs as cassiterite and always close 
to granite: almost all the tin is alluvial but the lode mines 
at Sungei Lembing, near Kuantan, are the deepest tin 
mines in the world. 

The population of British Malaya is over four millions. 
Three aboriginal races are found of which the oldest is the 
Semang, a tribe of negrito people with frizzy hair and of 
small stature who lead a nomadic existence in the forest 
and make no attempt at cultivation. The Semang are 
closely related to the negritic peoples found in the Andaman 
Islands, Sumatra and Palawan. Other pagan races found 
in Malaya are the Sakai and the rather similar Jakun. 
The modern Malays reached the Peninsula from Sumatra 
in comparatively recent times. Other nationalities con- 
stituting an appreciable proportion of the population are 
Chinese, Tamils and Europeans. 

SUMATRA 

The island of Sumatra has an area of some 167,000 square 
miles and is formed largely of strata of the Tertiary period, 
but contains a few formations anterior to this. A high 
mountain chain runs the length of the island parallel to the 
west coast: the mountains rise abruptly from the narrow 
coastal strip in the west but descend gradually to the wide 
plains in the east. This chain is a link in an archaic moun- 
tain system persisting to-day only in fragments in south 
Burma, the Andaman and Nicobar Islands, Sumatra, Java, 



MALAYSIA 9 

the Lesser Sunda Islands and the Moluccas. In both 
Sumatra and Java are numerous volcanic peaks, many of 
which are still active. 

The loftiest peak in Sumatra is Korinchi (12,484 feet), 
also known as Indrapura, and there are several summits 
in the neighbourhood of 10,000 feet. As would be anti- 
cipated from the geographical configuration of the island, 
the rivers on the west side of the main range are short, 
flowing in deep valleys, while those on the east are longer 
and slower : the east coast is fringed with mangrove swamps 
and many of the estuaries are constantly changing position. 
The most important river in Sumatra is the Asahan, which 
drains into Toba Meer, a large lake some forty-five miles 
long and fifteen miles broad, situated in the Battak moun- 
tains. In the centre of this lake is an island, Pulau Samosir; 
the greatest depth of the lake is about 1460 feet. Other 
important rivers are the Panei, Rokan, Kampar, Indragiri 
and Jambi: the last-named, which is the largest river in 
Sumatra, rises in Mount Korinchi and is navigable for some 
five hundred miles. 

In some regions of the highlands of Sumatra, particularly 
in parts of the Battak Plateau, lying more than 2000 feet 
above sea level, are large areas of savannah country covered 
with lalang (see page 30) ; on such areas little or nothing 
else will grow. 

The population of Sumatra is about six millions. The 
oldest race, the Kubus of the Jambi Mountains, are of 
negritic blood and small stature; they lead a nomadic 
existence in the forests. Of the Malay races which constitute 
the rest of the indigenous population, the Achinese occupy 
Achin in the north-west of the island, further south are the 
Gayos, and the Battaks are found in the Battak Plateau 
and in the mountains around Toba Meer. The Menangkabau 
Malays of the Padang Highlands still retain matriarchal 
law. Other Sumatran races of lesser importance found 
in the southern portions of the island are the Korinchis, 
Rejangs and the Jambi Malays. 

The most important agricultural crops are tobacco 
and rubber, but timber and gums are products also of 



10 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

consequence. Coal and petroleum occur in some quantity 
and the adjacent islands of Banka and Billiton have long 
been famed for the production of tin. 

JAVA 

Together with the island of Madura, Java has an area of 
about 51,000 square miles of which about forty per cent. 
of the total is under cultivation. The whole island is 
covered by deposits of the Tertiary and Quaternary ages, 
and is the most actively volcanic region in the world, over 
a quarter of the total area of the island being covered with 
volcanic rocks. 

The mountain range, running east and west, follows on 
the course of the main chain of Sumatra, and although the 
area of mountainous country is not nearly as extensive 
as is the case in the rest of Malaysia, there are several 
peaks above 9000 feet. The highest mountain in Java 
is Gunong Semero (12,300 feet). The rivers are of little 
importance: the Chi Tarum and the Chi Manuk flow into 
the sea on the north coast while the Chi Tanduwi enters the 
sea on the south. The north coast of the island is covered 
with nipah and mangrove areas, sandy stretches and shifting 
river mouths. 

Relative to the rest of Malaysia, the population of Java 
is enormous. The total population is in the neighbourhood 
of thirty-six millions; thirty-five millions belong to the 
Malay races, the most important of which are the Javanese 
proper, the Sundanese and Madurese. 

Rubber, rice and sugar are the most important crops 
in Java, but a large number of other products are cultivated, 
including coffee, tea, tobacco, copra, cinchona, indigo, 
spices and teak. The island is poor in metallic ores. 

BORNEO 

The island of Borneo (politically divisible into Dutch 
Borneo, Sarawak, British North Borneo and Brunei) has a 
total area of nearly 300,000 square miles and ranks after 
Greenland and Papua as the third largest island in the world. 




^ 



MALAYSIA II 

The geology of Borneo has not yet been investigated in 
any detail ; there appears to be a preponderance of rocks of 
Tertiary and Quaternary origin but a number of fossils 
have been found in some Secondary deposits. 

Borneo is a mountainous country although no range 
attains any great elevation. The mountain ranges radiate 
from an approximately central point, the Iran Mountains 
running to the north, the Upper Kapuas chain to the west, 
the Madei and Schwaner Mountains to the south-west and 
the rather low Batu Tampatung range to the east. The 
highest peak in Borneo, and in fact in the whole of Malaysia, 
is Mount Kinabalu (13,455 feet) situated in the north of 
British North Borneo: this peak is of considerable interest 
on account of its specialized flora and fauna. 5 The next 
highest Bornean mountain is Mount Mulu (9600 feet) in 
north Sarawak. 

A number of rivers in Borneo attain a considerable length : 
the Rejang in Sarawak flows into the South China Sea, 
and steamers can ascend this river for one hundred and 
fifty miles, while the river is navigable for native craft for 
five hundred miles. Other important rivers are the Kina- 
batangan in British North Borneo, the Kapuas which 
enters the sea on the west coast in Dutch Borneo and the 
Barito and Kutai flowing into the Java Sea and Macassar 
Strait respectively on the east. For the most part the 
coast line of Borneo consists of low alluvial lands, broken 
by occasional sandy stretches fringed by Casnarina trees, 
while in the neighbourhood of river mouths are mangrove 
and nipah swamps. 

The population of Borneo is about three millions. No 
negritic races are known and it is improbable that any remain. 
The indigenous population is Malayan and a great deal of 
intermixture appears to have taken place in recent times 
among the various races. Roughly, the Malayan races of 
Borneo can be divided into the Laut and the Dyaks; as the 
names imply, the first of these comprises the peoples living 
on the sea coasts while the latter are Mohammedans living 
in the interior. In British North Borneo the Dusuns, 
Bajans and Muruts constitute the most important tribes. 



12 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

The country is but little developed: rubber, sago and 
timber are the principal agricultural products and coal and 
petroleum are becoming increasingly important; metallic 
ores are scarce. 

BALI 

Besides the Larger Sunda Islands, Malaysia includes 
innumerable smaller islands, the largest of which are Bali 
and Palawan. Bali has an area of 2000 square miles and 
is mountainous, the highest peak being the volcanic Mount 
Agung (10,500 feet). Many devout Hindus fled to Bali 
from Java when the Arabs visited the latter island in the 
sixteenth century and employed coercive measures to 
convert the inhabitants to their belief; and the Balinese 
are still adherents to the Hindu faith. The population of 
the island is in the neighbourhood of one million and the 
Balinese have the reputation of being expert rice growers. 

PALAWAN 

Politically one of the Philippines, Palawan (area 4500 
square miles) appears to be more closely related faunistically 
to Malaysia than to Mindora. A mountain range between 
4000 and 5000 feet in height extends through the island 
where a dwarf, negrito people, the Aita, still survive. 

FAUNISTIC REGIONS 

In the natural course of evolution one species of plant 
or animal gives rise to others which, in turn, are the pre- 
cursors of further forms: each endeavours to extend its 
range of distribution as far as possible, but in most cases 
this can be effected only to a limited extent. A species of 
plant or animal on a remote oceanic island has but a slender 
chance of extending its range compared with one on a 
continental land mass. As a general rule, however, the 
geographical range of a species cannot be increased in- 
definitely because, sooner or later, the plant or animal in 
question arrives at a locality where climatic conditions are 
unsuitable for it, and it can penetrate further only if it is 
still in a plastic stage and able to adapt itself to meet the 



MALAYSIA 13 

altered conditions. Another reason for the abrupt con- 
clusion of an advance into new territory, such as we have 
described, may be the meeting of some impassable barrier 
such as the sea, a desert or a high mountain range. 

The existence of such barriers on the surface of the earth 
has resulted in the land area being divided into a number of 
well-defined faunistic regions, each of which has its own 
characteristic flora and fauna. While the passage from 
one region to another is not absolutely barred it appears 
that it took place very infrequently in the past but, 
owing to increased travelling facilities brought into being 
recently by man, transference of a species from one region 
to another is now by no means rare. There is no particular 
reason why a plant or animal from Brazil should not thrive 
in the eastern tropics once it has gained a footing; in fact 
it is surprising how frequently a plant or animal introduced 
from one region to another will multiply in the new area — 
often to the detriment of endemic species. In Table IV 
(page 32) are given the names of plants well known in 
Malaysia, many of which have been introduced within recent 
times from countries in other faunistic divisions. 

The four faunistic regions of the world are : the Palaearctic 
Region, comprising Europe, Africa north of the Sahara 
Desert and that part of Asia lying north of a line drawn 
from Baluchistan, through the Himalayas and across 
China to Shanghai; the Indo-Australian Region, 
comprising that part of Asia south of the Palaearctic region, 
the Malay Archipelago, Australia, New Zealand and the 
South Pacific Islands; the Hawaiian Islands properly belong 
to this region although they now contain a number of species 
of plants and animals introduced from America; the 
Ethiopian Region, constituting the continent of Africa south 
of the Sahara Desert; and the American Region, comprising 
the whole of the American continents. In the last-named 
region the transition from the tropical to the temperate 
zones is gradual and several tropical species extend far 
into the United States. 

Considering the varied geographical and climatic nature 
of the Indo-Australian region (the division which concerns 



14 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

us most closely), it is surprisingly homogeneous but admits 
of further division into a number of sub-regions, each of 
which shows greater uniformity. The Malaysian Sub- 
Region includes the Malay Peninsula, Sumatra, Borneo, 
Java, Bali, Palawan and the adjacent small islands — in 
fact, all that persists of the original Sundaland. It is a 
remarkable fact that even to-day, in most respects, the 
flora and fauna of Malaya are more closely allied to those 
of Sumatra than to those of south Burma, in spite of the 
gradual infiltration of species which must have been taking 
place for centuries. It might be anticipated that, with 
the passage of time, the flora and fauna of the Malay 
Peninsula would show less afinity with Sumatra and become 
more Burmese in character. This may be true but, probably, 
only within limits; for climatically the Malay Peninsula 
is an island and there are climatic factors operating which 
inhibit the establishment of certain Burmese elements in 
the Peninsula. In some respects the Langkawi Islands 
have more the faunistic characters of south Burma than of 
the Malay Peninsula. The Indian crow (Corvus splendens), 
so abundant in Burma, is very common on Pulau Langkawi, 
but, although increasing rapidly now, has made little head- 
way since it was introduced into the Malay Peninsula some 
forty years ago. Several species of insects occurring in 
the Langkawi Islands are definitely of Burmese origin 
and do not otherwise extend into the Malaysian sub-region. 

Introduced plants and animals usually have to contend 
with the native species and, although on occasion immigrants 
may be entirely successful in the struggle, cases must occur 
constantly where introduced species fail to become estab- 
lished in a new faunistic area. 

It is certain that many of the islands which pertain to 
Malaysia were never connected with either the mainland 
of Asia or Malaysia. Such islands are termed oceanic in 
contrast to continental islands which originated as part of a 
large land mass. The remote Christmas Island, nearly two 
hundred miles south of Java, is an instance of a Malaysian 
oceanic island. This island, known for its phosphatic 
deposits, has a flora and fauna containing many distinct 



MALAYSIA 



15 



forms, but all show a certain degree of affinity with species 
found in the Large Sunda Islands. 6 

CLIMATE 

Malaysia lies in the equatorial belt of low pressure and 
the chief characteristics of its climate, therefore, are high 
temperature and humidity and heavy rainfall. The seasonal 
changes are slight and are determined by variations in 
rainfall and not in temperature. 

The mean annual shade Temperature on the plains 
throughout Malaysia is in the neighbourhood of 8o° F. and 
the range of variation between the monthly means is between 
2° and 3 F. The mean temperature at Rangoon, in south 
Burma, is just under 8o° F., but the range of variation is 
over io°. In the primeval forest the temperature is some- 
what lower: in the Malay Peninsula 7 the temperature at 
the surface of the soil was found to be 77 F., and this value 
did not show a variation of more than i° F. during the 
course of a year. As the hills are ascended the temperature 
falls, as is shown in Table I, which gives figures obtained 

TABLE I 
The Variation in Temperature with Altitude in Java 



Altitude. 


Mean 
temperature. 


Minimum 
temperature. 


Maxii 
temper 


num 
ature. 


Metres. Feet. 


°C. 


°F. 


°C. 


°F. 


°C. 


°F. 



200 650 
1,000 3,280 
1,800 5,900 
2,600 8,530 
3,400 11,150 


26-5 
25-0 

20-0 
I 5 -0 
IO'O 

5-o 


and 
below. 


797 
77 >0 
68-o 

59-o 
50-0 
41-0 
32-0 
and 
below. 


22-0 
20-0 
I 5 -0 
IO'O 

50 




71-6 
68-o 
590 
50-0 
41-0 
32-0 


33-° 
3 o-o 
25-0 

20-0 
I50 

io-o 


91-4 
86-o 
77-0 
68-o 
59-o 
50-0 


4,300 14,10c; 
and above. 


(snow limit) 



C. J. Mohr, De Grond van Java en Sumatra (Amsterdam), 193°. P- "■ 

for Java: in general, throughout the world an increase in 
altitude of 300 feet is equivalent to a fall in the average 
temperature of 1° F. On the banks of Toba Meer in Sumatra, 



i6 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



at an altitude of 3770 feet, the mean tempeature is 70 F. 
The mean monthly temperatures in certain Malaysian 
localities are given in Table II. 

TABLE 11 

Mean Monthly Shade Temperatures in Malaysia 
(Degrees Fahrenheit.) 





5 >> 


O rt 


g 

TO 


bo rt 


Kock 

tra). 


2 ■ 

'> TO 


bo 
G • 

X TO 


M ■ 

2^ 


3^. 

to" 


TO . 

6 "2 




2 to 

<U TO 


TO JS 
Sf TO 


bo ,2 

G TO 

os 

3 
O 


TO £ 

Ph 3 


O Co 

+> 3 

Pn 


to > 


:> 

c * 

toS 


.2 G 
-t- l-i 

c 
opq 

Ph — 


G O 


-u G 

U 


Altitude 










































in feet. 


23 


10 


5460 


3 


3500 


23 


2366 


IO 


98 




January 


8o° 


78 


6o° 


79° 


69 


78° 


72 


78° 


79° 


8o° 


February 


8o° 


79° 


6i° 


8o° 


69° 


78 


72 


79° 


8o° 


8o° 


March 


8i° 


8o° 


62° 


79° 


70° 


79° 


72° 


79° 


8i° 


8o° 


April 


82 


8i° 


63° 


8o° 


70 


79° 


72° 


79° 


8i° 


8o° 


May 


82 


82 


64° 


8o° 


71° 


79° 


72 


8o° 


82 


79° 


June 


8i° 


8i° 


65° 


79° 


70 


79° 


72° 


8o° 


81° 


79° 


July 


8o° 


8i° 


6 4 ° 


79° 


69 


7 S° 


72 


8o° 


8i° 


78° 


August 


8o° 


8i° 


63° 


79° 


69 


79° 


72 


79° 


8i° 


78° 


September 


8o° 


8o° 


63° 


79° 


69 


8o° 


72 


79° 


8i° 


78° 


October 


8o° 


8o° 


62° 


79° 


70 


8o° 


73° 


79° 


8i° 


79° 


November 


79° 


79° 


62° 


79° 


69 


79° 


72 


78° 


8o° 


79° 


December 


79° 


79° 


60 ° 


79° 


69 


78° 


72° 


78° 


79° 


79° 


Range 


3° 


3° 


5° 


1° 


2° 


2° 


1° 


2° 


3° 


3° 


Mean an- 






















nual tem- 


8o° 


8o° 


62° 


79° 


69° 


79° 


72° 


79° 


8o° 


79° 


perature 























The annual Rainfall often differs very considerably 
in different localities as Table III shows ; nevertheless, it may 
be stated that the rainfall of most places on the plains in 
Malaysia is in the neighbourhood of 100 inches per annum. 

Of the meteorological stations in the Malay Peninsula, 
that at the " Cottage " (4500 feet) in the Taiping Hills has the 
heaviest rainfall — an average of 259 inches per annum 
(1912 to 1919), while Jelebu in Negri Sembilan has the 
lowest (60 inches per annum from 1905 to 1919). The west 
coast of Sumatra has an annual rainfall of 122 inches, the 
east coast 106 inches and the north coast 96 inches. The 
rainfall at Batavia is lower than at most places on the 



MALAYSIA 17 

plains of Java but an annual rainfall of over 270 inches has 
been recorded from Kranggan in the Javanese mountains: 
the rainfall in Borneo appears to be appreciably higher than 
in other parts of Malaysia. 

The intensity of the rainfall in the tropics is often a factor 
of considerable importance for, very frequently, during a 

TABLE III 

Mean Monthly Rainfall in Malaysia 
(Inches.) 







<u • 


a 

,3 • 




■ 

1—1 +j 




bo 


x ■ 


s 


en 




6 >> 


O rt 

Cm ^ 


H& 


be fS 




s ■ 


3^ 


3 

*-5 CD 


2*2 




5 d 
CD c3 




3 rt 
OS 

3 
O 


55. 


-a 
o52. 


rt > 


n 


.2 3 

3 

opq 


S 3 

3 O 

c3& 


+j 3 

.3 >-H 

u 


Altitude 
in feet. 






















23 


10 


5460 


3 


3500 


23 


2366 


10 


98 




January- 


3-9 


8-5 


20'7 


13-5 


9-2 


13-0 


7-6 


io-8 


19-4 


5-5 


February 


3-o 


6-i 


5-6 


99 


7-0 


136 


7-1 


7-9 


9-9 


12-5 


March 


47 


6-5 


7-5 


1 1 -9 


9-2 


7-8 


9-6 


9-8 


7-8 


9-0 


April 


7-0 


6-9 


io-6 


14-0 


IO-2 


4-8 


9-0 


io-8 


4-i 


6-2 


May 


II-O 


7-2 


11-2 


12-6 


7-6 


37 


5-2 


10-7 


5-i 


7-6 


June 


7-2 


6-7 


50 


13-0 


5-6 


3-6 


3-6 


8-7 


8-6 


34 


July 


8-9 


6-8 


7-0 


n-8 


3-8 


2-6 


2-6 


6-3 


100 


77 


August 


12-8 


8-5 


8-3 


13-7 


6-4 


i-3 


2-3 


8-9 


6-9 


2-3 


September 


19-0 


7-1 


n-6 


16-1 


6-4 


2-6 


3-6 


8-4 


9-5 


34 


October 


i6-i 


8-2 


23-9 


20-0 


9-0 


4-i 


6-7 


14-8 


IO-2 


2-2 


November 


109 


io-o 


18-2 


20-7 


9-0 


5-o 


8-9 


15-7 


16-4 


9-4 


December 


4-8 


10-4 


I5-I 


19-4 


10-4 


8-7 


8-5 


13-2 


19-3 


6-6 


Total 


109-3 


92-9 


144-7 


I 77 -6 


94-0 


70-9 


747 


125-9 


I27-2 


75-8 



heavy storm a large proportion of the rain is precipitated 
during a relatively short space of time. Haines has pointed 
out that each soil has a certain limit for the rate at which 
it can absorb rain by percolation; and it is the quantity 
which falls in excess of this that accumulates and inns 
over the soil surface and constitutes the major factor in 
erosion. 

In Burma the rainy season is from May to October during 
the south-west monsoon and practically the whole of the 
rain is precipitated during this period. In Malaysia no 
such seasonal differences can be marked although in the 
northern part of the Malay Peninsula there is a very slight 



10 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

tendency towards seasonal changes: in the Langkawi 
Islands and off the coast of Kedah the force of the south-west 
monsoon is felt occasionally. The north-east monsoon, 
which rages in the China Sea between November and March, 
is not without effect on the east coast of Malaya and for 
a month or two during the year Pulau Tioman and Pulau 
Aor are cut off from the mainland by rough seas. Violent 
squalls of short duration, known as "Sumatras," occur 
quite frequently in the Straits of Malacca. 

In equatorial regions, particularly those with the type 
of climate which occurs in Malaysia, variations in barometric 
pressure are regular besides being negligibly small: conse- 
quently altitudes can be determined by means of an aneroid 
barometer. 

THE WEATHERING OF ROCKS AND SOIL 
FORMATION 

The soil, comprising as it does an admixture of mineral 
fragments derived from the underlying rocks and particles 
of organic matter resulting from the decomposition of dead 
vegetation, is a highly complex material showing considerable 
variation in composition from one locality to another. 
At first sight it would be anticipated that the soil would 
be closely related to the rock formation underlying it and 
that a classificatory system would be based on such a 
relationship. It is now generally recognized that soils 
are related primarily to the climatic conditions under which 
they are formed, and only indirectly to the parent mineral 
material. In short, soil is the product of the action of 
climate upon rocks. 8 

The weathering of rocks, resulting in the production of 
minute mineral particles, is an essential process in soil 
formation: in temperate regions this disintegration is 
essentially physical, being effected by such factors as a wide 
range of temperature change (resulting in alternate con- 
traction and expansion of the rocks), the action of frost 
and glaciers and wind erosion but, undoubtedly, carbon 
dioxide dissolved in rain water plays some small part. 
In the humid tropics the weathering of rocks is almost 



MALAYSIA 19 

exclusively a chemical process ; in such regions the tempera- 
ture is high and, in consequence, the solvent action of rain 
water is much increased. Although the temperature falls 
as the hills are ascended, throughout almost the whole of 
Malaysia climatic conditions are such that, even at the 
highest altitudes, weathering is effected by chemical and 
not by physical means. 

LATERITE 

The fundamental importance of the relation between 
precipitation and evaporation in the weathering of rocks 
was first recognized by Hilgard 9 but the subject has been 
examined in detail by Mohr as far as Java and Sumatra 
are concerned. Three cases arise which can be classed 
as follows: — 

(a) Rainfall exceeds evaporation throughout the year. 

("Humid" regions.) 

Much of the water escapes by a downward movement 
through the soil and this percolating water dissolves soil 
gases, any soluble organic material and, to a lesser extent 
for a reason which will appear later, mineral salts. There is 
thus a continual leaching into the subsoil of mineral salts 
and any soluble organic matter, much of which finds its 
way eventually into rivers and then into the sea, and 
the whole process is one of soil impoverishment. 

(b) Evaporation exceeds rainfall throughout the year. ("Arid" 

regions.) 

In this case the rain falling on the soil during a shower 
does not percolate to any depth but soon passes upwards 
and is lost by evaporation. The ascending solution contains 
salts derived from the subsoil and underlying rock and this 
results in a concentration of soluble mineral material at 
the surface of the soil. It is interesting to note that all 
the great civilisations whose development was agricultural 
rather than intellectual have grown up in such arid regions 
as, for example, Peru, Mesopotamia, Egypt and Assyria. 



20 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

(c) Alternation of wet and dry seasons. 

These conditions obtain in temperate regions, such as 
Europe and North America, but, compared with the tropical 
zone, the rainfall is slight and the temperature low so that 
no appreciable upward and downward movements of soil 
material are to be expected. 

It is evident that Malaysia falls into the class (a) where 
rainfall exceeds evaporation and there is a continual down- 
ward movement of water resulting in soil impoverishment. 
Given an infinite expanse of primary forest, the method 
of agriculture employed by certain of the aboriginal races 
of Malaysia (e.g. the Sakai "ladang" method) is probably 
the correct one under the circumstances. A small area of 
forest is cleared and a few crops are grown but, as soon as 
the inevitable impoverishment of the soil sets in, the ladang 
is abandoned and the cultivators seek pastures new and 
leave time to deal with the forsaken clearings. Once a 
limit is set on the area of land available for cultivation, 
however, such methods cannot be employed to any appreci- 
able extent. 

In the humid tropics, where there is a constant downward 
movement of water through the soil as a result of the excess 
of rainfall over evaporation, the final product of weathering 
is remarkably similar throughout the world and is the 
yellowish- to brownish-red material underlying the top-soil, 
and so frequently seen exposed at the roadside and elsewhere. 
The ultimate product of such weathering is usually referred 
to as laterite, but this term was first used by Buchanan 10 
in 1807 to connote an indurated clay, full of cavities and 
pores and containing a very large quantity of iron in the 
form of red and yellow ochres, overlying the granite in 
Malabar; when first quarried this material could be cut 
into bricks and hardened by exposure. This quarried 
laterite is still used to a small extent in building and was 
employed by the Portuguese for the sixteenth-century 
church of St. Paul's at Malacca. The term is now generally 
applied to the weathered products formed in situ, rich in 
alumina and iron oxide, designated lixivium by Mohr, and 
this misuse of the word is so usual that there seems little 




N RP \. ING \S 



i'i in >N VG \INM LANDS] tDES 

SLOPE ON Mil ROADSIDE. 



I \ I I nil 



MALAYSIA 21 

purpose in attempting to restrict the term to its original 
meaning. The definition of laterite has been subject to 
much discussion and in this book the term is employed 
in its popular sense unless the contrary is stated. 11 

According to Mohr, the continuous action of warm rain 
water on rocks results in the removal of practically all 
materials except iron and aluminium oxides, though where 
an impervious substance such as quartz occurs it persists 
unchanged with the alumina and iron oxide in the laterite. 

The question of the formation of humus in the humid 
tropics is discussed on page 99, but it may be stated here 
that, under the usual conditions of temperature and humidity 
prevailing at low elevations in Malaysian forests, humus 
destruction takes place almost as fast as its formation so 
that, in general, practically no accumulation of organic 
matter takes place. Silica and kaolin are more soluble in 
water in the absence of carbon dioxide and organic matter 
than in the presence of these substances but with the 
oxides of iron and aluminium the reverse is the case. Thus, 
continual leaching by water containing little or no dissolved 
organic matter results in the Malaysian soils being impover- 
ished as far as silica and kaolin are concerned, while there 
is a gradual accumulation of iron oxide and alumina. Thus 
it comes about that laterite is so characteristic a feature of 
the humid tropics throughout the world, and it is on laterite 
slopes that landslides are so prevalent. The depth to which 
this type of weathering occurs is often remarkable and it is 
not extraordinary for road-making operations to disclose 
soft weathered mineral material at depths from 50 to 100 feet 
under primary forest. Completely weathered laterite is 
very infertile and of little use to the agriculturist. 

Iron and aluminium oxides are comparatively soluble 
in water containing organic matter, and it will often be 
observed that water in ditches in plantations on peaty soils 
which have been drained is deep reddish-brown, the colour 
being due largely to dissolved ferric material. 

Mohr states that in some parts of Java, where there is 
a tendency towards the alternation of wet and dry seasons, 
in the presence of alkaline salts the climatic conditions 



22 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

result in the formation of a black and not a red soil. Under 
such circumstances leaching occurs during the wet season 
and during the dry months the salts in solution are brought 
to the surface again where they react with the organic 
matter present to produce a black soil. With such soils 
the percolating water frequently contains dissolved organic 
matter and, in consequence, iron oxide is taken into solution 
and may be deposited as a solid concretion some depth 
below the surface. This secondary rock formation in the 
soil, however, occurs the world over but is usually very 
local: the deposition of material rich in silicic acid is a 
feature of arid regions while the formation of concretions 
rich in ferric hydroxide, such as "hard pan" and ortstein, 
is a world-wide phenomenon. 

The reverse of laterization is podsolization which takes 
place in the lowlands in temperate regions and, to a limited 
extent, in the high mountain zone of Malaysia. 12 In this 
process water laden with organic matter percolates through 
the soil resulting in a relative increase in silica in the upper 
horizon and a diminution in the content of iron oxide and 
alumina. 

CLASSIFICATION OF SOILS 

Although all mature soils formed under the same con- 
ditions of temperature and humidity approximate to the 
same composition in whatever part of the world they may 
have originated, it is evident that recently formed soils 
are much more closely related to the parent rocks from 
which they were derived. It has been stated that the 
composition of the soil is primarily a function of the climate 
but, nevertheless, in comparatively recent formations such 
as occur in Malaysia the relation between the soil and 
the parent rock may be sufficiently close to admit of a soil 
classification on this basis. An attempt at such a classi- 
fication of the soils of the Malay Peninsula has been made 
by Belgrave and Dennett. 13 A certain amount of soil 
cartography has been carried out in Sumatra and Java. 14 

According to Arrhenius, 16 continual leaching of soil in the 
humid tropics results in an increase in the degree of acidity, 



MALAYSIA 23 

so that the pH value 16 of a soil gives some indication of its 
age and degree of impoverishment. Martin found that 
low pH values are associated with high rainfall: in north 
Sudan, where the rainfall is low (about 3 inches per annum), 
the average reaction of the soils is about 9-5 but in Sierra 
Leone, under a rainfall of 180 inches per annum, the average 
value is 4-5. Malayan soils range from pH 47 to 5-5, 
while the coastal peat soils are even more acid. 17 Low 
pH values are associated with low figures for soluble 
potassium and phosphate. 18 

SUMMARY 

The Malay Peninsula, Sumatra, Borneo, Java, Palawan 
and the adjacent small islands, constituting the faunistic 
sub-region of Malaysia, were once united to form a continent 
(Sundaland) and still exhibit remarkable uniformity in 
geological formation, flora and fauna. As in other parts of 
the equatorial belt, the climate of Malaysia is characterized 
by a uniformly high temperature (8o° F.) and heavy rainfall 
(100 inches per annum) and the atmosphere is very humid: 
the temperature falls as the hills are ascended. 

Under these climatic conditions, weathering of the rocks 
is largely a chemical process, resulting in the formation of 
lateritic soils, characterized by low silica and high sesqui- 
oxide (A1 2 3 + Fe 2 3 ) content, and it is the presence of 
iron oxide which gives these soils a red or reddish-yellow 
colour. 

REFERENCES 

1. In contradistinction to the Lesser Sunda Islands comprising Lombok, 

Sumbawa, Sumba, Flores, and Timor; Bali is frequently included 
in the group although, faunistically, it pertains to Malaysia. 

2. C. B. Kloss, Bull. Raffles Mus., Singapore, 1929, No. 2, 1-10. 

3. H. N. Ridley, J. Roy. As. Soc, Straits Br., 1911, No. 57, 59. 

4. J. B. Scrivenor, Geology of Malaya, (London), 1931. 

5. H. M. l'endlebury and F. N. Chasen, J.F.M.S. Mus., 1932, 17, 1-38. 

6. F. N. Chasen et al., Bull. Raffles Mus., Singapore, 1933, No. 8, 51-101. 

7. W. B. Haines, Rubber Res. Inst. Malaya, Bull. No. 4, 1931; see also 

C. Braak, Het Klimaat van Nederlandsh Indie, 3 vols., with 
English summary by C. E. P. Brooks, in Koninkliyk Magnetisch 
en Mcteorologisch Observatorium te Batavia, iw.i [929 

8. E. C. J. Mohr, De Grond van Java en Sumatra, 2nd ed. ( \nisterdam), 

1930; see English translation entitled Tropical Soil-forming 
Processes and the Development of Tropical Soils with Special 
Reference to Java and Sumatra, by R. L. Pendleton (Peiping), 1933. 



24 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

9. E. W. Hilgard, Soils, their Formation, Properties, Composition, and 
Relations to Climate and Plant Growth in the Humid and Arid 
Regions (New York), 1906. 

10. F. Buchanan, A Journey from Madras through the Countries of Mysore, 

Canara and Malabar (London), 1807. 

11. F. J. Martin and H. C. Doyne (/. Agric. Sci., 1927, 17, 530-547) 

classified the Sierra Leone soils on the basis of the silica/alumina 
ratio of the clay fraction as follows : 

Laterite . . Si0 2 /Al 2 3 ratio 1-33 and under. 

Lateritic . . Si0 2 /Al 2 3 ratio between 1-33 and 2-0. 

Non-lateritic Si0 2 /Al 2 3 ratio 2-0 and above. 
Some investigators employ the Si0 2 /(A1 2 3 + Fe 2 3 ) ratio rather 
than the silica/alumina ratio, but Martin and Doyne do not 
regard iron as an essential constituent of laterite soils. Recent 
work by F. Hardy (/. Agric. Sci., 1931, 21, 150-166) indicates 
that it is the silicajfree alumina ratio which is important in this 
connection. 

M. S. Anderson and S. Mattson {U.S. Dept. Agric. Bull., 1452, 
p. 46 [1926]) have shown that a linear relationship exists between 
the ammonium ions absorbed by different clays and their 
Si0 2 /(A1 2 3 + Fe 2 3 ) ratios. In laterite and lateritic soils, 
where the silica/sesquioxide ratio is low, the absorption of am- 
monium ions is low so that it would be anticipated that free 
ammonia would be present in measurable quantity in the soils of 
the equatorial belt. R. G. H. Wilshaw [Malayan Agric. J., 1934, 
22, 4-24) has shown that such free ammonia does in fact exist 
in Malayan soils and can be removed by leaching. Clay becomes 
unstable in presence of high rainfall and, usually, is not present 
in large amounts in soils of the humid tropical regions. 

12. M. W. Senstius, Soil Research, 1930, 2, 10-56. 

13. W. N. C. Belgrave, Malayan Agric. J., 1929, 17, 175-178; J. H. 

Dennett, ibid., 17, 179-191; J. H. Dennett, ibid., 1933, 21, 
347-361. 

14. For soil maps see: Sumatra — J. H. Druif, Medan, Deli Proef station 

Meded., 2nd Ser., No. 75, 1932; J. Szemian, Soil Research, I933> 
3, 202-221. Java — J. T. White, Landbouw, 1930-31, 6, No. 3; 
V. J. Konigsberger, E. C. J. Mohr andG. A. Neeb, Arch. Suikerind., 
1931, 39, 601-636; E. C. J. Mohr, ibid., 1931, 39, 881-890. 

