Transcriber’s Notes
Text printed in italics has been transcribed _between underscores_;
underlined text ~between tildes~. ^x and _{x} represent a superscript
and subscript x respectively.
Uppercase letters between square brackets (such as [A]) refer to
footnotes (to be found directly underneath the paragraph or table),
numbers between square brackets (such as [1] or [1_a_] refer to
references at the end of the chapter(s) by the same author.
More Transcriber’s Notes may be found at the end of this text.
_~THE ROTHAMSTED MONOGRAPHS ON
AGRICULTURAL SCIENCE~_
EDITED BY
SIR E. J. RUSSELL, D.Sc. (LOND.), F.R.S.
THE MICRO-ORGANISMS OF THE SOIL
THE ROTHAMSTED MONOGRAPHS ON AGRICULTURAL SCIENCE.
EDITED BY SIR E. JOHN RUSSELL, D.Sc., F.R.S.
During the past ten years there have been marked developments in
knowledge of the relations between the soil and the growing plant. The
subject involves physical, biological, and chemical considerations, and
its ramifications are now so wide that they cannot be satisfactorily
dealt with in detail in any one book. These monographs collectively
cover the whole ground. In “Soil Conditions and Plant Growth” the
general outlines are presented: in the monographs the various divisions
are fully and critically dealt with by the Heads of the Departments
concerned at Rothamsted. A homogeneous treatment is thus secured that
will, it is hoped, much facilitate the use of the series.
SOIL CONDITIONS AND PLANT GROWTH, Fourth Edition. By SIR E. JOHN
RUSSELL, F.R.S. 16_s._ net.
The following volumes are in preparation:--
MANURING OF GRASS-LANDS
FOR HAY By WINIFRED E. BRENCHLEY, D.Sc., F.Z.S.
THE MICRO-ORGANISMS OF THE
SOIL By Sir E. JOHN RUSSELL, F.R.S., and
Members of the Biological Staff of the
Rothamsted Experimental Station.
SOIL PHYSICS By B. A. KEEN, B.Sc.
SOIL PROTOZOA By D. W. CUTLER, M.A., and L. M. CRUMP,
M.Sc.
SOIL BACTERIA By H. G. THORNTON, M.A.
SOIL FUNGI AND ALGÆ By W. B. BRIERLEY, S. T. JEWSON, B.Sc.,
and B. M. ROACH (Bristol), D.Sc.
CHEMICAL CHANGES IN
THE SOIL By H. J. PAGE, B.Sc.
LONGMANS, GREEN AND CO.,
LONDON, NEW YORK, TORONTO, BOMBAY, CALCUTTA, AND MADRAS.
THE MICRO-ORGANISMS
OF THE SOIL
BY
SIR E. JOHN RUSSELL, F.R.S.
AND
MEMBERS OF THE BIOLOGICAL STAFF OF THE
ROTHAMSTED EXPERIMENTAL STATION
_WITH DIAGRAMS_
LONGMANS, GREEN AND CO.
39 PATERNOSTER ROW, LONDON, E.C. 4
NEW YORK, TORONTO
BOMBAY, CALCUTTA and MADRAS
1923
_Made in Great Britain_
INTRODUCTION.
The purpose of this volume is to give the broad outlines of our present
knowledge of the relationships of the population of living organisms
in the soil to one another and to the surface vegetation. It is shown
that there is a close relationship with vegetation, the soil population
being dependent almost entirely on the growing plant for energy
material, while the plant is equally dependent on the activities of
the soil population for removing the residues of previous generations
of plants and for the continued production in the soil of simple
materials, such as nitrates, which are necessary to its growth. It is
also shown, however, that the soil population takes toll of the plant
nutrients and that some of its members may directly injure the growing
plant.
The soil population is so complex that it manifestly cannot be dealt
with as a whole in any detail by any one person, and at the same
time it plays so important a part in the soil economy that it must
be seriously studied. Team work therefore becomes indispensable, and
fortunately this has been rendered possible at Rothamsted.
Each group of organisms is here dealt with by the person primarily
responsible for that particular section of the work. The plan of the
book has been carefully discussed by all the authors, and the subject
matter has already been presented in a course of lectures given at
University College, London, under the auspices of the Botanical
Board of Studies of the London University. The interest shown in
these lectures leads us to hope that the subject may appeal to a
wider public, and above all to some of the younger investigators in
biological science. They will find it bristling with big scientific
problems, and those who pursue it have the satisfaction, which
increases as the years pass by, of knowing that their work is not only
of interest to themselves, but of great importance in ministering to
the intellectual and material needs of the whole community.
CONTENTS.
CHAP. PAGE
I. DEVELOPMENT OF THE IDEA OF A SOIL POPULATION 1
Sir E. JOHN RUSSELL, F.R.S., Director.
II. OCCURRENCE OF BACTERIA IN SOIL--ACTIVITIES CONNECTED WITH
THE ACQUIREMENT OF ENERGY 20
H. G. THORNTON, B.A., Head of the Department of
Bacteriology.
