G 6290SS20 LOZL &

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JEAN BAPTISTE JOSEPH DIEUDONNE BOUSSINGAULT
1802 - 1887

The Founder of Modern Agricultural Chemistry,

DSc (Loxp.), PRS.

teh:

$ AND PLANT GROWTH. .

Be 8

ments in knowledge of the relations between the soil an t
growing plant. The subject involves physical, biological,
chemical considerations, and its ramifications are now so V

“SOIL BACTERIA... By H. G. Tuornrton, M.A.

THE ROTHAMSTED MONOGRAPHS ON
AGRICULTURAL SCIENCE, _

Epitep sy Dr, E, J. RUSSELL, D.Sc., F.R.S.

DvRING the past ten years there have been marked ab |

that they cannot be satisfactorily dealt with in detail in any |
one book. These monographs collectively cover the whole 8
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 Depart- —
ments concerned at Rothamsted. A homogéneous treatment i

is thus secured that will, it is hoped, much facilitate the use of i

the series.

i 1

SOIL CONDITIONS ‘AND PLANT GROWTH, Fourth Edition. is

\
®

By Epwarp J. Russe, D.Sc. (Lonp.), F.R.S.

The following volumes are in preparation :—

SOIL PHYSICS... . ByB. A. Keen, BSe ee ;

SOIL PROTOZOA . By D. W. Curter, M.A.,, and )
L. M. Crump, M.Sc. ae

SOIL FUNGI AND ALGA:_ By W. B. Briertey, S. T. Jew-

\ son, B.Sc. and B. M. BrisToL, —

\ D.Sc.
\ CHEMICAL CHANGES IN

\ “PER SOIL . . .- By H.’J. Pace, BSc.

\ LONGMANS, GREEN AND CO.,
LONDON, NEW YORK, BOMBAY, CALCUTTA, AND MADRAS.

i
re)

b>

i
S/

SOIL CONDITIONS

AND

PLANT GROWTH
(299)

of BY

: h
)y) EDWARD J! RUSSELL, D.Sc. (Lonn.), F.R.S.

DIRECTOR OF THE ROTHAMSTED EXPERIMENTAL STATION, HARPENDEN

WITH ILLUSTRATIONS

FOURTH EDITION

Ps

ate

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LONGMANS, GREEN AND CO.

39 PATERNOSTER ROW, LONDON
FOURTH AVENUE & 20TH STREET, NEW YORK

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BOMBAY, CALCUTTA, AND MADRAS

1921

New gee » na
New Edition . . ay
Third Edition .

Agricultural Science) .

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PREFACE TO THE FOURTH EDITION.

Wen the revision of the present edition was begun it
became obvious that some fundamental change would
be necessary in the treatment. Since the first edition
was published in June, 1912, the subject has grown
enormously : it has now completely run away from the
166 pages that proved a sufficient allowance in those
days. Several courses were open; the one chosen was
to retain the general characteristics of the older editions,
dealing broadly with the whole subject, emphasising the
outlines and endeavouring to maintain a dispassionate
balance between the various parts. Detailed treatment
has been avoided, partly because it is now hopeless for
any one individual to attempt it, and partly because I
did not wish to overload the book and make it into a
huge ‘‘ Handbuch ” where the outlines are all obscured
and the perspective lost.

At the same time, however, the student must have
access to the more detailed treatment, and it would have
helped him little had I simply published a card index
and left him to find out the rest for himself.

The problem was solved by enlisting the sympathy
of the Heads of the Departments at Rothamsted, who
each undertook to write a monograph dealing with his
or her special branch, and which Messrs. Longmans will
publish uniformly with this volume. Each one has

discussed with me the corresponding sections of this
iy

vi SOIL CONDITIONS AND PLANT GROWTH

book, with the results that the book is much improved,
and a definite continuity is assured between this general
monograph and the detailed monographs to which it
will serve as a link. The whole series is to be called .
“The Rothamsted Monographs on Agricultural Science’.

It is hoped that this new arrangement will prove
satisfactory to students. It has the advantage that the
book still covers the whole ground, while remaining of
a manageable size, and that it is closely linked up with
a series of similar books, each dealing with separate
sections, in which more complete treatment and more
fully informed criticism are given than I could myself
undertake.

On the other hand, the necessity for freedom of
treatment, and the difficulty of placing such subjects as
Soil Physics or Soil Protozoa under the heading of
Biochemistry, compelled the withdrawal of the book
from the important ‘‘ Biochemical Series,” of which it has
hitherto been a member. And, although Drs. Plimmer
and Hopkins willingly consented, this course was not
taken without serious consideration and regret: it in-
volves the separation of the book from a very useful
series of volumes.

Perhaps the most striking feature of the past ten
years’ developments has been the increasing recognition
of the complexity of soil phenomena. The American
investigations have shown how complex are the physico-
chemical relations of the soil, the soil solution and the
plant. English work has shown that the soil population
is numerous and very varied. It is not sufficiently
recognised, however, that this complexity necessitates a
difference in method of investigation from that usually
adopted in scientific laboratories, and this I have tried
to bring out in the course of the book. A chemist

PREFACE TO THE FOURTH EDITION vii

dealing with a single substance can treat it in a certain
way, and be reasonably certain that the result obtained
is the direct consequence of the treatment. The soil
investigator has no such certainty: definite treatment
of a soil may be followed by a definite result, but there
may be no direct relationship with the factor under in-
vestigation—the result may be merely a reflex of some
far-reaching change produced in some other factor which
is entirely overlooked. Further, if observations are
attempted in the field it is impossible to ensure such
simple variation as the recorded data seem to suggest.
It follows that the ordinary laboratory method in which
factors are varied only one at the time requires con-
siderable modification when used for soil work.