For geological maps see: Malay Peninsula — J. B. Scrivenor, 
Geology of Malaya (London), 1931. Sumatra — H. Bucking, Zur 
Geologie von Nord und Ost Sumatra, Samml. Geol. Reichs-Mus., 
1904; D. J. Erb, Beitrage zur Geologie und Morphologie der 
sudlichen West-kiiste von Sumatra, Z. Ges. Erdk. Berlin, 1905. 
Java — R. D. M. Verbeek and R. Fennema, Geologische Beschrijving 
van Java en Madoera (Amsterdam), 1896. Borneo — T. Posewitz, 
Borneo, its Geology and Mineral Resources (London), 1892; F. 
Molengraaf, Geologische Verkinningstochten in Central Borneo 
(Leiden), 1900 (English translation, 1902). 

15. O. Arrhcnius, Arch. Suikerind., 1927, 35, 207. 

16. Th" p\\ v. iluc is numerically equal to the logarithm of the number of 

litres oJ solution containing 1 gram of hydrogen ion ; absolute 
neutr.ln . 1 mi responds to a pH value of 7, acid solutions have a 
pH value below 7, while alkaline solutions have a value above 7. 

17. W. N. C. Belgrave, Malayan Agric. J., 1928, 16, 289-295. 

18. F. J. Martin, Imp. Bur. Soil Sci., Tech. Communication No. 17, 1931. 

p. 21. 




epiphytes on ENTEROLOBIUM SAMAN. 



II. THE PLANT LIFE OF MALAYSIA 

Before the advent of civilized man, practically the whole 
of Malaysia was covered with dense, primeval forest, except 
for estuarine areas on some of the coasts which supported 
mangrove associations. The Malaysian forests are denser 
than those on the Asiatic mainland and differ in certain 
other respects: they are exceedingly damp and are richer 
in species of plants and animals than in individuals. 
Naturalists in temperate climates are prone to visualize 
the jungle of the humid tropics as sunlit forest in which a 
boundless variety of brightly-coloured birds and insects 
nourishes amid a luxuriant plant growth, but very rarely is 
such a picture realized. The tropic jungle is often sombre 
in the extreme, the canopy being so dense in some places 
that the sun never penetrates and consequently the atmos- 
phere is very moist. In the forest itself butterflies are 
not much in evidence; once the sun is up birds are rarely 
seen or heard and the feeling experienced by the solitary 
wanderer of being watched by invisible and, perhaps, 
hostile eyes tends somewhat to deepen the gloom and to 
fill him with vague apprehensions. Nevertheless the jungle 
is not usually silent except during the few moments that 
precede the breaking of a tropical storm. Sometimes the 
chatter of monkeys adds a lively note and they can be heard 
crashing through the tree-tops; squirrels are seen quite 
frequently and on almost any bright day the curiously 
monotonous and ventriloquial call of the cicada rings shrilly 
through the woods. Although large mammals are rarely 
seen, it is soon appreciated that they are not necessarily 
very distant and it behoves a solitary human to walk warily. 
To convey any adequate picture of the Malaysian forest 
is difficult but, for those who can appreciate the immensity 
of the natural forces at work and be aware of the great 
antiquity of many of the plant and animal associations, 
truly the forest is enchanted. 

25 



26 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

In primary forest, trees and large shrubs predominate 
and herbaceous plants are not common. Many of the 
tree-trunks are of enormous dimensions and are supported 
by large buttresses at the base, and some of the tallest 
trees pertaining to the Dipterocarpaceae are remarkable for 
the absence of branching. From the crowns of many 
giants of the forest trail creeping lianas and most of the 
larger trees bear numbers of such epiphytes as orchids and 
ferns on their boughs. Usually there are relatively few 
trees with trunks of small diameter, but fair-sized shrubs 
flourish where space permits and it is often difficult to make 
a passage through the forest on account of the many palms, 
such as rotan (Calamus), possessing hooked spines which 
impede progress. Of the spiny palms, the large bertam 
(Eugeissonia tristis) is remarkable in that it is practically 
stemless and grows in large clumps. Flowers are not 
encountered frequently, the reason being not that they are 
necessarily rarer than in temperate regions but that there 
is no definite flowering season and most plants blossom at 
rather irregular periods : moreover, the majority of the flowers 
are of small size and of a greenish hue and consequently 
are overlooked among so much foliage. On the other hand, 
trees are found occasionally which display a mass of brightly 
coloured flowers, constituting a powerful attraction to bees 
and to large and richly ornamented butterflies. In some 
localities, both on the plains and in the mountains, large 
patches of bamboo occur and in such areas these plants 
predominate very largely to the exclusion of other forms of 
vegetation. 

To the ordinary observer, with a limited knowledge of 
botany, there may appear to be little difference in the 
density or the species of plants at elevations between sea 
level and 5000 feet; but many plant species are confined 
to the plains while others occur only on the hills. Large 
tree ferns (Dicksonia, Cyathea and Alsophila species) are 
a feature of the forested hills at elevations of 2000 feet 
and above and the remarkable pitcher plants (Nepenthes) 
are rarely found in any profusion below 4000 feet. In 
some localities, however, a marked change in the flora can 







f 



wo* 

1 Li .*•! . 



RHIZOPHOR 1 l I'l I IN MANGROVE FOREST. NOTE THE PNEUMATOPHORES 
AND THE VIVIPAROUS FRUITS IN "I III- MUD IN nil FOREGROUND. 



THE PLANT LIFE OF MALAYSIA 27 

be detected when certain altitudes are reached. In parts 
of Pahang, at elevations of over 5000 feet the forest becomes 
less dense, the more massive trees disappear and certain 
species of flowering plants such as the Himalayan violet 
(Viola serpens), 1 Sanicula europea (a member of the wild 
parsley family), Ophiopogon intermedins and golden balsam 
(Impatiens oncidioides) , which are rarely seen below 4000 
feet, become evident : in fact, at this height, the forest begins 
to take on some of the character of European woodland. 
These Himalayan plant species occur only in a restricted 
area in the Malay Peninsula and the first three named are 
found at high altitudes in Sumatra and Java; recently 
V. serpens and 5. europea have been reported from Mount 
Tibang in the centre of Borneo. 

In primary jungle the forest floor is covered with a pall 
of damp dead leaves and twigs and it is surprising how 
noiselessly large animals and the aboriginal tribes move 
about. 

In Malaysia, the composition of the vegetation comprising 
primary forest bears little relation to the underlying geolo- 
gical formation but in some localities, such as the Langkawi 
Islands, the stunted growth of the forest on limestone 
compared with granite is striking. In the Malaysian forests 
the diurnal variations in temperature are but slight: the 
temperature of the soil is practically constant at 75 ° F. 
(24°C), the daily variation even at the surface being 
less than i°. 

MANGROVE FORESTS 

The mangrove forests fringing the coasts are of particular 
interest, although the flora is confined almost entirely to the 
Rhizophoraceae, Verbenaceae, Lythraceae and Meliaceae. 2 
Species in the first-named order predominate, their stilt- 
like roots and curious clusters of respiratory root suckers 
(pneumatophores) projecting from the mud constituting very 
characteristic features of the mangrove forests. The fruits 
of the Rhizophoraceae and a few species pertaining to other 
orders are viviparous, germinating while still united to the 
parent and then falling vertically into the mud beneath 



28 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

and taking root. Viviparity in plants is confined exclusively 
to species occurring in tropical mangrove swamps. 

The nipah palm (Nip a friiticans) is an important species 
found in mangrove plant associations, but, under natural 
conditions, pure stands do not occur to an appreciable extent 
in the Malay Peninsula as they do in North Borneo. The 
nipah palm is used as a source of alcohol in the Philippines 
and an attempt has been made to cultivate it for this 
purpose in Malaya, where its primary use was as a thatching 
material. 

The trees in mangrove forests do not attain the heights 
found in primeval jungle but the canopy is often sufficient 
to give considerable shade to the forest floor. There is a 
certain amount of local variation in the undergrowth: 
leguminous plants are scarce, in fact, Watson lists two 
species only. Mangrove forests constitute a source for the 
supply of firewood, the bulk of "bakau" firewood sold in 
Malaya being obtained from Rhizophora conjugate*, the domi- 
nant species in the usual plant association found in Malaysian 
mangrove areas. 

THE EFFECT OF CLEARING FOREST 

It is a matter of everyday experience that, even in the 
humid equatorial regions, forests do not arise shortly after 
cultivated land is abandoned but they consitute the climax 
in a series of plant associations which appear in turn when 
nature is given a free hand. The whole series of changes 
which precedes the establishment of normal forest 3 is very 
complex and considerable modifications may result from 
but slight changes in local conditions. In Western Europe, 
where land is rarely abandoned and left undisturbed for 
decades, the problem of secondary growth is of little con- 
sequence, but in the humid tropics, particularly at the present 
time, when large areas of cultivated land are being neglected, 
this question is of considerable importance. 

The ultimate consequences of forest clearing in Malaysia 
are of much interest and not without economic importance. 
Most of the clearing that has taken place in the Malay Penin- 
sula during the last decade or so has been for the purpose of 



THE PLANT LIFE OF MALAYSIA 20, 

opening up land for the growth of rubber, and the modus 
operandi has always been the same. Timber of economic 
value is removed and the remaining jungle growth is entirely 
felled, and burned as far as possible. The soil is thus ex- 
posed to the sun and bereft of all shade until such time as 
rubber seedlings are planted. The entire history of a rubber 
estate was, and in many cases still is, a prodigal squandering 
of natural resources; so that when an old plantation is 
finally abandoned the soil is so much impovished that many 
years must elapse before the plant association can bear any 
resemblance to the primeval forest which was ousted. 

Before considering in detail the reversion of a completely 
cleared area to primary jungle it is instructive to examine 
the results of partial clearing of forest, such as frequently 
takes place through the agency of Chinese wood-cutters in 
the Malay Peninsula. These wood-cutters are permitted 
to fell certain species of trees not exceeding a stated diameter 
and to do this effectively the smaller shrubs and undergrowth 
are removed, so that finally almost only the large trees are 
left, though the floor of the forest may still be shaded 
adequately. With such partial clearing, the nature of the 
secondary growth is to a large extent dependent on the 
degree of shade remaining, and if there are sufficient numbers 
of large trees to ensure the forest floor being still heavily 
shaded, ginger plants (Zingiberaceae) appear and soon 
become the dominant feature of the flora. As time goes 
on, other plants obtain a footing and the Zingiberaceae 
gradually drop out, but for a long time such forests arc 
characterized by a rather heavy surface growth, which 
disappears only when new trees and shrubs have attained 
reasonable dimensions. 

When clearing has been more drastic and the forest floor 
is left partially exposed to sunlight, the secondary growth 
follows a different course and, in these circumstances, the 
ground is soon densely covered with low-growing grasses, 
such as Andropogon aciculatus, which act as an effective 
check to other competitors. In the course of time low- 
growing shrubs such as Straits rhododendron (Melastoma 
polyanthum) appear, and once they obtain a footing and the 



30 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

forest floor is effectively shaded, the grass gradually dis- 
appears and the way lies open for larger shrubs and trees 
to become established. 

Shade is a factor of vital importance in determining the 
subsequent history of cleared jungle areas. In the first 
instance outlined above where ginger plants have predomin- 
ated in secondary growth, it is a matter of a few years only 
before the cleared area begins to resemble a forest again; 
and it seems probable that the vegetation following the 
Zingiberaceae largely comprises trees and shrubs which are 
definitely indigenous to the primeval forest. In cases where 
grass has become established, however, the subsequent cycle 
of change is very different and, for many years, the plants 
predominating are those which find no place in primary 
jungle. Incidentally, at the jungle edge there is usually a 
very profuse and tangled mass of low-growing vegetation, 
and the forest floor is so thickly covered that penetration 
is difficult and often impossible without the aid of a 
parang. Visitors to these tropics who have never entered 
the jungle are apt therefore to receive a somewhat false 
impression of its density. 

LALANG 

When the original jungle has been felled and cleared in 
the customary manner and the soil is left exposed to the 
sun, in a remarkably short space of time the whole of the 
abandoned area is covered with a dense growth of a tough, 
leathery spear-grass known as lalang [Imperata arundinacea) . 
This is the " alang-alang " of the Malays and once established 
its eradication is a matter of considerable difficulty: it 
grows rapidly, attaining a height of several feet, and for a 
year or two it flourishes to the total exclusion of all other 
plants. It appears to be remarkably immune from attack 
by insects and fungi and is reported to occur in all the 
humid tropical regions of the world: it is doubtful if it is 
indigenous to Malaysia. There is a generally accepted 
belief among the planting community in Malaya that lalang 
grows only on "good" soil; but it may well be that the 
reverse is the case and that it can do well on soil which is 




I M \.\<. GRASS IN A COCONl'l I'l WIATION. 



THE PLANT LIFE OF MALAYSIA 31 

unable to satisfy so readily the more exacting requirements 
of other forms of vegetation. Actually it would appear 
that the only condition necessary for the vigorous growth 
of lalang on the plains of Malaysia is a soil exposed to the 
sun. 

Once lalang has become established on a rubber plantation 
it can be removed only by digging out each separate plant 
by its root: it has been stated that, when lalang has been 
cut down or burnt, it subsequently flowers freely and there- 
after spreads in an alarming manner. It is generally 
accepted that lalang has a deleterious effect on the growth 
of rubber trees: there is no evidence to show that this 
plant is responsible for the elaboration of toxic substances 
but, according to Vageler 4 it vigorously promotes ortstein 
formation (see page 22) in acid soils. The risk of fire as 
a result of the presence of lalang is a real one as it burns 
furiously even when in vigorous growth, giving off great 
heat, and this fact, coupled with the difficulty of keeping the 
weed under control, demands steps to discourage its increase 
as far as possible on rubber estates. Lalang has a very 
dense and matted root system which is probably a deter- 
rent to sun-loving plants which might otherwise compete 
with this pest. Other plants appear in lalang areas only 
after a considerable lapse of time: Lantana aculeata and 
Straits rhododendron (Melastoma polyanthum) are among 
the first and most conspicuous of these, although the former 
is not native to Malaysia, having been introduced from 
Jamaica by Lady Raffles, according to some authorities. 
Gradually, larger shrubs appear and as the ground shade 
is increased the lalang growth is reduced; but this plant 
maintains its stranglehold on the soil for some years before 
it ceases to be the dominant species of the flora. 

In the Malay Peninsula lalang does not appear to flourish 
on the hills much above 3000 feet, but vast tracts of the 
Battak plateau in Sumatra at elevations of about 4000 
feet are covered with it. When primary jungle is removed 
on the Malayan hills, the secondary growth usually consists 
largely of bracken {Gleichenia linearis) but a dense and low- 
growing grass appears in some situations. 



32 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



Imperata arundinacea and Lantana aculeata are two of the 
commonest weeds in Malaysia and it is certainly curious 
how the entry of a plant or animal into a new faunistic 
region is so often fraught with disastrous consequences. A 
few of the many similar cases can be instanced: the Euro- 
pean rabbit which has become a pest in Australia ; a Japanese 
cockchafer beetle which does untold damage annually to 
certain crops in North America and the American musk rat 

TABLE IV 
Common Malaysian Plants with Country of Origin 



Name. 


Family. 


Country of origin. 


Allamanda cathartica 


Apocynaceae 


Tropical America 


Andropogon aciculatus 


Gramineae 


Tropical Asia to 


Love grass 




Australia 


Antigonon leptopus 


Polygonaceae 


Tropical America 


Honolulu creeper 






Asclepias curassavica 


Asclepiadeae 


West Indies 


Bougainvillea glabra 


Nyctagineae 


Tropical America 


Caesalpinia pulcherrima 


Leguminosae 


West Indies 


Peacock flower 






Casuarina equisetifolia 


Casuarineae 


North Australia 


Crotalaria anagyroides 


Leguminosae 


Tropical America 


Eichhornia crassipes 


Pontederiaceae 


South America 


Water hyacinth 






Enterolobium saman 


Leguminosae 


Tropical America 


Rain tree 






Fagraea fragrans 


Loganiaceae 


Malaysia 


Hibiscus rosa- sinensis 


Malvaceae 


Probably Malaysia 


Garden hibiscus 






Ipomoea learii 


Convolvulaceae 


Tropical America 


Morning glory 






Lantana aculeata 


Verbenaceae 


Tropical America 


Melastoma polyanthum 


Melastomaceae 


Probably Malaysia 


Straits rhododendron 






Mimosa pudica 


Leguminosae 


Tropical America 


Sensitive plant 






Plumeria acutifolia 


Apocynaceae 


Tropical America 


Temple flower 






Ravenala madagascariensis 


Scitamineae 


Madagascar 


Traveller's palm 







which is becoming a menace in western Europe. The giant 
snail {Achatina fullica) was introduced into India from Africa 
over seventy years ago and in about 1900 the species became 
established in Ceylon : before 1922 it was found in the Malay 
Peninsula and, during the last few years, it has spread 



THE PLANT LIFE OF MALAYSIA 33 

rapidly there to become an important pest in gardens. The 
giant snail reached Sarawak in 1928 and was already a 
pest by 1931: during a fortnight in October, 1931, approxi- 
mately half a million snails and twenty million eggs were 
destroyed. 5 Mimosa pudica, the sensitive plant, which is 
so abundant in grassy, sunlit areas in Malaysia, was intro- 
duced from America. 

It is often averred that the reason for introduced species 
of plants and animals becoming so prolific in a new faunistic 
region is to be found in the absence of their natural checks. 
It is evident that such an explanation accounts for very 
few cases: the subject is one of economic importance and 
another explanation is suggested on page 68. 

It is not generally realized what a large proportion of the 
conspicuous plants growing in gardens and waste places in 
Malaysia have been introduced from other parts of the world : 
a list of some of the commoner plants with the country of 
origin is given in Table IV. 

SUMMARY 

The climax of the vegetation in Malaysia is the tropical 
rain forest, characterized by a number of storeys, of which 
the topmost provides a dense canopy. The tallest forest 
trees, which pertain to the family Dipterocarpaceae, are 
notable for the complete absence of branching until the 
crown is reached. An abundance of epiphytes and trailing 
lianas contribute to the riotous growth of the jungle. A 
feature of these forests is the paucity of individuals repre- 
senting the numerous species. 

When primary forest is felled and cleared, in the absence 
of cultivation, there first appears the pernicious lalang grass 
(Imperata arundinacea) , and a year or two elapses before 
low-growing shrubs and leguminous plants gain a footing. 
Eventually the lalang dies out and trees come in, but many 
decades must pass before such secondary associations bear 
any resemblance to primeval forest. 

Apart from cultivated crops, the only other vegetation 
type which covers any area in Malaya is the mangrove 
association, found in muddy estuaries and elsewhere on 



34 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

sheltered coasts. In this community, the Rhizophoraceae 
predominate. Many of the trees in mangrove swamps are 
of particular interest on account of their stilt-like roots, 
pneumatophores projecting above the mud and viviparous 
seeds. 

REFERENCES 

i. According to H. N. Ridley (The Flora of the Malay Peninsula, 1922- 
1925, 5 vols, Ashford, Kent) this species formerly occurred on 
Penang Hill at 3000 feet, but is now extinct. 

2. J. G. Watson, "Mangrove Forests of the Malay Peninsula," Malayan 

Forest Records, 1928, No. 6. 

3. Normal forest is that in which the distribution of the age classes is 

in the proportion required to produce the same yield perpetually : 
A. H. M. Barrington, Burma Forest Bull., 1931, No. 25. 

4. P. Vageler, An Introduction to Tropical Soils, translated by H. Greene 

(London), 1933. 

5. V. H. C. Jarrett, Hongkong Naturalist, 1931, 2, 262-264. 



III. THE SOIL FAUNA 

On old coconut and rubber plantations in Malaysia, where 
the bulk of the original soil organic matter has long since 
disappeared, dead rubber or coconut leaves offer no great 
attractions to insects or other small invertebrates, and such 
areas are usually remarkable for the paucity of the soil 
fauna. A different state of affairs exists in the soil under 
forest for, although at first sight it does not appear to be 
teeming with life, closer inspection shows that a considerable 
soil population exists. It appears that the soil macro-fauna 
presents an analogous case to the tropical forest fauna in 
that a large number of species are represented by com- 
paratively small numbers of individuals. 1 Many animals 
pass the whole of their lives in the soil and many more 
spend some part of it below ground. Most of the larger 
soil invertebrates are found under the roots of trees and 
shrubs and in the neighbourhood of buried timber. 

Invertebrates affect the soil in which they live in several 
ways : — 

(a) their passage through the soil results in improved 
aeration and, possibly, in better distribution of the 
plant nutrients present; 

(b) the deposition of their excreta indicates a quick return 
to the soil of organic and mineral matter which would 
otherwise be retained in growing vegetation ; 

(c) in cultivated areas they may damage certain crops. 

In considering the soil macro-fauna, the subject will be 
dealt with under three heads: 

(a) primary forest ; 

(b) cultivated areas ; 

(c) mangrove swamps. 

35 



36 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

While (b) is the most important from the agricultural stand- 
point, the fauna of the primeval forest is much richer and 
will be considered in greater detail. 

PRIMARY FOREST 

Apart from a very few species of mammals there are no 
vertebrates normally resident in the forest soil in sufficient 
numbers to play a role of any importance. In the Arthro- 
poda, some orders of insects, notably Lepidoptera (butterflies 
and moths), Hymenoptera (ants, bees and wasps), Coleoptera 
(beetles), and Diptera (two- winged flies) are represented by 
many species passing the early stages under ground, but 
several species of ants and beetles spend all their lives on 
or just below the surface. From the standpoint of the soil 
biologist, the most important insects are the Isoptera 
(termites), which creatures, to a certain extent, take the 
place occupied by earthworms in colder climates. 

Termites. 

Termites, popularly known as "white ants," live in 
communities much in the same manner as do certain species 
of bees and ants. The nest is known as the termitarium 
and contains numbers of individuals belonging to different 
castes, the members of which serve the rest of the com- 
munity in the manner to which they are each peculiarly 
fitted. The winged forms, which are only too familiar at 
the lights of bungalows during certain periods of the year, 
comprise males and females which are sexed in the usual 
manner. After the nuptial flight a male and female, or 
rather "king" and "queen," establish a colony and, from 
the ova produced by the queen, individuals are born in 
which the sex organs are aborted. The majority of these 
sexless termites are "workers" whose functions comprise 
constructing and attending to the termitarium, feeding the 
royal pair and the immature forms {nymphs) and, in the 
case of certain species, cultivating the fungus gardens. 
The soft-bodied workers are quite blind, and have to rely 
entirely for their protection on the "soldiers." The latter 
have prominent, hard heads and are equipped with powerful 



THE SOIL FAUNA 37 

mandibles: their primary function is to keep off invaders 
while the workers repair any breaches in the walls of the 
termitarium. 

Once a colony is established the queen produces ova at a 
prodigious rate ; the abdominal segments of her body become 
greatly distended and often she attains an enormous size. 
For the rest of her life she remains imprisoned in the royal 
cell with the king, whose sole function is to fertilize the 
queen. At some periods of the year the sexed winged forms 
are produced in vast numbers and, when certain climatic 
conditions obtain, they leave the termitarium together at 
dusk and the comparatively small proportion that escape 
destruction establish new termitaria. 

Some one hundred and forty species of termites are 
already known from Malaysia and some of them are charac- 
terized by the very distinctive nature of the termitaria 
constructed. 2 Many species appear to reside in decaying 
timber, others nest deep underground and a few species 
are mound builders. Some of the mounds constructed 
attain a height of 5 or 6 feet, but the mound builder most 
frequently encountered in Malaysia is Macrotermes gilvus 
of which the mound is low and, when covered by vegetation, 
is not conspicuous. 

The sexless castes of termites abhor the daylight and 
when it is necessary for them to travel from one spot to 
another a mud tunnel is constructed under cover of darkness. 
The termites resident in the soil play an important role 
in effecting the decomposition of fallen timber. Such 
material has a high carbon/nitrogen ratio and its decom- 
position by bacterial or fungal activity would not only be 
slow but would entail the temporary loss of a certain 
proportion of the soil nitrogen in the cells of the micro- 
organisms. Although the chemical effects of termites in 
the soil have not been investigated in any detail, it seems 
tolerably certain that termites are the most active agents 
in bringing about the decomposition of timber in the forest. 
It has been shown that, in the course of the digestion of 
wood by the Australian mound builder Eidermes exitiosus, 
a considerable decomposition of the cellulose occurs but 



38 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

the lignin remains almost unchanged. 3 The ceaseless 
movements of termites through the soil hasten the decom- 
position of decaying vegetation effected by microbiological 
activity, by increasing the aeration. 

The small termites which infest the timber of dwelling 
houses appertain to the genus Cryptotermes. 



Other Insects 

Ants are important for much the same reasons as are 
termites, but their good works must be assessed at a much 
lower value. Apart from their movements which assist 
the aeration of decaying vegetation, they serve a very 
useful purpose in removing carrion and all dead animal 
matter. The speed with which these creatures work has 
to be seen to be believed and, at any time, immediately an 
insect falls helpless on the forest floor it is removed by ants. 
It is very unusual indeed to find any dead creature or 
excrement in the forest and the rapid return of the nitrogen 
in such material to the soil is attributable to the activities 
of the ants. In short, ants are responsible for the speedy de- 
composition of animal organic matter while termites effect 
the breakdown of some of the more refractory vegetable 
materials. Ants do not confine their attentions exclusively 
to dead animal bodies, many species preying on other 
insects: in fact, the pugnacious tree ant or "kerengga" 
(Oecophylla smaragdina) was employed to clear cultivated 
crops of insect pests until it was discovered that the remedy 
was as bad as the disease. 

The other insects occurring in the soil are probably of 
relatively small importance. Larvae of wood-boring beetles 
possibly play some small part in the cycle whereby timber 
is transformed into a form in which it can be utilized as 
plant food. The grubs of many species of beetles, however, 
are to be found below ground and their presence is un- 
desirable only when they cause damage to growing vegetation 
by feeding on the roots: the dung beetles (Coprinae) enrich 
the soil by burying excreta and the sexton beetles (Silphidae) 
help to bring about the rapid disintegration of animal 



THE SOIL FAUNA 39 

carcases. Cockroaches (Blattidae) and earwigs (Forfi- 
culidae) are not uncommon among the debris on the soil 
surface: to many lepidopterous larvae and pupae the soil 
serves as a refuge. 

Earthworms 

Although earthworms (Oligichaeta) occur commonly in 
Burma, they are by no means abundant in Malaysia and 
they can play no essential part in the equatorial tropics in 
increasing the fertility of the soil in which they live. The 
commonest Malayan species, Pontoscolex corethrurus, is 
generally distributed and in habits resembles the common 
European species of Lumbricus. Occasionally these animals 
may be turned up in some numbers but usually only odd 
examples are seen: they are much more abundant in the 
Langkawi Islands than on the mainland of the Malay 
Peninsula, but it has already been pointed out that north 
Kedah (including the Langkawi Islands) shows affinities with 
south Burma as regards both flora and fauna. Incidentally, 
Pontoscolex corethrurus is common in south Burma. 
Dammerman reported the Enchytraeid Fridericia bulbosa 
as a common species in Sumatra and Java, occurring from 
the seashore to the top of Mount Gedeh (9800 feet). A few 
Malaysian species of earthworms attain enormous dimensions ; 
the Sumatran species Moniligaster houteni often exceeds a 
yard in length. 

In temperate regions earthworms are able to raise the 
level of the soil by throwing up wormcasts 4 : they are 
sensitive to the reaction of the soil and do best when the 
pH is not more acid than 6-o. 

The minute, unsegmented roundworms or eelworms 
(Nematoda) are abundant in the surface layer of tropical 
soils, but they have attracted little attention in these regions. 
Many of the species are free-living saprophytes, utilizing 
dead vegetation and playing a part in the transformation 
of soil organic matter and in improving the soil aeration. 
Some species consume micro-organisms and others, in 



40 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



cultivated areas, often effect considerable damage to 
cultivated crops. 

Woodlice (Oniscoida) are the commonest representatives 
of the Crustacea in the soil surface debris and, of the Myria- 
poda, centipedes (Chilopoda) and millipedes (Diplopoda) 
are fairly common. Ground spiders (Arachnida) are to be 
found almost everywhere and the same may be said of some 
of the smaller species of snails (Mollusca). 1 

The character and concentration of the forest soil fauna 
varies with altitude (see Table V) ; in general, species which 
are characteristic of the tropics tend to disappear as the 

TABLE V 
The Distribution of the Soil Fauna with Altitude in Java 



Height in feet on Mount Gedeh, 
Java. 


Average number of species found 

in debris on 1 square metre of 

soil surface. 


45°°-55°° 

6500-8000 

9800 


53 
3° 
11 



K. W. Dammerman, Trenbia, 1925, 6, 107-139. 

mountains are ascended. Termites are absent above 
5000 feet, ants disappear at about 6500 feet and cockroaches 
are not found above 8000 feet. The woodlice decrease in 
numbers as the mountains are ascended, but sandhoppers 
(Orchestia) are most numerous between 4500 and 8000 feet. 
Mites are not found above 8000 feet but centipedes, milli- 
pedes, ground snails and ground spiders occur at the summit 
of Mount Gedeh (9800 feet). 1 



CULTIVATED AREAS 

On the older rubber estates, many of which have been 
" clean- weeded " for a number of years, the soils have a 
hard surface layer and are deficient in humic material. 
The rubber leaves decompose but slowly, giving only a 
shallow surface layer so that, generally, rubber plantations 
have little to offer to insects. Nevertheless, any soil 
insects which stray to rubber estates have a clear field 



THE SOIL FAUNA 41 

and are not liable to suffer heavily from attack by natural 
enemies. 

Ants are common on rubber estates, as they are almost 
everywhere in the humid tropics, and nest in old timber or 
under the soil surface ; but their presence is of little account 
for most of them travel above ground were possible. The 
presence of buried timber on estates, coupled with the old 
" clean- weeding " methods, makes attractive conditions for 
certain species of termites, some of which can become a 
veritable scourge, having been known as a pest to rubber 
trees from the earliest planting days. 

Two species of termites are of importance on rubber 
estates: one of these is the common and widely distributed 
mound builder, Macrotermes gilvus, and there appear to 
be no reliable records of its having perpetrated any damage 
to rubber. This species favours rather low-lying land on 
or in the vicinity of rubber estates, and the low mounds 
are hardly noticeable where there is a good cover. The 
nest is honeycombed with tunnels and contains a number 
of fungoid, sponge-like masses, the "fungus gardens," 
which are constructed by the workers from partially digested 
wood and leaves and used as food for the young. In the 
queen the abdomen is often greatly distended and may 
attain a length of two and a half inches. Much money 
has been wasted in Malaya in attempts to eradicate M. gilvus 
from rubber plantations under the mistaken notion that 
this insect is responsible for the damage caused by the 
following species. In any case the method of paying a 
bonus to coolies for each queen found is futile, as new queens 
can easily be reared! 

Coptotermes curvignathus is a serious pest of rubber both 
in Sumatra and in the Malay Peninsula. 5 - 6 It has often 
been recorded in Malaya as causing damage to living rubber 
trees, but there appears to be a general impression that the 
species confines its attention to trees which are diseased: 
it can now be stated that there is no justification for the 
belief that healthy trees are immune from attack by 
C. curvignathus. The species is common where it occurs 
and shows a decided preference for damp situations, being 



42 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

found on low-lying land where moist conditions obtain. 
It has been found attacking young rubber showing no signs 
of disease and, if left undisturbed, can inflict fatal injury 
within three or four weeks. The method of attack is as 
follows: first, a mud tunnel is constructed on the trunk 
upwards for a distance of several feet and this tunnel is 
gradually extended laterally until the whole of the lower 
portion of the trunk is encased with mud; there can be 
little doubt that these pests reach the tree by means of a 
subterranean tunnel. A curious point about the initial 
stages of the attack is that, during the daytime, the mud- 
encased trunk contains a great preponderance of soldiers 
over workers. 

Once the mud wall has been completed, the bark is attacked 
simultaneously at several points, but most strongly near the 
base of the trunk. The exudation of latex does not appear 
to deter the invaders and the attack is usually continued 
until the whole of the lower part of the trunk is denuded 
of bark. Then the wood is penetrated and numbers of 
long vertical tunnels are constructed: this is followed by 
an attack on the root system by which time the tree is 
injured beyond recovery. When Hevea "buddings" are 
subjected to the ravages of C. curvignathus, the upper limit 
of the mud wall may not extend beyond the point of union 
of the stock and scion and the termites may concentrate 
their attack upon the snag. When, as is frequently the case, 
there is a good growth of cover it is difficult to detect trees 
which have been attacked until irreparable damage has 
been effected. From the writer's observations, C. curvig- 
nathus can kill three-year old trees within a month and, 
moreover, if treatment is not applied within a fortnight of 
the first appearance of the termites it is unlikely to be of 
any avail later on. 7 

It is probable that these assaults are carried out by 
occupants from a central nest which may be situated some 
distance underground. The fact that the large numbers of 
soldiers found in the early stages of an attack subsequently 
give place to a preponderance of workers indicates that 
communications are maintained between the termites on 



THE SOIL FAUNA 43 

the tree and those in a central termitarium. The queens 
of C. curvignathus have been rarely found and there seems 
to be no evidence to support the suggestion that this species 
makes permanent nests in young rubber trees. Once this 
pest has become established its complete eradication is 
hardly practicable ; formerly, removal of all buried timber was 
advocated, but there is no reason for supposing that this 
constitutes an effective means of control. On estates where 
this costly procedure is practised the timber is collected 
and burnt, but, during the process of collecting and stacking, 
the termites are disturbed and leave the timber before any 
measures are taken to encompass their destruction. 

Another species of termite, Eutermes hirtiventris, has been 
found attacking adult rubber trees in Malaya in a manner 
very similar to that employed by C. curvignathus. 1 

During the last few years a few rubber estates in the 
Malay Peninsula have been visited by the cockchafer beetle, 
Psilopholis grandis, which appeared in numbers on an estate 
in Java some years ago. The ova are deposited in the soil 
and the larvae spend the whole of their lives underground, 
feeding on the roots of growing plants. On estates where 
these beetles are numerous the march of the grubs under- 
ground can be traced by the decay of the vegetation whose 
roots have been devoured. In the presence of this pest it 
is impossible to establish a cover. In Malaya two primary 
parasites of the beetle are known: these are the Scoliid 
wasps, Campsomeris javana and C. pulchrivestita. In 
both cases the female wasps deposit an egg on the well- 
grown beetle grub and the latter is devoured in a few days 
after the wasp larva has hatched; a Bombyliid fly, Hypera- 
lonia tantalus, is a hyperparasite, its grubs feeding on 
those of the wasps. 8 

Earthworms are usually of only casual occurrence in 
estate soils in the Malay Peninsula, but when present they 
may occur in some numbers over a small area. Earthworms 
on plantations are generally confined to rather damp 
situations where the soil is soft: their presence does not 
necessarily indicate a good soil, for these animals were 
found in some abundance on an old estate in Malacca where 



44 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

the soil was almost devoid of organic matter. All the 
earthworms found on rubber estates in Malaya by the writer 
are referable to Pontoscolex corethrurus. 