III. CONDITIONS AFFECTING BACTERIAL ACTIVITIES IN THE
SOIL--ACTIVITIES CONNECTED WITH THE INTAKE OF PROTEIN
BUILDING MATERIALS 39
H. G. THORNTON, B.A., Head of the Department of
Bacteriology.
IV. PROTOZOA OF THE SOIL, I. 66
D. W. CUTLER, M.A., Head of the Department of
Protozoology.
V. PROTOZOA OF THE SOIL, II. 77
D. W. CUTLER, M.A., Head of the Department of
Protozoology.
VI. SOIL ALGÆ 99
B. MURIEL BRISTOL, D.Sc., Algologist.
VII. SOIL FUNGI--THE OCCURRENCE OF FUNGI IN THE SOIL 118
W. B. BRIERLEY, D.Sc., Head of the Department of
Mycology.
VIII. SOIL FUNGI--THE LIFE OF FUNGI IN THE SOIL 131
W. B. BRIERLEY, D.Sc., Head of the Department of
Mycology.
IX. THE INVERTEBRATE FAUNA OF THE SOIL (OTHER THAN PROTOZOA) 147
A. D. IMMS, D.Sc., Head of the Department of Entomology.
X. THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR
RELATION TO THE GROWING PLANT 164
Sir E. JOHN RUSSELL, F.R.S., Director.
INDEX 181
CHAPTER I.
THE DEVELOPMENT OF THE IDEA OF A SOIL POPULATION.
From the earliest times agriculturists have been familiar with the idea
that decomposition of vegetable and animal matter takes place in the
soil, and that the process is intimately connected with soil fertility.
By the middle of the nineteenth century three different ways were known
in which the decomposition occurred. One had been since early times
specially associated with soil fertility, in that it gave rise to
humus, the black sticky substance in farmyard manure or in soil--which
was supposed up to 1840 to be the special food of plants. No good
account of the process or of the conditions in which it occurred is,
however, given by the older writers.
A second resulted in the formation of nitrates. This process became
known as nitrification: it was described by Georgius Agricola
(1494-1555) in his book “De Re Metallica,” and it was of great
importance in the seventeenth and eighteenth centuries, because it
was used for the manufacture of gunpowder in the great wars of that
period. The conditions for the making of successful nitre beds were so
thoroughly investigated that little fresh knowledge was added to that
of 1770[A] until quite recently. This process, however, was not usually
associated with soil fertility, although both Glauber (1656) and Mayow
(1674) had insisted on the connection.
[A] See the remarkable collection of papers entitled
“Instructions sur l’établissement des nitrières,” publié par les
Régisseurs-généraux des Poudres et Salpêtre. Paris, 1777.
A third type of decomposition was brought into prominence by Liebig
in 1840.[7][B] Reviewing the decomposition of organic matter in the
light of the newer chemistry, he concluded that the process was a
slow chemical oxidation, to which he gave the name “Eremacausis.”
He recognised that humus was formed, but he regarded it only as an
intermediate product, and emphatically denied its importance in
soil fertility. The true fertility agents, in his view, were the
final products--CO₂, potassium and other alkaline salts, phosphates,
silicates, etc. He went on to argue brilliantly that instead of
applying farmyard or similar manures to the soil it was altogether
quicker and better to apply these mineral compounds obtained from
other sources than to wait for the slow process of liberation as the
result of decomposition. For some reason, difficult to understand, he
overlooked nitrification and the part that nitrates might play in soil
fertility. Lawes and Gilbert[6] were much attracted by this new idea;
they showed that it was incomplete because it took no account of the
necessity for supplying nitrogen compounds to the crop. When ammonium
salts were added to Liebig’s ash constituents the resulting mixture had
almost as good a fertilising effect as farmyard manure. Lawes at once
saw the enormous practical importance of this discovery, and set up
a factory for the manufacture of artificial fertilisers. He did not,
however, follow it up more closely on the scientific side.
[B] The numbers refer to the short bibliography on p. 18.
Both Lawes and Gilbert were in constant touch with the idea of
decomposition in the soil, and they attached so much importance to
nitrogen compounds in plant nutrition that it is not easy to understand
how they missed the connection with nitrification. But they did so, and
like other English and German workers of the day, considered that plant
roots assimilated their nitrogen as ammonia. For the first ten years of
the history of Rothamsted only few experiments with nitrates were made,
and not till thirty-five years had elapsed were they systematically
studied.
It was by Boussingault[2] and in France that the connection between
nitrification and soil fertility was first recognised. The news came
to England, but it was not accepted, although Way, one of the most
brilliant agricultural chemists of his time, showed that nitrates
were formed in soils to which nitrogenous fertilisers were added, and
that they were comparable in their fertiliser effects with ammonium
salts.[12] “The French chemists,” he wrote in 1856, “are going further,
several of them now advocating the view that it is in the form of
nitric acid that plants make use of compounds of nitrogen. With this
view I do not at present coincide, and it is sufficient here to admit
that nitric acid in the form of nitrates has at least a very high value
as manure.” Indeed, Kuhlmann went so far as to argue that the nitrates
found in the soil were there reduced to ammonia before assimilation
by plants could take place. The water-culture work of the plant
physiologists of the ’sixties finally showed the correctness of the
French view.