The methods in use at Rothamsted fall into two
groups :— :

1. Observations are made in natural conditions as ac-
curately as is feasible, and repeated sufficiently frequently
to allow of treatment by modern statistical methods.
These enable the investigator to study the variations,
and hence to make deductions as to the numbers and
properties of the factors involved. The factors can then
be studied in the laboratory as single factors, using more
precise methods and more rigid controls than are possible
in the field.

2. Experiments are made on the soil, and from the
results deductions are drawn as to the probable nature
of some new factor. Direct experiment is then made
to test the operation of the factor in the field, and precise
laboratory experiments are also undertaken.

Further, just as the ordinary methods of investigation
are insufficient, so also the customary divisions of science
cannot be rigidly maintained in soil work. The chemist
is constantly confronted with physical and_ biological

viii SOIL CONDITIONS AND PLANT GROWTH

problems: the biologist constantly needs the help of
the statistician, the physiologist and chemist: most of
the work is essentially “team work,” requiring close
co-operation between experts in different branches ot
science. 3 .

The recognition of these considerations, and the fact
that the Ministry of Agriculture has now, through the
Development Fund, enabled Rothamsted to interest
some of the more promising of the younger scientific
workers in agricultural problems, justifies the hope that
the future will see even greater advances than the
past.

In preparing this edition thanks are due to my
colleagues at Rothamsted who have read sections or
chapters of the book and made useful comments, es-
pecially to Miss Aslin, Dr. B. Muriel Bristol, Miss.
_Jewson, Messrs. W. B. Brierley, D. W. Cutler, E. M.-
Crowther, B. A. Keen, and H. J. Page; to Professor
V..H. Blackman, Mr. G. W. Robinson, of University
College, Bangor, and Dr. E. J. Salisbury; to M.
Georges Matisse, of Paris, who is translating this
edition into French; and in America to Dr. H. L.
Walster for a valuable critical review and list of refer-
ences, and others for useful suggestions. The book has
been helped by so many that I hope it will still find
friends in spite of its larger size.

PREFACE TO THE THIRD EDITION.

CONSIDERABLE alterations have been made in the text
and a new chapter has been added discussing the col-
loidal properties of the soil. It is abundantly clear that
the soil investigator of the future will have to be
thoroughly familiar with the ways of colloids, and I fully
expect that much of the older work will require careful
re-examination in the light of what has been done in
this direction by chemists and physicists.

Although the volume has necessarily expanded |
have tried to keep it as a monograph: I have not
attempted to turn it into an extended card index by re- ©
ferring to every paper published on the subject since the
first edition came out. Many papers have been omitted ;
the guiding principle has been to include only those that
brought in some new idea or profoundly modified an old
one. Some of the papers omitted from the last edition
have been included in this because they now fall into
their place, while before they did not. Doubtless this
will happen again.

Continued progress is being made. Since the book
was first begun two Journals have sprung up devoted
entirely to soil: Soz/ Sczence, under the editorship of
the indefatigable J. G. Lipman, and the /xternational
Mitterlungen fiir Bodenkunde. Another Journal, the
Journal of Ecology, has also arisen and is vigorously
developing another aspect of the same subject, while the
older agricultural journals are finding more and more of
their space taken up by soil papers. The subject now
only lacks a name, and though many have been pro-
posed—pedology, agrogeology, edaphology, etc.—I have
not felt drawn to any of them.

CONTENTS.

CHAPTER
I. HistoricAL AND INTRODUCTORY M “ = d

Il. Sou CONDITIONS AFFECTING PLANT GROWTH - -

Ill. (THe Composition or THE SOL \ - - 3 -~ -

IV. THe CoL.tomaL PROPERTIES OF SOIL -. . \

V. THe Carron and NirrRoGEN CYCLES IN THE SOIL

VI. THE BioLocicaL ConpDiITIONS IN THE SOIL é

VIL. THe Micro-Orcanic PoPpuULATION OF THE SOIL AND
Irs RELATION TO THE GROWTH OF PLANTS -

Vill. Tue Soi, in RELATION TO PLANT GROWTH /- -

aes TX, Sor ANALYSIS AND ITS’ INTERPRETATION 2 n -
i SNE ee or r ‘

APPENDICES.
I. Tue Mertuops or Soi, ANALYSIS” - - 7 =

“4 Il. AMounts or Various SUBSTANCES ABSORBED FROM
THE SOIL BY THE COMMON AGRICULTURAL CROPS
OF ENGLAND~ - - - - - - -

Ill. A SeLecTepD BIBLIOGRAPHY OF PAPERS ON Som CoNn-
DITIONS AND PLANT GROWTH - - - -

-AutTHorR INDEX * “ = “ ‘ 2 = ‘

Supject INDEX > 3 ; i i mi d

PAGE

301

322

347

357

359
387
395

List: OF: PLATES:

Portrait or J. B. J. Dieuponné BoussIncauLt (1802-.

Fic.

_ Fic.

Fic.

Fic.

1887) < - . - - - - Frontispiece

TO FACE PAGE
9.—INFLUENCE OF MEDIUM ON ROOT DEVELOPMENT OF
LUPINES - - - . - - - - a eS

I1I.—SWEDES FROM AGDELL FIELD, BROADBALK: UN-
MANURED; SUPERPHOSPHATES AND POTASSIC FERTILISERS;
AND COMPLETE FERTILISER - - - - - os

30A.—POOR GRASS PASTURE, UNTREATED PLOT,
HORNDON-ON-THE-HILL, EssEx (LONDON CLAY) - Icio

30B.—ADJOINING PLOT, BUT TREATED witH |312 & 313
GAFSA PHOSPHATE, FEB. 27TH, 1918 - ‘

xii

CHAPTER I.
HISTORICAL AND INTRODUCTORY.