In secondary growth the soil fauna is a mean between 
that found in the soil of the primeval forest and that under 
rubber, but there is no very apparent relationship between 
the soil surface fauna and that of the vegetation: the 
surface fauna of bamboo areas is poor, but that under 
mangrove is intermediate between that of primary and 
secondary jungle. In most cases, the soil under secondary 
growth was originally exposed to the sun and, in addition 
to the loss of soil organic matter, has suffered from a hard- 
baked surface. Both factors make for a much reduced 
soil fauna but, with the reappearance of a vegetative cover, 
soil conditions gradually improve until eventually the soil 
fauna is comparable with that of the primary forest. 

MANGROVE FORESTS 

The mangrove swamps have a soil population quite 
different from that of the primary forest or the cultivated 
plantation and the animals most in evidence are the 
Crustacea. The soil is pitted with holes and these are 
particularly numerous in tidal areas or on the banks of 
streams. 

The larger holes are surprisingly deep and tenanted by 
various species of crabs which emerge at low tide to seek 
their prey, consisting of sand-hoppers and smaller creatures. 
The small and brightly coloured fiddler crabs (Ocypodidae), 
in which one of the claws is enormously enlarged in the male, 
may often be seen on the mud in hundreds but, at the 
slightest sign of danger (probably when they feel any 
vibration through the ground), they dart back into their 
holes or dig down into the soft wet sand with surprising 
rapidity. 9 The chimney prawn {Thalassima anomala) also 
lives in holes in the mangrove areas and throws up soil 
round the entrace to make a "chimney." The amphibious 
mud fishes (Boliophthalmus and Periophthalmus) seem to be 
equally at home both in the water and out of it. Hermit 



THE SOIL FAUNA 45 

crabs (Paguridea) are often abundant some distance from 
the water. 

The subterranean crustacean tunnels occupy a considerable 
volume but apparently each creature has its own particular 
burrow and builds a new one only under stress of unusual 
circumstances, so it is doubtful if the tunnels represent 
any considerable soil movement. 

SUMMARY 

In the forests of Malaysia, termites (Isoptera) play an 
essential part in the extent to which they effect the destruc- 
tion of timber and other plant debris. Ants (Hymenoptera) 
are important also in that they are responsible for removal 
of dead animal bodies and excreta. 

Earthworms are usually scarce in the soils of Malaya 
although, occasionally, Pontoscolex corethrurus occurs locally 
in some abundance. 

The soil fauna of the mangrove association differs entirely 
from that of primary or secondary forest as it consists 
largely of burrowing species of Crustacea. 



REFERENCES 

K. W. Dammerman, Treubia, 1925, 6, 107-139. 

O. John, Treubia, 1925, 6, 360-419. 

W. E. Cohen, /. Council Sci. and Indus. Res., Australia, 1933, 6, 

166-169. 
C. H. Bornebusch, The Fauna of Forest Soil (Copenhagen), 1930. 

K. W. Dammerman, The Agricultural Zoology of the Malay Archi- 
pelago (Amsterdam), 1929. 
H. M. Pendlebury, Malayan Forest Records, 1930, No. 8, 45-56. 
A. S. Corbet, unpublished data. 

H. M. Pendlebury, J.F.M.S. Mus., 1932, 17, 210-211. 
J. Verwey, Treubia, 1930, 12, 2. 



IV. SOIL MICRO-ORGANISMS 

From the agricultural standpoint, the most important 
groups of soil micro-organisms are Bacteria, Fungi and 
Protozoa and the two first-named are much more active 
than are the Protozoa. In temperate regions, the numbers 
of bacteria present in the soil greatly exceed those of the 
fungi and, moreover, their activities are generally of a 
much more fundamental nature. 

Bacteria possess several characteristics which facilitate 
their division into broad groups. Thus, they may be 
divided into autotrophic, prototrophic and heterotrophic 
bacteria: those in the first group derive energy from purely 
inorganic substances, such as metal salts and carbon 
dioxide, prototrophic bacteria utilize the elements them- 
selves, while the heterotrophic organisms subsist on complex 
organic compounds which have been elaborated by other 
organisms. Autotrophic and prototrophic bacteria are far 
less numerous than heterotrophic species, but, in many 
instances, their activities are of vital importance. A few 
species of prototrophic soil bacteria possess the faculty of 
fixing atmospheric nitrogen. 

A few moulds and yeasts are autotrophic in nature but 
probably all the soil fungi are heterotrophic and most are 
facultative saprophytes, living on dead organic material, 
rather than parasites which live in or on living organisms. 
Their importance in temperate climates has not been over- 
rated but it is evident that they play a much more im- 
portant role in the humid tropics than in the rest of the world. 
They are responsible for the decomposition of large amounts 
of soil organic matter and it has long been known that they 
take an active part in the destruction of cellulose in the soil. 
Certain fungal species form a symbiotic union with the 
roots of some plants and it has been shown, in the case of 
the European pine (Pinus silvestris), that satisfactory 
growth is never attained in the absence of these organisms: 

4 6 



SOIL MICRO-ORGANISMS 47 

such fungi are known as Mycorrhiza, and it has been claimed 
that some of them are able to effect nitrogen fixation. 

Protozoa are unicellular, motile animals, represented in 
the soil by a large number of species: little attention was 
paid to their presence in the soil until Russell and 
Hutchinson 1 showed that they utilized bacteria as food, 
and suggested that, by setting a limit to bacterial activity, 
they diminished soil fertility. Although this view has not 
met with universal acceptance, it has been found that 
bacterial numbers in the soil vary inversely as the numbers 
of protozoa present. 2 

FACTORS AFFECTING THE ABUNDANCE OF 
MICRO-ORGANISMS IN THE SOIL 

For agricultural purposes a soil is evaluated largely on 
the basis of the organic matter present and its rate of 
decomposition, and these factors are included in the rather 
vague term "fertility." The amount of organic matter 
present in a soil depends on its previous history and on the 
climatic factors, temperature, humidity and insolation: the 
rate of decomposition of soil organic matter is correlated 
with the concentration of micro-organisms present which, 
in turn, is conditioned by a number of factors, such as the 
size of the soil particles, degree of aeration, mineral con- 
stituents, soil reaction, moisture, temperature and the 
vegetation growing above the soil. Many of the factors 
affecting the numbers of bacteria present can be controlled, 
and it is of practical importance to consider precisely how 
these control the soil micro-flora. 

(a) Temperature. — For all species of bacteria and fungi 
there is an optimum temperature which may differ con- 
siderably from one species to another. Although, as a 
general rule, fungi have an optimum temperature close to 
that of bacteria, they are able to withstand greater tem- 
perature ranges and to endure high temperatures for longer 
periods. Most species of micro-organisms have a tem- 
perature range between io° and 45 C. and these are termed 
mesophilic: thermophilic organisms work at temperatures 



48 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

above 45 ° and psychrophilic organisms are active between 
5 and 30 C. The very few truly thermophilic bacterial 
species occur in the neighbourhood of hot springs. The 
optimum temperature for most species of soil micro- 
organisms lies between 25 and 37 C. 

(b) Soil moisture. — The species of bacteria and fungi 
normally occurring in soil are not favoured by arid conditions 
but, during water-logging, the aerobic species suffer owing 
to insufficient aeration. The optimum degree of soil 
moisture for most aerobic species of micro-organisms appears 
to be somewhere between 50 and 70 per cent, of the saturation 
value: this is also the optimum value for the growth of 
vegetation. 

(c) Aeration. — Most of the decompositions effected by 
micro-organisms in Malaysian soils are essentially oxidation 
processes requiring the presence of an adequate supply of 
free oxygen. The formation of nitrate from ammonia, 
whether by bacterial, chemical or photochemical means, 
can take place only in the presence of a sufficient supply 
of oxygen and if this supply is withheld the oxidation 
processes are inhibited. 

(d) Soil reaction. — Species of micro-organisms are usually 
very sensitive regarding the reaction of the medium in 
which they live: normally an optimum pK value is found 
(certain species of yeasts have two or even three optimum 
pK values) and the pH range tolerated varies from one 
species to another. In temperate regions it is usual for 
the more acid soils to have a greater proportion of fungi 
to bacteria than is the case with less acid soils: this is not 
necessarily due to soil fungi having acidophilic characters, 
but is accounted for by the fungi having a much wider 
pH range than is usual with bacteria. The tropical soils 
of Malaysia are more acid than those of western Europe and 
North America, and have a high proportion of fungi to 
bacteria. 

The "potato scab" of temperate regions, caused by 
Actinomyces scabies, will not develop if the soil reaction is 
more acid than pH 4-8, while Plasmodiophora brassicae, 



SOIL MICRO-ORGANISMS 49 

the organism responsible for "club root" in cabbages, is 
eradicated when the pH value of the soil is raised to 7-2. 
In temperate countries, the nitrogen-fixing Azotobacter is 
rarely found in soils with a reaction more acid than />H6 
but it appears to be widely distributed in Malaysian soils, 
which normally have a more acid reaction. 

(e) Mineral content of the soil. — Certain elements are 
necessary for the growth and reproduction of even the 
lowest forms of life, although it is surprising how bacteria 
and fungi will grow when it is evident that anything in the 
nature of nutrient can be present only in very small amount. 
Apart from carbon, hydrogen, nitrogen, oxygen, sulphur, 
phosphorus and potassium, the presence of minute amounts 
of other elements is essential; but in most cases these 
requirements can be satisfied by the traces present in water. 
It is improbable that the absence of certain essential metals 
ever sets a limit on bacterial activity in Malaysian soils. 

(/) Vegetation. — The relation between the micro-flora of 
the soil and the vegetation growing above it is only an 
indirect one except in special cases, such as that of the 
nitrogen-fixing Rhizobium bacteria, which are associated 
with the formation of nodules on the roots of leguminous 
plants. In many cases the Rhizobium strains have proved 
to be specific, the organisms producing nodules on one plant 
species being unable to do so on other legumes. It has 
recently been shown that atmospheric nitrogen is fixed by 
bacteria living symbiotically in the root nodules of Casuarina 
equisetifolia. 3 

(g) Depth. — The concentration of micro-organisms in the 
soil varies inversely with the depth below the surface, but 
practically no organisms are found below the level to which 
soil humic matter has penetrated. In Malaysia, the depth 
of the top-soil is much less than in temperate countries: 
a considerable amount of local variation is found but, even 
in the virgin forest where a state of equilibrium has existed 
for centuries, when the carpet of partially decomposed 
leaves and twigs is removed, it is seen that the top-soil 
rarely extends for more than a few inches. When a top- 



50 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



soil layer is encountered which can be measured in feet it 
is safe to conclude that the site was subject to water-logging 
at no remote date. 

On many of the older rubber plantations, where the soil 
has been treated with but scant respect for a number of 
years, a top-soil layer hardly exists. 

In Malaysia the maximum numbers of soil micro-organisms 
occur at a depth of one or two inches: the surface is not 
rich in life, doubtless on account of the sterilizing action 
of the sun. In Table VI are given some representative 
results obtained from plate counts of soil micro-organisms 
at varying depths. 

TABLE VI 

The Variation in Numbers of Micro-organisms with Depth in the 
Malay Peninsula 

(a) Soil under primary forest. 



Depth below surface in 
inches. 


Numbers of micro-organisms per 
i g. moist soil. 


o-5 
35 
8-5 

II-O 


420,000 

443,000 

142,000 

51,000 



(b) Soil under primary forest : the site was remarkable for the presence 
of earthworms [Pontoscolex corethrurus) . 



Depth below surface in 
inches. 


Numbers of micro-organisms per 
1 g. moist soil. 


10 

3° 
60 
90 


813,000 

1,160,000 

181,000 

143,000 



(c) Sungei Buloh Experiment Station: soil under a cover of Centrosetna 
pubescens. On the block from which these samples were taken the top- 
soil was unusually deep, and it was known that the area had been water- 
logged before it was cleared of forest. 



Depth below surface in 
inches. 


Numbers of micro-organisms per 
1 g. moist soil. 


10 

60 

120 

18-0 


179,000 

35.000 
18,000 
19,000 



SOIL MICRO-ORGANISMS 51 

THE NUMBERS OF MICRO-ORGANISMS IN 
THE SOIL 

It has long been recognized that a relation exists between 
the micro-organic population of the soil and its fertility, 
and a certain amount of research work has been carried 
out on this subject during recent years. Several methods 
have been elaborated for the estimation of the total bacterial 
population in a sample of soil, but no one of these is entirely 
satisfactory; the results obtained by the different methods 
exhibit such considerable divergences that an estimate of 
the numbers of bacteria present in a soil sample is of no 
value unless it is known by which method it was determined. 

The two most important methods in use for estimating 
the bacterial numbers in the soil are: — 

(a) direct counting of the stained organisms under the 
microscope ; 

(b) the plate count method, in which the colonies 
developing on an agar plate as a result of inoculation of a 
given volume of soil suspension are enumerated. 

The first method gives values many times greater than 
those obtained by the second procedure, but it is known 
that dead organisms are included in a direct count: on the 
other hand, the plate count method shows only colonies 
formed by aerobic micro-organisms able to grow on the 
particular type of medium employed. It has to be assumed 
that each colony on the plate represents a single organism in 
the inoculum, but it is generally recognized that this 
assumption is rather wide of the mark. 4 Nitrifying or- 
ganisms, Azotobacter, Rhizobium and obligate anaerobes are 
not determined in the usual plate count method. 

As a routine method of estimating the actual numbers of 
micro-organisms in soil the plate count method has not 
been surpassed and its reliability has been investigated by 
Fisher, Thornton and Mackenzie, 5 and by Waksman. 6 
Nevertheless, certain drawbacks are inherent in both 
methods. The direct count method is hardly suitable 
when large numbers of determinations have to be made, 



52 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

and the uncertainty of the significance of the results is such 
that it is rarely employed in agricultural work. The plate 
count method is tedious and it is essential that the platings 
be carried out in replicate : a few isolated results are valueless 
and may be misleading. By the use of Thornton's medium, 7 
the growth of bacteria is encouraged at the expense of the 
fungi, but not infrequently the whole of a plate may be 
covered with a fungoid growth which prevents accurate 
counting of the bacterial colonies present. 

In temperate regions it is customary to enumerate only 
the bacteria and to ignore the presence of fungi, or to employ 
a special plating medium if it is desired to enumerate the 
latter. In the humid tropical regions, however, fungi 
predominate over bacteria to such an extent that, even by 
the use of Thornton's medium, it is impossible to obtain 
plates free from fungi. Actually, fungi play such an 
important role in effecting the decomposition of the soil 
organic matter in the humid tropics, that any determinations 
of the soil micro-organisms which failed to take them into 
account would be misleading. 

In Europe and North America the numbers of bacteria 
per gram of soil, as found by the plate count method, vary 
between several thousands and some millions, but almost 
all determinations have been carried out on cultivated soils. 
Having regard to the abundant native flora and fauna of 
Malaysia it might be anticipated that the soil would habour 
a rich micro-flora, but this is not realized in practice. A 
study of the concentration of micro-organisms in the soil 
in the Malay Peninsula has shown that the numbers are 
surprisingly constant, having a value of about 500,000 
micro-organisms per 1 gram of moist soil. Almost 
invariably fungi predominate over bacteria and the results 
show no significant difference between the numbers present 
in cleared land and in soil under primary forest, suggesting 
that the fluctuations observed in bacterial numbers in the 
soils of temperate regions are largely reflections of changes 
in temperature and soil moisture content. 8 

An experiment was carried out in which the numbers of 
soil micro-organisms were determined, by the plate count 



SOIL MICRO-ORGANISMS 53 

method, at approximately weekly intervals on a block at 
the Rubber Research Institute Experiment Station in 
Selangor. Counts were made while the block was still 
under forest and were continued during the processes of 
forest clearing, " burning-off , " and planting with rubber 
seedlings and a leguminous cover. The experiment was 
continued for two years and the numbers of micro-organisms 
showed significant differences from the value of 500,000 
only during the course of "burning-off" operations, when 
the soil was impregnated with wood ash. That the increased 
population was attributable to the presence of the alkaline 
wood ash and not to any "partial sterilization" effects is 
certain. 

On a few sites in the Malay Peninsula, such as in peat 
areas and in mangrove swamps, low counts were obtained; 
but such results would be anticipated where the usual 
aerobic conditions are lacking. Anaerobic micro-organisms 
are rarely found in forest and cultivated soils in Malaysia. 

Previously it had been assumed that removal of the forest 
cover resulted in an increased microbiological population of 
the soil as a consequence of the increased soil temperature ; 
it is now known that such is not the case and the question 
of the chemical and bacteriological effects of clearing forest 
is discussed on page 103. 

BACTERIA 

All bacteria are microscopic in size and may be classed 
as coccoid (i.e. spherical), rod-shaped or spiral-shaped. 
Although all bacteria exhibit a certain degree of variation 
with regard to shape and size, such differences are not 
usually considerable so that information concerning the 
dimensions of a species is often an aid to its identity. An 
average rod-shaped organism is about 0-5 /x in breadth 
and from 3 to 4 n in length, n being the symbol for the 
micron which is 1/1000 mm. or about 1/25000 inch. 

The variation in form shown by some species of bacteria 
has rendered their classification a matter of difficulty and 
uncertainty. With young cultures of some species, not only 
is a certain amount of individual variation apparent but, 



54 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

perhaps, all the organisms present may exist either as rods 
or cocci, according to the conditions which obtain. A 
Malayan species of Hevea latex bacteria, Micrococcus 
chersonesia Cbt., was found first as a coccus, but it gradually 
changed to a rod form; subsequent plating resulted in the 
reappearance of the coccus form which again reverted to 
the rod form, but eventually the culture lost this power of 
transformation and the rod form persisted. Such variation 
is termed pleomorphism or dissociation. With old cultures, 
large aberrant forms may be present and, on occasion, 
hardly two of the organisms are alike: such are known as 
involution forms. The variation in bacteria is frequently 
ill-defined and some confusion exists with regard to the 
nomenclature of the subject: it seems more than probable, 
however, that the nature of the variation is similar to that 
exhibited by macro-organisms, a certain amount of true 
variability being associated with a young and vigorous 
culture, while the appearance of large and bizarre involution 
forms in old cultures has a parallel in the tendency shown 
by old species (in the phylogenetic sense), such as the 
Proboscidea (elephants) , for individuals to become fewer and 
larger. 

Although the subject has not yet been investigated very 
thoroughly, it appears that the life cycles of bacteria are 
usually relatively simple, showing none of the complex 
transformations exhibited by many microscopic animals. 
With some species, notably Rhizobium leguminosarum, the 
life cycle is highly complicated and the organism passes 
through a number of different stages. 9 

Reproduction is achieved by a process of cell division, 
a single unit dividing into two and thus giving rise to two 
daughter cells which further divide and so on. Given 
favourable circumstances the generation time may be as 
brief as 15 minutes. There is some evidence to show that 
union of two cells may occur at rare intervals. 

Structure of Bacteria 

A biological cell comprises a nucleus, a semi-fluid material, 
largely protein in nature and known as cytoplasm, and the 



SOIL MICRO-ORGANISMS 55 

cell membrane. Until recently no convincing proof had 
been submitted of the presence of a nucleus in bacteria but, 
by means of a new staining technique, Stoughton 10 has now 
demonstrated the existence of what appears to be nuclear 
structures in the plant pathogens Phytomonas malvacearum 
Erw. Smith. The cell membrane is a protective covering 
secreted by the cytoplasm and behaves as a semi-permeable 
membrane through which liquid food and waste products 
pass. Some bacteria have also a gelatinous covering and 
it is on account of the mucilaginous nature of this capsule 
that cells of certain species cohere to form a slimy mass 
known as zooglcea. Locomotion is achieved by the movement 
of fine cilia situated on the cell wall and termed flagella, 
but not all species of bacteria are motile. 

Spore formation takes place in some species whereby 
the organisms are enabled to survive adverse conditions, 
such as heat and desiccation : in sporulation the cell shrinks 
and becomes covered with a hard film formed from a 
secretion of the cytoplasm. In spore-forming organisms, 
an endospore is present in the normal cell and, in some 
species, becomes greatly enlarged at the expense of the 
rest of the cell when sporulation occurs. 

Destruction of bacteria is effected by methods of steriliza- 
tion and disinfection; the former term denoting the total 
destruction of all organisms present, while the latter usually 
applies only to the elimination of disease-bearing species. 
Sterilization is effected by the action of heat or chemicals: 
spore-forming organisms are more difficult to destroy on 
account of their increased power of resistance. It has been 
shown that the course of the death of bacteria by dis- 
infection processes follows the monomolecular law: 

—dyjdt = ky, 

where y represents the number of organisms surviving at 
time t and k is a constant. 11 That is to say, the rate of 
death is proportional to the number of surviving organisms. 
This decrease in numbers of bacteria as a result of exposure 
to adverse conditions follows the usual equation representing 
the phase of decline in the normal bacterial growth curve 
{see page 62). 



56 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Bacterial Metabolism 

The possession of chlorophyll by green plants enables them 
to utilize energy in sunlight and so effect the elaboration of 
complex organic substances from carbon dioxide, water, 
and simple nitrogen salts absorbed by the root system. 
Animals obtain energy by chemical decomposition of 
complex organic materials into simple substances, and 
bacteria and fungi, lacking chlorophyll, also derive the 
energy necessary for growth and reproduction entirely 
from chemical sources. The vast majority of species of 
bacteria and fungi obtain carbon and nitrogen from complex 
substances elaborated by plants and animals. 

The chemical action by bacteria on certain groups of 
compounds has been studied in detail and these reactions 
often afford a ready aid to identification. Sugars are 
readily decomposed by most species of bacteria, giving 
rise to organic acids, alcohols, carbon dioxide and other 
compounds, and such fermentations are accompanied by 
the liberation of energy. The disruption of glucose to 
lactic acid, for example, results in the liberation of 18200 
calories per 1 g.-molecule of glucose: 

C 6 H 12 6 = 2 CH 3 -CHOH-COOH + 18200 calories. 

(Glucose.) (Lactic Acid). 

The oxidation of lactic acid to carbon dioxide and water is 
accompanied by the release of 324000 calories: 

CH 3 -CHOH-COOH+6O=3CO 2 +3H 2 O+324O00 calories. 

(Lactic Acid.) 

Many complex carbohydrates are subject to attack by 
bacteria, with the production of varied products of decom- 
position. As a rule, amino acids and proteins are readily 
disrupted by bacteria and a variety of compounds appear, 
although ammonia and fatty acids are frequently the final 
products of the reaction. 

The oxidation of ammonia to nitrous acid by Nitrosomonas- 
takes place according to the equation: 

NH 3 +30=HN0 2 +H 2 0+7886o calories 



SOIL MICRO-ORGANISMS 57 

and this balance of 78860 calories per 1 g.-molecule of 
ammonia is utilized by the micro-organisms responsible for 
the change. In the same way, Nitrobacter can oxidize 
nitrous acid to nitric acid according to the scheme: 

HN0 2 +0=HN0 3 +i8300 calories. 

Some oxidation processes can be effected in the absence 
of air, the oxygen required being taken from a suitable 
oxygenated compound present. Thus the oxygen necessary 
for the oxidation of lactic acid may be obtained by the 
reduction of nitrate according to the reactions given below, 
which result in a gain in energy of (324000 — 132000) = 
192000 calories per 1 g.-molecule of lactic acid decomposed : 

CH 3 -CHOH-COOH + 60 = 3C0 2 + 3H 2 + 324000 calories 
6KNO3 =6KN0 2 + 60 — 132000 calories. 

Some bacteria can obtain carbon from carbon dioxide 
and such is the case with the nitrifying organisms. With 
Nitrosomonas, for instance, the energy released by the 
oxidation of ammonia to nitrous acid is utilized in the 
synthesis of glucose from carbon dioxide: 

NH 3 +30=HN0 2 +H 2 0+7886o calories 
6C0 2 +6H 2 0=C 6 H 12 6 +60 2 - 672000 calories. 12 

Enzymes 
The chemical reactions effected by bacteria are brought 
about by means of enzymes which are present in, or on the 
surface of, the bacterial cells. The precise nature of enzymes 
is still obscure but, in general, they may be regarded as 
somewhat unstable catalysts which are able to effect the 
decomposition of complex organic substances: a small 
amount of enzyme may be responsible for the transformation 
of large quantities of material. In most cases, enzymes 
appear to be specific in their actions; thus cellulose can be 
decomposed by the enzyme cellulase but not by lactase, 
the enzyme responsible for the oxidation of lactic acid. 
Protein hydrolysis is effected by a class of enzymes termed 
proteases. 



58 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Classification of Bacteria 

The classification of bacteria is a question of difficulty 
particularly as some species can occur both as rods and as 
cocci. The most satisfactory classification yet evolved is 
that of Bergey and the American Society of Bacteriologists. 13 
Bacteria are classified not only according to their mor- 
phological characters, such as shape, presence or absence 
of endospores, motility, etc., but also on the basis of their 
behaviour with certain chemicals and stains. In general, 
bacteria are arranged in orders and families on account of 
morphological characters and in genera and species according 
to their chemical activities. 

Bergey's scheme of classification is considered in the 
appendix in so far as commonly occurring soil bacteria are 
concerned. 

FUNGI 

Fungi are multicellular plants which resemble bacteria 
in that they lack chlorophyll and so are unable to affect 
the formation of cell material by the process of photo- 
synthesis: the energy for growth and reproduction is 
obtained by the disintegration of substances of high energy 
content to compounds of low energy value. The fungi are 
composed of vegetative (or assimilative) and reproductive 
cells; the fungus body (or thallus) comprises a number of 
thread-like hyphae, usually branching to give a network of 
mycelium. In the higher fungi (Ascomycetes and Basidi- 
omycetes) the hyphae are divided into cells of variable 
length by transverse membranes known as septa; the lower 
fungi (Phycomycetes) are non-septate. 

Reproduction is effected by spores and the process of 
spore formation may be asexual or sexual. Asexual spore 
production occurs in all fungi, while sexual spores are found 
mainly in the Phycomycetes. Asexual reproduction can 
take place according to two different processes: — 

(a) Endospores result from division of the protoplasm 
within a mother-cell, when the encasing cell is termed a 
sporangium, and the hyphae bearing the sporangia are 
known as sporangiophores ; 



SOIL MICRO-ORGANISMS 59 

(b) A conidium is a single cell formed at the tip of a 
hypha, which latter is then termed a conidiophore. Fre- 
quently one conidium is formed after another on the same 
hypha, so that a chain of spores results. 

Although fungi are of considerable importance in the soil, 
and more so in humid tropical regions than in temperate 
zones, comparatively little is known regarding tropical soil 
fungi. Fungi play an important part in the decomposition 
of soil organic matter, and their multiplication in the soil 
results in the immobilization of a certain amount of plant 
nutrient material. In the soils of Malaysia, fungi pre- 
dominate over bacteria and, taking into consideration the 
fact that each individual fungus occupies a much greater 
volume than does a single bacterial cell, it is evident that 
a considerable proportion of the total volume of micro- 
organisms present in such soils consists of fungal material. 14 

Classification of Fungi 

The classification of fungi is based on the presence or 
absence of septa, the structure of the asexual fruiting bodies 
and the nature of sexual reproduction, when this method 
is present. A classification scheme is briefly considered in 
the appendix. 

Waksman 15 investigated the soil fungi and examined soil 
samples from various localities in North America and from 
Alaska and the Hawaiian Islands and found certain species 
to be widely distributed. The Mucorineae, and to a lesser 
extent the Penicillium, are more largely represented in 
cooler climates while Aspergillus is more abundant in 
warmer regions. Trichoderma species are found exten- 
sively in the more acid and water-logged soils. 

PROTOZOA 

Although protozoa occur in the soil in some numbers 
their functions are not yet completely understood: many 
feed on living bacteria, while others are able to utilize 
organic and inorganic materials in the soil. The unicellular 
Protozoa are divided into four classes: the Sarcodina, 



60 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Mastigophora and Infusoria are differentiated according to 
their locomotory organs and all are represented by species 
occurring in the soil, while the Sporozoa, typified by the 
malaria parasite, are parasitic and without organs of 
locomotion. 16 

If, as has been surmised, the intestinal protozoa found 
in wood-destroying termites are able to fix atmospheric 
nitrogen, the importance of these creatures in tropical 
soils may be considerable. 17 

SUMMARY 

The most important groups of soil micro-organisms are 
bacteria, fungi and protozoa and, in soils in temperate 
regions, bacteria occur in greatest numbers while fungi 
predominate in the soils of the equatorial belt. 

In general, the number of soil micro-organisms present 
depends on temperature, moisture, aeration, soil reaction 
and depth, there being an optimum for each of these factors. 
In Malaysia, however, conditions are very uniform and the 
concentration of soil micro-organisms (as determined by the 
plate count method) is constant under all usual aerobic 
conditions. 

Anaerobic micro-organisms are rarely found in Malayan 
soils except under swamp conditions. 

REFERENCES 

i. E. J. Russell and H. B. Hutchinson, /. Agric. Set., 1909, 3, 111-144; 
ibid., 1913, 5, 152-221; E. J. Russell, Proc. Roy. Soc, B, 1915, 
89, 76-82. 

2. D. W. Cutler, L. M. Crump and H. Sandon, Phil. Trans. Roy. Soc, B, 

1922, 211, 317-350. 

3. R. N. Aldrich-Blake, Oxford Forestry Mem., 1932, 14, 1-20. 

4. C. L. Whittles, /. Agric. Sci., 1923, 13, 18-48. 

5. R. A. Fisher, H. G. Thornton and W. A. Mackenzie, Ann. Appl. 

Biol., 1922, 9, 325-359. 

6. S. A. Waksman, Soil Sci., 1922, 14, 81-101; 283-298. 

7. H. G. Thornton, Ann. Appl. Biol., 1922, 9, 241-247. 

8. A. S. Corbet, Soil Sci., 1934, 38, 407-416. 

9. H. G. Thornton and N. Gangulee, Proc. Roy. Soc, B, 1926, 99, 

427-45I- 



SOIL MICRO-ORGANISMS 6l 

10. R. H. Stoughton, Proc. Roy. Soc, B, 1929, 105, 469. 

"• ° r a = _l_ .log a 

h - h y% 

12. See A. Harden, Chapter VI of A System of Bacteriology in Relation 

to Medicine, Vol. I, pp. 208-262 (London), 1930. 

13. D. H. Bergey, Manual of Determinative Bacteriology (London), 

4th ed., 1934. 

14. A. S. Corbet, /. Rubber Res. Inst. Malaya, 1931, 3, 5. 

15. S. A. Waksman, Soil Sci., 1917, 3, 565-589. 

16. H. Sandon, The Composition and Distribution of the Protozoan Fauna 

of the Soil (Edinburgh and London), 1927. 

17. L. R. Cleveland, Biol. Bull., 1925, 48, 292. See also A. D. Imms. 

Recent Advances in Entomology (London), 1931, p. 215. 



V. THE BACTERIAL GROWTH CURVE 

When a few bacteria, in pure culture, are introduced into a 
nutrient medium suitable for their growth and reproduction 
there is, at first, a rapid increase in population which is 
followed by a slow decline in numbers until, finally, the 
whole colony is extinct. The curve obtained by plotting 
the population at any instant against the time elapsed 
throughout the life of the colony conforms to a general type, 
of which an example is given in Fig. 3, provided that the 
temperature is maintained constant and there is no con- 
tamination from external sources. 

It is evident that no one simple equation can be found 
to express a skew curve of this nature, but the curve admits 
of division into a number of fairly well-defined portions. 
Writers are by no means agreed on the division and nomen- 
clature of the phases constituting these growth curves and, 
for the present purpose, we shall follow Topley and Wilson, 1 
who recognize four growth phases: — 

(a) lag phase (A-B in Fig. 3) ; 

(b) logarithmic phase (B-C in Fig. 3) ; 

(c) stationary phase (C-D in Fig. 3) ; 

(d) phase of decline (D-E in Fig. 3). 

In a general way, the growth phases mentioned can be 
recognized in all bacterial growth curves but, in some cases, 
the lag phase is of very brief duration, and the decline phase 
admits of further subdivision. 2 It is a matter of interest 
that the appearance of the phase of decline is not necessarily 
the result of a deficiency in the food supply, but, in the case 
of bacteria, it may be due to the presence of inhibitory 
products of metabolism. 

During the lag phase, the duration of which varies 
considerably, the organism is becoming inured to a new set 
of conditions; in the illustration given in Fig. 3 this phase 

62 



THE BACTERIAL GROWTH CURVE 63 

occupies but a small portion of the whole curve. Once an 
organism has adapted itself to altered conditions a rapid rise 
in the population takes place, and the logarithmic phase is 
remarkable chiefly for the great increase in numbers in a 
relatively short space of time. During this growth phase, 
the rate of increase of bacteria at any instant varies directly 
as the number of organisms present and can be represented 
by the equation: 

db/dt = kb, 

where b is the number of micro-organisms present at time t 
and k is a constant. The relationship may also be expressed : 

b = Ae M , 

where e is the base of natural logarithms and equal to 2-718, 
A is a constant and b, t and k have the same significance 
as before. 

It appears to be the rule that the stationary phase, 
during which the population remains approximately con- 
stant, is relatively of very short duration. The phase of 
decline occupies a comparatively large portion of the whole 
growth curve, as the rate of decrease is usually slow com- 
pared with the rapid rise in population characteristic of the 
logarithmic phase. The phase of decline is also logarithmic, 
and can be expressed by the equation: 

- db/dt = k'b 
or b = Ae k>t , 

where the symbols have the same significance as before. 
It is well known that, in the presence of disinfectants, the 
death rate of bacteria is a function of the number of cells 
surviving, and the ultimate process of extinction may be 
very slow indeed (see page 55). It is sometimes a matter of 
difficulty to define growth phases accurately and, in general, 
the transition from one phase to another is abrupt and not 
gradual. 

THE RISE AND FALL IN 
POPULATIONS 

It seems evident that the laws which regulate the rise and 
fall of bacterial populations are of very general application. 