Even when the importance of nitrification was realised its mechanism
was not understood: some thought it was chemical, some physical.
Again the explanation came from France. Pasteur in 1862 had expressed
the view that nitrification would probably be a biological action,
since purely chemical oxidation of organic matter was of very limited
occurrence. “Pénétrés de ces idées,” as Schloesing tells us, he and
Müntz in a memorable investigation cleared up the whole problem, and
in 1877 opened the way to a most fruitful field of research.[10] The
formal description is given in his papers in the “Comptes Rendus,”
but a more lively account is given in his lectures before the _École
d’application des Manufacteurs de l’état_, which, though not printed,
were collected and issued in script by his distinguished son, and a
copy of this work is among the treasures of the Rothamsted Library.
He had been asked to study the purification of sewage, and he and Müntz
showed that it was bound up with nitrification. The process was slow in
starting, then it proceeded rapidly. Why, they asked, was the delay?
There should be none if the process were physical or chemical, and the
fact that it occurred strongly suggested biological action. The process
was stopped by chloroform vapour, but could be restarted after the
removal of the vapour by the addition of a little fresh soil.
The importance of this work in connection with soil fertility
was immediately realised by Warington, who had recently come to
Rothamsted.[11] He quickly confirmed the result, and made the valuable
discovery that two stages were involved--the conversion of ammonia to
a nitrite by one organism, and of the nitrite to nitrate by another.
He made long and persistent attempts to isolate the organisms from the
soil, using the best technique of his time, but though he found many
bacteria none of them could nitrify ammonium salts; yet the soil did
it easily. For years he continued his efforts to find the nitrifying
organism, but always failed. His health was not good, his life at
Rothamsted was not happy owing to disagreements with Gilbert, and
although his other research work was succeeding, this investigation
on which he had set his heart was not coming out; bacterial technique
was not yet sufficiently far advanced. Ten bitter, disappointing years
passed, and the crown of disappointment came when Winogradsky, a young
bacteriologist in Paris, changed the technique and succeeded at once in
isolating both the nitrite and the nitrate-forming organisms.[13]
The numerous bacteria found by Warington in the soil suggested the
presence of a soil population, and this idea was greatly strengthened
by another line of investigation which was being followed up in France.
Boussingault had shown that soils absorb oxygen and give out carbon
dioxide; Schloesing extended this discovery, as also did Wollny. It
was concluded that oxidation was the result of the activities of the
soil organisms in decomposing the organic matter of the soil, and thus
preparing the way for the nitrifying organisms.
A third important function of soil bacteria was revealed by
Berthelot.[1] It was known that considerable loss of nitrogen from the
soil took place as the result of the conversion of nitrogen compounds
into nitrates, which were subsequently washed out in the drainage
water. It followed inevitably that the stock of nitrogen compounds in
the soil must long ago have become exhausted had there been no addition
of nitrogen compounds to the soil. Berthelot argued that there must be
fixation of atmospheric nitrogen, and, following the prevailing trend
of thought in France, he attributed it to bacteria. He confirmed the
anticipation by exposing soil to air in such conditions that dust,
rain, etc., were excluded, and he found an increase in the percentage
of nitrogen.
Looking back over the work, it is difficult to understand the result.
The fixation of nitrogen is a process that absorbs energy, and
should have necessitated some source of energy, which apparently was
not supplied. But in spite of this drawback the investigation was
immediately fruitful in that it gave the key to another problem which
had long puzzled agriculturists.
It had long been known that the growth of leguminous crops, unlike that
of others, enriched the ground,[C] and Lawes and Gilbert had shown
that this was due to an increase of soil nitrogen. But no explanation
could be found till Hellriegel and Wilfarth solved the problem.[4] In
studying the nitrogen nutrition of gramineous and leguminous crops,
they discovered that the gramineous plants died in absence of nitrate,
and in its presence made growth which increased regularly with nitrate
supply; while leguminous plants sometimes died and sometimes flourished
in absence of nitrate, and behaved equally erratically with increasing
nitrate supply. When the plants flourished nodules were invariably
present on the roots, but not otherwise. They concluded, therefore,
that the nitrogen nutrition of leguminous plants differed from that
of the gramineæ, and depended on some factor which sometimes came
into their experiments and sometimes did not, and, in any case, was
associated with the nodule. Knowing that the nodules on the roots of
leguminous plants contained bacteria-like bodies, and remembering
Berthelot’s results, they explored the possibility of bacterial
fixation. They sterilised the sand and found that peas invariably
failed to develop nodules and died, but after adding a little garden
soil nodules were found and vigorous growth was obtained.
[C] “Of the leguminous plants the bean best reinvigorates the ground
... because the plant is of loose growth and rots easily, wherefore
the people of Macedonia and Thessaly turn over the ground when it
is in flower” (i.e. dig it into the ground if the soil is poor).
Theophrastus, “Enquiry into Plants,” bk. viii. 2, and bk. ix. I. This
book is of profound interest to agriculturists and botanists. An
excellent translation by Sir Arthur Hort is now available. (Loeb’s
Classical Library.)