IN all ages the growth of plants has interested thoughtful men.
The mystery of the change of an apparently lifeless seed to a
vigorous growing plant never loses its freshness, and consti-
tutes, indeed, no small part of the charm of gardening, The
economic problems are of vital importance, and become more
and more urgent as time goes on and populations increase and
their needs become more complex.

There was an extensive literature on agriculture in Roman
times which maintained a pre-eminent position until compara-
tively recently. In this we find collected many of the facts
which it has subsequently been the business of agricultural
chemists to classify and explain. The Roman literature was
collected and condensed into one volume about the year 1240
by a senator of Bologna, Petrus Crescentius, whose book! was
‘one of the most popular treatises on agriculture of any time,
being frequently copied, and in the early days of printing,
passing through many editions—some of them very handsome,
and ultimately giving rise to the large standard European
treatises of the sixteenth and seventeenth centuries. Many
- other agricultural books appeared in the fifteenth and early
sixteenth centuries, notably in Italy, and later in France. In
some of these are found certain ingenious speculations that

have been justified by later work. Such, for instance, is.
Palissy’s remarkable statement in 1563 (222)*: ‘“ You will
admit that when you bring dung into 'the field it is to return
to the soil something that has been taken away. . . . Whena
1 De agricultura vulgare, Augsburg, 1471, and many subsequent editions.

2 The numbers in brackets refer to the Bibliography at the end of the book.
, I

2 SOIL CONDITIONS AND PLANT GROWTH

plant is burned it is reduced to a salty ash called alcaly by
apothecaries and philosophers. . . . Every sort of plant with-
out exception contains some kind of salt. Have you not seen
certain labourers when sowing a field with wheat for the second
year in succession, burn the unused wheat straw which had
been taken from the field? In the ashes will be found the
salt that the straw took out of the soil ; if this is put back the
soil is improved. Being burnt on the ground it serves as
manure because it returns to the soil those substances that had
been taken away.” But for every speculation that has been
confirmed will be found many that have not, and the begin-
nings of agricultural chemistry must be sought later, when men
had learnt the necessity for carrying on experiments.

The Search for the “ Principle” of Vegetation, 1630-1750.

The earlier investigators sought for a ‘“‘ principle” of
vegetation to account for the phenomena of soil fertility and
plant growth. The great Lord Bacon (8) believed that water
formed the “ principal nourishment ” of plants, the purpose of
the soil being to keep them upright and protect them from
excessive cold or heat, but he also considered that each plant
drew a “particular juyce” from the soil for its sustenance,
thereby impoverishing the soil for that particular plant and
similar ones, but not necessarily for other plants. Van Hel-
mont regarded water as the sole nutrient for plants, and his son
thus records his famous Brussels experiment (131): “I took
an earthen vessel in which I put 200 pounds of soil dried in an
oven, then I moistened with rain water and pressed hard into
it a shoot of willow weighing 5 pounds. After exactly five
years the tree that had grown up weighed 169 pounds and
about three ounces. But the vessel had never received anything
but rain water or distilled water to moisten the soil when this
was necessary, and it remained full of soil, which was still
. tightly packed, and, lest any dust from outside should get into
the soil, it was covered with a sheet of iron coated with tin
but perforated with many holes. I did not take the weight of
the leaves that fell in the autumn. In the end I dried the soil

HISTORICAL AND INTRODUCTORY 3

once more and got the same 200 pounds that I started with,
less about two ounces, Therefore the 164 pounds of wood,
bark, and root, arose from the water alone.”

The experiment is simple and convincing, and satisfied
Boyle (50), who repeated it with “squash, a kind of Indian
pompion ” and obtained similar results. Boyle further dis-
tilled the plants and concluded, quite justifiably from his prem-
ises, that the products obtained, “ salt, spirit, earth, and even
oil (though that be thought of all bodies the most opposite to
water), may be produced out of water”. Nevertheless, the
conclusion is incorrect, because two factors had escaped Van
Helmont’s notice—the parts played by the air and by the
missing two ounces of soil. But the history of this experiment
_ is thoroughly typical of experiments in agricultural chemistry
generally : in no other subject is it so easy to overlook a vital
factor and draw from good experiments a conclusion that
appears to be absolutely sound, but is in reality entirely
wrong. »

Some years later—about 1650—Glauber (107) set up the
hypothesis that saltpetre is the “principle” of vegetation.
_ Having obtained saltpetre from the earth cleared out from
cattle sheds, he argued that it must have come from the urine
or droppings of the animals, and must, therefore, be contained
in the animal’s food, ze. in plants. He also found that addi-
tions of saltpetre to the soil produced enormous increases in
crop. He connected these two observations and supposed
that saltpetre is the essential principle of vegetation. The
fertility of the soil and the value of manures (he mentions dung,
feathers, hair, horn, bones, cloth cuttings) are entirely due to
saltpetre.

This view was supported by Mayow’s experiments (195).
He estimated the amounts of nitre in the soil at different times
of the year, and showed that it occurs in greatest quantity in
spring when plants are just beginning to grow, but is not to
be found ‘in soil on which plants grow abundantly, the reason
being that all the nitre of the soil is sucked out by the plants”.
Kiilbel (quoted in 293), on the other hand, regarded a magma

r*

4 SOIL CONDITIONS AND PLANT GROWTH

unguinosum obtainable from humus as the “ principle” sought
for.