64 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

for it is difficult to believe that the growth of populations 
of higher plants and animals is not governed by the same 
laws. 3 It is not always easy to measure changes in plant 
and animal populations over lengthy periods; but the data 
available support the suggestion that a biological population 
which can be regarded as being maintained in a closed 
system at a constant temperature first increases and then 
declines in numbers according to the laws expressed in the 
bacterial growth curve. 









c,--. 






I 












/ x x 














/ 


» > 












/ 
/ 


s 












/ / 


* 


X 

\ 










' c s 




\ 
% 




1 A 


B^ 


/ 






\ 

X 





TIME IN DAYS 
organisms produced ■ 



Fig. 3. The Growth of a Population of Yeast Cells. 
(Carlson's Data.) 

The full line represents the summation curve showing the total number 
of organisms produced during the experiment. The dotted line is the 
usual bacterial growth curve and depicts the number of living organisms 
present at any given time. 

In both curves A-B represents the lag phase, B-C the logarithmic 
ncrease phase, C-D the stationary phase, and D-E the phase of decline. 

Here must be mentioned a point frequently overlooked 
when comparisons are made between populations of higher 
plants and animals and those of micro-organisms. The 
numbers of bacteria in a culture medium are usually deter- 
mined by counting the cells present, in the same way that 
measurements of animal populations may be made by 



THE BACTERIAL GROWTH CURVE 



65 



counting heads, but when the strength of a colony is estimated 
by measurement of some accumulating product of metabol- 
ism, such as carbon dioxide or nitrate (to which contributions 
have been made by organisms both living and dead) , what 
is being evaluated is not the actual population at the time 
of measurement but the total numbers (living and dead) 
which have existed since the foundation of the colony. 
The plot of the total number of organisms which have 

TABLE VII 
Growth of a Population of Yeast Cells (Carlson's Data) 



Days of 


Daily growth 


Total quantity 


growth. 


of yeast. 


of yeast. 





— 


9-6 


1 


8 7 


18-3 


2 


10-7 


29-0 


3 


18-2 


47-2 


4 


23-9 


71-1 


5 


48-0 


119-1 


6 


55-5 


1746 


7 


82-7 


257-3 


8 


93-4 


350-7 


9 


90-3 


441-0 


10 


72-3 


5133 


11 


46-4 


5597 


12 


35-i 


594-8 


'3 


34-6 


629-4 


'4 


114 


640-8 


15 


10-3 


651-1 


16 


4-8 


655-9 


'7 


3-7 


659-6 


18 


2-2 


66i-8 



During the logarithmic phase (2nd to 7th day) and the phase of decline 
(9th to 17th day), the plot of the time against the logarithm of the daily 
growth is a straight line. 



existed in a microbial colony against the times elapsed, 
gives a summation curve of which an example is shown in 
Fig. 3. This corresponds to the bacterial growth curve 
depicted in the same figure, and the data from which these 
two curves are constructed are given in Table VII. In the 
same way the plot of the total number of trees killed by a 
fungus epidemic, and the growth of rubber fruits after 



66 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

pollination, to take illustrations at random, are examples 
of summation curves. Such can be transposed to ordinary 
growth curves by plotting the numbers of trees killed or 
the fruit growth in unit intervals of time against the time 
elapsed. 

It seems highly probable that when a species of plant or 
animal is introduced into a new habitat, its initial rise and 
subsequent decline in population follow the bacterial 
growth curve, for the restrictions of a closed system and a 
constant temperature usually apply. Many species reaching 
new lands in which climatic conditions are suitable for them, 
find themselves unable to compete successfully with the 
native population and so never gain a footing; but once a 
species has become established, although certain local and 
seasonal fluctuations in population may occur, there is 
little doubt that, under natural conditions, the whole history 
of the introduced species closely conforms to the bacterial 
growth curve. When an organism is introduced into a 
new habitat it seems possible that the duration of the lag 
phase depends primarily on the phase of the organism in its 
former habitat. A species in the logarithmic phase will 
exhibit a much curtailed lag phase in new surroundings 
compared with that shown by a species formerly in the 
decline phase. 

BIOTIC POTENTIAL AND 
ENVIRONMENTAL RESISTANCE 

It is interesting to consider why, under conditions of a 
closed system and a constant temperature, all populations 
(bacterial, plant or animal) do not continue indefinitely 
along the logarithmic phase of increase. Doubtless, this 
phase is terminated for different reasons with different 
species but, in general terms, the explanation appears to 
be as follows. A plant or animal appearing in a new 
faunistic region may encounter conditions differing con- 
siderably from those obtaining in its original habitat. The 
immigrant species may possess a certain degree of adapta- 
bility or plasticity and may be able to change its mode of 
life to meet the altered circumstances. If the degree of 



THE BACTERIAL GROWTH CURVE 67 

adaptability is small the chances are heavily against the 
species becoming established in its new surroundings. 

It has been pointed out 3 that a distinction may be drawn 
between species of plants and animals which are phylo- 
genetically "old" and "young," the latter being those 
which are in the phase of increase in the habitat under 
consideration, while the former are those which have passed 
the stationary phase and are declining in numbers. Certain 
characteristics are associated with this age distinction; in 
general, in an "old" species the geographical races exhibit 
definite and constant differences and individual variation is 
slight, while a "young" species is characterized by slight 
geographical but marked individual variation. With 
Malaysian butterflies it has been found, as far as tested, 
that when there is a great preponderance of males to females, 
usually the other characteristics are those associated with 
species in the phase of decline. 

It is evident that, during the logarithmic phase, organisms 
possess a degree of adaptability which is gradually lost as 
the species proceeds along the road to stability and then 
on to decline and decay; at the same time, conditions 
prevailing in the habitat are gradually changing, and these 
will demand adaptations in the organisms. The power of 
adaptability possessed by an organism and the continuous 
alterations in the habitat correspond to the terms biotic 
potential and environmental resistance respectively employed 
by Chapman, 4 who supposes that equilibrium is attained 
when B = R 

where B represents the biotic potential and R is the environ- 
mental resistance. 

During the declining stages in the life of a species, not 
infrequently, a comparatively large amount of the total life 
mass is concentrated in a few individuals. The dying 
species of Rhinocerus are large and bulky and comparatively 
few in numbers, so that the loss of a few individuals has 
important effects on the future racial history. The forma- 
tion of the curious, enlarged involution forms of bacterial 
cells found in old cultures appears to be a parallel case. 



68 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

PESTS AND DISEASES 

Instances of the general applicability of the bacterial 
growth curve are very frequently encountered in the 
equatorial regions, where life cycles proceed at a much 
greater speed than in temperate countries. 

It has been shown that the incidence of Fomes lignosus 
on rubber estates, if uncontrolled, follows a curve of the 
type under consideration. During the initial stages of 
infection the spread of the disease may be slow, but this 
is followed by the increase phase when growth is rapid; 
latterly, as the biotic potential of the causal organism falls 
off, the decline phase is entered and finally the disease 
almost, if not entirely, disappears. Rarely is treatment 
accorded in the early and more virulent stages of the 
disease, but when control measures are set in train during 
the phase of decrease it is, of course, debatable how much of 
the final success can be fairly attributed to the treatment! 
It is not generally realized, perhaps, that all bacterial and 
fungal diseases and plant and animal pests are more 
amenable to control at some phases than at others. The 
correct time to apply remedial measures is during the 
initial lag phase, but such a counsel of perfection is rarely 
within the bounds of practical politics. Once the initial 
stage has been passed and the disease-bearing organism has 
become established in a new habitat, the difficulty of 
eradication is increased although, of course, the logarithmic 
phase of increase can usually be curtailed by treatment. 
Mycological experiments rarely take these considerations 
into account, so that it is hardly surprising that treatment 
for specific diseases is found to be much more efficacious in 
some cases than in others. 

There is little doubt that the outbreak of an insect pest 
is governed by the same considerations. 

SUMMARY 

When bacteria are grown in pure strain at a constant 
temperature, after an initial lag period of variable duration, 
there is a rapid increase in the population until, following a 



THE BACTERIAL GROWTH CURVE 69 

brief stationary phase, the numbers decline until the whole 
colony becomes extinct. Relative abundance and marked 
individual variability are associated with the increase phase, 
in contrast to the lesser numbers and diminished variation 
characteristic of the decline phase. 

It appears that plant and animal populations follow these 
same growth laws, under conditions of a constant mean 
annual temperature and a closed system. The sudden 
appearance and gradual decline of pests and diseases on 
plantations are illustrative of the same laws. 

REFERENCES 

1. W. W. C. Topley and G. S. Wilson, The Principles of Bacteriology and 

Immunology (London), 1931, Vol. I, p. 73. 

2. C.-E. A. Winslow, Newer Knowledge of Bacteriology and Immunology 

(Chicago), 1928, p. 56, recognizes five phases: 

Phase of adjustment ( = Topley and Wilson's lag phase). 
Phase of increase (= Top ley and Wilson's logarithmic phase). 
Phase of crisis ( = Topley and Wilson's stationary phase), 
Phase of decrease (=Topley and Wilson's phase of decline), and 
Phase of readjustment. 

3. A. S. Corbet, Nature, 1933, 131, 61-62. 

4. F N. Chapman, Ecology, 1928, 9, ill. 



VI. THE SOIL ORGANIC MATTER 

The soil particles are composed of water, mineral salts and 
organic matter, but the customary method of examining 
soil in the laboratory is concerned with determination of 
the moisture content and with mechanical and chemical 
analyses. In ordinary analysis, the moisture is found by 
estimation of the loss in weight, after drying at a temperature 
slightly above the boiling point of water for some hours; 
for a very rough and ready measure of the organic matter 
content may be taken the loss on ignition, after correction 
for the moisture present. 

The method of Mechanical Analysis is concerned with 
the size of the soil particles, and consists of separating the 
mineral portion of the soil into fractions according to the 
diameter of the particles. A knowledge of the texture of 
the soil is of value to the agriculturist, but the information 
acquired as a result of mechanical analysis is frequently out 
of proportion to the labour entailed. 

Of the soil fractions, clay is by far the most important and 
it alone is chemically reactive. Essentially, clay is an 
aluminosilicic acid of complex composition: it is a colloid 
and has the power of adsorbing certain electro-positive ions 
to form a loose complex. Metal ions thus held by clay 
particles can be replaced by other ions of a basic nature and 
the whole process, which is largely a surface phenomenon, 
is known as base exchange. Certain of the physical properties 
of clay, such as power of retaining water, stickiness and 
capacity to swell or shrink according to the moisture content, 
have an important bearing on the question of soil fertility. 

Base-adsorbing powers are possessed also by humus 
which, weight for weight, are more important than those 
of clay. 1 

Chemical analyses of soil are concerned primarily with 
measurements of nitrogen, potassium and of phosphorus 
as phosphate — in fact, with the elements which may prove 

70 



THE SOIL ORGANIC MATTER 71 

to be limiting factors in crop production — but it is evident 
that, in the humid tropics, the mineral portion of the soil 
is of considerable importance as its rate of decomposition 
may be increased by cultural operations. 

It is the presence of organic matter which differentiates 
soil from other naturally occurring, finely powdered materials, 
such as sand, and usually the fertility of the soil is intimately 
connected with its organic matter content. 

Under conditions of equilibrium, such as exist in the 
forests of Malaysia, and in fact under all natural conditions 
where no additions of organic material are made in the 
form of fertilizers, the soil organic matter is almost entirely 
derived from the vegetation which covers the forest floor. 
The excreta and dead bodies of animals add their quota, 
but this contribution is small compared with that obtained 
from the vegetation. 

THE DECOMPOSITION 
OF SOIL ORGANIC MATTER 

The Chemical Composition of Plants 

The principal constituents of plants are cellulose, hemi- 
celluloses, lignin, proteins, simple carbohydrates (sugars 
and starch), fats, waxes, oils, gums and mineral salts. 
The chemical composition of plants varies according to age 
and species : as a plant grows the proportion of cellulose and 
lignin increases considerably, while the amount of protein 
and water-soluble material decreases. Some plants contain 
very characteristic substances such as alkaloids, latex, etc., 
of quite different chemical composition, so it would not be 
anticipated that all plants would decay to give a product of 
definite and constant composition. 

Regarding the formation of soil organic matter the 
water-soluble substances present in plants (simple carbo- 
hydrates such as glucose and fructose, amino acids and 
nitrogen-free organic acids) are of minor importance, as 
they are rapidly decomposed by the soil micro-organisms 
and so take no part in the formation of soil humic matter. 
Cellulose is a condensation product of glucose, insoluble 



72 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

in water and not resolved by dilute acids, and is the chief 
constituent of fibrous and woody portions of plants ; materials 
such as flax and hemp are largely composed of pure cellulose. 
In the soil, in temperate regions, cellulose decomposition 
is effected by the agency of aerobic and anaerobic bacteria, 
actinomyces and fungi, but, in the acid soils of the humid 
tropics, it is probable that fungi and termites are mainly 
responsible for the disruption of cellulose. The destruction 
of cellulose by animals may be due to the presence of in- 
testinal enzymes or it may be effected by bacteria or protozoa 
in the digestive tract. 2 The hemicelluloses are condensation 
products of simple carbohydrates (hexoses and pentoses) 
with uronic acid compounds such as pectin ; all hemicelluloses 
are hydrolysed by dilute mineral acids, yielding hexoses 
and pentoses, and are readily decomposed by soil micro- 
organisms. 

The chemical nature of lignin is not yet fully understood 3 ; 
it is known that this substance contains aldehyde, hydroxyl 
and methoxyl groupings and the presence of the last- 
named has been utilized for the quantitative estimation 
of lignin. Lignin enters into some kind of association 
with cellulose and, in this form, occurs in the tissues of the 
growing plant ; in the soil it is decomposed but slowly and 
so tends to accumulate. 

The most important nitrogen compounds in the plant are 
proteins, substances composed of chains of amino acids and 
differing considerably from one another in their resistance 
to attack by micro-organisms. The simple amino acids 
such as glycine, CH 2 (NH 2 )-COOH, and aspartic acid, 
HOOC-CH 2 -CH(NH 2 )-COOH, readily undergo decomposition 
with formation of a variety of simple compounds such as 
ammonia, organic acids and alcohol. Pure proteins contain 
about 15 to 19 per cent, of nitrogen and, in the laboratory, 
their estimation is carried out by heating with sulphuric 
acid, collection of ammonia thus formed and multiplication 
of the nitrogen figure obtained from the ammonia deter- 
mination by 6-25. 

The remaining plant constituents, such as fats, waxes 
and oils which are esters of alcohols and higher fatty acids, 



THE SOIL ORGANIC MATTER 73 

are of small importance in the formation of soil organic 
matter under normal aerobic conditions. Cellulose, lignin 
and pentosans (the most commonly occurring hemicelluloses) 
constitute between seventy and eighty per cent, of the total 
plant substances. 

The Composition of Soil Organic Matter 

Some confusion exists with regard to the nomenclature 
employed by different authors for the soil constituents. 
It is, perhaps, rather misleading to speak of "soil con- 
stituents," for the soil organic matter must be considered 
as being divisible into groups of related substances rather 
than as comprising a number of well-defined chemical 
entities. The total organic matter in the soil is referred to 
as the soil organic matter: the dark-coloured, colloidal 
organic matter, characteristic of the soil, is termed humic 
matter, while the remaining colourless organic material, 
which is largely soluble, and the undecomposed parts of 
plant and animal residues are regarded as the non-humic 
matter. 

The different groups of the organic constituents of the 
soil decompose at different rates, so there is an accumulation 
of the most resistant materials. Between seventy and 
eighty per cent, of the soil organic matter consists essentially 
of a dark-coloured, colloidal complex of lignin and protein 
which undergoes disintegration very slowly. 4 - 6 The exact 
nature of the union between these substances (or groups of 
substances) is not known, but evidently the combination 
is a very intimate one. It has been shown that protein 
substances which are readily decomposed by bacteria 
become resistant to the action of micro-organisms in 
presence of lignin. 6 Lignin contains no nitrogen so that, 
essentially, the soil humic matter consists of nitrogen-free 
lignin and the nitrogen-containing protein complex. Humic 
matter, obtained from different types of soils receiving 
different manurial treatments, is remarkably uniform in 
general properties but, nevertheless, its constitution varies 
between fairly wide limits. 



74 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

The slow disintegration of the ligno-protein complex 
in the soil is effected by bacteria and actinomyces, and also 
by some species of higher fungi (Hymenomycetes). 

The colourless non-humic matter forms a comparatively 
small proportion of the total soil organic matter and the 
greater part of the nitrogen present in this group occurs in 
the form of peptides. 

In moist soils, fats are slowly decomposed but, in dry 
soil, decomposition may be inhibited altogether; waxes 
are even more resistant to microbiological attack. It has 
been suggested that soil exhaustion may be due to an 
accumulation of fats and waxes, 7 but, although this view 
has not been generally accepted, it may well be that decom- 
position of some of the soil humic matter is delayed owing 
to a coating of fats and waxes on the soil particles. It is 
well known that increased bacterial activity follows treat- 
ment of soil by fat solvents, such as chloroform; in 
Table VIII is shown the result of extracting a Malayan 
soil with chloroform for several days. 

TABLE VIII 

Increase in Microbiological Activity as a Result of Extraction 
of Soil with Chloroform 



F (mg. CO a per ioo g. of 
soil per day). 



Before extraction with chloroform 
After extraction with chloroform 



n-3 

I4'3 



The Carbon : Nitrogen Ratio 

The course and speed of the decomposition in the soil 
of such materials as cellulose, hemicelluloses and the ligno- 
protein complexes, are influenced by the carbon/nitrogen 
ratio. With most cultivated soils this ratio approaches 
a value of n but, clearly, it depends on the relation between 
temperature and rate of decomposition of plant residues. 
In arctic regions, where the soil contains much undecomposed 
vegetation, the C/N ratio is necessarily higher than in the 



THE SOIL ORGANIC MATTER 75 

equatorial belt where decomposition of all plant and animal 
remains is rapid. There is a definite C/N ratio in the 
material constituting microbial cells, so that the presence 
of a certain amount of nitrogen is essential for growth and 
reproduction of micro-organisms, and in the complete 
absence of this element no biological decomposition can 
take place. In organic materials any nitrogen present in 
excess of the requirements of the micro-organisms effecting 
the decomposition appears finally as ammonia or nitrate 
or both; with a deficiency of this element decomposition is 
retarded, perhaps even inhibited. 8 It is well established that 
the addition to the soil of a simple carbohydrate, such as 
glucose, is followed by the disappearance of any nitrate 
present. The explanation of this is that, although glucose 
constitutes a readily available source of energy for micro- 
organisms, growth can take place only if sufficient nitrogen 
is present. In the soil the most readily available nitrogen 
occurs in the form of nitrates, so these compounds are 
utilized immediately by the organisms to make good the 
nitrogen deficiency during the disruption of the carbohydrate. 
This immobilization of the soil nitrogen, consequent 
upon the addition of a substance such as glucose, is quite 
temporary and the nitrogen is returned to the soil in due 
course as the micro-organisms perish and suffer decom- 
position by their fellows. The transitory nature of the 
phenomenon is apt to be overlooked and in perennial crops, 
such as rubber and coconuts, such an immobilization process 
is not without advantages. Another case in point is the 
rotting of timber, where the C/N ratio is very high, and 
for the decomposition to proceed apace, whether by the 
agency of termites or of micro-organisms, reserves of 
nitrogen must be present. Any nitrogen used may remain 
locked up for a considerable period, but it will be returned 
to the soil ultimately. In tropical Malaysia, the rotting 
of timber in the forest is effected very largely by the agency 
of termites; little is known concerning the metabolism of 
these insects, but it appears that the cellulose in timber 
may be decomposed while the lignin remains almost 
unaltered. 9 



76 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

PEAT 

Peat formation results when the soil organic matter 
decomposes in the absence of adequate supplies of oxygen. 
Such conditions obtain with water-logging, when the 
activities of aerobic species of bacteria and fungi are in- 
hibited and decomposition is effected by anaerobic organisms. 
Disruption under anaerobic conditions is much slower than 
the usual aerobic process. 

In the normal course of events on the lowlands of Malaysia 
there is very little accumulation of organic matter in the 
soil, for humus decomposition takes place as fast as humus 
formation. Peat beds evidently represent an accumulation 
of organic matter and it was formerly thought that such 
could not occur in equatorial regions. According to Mohr, 10 
peat formation can take place only in the absence of calcium, 
when the acids formed as a result of cellulose disintegration 
are not removed, but gradually accumulate and so cause a 
slowing down in microbiological activity. In the presence 
of lime, the calcium salts of the acids are formed and many 
of these are precipitated and so removed from solution. 

In Java, true peat is found only in the lime-deficient soils 
of Bantam, but along the east coast of Sumatra and the 
west coast of the Malay Peninsula are considerable areas of 
peat. Peat soil from the Malayan coastal belt has been 
examined by Dennett 11 who found the moisture content to 
be about 80 per cent, and 80 per cent, of the dry matter to 
be organic in nature; the pH value is about 3-0 or below. 
Deposits of peat occur on some of the Malayan hills, notably 
on Gunong Batu Brinchang at an elevation of from 5000 
to 5500 feet. 

The nature of peat varies considerably with the vegetation 
from which it originated and, in temperate regions, peat 
soils can be classified according to the vegetation growing 
above them: in Malaysia it is out of the question to attempt 
any such classification with the data at present available. 
Peat contains a considerable proportion of water and marked 
shrinkage occurs on drying, with consequent drop in the 
level of the soil. This is demonstrated on several rubber 



THE SOIL ORGANIC MATTER 77 

plantations in west Selangor where drainage methods have 
been applied to peat soils and the ensuing soil shrinkage 
has caused the trees to keel over owing to insufficient 
support to the root system. On peat areas in Malaya, 
permanent crops usually thrive during their early stages, 
but subsequently they are apt to become sickly; if peat is 
thoroughly drained before planting, it shrinks to a dry 
powder which cannot be reconditioned. 11 

Waksman and Purvis 12 have found that peat usually 
has an abundant anaerobic micro-flora which increases with 
depth, and the rate of decomposition is profoundly affected 
by the moisture content. In a sample of lowmoor peat 
examined by these authors, it was found that the peat 
decomposed as a whole without any chemical complexes 
disappearing more rapidly than others, indicating that a 
condition of equilibrium had been attained. A tendency 
was noticed, however, for the rather more rapid disappearance 
of the non-nitrogenous complexes leading to an increase 
in the nitrogen content of the residual peat. Drying of peat, 
followed by remoistening, resulted in an increased speed of 
disintegration. 

Peat soils in Malaysia are remarkable for the almost 
complete absence of aerobic bacteria and the very sparse 
population of aerobic fungi. 

On plantations in Malaysia, the water in ditches draining 
peat areas is always of a deep reddish-brown colour, as water 
percolating through soil rich in organic matter is able to 
dissolve iron salts, and a portion of the iron is subsequently 
precipitated as dark reddish-brown ferric hydroxide. A 
number of thread-like bacteria (appertaining to the so-called 
"higher bacteria"), of which the best known isCrenothrix, 
are found in these iron-containing waters and deposit iron 
oxide as a product of their metabolism. 

THE EVOLUTION OF CARBON DIOXIDE AS 

AN INDEX OF SOIL MICROBIOLOGICAL 

ACTIVITY 

The final products of decomposition of the soil organic 
matter are carbon dioxide, ammonia and water. Almost 



78 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

all the carbon dioxide evolved results from the activities 
of soil micro-organisms, although on calcareous soils a small 
amount may be attributable to the destruction of mineral 
carbonate by organic acids. This suggests that a measure 
of the microbiological activity of the soil might be obtained 
by determination of the carbon dioxide or ammonia liberated ; 
in actual practice, ammonia formation can be assessed only 
with difficulty on account of the possibility of part of it 
having been oxidized to nitrite and nitrate by biological 
or other means. 

The idea of following the course of the decomposition of 
soil organic matter, or of estimating the soil microbiological 
activity, by laboratory measurement of the carbon dioxide 
evolved seems to have originated with Kissling and 
Fleischer, 13 but the subject has been examined in some 
detail by Lemmermann and Weissmann, 14 and by the 
writer. 15 The evolution of carbon dioxide from soils in the 
laboratory is effected by micro-organisms passing through 
the phase of decline (see page 62), and, during this period, 
the rate of gas evolution is not strictly proportional to the 
numbers of micro-organisms present. This being so, the 
plate count method can hardly be as efficient for estimating 
soil microbiological activity as a method depending on the 
measurement of the rate of formation of some product of 
metabolism, such as carbon dioxide. Nevertheless, carbon 
dioxide is not liberated as a result of the activities of 
Nitrosomonas and Nitrobacter, but neither are these organisms 
determined in the ordinary plate count. 

The liberation of carbon dioxide from soil samples in the 
laboratory as a result of microbiological activity is expressed 
by the equation : y = F ^ 

where Y represents the total yield of carbon dioxide liberated 
in time t, and m and F are constants. Of these constants, 
m is a measure of the retardation in the rate of gas evolution 
due to laboratory conditions while F is the carbon dioxide 
evolved in the first time unit, that is, if t be the time in days, 
F is the yield of gas on the first day. This factor F has a 
practical significance. 



THE SOIL ORGANIC MATTER 



79 



In tropical Malaysia the soil temperature in the forest 
is constant and the mean daily temperature of soil in more 
open situations is sensibly constant. This being so it might 
be anticipated that, under normal circumstance, the numbers 
of soil micro-organisms and the microbiological activity of 
the soil would be constant. Such appears to be the case 
(see page 52), so that a knowledge of the amount of carbon 
dioxide liberated daily from the soil in situ will be of value, 
particularly for comparative purposes. This is where the 
factor F in the above equation is of practical importance, 
for the daily yield of gas from a soil sample in the laboratory 
on the first day must be very close to the actual value in 
the field. Usually it is not sufficient to measure the carbon 
dioxide evolved during the first 24 hours after sampling 
as a certain degree of irregularity is often evident during the 
short initial period, but if the measurements are continued 
until it is certain that the soil micro-flora have settled down, 
results are obtained which are reproducible within reasonable 
limits. 

In Table IX are given some of the values of m and F 
obtained from soils in the Malay Peninsula. 



TABLE IX 



m and F Values of Soils from 
Temperature 26 


[•he Malay 
-29 C. 


Peninsula: 


Origin of sample. 


0/ 

/O 

Moisture. 


m. 


F. 


Primary forest on the plains 

Mangrove forest 

Rubber plantation 

Rubber plantation where a peat soil had 

been dressed with lime some three 

years previously 


ti-7 
36-2 
32-8 

35° 


o-55 
0-85 
0-63 

0-58 


191 

6-5 

131 

20-7 



F represents the daily yield of carbon dioxide in milligrams per 100 grams 
of moisture-free soil at the beginning of the experiment. 

It was found that the average value for F for the soils 
of the Malay Peninsula was in the neighbourhood of 15. 
Data obtained by Stoklasa 16 showed the carbon dioxide 
(in milligrams) produced in 24 hours from 100 grams 



80 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

of soil at 20° C. to be i-o for meadow soil, between 0*9 and 
i-6 for a forest soil, and about 5-0 for fertile cultivated soils. 
From these figures it appears that carbon dioxide evolution 
in Malaysian soils is some ten times greater than in temperate 
regions during the summer months, and this in spite of the 
microbiological population being considerably smaller. It is 
possible that micro-organisms work more efficiently in 
equatorial regions in the absence of fluctuations in tem- 
perature but, in actual fact, the difference in total population 
is more apparent than real; in the humid tropics fungi 
predominate and play an important part in the decom- 
position of soil organic matter so that, although the soils 
under consideration contain fewer micro-organisms per 
gram than do soils in temperate regions, they contain a 
relatively greater weight of micro-organisms. 

THE RELATION BETWEEN SOIL AND MALARIA 

The nature of an aquatic flora or fauna is largely deter- 
mined by the composition of the water, which, in turn, 
is conditioned by the underlying soil. In the sea and in 
large lakes the composition of the bed is relatively un- 
important but, in ponds and puddles, the volume of water 
is small compared with the area of the underlying soil 
surface. It is hardly surprising, therefore, to find a relation- 
ship existing between the incidence of malaria and the 
nature of the soil. There appears to be no correlation 
between pYL and malaria, and the effect of the geological 
formation beneath the soil is slight. 

In the Malay Peninsula the incidence of malaria increases 
from the coast inland and reaches a maximum intensity 
at the foothills. The malaria-bearing anophelines prefer 
to breed in pure water, and Williamson 17 has adduced 
evidence showing that the oxidation-reduction potential of 
a water has an important bearing on the presence of malaria. 
In water-logged soils, and in stagnant water containing 
decaying vegetable matter, conditions are unfavourable for 
malaria-bearing anopheline larvae and, in this connection, 
it is interesting to note that mangrove forests are by no 
means as malarial as might be anticipated. 



THE SOIL ORGANIC MATTER Ol 

It seems safe to conclude that the numbers of anophelines 
are reduced on sites where anaerobic biological processes 
are in operation. Shallow and stagnant pools do not find 
favour with malaria-bearing mosquitoes, and it has been 
shown that there is less malaria on the coastal peat soil 
of Malacca than further inland. Recent work by Williamson 
strongly suggests the possibility of reducing anophelines, 
especially the dangerous pure water species, by contamina- 
tion of breeding places with organic debris. 

SUMMARY 

The most important plant constituents are cellulose, 
hemiceiluloses, lignin and protein and the first two constitute 
about three-quarters of the total plant substances; much 
smaller amounts of simple carbohydrates, fats, waxes and 
mineral salts occur. 

In the soil most of the plant remains are decomposed 
rapidly by micro-organisms, but lignin and protein combine 
to form a complex which is resistant to attack and so 
accumulates. Although, normally, proteins are readily 
decomposed by bacterial activity, when combined with 
lignin disruption by biological agency is very slow. Almost 
all the soil nitrogen is present in protein complexes. 

Peat results when the decomposition of soil organic matter 
proceeds in the absence of adequate aeration (for example, 
under conditions of water-logging), and peat decomposition 
is effected largely by anaerobic species of micro-organisms. 

Measurement of carbon dioxide evolution shows that 
microbiological activity in Malayan soils is some ten times 
greater than that in temperate regions during the summer 
months. 

REFERENCES 

i. P. E. Turner, /. Agric. Sci., 1932, 22, 83. 

2. K. Mansour and J. J. Mansour-Bek, Proc. K. Akad. Wetensch. 

Amsterdam, 1933, 36, 795-799- 

3. M. Phillips, "The Chemistry of Lignin," Chem.Rev., 1934. 14 - io 3- t 7°- 

4. H. J. Page, /. Agric. Sci., 1930, 20, 455-459'. C. W. B. Arnold and 

H. J. Page, ibid., 1930, 20, 460-477; M. M. S. du Toit and H. J. 
Page, ibid., 1930, 20, 478-488; 193^. 22, 115-125; H. J. Page, 
ibid., 1932, 22, 291-296; R. P. Hobson and H. J. Page, ibid., 
1932, 22, 297-299, 497-515. 5 l6 "5 26 - 



82 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

5. S. A. Waksman and K. R. N. Iyer, Soil Sci., 1932, 34, 43-69, 71-79. 

6. Waksman and Iyer (loc. cit.) prepared artificial " ligno-proteinates " 

in the laboratory, with properties close to those of soil humic 
matter. 

7. R. Greig-Smith, P. Linn. Soc, N.S.W., 1912, 37, p. 735. 

8. S. A. Waksman and R. L. Starkey, The Soil and the Microbe (New 

York), 1 93 1, p. 96. 

9. W. E. Cohen, /. Council Sci. and Indus. Res. (Aust.), 1933, 6, 166-169. 

10. E. C. J. Mohr, De Grond van Java en Sumatra, 2nd ed. (Amsterdam), 

1930. 

11. J. H. Dennett, Malayan Agric. J., 1932, 20, 298-303. 

12. S. A. Waksman and E. R. Purvis, Soil Sci., 1932, 34, 95-113, 323-336 

13. R. Kissling and M. Fleischer, Landw. Jahrb., 1891, 20, 876-889. 

14. O. Lemmermann and H. Weissmann, Z. Pflanz. Dung., 1924, A3, 

387-395- 

15. A. S. Corbet, Soil Sci., 1934, 37, 109-115. 

16. J. Stoklasa, quoted in S. A. Waksman and R. L. Starkey, The Soil 

and the Microbe (New York), 193 1, p. 158. 

17. K. B. Williamson, Malayan Medical J., 1928, 3, No. 4; Trans. Perak 

Br. Engin. Assoc. Malaya, 1934, 9, 150-161. 



VII. THE NITROGEN CYCLE 

Of the elements concerned in the biological changes which 
take place during the life of the growing plant, usually 
nitrogen is of most interest to the practical agriculturist, 
as it so frequently proves to be the limiting factor in crop 
production. Innumerable complex nitrogen compounds 
are present in the soil and in growing vegetation; as the 
plant ages new compounds are synthesized while others 
disappear, and small local variations in temperature, 
moisture content, etc., doubtless result in changes in the 
composition of the nitrogenous substances present in the 
soil. Change is an inevitable consequence of growth; in 
the ever-enduring forest there is little stability in the 
chemical composition of the vegetation, and the equilibrium 
existing in tropical forests is strictly dynamic and not 
static. It is instructive to consider the nitrogen trans- 
formations in some detail and to follow the essential changes 
occurring in so far as they affect the nitrogen atoms in 
circulation; in this cycle the air, growing plants, dead 
vegetation, the soil and the soil fauna all function as 
reservoirs of nitrogen. 

THE NITROGEN CYCLE 

The evidence available at present indicates that nitrogen 
can serve as plant nutrient material only when present in 
solution in the form of nitrates or, to a lesser extent, as 
ammonium salts. It appears that the intake of ammonium 
ions by plants is greatest in the early stages while nitrate 
absorption attains a maximum at the time of flowering, 
but both forms of nitrogen are taken up throughout the 
growth period. 1 The soil solution is absorbed by the root 
hairs of the plant, and in the leaves the nitrogen compounds 
are elaborated into highly complex organic substances, 
mainly protein in nature; the plant nitrogen thus remains 
immobilized until returned to the soil in the substance of 
seeds, dead leaves, etc. Dead vegetation is transformed 

83 



84 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

to soil humus by a series of changes effected through the 
agency of micro-organisms, and further microbiological 
activity results in disintegration of the humic material to 
ammonia, carbon dioxide and water. Oxidation of am- 
monia by biological or chemical means results in the forma- 
tion of nitrate, which may be taken up by the growing plant, 
leached into the drainage water or, perhaps, under certain 
conditions, reduced to ammonia. The essential changes, 
therefore, in the nitrogen cycle are: 

Ammonium ancH Nitrogen Nitrogen Nitrogen 1 Ammonium 

>->compounds-> compounds-^ in soil -> I and 
nitrate ions J in plants in dead organic 1 nitrate 

vegetation matter lions. 