Chemical analysis showed considerable fixation of gaseous nitrogen,
which Hellriegel associated with the nodule organism. This has proved
to be correct, and the fixation of nitrogen by bacteria is now a
well-recognised process, the conditions of which are being thoroughly
worked out. Two types of organisms are known--those associated with
leguminous plants, and those living in a free and independent state in
the soil. Of the latter the Clostridium, isolated by Winogradsky, is
anaerobic, and the Azotobacter of Beijerinck is aerobic. The essential
conditions are that a source of energy must be supplied--usually
given as sugar--that the medium must not be acid, and that sufficient
phosphate must be present.
All this brilliant work had been accomplished in the short space of the
ten years 1880 to 1890. The inspiration had in each instance come from
France, and is traceable direct to Pasteur, although coming long after
his own work on bacteriology. It is impossible for us now to realise
the thrill of wonder and astonishment with which students, teachers,
and writers of those days learned that the nutrition of plants, and
therefore the growth of crops and the feeding of themselves, was
largely the result of the activity of bacteria in the dark recesses of
the soil. It is not surprising that the ideas were pushed somewhat too
far, that the soil population was regarded as solely bacterial, and
that important chemical and physical changes were sometimes overlooked.
Gradually there came the inevitable reaction and a somewhat changed
outlook. Continued examination showed the presence in soil of almost
every kind of bacteria for which search was made. Some of them were
almost certainly in the resting condition as spores, and the new
generation of workers had an uneasy feeling that the case for the
overwhelming importance of bacteria in the economy of the soil was
not too well founded. It was shown that the decomposition of nitrogen
compounds to form ammonia would take place without micro-organisms if,
as presumably would happen, the plant enzymes continued to act after
they got into the soil. Even the oxidation of ammonia to nitrate--the
great stronghold of the biological school--was accomplished by chemical
agents. The fixation of nitrogen in soil conditions was beyond the
power of chemists to achieve, and here it was universally agreed that
bacteria were the active agents. And finally, chemists were themselves
bringing into prominence a set of bodies--the colloids--whose
remarkable properties seemed indefinitely expansible, and were in
addition sufficiently incomprehensible to the ordinary student to
attain much of the magnificence of the unknown.
All the time, however, a faithful body of workers was busy exploring
the ground already won, improving the technique, making counts of the
numbers of bacteria in the soil, and trying to measure the amount of
bacterial activity. Much of the value of this work was limited by the
circumstance that the bacteria were regarded as more or less constant
in numbers and activities, so that a single determination was supposed
to characterise the position in a given soil.
This was the condition of the subject when it was seriously taken up at
Rothamsted. Before turning to agriculture, the writer had been studying
the mechanism of certain slow chemical oxidations, and one of his first
experiments in agriculture was to examine the phenomena of oxidation
in soil. The results accorded with the biological explanation of
Schloesing: when the soil was completely sterilised oxidation almost
ceased. But the striking discovery was made, as the result of an
accident to an autoclave, that partial sterilisation increased the
rate of oxidation, and therefore presumably the bacterial activity.
This remarkable phenomenon had, however, already been observed, and
it had been shown that both bacterial numbers and soil fertility
were increased thereby. A full investigation was started in 1907 by
Dr. Hutchinson and the writer.[9] From the outset the phenomena were
recognised as dynamic and not static, and the rates of change were
always determined: thus the bacterial numbers, the nitrate and ammonia
present were estimated after the several periods. Close study of the
curves showed that the chemical and bacterial changes were sufficiently
alike to justify the view that bacteria were in the main the causes
of the production of ammonia and of nitrate; although non-biological
chemical action was not excluded, there was no evidence that it played
any great part. Thus the importance of micro-organisms in the soil was
demonstrated.
The factor causing the increased bacterial numbers after partial
sterilisation was studied by finding out what agents would, and what
would not, allow the numbers to increase, e.g. it was found that the
bacterial increases became possible when soil had been heated at 56°
C., but not at 40° C. Again, it was shown that the high numbers in
partially sterilised soils rose for a time even higher if a little
fresh untreated soil were incorporated into the partially sterilised
soil, but afterwards they fell considerably. Putting all the results
together, it appeared that some biological cause was at work depressing
the numbers of bacteria in normal soils, but not--or not so much--in
the partially sterilised soils. Studied in detail, the data suggested
protozoa as the agent keeping down bacterial numbers, and they were
found in the untreated, but not in the treated, soils. The hypothesis
was therefore put forward that bacteria are not the only members of the
soil population, but that protozoa are also present keeping them in
check, and therefore adversely affecting the production of plant food.
This conclusion aroused considerable controversy. It was maintained
that protozoa were not normal inhabitants of the soil, but only
occasional visitants, and, in any case, they were only there as cysts;
the soil conditions, it was urged, were not suitable to large organisms
like protozoa. The objection was not to be treated lightly, but, on
the other hand, the experiments seemed quite sound. As neither Dr.