The most accurate work in this period was published by
John Woodward, in a remarkable paper in 1699 (321). Set-
ting out from the experiments of Van Helmont and of Boyle,
but apparently knowing nothing of the work of Glauber and
of Mayow, he grew spearmint in water obtained from various
sources with the following results among others :—

Weight of Plants. ad Expense of Proportion of Of
Source of Water. ained in ater (i.e. ncrease 0
When put | When 77 days. ah Yi ae ¥ ros
in. taken out. | °

2 _| Grains. Grains. Grains. Grains. .
Rain water . ¢ 28} 452 174 3004. I to 17133
River Thames i 28 54 26 2493 Ito 953%
Hyde Park conduit IIo 249 139 13140 Ito 9475

+

” 9 ”

1% ozs. garden mould 92 376 284 14950 Ito 52382

Now all these plants had abundance of water, therefore all
should have made equal growth had nothing more been needed,
The amount of growth, however, increased with the impurity
of the water. ‘ Vegetables,” he concludes, “are not formed
of water, but of a certain peculiar terrestrial matter. It has
been shown that there is a considerable quantity of this matter
contained in rain, spring, and river water, that the greatest
part of the fluid mass that ascends up into plants, does not.
settle there but passes through their pores and exhales up into
the atmosphere: that a great part of the terrestrial matter,
mixed with the water, passes up into the plant along with it,
and that the plant is more or less augmented in proportion as
the water contains a greater or less quantity of that matter;
from all of which we may reasonably infer, that earth, and not
water, is the matter that constitutes vegetables.”

He discusses the use of manures and the fertility of the
soil from this point of view, attributing the well-known falling —
off in crop yield when plants are grown for successive years on

AISTORICAL AND INTRODUCTORY 5

unmanured land to the circumstance that “the vegetable
matter that it at first abounded in being extracted from it by
those successive crops, is most of it borne off. . . . The land
may be brought to produce another series of the same vege-
tables, but not until it is supplied with a new fund of matter,
of like sort with that it at first contained; which supply is
made several ways, either by the ground’s being fallow some
time, until the rain has poured down a fresh stock upon it ; or
by the tiller’s care in manuring it.” The best manures, he
continues, are parts either of vegetables or of animals; which
ultimately are derived from vegetables.

-In his celebrated textbook of chemistry Boerhaave (41)
taught that plants absorb the juices of the earth and then work
them up into food. The raw material, the “prime radical
juice of vegetables, is a compound from all the three kingdoms,
wiz. fossil bodies and putrified parts of anzmals and vege-
tables”. This “we look upon as the chyle of the plant ; being
chiefly found in the first order of vessels, vzz., in the roots and
the body of the plant, which answers to the stomach and intes-
tines of an animal ”.

For many years no such outstanding work as that of
Glauber and Woodward was published, if we except Hales’ ,
Vegetable Staticks (119), the interest of which is physiological
rather than agricultural.' Advances were, however, being
made in agricultural practice. One of the most important
was the introduction of the drill and the horse hoe by Jethro
Tull (286), an Oxford man of a strongly practical turn of
mind, who insisted on the vital importance of getting the soil
into a fine crumbly state for plant growth. Tull was more
than an inventor ; he discussed in most picturesque language
the sources of fertility in the soil. In his view it was not the
juices of the earth, but the very minute particles of soil
loosened by the action of moisture, that constituted the
“proper pabulum” of plants. The pressure caused by the
swelling of the growing roots forced these particles into the
“lacteal mouths of the roots,’ where they entered the circula-
tory system. All plants lived on these particles, ze. on the

1 He shows, however, that air is “‘ wrought into the composition ” of plants.

6 SOIL CONDITIONS AND PLANT GROWTH

same kind of food ; it was incorrect to assert, as some had
done, that different kinds of plants fed as differently as
horses and dogs, each taking its appropriate food and no
other. Plants will take in anything that comes their way,
good or bad. A rotation of crops is not a necessity, but only
a convenience. Conversely, any soil will nourish any plant
if the temperature and water supply are properly regulated.
Hoeing increased the surface of the soil or the “ pasture of
the plant,” and also enabled the soil better to absorb the
nutritious vapours condensed from the air. Dung acted in
the same way, but was more costly and less efficient.

So much were Tull’s writings esteemed, Cobbett tells us,
that they were “ plundered by English writers not a few and
by Scotch in whole bandittis ”.

The position at the end of this period cannot better be
summed up than in Tull’s own words: “ It is agreed that all
the following materials contribute in some manner to the in-
crease of plants, but it is disputed which of them is that very
increase or food: (1) nitre, (2) water, (3) air, (4) fire, (5)
earth ”.

The Search for Plant Nutrients.
1. The Phlogistic Period, 1750-1800.

Great interest was taken in agriculture in this country dur-
ing the latter half of the eighteenth century. “ The farming
tribe,” writes Arthur Young during this period, ‘‘is now made
up of all ranks, from a duke to an apprentice.” Many experi-
ments were conducted, facts were accumulated, books written,
and societies formed for promoting agriculture. The Edin-
burgh Society, established in 1755 for the improvement of arts
and manufactures, induced Francis Home (137) “to try how
far chymistry will go in settling the principles of agriculture ”.
The whole art of agriculture, he says, centres in one point : the
nourishing of plants. - Investigation of fertile soils showed that
they contain oil, which is therefore a food of plants. But
when a soil has been exhausted by cropping, it recovers its

HISTORICAL AND INTRODUCTORY 7

fertility on exposure to air,! which therefore supplies another
food. Home made pot experiments to ascertain the effect of
various substances on plant growth. “The more they (ze.
farmers) know of the effects of different bodies on plants, the
greater chance they have to discover the nourishment of plants,
at least this is the only road.” Saltpetre, Epsom salt, vitriol-
ated tartar (ze. potassium sulphate) all lead to increased plant
growth, yet they are three distinct salts. Olive oil was also
useful. It is thus clear that plant food is not one thing only,
but several; he enumerates six: air, water, earth, salts of
different kinds, oil, and fire in a fixed state. As further proof
he shows that “ all vegetables and vegetable juices afford those
very principles, and no other, by all the chymical- experi-
ments which have yet been made on them with or without
fire”, ;