As a rule, the ammonium- and nitrate-nitrogen together 
comprise less than 1 per cent, of the total soil nitrogen 
but, although a considerable volume of research work has 
been carried out on the changes undergone by this "in- 
organic" nitrogen, precise information concerning the 
mechanism of some of the transformations is still lacking. 
In temperate regions the proportions of nitrogen dis- 
tributed between living vegetation, dead leaves, soil micro- 
organisms, etc., depend largely on the season, but in the 
primeval forests of Malaysia seasonal variation is inap- 
preciable, the temperature is constant throughout the year, 
and it is safe to say that the proportions of nitrogen present 
in leaves, animals, micro-organisms and dead vegetation 
are constant and will remain so until disturbed by external 
forces. There are reasons for believing that, in the forests 
on the plains of Malaysia, the rate of decomposition of the 
soil organic matter is equal to the rate of formation so that, 
as a whole, the forest is neither losing nor gaining nitrogen. 
In soil in more open situations humus destruction outpaces 
its formation. In considering the nitrogen cycle in the 
equatorial tropics it is necessary to differentiate clearly 
between soil under forest and soil under secondary growth 
or crops. In the first case a condition of equilibrium is 
attained (Fig. 4), but in the second the soil may be losing 
or gaining nitrogen according to circumstances; a newly- 
cleared area will lose nitrogen (Fig. 5), but when such an 



THE NITROGEN CYCLE 85 

area is abandoned — in other words, when the surface is 
shaded and there is a return of dead vegetation to the soil — 
there is a slow reversion to the original condition, which 



N (ANIMALS) 



N (VEGETATION) 
A 



N (DEAD BODIES) 
N (EXCRETA) 



Ants 



Micro-organisms 
Termites 



N (DEAD VEGETATION) ^ N (HUMIC MATTER) 



Acid soils 



Nitrosomonas 



N (NITRIC ACID) <- 



N (NITROUS ACID) <- 



N (AMMONIA) 



Fig. 4. The Nitrogen Cycle in Soil under Primeval Forest. 

A condition of dynamic equilibrium is attained and any slight losses of 
nitrogen as nitrate in the drainage water are probably made up by the 
nitrogen compounds present in rain water. 



N (HUMIC MATTER) 



Micro-organisms 
Ultraviolet light ? 



i w °/, 



*J>, 



'gh t 






-^ N (AMMONIA) 



N (NITROUS ACID) 



■5 j£ 
u 

< 



• 1 



N (GASEOUS) 



N (NITRIC ACID) 



Y 
Lost in the air 



Lost In the Some lost In the 

drainage water drainage water 

Fig. 5. The Nitrogen Cycle in Soil cleared of Forest and exposed 

to the Sun. 

A continual loss of nitrogen occurs until a new and lower level is reached. 
The only slight additions are due to ammonia and nitric acid in rain water. 



86 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



entails a very gradual accumulation of nitrogen (Fig. 6). 
The reason for these distinctions will be made clear in the 
next chapter. 

N (AIR) 



Micro-organisms 
Termites 



e a 



2© 



•<* 



Acid soils 




N (VEGETATION) > N (DEAD VEGETATION) >N (HUMIC MATTER) 

g -C 

C ~ 

m •»■ 
ao_4> 

9 2 

O n 

'Nitrosomonas Z ^> 
Ultraviolet light ^ 

N (NITRIC ACID) ^ N (NITROUS ACID) < N (AMMONIA) 

Fig. 6. The Nitrogen Cycle in Soil under Secondary Growth. 

Following clearing, increased shading results in a gradual replenishment 
of the nitrogen lost as a consequence of exposure of soil to the sun. The 
gain in nitrogen is due to the activities of nitrogen-fixing bacteria. 

In view of the heavy rainfall in the humid tropics it is 
inevitable that a certain amount of nitrate should be lost 
as a consequence of drainage water finding its way into 
rivers, but it is improbable that, under the conditions of 
equilibrium in the forest, any losses are attributable to the 
evolution of gaseous nitrogen. To remedy the deficit, 
slight though it may be, there must be present some means 
of making additions of nitrogen to the system. It seems 
probable that rainfall is most important in this connection, 
for some 40 to 50 pounds per acre of nitric acid, and smaller 
quantities of ammonia, are returned to the soil by this 
means in the course of a year. It seems probable that 
nitrogen-fixing organisms, both symbiotic and non-sym- 
biotic, are rare in forest soils; witness the scarcity of 
low-growing legumes in the shade of the Malaysian 
forests. 

In the soil organic matter the C/N ratio is in the neigh- 
bourhood of 11, and it is evident that, if this ratio is to 
remain constant, some 11 parts of carbon must be oxidized 



THE NITROGEN CYCLE 87 

to carbon dioxide for every part of nitrogen converted to 
ammonia or nitrate during the process of decomposition. 
In the primeval forest a large proportion of the vegetation 
destined to become soil organic matter is in the form of 
timber, a material with a very high C/N ratio ; in Malaysian 
soils the C/N ratio lies between 33 (peat) and about 9, so it 
is clear that much carbon must escape as carbon dioxide 
during the transformation of dead vegetation to soil humus. 

INORGANIC NITROGEN COMPOUNDS 
IN THE SOIL 

The best known transformations in the nitrogen cycle are 
those concerned with the formation of ammonia and nitric 
acid. When the pB. value of a soil is above 7 the last- 
named compound is present as nitrate but, in the typical 
soils of Malaysia, the pB. is always much below 7 so that 
acids rather than their salts persist. 

It has long been known that ammonium salts are oxidized 
to nitrous acid and then to nitric acid by the agency of soil 
bacteria, and it was formerly thought that the process was 
entirely biological in nature; it has been shown recently 
that oxidation of ammonia in the soil may be effected by 
purely chemical means. 2 - 3 Nitrification consists of two 
important stages: 

(a) the oxidation of ammonia to nitrous acid, 

NH 3 > HN0 2 

(b) the oxidation of nitrous acid to nitric acid, 

HN0 2 > HNO3. 

(a) The reaction Ammonia > Nitrous acid. 

The conversion of ammonia to nitrous acid necessitates 
the presence of a sufficient supply of oxygen so that the 
reaction can proceed according to the equation: 

NH 3 + 30 = HNO a + H 2 0. 
In soils in temperate regions this first stage in nitrification 
is effected largely, if not almost entirely, by Nitrosomonas, 
although it appears that a few other micro-organisms are 



88 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

able to produce small amounts of nitrous acid from am- 
monium salts. Nitrosomonas is particularly sensitive to 
the reaction of the medium in which it works; but Gaarder 
and Hagem 4 in Norway obtained weak nitrification by 
biological agency in media of pK below 2, and it may be 
that here, as well as in the acid soils of Malaysia, species or 
strains of Nitrosomonas occur which can tolerate acid 
conditions. 

It has been established by Dhar and his co-workers that 
ammonia is converted to nitrous acid in the presence of 
sunlight and an adequate supply of oxygen and in the 
complete absence of micro-organisms; the writer has shown 
that nitrite formation can be effected in soils through 
exposure to ultraviolet light. There is a further type of 
chemical nitrification which is not yet clearly understood; 
it appears that nitrite formation can proceed in soil in the 
absence of micro-organisms and of light, 5 and the writer 
has found that measurable amounts of nitrite can accumu- 
late in soil subjected to a process of alternate wetting and 
drying-out. 

There can now be little doubt that these chemical means 
of nitrite production are important, under certain con- 
ditions, in tropical soils. Although the evidence is in- 
complete, it is probable that the formation of nitrous acid 
in the soil proceeds by biological agency in the primeval 
forest while it may be effected largely by chemical means 
in soil on cleared areas exposed to the sun. 

The expression 

NH 3 > HN0 2 

represents the final result of a series of reactions. It has 
been established by the writer that hydroxylamine and 
hyponitrous acid can occur as intermediate compounds 
during the oxidation of ammonia to nitrous acid, so that 
the complete cycle of changes may be shown as under: 

NH 3 > NH 2 OH > H 2 N 2 2 > HN0 2 

\ nun. .ii i.( Hydroxylamine. Hyponitrous acid. Nitrous acid. 

As hydroxylamine is unstable at />H values above 5-0 and 
in the presence of nitrite, its existence in the soil can only be 



THE NITROGEN CYCLE 89 

ephemeral. On the other hand, hyponitrous acid, or its 
calcium salt, may occur in soil in measurable amount, but 
whether its presence is of any importance in the soils of 
equatorial regions remains to be investigated. Normally, 
hyponitrous acid escapes detection in soil during routine 
analysis, for on warming it decomposes to nitrous oxide 
gas and water 

H 2 N 2 2 = N 2 + H 2 0. 

The information available at present indicates that, 
when the oxidation of ammonia to nitrous acid takes place, 
at usual temperatures and in neutral or slightly alkaline 
media, by biological or photochemical agency, there is no 
loss of nitrogen in any gaseous form, the whole reaction 
proceeding quantitatively. 

(b) The Reaction Nitrous Acid > Nitric Acid. 

The oxidation of nitrous acid to nitric acid requires 
oxygen in accordance with the equation: 

HN0 2 + O = HNO3. 

In the soil this oxidation can be effected by Nitrobacter, 
an organism which is sensitive to the reaction of the medium, 
although not so intolerant of acid conditions as Nitrosomonas. 
Until quite recently, it was supposed that this second stage 
in nitrification was always carried out by bacteria, but it 
has been demonstrated conclusively that nitrous acid is 
converted to nitric acid by chemical means in acid media. 
This nitrite loss may not be appreciable at pH values much 
above 5 but, under more acid conditions, the transformation 
is rapid. The relation between nitrite oxidation and the 
pH of the medium is shown in Fig. y. 3 

In the presence of sunlight nitrates are reduced to 
nitrites, so that nitrite constitutes the final product when 
ammonium salts or nitrates are exposed to light; under 
acid conditions, of course, this nitrate reduction cannot 
become effective. 

From the foregoing, it would appear that Nitrobacter 
can be responsible for the presence of nitrate only in soils 
whose pH number is well above 5, while in more acid media 



90 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

the oxidation of nitrous acid is purely of a chemical nature. 
Many observers agree that Nitrosomonas is of more frequent 
occurrence in soils than is Nitrobacter. In the acid soils of 
Malaysia, it is probable that nitrification is effected by both 
biological and chemical agency. Under the shade of the 
primeval forest, nitrite formation in the soil may be 




40 60 80 100 

PERCENTAGE LOSS OF NITRITE 

Fig. 7. The Transformation of Nitrite to Nitrate at varying 
pU Values. (A. S. Corbet.) 

The curve represents the amount of nitrite (0-22 gram potassium nitrite 
per 100 ml. solution) oxidized to nitrate at 32 C in 8 days. 

attributed to microbiological activity and the further 
oxidation to nitrate accounted for by the acidity of the 
medium. In cleared areas exposed to the tropical sun, 
Nitrosomonas or some allied organism may be responsible 
for the appearance of nitrous acid but, at least at the soil 
surface, this compound can result from the operation of 
chemical processes. As in forest soils, the pH value on 
cleared soils is usually sufficiently low to ensure immediate 
oxidation of nitrous acid to nitric acid. 

Doubt has been expressed as to whether nitrification is as 
important in forest as in cultivated soils: it is well known 
that, in temperate regions, fungi are relatively more 
abundant than bacteria in forest soils, and many species 
of the former arc associated with production of ammonia. 



THE NITROGEN CYCLE 91 

Aaltonen considered that, unlike other plants and shrubs, 
forest trees may not receive nitrogen in the form of nitrate. 6 
In this connection it is interesting to note that ammonium 
sulphate has usually a more stimulating effect on the 
growth of Hevea brasiliensis than has nitrate. 

Although the photochemical reduction of nitrate to 
nitrite takes place quantitatively, the biological process of 
denitrification results in the loss of nitrogen in a gaseous 
form. It does not appear probable that denitrification 
processes are responsible for any serious losses of nitrogen 
from the soils of Malaysia, except under conditions of 
water-logging for, excepting swamp and peat soils, anaerobic 
micro-organisms are scarce in Malaysian soils and denitri- 
fication is an anaerobic process. 

LOSSES AND GAINS OF NITROGEN 

It has already been pointed out that, in the rain forests 
of Malaysia, gains or losses of nitrogen in the soil must be 
inappreciable compared with the total quantity of this 
element which is involved. And, moreover, any losses of 
nitrogen which occur as a result of nitrates finding their 
way into the drainage water and thence into streams and 
rivers are probably balanced by nitrogen returned to the 
system in rain water. 

The case is very different with soils exposed to the action 
of the elements, however, for here raising or lowering of 
the nitrogen content of the soil will take place according 
to circumstances. 

Additions of nitrogen to the soil are the result of activities 
of both symbiotic and free-living micro-organisms. The 
role played by nodule bacteria found on the roots of legumes 
has been mentioned already (page 49), but other nitrogen- 
fixing bacteria occur on the leaves of certain plants and on 
the roots of some non-leguminous plants. Shrubs and low- 
growing plants pertaining to the family Leguminosae are 
rare in the Malaysian forests, and it appears probable that 
they play a minor part in the nitrogen cycle under such 
circumstances. In the tropics in general, leguminous 
plants are found in sunny situations, and it is here that their 



92 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

offices in fixing atmospheric nitrogen are needed most, for 
nitrogen losses are considerable when the soil loses the 
shade offered by forest trees. 

Of the two important groups of non-symbiotic nitrogen- 
fixing bacteria, of which Azotobacter and Clostridium may be 
taken as representative examples, the first-named is sensitive 
to acid conditions and occurs more frequently in cultivated 
than in virgin soils. It has been stated that the pYL limit 
for the occurrence of Azotobacter is 6-o, yet the presence of 
this organism has been demonstrated in many Javanese 
and Indian soils. 7 Altson has isolated Azotobacter from 
typical Malayan soils and has shown that isolations were 
able to fix nitrogen in culture. 8 Of the anaerobic nitrogen- 
fixing organisms, Clostridium butyricum Prazm. (= C. 
pastorianum Win.) is the best known; the optimum tem- 
perature for its development is between 28 and 30 C, and, 
although the optimum pH is almost at the neutral point, 
this organism has been found in soils as acid as pH 5-0. 
Practically nothing is known regarding the occurrence of 
Clostridium in Malaysian soils. 

The fact that Azotobacter and Rhizobium are more 
resistant to ultraviolet light than most species of bacteria 
is probably not without significance, as these organisms are 
more active in exposed soils than under the dense shade of 
primeval forest. 9 - 10 

It is generally conceded that the nitrogen losses which 
take place in soils recently cleared of cover and exposed 
to the sun may be very considerable. Those interested in 
gardening in Malaysia are only too well aware of the 
necessity of constantly replenishing the supplies of organic 
matter in the soil. From work carried out by the writer, 
it appears that the nitrogen escapes from the soil mainly 
as nitrogen gas and that comparatively small amounts are 
lost as ammonia and nitrate. 11 That such losses of gaseous 
nitrogen are serious there can be no question, and the 
theoretical and practical aspects of the matter are discussed 
in the following chapters. 

The nitrogen status of the soil appears to be related, not 
only to the mean annual temperature and humidity, but 







z 

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i< 

Z o, 

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THE NITROGEN CYCLE 93 

also to the amount of solar radiation received and, when 
equilibrium is attained, cultivated and exposed soils in 
Malaysia have a lower nitrogen content than those of the 
well-shaded forest. However, under natural conditions in 
soils exposed to the glare of the sun, it is not long before 
the soil temperature and insolation are reduced and the 
surface shading increased as a result of the growth of 
vegetation — be it only lalang — and both symbiotic and 
non-symbiotic nitrogen-fixing bacteria come into operation. 
In Malaysia, leguminous plants are always to be found in 
open sunny situations, although it must be emphasized that 
not all species of the family harbour the Rhizobium nodule- 
forming bacteria on their root systems. Nodules are 
present in the subfamily Mimosaceae, but all the Caesal- 
piniaceae examined by various investigators have been 
found to lack nodules. 12 It has been suggested that the 
absence of nodules on various leguminous plants does not 
necessarily preclude the presence of nitrogen-fixing bacteria 
within the root tissues but, during a careful investigation 
of Cassia tora in the Hawaiian Islands, micro-organisms 
were not found in any of the tissues examined. 13 In 
Malaysia, the sensitive plant (Mimosa pudica) always 
appears at roadsides and in neglected corners of gardens, 
and it is known that this plant harbours nodule-forming 
bacteria. Many of the leguminous plants employed as 
covers on rubber plantations, such as Crotalaria anagyroides, 
Dolichos hosei, Centrosema pubescens, and C. plumieri, have 
been examined by the writer and found to possess nodules 
on the root systems. 

From the foregoing it might be thought that the process 
of restoring to the soil the nitrogen lost by wrong and 
extravagant methods of clearing and cultivation might be 
left to nature : this may be true ; but the process takes many 
decades and there is no doubt that the most economical 
procedure is to conserve the nitrogen already in the soil as 
far as possible, rather than to look round for methods of 
making good the depletions. 

The nitrogen cycle in soil under secondary growth following 
clearing is shown diagrammatically in Fig. 6. In Fig. 5 



94 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

is shown the probable course of the nitrogen cycle when 
soil cleared of forest is exposed to sun and rain in the 
absence of cover. To summarize, it may be said that, 
except for ammonia and nitric acid in the rain water, any 
increase in soil nitrogen depends on biological activity, 
while, under usual circumstances in Malaysia, losses appear 
to be due mainly, if not entirely, to physico-chemical 
processes. 

THE RECOLONIZATON OF KRAKATAU 

At this point it is interesting to consider the recolonization 
of the Krakatau Islands in the Sunda Strait. It seems most 
improbable that any forms of life survived the terrific 
eruptions which occurred in August, 1883, and left the islands 
covered with a layer of hot ashes and pumice to a depth of 
30 to 60 metres. 14 At the present time, the islands are 
clothed with forest and have quite a considerable flora and 
fauna; nevertheless, the inland forest is not the primary 
forest association of the lowlands of Malaysia, but appears 
to have rather the character of an advanced secondary 
association. 

It might be supposed that nitrogen-fixing micro-organisms 
were the pioneers in the recolonization of Krakatau, but it 
seems more probable that the nitrogen reaching the islands 
in the rainfall is sufficient for the requirements of small, 
stray, wind-borne organisms which could easily maintain 
a footing when once established. With an annual rainfall 
of 100 inches, the nitrogen deposited as nitric acid and 
ammonia in this part of the world is of the order of 
24 pounds per acre per annum. 15 

The plant associations on Krakatau have been those 
which would have occurred on the larger islands of Malaysia 
<lming the reversion of cleared land to forest. Earlier 
observers reported the presence of large tracts of lalang 
(Imperata arnndinacea) and elephant grass (Saccharum 
spontaneum) and, later, there appeared the common 
plants of secondary associations, such as Melastoma mala- 
bathricum, stag-horn moss (Lycopodium cermmm), the fern 
Blechnum orientate, Casuarina equisetifolia and Lanlana 



THE NITROGEN CYCLE 95 

camara. It is of particular interest that Ernst reported 
in 1906 that, both on the strand and in the inland zone, the 
number of individual plants pertaining to the Leguminosae 
almost equalled that of all the arborescent and shrubby 
plants belonging to other families. 16 

In 1928, the numbers of terrestrial plant and animal 
species recorded from the Krakatau Islands were 276 and 
751 respectively. 

SUMMARY 

The nitrogenous material in the soil is decomposed by 
biological agency to ammonia and nitric acid and these 
compounds are absorbed by the root systems of growing 
vegetation. 

It appears that a state of dynamic equilibrium exists in 
the primeval forests of Malaysia and the slight nitrogen 
losses due to leaching of nitrate are balanced by ammonia 
and nitric acid returned to the system in the rain water. 

In cleared areas exposed to the sun, nitrogen disappears 
rapidly from the soil until a new and lower level is reached ; 
with the subsequent appearance of a vegetative cover, and 
consequent shading of the soil, there is a gradual increase 
in the nitrogen content of the soil as a result of the activities 
of symbiotic and free-living nitrogen-fixing bacteria. 

The nitrogen losses from tropical soils exposed to the sun 
appear to be effected by a chemical mechanism, entailing 
release of nitrogen gas, with smaller losses of ammonium 
salts and nitrates in the drainage water. Whether a soil 
is gaining or losing nitrogen depends largely on the amount 
of solar radiation received. 

REFERENCES 

t. A. L. Stahl and J. W. Shive, Soil Sci., 1933, 35, 375-399, 469-483. 

2. G. G. Rao and N. R. Dhar, Soil Sci., 1931, 31, 379-384; G. G. Rao, 

1934. 38, I43-I59- 

3. A. S. Corbet, Biochem. J., 1934, 28, 1575-1582; ibid., 1935 (in the 

press) . 

4. T. Gaarder and O. Hagem, Bergens Mus. Aarbok, 1919-1920; Naturv. 

raehke, No. 6, 14. 

5. G. de Rossi, Soc. Intern. Microbiol., Bollet. Scz. Ttal., 1 <»33. 5, 132-136. 

6. V. T. Aaltonen, Inst. Quaest. Forest. Finland, ed [O, Helsinki, 1920. 



96 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



7. J. Groenewege, Arch. Suikerind., 1913, 21, 790-793. 

8. R. G. H. Wilshaw, Malayan Agric. /., 1934, 22, 22. 

9. J. Stoklasa, Centbl. Bakt. (etc.), Abt. 2, 1912, 31, 477-495. 

10. W. A. Albrecht and L. W. Turk, Missouri Agric. Expt. Sta. Bull., 

1930, 285, 112. 

11. A. S. Corbet, Soil Sci. (in the press). 

12. L. T. Leonard, Soil Sci., 1925, 20, 165-167. 

13. E. K. Allen and O. N. Allen, Amer. J. Bot., 1933, 20, 79. 

14. W. S. Bristowe, Proc. Zool. Soc, 1931, 1387-1400; a useful summary 

of the history and biology of the Krakatau Islands is given. 

15. This value was found by calculation from data obtained in Indo- 

China by G. Capus (see P. Vageler, An Introduction to Tropical 
Soils, English trans, by H. Greene (London), 1933). 

16. Some soil samples taken from Krakatau in 1906 were submitted to 

a bacteriological examination by de Kruijff; Bacillus mycoides 
Fliigge and Bacillus megatherium de Bary were stated to be 
abundant, Pseudomonas fluorescens Mig. was found among the 
putrefactive bacteria. Rhizobium leguminosarum Frk. was 
present, nodules being found on the roots of Vigna, Canavalia 
and Erythrina, but Clostridium butyricum Prazm. and Azotobacter 
chroococcum Beij. were not detected. However, a new nitrogen- 
fixing organism, described as Bacterium krakataui de Kruijff, 
but probably a species of Azotobacter, was isolated from pumice 
stone and from forest soil. (See A. Ernst, New Flora of the 
Volcanic Island of Krakatau, English trans, by A. C. Seward 
(Cambridge), 1908, and E. de Kruijff, Bull. Dep. Agric. Ind. Ne'er., 
1906, No. 4.) 



VIII. JENNY'S LAW 

Although it has long been known that the distribution 
of organic matter in the soil is closely related to climatic 
conditions, it is only within the last few years that the 
subject has been approached from the mathematical stand- 
point. Jenny 1 first showed the nitrogen and organic matter 
content of the soil to be functions of the temperature and 
humidity, and as his conclusions are of vital importance 
to tropical agriculture, they are considered here in some 
detail. 

According to Jenny, when climate is considered in relation 
to soil conditions it is found to consist of three primary 
factors, temperature, precipitation and evaporation, but 
the ratio of the last two may be expressed as the humidity 
factor. The nitrogen content of the soil is a function of 
the mean annual temperature and the humidity, or expressed 
mathematically : 

N=f(t.H), 

where N is the nitrogen content of the soil, t the mean an- 
nual temperature and H the humidity factor 

(precipitation\ 
evaporation / 

There are many regions on the earth's surface where the 
ratio precipitation/evaporation is constant: this condition 
obtains practically throughout Malaysia and Jenny based 
his calculations on belts of constant humidity in the United 
States of America. When the humidity factor is constant 
the equation becomes 

N H = /(*), 
but this gives no information concerning the precise nature 
of the relationship. Biological processes are commonly 
represented by sigmoid curves and it would be anticipated 
that the relation between the nitrogen content of the soil 
and the mean annual temperature would be represented by 
such a curve. Jenny has shown that, for soils in North 

97 



0,8 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

America in the same humidity belts, the relation between 
the soil nitrogen and the mean annual temperature is 
exponential and can be expressed by the equation: 

N H = a , 

(I + e^) 

where N H and t have the same significance as before, e is 
the base of natural logarithms and equal to 2-718, and a 
and k are constants. This equation is regarded as a first 
approximation of a fundamental law and it is stated that 
the nitrogen content of the soil decreases exponentially 
with increasing temperature, provided that the same 
humidity factors are operative. 

The same equation expresses the relation between the 
organic matter (or carbon 2 ) content of the soil and the 
temperature, but the numerical values of the constants a 
and k are different for the C/N ratio decreases with increasing 
temperature. Jenny has shown that this ratio decreases 
from 15 to 9 in passing from north to south in North America : 
in Malaysian soils this ratio appears to be higher than would 
be anticipated from these considerations, but it may well 
be that this anomaly is more apparent than real. 

Jenny has shown that the nitrogen content of the soil 
increases with increasing humidity, if the temperature is 
maintained constant, but in tropical and subtropical soils 
the high temperature keeps the soil nitrogen at so low a 
level that moisture conditions are of very secondary impor- 
tance. It is evident that the law governing the nitrogen- 
temperature relationship and the variation in the C/N ratio 
discussed above can be operative only within certain limits, 
for microbiological activity is inhibited both at low and at 
high temperatures, and beyond a certain temperature range 
the equation given on this page cannot apply. With low 
temperatures, when microbiological activity in the soil is 
slight, the C/N ratio approaches that of the overlying 
vegetation. 

The enunciation by Jenny of these laws relating soil 
organic matter with the mean annual temperature and 
humidity marks a very great advance in soil science, but 



JENNY S LAW 99 

there appears to be no doubt that a further factor, that of 
insolation, is important in determining the nitrogen status 
of any given soil. In general, there is parallelism between 
the mean annual temperature and the solar radiation re- 
ceived per annum and so the effects of sunlight are often 
obscured; nevertheless, high temperature unaccompanied 
by exposure to strong sunlight is of itself insufficient to 
produce the effects predicted by Jenny's law. In so far 
as these considerations apply to Malaysia, it appears that 
Jenny's theorem should be re-stated as follows: the nitro- 
gen AND ORGANIC MATTER CONTENT OF THE SOIL DECREASE 
WITH INCREASING INSOLATION AND TEMPERATURE, PROVIDED 
THAT THE SAME CONDITIONS OF HUMIDITY OBTAIN. 

THE RELATION BETWEEN TEMPERATURE 

AND HUMUS FORMATION AND 

DECOMPOSITION 

The optimum temperature for humus accumulation differs 
from that for humus decomposition so that, in presence of 
an adequate supply of moisture and air, the extent to which 
organic matter accumulates in the soil depends on the mean 
annual temperature. 3 Humus formation is favoured by a 
luxuriant growth of vegetation and, in turn, a rich macro- 
flora is promoted by strong sunlight (which ensures vigorous 
assimilation), a fairly high temperature and a sufficiency of 
moisture. Thus humic formation may be taken as corres- 
ponding to the curve A in Fig. 8 which represents the relation 
between vegetative growth and temperature; plant growth 
is inhibited both at low and at high temperatures and has 
an optimum in the neighbourhood of 20°-25° C. 

Humus decomposition is effected largely by the soil micro- 
organisms, whose activities are favoured by high temperature 
and adequate moisture supplies but are not directly 
dependent on solar radiation. The soil micro-flora cannot 
withstand extreme temperatures and have an optimum 
between 30 and 35 C. The relation between humus decay 
and temperature is depicted by curve B in the figure. The 
graphs for humus formation and humus decomposition 



100 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

intersect at about 25 C. as at this temperature the two 
processes proceed at the same speed and there is no accumula- 
tion of organic matter in the soil. At temperatures below 
20°-25° C. humus formation outpaces its decay and at 
temperatures above 25 C. the reverse is the case: these 

7S°F 3S°F. 



Temperature of soil under foresc 



Temperature of soil under cleared conditions 




Fig. 8. The Relation between Temperature and Humus Formation 
and Decomposition. (Adapted from E. C. J. Mohr.) 

The graph (A) represents the growth of vegetation, showing that, in 
general, plant growth ranges from 5 to 45 C, with an optimum at 
20°-25° C. This curve also portrays the conditions under which humus 
formation can take place. The curve (B) shows the relation between 
microbiological activity and temperature, the range being from 5 to 
about 6o°C., with an optimum between 30 and 35° C. Humus decom- 
position depends on the activities of soil micro-organisms and so curve 
(B) expresses the conditions under which this process can take place. 

It is evident that humus accumulation can take place only at tem- 
peratures below 25 C. At 25 C. humus formation and decay proceed 
at the same rate and so the organic matter content of the soil remains 
constant. 

temperatures are in the nature of approximations but they 
may be taken as correct for all practical purposes. The im- 
portant fact emerges that there can be no accumulation 
of organic matter in the soil at temperatures above 
25° C. This generalization, however, does not apply to 
water-logged and peat soils where anaerobic conditions 
obtain. 



JJENNY S LAW 10 1 

THE NITROGEN CONTENT OF MALAYSIAN 

SOILS 

Application of the principles discussed in this chapter to 
the soils of Malaysia yields results of considerable importance 
to the practical agriculturalist. The humidity factor may 
be regarded as constant so that, in Malaysia, the nitrogen 
and organic matter content of the soil are related to the 
insolation and the temperature. In the forests on the plains 
of the Malay Peninsula the soil temperature at or near the 
surface is slightly below 25 C, the daily variation being 
less than i°, and these results may be taken as being of 
general application throughout Malaysia: in such soils the 
rates of humus formation and decomposition are equal so 
that there is neither loss nor gain of humus. 4 As the hills 
are ascended the temperature falls and it would be expected 
that the organic matter content of the soil would increase. 
Observation shows that this is actually the case, but the 
precise relationship between the nitrogen content of the 
soil and altitude has not yet been studied in Malaysia. 
It has been stated that, in general, a fall of io° C. in the 
mean annual temperature results in increasing the average 
nitrogen content of the soil some two- or three-fold. The 
relation between elevation and nitrogen content of the soil 
in Colorado has been examined by Hockensmith and Tucker 
and their results show that, at altitudes between 5000 and 
10,500 feet, an exponential relationship exists, as would 
be deduced from Jenny's law. 5 It is probable that the C/N 
ratio is slightly higher on the forested hills of Malaysia than 
on the plains for, at high altitudes, conditions tend to ap- 
proach those which obtain in temperate regions. 

On the plains, where the high temperature militates 
against the accumulation of humus, the organic matter 
content of the soil is low, and to quote Rahmann, "Tropical 
forest works with a small capital of nutrients and a rapid 
turnover." When the dense primeval forest is envisaged, it 
seems remarkable that the soil can be so poor in organic 
material; yet it is not uncommon to find patches of jungle 



102 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

soil entirely devoid of humus. Where there is any con- 
siderable layer of humus the explanation is to be sought in 
the presence, now or formerly, of factors inhibiting aerobic 
decomposition. In the forest extensive areas of marsh-land 
are encountered and often local conditions obtain which 
result in small areas being more or less permanently water- 
logged, and it is here that decomposition proceeds by slow 
anaerobic processes. Several observers have drawn atten- 
tion to the occasional occurrence of quantities of humic 
matter under lalang, but there is no reason for supposing 
that the explanation of this apparent anomaly is other 
than that given above, for the presence of this weed usually 
indicates recent clearing and, therefore, the possibility of 
a water-logged site having been drained. 

The forested plains of Malaysia are of particular interest 
to the soil microbiologist for the soil temperature is constant, 
and this temperature is that at which humus formation and 
decomposition proceed at the same speed. As the hills are 
ascended the mean annual temperature decreases and humus 
accumulation becomes possible. It is apparent that there 
is some mechanism in operation maintaining the soil nitro- 
gen and carbon at a definite level according to the soil 
temperature and amount of insolation received, and it is 
now possible to envisage clearly the consequences of felling 
jungle and exposing the soil to the sun during clearing 
operations. Such a procedure has immediate and important 
results: the temperature is raised and the soil is exposed to 
the direct action of the sun and, for the time being, there 
is a complete cessation of any addition of organic material 
to the soil system. 

When clearing takes place on the plains, as it usually 
does, before equilibrium can be attained the organic matter 
content of the exposed soil must fall to a new level in accord- 
ance with the considerations advanced above. That is to 
say, the increase in insolation and temperature, consequent 
upon felling and clearing, is followed by a loss of humic 
matter and such losses continue until the level for the new 
set of conditions is reached. It is evident that, under such 
circumstances, addition of nitrogen to the soil can have only 



JENNY S LAW 103 

a transient effect ; in fact, in the absence of growing vegeta- 
tion such nitrogen can neither be utilized by the plant nor 
retained by the soil particles but is speedily lost. 

LOSSES OF NITROGEN FROM 
CLEARED AREAS 

It is now being realized that the loss of nitrogen which 
follows increased soil temperature and exposure to the sun 
may be very considerable, and Wilshaw has suggested that, 
under such conditions, decomposition of soil organic matter 
proceeds at a far greater rate than is generally supposed. 6 
Recently, this investigator has shown that, in soil exposed 
in pots in Malaya, the total nitrogen content (as determined 
"by the Kjeldahl method) dropped from 0-170 to 0-105 per 
cent, in seventeen weeks. 7 

The precise manner in which any nitrogen present in 
excess over that which can be retained under given conditions 
of insolation and temperature escapes from the soil is a 
matter of paramount importance. In this connection it 
must be remembered that the soil systems on most rubber 
estates are not in a state of equilibrium: such is certainly 
not the case with the older clean-weeded plantations where 
almost all soil organic matter has long since vanished, but 
the reason for this is considered on page 105. In the past 
it has been generally assumed that any increase in soil 
temperature results in a speeding up of the microbiological 
processes whereby the humus is rapidly oxidized to carbon 
dioxide, nitrate and water, but it is now known that this 
explanation is not correct, for the writer has shown that no 
significant changes occur in the soil microbiological popula- 
tion as a result of forest clearing. 8 Humus decomposition 
is effected by aerobic species of fungi and bacteria which 
normally appear on agar plates, and there can be no reason 
for supposing that the opening-up of forest (with consequent 
increase of soil temperature and exposure of the soil surface 
to the sun) results in the appearance of large numbers of 
micro-organisms able to decompose soil organic matter but 
incapable of growing on ordinary culture media. 