Hutchinson nor the writer were protozoologists, Dr. T. Goodey and
(after he left) Mr. Kenneth R. Lewin were invited to try and find
out, quite independently of the partial sterilisation investigation,
whether protozoa are normal inhabitants of the soil, and if so, whether
they are in a trophic condition, and what is their mode of life and
their relation to soil bacteria. Had it turned out that protozoa had
nothing to do with the matter, search would have been made for some
other organism. Goodey showed that the ciliates were not particularly
important; Lewin soon demonstrated the existence of trophic amœbæ and
flagellates. Unfortunately he was killed in the war before he had got
far with the work. After the Armistice, Mr. Cutler accepted charge of
the work: he will himself relate in Chapters IV. and V. what he has
done.
At first sight it might be thought comparatively easy to settle a
question of this kind by examining soil under a microscope or by
sterilising it and introducing successively bacteria and known types
of protozoa. Unfortunately neither method is simple in practice. It
is impossible to look into the soil with a microscope, and methods of
teasing-out small pieces of soil on a slide under the high, or even the
low power, give no information, because the particles of soil have the
remarkable power of attracting and firmly retaining protozoa, and no
doubt bacteria as well; indeed, for protozoa (which have been the more
fully investigated) there seems to be something not unlike a saturation
capacity (see Fig. 9, p. 78). Further, complete sterilisation of soil
cannot be effected without at the same time altering its chemical and
physical properties, and changing it as a habitat for micro-organisms.
Cutler has, however, overcome the difficulties and shown that the
introduction of protozoa into soils sterilised and then reinfected with
bacteria considerably reduces the numbers of these organisms.
The method adopted, therefore, is to take a census of population and
of production. Counting methods are elaborated, and estimates as
accurate as possible are made of the numbers of the various organisms
in a natural field soil at stated intervals. Simultaneously, wherever
possible some measure is taken of the work done. The details of the
census are finally arranged in consultation with the Statistical
Department, to ensure that the data shall possess adequate statistical
value. From the results it is possible to adduce information of great
value as to the life of the population, the influence of external
conditions, etc.
The most important investigation of this kind carried out at Rothamsted
was organised by Mr. Cutler.[3] A team of six workers was assembled,
and for 365 days without a break they counted every day the ciliates,
the amœbæ, the flagellates, and the bacteria in a plot of arable
ground, distinguishing no less than seventeen different kinds of
protozoa. The conclusions arrived at were carefully tested by the
Statistical Department.
Of the protozoa the flagellates were found to be the most numerous,
the amœbæ came next, and the ciliates were by far the fewest. The
numbers of each organism varied from day to day in a way that
showed conclusively the essentially trophic nature of the protozoan
population. The numbers of amœbæ--especially _Dimastigamœba_ and of
a species called α--were sharply related to the numbers of bacteria:
when the amœbae were numerous the bacteria were few, and vice versa.
Detailed examination showed that the amœbæ were probably the cause
of the fluctuations in the bacterial numbers, but Mr. Cutler has not
yet been able to find why the amœbæ fluctuated; it does not appear
that temperature, moisture content, air supply or food supply were
determining causes. The flagellates and ciliates also showed large
fluctuations, amounting in one case--_Oicomonas_--to a definite
periodicity, apparently, however, not related to bacterial numbers, or,
so far as can be seen, to external conditions of moisture, temperature
and food supply, and showing no agreement with the fluctuations of the
amœbæ. However, one cannot be certain that lack of agreement between
curves expressing protozoan numbers and physical factors implies
absence of causal relationships: the observations (though the best that
can yet be made) are admittedly not complete. If we saw only the end of
the bough of a tree, and could see no connection with a trunk, we might
have much difficulty in finding relationships between its motion and
the wind; whatever the direction of the wind it would move backwards
and forwards in much the same way, and even when the wind was blowing
along the plane of its motion it would just as often move against the
wind as with it.
Meanwhile evidence was obtained that the twenty-four hour interval
adopted by the protozoological staff was too long for bacteria, and
accordingly the Bacteriological Department, under Mr. Thornton, refined
the method still further. Bacterial counts were made every two hours,
day and night, for several periods of sixty or eighty hours without
a break. The shape of the curve suggests that two hours is probably
close enough, and for the present counts at shorter intervals are not
contemplated. But there is at least one maximum and one minimum in the
day, although the bacterial day does not apparently correspond with
ours, nor can any relationship be traced with the diurnal temperature
curve.
The nitrate content of the soil was simultaneously determined by Mr.
Page and found to vary from hour to hour, but the variations did not
sharply correspond with the bacterial numbers; this, however, would not
necessarily be expected. The production of nitrate involves various
stages, and any lag would throw the nitrate and bacterial curves out
of agreement. There is a suggestion of a lag, but more counts are
necessary before it can be regarded as established.
Examination of these and other nitrate curves obtained at Rothamsted
has brought out another remarkable phenomenon. No crop is growing on
these plots, and no rain fell during the eighty hours, yet nitrate
is disappearing for a considerable part of the time. Where is it
going to? At present the simplest explanation seems to be that it is
taken up by micro-organisms. A similar conclusion had to be drawn
from a study of the nitrogen exhaustion of the soil. The whole of the
nitrate theoretically obtainable from the organic matter of the soil
is not obtained in the course of hours or even days; in one of our
experiments at Rothamsted nitrification is still going on, and is far
from complete, even after a lapse of fifty-three years. The explanation
at present offered is that part of the nitrate is constantly being
absorbed by micro-organisms and regenerated later on.