The book is a great advance on anything that had gone
before it, not only because it recognises that plant nutrition
depends on several factors, but because it indicates so clearly
the two methods to be followed in studying the problem—pot
cultures and plant analysis. Subsequent investigators, Wal-
lerius (293), the Earl of Dundonald (90), and Kirwan (149)
added new details but no new principles. The problem indeed
was carried as far as was possible until further advances were
made in plant physiology and in chemistry. The writers just
mentioned are, however, too important to be passed over com-
pletely. Wallerius, in 1761, professor of chemistry at Upsala,
after analysing plants to discover the materials on which they
live, and arguing that (Vutritio non fieri potest a rebus hetero-
genets, sed homogenezs, concludes that humus, being homogeneis,
is the source of their food—the mutritéva—while the other soil
constituents are instrumentala, making the proper food mix-
ture, dissolving and attenuating it, till it can enter the plant
root. Thus chalk and probably salts help in dissolving the
“fatness” of the humus. Clay helps to retain the “ fatness ”
and prevent it being washed away by rain: sand keeps the
soil open and pervious to air, The Earl of Dundonald, in

1 Recorded by most early writers, ¢.g. Evelyn (Terra, 1674) (96).

8 SOIL CONDITIONS AND PLANT GROWTH

1795, adds alkaline phosphates to the list of nutritive salts,
but he attaches chief importance to humus as plant food.
The “oxygenation” process going on in the soil makes the
organic matter insoluble and theréfore useless for the plant;
lime, “alkalis and other saline substances” dissolve it and
change it to plant food ; hence these substances should be used
alternately with dung as manure. Manures were thus divided,
as by Wallerius, into two classes : those that afford Pian food,
-and those that have some indirect effect.

Throughout this period it was believed that plants could
generate alkalis. ‘‘ Alkalies,” wrote Kirwan in 1796, “‘seem
to be the product of the vegetable process, for either none, or
scarce any, is found in the soils, or in rain water.” In like
manner Lampadius thought he had proved that plants could
generate silica. The theory that plants agreed in all essentials
with animals was still accepted by many men of science ; some
interesting developments were made by Erasmus Darwin in
1803 (77). |

Between 1770 and 1800 work was done on the effects of
vegetation on air that was destined to revolutionise the ideas
of the function of plants in the economy of Nature, but its
agricultural significance was not recognised until later. In
1771 Priestley (230), knowing that the atmosphere becomes
vitiated by animal respiration, combustion, putrefaction, etc.,
and realising that some natural purification must go on, or life
would not longer be possible, was led to try the effect of sprigs
of living mint on vitiated air. He found that the mint made
the air. purer, and. concludes “that plants, instead of affecting
the air in the same manner with animal respiration, reverse the
effects of breathing, and tend to keep the atmosphere pure and
wholesome, when it is become noxious in consequence of
animals either living, or breathing, or dying, and putrefying in
it”. But he had not yet discovered oxygen, and so could not
give precision to his discovery: and when, later on, he did
discover oxygen and learn how to estimate it, he unfortunately
failed to confirm his earlier results because he overlooked a:
vital factor, the necessity of light. He was therefore unable to

HISTORICAL AND INTRODUCTORY 9

answer Scheele, who had insisted that plants, like animals,
vitiate the air. It was Ingen-Housz (142) who reconciled both
~ views and showed that purification goes on in light only, whilst
vitiation takes place in the darkness. Jean Senebier at Geneva
had also arrived at the same result. He also studied the con-
verse problem—the effect of air on the plant, and in 1782
argued (259) that the increased weight of the tree in Van
- Helmont’s experiment (p. 2) came from the fixed air. “Si
donc l’air fixe, dissous dans l'eau de l’atmosphére, se combine
dans la parenchyme avec la lumiére et tous les autres élémens
de la plante; si le phlogistique de cet air fixe est sirement
précipité dans les organes de la plante, si ce précipité reste,
comme on le voit, puisque cet air fixe sort des plantes sous la
forme d’air déphlogistiqué, il est clair que l’air fixe, combiné
dans la plante avec la lumiére, y laisse une mati¢re qui n’y
seroit pas, et mes expériences sur |’étiolement suffisent pour le
démontrer.” Later on Senebier translated his work into the
modern terms of Lavoisier’s system.

2. The Modern Period, 1800-1860.

(a) The Foundation of Plant Phystology.—We have seen
that Home in 1756 pushed his inquiries as far as the methods
in vogue would permit, and in consequence no marked advance
‘was made for forty years. A new method was wanted before
further progress could be made, or before the new idea intro-
duced by Senebier could be developed. Fortunately, this was
soon forthcoming. To Théodore de Saussure, in 1804 (244),
son of the well-known de Saussure of Geneva, is due the quan-
titative experimental method which more than anything else
has made modern agricultural chemistry possible : which formed
the basis of subsequent work by Boussingault, Liebig, Lawes
and Gilbert, and indeed still remains our safest method of in-
vestigation. Senebier tells us that the elder de Saussure was
well acquainted with his work, and it is therefore not surpris-
ing that the son attacked two problems that Senebier had also
studied—the effect of air on plants and the nature and origin