If this organic matter disappearance is not biological in 



104 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

nature it must be a physical or chemical process, but the 
most important physical factor is erosion and it is known that 
the amount of organic material lost from the soil by such 
mechanical means does not nearly account for all the losses. 
It becomes evident then that the mechanism responsible 
for the losses is chemical in nature : mere exposure of soil to 
high temperature in a humid atmosphere does not result in 
conversion of humic material to a soluble form which may 
be lost in the drainage water, nor does an alternating process 
of drying and washing-out, such as occurs almost daily in 
newly-cleared areas in equatorial regions, effect any ap- 
preciable transformation of soil organic matter into soluble 
form. Almost the only possibility left is that increased 
insolation is responsible for a photochemical change which 
results in humic matter being broken down to carbon dioxide 
and nitrogen, together with smaller quantities of ammonia 
and nitrate, and experimental evidence has been adduced 
which shows that this explanation is almost certainly the 
correct one. 9 Wilshaw opined that nitrogen was probably 
lost from the soil in gaseous form but he attributed the 
losses to increased bacterial activity. The precise nature of 
this chemical mechanism, which results in losses of nitrogen 
from exposed soils, is obscure but it has been shown by the 
writer in the laboratory that such nitrogen losses occur 
rapidly when soil is exposed to ultraviolet light in the 
presence of free alumina. Under the experimental conditions 
described, the fall in the nitrogen level is not continuous 
but ceases when a certain value has been reached, which is 
in accord with the views developed earlier in this chapter. 
The results obtained by Hardy show that free alumina is 
present in quantity in the lateritic soils of the equatorial 
tropics. 10 It is possible that the release of elementary 
nitrogen from the soil, under the tropical conditions con- 
sidered, is in some way connected with the formation of 
hydrogen peroxide by the the action of ultraviolet light. 
Nevertheless, the difficulty in accepting this as a complete 
explanation of the phenomenon is the apparent failure of 
ultraviolet light to penetrate any distance below the surface 
of the soil. 



JENNY S LAW 105 

It is apparent from the foregoing that, in the absence of 
shading, it is not possible to increase the nitrogen content 
of soils on the plains of Malaysia beyond a certain low level, 
which is below that normally found in soils under forest. 
In the surface soils of the sugar cane districts in Java, for 
instance, the average amount of nitrogen present is 0-076 
per cent. 11 Addition of nitrogenous matter, whether in the 
form of inorganic fertilizers or as green manure, cannot 
result in a permanent increase in the nitrogen status of the 
soil, and such additions can be justified only in the presence 
of growing vegetation ready to assimilate immediately the 
products of decomposition. Even so, a large proportion 
of the nitrogen in added manure may disappear into the 
air in the form of elementary nitrogen. 

When a cleared area is left to recuperate there is, usually, 
an immediate growth of lalang grass, followed subsequently 
by the appearance of other weeds and shrubs ; in this manner 
the soil temperature and the amount of solar radiation 
received are lowered, so that it is possible for the soil grad- 
ually to retain slightly increasing amounts of nitrogen. In 
areas under secondary associations, almost invariably there 
is a good growth of species of Leguminosae known to bear 
root nodules as a consequence of the presence of nitrogen- 
fixing bacteria, and it appears also that certain non-sym- 
biotic organisms, such as Azotobacter, are partly responsible 
for the return of nitrogen to the soil when temperature and 
shade conditions permit of restoration to a higher level. 
In Fig. 9 is given a diagrammatic representation of the course 
of the nitrogen changes in the soil when primary forest is 
cleared, and then subsequently allowed to revert to 
jungle. 

On old planations, which have suffered severely from the 
application of clean-weeding principles, it is improbable that 
the nitrogen content of the soil is as high as it should be 
under the conditions of shading and lowered temperature 
which normally obtain. The reason for this is not far to 
seek — although the loss of nitrogen and organic matter 
consequent upon exposure of soil to the sun is a chemical 
reaction, the reverse process is biological in nature, and 



io6 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



humus formation and nitrogen fixation cannot take place 
in an absolutely sterile soil. 



Forest felled and 
cleared, and soil 
exposed to the sun 




Fig. q. A Diagrammatic Representation of the Changes in the 

Nitrogen Content of the Soil when Primary Forest is cleared 

and then subsequently allowed to revert to jungle. 

Under primary forest the nitrogen content of the soil is constant. 
When the forest is felled and cleared and the soil is exposed to the sun 
there is a rapid loss of nitrogen (mainly in the gaseous form). When 
cleared land is abandoned the degree of soil shading is gradually increased 
with succeeding secondary plant associations and the nitrogen content 
of the soil slowly returns to its normal value for forest soil. 



SUMMARY 

According to Jenny, the nitrogen (and organic matter) 
content of the soil varies inversely with the mean annual 
temperature and directly with the humidity but it is now 
evident that, in Malaysia, the nitrogen content of the soil 
decreases with the amount of solar radiation received. 

It has been shown also that at temperatures above 25 C. 
humus decomposition outpaces its formation but at tem- 
peratures below 25 ° C. there is a gradual accumulation of 
soil organic matter. 

The soil temperature in the Malaysian forests is 25 , so 
that the nitrogen content remains constant. When the 
insolation is increased by felling and clearing forest, the 
nitrogen content of the soil drops to a new and lower level 
and can only be permanently increased above this value 
in the presence of adequate shading. 



jenny's law 107 

references 

1. H. Jenny, /. Amer. Soc. Agronomy, 1928, 20, 900-912; Soil Sci., 

1929, 27, 169-188; ibid., 1930, 29, 193-206; /. Phys. Chem., 

1930, 34, 1053-1057; Missouri Sta. Res. Bull., No. 152, 1930; 
Soil Sci., 1931, 31, 247-252; Proc. Sec. Int. Congr. Soil Sci., 
Comm. iii, 1932, 120-131. 

2. The soil organic matter = organic carbon x 1-74. 

3. E. C. J. Mohr, De Grond van Java en Sumatra, 2nd ed. (Amsterdam), 

1930. 

4. Actually, the balance is slightly in favour of humus formation. 

5. R. D. Hockensmith and E. Tucker, Soil Sci., 1933, 36, 41-45. 

6. H. E. F. Savage and R. G. H. Wilshaw, Dept. Agric. 5.5. and F.M.S., 

Bull. Sci. Ser., No. 10, 1932, p. 14. 

7. R. G. H. Wilshaw, Malayan Agric. J., 1934. 22 > 4~ 2 4- 

8. A. S. Corbet, Soil Sci., 1934, 38, 407-416. 

9. A. S. Corbet, Soil Sci. (in the press). 

10. F. Hardy, /. Agric. Sci., 1931, 21, 150-166. 

11. C. H. Harreveld-Lako, De Eigenschappen van de Suikerrietgronden op 

Java (Groningen), 1928; see English translation entitled The 
Properties of Sugar Cane Soils in Java, by R. L. Pendleton 
(Peiping), 1932, p. 70. 



IX. SOME PRACTICAL 
CONSIDERATIONS 

THE BURNING OF CLEARED LAND 

Some controversy has centred around the question as to 
whether timber and other forest debris on newly-cleared 
land should be burned or allowed to decay on or near the 
site. In parts of tropical Africa, this burning of forest 
waste on cleared land prior to cultivation is practised as a 
religious rite. In tropical Asia, and particularly in the 
Malay Peninsula, the custom of burning timber and dead 
vegetation on land cleared for agricultural purposes is 
followed almost invariably, but there is no reason for sup- 
posing that the practice is the outcome of detailed con- 
sideration of all the factors involved. Among the more 
enlightened sections of planting communities, " burning-off " 
is advocated primarily on account of the fear that decaying 
timber may be a reservoir for pathogenic fungi, later to 
prove troublesome to the crop; but to some extent also as 
a result of a desire to have plantations neat and orderly. 
At first sight the process strikes one as being a squandering 
of natural resources, but local conditions are such that 
removal of timber from the site of clearing is not an economic 
possibility. It is not generally appreciated that very few 
of the trees of a tropical rain forest produce timber of 
commercial value, so that the issue is really concerned with 
burning the timber after felling or permitting it to decay 
in situ. The practice of burning cleared land has been 
vigorously defended by Milne who has advanced the 
following points in favour of this procedure 1 : — 

(a) The mineral constituents of the forest debris are 
retained on the site. 

(b) Addition of a large quantity of organic matter of high 
C/N status is undesirable as its decomposition will entail 
the immobilization of, perhaps, the whole of the available 
nitrogen of the soil. 

1 08 



SOME PRACTICAL CONSIDERATIONS IO9 

(c) The destruction of the soil humic matter under the 
usual conditions of burning is inappreciable. 

(d) Reduction in the numbers of soil organisms as a 
consequence of burning may have the beneficial effects of 
a partial sterilization. 

(e) Although there will be some permanent loss of sulphur 
and phosphorus in the volatile products of combustion, the 
nitrogen loss due to a similar cause may be very slight on the 
balances of all the processes involved. 

(/) The ground is cleared of trash which would support a 
dubious population of fungi and termites and, perhaps, 
encourage fires. 

In discussing the subject the fate of the mineral salts 
and soil organic matter will be considered before dealing 
with the question of danger from pests and diseases. 

THE EFFECT OF BURNING ON THE 
SOIL MINERAL SALTS 

Pitcaithly and Worley have drawn attention to the 
important role played by large, deep-rooted, forest trees in 
New Zealand in bringing to the surface mineral matter 
essential to plant growth. 2 Destruction of the trees places 
the source of the supply of minerals out of reach so that 
the existing stock has to be conserved or the depletions have 
to be made good by the application of fertilizers. 

If vegetable waste is allowed to remain and decay on the 
site of clearing, mineral constituents are gradually returned 
to the soil under conditions whereby they stand every chance 
of being assimilated by a growing crop or cover. When 
vegetation is reduced to ashes by burning there is an 
immediate increase in the pK value of the soil, and a very- 
temporary increase in the population of soil micro-organisms 
as a consequence of the presence of wood ash. In the 
absence of growing vegetation, the potassium present in 
the wood ashes 3 is adsorbed by the soil colloids and it is 
improbable that any appreciable losses of metal ions occur 
as a result of base exchange ; under the usual circumstances, 
losses of mineral plant nutrients by leaching must be slight. 



110 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Destruction of the plant debris by burning renders the 
mineral constituents immediately available, and whether this 
is desirable or otherwise depends, to a large extent, on the 
crop following. In any case, depletions in the stock of 
mineral salts by a succession of annual crops will ultimately 
necessitate the application of fertilizers. 

BURNING AND THE SOIL ORGANIC MATTER 

The dead vegetation left after felling consists largely of 
stems, branches and tree trunks, all of which have a high 
C/N ratio. Figures recently published show the C/N ratio 
for certain typical low-growing Malayan plants to range 
between 21 and 10 for the leaves and between 58 and 14 
for the stems, 4 but the ratio for timber is not less than 200. 
It is well known that decay of organic matter of low nitrogen 
status, by microbiological agency, results in the immobiliza- 
tion of a certain amount of the nitrogen present in the soil, 
for the growth and multiplication of micro-organisms 
demands a supply of nutrients in which the C/N ratio is 
approximately 10 for fungi and 5 for bacteria. If micro- 
organisms effected the decomposition of timber it would 
certainly be necessary for them to remove nitrogen from 
the soil to make good the deficiency of this element in the 
material being disrupted. With perennial crops, however, 
this aspect of the question is far from being a disadvantage ; 
in fact, it may be considered as a reason for allowing the 
timber to decay on the site, for this temporary immobilization 
of soil nitrogen on recently cleared land is desirable, par- 
ticularly when a permanent crop, such as rubber, is to be 
established. It has been pointed out already (page 99) 
that exposure of the soil to the sun results in a diminution 
in the total nitrogen content, and this loss will be accentuated 
when no replenishment of organic matter takes place for 
some years. Under such circumstances, the position is 
that the soil is not only "living on capital" as it were, but 
is doing so at an increased rate of expenditure. It has 
been shown that the process of burning is responsible for a 
greatly increased bacterial population at or near the site 
of firing, but) although this increase is very ephemeral, it 




S o 



SOME PRACTICAL CONSIDERATIONS III 

must account for the breakdown of a small amount of humic 
matter. In the absence of vegetation the nitrates elaborated 
as a consequence of biological and chemical activity will not re- 
main long in the soil but will disappear in the drainage water. 

It seems clear that a process whereby nitrogen is retained 
in the soil during an initial period, when heavy losses are 
inevitable, and is gradually returned to the soil at some later 
date, when the available supplies of this element have been 
depleted, is of considerable economic value when dealing 
with a perennial crop and the alleged disadvantages of the 
"natural decay" method should be investigated very com- 
pletely before the practice is condemned as unworkable. 

In actual fact, however, it appears that micro-organisms 
play a very insignificant part in the decompositions of 
timber in equatorial regions, such decay being accomplished 
very largely, if not entirely, by termites. It is believed that 
these insects obtain nitrogen supplies through the agency 
of nitrogen-fixing intestinal protozoa and, if this is so, 
decaying timber must be placed on the credit and not on 
the debit side when drawing up a nitrogen balance sheet. 5 

THE EFFECT OF BURNING ON THE SOIL 
MICRO-FLORA 

It has been stated on page 103 that the generally accepted 
view that increased temperature, consequent on clearing 
forest, results in a speeding-up of the microbiological pro- 
cesses responsible for decomposition of soil organic matter 
can no longer be upheld. Forest clearing in Malaysia is 
without effect on the numbers of soil micro-organisms, as 
determined by the plate count method. 

The effects of the heat generated by burning brushwood 
are very localized, and as Milne observes, the destruction 
of humic matter under a running fire is inappreciable. The 
actual burning operations are also without effect on the 
soil micro-flora, but addition of wood ash to the soil is re- 
flected in a greatly increased microbiological population 
though this phenomenon is strictly localized and of short 
duration. Under no circumstances is there a reduction in 
the numbers of soil micro-organisms. 



112 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

PESTS AND DISEASES HARBOURED BY 
DECAYING TIMBER 

From the economic standpoint, the question of danger 
to a growing crop from pests and diseases harboured by 
timber and plant debris left to decay in situ is a matter of 
prime importance. Loss as a result of infection by patho- 
genic fungi will, in general, be much more serious than damage 
inflicted by termites. 

Dr. J. R. Weir, of the Rubber Research Institute of 
Malaya, arranged a series of experiments at the Institute's 
Experiment Station in which direct comparison of the 
incidence of root disease on Hevea brasiliensis under clean- 
cleared and uncleared conditions was possible. These ex- 
periments were in progress for rather more than three years 
before the results were analysed by Napper, 6 who was 
concerned mainly with the presence of Fomes lignosus, one 
of the two most destructive root diseases of the rubber tree 
in Malaysia and often responsible for heavy losses during 
the first five years in the life of a plantation. 

It has been tactily assumed that the incidence of root 
disease in a planted area varies directly with the total 
amount of timber remaining in the soil at the time of planting, 
and so the larger the proportion of the timber removed at 
this time, the lower will be the subsequent losses due to 
root disease. The acceptance of this principle entailed the 
burning-off of dead vegetation, followed by removal of the 
remaining debris as far as possible. It was considered 
uneconomic to unearth the last remnants of buried timber 
as these could hardly be responsible for any considerable 
incidence of disease. 

The results obtained by Napper appear to show that, up 
to the age of three years, rubber trees are exposed to a 
greater risk of infection from Fomes lignosus in an area from 
which surface timber and jungle stumps have been removed 
than in an uncleared area. It was found also that the 
amount of disease was less under a cover than on a clean- 
weeded area and still less where secondary forest growth was 



SOME PRACTICAL CONSIDERATIONS 113 

established as a cover. Although the published results 
appear to be definite, it is too early to conclude that infection 
by pathogenic fungi is invariably less in uncleared than in 
cleared sites at all ages in the life of a plantation : neverthe- 
less, it seems probable that further work along these lines 
may show that " burning-off " and "clean-clearing" are 
practices which have little to commend them on economic 
grounds. 

Napper supposes that Fomes lignosus is present in the 
jungle stand before clearing, and that complete removal of 
all the timber before planting would eliminate entirely any 
danger of infection. Such a counsel of perfection, however, 
is quite beyond the bounds of practical politics and a certain 
amount of timber must always be left in the soil. Under the 
usual conditions of clean-clearing, the food material avail- 
able for the fungus is small and soon exhausted ; this results 
in the formation of rhizomorphs which wander through the 
soil until they encounter further food material which — under 
the usual planting conditions in Malaysia — will be the roots 
of a rubber tree. The presence of a woody plant, such as 
Crotalaria, known to be susceptible to attack from Fomes, 
tends to reduce the incidence of disease on the rubber tree. 
According to Napper, disease on a new clearing rises to a 
maximum between the second and third years after burning- 
off and then drops to a comparatively low figure. 

However, Napper's findings have met with a certain 
amount of criticism by de Jong, 7 who considers that Napper's 
method of estimating the status of the disease is open to 
question. De Jong found Fomes lignosus universally present 
in decayed root systems of all rubber trees up to 22 years 
on the H.A.P.M. estates in east Sumatra: it appears that the 
fungus is only weakly parasitic to Hevea brasiliensis and 
the presence of mycelium on the root system by no means 
augurs the subsequent death of the tree. 

A great deal of confusion has persisted with regard to 
termites on rubber estates; it was formerly supposed that 
certain species known to be pests on plantations constructed 
termitaria in decaying tree stumps or built mounds just 
under the surface of the soil and, from these bases, made 



114 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

attacks periodically on nearby rubber trees. It is now clear 
that Coptotermes curvignathus is the species which so fre- 
quently attacks young rubber with such disastrous results. 
It has already been pointed out that this termite constructs 
its termitarium deep underground and its complete eradi- 
cation is difficult, if not impossible: the termites living in 
decaying timber and mounds have little or no interest in 
Hevea brasiliensis. 

From the above consideration of the question of the 
advisability of burning cleared land, it will be seen that, 
in general, the practice has no special points to cgmmend it. 
In fact, of the reasons in favour of the procedure given by 
Milne (page 108), (d) is not valid, and (b) and (/) hardly so, 
in equatorial regions. As regards (a), burning has the 
advantage of making certain nutrient minerals immediately 
available to the crop following, but in this connection the 
time of planting after burning may be of some significance. 
It is almost certain that losses of humus and nitrogen from 
the soil as a result of burning, (c) and (e), are inappreciable, 
as stated by Milne. 

Of course, a question of this nature cannot be decided 
entirely on theoretical grounds for such matters as the 
crop following and clearing costs must be considered. In 
some countries with certain crops, for example, the cultiva- 
tion of sugar cane in Trinidad and the Hawaiian Islands, 
planting is preceded by ploughing and, under these circum- 
stances, burning of plant debris is essential. 

LEGUMINOUS COVER PLANTS 

In the early days of rubber cultivation it was customary 
to plant a "catch crop" between the rows of rubber trees; 
usually coffee was grown, but mainly as a result of difficulties 
with pests and diseases, this policy was abandoned in favour 
of one of "clean-weeding." In spite of obvious drawbacks 
the practice of clean-weeding had powerful advocates and 
for a long period in planting circles in the Malay Peninsula 
it was regarded as the only correct procedure. In these 
unenlightened times rubber estates were denuded of all 



SOME PRACTICAL CONSIDERATIONS 115 

vegetation save rubber trees, but in the course of time, 
the evil consequences of the method became apparent, 
when plantations were left with hard-baked, impoverished 
soils and, often, on hillsides, the whole of the top-soil dis- 
appeared as a result of rainfall erosion. The failure of 
clean-weeding caused a reversion in planting policy and 
cover plants were now grown, primarily with a view to pro- 
tecting the soil against erosion. The efficiency of covers 
as soil retainers is now undisputed, and Belgrave has shown 
that, in Malaya, no significant diminution in soil moisture 
content occurs in the presence of covers. 8 The activity of 
nitrogen-fixing bacteria associated with the root nodules of 
leguminous plants was well known and it was suggested, 
quite reasonably, that low-growing species of Leguminosae 
might be employed advantageously as covers as they might 
do much towards reconditioning the soil on old rubber 
plantations. 

As far as the Malay Peninsula was concerned the selection 
of a suitable legume species was not easy, as the cover was 
required to grow profusely on newly-cleared forest soil, 
exposed to the full glare of the sun, and then to continue 
to thrive under the shade of mature rubber trees. The 
ideal leguminous cover plant for rubber plantations in 
Malaya has not been found, although the problem has been 
largely solved in the Large Sunda Islands. 

From first principles it would not have been anticipated 
that introduction of species of Leguminosae into the Malay 
Peninsula from the Sunda Islands would have been an un- 
qualified success. It has been pointed out that in times not 
very remote the Large Sunda Islands, together with the 
present Malay Peninsula, formed a unified land mass and 
so shared a common flora and fauna (page 3). There is 
no reason for believing that the life of any particular species 
of plant or animal is infinite: on the contrary, it seems evi- 
dent that the life of any plant species can be represented by 
a "life curve" in which an increasing population is followed 
by a decline and then by extinction (page 63). It is a fact 
that certain species of Leguminosae occur to-day in the 
Sunda Islands which have not yet been found in Malaya, 



Il6 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

and the explanation of this absence from the Peninsula 
may be either that: 

(a) the plant formerly occurred in Malaya but has since 
died out naturally, or that 

(b) the plant never became established in the Peninsula, 
possibly because climatic or soil conditions were 
unsuitable. 

In either circumstance, no strong case can be made out 
for introducing leguminous plants into Malaya from the 
Sunda Islands. In point of fact, the policy of bringing 
cover plants to Malaya from Borneo and Java has proved 
to be a failure, and probably a more costly one than is 
generally realized. On a few plantations some covers have 
done well, but there is little doubt that the policy in favour 
of employing leguminous covers is being abandoned as a 
general practice. 

In Malaysia very few leguminous plants of low habit are 
to be found growing in the shade of the primeval forest : it has 
been indicated already (page 91) that apparently nodule- 
bearing legumes grow where they are most needed, that is, 
in open sunny situations, so there is little hope of finding 
in Malaya or Malaysia a leguminous plant suitable for the 
shade of rubber estates in the Peninsula. It is possible, 
but hardly probable, that such plants may be found in 
equatorial Africa or tropical America and, if so, their 
introduction into the East Indies may be attended with 
some measure of success. 

The family Leguminosae is divided into three subfamilies : 
Mimosaceae (including Mimosa, Acacia, etc.), Caesalpini- 
aceae (comprising Cassia, Caesalpinia, etc.) and Papilioni- 
ceae. The last-named subfamily includes all the common 
American and European genera, such as Lupinus, Pisum, 
Trifolium, Wistaria, Ulex, Cytisus and Genista, and almost 
all members are known to possess root nodules. Con- 
siderably less is known concerning the formation of root 
nodules with the remaining two subfamilies: no nodules 
have been found on the roots of many species of Cassia and 
Caesalpinia, but nodule-formation appears to be the rule 




>-. • r ■ 




FOUR YEAR OLD RUBBER WITH A COVER OF 

CALAPOGONIUM MUCUNOIDES. 



SOME PRACTICAL CONSIDERATIONS 117 

in the Mimosaceae (Mimosa pudica, the sensitive plant, has 
nodules 9 ). Although never known to bear nodules, Cassia 
tora is used in the Hawaiian Islands as a green manuring 
crop with beneficial results: in this case it was considered 
possible that, although no nodules were formed, nitrogen- 
fixing bacteria might be present within the root tissue but 
it has been demonstrated that this is not so. 10 It is evident 
that the mere selection of a legume as a cover plant does not 
ensure the presence of a nitrogen-fixing mechanism in the 
underlying soil. 

Of the leguminous plants employed as covers in Malaysia 
the majority are creepers which often obtain a stranglehold on 
other low-growing vegetation, and so are useful in eliminating 
weeds. It is desirable that the covers should thrive under 
shade but few satisfy this requirement : most of the legumin- 
ous covers employed have deep and well-developed root 
systems, the presence of which doubtless serves a useful 
purpose in opening up the soil, which may have become 
closely-packed on old estates where the movements of 
coolies have not been restricted to paths. 

The more important cover plants in use in the Malay 
Peninsula at present are: — 

(a) Creepers 

Calapogonium mucunoides : useful with young rubber but 
dies out under shade. A native of tropical America, 
it was found growing wild on the east coast of Sumatra 
and in Java some years ago, and was introduced into 
the Malay Peninsula about 1924. 

Indigofera endecaphylla : used with young rubber but apt 
to die back in dry weather, and of little use under 
shade. Native to South India at an altitude of 3000 
feet. 

Centrosema pubescens : usually grows well in open situations 
and is not easily established under shade ; nevertheless, 
it is reported as being able to continue to grow under 
shade if well established with young rubber. With 
this and the following species, the leaves close during 
the brightest parts of the day. 



Il8 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

Centrosema plumieri : can grow well in open situations but 
is less satisfactory than C. pubescens. The Centrosema 
species are native to South America, but C. pubescens 
was found wild in Java in 1921. 

Dolichos hosei (= Vigna oligosperma) : the "Sarawak bean" 
often does well under heavy shade and is the leguminous 
cover which has grown best under the shade of mature 
rubber in the Malay Peninsula. The root system is 
shallower than usual with most plants of this group. 
D. hosei occurs wild in the Large Sunda Islands but 
appears to have been absent from Malaya until intro- 
duced from Borneo. It was first employed as a cover 
on rubber plantations in Sarawak, and it was formerly 
believed that the species was confined to Borneo : later, 
it was discovered that Vigna oligosperma from Java 
and Sumatra was identical with D. hosei. 

Peuraria phaseoloides (— P. javanica): can be used under 
shade and prefers heavy soil. It is indigenous to Malay- 
sia, being found growing over low shrubs in secondary 
associations on the plains. 

Mimosa invisa: formerly enjoyed a measure of popularity 
but has a decided disadvantage in the fact that it 
harbours various species of insects. 

(b) Shrubs 

Crotalaria anagyroides: a tall shrub which can attain a 
height of some twelve to fourteen feet and usually 
covered profusely with yellow flowers. It grows well 
in open situations and can survive dry periods; in 
Malaysia it grows at altitudes up to 5000 feet but has 
a tendency to die back after two years. The root 
system is known definitely to have nodules. Native 
to tropical America. 

Several other species of Crotalaria are cultivated in 
Malaysia but C. anagyroides is most generally employed. 

Tephrosia Candida: can thrive under light shade although 
it prefers open situations. In Sumatra it is employed 
as a light shade and green manure for tea: it is native 



SOME PRACTICAL CONSIDERATIONS II9 

to tropical Asia and occurs on the plains and at alti- 
tudes up to 5000 feet. 
Leucaena glauca: appears to grow fairly well in open situ- 
ations. 

Non-leguminous plants employed as covers for rubber 
plantations are: — 

Passiflora foetida (Family Passifloraceae) : used extensively 
as a cover on new clearings as its creeping habit renders 
it particularly useful on hillsides. Although it grows 
well at the start, it disappears with the development 
of shade from the young rubber trees. Originally con- 
fined to tropical America, this species is now distributed 
throughout the equatorial belt. 

Mikania scandens (Family Compositae) : a native creeping 
plant, occurring on the forest edge at all elevations up 
to 4000 feet. It can thrive under light shade but 
cannot withstand drought. 

Marremia vitifolia (Family Convolvulaceae) : is a native plant 
which has recently been employed successfully as a 
cover under young rubber in Kedah. 

The subject of leguminous cover plants in Malaya has 
been dealt with in a comprehensive manner by Bunting 
and Milsum. 11 

With most leguminous cover plants cultivated in Malaya 
root nodules are usually present, although small in size 
and few in number, and it has been suggested repeatedly 
that nodule formation might be improved by inoculation 
with more vigorous strains of nodule-forming bacteria. A 
few experiments carried out by the writer with Dolichos 
hosei showed that inoculation alone produced no visible 
effect when small nodules were already present, but a tem- 
porary increase in the size and numbers of nodules followed 
the application of wood ash. 

It has been mentioned already that, under natural con- 
ditions in Malaysia, a state of dynamic equilibrium exists 
in the soil under the shade of the primeval forest and it 
appears that leguminous plants play but a small part in 
maintaining this equilibrium. It may well be that the 



120 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

nitrogen-fixing mechanism of these tropical legumes is 
unaccustomed to working under shade and so operates very 
inefficiently on plantations under mature rubber. If this 
is so there is no reason why leguminous rather than other 
plants should be employed as covers. 

It has been emphasized repeatedly that, to obtain any 
benefit from the growth of covers, they must be turned into 
the soil at intervals; but recent work indicates that this 
view is, perhaps, hardly correct. The digging-in of cover 
plants on a rubber estate does not result in any permanent 
increase in the nitrogen content of the soil, in fact, rather 
is the reverse the case. It has been mentioned on page 99 
that the nitrogen content of the soil is dependent on a number 
of climatic factors and cannot be permanently increased 
above a certain value, except by effecting alterations in these 
climatic factors. Addition of organic matter to the soil 
(e.g. digging-in of cover plants), under the same degree of 
shading as before, means that the added material is soon 
decomposed and completely disappears and any growing 
crop gains only a temporary benefit. The removal of grow- 
ing cover, moreover, results in increased exposure of the soil 
to the sun so that the final value of the nitrogen content of 
the soil is probably lower than that which obtained before 
the cover was cut down. Under some conditions, of course, 
it is essential to cut the cover. 

If the long view be taken, there can be little doubt that 
it is advisable to maintain rather than to dig in a growing 
cover, for it may be supposed that the soil nitrogen content 
will be slowly but surely increased by addition of dead leaves, 
etc. as the shade is increased. It has already been pointed 
out that a fall in the potential nitrogen level of the soil as 
a result of removal of shade appears to be accompanied by 
release of the excess nitrogen largely in the gaseous form, 
in which state it cannot be assimilated by growing vegetation. 

NATURAL COVERS 

EVILS OF CLEAN-WEEDING 

Although the question of the use of covers for rubber 
plantations has been freely debated in the past, the practical 



SOME PRACTICAL CONSIDERATIONS 121 

result has been that the pendulum of popular planting 
opinion has swung from one extreme to the other. The days 
of "catch crops " were characterized by efforts to extract the 
maximum profit from the soil in the minimum space of time, 
regardless of any consideration for the future, and the ex- 
tremes to which the vogue of " clean- weeding" were carried 
were possibly attributable to experience of the evil effects 
resulting from the admittance of lalang. Although the 
disastrous consequences of clean-weeding are now only too 
evident, it is surprising that the practice was continued 
for so many years before being seriously questioned. 

The deleterious effects of clean-weeding may be recapitu- 
lated: — 

(a) removal of shade results in a lowering of the potential 
nitrogen and organic matter content of the soil; 

(b) a hard-baked surface lacking an adequate moisture 
content is an evil which continually worsens as a 
result of the unrestricted movements of coolies, as 
has been pointed out by Haines; 

(c) the absence of surface vegetation and soil humic 
matter may mean that any nitrates are leached out 
by the intensive rainfall; 

(d) on hilly land considerable losses of soil occur as a 
result of erosion. 

Once the consequences of a policy of clean-weeding were 
generally realized an attempt was made to meet the situation 
by the cultivation of some single plant species, usually one 
of the Leguminosae, as a cover. This idea of cultivation 
of covers in pure strain met with indifferent success in 
Malaya, although promising results were often obtained in 
Sumatra and Java. On the older plantations in the Malay 
Peninsula, where the top-soil had been non-existent for 
decades, attempts to cultivate leguminous covers usually 
proved a dismal failure. 

THE BIRKEMOSE SYSTEM 

It appears that the so-called "forestry system" was first 
successfully demonstrated by Mr. F. Birkemose in Pahang, 



122 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

and on the particular estate the method proved to be such 
a striking success that there followed a desire on the part 
of Malayan planters to see how far the system was of general 
applicability. 

When the experiment was started on the Pahang planta- 
tion the clay soil had a hard-baked surface, which was 
generally bare but carried a cover of stag-horn moss (Lyco- 
podium cernuum) in some places. The soil was probably 
quite acid, and it is safe to say that microbiological life 
must have been at a minimum. In the beginning of the 
experiment any weeds which could gain a footing under 
the inauspicious circumstances and any rubber seedlings 
which germinated were given every encouragement, although 
an exception was made in the case of lalang grass which was 
rigorously excluded. At intervals the weeds were pulled 
up, or slashed down, and left to decay on the surface of 
the soil and, where considered necessary, the acidity of the 
soil was reduced by a light application of lime. In the 
course of time, as soil conditions became more kindly, 
further weed species appeared and gradually humus forma- 
tion was set up and conditions became favourable for the 
normal microbiological processes of the soil to become 
operative again. Once a humus layer was obtained progress 
was more rapid, and a certain amount of selection could be 
exercised with regard to the species of plants allowed to 
obtain a footing, but weeds with a large and matted root 
system were excluded. Attention was paid to the condi- 
tions of light and shade, and subsequently earthworms were 
introduced and they increased rapidly. The soil containing 
an abundance of earthworms was generally friable, well- 
aerated and comparatively rich in humic matter. As the 
plants themselves are good indicators of soil conditions, 
once the normal cycle has been restored it can be maintained. 
This, in brief, is the Birkemose system employed in Pahang 
and there can be little doubt that its successful demonstration 
will prove to be a landmark in planting history in the Malay 
Peninsula. 

In many respects the scheme is revolutionary and, natural- 
ly enough, has come in for some severe criticism: the subject 




stag-iiorn moss {LYCO/'ODII'M CERNUUM) 



SOME PRACTICAL CONSIDERATIONS 1 23 

has been given a preliminar}/ examination by the Rubber 
Research Institute of Malaya 12 and it is of sufficient interest 
to merit detailed examination here. Some very obvious 
advantages of the system are immediately evident. The 
natural habitat of Hevea brasiliensis is the humid forest of 
tropical South America and a procedure which has for its 
object the maintenance of conditions approaching those of 
the natural habitat of the tree is clearly to be encouraged; 
the establishment of a natural cover at a desired level is 
a first essential. 

The restoration of a cycle entailing growth and decay 
of vegetation will ensure continual addition of organic 
matter to the soil, the growth of a light cover will result 
in the surface of the soil being adequately but not too densely 
shaded from the sun, and the soil will lose its hard-baked 
appearance and retain moisture and mineral salts. It has 
been pointed out already that in regions such as Malaysia, 
where the rainfall always exceeds the evaporation, there is 
a continual loss of soluble material downwards through the 
soil. If these water-soluble substances are retained in 
growing vegetation at the surface, although temporarily 
removed from circulation, they are lost to the main crop 
only for a time. 