Now what organisms could be supposed to absorb nitrates from the soil?
Certain bacteria and fungi are known to utilise nitrates, and one
naturally thinks of algæ as possible agents also. Dr. Muriel Bristol
was therefore invited to study the algæ of the soil. Her account is
given in Chapter VI. She has found them not only on the surface, but
scattered throughout the body of the soil, even in the darkness of 4
inches, 5 inches, or 6 inches depth, where no light can ever penetrate,
and where photosynthesis as we understand it could not possibly take
place. Some modification in their mode of life is clearly necessary,
and it may well happen that they are living saprophytically. Dr.
Bristol has not yet, however, been able to count the algæ in the soil
with any certainty, although she has made some estimates of the numbers.
The quantitative work on the soil population indicates other
possibilities which are being investigated. There is not only a daily
fluctuation in the numbers, but so far as measurements have gone,
a seasonal one also. There seems to be some considerable uplift in
numbers of bacteria, protozoa, and possibly algæ and fungi in the
spring-time, followed by a fall in summer, a rise in autumn, and a
fall again in winter. At present we are unable to account for the
phenomenon, nor can we be sure that it is general until many more data
are accumulated.
In the cases of the protozoa and the algæ, there was a definite reason
for seeking them in the soil.
Another section of the population, the fungi, was simply found, and
at present we have only limited views as to their function. The older
workers considered that they predominated in acid soils, while bacteria
predominated in neutral soils. Present-day workers have shown that
fungi, including actinomycetes, are normal inhabitants of all soils.
The attempts at quantitative estimations are seriously complicated
by the fact that during the manipulations a single piece of mycelium
may break into fragments, each of which would count as one, while a
single cluster of spores might be counted as thousands. Little progress
has therefore been made on the quantitative lines which have been so
fruitful with protozoa. Dr. Brierley gives, in Chapters VII. and VIII.,
a critical account of the work done on fungi.
In addition to the organisms already considered there are others of
larger size. The nematodes are almost visible to the unaided eye,
most of them are free living and probably help in the disintegration
of plant residues, though a few are parasitic on living plants and do
much injury to clover, oats, and less frequently to onions, bulbs, and
potatoes. Further, there are insects, myriapods and others, the effects
of which in the soil are not fully known. Special importance attaches
to the earthworms, not only because they are the largest in size and
in aggregate weight of the soil population, but because of the great
part they play in aerating the soil, gradually turning it over and
bringing about an intimate admixture with dead plant residues, as first
demonstrated by Darwin. Earthworms are the great distributors of energy
material to the microscopic population. Systematic quantitative work
on these larger forms is only of recent date, and Dr. Imms, in Chapter
IX., discusses our present knowledge.
TABLE I.
SOIL POPULATION, ROTHAMSTED, 1922.
(The figures for algæ and fungi are first approximations only, and have
considerably less value than those for bacteria and protozoa.)
+----------------------+-----------+--------------------------------+
| | |Approximate Weight per Acre of--|
| | +---------+----------+-----------+
| | Numbers | Living |Dry Matter| Nitrogen |
| | per Gram | Organ- |in Organ- | in Organ- |
| | of Soil. | isms. | isms. | isms. |
+----------------------+-----------+---------+----------+-----------+
|_Bacteria_-- | | lb. | lb. | lb. |
| High level |45,000,000 | 50} | 2 | 0·2 |
| Low level |22,500,000 | 25} | | |
|_Protozoa_-- | | | | |
| _Ciliates_-- | | | | |
| High level | 1,000 | -- | -- | -- |
| Low level | 100 | -- | -- | -- |
| _Amœbæ_-- | | | | |
| High level | 280,000 | 320} | 12 | 1·2 |
| Low level | 150,000 | 170} | | |
| _Flagellates_-- | | | | |
| High level | 770,000 | 190} | 7 | 0·7 |
| Low level | 350,000 | 85} | | |
| _Algæ_ | | | | |
| (not blue-green)| [100,000]| 125 | 6 | 0·6 |
| Blue-green | Not known.| | Say 6 | Say 0·6 |
| _Fungi_-- | | | | |
| High level |[1,500,000]| 1700} | 60 | 6·0 |
| Low level | [700,000]| 800} | | |
| | | +----------+-----------+
| | | | 93 | 9·3 |
| | | |= 4 parts nitrogen per|
| | | |1,000,000 of soil. |
+----------------------+-----------+---------+----------------------+
+--------------------------------------
| LARGER ORGANISMS.