Ke) SOIL CONDITIONS AND PLANT GROWTH

of salts in plants. De Saussure grew plants in air or in known
mixtures of air and carbon dioxide, and measured the gas
changes by eudiometric analysis and the changes in the plant
by “‘carbonisation”. He was thus able to demonstrate the
central fact of plant respiration—the absorption of oxygen and
the evolution of carbon dioxide, and further to show the de-
composition of carbon dioxide and evolution of oxygen in
light. Carbon dioxide in small quantities was a vital neces-
sity for plants, and they. perished if it was artificially removed
from the air. It furnished them not only with carbon, but
also with some oxygen. Water is also decomposed and fixed
by plants, On comparing the amount of dry matter gained
from these sources with the amount of material that can enter
through the roots even under the most favourable conditions,
he concludes that the soil furnished only a very small part of
the plant food. Small as it is, however, this part is indispens-
able: it supplies nitrogen—«ne partie essentielle des végétaux
—which, as he had shown, was not assimilated direct from the
air ; and also ash constituents, gud peuvent contribuer a former,
comme dans les animaux, leur parties solides ou osseuses. Fur-
ther, he shows that the root is not a mere filter allowing any
and every liquid to enter the plant ; it has a special action and
takes in water more readily than dissolved matter, thus effect-
ing a concentration of the solution surrounding it; different
salts, also, are absorbed to a different extent. Passing next
to the composition of the plant ash, he shows that it is not
constant, but varies with the nature of the soil and the age of
the plant ; it consists mainly, however, of alkalis and phos-
phates. All the constituents of the ash occur in humus. Ifa
plant is grown from seed in water there is no gain in ash: the
amount found at the end of the plant’s growth is the same as
was present in the seed excepting fora relatively small amount
falling on the plant as dust. Thus he disposes finally of the
idea that the plant generated potash.

After the somewhat lengthy and often wearisome works of
the earlier writers it is very refreshing to turn to de Saussure’s
concise and logical arguments and the ample verification he

HISTORICAL AND INTRODUCTORY II

gives at every stage. But for years his teachings were not
accepted, nor were his methods followed.

The two great books on agricultural chemistry then
current still belonged to the old period. Thaer and Davy,
while much in advance of Wallerius, the textbook writer of
1761, nevertheless did not realise the fundamental change
introduced by de Saussure; it has always-been the fate of
agricultural science to lag behind pure science. Thaer pub-
lished his Grundsdtze der rationellen Landwirtschaft in 1809-
1812: it had a great success on the Continent as a good,
practical handbook, and was translated into English as late
as 1844 by Cuthbert Johnson. Davy’s book (79) grew out of
the lectures which he gave annually at the Royal Institu-
tion on agricultural chemistry between 1802 and 1812; it
was published in 1813, and forms the last textbook of the
older period. Whilst no great advance was made by Davy
himself (indeed his views are distinctly behind those of de
Saussure) he carefully sifted the facts and hypotheses of
previous writers, and gives us an account, which, however de-
fective in places, represents the best accepted knowledge of
the time, set out in the new chemical language. He does not
accept de Saussure’s conclusion that plants obtain their carbon
chiefly from the carbonic acid of the air: some plants, he says,
appear to be supplied with carbon chiefly from this source
but in general he supposes the carbon to be taken in through
the roots. Oils are good manures because of the carbon and
hydrogen they contain; soot is valuable, because its carbon
is “in a state in which it is capable of being rendered soluble
by the action of oxygen and water”’. Lime is useful because
it dissolves hard vegetable matter, Once the organic matter
has dissolved there is no advantage in letting it decompose
further: putrid urine is less useful as manure than fresh
urine, whilst it is quite wrong to cause farmyard manure to
ferment before it is applied to the land. All these ideas
have long been given up, and indeed there never was any
sound experimental evidence to support them. It is even
arguable that they would not have persisted so long as they

12 SOIL CONDITIONS AND PLANT GROWTH

did had it,not been for Davy’s high reputation. His insistence
on the importance of the physical properties of soils—their
relationship to heat and to water—was more fortunate and
marks the beginning of soil physics, afterwards developed con-
siderably by Schiibler (254). On the Continent, to an even —
greater extent than in England, it was held that plants drew
their carbon from the soil and lived on humus, a view sup-
ported by the very high authority of Berzelius.’
(6) The Foundation of Agricultural Science.—Hitherto ex-
periments had been conducted either in the laboratory or in
small pots: about 1834, however, Boussingault, who was
already known as an adventurous traveller in South America,
began a series of field experiments on his farm at Bechelbronn
in Alsace. These were the first of their kind: to Boussin-
gault, therefore, belongs the honour of having introduced the
method by which the new agricultural science was to be de-
veloped. He reintroduced the quantitative methods of de
Saussure, weighed and analysed the manures used and the
crops obtained, and at the end of the rotation drew up a bal-

ance sheet, showing how far the manure had satisfied the needs _
of the crop and how far other sources of supply—air, rain, and
soil—had been drawn upon. The results of one experiment
are given in Table I. on the opposite page. At the end of the
period the soil had returned to its original state of productive-
ness, hence the dry matter, carbon, hydrogen, and oxygen not
accounted for by the manure must have been supplied by the
air and rain, and not by the soil. On the other hand, the
manure afforded more mineral matter than the crop took off,
the balance remaining in the soil. Other things being equal,
he argued that the best rotation is one which yields the great-
est amount of organic matter over and above what is present
in the manure. No fewer than five rotations were studied,
but it’ will suffice to set out only the nitrogen statistics (Table
II. on the opposite page), which show a marked gain of nitrogen

1J. J. Berzelius, Lehrbuch d. chemie, ubersetz. v.-F. Wohler, 3 Aufl., 1839,,
Bd, 8.

Matter.
t *

meee, S| | | 3872) 5357°7|'. 184'0| *:%376°7| — 53°09 199°8
| 2. Wheat . Q .| 3006) 143r°6| 164°4|] x2r4'qg| 31°3 163°8
3. Clover hay . -| 4029} 1909°7|; 201°5| 1523°0| 84°6 310°2
| 4. Wheat. ; -| 4208} 2004'2| 230°0| 1700°7] 43°38 229°3

Turnips (catch
_ crop) . 716 307'°2 39°3 3029} 12°2 54°4

HISTORICAL AND INTRODUCTORY 13

Tapue I.—Sratistics or A RoraTion. BoussinGAuLt (46).