In addition to increased humic matter and moisture and 
the return of normal microbiological activity in the soil, 
there is a further important consequence of the "forestry" 
system which is that, as a result of increased shading of the 
soil surface, the potential organic matter content will be 
raised. Conditions will then be such that any extra nitrogen 
added to the soil will be retained in the normal cycle until 
the nitrogen content of the soil reaches the maximum value 
possible under the conditions of temperature and insolation 
which obtain. This point is of considerable importance, 
for it corresponds to an increase in capital of which the 
interest will be available for use by the cultivated crop. 

Much research work remains to be carried out regarding 
the plant associations which appear under the conditions 
described. The term "natural covers," which is used so 
frequently in discussions on the Birkemose system, is not, 



124 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

perhaps, a very happy one as many of the plants which are 
among the first to appear on old clean-weeded areas are 
introduced species and not indigenous to Malaysia (see 
Table IV). 

Some experiments have been carried out in Malaya with 
bamboo as a cover for rubber 13 and the idea merits further 
investigation, but it is possible that the dense litter thrown 
off by bamboo does not make for ideal soil conditions. The 
use of bamboo in pure stand, however, must not be con- 
founded with the Birkemose system, the essential feature of 
which is to encourage the growth of a natural plant associa- 
tion. Bamboo has been successfully employed as a means 
of eradicating stag-horn moss in the Hawaiian Islands. 14 

DEFECTS OF THE SYSTEM 

Two serious criticisms have been levelled at the "natural 
cover" system: one is on account of the possibility of in- 
creased disease to the rubber trees and the second is con- 
cerned with disease to human beings. The first may not 
be serious but it may well be that the system will stand 
or fall on the second count. 

It is considered in some quarters that the "forestry" 
conditions described will encourage the growth and spread 
of pathogenic fungi. One of the features of the system is 
that old or diseased trees are cut off level with the ground 
and the root system is allowed to decay in situ: it has been 
pointed out that such a procedure may result in serious 
cumulative effects. The plant pathologist is within his 
rights in not minimizing the danger from this source, but 
it would have to be very grave to merit condemnation of 
the method without further investigation. Taking a broad 
view of the question, it is quite possible that disease will 
decrease rather than increase under the Birkemose method. 
It has been pointed out already (page 68) that, in general, 
the outbreak of an epidemic of any nature follows the 
bacterial growth curve, and it may be that the occurrence 
of epidemics is suggestive of artificial conditions. It is 
hardly probable now that any of the well known fungal 



SOME PRACTICAL CONSIDERATIONS 125 

diseases of the rubber tree would break out with greatly 
increased virulence: in fact, with a reversion to more natural 
conditions it would be anticipated that anything in the 
nature of epidemics would tend to disappear and that the 
growth curve for fungal diseases would tend rather to follow 
a straight line than to exhibit a number of crests and troughs 
corresponding to rises and falls in the population of disease- 
bearing organisms. That is to say, a certain amount of 
disease will be present always as a normal factor and as 
such it must be accepted. In mixed forests it is surely 
unusual for all the trees of any one species to die as a 
consequence of the appearance of a fungal epidemic. 

Another point which must not be overlooked with regard 
to the question of natural covers and disease is that under 
the new system different soil conditions must be envisaged 
and it may be that pathogenic fungi will prove to be less 
resistant in less acid soils. De Jong could find no direct 
relationship between the incidence of Fomes lignosus on 
rubber trees and the reaction of the soil but, otherwise, little 
or no work appears to have been carried out on the relation 
between the pH of the Malaysian soils and the incidence of 
fungus diseases. 7 All things considered, however, it would 
appear that, as a condition of equilibrium in the natural 
cycle on an estate is approached, there will be less likelihood 
of any one species of plant or animal increasing prodigiously 
at the expense of others. 

The second objection that forestry methods may make 
for conditions inimical to human life is a much more serious 
one. It must be conceded that if natural growth is allowed 
to attain any dimensions it will harbour a population of wild 
life which will include small mammals, and this being so, 
the danger to humans from Japanese river fever and tropical 
typhus may be very real. In both these infections rats 
act as reservoirs and the disease is transmitted to man by 
mites. It has been submitted that the forestry method will 
encourage rodents and other small mammals able to carry 
these diseases by affording them protection, and it is known 
already that Japanese river fever and tropical typhus are 
often associated with the presence of cover plants and forest 



126 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

growths. This suggests that a cover of a shrubby nature 
is preferable to one which covers the ground. 

It is perhaps hardly necessary to suggest that a dense, 
uncontrolled cover of secondary growth may constitute 
a serious danger to a labour force if the possibility of lurking 
tigers is ever present. 

From the standpoint of the agriculturalist, the Birkemose 
system has much to commend it, and it is to be hoped that 
it will not be condemned at the outset by unfair or preju- 
diced criticism; nevertheless, the question of disease both 
to the rubber tree and to human beings is an important 
aspect of the matter which requires thorough investigation. 

THE EFFECT OF FERTILIZERS ON THE SOIL 

Fertilizers are added to the soil with the primary object 
of supplying additional nutrient material to a growing crop, 
but their presence may prove to be advantageous in three 
ways : — 

(a) by supplying plant nutrient; 

(b) by interaction with substances already present re- 
sulting in the appearance of a third compound; 

(c) by altering the physical condition of the soil in 
certain respects. 

It would appear that the most usual result is that des- 
cribed under (a), but here two cases have to be differentiated. 
Certain inorganic substances, such as nitrates and ammonium 
salts, are assimilated directly by the root system of the grow- 
ing plant, but other compounds, such as urea, cannot be 
utilized by the plant until a certain amount of decomposition 
has been effected by microbiological or chemical means. 
It is only within the last few years that it has been estab- 
lished beyond question that ammonium salts can be assimi- 
lated as such by plants ; formerly it was believed that nitro- 
gen present in ammonium salts could be absorbed by growing 
vegetation only after oxidation to nitrate. 

As an instance of the interaction of a mineral salt employed 
as a fertilizer with a substance already present in the soil 



SOME PRACTICAL CONSIDERATIONS 127 

it may be mentioned that the addition of a potassium 
salt to certain soils is followed by release of adsorbed cal- 
cium, so that a supply of this latter element is rendered 
available to the plant. Fertilizers may affect the physical 
condition of the soil in a number of ways ; addition of alkali 
salts to a clay soil renders it "sticky" and certain salts are 
not without effect on the pH value of the soil, either im- 
mediately or subsequently. Very little lime is required, 
however, economically to counteract the effect of ammonium 
salts in lowering the pH of the soil. 15 

Almost invariably manuring is carried out with a strictly 
commercial end in view, so that the subject has always to 
be considered from an economic standpoint. Although the 
yield of a crop may be increased by application of a fertilizer 
to the soil, it is evident that it cannot be augmented beyond 
a certain maximum : in fact, owing to the toxicity of many 
salts in high concentration, addition of excessive amounts 
of fertilizer may have a deleterious effect. In general, 
manuring is most successful when it is confined to the addi- 
tion of certain substances known to be lacking from the soil 
under examination: thus in many Malayan soils the phos- 
phate content becomes a limiting factor in crop production 
so that application of a dressing of superphosphate has often 
a marked effect on crop yields. The mere addition to the 
soil of materials removed in the crop, however, is not 
sufficient to ensure maintenance of a high measure of 
fertility. 

In the presence of excess of nitrogenous manures the 
leaves of plants become large and dark green in colour and 
are particularly prone to attack by insect pests; in the 
absence of available nitrogen, growth is stunted and the 
leaves assume a yellowish colour. Starvation of potassium 
is characterized by the leaves yellowing at the tip and by 
physiological derangement: to a certain extent potassium 
acts as a corrective when excessive amounts of nitrogen 
are present, and it is well known that the full effect of either 
nitrogenous or potassium fertilizers is apparent only when 
the supply of the other is adequate. The lack of sufficient 
supplies of phosphate is associated with a stunted root 



128 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

system, while the ripening of seed and fruit is hastened by 
addition of phosphate. 16 

MANURING OF RUBBER 

With regard to the application of fertilizers to annual 
crops in the tropics it is possible that the governing principles 
do not differ widely from those in temperate regions. For 
example, if lack of phosphate were found to be the limiting 
factor in crop production then, other conditions being 
normal, application of superphosphate in suitable quantity 
should result in the production of an increased yield. 
Nevertheless, in sun-baked tropical soils on old plantations 
where humic matter is non-existent, any fertilizers depending 
on biological agency to render them available to plants 
may be lost by leaching before being brought to a condition 
in which they can be assimilated by the growing crop. 

It is on annual crops that manuring is most likely to be 
practised extensively and in this case the success or failure 
of the procedure is soon evident. The question of manuring 
perennial crops such as rubber and coconuts is more com- 
plicated. Nitrogenous fertilizers are most frequently used 
and the situation here merits detailed consideration. 

From the discussion in Chapter VIII, it is apparent that 
the nitrogen content of the soil depends primarily on the 
mean annual values of the insolation, temperature and 
humidity. As far as Malaysia is concerned, it appears that 
insolation is the most important of these climatic factors 
and, if the amount of solar radiation reaching the soil be 
increased by any means, loss of nitrogen ensues until a new 
and lower level is reached. 

Speaking in general terms then, it seems safe to say that, 
in the soils under consideration, the nitrogen content varies 
inversely with the amount of solar radiation received and 
it is evident that effective shading must be a prelude to 
any efforts to raise the nitrogen status. In the absence of 
such shading, any nitrogen added to the soil in excess of 
that determined by the relationship just mentioned will be 
lost as speedily as the mechanism operating will permit. 

There is abundant evidence to show that no permanent 



SOME PRACTICAL CONSIDERATIONS 120, 

increase in the nitrogen status of Malaysian soils can follow 
the application of nitrogenous fertilizers, unless conditions 
are otherwise favourable for the soil to accumulate nitrogen. 
According to the views just advanced, addition of nitro- 
genous material, such as urea or sulphate of ammonia, to 
the soil in exposed situations must be followed by losses of 
nitrogen (as elementary nitrogen, ammonium salts and 
nitrates) until the nitrogen content of the soil has fallen 
to its former level. It is true, however, that in spite of 
these losses growing vegetation will derive some benefit as 
a result of the presence of increased amounts of ammonium 
and nitrate ions in the soil solution, but the proportion of 
the fertilizer actually utilized by the plant may be sur- 
prisingly low. 

Doubtless, these losses are not so rapid when nitrogen 
compounds are added to soil under the shade of mature 
rubber trees, but a difficulty here is that, on old plantations, 
the soil is so impoverished of organic matter that, in spite of 
increasing shade, the biological mechanism for making 
additions of nitrogen to the soil is absent and so the nitrogen 
content of the soil remains at a low level. On the other 
hand, although this application of fertilizers to Hevea may 
not result in any permanent increase in the nitrogen status, 
the added material will remain in the soil for a longer period 
and so give the roots of the trees a chance of utilizing some 
proportion of the extra nutrients present. 

The question of the most satisfactory method of adding 
fertilizers to perennial crops in the humid tropics cannot 
be regarded as settled for, while the foregoing discussion 
suggests that it would be advisable to apply the fertilizer 
in a number of small applications, other considerations 
point to there being a minimum dose below which the manure 
is without effect. 

Interesting results witli rubber on the east coast of 
Sumatra have been reported by Grantham, who obtained 
responses to fertilizer treatment only on estates on "white" 
alluvial soil, where the soil is known to be poor in plant food 
material, and where rubber begins to decline rapidly when 
some five or six years old. On the other hand, prolonged 



130 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

experiments have shown that certain "red" volcanic soils, 
exhibiting no initial response, began to give economic 
returns for manuring after 15 years, at which age deteriora- 
tion normally sets in on this type of soil. 17 Results obtained 
with rubber in Java have been much less spectacular and 
the improvement slower. 18 

In the Malay Peninsula, results have been almost uniformly 
disappointing and, in general, only with very poor and 
backward rubber is there any noticeable response to treat- 
ment with fertilizers. As Haines has pointed out, the 
amount of plant nutrient material removed annually in the 
latex is almost inappreciable, so that there need be no 
inevitability about approaching soil exhaustion such as 
occurs with annual crops. 19 Nevertheless, as far as nitrogen 
is concerned, the present procedure as regards felling, 
clearing and planting is such that the eventual approach 
of soil exhaustion is assured, although it may be long 
delayed. And when this period of sterility does appear it 
is not to be expected that a dose or two of nitrogenous 
fertilizer will restore the status quo\ At the risk of being 
tedious, it may be as well to emphasize that, once equilibrium 
has been attained, no amount of fertilizer treatment can 
permanently raise the nitrogen content of the soil — unless 
shading is increased and conditions are such that a return 
of normal biological processes is possible. 

GARDEN SOILS 

It is the usual experience of residents in humid tropical 
regions to find that addition of some kind of fertilizer, 
such as bullock manure, to the garden soil is essential every 
two or three months, if even moderate results are to be 
obtained with flower or vegetable cultivation. Unfortunately 
it is only too well known that the disappearance of this 
added manure is very rapid. From the discussion in 
Chapter VIII it is evident that these facts admit of a ready 
explanation: it has been pointed out that 

{a) the nitrogen and organic matter content of the soil 
varies inversely with the insolation and temperature, 
and 




rAPPINO \ Rl HI'. 1 ■ K IK I I I Ills I'l \\ l Alios HAS 1. 1 I N (II \\ WEEDED 

AND sill I'lls iiwi i-.l.l S KMI'LOYKl) IN VITEMPTS I" CONSERVE THE 

EXPOSED rOP-SOIL FROM RAINFALL EROSION. 



SOME PRACTICAL CONSIDERATIONS I31 

(b) at temperatures above 25 C. humus decomposition 
proceeds at a greater rate than its formation. 

The mean temperature of the soil in gardens on the plains is 
above 25 C. and it is clear, therefore, that in such soils 
there can be no accumulation of humic matter and, moreover, 
the nitrogen and organic matter content of the soil will 
readjust itself so that it remains at a low level. Repeated 
replenishment of soil nitrogen by artificial methods is a 
costly and unsatisfactory proceeding, but it is the only 
practical method of ensuring a temporary restoration of 
soil fertility under the circumstances described. It is 
essential to keep the soil shaded if humic matter is to be 
retained and, although it is not practicable to have a garden 
entirely under shade, a certain amount of soil conservation 
can be effected by retaining shade trees and encouraging 
suitable shrubs. 

Fagraea fragrans (Malay "Tembusa") is a suitable shrub 
for waste patches in gardens in Malaya, for it grows rapidly 
on poor soil and produces shade conditions which make 
control of Mimosa pudica and lalang an easy matter: a 
drawback to F. fragrans is that it seems to have a fatal 
attraction for the tree ant or "kerengga" (Oecophylla 
smaragdina) and usually harbours an inordinate number of 
nests of this pest. 

SUMMARY 

The practical aspects of such matters as burning of 
cleared land, leguminous and natural covers and the 
employment of fertilizers are considered in the light of 
the views developed in the earlier chapters of the book. 

REFERENCES 

1. G. Milne, Tropical Agriculture, 1933, 10, 179 (Abstract). 

2. N. P. Pitcaithly and F. P. Worley, /. Agric. Sci., 1933, 23, 206. 

3. E. Thorpe, A Dictionary of Applied Chemistry (London), 1927, 

Vol. I, gives the composition of air-dried oak wood as: Water 
15-0, ash 0-51, K 8 005, Na s O 0-02, MgO 002, CaO 037, P 2 O s 
0-03, S0 3 001, and Si0 2 001 per cent. 

4. C. G. Akhurst, J . Rubber Res. Inst. Malaya, 1932, 4, 131-139. 



132 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

5. L. R. Cleveland, Biol. Bull., 1925, 48, 292. 

6. R. P. N. Napper, /. Rubber Res. Inst. Malaya, 1932, 4, 5-33. 

7. W. H. de Jong, Arch. Rubbercultuur, 1933, 17, 83-104. 

8. W. N. C. Belgrave, Malayan Agric. J., 1930, 18, 492-496. 

9. L. T. Leonard, 5oj7 Sci., 1925, 20, 165-167. 

10. E. K. Allen and O. N. Allen, Amer. J. Botany, 1933, 20, 79-84. 

11. B. Bunting and J. Milsum, Malayan Agric. J., 1928, 16, 256-280. 

12. /. Rubber Res. Inst. Malaya, 1932, 4, 54-64. 

13. N. Fish, Planter, 1932, 13, 89. 

14. C. S. Judd, Hawaii. Forester and Agric, 1927, 24, 54-55. 

15. P. E. Turner, Tropical Agriculture, 1932, 9, 52. 

16. E. J. Russell, Soil Conditions and Plant Growth (London), 1932. 

17. J. Grantham, Arch. Rubbercultuur, 1924, 8, 501-526; ibid., 1927, 11, 

472-478; ibid., 1930, 14, 345-350; J. F. Schmole, ibid., 1926, 
10, 233-238. 

18. W. C. van Heusden and J. S. Vollema, Arch. Rubbercultuur, 1931, 15, 

125-146. 

19. W. B. Haines and C. F. Flint, /. Rubber Res. Inst. Malaya, 1932, 3, 

57-93- 



APPENDIX 

I. STANDARD METHODS EMPLOYED FOR THE 
EXAMINATION OF SOILS 

Mechanical analysis. 

In the method of mechanical analysis now generally followed 
by British chemists the following classes are distinguished 1 : 

particle size above 2 mm. in diameter : gravel 

particle size between 2 mm. and 0-2 mm. : coarse sand 
particle size between 0-2 mm. and 0-02 mm. : fine sand 
particle size between 0-02 mm. and 0-002 mm. : silt 

particle size below 0-002 mm. in diameter : clay. 

In the Netherlands Indies, the soil fractions are divided 
somewhat differently : gravel particles are above 2 mm. in 
diameter, sand particles between 2 mm. and 0-05 mm., silt 
particles between 0-05 mm. and 0-005 mm- an( i clay particles 
below 0-005 mm - m diameter. 

Soil moisture. 

The soil moisture is estimated by weighing out 10 grams of 
soil in a weighing bottle and estimating the loss in weight, after 
drying to a constant weight in an oven at 105 C. UsuaLly 
twenty-four hours will suffice to give a constant weight. 

Loss on ignition. 

The loss on ignition is used as a rough method of estimating 
the organic matter present in soils, but if calcium carbonate is 
present there is a loss of carbon dioxide due to the decomposition 
of this compound. The material is ignited to a constant weight 
in a crucible and the loss in weight taken as the organic matter. 
When calcium carbonate is present the ignited material is 
moistened with ammonium carbonate solution and heated gently, 
when calcium carbonate is re-formed, and the loss in weight is 
due only to organic matter and "combined water." 2 

Soil reaction. 

The soil reaction can be determined roughly by colorimetric 
means, e.g. by use of the B.D.H. soil indicator and "Sofnol," 

133 



134 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

but the results obtained are without precision. The quinhydrone 
electrode method 3 is of general applicability at pH values below 8 : 
in presence of manganese, however, the results are high 4 and 
more accurate values are obtained with the hydrogen electrode. 5 

Total nitrogen. 

It is customary to determine the total nitrogen present in soil 
by a modification of the Kjeldahl method. The air-dry soil is 
ground to pass through a i mm. sieve and then 20 grams are 
weighed into a Kjeldahl flask, followed by 50 ml. of water. 
After standing for about half-an-hour, 30 ml. of concentrated 
sulphuric acid, 10 grams of potassium or anhydrous sodium 
sulphate and 1 gram of copper sulphate are added. On com- 
pletion of the digestion process, which takes about 5 hours, the 
liquid is poured into a distillation flask containing distilled water 
and, after addition of excess of 50 per cent, sodium hydroxide, 
the ammonia present is distilled into standard sulphuric acid. 
The excess of acid is determined by titration with sodium 
hydroxide, in presence of methyl red as indicator. 

The estimations should be carried out in quadruplicate, but 
it is more satisfactory to make five or six parallel determinations. 

Estimation of ammonia in soil. 6 

To 100 grams of fresh soil (not more than 50 grams in the case 
of peat) in a bottle are added 100 ml. of 2 N potassium chloride 
solution, sufficient hydrochloric acid to give the suspension a 
reaction of pH i, 7 and water sufficient to make the volume of the 
extraction medium up to 200 ml. After addition of a few drops 
of toluene, 8 the bottle is closed with a tightly-fitting rubber 
bung and shaken in a shaking machine for 1 hour. 

The contents of the bottle are then transferred to a large dis- 
tillation flask to which are added 200 ml. of distilled water, a 
fragment or two of pumice and 4 grams of magnesia. The neck 
of the distilling flask is fitted with an ammonia trap, and the 
contents of the flask are distilled into a receiving flask contain- 
ing a known volume of approximately 0-075 N sulphuric acid. 
When about 200 ml. of liquid have passed over, the receiving 
flask is removed, the contents are boiled for about 20 minutes to 
drive off carbon dioxide, and then titrated with 0-075 N sodium 
hydroxide, using methyl red as indicator. 

The whole analysis should be carried out in duplicate and a 
control analysis should be made in which 100 ml. of the extraction 



APPENDIX 135 

medium are made up to 300 ml. with distilled water and distilled 
with 4 grams of magnesia. 

The moisture content of the soil must be estimated in a further 
sample and allowed for in calculating the ammonia-nitrogen 
present. In the case of soils rich in calcium carbonate, smaller 
samples must be taken and stronger hydrochloric acid employed. 

Estimation of nitrite and nitrate. 

The nitrite- and nitrate-nitrogen can be determined in the 
soil sample used for the estimation of ammonia. After distilla- 
tion of the ammonia a further 200 ml. of distilled water are added, 
together with 3 grams of Devarda's alloy and 1 ml. of ethyl 
alcohol, and the contents of the flask are distilled into 0-075 N. 
sulphuric acid as before. Any nitrite and nitrate present is 
reduced to ammonia by the alloy and the acid neutralized is 
estimated by titration with sodium hydroxide as before. 

In soils of the humid tropics nitrites are rarely found so the 
ammonia found after reduction by Devarda's alloy usually 
represents nitrate-nitrogen only. If nitrite is present (test with 
the Griess reagent 9 ) it must be estimated in a separate sample. 

Estimation of nitrite in soil. 

About 20 grams of fresh soil are taken and extracted with 
too ml. of distilled water. After filtration, 25 ml. of solution 
are taken and made up to a definite volume, say 50 or 100 ml., 
after addition of 2 ml. of each of the Griess reagents, 9 and the 
colour produced is compared in a colorimeter with that obtained 
from a standard nitrite solution containing the same quantity of 
the Griess reagent. In this estimation the colours should be 
compared after a definite time has elapsed since preparation of 
the solutions, say 8 or 10 minutes. 

In the presence of clay, the aqueous filtrate obtained after 
extraction of the soil may be cloudy and in such a case the 
procedure must be modified; to a known weight of fresh soil is 
added approximately the same volume of a 0-5 per cent, solution 
of aluminium sulphate and the mixture is shaken for ten minutes. 
Then about 1 gram of calcium hydroxide is added and after further 
shaking the mixture is left to stand; the soil settles quickly, 
and the clear extract can be removed by filtration and used for 
the estimation of nitrite. 10 

Estimation of the numbers of soil micro-organisms. 

The procedure described below was found to giw satisfactory 



I36 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

results with soil samples in the Malay Peninsula. It must be 
noted that the method assumes that 

(a) each colony which develops on the plate represents a 
single organism; 

(b) every viable organism in the sample which falls on the 
plate forms a colony. 

In practice neither condition is fulfilled. 

Sampling. The composition of the soil is so variable that 
results based on a single sample are almost valueless. Sampling 
is best carried out by a set of metal tubes, two inches in diameter 
and some six or eight inches in length; for most experimental 
purposes sampling should be carried out to the same depth, 
which may conveniently be four or six inches. Accordingly a 
mark is filed four or six inches from one end of the tube, and, 
when sampling, the tube is driven into the ground until this file 
mark is flush with the surface. The practice of bulking three 
or four cores is indefensible on mathematical grounds, although 
no serious error is introduced when the concentration of micro- 
organisms does not differ widely from one sample to another. 
At least two samples should be taken. 

It is a good plan to carry out the plating operations when the 
samples are of the same age ; twenty-four hours after sampling is 
usually a convenient time, as it permits examination of samples 
taken at some distance from the laboratory. 

Preparation of the plates. 2-5 grams of soil are weighed out 
(at this stage it is unnecessary to adopt any precautions against 
contamination by stray organisms) and added to 250 ml. of 
sterile water 11 contained in a 500 ml. flask. This flask, which 
previously had a wad of cotton wool in the neck, is now closed 
by a sterile rubber bung. If the soil has not been sieved, at this 
point it may be advisable to insert a sterile glass rod into the 
flask and grind any lumps of soil to powder. The flask is now 
shaken by hand so that an arc about 3 feet in length is described 
fifty times. 

The flask contains 1 gram of soil per 100 ml. of liquid, and it 
will be necessary to make higher dilutions. Ten ml. are taken 
out by a sterile pipette and placed in a bottle containing 90 ml. 
of sterile water (or saline) ; the solution is sucked up and down 
the pipette once, then the bottle is closed by a sterile rubber 
bung and shaken by hand until complete admixture has taken 



APPENDIX 137 

place. This results in a dilution of 1/1000, which is usually 
sufficient in Malaya; nevertheless, in starting a series of plate 
counts it is well to make dilutions of 1/1000, 1/10,000 and 
1/100,000, and the higher dilutions can be omitted subsequently, 
if found to be unnecessary. 

Finally, 1 ml. of the dilutions employed is taken in a sterile 
1 ml. pipette and placed in a sterile petri dish, and this is followed 
by addition from a test-tube of 10 ml. of agar medium which 
has been melted in a water bath and then cooled to 42 ° C. At 
least four platings of each dilution of each sample should be made 
and, immediately after addition of the agar, the plate should be 
given a circular motion to ensure proper mixing of the soil 
solution and agar. The petri dishes are then set aside to cool 
and are inverted when solid and placed in an incubator. In 
Malaya the plates were incubated at laboratory temperature 
(26°-29° C.) and the colonies counted after five days. If counting 
is delayed beyond this period it will be found that many of the 
plates are useless on account of being overgrown with fungal 
colonies. 

The counting is carried out by inverting the petri dishes, 
ruling two diameters at right angles on the plate containing the 
agar, and counting the colonies in each quarter. The actual 
counting is carried out with a hand lens (magnification X4), 
and each colony is marked with an ink dot as counted. If the 
same lens is used throughout the investigation there is no need 
to subject the eyes to a severe strain in an endeavour to 
enumerate colonies not clearly visible with the lens. 

From the foregoing it will be realized that all colonies should 
be counted unless it is specially desired to distinguish between 
fungi and bacteria. The total plate count recorded is that 
developing from 1 gram of moist or dry soil; if results are 
expressed on a dry basis the moisture content of the soil must be 
determined. 

Sterilization of apparatus. If much bacteriological work is 
to be carried out it is advisable at the start to arrange the 
sterilization processes so that they are as automatic as possible. 

The petri dishes may be sterilized by heating in an air-oven 
at 160 C. for two hours, and then wrapped in grease-proof paper, 
or a number may be placed in a proper metal petri dish-container 
and the whole sterilized in the autoclave between one and two 
atmospheres (120 to 130 C.) for thirty minutes. The rubber 
bungs may be placed in a large beaker of water, covered with 



138 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

stiff paper, and sterilized in the autoclave. Pipettes are con- 
veniently sterilized just before use in an instrument sterilizer 
and removed by means of forceps passed through the flame a 
few times immediately before use. The flasks and bottles 
containing water or saline are closed by a plug of cotton wool, 
over which is placed a covering of paper secured by an elastic 
band, and heated in the autoclave at 120 to 130 C. for half- 
an-hour. It is a great convenience if the pipettes employed are 
graduated right up to the tip. 

The process of autoclaving results in a certain loss of water 
from the flasks and bottles, but this difficulty can be overcome 
by a simple preliminary experiment to determine the approximate 
loss of water from both the 250 and 90 ml. lots and adding the 
extra amount of liquid required before sterilization. 

is 



Thornton's agar count medium. 


Thornton's medium 12 


composed 


of the following constituents: 








Dipotassium hydrogen phosphate 
Potassium nitrate 


i-og. 
o-5 g- 




Magnesium sulphate 
Calcium chloride 






0-2 g. 

o-ig. 




Potassium chloride 






o-ig. 




Sodium chloride 






o-ig. 




Asparagine 
Ferric chloride 






o-5 g- 

0-002 g. 




Agar 
Distilled water 




to 


15 g- 

i litre. 



(Note. In equatorial regions it may be necessary to add more 
than 15 grams of agar to ensure solidification of the medium and 
the minimum quantity necessary is best found by trial.) 

The salts are dissolved in water, then the agar is added, 
and the whole is melted in the steamer. The mixture is filtered 
through a thin layer of cotton wool in a Buchner funnel and 
1 gram of mannitol is added to the nitrate. The reaction of the 
medium is adjusted to pH 7-3 with sodium hydroxide solution 
by means of brom-thymol-blue as indicator. The medium is 
tubed in 10 ml. lots and autoclaved at 120 C. for fifteen minutes. 

Interpretation of the results of plate counts. The significance 
of the results can be accurately assessed only by submitting 
them to a statistical analysis. 

Before starting an experiment, it is well to examine the 
variability of the soil by making plate counts on a large number 



APPENDIX 139 

of samples taken simultaneously, and then to determine the 
error of the method by making a number of separate deter- 
minations on the same sample. In both cases the standard 
error of the determinations can be found and will be useful 
subsequently; in neither case should it exceed 20 per cent. 

The arithmetic mean is found by dividing the sum of the 
colonies on all the parallel plates for a particular sample by the 
number of plates. Thus, 

m = — , 

n 

where a is the number of colonies on a plate and n is the total 
number of plates. 

The geometric mean is the arithmetic value which corresponds 
to the mean of the values of the logarithms. 

The standard error is given by the formula : 

IH(m— a) 2 



S.E. = 



n(n- 



where m is the arithmetic mean, a is the number of colonies on 
the plate, and n is the number of plates. 

On the P = 0-05 level of significance, two values are 
significantly different from each other when the difference 
between them is greater than three times the standard error: 
that is to say, two values which were not significantly different 
could show a difference of three times the standard error by 
chance only once in twenty times. 

If the frequency distribution of a number of platings of the 
same soil sample be plotted, a T-shaped curve is obtained, but if 
the logarithms of these values are so treated the usual bell- 
shaped curve, characteristic of a normally distributed population, 
is obtained. During the period between sampling and analysis 
the soil micro-organisms are passing through the phase of decline 
when the decrease in numbers is logarithmic, so that it seems 
safe to say that the geometric, and not the arithmetic, mean 
should be taken when averaging the results of a number of 
different samples. The mean value for a number of platings of 
the same sample should be averaged in the usual way from the 
actual values. 13 

In Fig. 10 are shown the distribution frequencies of the 
logarithms of the whole of the "bacterial counts" obtained by 
the writer for soils in the Malay Peninsula. The histogram is 



140 



BIOLOGICAL PROCESSES IN TROPICAL SOILS 



characteristic of a normally distributed population, if some eight 
or nine high values (all taken during a period of " burning-off " 
on an area recently cleared of forest) be excepted. Excluding 
these "burning-off" samples, the mean value of the logarithms 
of the counts coincides with the mode of the curve. This 




logarithms of •bacterial -- counts in thousands 
Fig. 10. The Distribution Frequencies of the Logarithms of 
"Bacterial Counts" made in Malaya with Soil under Forest, 
under Secondary Growth, and Clean-weeded and Exposed to 

the Sun. 

corresponds to a value of 500,000 micro-organisms per 1 gram of 
soil, while the mode of the curve for the samples taken during 
the "burning-off" period corresponds to a value of 5 million 
micro-organisms per 1 gram of soil. 



II. CLASSIFICATION OF BACTERIA 

Bergey's scheme of classification of bacteria is briefly ap- 
pended in so far as commonly occurring soil bacteria are 
concerned. 






appendix 14i 

Class Schizomycetes 

Order Eubacteriales 

This order comprises all bacteria, except the Actinomyces and 
allied forms and the alga-like "higher bacteria." The cells occur 
as spheres or as straight or curved rods; they are motile or non- 
motile and some species possess endospores. 

FAMILY NITROBACTEREAE 

Cells usually rod-shaped and motile or non-motile ; endospores 
not present. All are obligate aerobes, securing energy by the 
oxidation of carbon, hydrogen or nitrogen, or of simple com- 
pounds of these. Two tribes are recognized: 

(a) Tribe Nitrobactereae, in which energy is secured by 
oxidation of simple compounds of carbon and nitrogen, e.g. 
alcohol, nitrites. This group contains some important and 
interesting forms. Nitrosomonas [eurofiaea Win.] 14 effects the 
oxidation of ammonia to nitrous acid and is an important 
factor in soil fertility, particularly in temperate regions. It is 
sensitive to pH changes, growing best between pH 7-6 and 8-i; 
it cannot be cultivated in the laboratory in presence of more 
than small amounts of organic matter. The organism is strictly 
aerobic, requiring oxygen in accordance with the equation: 

NH 3 + 30 = HN0 2 + H 2 0. 
Nitrococcus [nitrosus (Mig.)] is a coccoid-like form of lesser 
importance which also effects the oxidation of ammonia to 
nitrous acid; it may prove to be identical with Nitrosomonas. 

Nitrobacter [winogradskyi Buch.] is a rod-shaped, non- 
motile organism which brings about the oxidation of nitrous 
acid to nitric acid: 

2HN0 2 + 2 = 2HNO3. 

It works best at pH values ranging between 6-8 and 7-4; often 
it appears to be scarce or absent from soils, and it is probable 
that its importance in the nitrogen cycle in the soil is over-rated. 
Thiobacillus [thioparus Beij.] is a small rod-like organism 
responsible for the oxidation of sulphur or simple sulphur com- 
pounds. 

(b) Tribe Azotobactereae, containing nitrogen-fixing organ- 
isms. Azotobacter (chroococcum Beij.] is a large obligate 
aerobe occurring as rods or cocci. In a medium deficient in 
nitrogen and in the presence of carbohydrates it can fix nitrogen 



142 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

from the air. It is reported to work only in media with a.pH 
value above 6-o, but it appears to be widely distributed in the 
cultivated soils of Malaysia. The sensitivity of Azotobacter to 
acidity and to the presence of phosphate has led to the use of 
this organism as a means of assessing soil fertility. 15 It seems that 
Azotobacter cannot develop in the absence of iron compounds. 