+-----------------+-------------------+
| | |
| | +
| | Numbers |
| | per Acre.[D] |
| +---------+---------+
| | | Unma- |
| | Manured.| nured. |
+-----------------+---------+---------+
|_Oligochaeta_ | | |
| (_Limicolae_)--| | |
| Nematoda, etc. |3,609,000| 794,000|
| Myriapoda |1,781,000| 879,000|
| Insects |7,727,000|2,475,000|
| Earthworms |1,010,000| 458,000|
+-----------------+---------+---------+
|
+--------------------------------------
+-----------------------------------------------------------+
| LARGER ORGANISMS. |
+-----------------+-----------------------------------------+
| | Approximate Weight per Acre of-- |
| +-------------+-------------+-------------+
| | Living | Dry Matter | Nitrogen in |
| | Organisms. |in Organisms.| Organisms. |
| +------+------+------+------+------+------+
| | Ma- | Unma-| Ma- | Unma-| Ma- |Unma- |
| |nured.|nured.|nured.|nured.|nured.|nured.|
+-----------------+------+------+------+------+------+------+
|_Oligochaeta_ | | | | | | |
| (_Limicolae_)--| lb. | lb. | lb. | lb. | lb. | lb. |
| Nematoda, etc. | 9 | 2 | 3 | 1 | -- | -- |
| Myriapoda | 203 | 99 | 85 | 42 | 4 | 2 |
| Insects | 34 | 16 | 14 | 6 | 1 | 1 |
| Earthworms | 472 | 217 | 108 | 50 | 10 | 5 |
+-----------------+------+------+------+------+------+------+
| Total | 210 | 99 | 15 | 9 |
+-------------------------------+------+------+------+------+
Total organic matter (dry weight) in this soil = 126,000 lb. per acre.
Total nitrogen = 5700 lb. per acre. (1 lb. nitrogen per acre = 0·4
parts per 1,000,000 of soil.)
[D] To a depth of 9 inches. The weight of soil is approximately
1,000,000 kilos.
Are there any other members of the soil population that are of
importance? As already shown, the method of investigating the soil
population in use at Rothamsted is to find by chemical methods the
changes going on in the soil; to find by biological methods what
organisms are capable of bringing about these changes; and then to
complete the chain of evidence by tracing the relationships between
the numbers or activities of these organisms and the amount of change
produced. The list as we know it to-day is given in Table I.
The method, however, does not indicate whether the account is fairly
complete, or whether there are other organisms to be found. We might,
of course, trust to empirical hunting for organisms, or to chance
discoveries such as led Goodey to find the mysterious Proteomyxan
Rhizopods, which cannot yet be cultured with certainty, so that they
are rarely found by soil workers. It is possible that there are many
such organisms, and it is even conceivable that these unknown forms far
outnumber the known. The defect of the present method is that it always
leaves us in doubt as to the completeness of the list, and so we may
have to devise another.
Reverting to Table I., it obviously serves no purpose to add the
numbers of all the organisms together. We can add up the weights of
living organisms, of their dry matter or nitrogen, so as to form some
idea of the proportion of living to non-living organic matter, and this
helps us to visualise the different groups and place them according
to their respective masses. But a much better basis for comparing the
activities of the different groups would be afforded by the respective
amounts of energy they transform, if these could be determined. It is
proposed to attempt such measurements at Rothamsted. The results when
added would give the sum of the energy changes effected by the soil
population as we know it: the figure could be compared with the total
energy change in the soil itself as determined in a calorimeter. If
the two figures are of the same order of magnitude, we shall know that
our list is fairly well complete; if they are widely different, search
must be made for the missing energy transformers. There are, of course,
serious experimental difficulties to be overcome, but we believe the
energy relationships will afford the best basis for further work on the
soil population.
Finally, it is necessary to refer to the physical conditions obtaining
in the soil. These make it a much better habitat for organisms than one
might expect. At first sight one thinks of the soil as a purely mineral
mass. This view is entirely incorrect. Soil contains a considerable
amount of plant residues, rich in energy, and of air and water. The
usual method of stating the composition of the soil is by weight, but
this is misleading to the biologist because the mineral matter has a
density some two and a half times that of water and three times that
of the organic matter. For biological purposes composition by volume
is much more useful, and when stated in this way the figures are very
different from those ordinarily given. Table II. gives the results for
two Broadbalk arable plots, one unmanured and the other dunged; it
includes also a pasture soil.
The first requirement of the soil population is a supply of energy,
without which it cannot live at all. All our evidence shows that the
magnitude of the population is limited by the quantity of energy
available. The percentage by weight of the organic matter is about
two to four or five, and the percentage by volume runs about four
to twelve. Not all of this, however, is of equal value as source of
energy. About one-half is fairly easily soluble in alkalis, and may
or may not be of special value, but about one-quarter is probably too
stable to be of use to soil organisms.
A second requirement is water with which in this country the soil is
usually tolerably well provided. Even in prolonged dry weather the soil
is moist at a depth of 3 inches below the surface. It is not uncommon
to find 10 per cent. or 20 per cent. by volume of water present, spread
in a thin film over all the particles, and completely saturating the
soil atmosphere.
TABLE II.
VOLUME OF AIR, WATER AND ORGANIC MATTER IN 100 VOLUMES OF ROTHAMSTED
SOIL.