¥

Weight in kilograms per hectare of

ps Carbon. |Hydrogen.| Oxygen. |Nitrogen.| Mineral

Rs Oats s , .| 2347| 1182°3 137°3 890°9| 284 108'0

4

Total during rotation | 17478} 8192°7| 956°5| 7oog'o| 254'2| 1065'5
Addedin manure .| ro161| 3637°6/ 426°3| 2621°5| 203:2| 3271'9

Difference not ac- |+ 7317|+ 4555°I| + 529°7|+ 4387°5| + 51°0 |— 22064
counted for, taken
from air, rain, or soil.

tooo kilograms per hectare = 16 cwt. per acre.

TaBLE II.—NITROGEN STATISIICS OF VARIOUS ROTATIONS.
BoussINGAULT (46).

Kilograms per hectare.

Excess in Crop over that

Rotation. supplied in Manure.

Nitrogen in| Nitrogen
Manure. in Crop.

Per Rotation.| Per Annum.

(x) Potatoes, (2) wheat, (3) clover,

4) wheat, turnips,! (5) oats ells SOBA 1 2507 47°5 9°5
(1), Beets, (2) wheat, (3) clover,
4) wheat, turnips, (5) oats - | 203°2 254°2 51I°0 r0'2
(x) Potatoes, (2) wheat, (3) clover,
Q) wheat, turnips,'(5) peas, (6)rye | 243°8 353°6 109°8 18°3
Jerusalem artichokes,two years | 188°2 274°2 860 43°02
(z) Dunged fallow, (2) wheat,
) wheat . ‘ : ; ; 82°8 87°4 4°6 zs
_ Lucerne, five years A -| 224°0 | 10780 854 170°8

1 Catch crop, i.e. taken in autumn after the wheat. __
* This crop does not belong to the leguminosz, but it is possible that the
nitrogen came from the soil, and that impoverishment was going on,

14 SOIL CONDITIONS AND PLANT GROWTH

when the newer rotations are adopted, but not where wheat
only is grown.

Now the rotation has not impoverished the soil, hence he
concludes that ‘“l’azote peut entrer directement dans lorgan-
isme des plantes, si leur parties vertes sont aptes a le fixer”.
Boussingault’s work covers the Whole range of agriculture and
deals with the composition of crops at different stages of their
growth, with soils, and with problems in animal nutrition
Unfortunately the classic farm of Bechelbronn did not remain
a centre of agricultural research and the experiments came to
anend. Some of the work was summarised by Dumas in a
very striking essay (88, see also 47) that has been curiously
overlooked by agricultural chemists.

During this period (1830 to 1840) Carl Sprengel was
studying the ash constituents of plants, which he considered _
were probably essential to nutrition (270). Schiibler was
working at soil physics (254), and a good deal of other work
was quietly being done. No particularly important discoveries
were being made, no controversies were going on, and no
great amount of interest was taken in the subject.

But all this was changed in 1840 when Liebig’s famous -
report to the British Association upon the state of organic
chemistry, afterwards published as Chemistry in its Application
to Agriculture and Physiology (174a), came like a thunderbolt
upon the world of science. With polished invective and a fine
sarcasm he holds up to scorn the plant physiologists of his day
for their continued adhesion, in spite of accumulated evidence,
to the view that plants derive their carbon from the soil and
not from the carbonic acid of the air. ‘All explanatioris of
chemists must remain without fruit, and useless, because, even
to the great leaders in physiology, carbonic acid, ammonia,
acids, and bases, are sounds without meaning, words without
sense, terms of an unknown language, which awake no thoughts
and no associations.” The experiments quoted by the physi-
ologists in support of their view are all ‘‘ valueless for the de-
cision of any question”. ‘These experiments are considered
by them as convincing proofs, whilst they are fitted only to

HISTORICAL AND INTRODUCTORY 15

awake pity.” Liebig’s ridicule did what neither de Saussure’s
nor Boussingault’s logic had done: it finally killed the humus
theory. Only the boldest would have ventured after this to
assert that plants derive their carbon from any source other
than carbon dioxide, although it must be admitted that we have
no proof that plants really do obtain all their carbon in this
way. Thirty years later, in fact, Grandeau (112) adduced
evidence that humus may, after all, contribute something to
the carbon supply, and his view still finds acceptance in
France ;! for this also, however, convincing proof is lacking.
But for the time carbon dioxide was considered to be the sole
source of the carbon of plants. Hydrogen and oxygen came
from water, and nitrogen from ammonia. Certain mineral
substances were essential: alkalis were needed for neutrali-
sation of the acids made'by plants in the course of their vital
processes, phosphates were necessary for seed formation, and
potassium silicates for the development of grasses and cereals.
The evidence lay in the composition of the ash: plants might
absorb anything soluble from the soil, but they excreted from
their roots whatever was non-essential. The fact of a sub-
stance being present was therefore sufficient proof of its
necessity.

Plants, Liebig argued, have an inexhaustible supply of
carbonic acid in the air. But time is saved in the early stages
of plant growth if carbonic acid is being generated in the soil,
for it enters the plant root and affords extra nutriment over
and above what the small leaves are taking in. Hence a
supply of humus, which continuously yields carbonic acid, is
advantageous, Further, the carbonic acid attacks and dissolves
some of the alkali compounds of the soil and thus increases
the mineral food supply. The true function of humus is to
evolve carbonic acid.

The alkali compounds of the soil are not all equally soluble.
A weathering process has to go on, which is facilitated by
liming and cultivation, whereby the comparatively insoluble

1See e.g. L. Cailletet (64), Jules Lefévre (169), and J. Laurent, Rev. gén,
bot., 1904, 16, 14.

16 SOIL CONDITIONS AND PLANT GROWTH —

compounds are broken down to a more soluble state. The
final solution is effected by acetic acid excreted by the plant
root, and the dissolved material now enters the root.