The species of Rhizobium [leguminosarum Frk., better 
known as radicicola Beij.] are responsible for nodule formation 
on the roots of leguminous plants and are able to fix atmospheric 
nitrogen. It appears probable that nodule formation and 
nitrogen fixation occur largely where there is a deficiency in the 
nitrogen level. The species of Rhizobium are obligate aerobes, 
having a complicated life cycle ; at one stage flagella are developed 
and the motile organisms are able to move rapidly through the 
soil. Entrance to a growing plant is effected through the 
unicellular root hairs. 16 

FAMILY COCCACEAE 

Cells spherical and division takes place in one, two or three 
planes. Motility is rare and endospores are usually absent. 
The metabolism is complex, involving the utilization of amino 
acids and carbohydrates. 

The Tribe Micrococceae comprises species which grow well on 
artificial media under aerobic conditions and many form pig- 
ments. The genus Micrococcus [luteus (Schr.)] contains 
a large number of species, many of which are found in the soil; 
in some cases yellow pigments are produced while in the allied 
genus Rhodococcus [rhodochrous Zopf] the pigment is a 
bright red. 

FAMILY BACTERIACEAE 

Rod-shaped organisms which may be motile or non-motile; 
endospores are absent and metabolism is complex. 

(a) Tribe Chromobactereae is characterized by the production 
of pigment. Serratia [marcescens Bizio] comprises small 
aerobic organisms which produce red or pink pigments. The 
larger species of Flavobacterium [aquatilis (Frkd.)] and 
Chromobacterium [violaceum Bergon.] produce yellow- and 
violet-coloured pigments respectively. Pseudomonas [aeruginosa 
(><hr.)] produces a water-soluble pigment of a green or blue 
hue. 



APPENDIX 143 

(b) Tribe Protaminobacterieae contains the single genus 
Protaminobacter [alboflavum den Dooren de Jong], in which 
the species are able to attack substances containing amino 
groups. 

(c) Tribe Cellulomonadeae, containing the single genus 
Cellulomonas [biatotea (Kell.)], is important in that its 
members effect the aerobic destruction of cellulose in the soil: 
these organisms may be motile or non-motile and chromogenic 
or non-chromogenic. Under very acid conditions in the soil 
(pH 4 or less), however, cellulose decomposition occurs largely 
as a result of the activities of fungi (Trichoderma and Penicillium). 17 

(d) Tribe Achromobacterieae contains the single genus 
Achromobacter [liquefaciens (Eisenb.)], which comprises 
rods which may be motile or non-motile and are rather variable 
in size and in cultural characters. 

(e) The Tribe Erwineae consists of a number of plant pathogens. 

FAMILY BACILLACEAE 

Rods producing endospores. Only two genera have been 
separated. 

Bacillus [subtilis (Ehrenb.)] consists of a number of aerobic 
forms which are mostly saprophytes. The well known "Bacillus 
coli" is a species of Escherichia pertaining to the previous family. 

Clostridium [butyricum Prazm.] comprises anaerobes and 
micro-aerophiles which are often parasitic. Many of the species 
are pathogenic and the well known CI. botulinum v. Ermen. 
produces one of the most powerful toxins known. Several 
species of the genus occur in the soil and are able to fix atmos- 
pheric nitrogen ; a few forms can effect the anaerobic destruction 
of cellulose. 

Order Actinomycetales. 

In this order the cells are usually elongated and frequently 
filamentous, with a decided tendency towards the development 
of branches. Endospores are not produced but conidia (i.e. 
reproductive cells) are found in some genera. The organisms 
are non-motile and usually aerobic; the metabolism is complex. 
Some species form mould-like colonies. 

FAMILY ACTINOMYCETACEAE 

Comprises a number of filamentous forms which are often 
branched and sometimes forming mycelium (i.e. a. network of 



144 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

thread-like hyphae) ; conidia are present in some species. It 
has been shown that the Actinomyces in the soil in India are 
present almost exclusively as conidia, and vegetative mycelia are 
found only when the organism is mixed with undecomposed 
plant remains. 18 

The genus Actinomyces [bovis Harz] contains a large 
number of forms, but the specific identity of many of them is 
open to question. 

FAMILY MYCOBACTERIACEAE 

Straight or curved rods which are rarely filamentous: conidia 
absent. 

The genus Mycobacterium [tuberculosis (Koch)] contains 
several aerobic species, mostly pathogenic to animals and of 
little importance in the soil. The species of Mycoplana 
[dimorpha Gray and Thornton] are able to decompose phenol: 
the recently described genera Cytophaga [hutchinsoni Win.], 
Cell vibrio [ochraceus Win.] and Cellfalcicula [viridis Win.] 
may prove to be important as they contain a number of soil 
organisms which decompose cellulose. 

Order Chlamydobacteriales 

FAMILY CHLAMYDOBACTERIACEAE 

The alga-like "higher bacteria" usually occur as filaments 
consisting of a number of cells enclosed in a sheath. The 
species of Leptothrix [ochracea Kiitz.] and Crenothrix 
[polyspora Cohn] occur in iron-containing waters, and frequently 
the sheath is coloured dark reddish-brown on account of the 
deposition of ferric hydroxide. 

III. CLASSIFICATION OF FUNGI 

It is usual to divide the Fungi into the classes Phycomycetes 
and Eumycetes (comprising the sub-classes Ascomycetes and 
Basidiomycetes) and the Fungi Imperfecti. The characters 
of these groups are briefly outlined below. 

CLASS PHYCOMYCETES 

The two types of sexual reproduction found in the "algal" 
fungi form the basis for the differentiation of the two orders 
Zygomycetes and Oomycetes. In the first-named group, the 
sexual organs are similar and zygospores are formed by the con- 
jugation of cells from different individuals; asexual reproduction 



APPENDIX 145 

is effected by non-motile endospores. The Oomycetes are 
characterized by well-differentiated reproductive organs, a 
large female element uniting with a smaller male body to form 
an oospore. Asexual generation in the Oomycetes is effected by 
conidia and also by motile endospores termed zoospores. 

Many of the best known soil fungi, such as Mucor, Rhizopus 
and Zygorhynchus of the family Mucorineae, pertain to the 
Zygomycetes, and of the few species parasitic on plants and 
insects, the house fly fungus, Empusa muscae, may be instanced. 

Most of the Oomycetes are parasitic on plants, and some of 
the most destructive fungi known belong to this order. 
Phytophthora infest ans is responsible for potato disease and 
Plasmopara viticola produces the "false mildew" on the leaves 
and fruit of the grape vine. 

CLASS EUMYCETES 

In the important sub-class Ascomycetes asexual multi- 
plication takes place by means of conidia. When sexual 
generation occurs, conjugation of the sex elements results in 
the formation of an ascus, which is a club- or oval-shaped 
sporangium containing a definite number (usually eight) of 
ascospores. In few instances have the sexual methods of 
reproduction in the Ascomycetes been investigated. 

This sub-class contains many important forms, among which 
may be mentioned Aspergillus (found on damp bread and 
vegetables), the blue-green mould Penicillium, the ergot-producing 
Claviceps purpurea, Taphrina (responsible for the formation of 
" witches' brooms ") and the well known yeast fungi (Saccharo- 
mycetes). 

In the sub-class Basidiomycetes asexual reproduction 
occurs by conidia formation; the more or less distinctive 
conidiophore is termed a basidium and basidiospores are produced 
by a process of budding. Evidence of sexuality among the 
Basidiomycetes was lacking until quite recently when bisexuality 
was established in two members of the sub-class. 19 

Fomes lignosus 20 and Gandoderma pseud of err eum, which are 
responsible for root diseases on Hevea brasiliensis, pertain to this 
group, and the fructifications of the Hymenomycetes are known 
as toadstools. 

FUNGI IMPERFECTI 

As "Fungi Imperfecti" are grouped all forms not known to 
produce fruits characteristic of the other groups and, con- 
sequently, much diversity of form is shown. It is believed that 



I46 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

many conidial forms of the Ascomycetes are included here, and 
it is probable that further study of the group will reduce its 
numbers. Important soil genera pertaining to the division are 
Oidium, Trichoderma and Fusarium. 

Many tropical plant pathogens belong to this group, among 
which may be mentioned Oidium heveae, causing secondary leaf 
fall in rubber trees, Helminthosporium incurvatum, which attacks 
the leaves of the coconut palm and Fusarium cubense, causing 
Panama disease of bananas. 

IV. CONVERSION FACTORS 

For conversion of degrees Fahrenheit to degrees Centigrade: 
(i) subtract 32 from the Fahrenheit value, 
(ii) multiply by 5, 
(iii) divide by 9. 

For conversion of degrees Centigrade to degrees Fahrenheit: 
(i) multiply the Centigrade value by 9, 
(ii) divide by 5, 
(iii) add on 32. 

For conversion of metres to feet : 

multiply by 3-28. 
For conversion of feet to centimetres: 

multiply by 30-48. 

V. BIBLIOGRAPHY 

The following bibliography, which is not intended to be more 
than indicative, may be useful to those interested in soil conditions 
in the eastern tropics. 

E. J. Russell. Soil Conditions and Plant Growth, 6th Ed. 
(London), 1932. 

S. A. Waksman. Principles of Soil Microbiology, 2nd Ed. 
(London), 1931. 

E. C. J. Mohr. De Grond van Java en Sumatra, 2nd Ed. 
(Amsterdam), 1930. See English translation entitled 
Tropical Soil-forming Processes and the Development of 
Tropical Soils, with Special Reference to Java and 
Sumatra, by R. L. Pendleton (Peiping), 1933. 

H. A. Tempany and G. E. Mann. Principles of Tropical 
Agriculture (Kuala Lumpur, F.M.S.), 1930. 



APPENDIX I47 

K. W. Dammerman. The Agricultural Zoology of the Malay 
Archipelago (Amsterdam), 1929. 

B. A. Keen. The Physical Properties of Soils (London), 1931. 

C. H. van Harreveld-Lako. De Eigenschappen van de 

Suikerrietgronden op Java (Groningen), 1928. See 
English translation entitled The Properties of Sugar 
Cane Soils in Java, by R. L. Pendleton (Peiping), 1932. 

A. R. Wallace. The Malay Archipelago (London), 1922. 

REFERENCES 

1. "The Mechanical Analysis of Soils; a Report on the Present Position, 

and Recommendations for a new Official Method," by a Sub- 
Committee of the Agricultural Education Association, J . Agric. 
Sci., 1926, 14, 123-144. 

2. G. W. Robinson, Soils, their Origin, Constitution and Classification 

(London), 1932, p. 377. 

3. E. Billman, /. Agric. Sci., 1924, 14, 232-239. 

4. S. G. Heintze and E. M. Crowther, Trans. Second Comm. Int. Soc. 

Sci., 1929, A, 102-111. 

5. D. J. Hissink, Soil Research, 1930, 2, 77-139. 

6. C. Olsen, Compt. Rend. Trav. Labor. Carlsberg, 1929, 17, No. 15, 1-20. 

7. The amount of hydrochloric acid necessary to make the pH value 1 

is found as follows : A 7 acid is added from a burette until a drop of 
the suspension gives a clean blue colour with Congo red paper 
which does not alter on standing for five minutes; this brings 
the pH value to 2-0, and a further 20 ml. of N acid are then added. 

8. It is doubtful if the addition of toluene has any significance. 

9. For the Griess reagent two solutions are prepared: 

(a) 10 g. of sulphanilic acid dissolved in 1000 ml. of 30 per cent. 
(by volume) acetic acid by heating on the water bath. 

(b) 3 g. of a-naphthylamine boiled with 700 ml. of water for two 
minutes, then filtered and 300 ml. of glacial acetic acid added. 
The two solutions are kept separate and about 2 ml. of each are 
added to the solution to be tested. In the presence of nitrite 
a bright carmine colour is developed. 

R. P. Bartholomew, Soil Sci., 1928, 25, 393-398. 

Or sterile saline, prepared according to the formula: 5 g. sodium 
chloride and 1 g. magnesium sulphate in 1 litre of water. 

H. G. Thornton, Ann. Appl. Biol., 1922, 9, 241-247. 

13. A. H. Robertson, J. Bad., 1932, 23, 123-134; A. S. Corbet, Soil Sci., 

1934, 38, 407-416. 

14. The type species of each genus is placed in square brackets after each 

generic name. 

15. H. R. Christensen and O. H. Larsen, Centr. Bakt., Abt. II, 191 1, 29, 

347; H. R. Christensen, Soil Sci., 1923, 15, 329. 

1''. II. G. Thornton, Imper. Bur. Soil Sci., Tech. Communication No. 20, 
1931. 



148 BIOLOGICAL PROCESSES IN TROPICAL SOILS 

17. H. L. Jensen, /. Agric. Set., 1931, 21, 38-80. 

18. V. Subrahmanyan and R. V. Norris, /. Ind. Inst. Set., 1929, 12a, 

53-5 o ; V. Subrahmanyan, ibid., 57-68; M. Ganesha Rao and 
V. Subrahmanyan, ibid., 253-273. 

19. S. R. Bose, La Cellule, 1934, 42, 249-266. 

20. Some difference of opinion exists with regard to the correct name for 

the fungus parasitic to the rubber tree in the eastern tropics and 
known to Malayan planters as Fomes lignosus. W. H. de Jong 
(Arch. Rubbercultuur , 1933, 17, 83-104) refers to it as Rigidoporus 
microporus (Swartz) van Overeem ( = Fomes lignosus Klotzsch). 



AUTHOR INDEX 



Aaltonen, V. T., 95. 
Akhurst, C. G., 131. 
Albrecht, W. A., 96. 
Aldrich-Blake, R. N., 60. 
Allen, E. K., 96, 132. 
Allen, O. N., 96, 132. 
Altson, R. A., 92. 
Anderson, M. S., 24. 
Arnold, C. W. B., 81. 
Arrhenius, O., 24. 

Barrington, A. H. M., 34. 
Bartholomew, R. P., 147. 
Belgrave, W. N. C, 24, 132. 
Bergey, D. H., 61. 
Billman, E., 147. 
Birkemose, F., 121. 
Bornebusch, C. H., 45. 
Bose, S. R., 148. 
Braak, C, 23. 
Bristowe, W. S., 96. 
Brooks, C. E. P., 23. 
Buchanan, F., 24. 
Bucking, H., 24. 
Bunting, B., 132. 

Capus, G., 96. 

Chapman, R. N., 69. 

Chasen, F. N., 23. 

Christensen, H. R., 147. 

Cleveland, L. R., 61, 132. 

Cohen, W. E., 45, 82. 

Corbet, A. S., 45, 60, 61, 69, 95, 96, 

107, 147. 
Crowther, E. M., 147. 
Crump, L. M., 60. 
Cutler, D. W., 60. 

Dammerman, K. W., 40, 45, 147. 
Dennett, J. H., 24, 82. 
Dhar, N. R., 95. 
Doyne, H. C, 24. 
Druif, J. H., 24. 

Erb, D. J., 24. 
Ernst, A., 96. 

Fennema, R., 24. 
Fish, N., 132. 
Fisher, R. A., 60. 
Fleischer, M. ( 82. 
Flint, C. F., 132. 

Gaarder, T., 95. 
Gangulee, N., 60. 



Grantham, J., 132. 
Greene, H., 96. 
Greig-Smith, R., 82. 
Groenewege, J., 96. 

Hagem, O., 95. 

Haines, W. B., 23, 132. 

Harden, A., 61. 

Hardy, F., 24, 107. 

van Harreveld-Lako, C. H., 107, 

147- 
Heintze, S. G., 147. 
van Heusden, W. C., 132. 
Hilgard, E. W., 24. 
Hissink, D. J., 147. 
Hobson, R. P., 81. 
Hockensmith, R. D., 107. 
Hutchinson, H. B., 60. 

Imms, A. D., 61. 
Iyer, K. R. N., 82. 

Jarrett, V. H. C, 34. 
Jenny, H., 107. 
Jensen, H. L., 148. 
John, O., 45. 
de Jong, W. H., 132, 148. 
Judd, C. S., 132. 

Keen, B. A., 147. 
Kissling, R., 82. 
Kloss, C. B., 23. 
Konigsberger, V. J., 24. 
de Kruijff, E., 96. 

Larsen, O. H., 147. 
Lemmermann, O., 82. 
Leonard, L. T., 96, 132. 

Mackenzie, W. A., 60. 

Mann, G. E., 146. 

Mansour-Bek, J. J., 81. 

Mansour, K., 81. 

Martin, F. J., 24. 

Mattson, S., 24. 

Milne, G., 131. 

Milsum, J., 132. 

Mohr, E. C. J., 15, 23, 107, 146. 

Molengraaf, F., 24. 

Napper, R. N. P., 132. 
Neeb, G. A., 24. 
Norris, R. V., 148. 



Olsen, C, 147. 



149 



i5o 



AUTHOR INDEX 



Page, H. J., 81. 
Pendlebury, H. M., 23, 45. 
Pendleton, R. L., 23, 107, 146, 147. 
Phillips, M., 81. 
Pitcaithly, N. P., 131. 
Posewitz, T., 24. 
Purvis, E. R., 82. 

Rao, G. G., 95. 
Rao, M. G., 148. 
Ridley, H. N., 23, 34. 
Robertson, A. H., 147. 
Robinson, G. W., 147. 
de Rossi, G., 95. 
Russell, E. J., 60, 132, 146. 

Sandon, H., 60, 61. 
Savage, H. E. F., 107. 
Schmole, J. F., 132. 
Scrivenor, J. B., 23, 24. 
Senstius, M. W., 24. 
Seward, A. C, 96. 
Shive, J. W., 95. 
Stahl, A. L., 95. 
Starkey, R. L., 82. 
Stoklasa, J., 82, 96. 
Stoughton, R. H., 61. 
Subrahmanyan, V., 148. 



Szemian, J., 24. 

Tempany, H. A., 
Thornton, H. G., 
Thorpe, E., 131. 
du Toit, M. M. S 
Topley, W. W. C 
Tucker, E., 107. 
Turk, L. W., 96 
Turner, P. E 



146. 
60, 147. 

, 81. 
, 69. 



81, 132. 



Vageler, P., 34, 96. 
Verbeek, R. D. M., 24. 
Verwey, J., 45. 
Vollema, J. S., 132. 

Waksman, S. A., 60, 61, 82, 146. 
Wallace, A. R., 147. 
"Watson, J. G., 34. 
Weir, J. R., 112. 
Weissmann, H., 82. 
White, J. T., 24. 
Whittles, C. L., 60. 
Williamson, K. B., 82. 
Wilshaw, R. G. H., 24, 96, 107. 
Wilson, G. S., 69. 
Winslow, C.-E. A., 69. 
Worley, F. P., 131. 



SUBJECT INDEX 



Acidity, 22, 24. 
Aeration of soil, 48. 
Aerobic processes, 48. 
Agar medium, 138. 
Altitude, 15, 40. 
Aluminium oxide, 21, 24, 104. 
Ammonia, 24, 134, 141. 
Anaerobic processes, 77, 91. 
Analysis of soil, 133. 
Ants, 38, 41. 
Ant, tree, 38, 131. 
Apparatus, 137. 

Bacteria, 46, 53. 

classification of, 58, 140. 

death of, 55. 

energy requirements of, 56. 

growth of, 62. 

numbers of soil, 51, 135. 

reproduction of, 54. 

structure of, 54. 
Bacterial 

growth phases, 62, 69. 

metabolism, 56. 
Bah, 12, 23. 
Bamboo, 124. 
Base exchange, 70. 
Battak Plateau, 9, 31. 
Beetle 

cockchafer, 32, 43. 

dung, 38. 

sexton, 38. 
Bertam palm, 26. 
Biotic potential, 66. 
Birkemose system, 121. 
Bombyliid fly, 43. 
Borneo, 10. 
Bracken, 31. 
Burning-off forest debris, 53, 108. 

Carbon 

dioxide from soil, 77. 

nitrogen ratio, 74, 86, 98, no. 
Catch crops, 114. 
Cellulose, 71, 143, 144. 
Centipedes, 40. 
Chimney prawn, 44. 
Chloroform extraction of soil, 74. 
Christmas Island, 14. 
Classification of 

bacteria, 58, 140. 

fungi, 59, 144. 

soils, 22. 
Clean-clearing, 112. 
Ciean-weeding, 105, 120. 



Cleared land, 105, 108. 

Clearing of forest, 28. 

Climate, 15. 

Cockroach, 39. 

Coffee, 114. 

Colonies of bacteria, 51, 62. 

Cover plants, 114. 

Covers, natural, 120. 

Crab 

fiddler, 44. 

hermit, 44. 
Creepers, 117. 

Curve, bacterial growth, 62. 
Cytoplasm, 54. 

Death of bacteria, 55. 

Denitrification, 91. 

Depth of soil and micro-flora, 49. 

Diseases, 68. 

Disinfection, 55. 

Earthworms, 39, 43, 50, 122. 

Earwig, 39. 

Energy requirements of bacteria, 

56. 
Environmental resistance, 66. 
Enzymes, 57. 
Epiphytes, 26. 
Erosion, soil, 17. 
Evaporation, 97. 

Fats, 72, 74. 

Fauna, soil, 35. 

Faunistic regions, 12. 

Ferric hydroxide, 21, 24, 77. 

Fertilizers, 126. 

Fiddler crab, 44. 

Fish, mud, 44. 

Flowers, 26. 

Fly, Bombyliid, 43. 

Forest 

clearing, 28. 

normal, 34. 

primary, 25, 106. 

secondary, 29, 30, 31. 
Fungi, 58. 

classification of, 59, 144. 

Imperfecta 145. 

reproduction of, 58, 144. 

soil, 59. 

Garden soils, 130. 
Geological 

eras, 1. 

maps, 24. 



151 



152 

Geology of 

Borneo, n. 

Java, 10. 

Malaya, 6. 

Sumatra, 8. 
Giant snail, 32. 
Ginger, 29. 
Golden balsam, 27. 
Grasses, 29. 
Ground spiders, 40. 
Growth of bacteria, 62. 

Hard pan, 22. 
Hemicellulose, 72. 
Higher bacteria, 77, 144. 
Himalaya Mountains, 3. 
Humidity, 15. 

factor, 97. 
Humus, 73, 99. 
Hydroxylamine, 86. 
Hyponitrous acid, 87. 

Ice age, 3. 

Indo- Australian region, 13. 

Insolation, 88, 93, 99, 104, 128, 

Iron 

bacteria, 77. 

hydroxide, 21, 24, 77. 
Islands, 14. 
Islands of 

Malaya, 7. 

Sumatra, 5, 10. 

Japanese river fever, 125. 
Java, 10, 130. 
Jenny's law, 97. 

Kaolin, 21. 
Krakatau, 94. 

Lalang, 9, 30, 94, 102, 105, 131. 
Langkawi Islands, 7, 14, 18. 
Large Sunda Islands, 4. 
Laterite, 19, 20, 24. 
Laterization, 21. 
Leaching, 19, 84, 86, 108. 
Leguminous 

cover plants, 93, 114. 

plants, 93, 95, 105, 115. 
Lesser Sunda Islands, 23. 
Lignin, 72. 
Limestone, 6. 
Lixivium, 20. 
Loss on ignition, 133. 

Malaria, 80. 
Malay 

Archipelago, 5. 

Peninsula, 6. 
Malaya, 5. 
Malaysia, 1, 5, 14. 



SUBJECT INDEX 






Malaysian plants, 32. 
Mangrove forests, 7, 27, 44. 
Manuring of rubber, 128. 
Maps 

geological, 24. 

soil, 24. 
Metabolism, bacterial, 56. 
Micron, 53. 
Micro-organisms 

numbers of soil, 50, 51, 135. 

soil, 46. 
Millipedes, 40. 
Mineral salts, soil, 49 
Moisture, soil, 48, 133. 
Monsoon, 17. 
Mountains of 

Borneo, 1 1 . 

Java, 10. 

Malaya, 6. 

Sumatra, 9. 
Mud fish, 44. 
Mycorrhiza, 47. 

Natural covers, 120. 

Negritoes, 5. 

Nematodes, 39. 

Nipah palm, 28. 

Nitrate in soil, estimation of, 135. 

Nitric acid, 57, 87, 141. 

Nitrite in soil, estimation of, 135. 

Nitrogen 

cycle, 83. 

fixation, 60, 91, 111, 141. 

fixing bacteria, 86, 91, 105, 141, 143 

gas, 92, 104, 129. 

in soil, estimation of, 134. 

losses and gains of, 91. 
Nitrogenous fertilizers, 127, 128. 
Nitrous acid, 56, 57, 87, 141. 
Nodule formation, 93, 142. 
Normal forest, 34. 
Nucleus, 54. 

Numbers of soil micro-organisms, 
5°. 51. 135- 

Oils, 72. 

Organic matter 

decomposition of soil, 71, 99, 
formation of soil, 71, 99. 
in soil, 70. 

Ortstein, 22. 

Palawan, 12. 
Parasitism, 46, 145. 
Peat, 76. 
Peoples of 

Bali, 12. 

Borneo, 11. 

Java, 10. 



SUBJECT INDEX 



153 



Peoples of 

Malaya, 8. 

Palawan, 12. 

Sumatra, 9. 
Pests, 68. 

pH, 23, 24, 89, 125, 141. 
Phases, bacterial growth, 62, 69, 78, 
Phosphate, 23, 127. 

as a fertilizer, 127. 
Photochemical reactions, 88, 104. 
Pitcher plant, 26. 
Plants 

chemical composition of, 71. 

Malaysian, 32. 
Pneumatophores, 27. 
Podsolization, 22. 
Populations, 63. 

Population of yeast cells, 64, 65. 
Potash, 23, 109. 

as a fertilizer, 127. 
Potential, biotic, 66. 
Prawn, chimney, 44. 
Pressure, 18. 
Primary forest, 25, 106. 
Protein, 72. 
Protozoa, 47, 59. 

Rain water, 86. 

Rainfall, 16. 

Rats, 125. 

Reaction, soil, 22, 23, 48, 133. 

Reproduction of 

bacteria, 54. 

fungi, 58, 144. 
Resistance, environmental, 66. 
Rivers of 

Borneo, 11. 

Java, 10. 

Malaya, 7. 

Sumatra, 9. 
Rocks, weathering of, 18. 
Rotan, 26. 
Rubber, 42, 112, 124, 128, 146. 

manuring of, 128. 

Salts, soil mineral, 109. 

Sandhoppers, 40. 

Saprophytism, 46. 

Sarawak bean, 118. 

Scoliid wasps, 43. 

Seasons, 15, 17. 

Secondary forest, 29, 30, 31, 94. 

Sensitive plant, 32, 33, 93, 131. 

Shade, 105, 120, 121, 122. 

Silica, 24. 

Snail, giant, 32. 

Snails, 40. 

Soil 

and malaria, 80. 

aeration, 48. 



Soil 

ammonia, 24, 134. 

analysis, 133. 

bacteria, 46, 140. 

chloroform extraction of, 74. 

classification, 22. 

erosion, 17. 

fauna, 35. 

fertilizers, 126. 

formation, 18. 

fungi, 46, 59, 145. 

maps, 24. 

micro-organisms, numbers of, 50, 

5i. 135- 

mineral salts, 49, 108. 

moisture, 48, 133. 

nitrate, 87, 135. 

nitrite, 87, 135. 

nitrogen, 83, 97, 128, 134. 

organic matter, 70, 99, 101. 
decomposition of, 71, 99. 
formation of, 73, 99. 

protozoa, 47, 59. 

reaction, 22, 23, 48, 125, 133. 

temperature, 47, 79, 99, 101. 

texture, 70. 
Soils, garden, 130. 
Spiders, ground, 40. 
Stag-horn moss, 94, 122. 
Sterilization, 55. 
Straits rhododendron, 31, 94. 
Structure of bacteria, 54. 
Sugar cane, 114. 
Sumatra, 8, 129. 
Sundaland, 3, 4, 5. 
Sunda shelf, 4. 
System, Birkemose, 121. 

Temperature, 15, 99. 
Termites, 36, 41, m, 114. 
Timber 

C/N ratio of, 1 10. 

decomposition of, 75. 
Tree ferns, 26. 
Tropical typhus, 125. 

Ultraviolet light, viii, 88, 92, 104. 

Vegetation and soil micro-flora, 49. 
Violet, 27. 
Viviparity, 27. 

Wasps, Scoliid, 43. 
Waxes, 72, 74. 
Weathering of rocks, 18. 
Weeding, clean, 105, 120. 
Woodlice, 40. 

Yeast cells, population of, 64, 65. 



INDEX TO ANIMALS 



Achatina fullica, 32. 
Arachnida, 40. 

Blattidae, 39. 
Boliophthalmus, 44. 

Campsomeris 

javana, 43. 

pulchrivestita, 43. 
Chilopoda, 40. 
Coprinae, 38. 
Coptotermes curvignathus , 41, 42, 43, 

114. 
Corvus splendens, 14. 
Cryptotermes, 38. 

Diplopoda, 40. 

Eutermes 
exitiosus, 37. 
hirtiventris, 43. 

Forficulidae, 39. 
Fridericia bulbosa, 39. 

Hymenoptera, 36, 45. 
Hyperalonia tantalus, 43. 



Isoptera, 36, 45. 

Macrotermes gilvus, 37, 41. 
Mollusca, 40. 
Moniligaster houteni, 39. 
Myriapoda, 40. 

Nematoda, 39. 

Ocypodidae, 44. 

Oecophylla smaragdina, 38, 131. 

Oligochaeta, 39. 

Oniscoida, 40. 

Orchestia, 40. 

Paguridea, 45. 
Periophthalmus, 44. 
Pithecanthropus erectus, 5. 
Pontoscolex corethrurus, 39, 44, 45, 

50. 
Protozoa, 47, 59. 
Psilopholis grandis, 43. 

Silphidae, 38. 

Thalassima anomala,^. 

Xiphosura, 1. 



'54 



INDEX TO PLANTS 



Achromobacter liquefaciens, 143. 
Actinomycetaceae, 143. 
Actinomycetales, 143. 
Actinomyces bovis, 144. 
Actinomyces scabies, 48. 
Allamanda cathartica, 32. 
Alsophila, 26. 

Andropogon aciculatus, 29, 32. 
Antigonon leptopus, 32. 
Asclepias curassavica, 32. 
Ascomycetes, 145, 146. 
Aspergillus, 59, 145. 
Azotobacter chroococcum, 96, 141. 

Bacillaceae, 143. 
Bacillus 

megatherium, 96. 

mycoides, 96. 

subtilis, 143. 
Bacteriaceae, 142. 
Bacterium krakataui, 96. 
Basidiomycetes, 145. 
Blechnum orientate, 94. 
Bougainvillea glabra, 32. 

Caesalpinia pulcherrima, 32. 

Calamus, 26. 

Calapogonium mucunoides, 117. 

Canavalia, 96. 

Cassia fora, 93. 

Casuarina equisetifolia, 32, 49, 94. 

Cellfalcicula viridis, 144. 

Cellulomonas biatotea, 143. 

Cellvibrio ochraceus, 144. 

Cewfrosema 

plumieri, 93, 117. 

pubescens, 50, 93, 117. 
Chlamydobacteriaceae, 144. 
Chlamydobacteriales, 144. 
Chromobacterium violaceum, 142. 
Claviceps purpurea, 145. 
C/osfr'ia'iwm 

botulinum, 143. 

butyricum, 92, 96, 143. 
Coccaceae, 142. 
Crenothrix polyspora, 77, 144. 
Crotalaria anagyroides, 32, 93, 11S 
Cyathea, 26. 
Cytophaga hutchinsoni, 144. 

Dicksonia, 26. 

Dolichos hosei, 93, n8, 119. 

Eichhornia crassipes, 32. 
Empusa muscae, 145. 



Enterolobium saman, 32. 
Erythrina, 96. 
Escherichia coli, 143. 
Eubacteriales, 141. 
Eugeissonia tristis, 26. 
Eumycetes, 145. 

Fagraea fragrans, 32, 131. 
Flavobacterium aquatilis, 143. 
Fomes lignosus, 112, 113, 125, 145, 

148. 
Fungi Imperfecti, 145. 
Fusarium cubense, 146. 

Gandoderma pseudoferreum, 145. 
Gleichenia linearis, 31. 

Helminthosporium incurvatum, 146. 
Hevea brasiliensis, 91, 112, 113, 114, 

123. 145- 
Hibiscus rosa-sinensis, 32. 
Hymenomycetes, 74. 

Impatiens oncidioides, 27. 
Imperata arundinacea, 30, 32, 33, 94. 
Indigofera endecaphylla, 117. 
ipomoea learii, 32. 

Law/awa 

aculeata, 31, 32. 

camara, 95. 
Leguminosae, 116. 
Leptothrix ochracea, 144. 
Leucaena glauca, 119. 
Lycopodium cernuum, 94, 122. 

Marremia vitifolia, rig. 
Me/asiowa 

malabathricum, 94. 

polyanthum, 29, 31, 32. 
Microcoecws 

chersonesia, 54. 

luteus, 142. 
Mikania scandens, 119. 
Mimosa 

invisa, 118. 

pudica, 32, 33, 93, 117, 131. 
Mucor, 145. 
Mycobacteriaceae, 144. 
Mycobacterium tuberculus, 144. 
Mycoplana dimorpha, 144. 

Nepenthes, 26. 
Nipa fruticans, 28. 
Nitrobacter winogradskyi, 141. 



155 



156 



INDEX TO PLANTS 



Nitrobactereae, 141. 
Nitrococcus nitrosus, 141. 
Nitrosomonas europaea, 141. 

Oidium heveae, 146. 
Oomycetes, 144. 
Ophiopogon intermedins, 27. 

Passiflora foetida, 119. 
Penicillium, 59, 143, 145. 
Peuraria phaseoloides, 118. 
Phycomycetes, 144. 
Phytomonas tnalvacearum, 55. 
Phytophthora infestans, 145. 
Plasmodiphora brassicae, 48. 
Plasmopara viticola, 145. 
Plnmeria acutifolia, 32. 
Protaminobacter alboflavum, 143. 

aeruginosa, 142. 
fluorescens, 96. 

Ravenala madagascariensis, 32. 



Rhizobium leguminosarum, 54, 96, 

142. 
Rhizopora conjugata, 28. 
Rhizopus, 145. 

Rhodococcus rhodochrous, 142. 
Rigidoporus microporus, 148. 

Saccharomycetes, 145. 
Saccharum spontaneum, 94. 
Sanicula europea, 27. 
Serratia marcescens, 142. 

Taphrina, 145. 
Tephrosia Candida, 118. 
Thiobacillus Ihioparus, 141. 
Trichoderma, 59, 143. 

Vigna, 96. 

oligosperma, 118. 
Ffo/a serpens, 27. 

Zingiberaceae, 29. 
Zygomycetes, 144. 
Zygorhychus, 145. 



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