+---+-----------------+-----------+--------------+
| | | |In Pore Space.|
| | | |Values Common-|
| | Solid Matter. | | ly Obtained. |
| +--------+--------+ +-------+------+
| |Mineral.|Organic.|Pore Space.| Water.| Air. |
+---+--------+--------+-----------+-------+------+
|(1)| 62 | 4 | 34 | 23 | 11 |
|(2)| 51 | 11 | 38 | 30 | 8 |
|(3)| 41 | 12 | 47 | 40 | 7 |
+---+--------+--------+-----------+-------+------+
(1) Arable, no manure applied to soil.
(2) Arable, dung applied to soil.
(3) Pasture.
The air supply is usually adequate owing to the rapidity with which
diffusion takes place. Except when the soil is water-logged, the
atmosphere differs but little from that of the one we breathe. There
is more CO₂, but only a little less oxygen.[8] The mean temperature is
higher than one would expect, being distinctly above that of the air,
while the fluctuations in temperature are less.[5]
The reaction in normal soils is neutral to faintly alkaline; _p_H
values of nearly 8 are not uncommon. Results from certain English soils
are shown on p. 18.
The soil reaction is not easily altered. A considerable amount of acid
must accumulate before any marked increase in intensity of _p_H value
occurs; in other words, the soil is well buffered. The same can be said
of temperature, of water, and of energy supply. Like the reaction, they
alter but slowly, so that organisms have considerable time in which to
adapt themselves to the change.
HYDROGEN ION CONCENTRATION AND SOIL FERTILITY.
_p_H
Alkaline 10 --+-- Sterile: Alkali soil.
↑ |
| 9 --+--
| |
| 8 --+-- Fertile: Arable.
| |
Neutral 7 --+--
| |
| 6 --+--
| |
| 5 --+-- Potato Scab fails.
| | Nitrification hindered.
| 4 --+-- Barley fails.
↓ |
Acid 3 --+-- Sterile: Peat.
A SELECTED BIBLIOGRAPHY.
[1] Berthelot, Marcellin, “Fixation directe de l’azote atmosphérique
libre par certains terrains argileux,” Compt. Rend., 1885, ci.,
775-84.
[2] Boussingault, J. B., and Léwy, “Sur la composition de l’air
confiné dans la terre végétale,” Ann. Chim. Phys., 1853, xxxvii.,
5-50.
[3] Cutler, D. W., Crump, L. M., and Sandon, H., “A Quantitative
Investigation of the Bacterial and Protozoan Population of the Soil,
with an Account of the Protozoan Fauna,” Phil. Trans. Roy. Soc.,
Series B, 1922, ccxi., 317-50.
[4] Hellriegel, H., and Wilfarth, H., “Untersuchungen über die
Stickstoffnahrung der Gramineen und Leguminosen,” Zeitsch. des
Vereins f. d. Rübenzucker-Industrie, 1888.
[5] Keen, B. A., and Russell, E. J., “The Factors determining Soil
Temperature,” Journ. Agric. Sci., 1921, xi., 211-37.
[6] Lawes, J. B., and Gilbert, J. H., “On Agricultural Chemistry,
Especially in Relation to the Mineral Theory of Baron Liebig,” Journ.
Roy. Agric. Soc., 1851, xii., 1-40.
[7] Liebig, Justus, “Chemistry in its Application to Agriculture
and Physiology,” 1st and 2nd editions (1840 and 1841), 3rd and 4th
editions (1843 and 1847); “Natural Laws of Husbandry,” 1863.
[8] Russell, E. J., and Appleyard, A., “The Composition of the Soil
Atmosphere,” Journ. Agric. Sci., 1915, vii., 1-48; 1917, viii.,
385-417.
[9] Russell, E. J., and Hutchinson, H. B., “The Effect of Partial
Sterilisation of Soil on the Production of Plant Food,” Journ. Agric.
Sci., 1909, iii., 111-14; Part II., Journ. Agric. Sci., 1913, v.,
152-221.
[10] Schloesing, Th., and Müntz, A., “Sur la Nitrification par les
ferments organisés,” Compt. Rend., 1877, lxxxiv., 301-3; 1877,
lxxxv., 1018-20; and 1878, lxxxvi., 892-5. “Leçons de chimie
agricole,” 1883.
[11] Warington, R., “On Nitrification,” Part I., Journ. Chem. Soc.,
1878, xxxiii., 44-51; Part II, Journ. Chem. Soc., 1879, xxxv.,
429-56; Part III., Journ. Chem. Soc., 1884, xlv., 637-72; Part IV.,
Journ. Chem. Soc., 1891, lix., 484-529.
[12] Way, J. T., “On the Composition of the Waters of Land Drainage
and of Rain,” Journ. Roy. Agric. Soc., 1856, xvii., 123-62.
[13] Winogradsky, S., “Recherches sur les organismes de la
nitrification,” Ann. de l’Inst. Pasteur, 1890, iv., 1^e Mémoire,
213-31; 2^e Mémoire, 257-75; 3^e Mémoire, 760-71.
“Recherches sur l’assimilation de l’azote libre de l’atmosphère par
les microbes.” Arch. des Sci. Biolog. St. Petersburg, 1895, iii,
297-352.
For further details and fuller bibliography, see E. J. Russell, “Soil
Conditions and Plant Growth,” Longmans, Green & Co.