BIOLOGICAL PROCESSES
IN TROPICAL SOILS
A. STEVEN CORBET
^CMILLAN LIBRARY
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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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o <:
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ij
i<
Z o,
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^ z
Id
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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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