The nitrogen is taken up as ammonia, which may come
from the soil, from added manure, or from the air. In order
that a soil may remain fertile it is necessary and sufficient to
return in the form of manure the mineral constituents and the
nitrogen that have been taken away. When sufficient crop
analyses have been made it will be possible to draw up tables
showing the farmer precisely what he must add in any par-
ticular case. ae

An artificial manure known as Liebig’s patent manure was
made up-on these lines and placed on the market. :

_ Liebig’s book was meant to attract attention to the subject,
and it did; it rapidly went through several editions, and as
time went on Liebig developed his thesis, and gave it a quan-
titative form: ‘‘The crops on a field diminish or increase in
exact proportion to the diminution or increase of the mineral
substances conveyed to itin manure”. He further adds what
afterwards became known as the Law of the Minimum,’ “ by
the deficiency or absence of ome necessary constituent, all the
others being present, the soil is rendered barren for all those
crops to the life of which ¢haz one constituent is indispensable ”.
These and other amplifications in the third edition, 1843, gave
rise to. much controversy. So much did Liebig insist, and
quite rightly, on the necessity for alkalis and phosphates, and
so impressed was he by the gain of nitrogen in meadow land
supplied with alkalis and phosphates alone, and by the con-
tinued fertility of some of the fields of Virginia and Hungary
and the meadows of Holland, that he began more and more
to regard the atmosphere as the source of nitrogen for plants. —
Some of the passages of the first and second editions urging
the necessity of ammoniacal manures were deleted from the
‘third and latér editions.’ “If the soil be suitable, if it con-

1The underlying principle was not discovered by Liebig, having already
been enunciated by political economists of the Malthus School. He was, how-
ever, the first to apply it to plant nutrition.

HISTORICAL AND INTRODUCTORY 17

tains a sufficient quantity of alkalis, phosphates, and sulphates,
nothing will be wanting. The plants will derive their
ammonia from the atmosphere as they do carbonic acid,” he
writes in the Farmer's Magazine.’ Ash analysis led him to
consider the turnip as one of the plants “which contain the
least amount of phosphates and therefore require the smallest
quantity for their development”. These and other practical
deductions were seized. upon and shown to be erroneous by
Lawes (161-162) who had for some years been conducting
vegetation experiments. Lawes does not discuss the theory
as such, but tests the deductions Liebig himself draws and
finds them wrong. Further trouble was in store for Liebig ;
his patent manure when tried in practice ad failed. This
was unfortunate, and the impression in England at any rate
was, in Philip Pusey’s words: “The mineral theory, too
hastily adopted by Liebig, namely, that crops rise and fall in
direct proportion to the quantity of mineral substances present
in the soil, or to the addition or abstraction of these sub-
stances which are added in the manure, has received its
death-blow from the experiments of Mr Lawes”.

And yet the failure of the patent manure was not entirely
the fault of the theory, but only affords further proof of the
numerous pitfalls of the subject. The manure was sound in
that it contained potassium compounds and phosphates (it
ought, of course, to have contained nitrogen compounds), but
it was unfortunately rendered insoluble by fusion with lime
and calcium phosphate so that it should not too readily wash
gut in the drainage water. Not till Way had shown in 1850
that soz/ precipitates soluble salts of ammonium, potassium and
phosphates was the futility of the fusion process discovered,
and Liebig saw the error he had made (1744).

Meanwhile the great field experiments at Rothamsted had
been started by Lawes and Gilbert in 1843. These experiments
were conducted on the same general lines as those begun

1 Farmer’s Magazine, 1847, vol. xvi., p. 511. A good summary of Liebig’s.
position is given in his Letters on Chemistry, 34th letter, 3rd edition, p. 519,
1851.

2

18 SOIL CONDITIONS AND PLANT GROWTH

earlier by Boussingault, but they have the advantage that they
are still going on, having been continued year after year on the
same ground without alteration, except in occasional details,
since 1852. The mass of data now accumulated is consider-
able and it is being treated by modern statistical methods,
Certain conclusions are so obvious, however, that they can
be drawn on mere inspection of the data. By 1855 the fol-
lowing points were definitely settled (166c) :—

(1) Crops require phosphates and salts of the alkalis, but
the composition of the ash does not afford reliable information
as to the amounts of each constituent needed, e.g. turnips re-
quire large amounts of phosphates, although only little is
present in their ash. Some of the results are :—

Composition of ash, per cent. Yield of turnips, tons per acre (1843)—
(1860 crop)— Unmanured ; ot een
K,O 44'8 Superphosphate . : ; - ess
PsO5) 5-3 <tr’ ‘3 + potassic salts rI1°9

(2) Non-leguminous crops require a supply of some nitro-
genous compounds, nitrates and ammonium salts being almost
equally good. Without an adequate supply no increases of
growth are obtained, even when ash constituents are added.
The amount of ammonia obtainable from the atmosphere is
insufficient for the needs of crops. LLeguminous crops behaved
abnormally.

(3) Soil fertility may be maintained for some years at
least by means of artificial manures.

(4) The beneficial effect of fallowing lies in the increase
brought about in the available nitrogen compounds in the
soil.

Although many of Liebig’s statements were shown to be
wrong, the main outline of his theory as first enunciated
stands. It is no detraction that de Saussure had earlier
published a somewhat similar, but less definite view of
nutrition: Liebig had brought matters to a head and made
men look at their cherished, but unexamined, convictions.
The effect of the stimulus he gave can hardly be over-

HISTORICAL AND INTRODUCTORY 19

estimated, and before he had finished, the essential facts of
plant nutrition were settled and the lines were laid down
along which scientific manuring was to be developed. The
water cultures of Knop and other plant physiologists showed
conclusively that potassium, magnesium, calcium, iron, phos-
phorus, along with sulphur, carbon, nitrogen, hydrogen, and
oxygen are all necessary for plant life. The list differs from
Liebig’s only in the addition of iron and the withdrawal of
silica; but even silica, although not strictly essential, is ad-
vantageous to cereals.

In two directions, however, the controversies went on for
“many years. Farmers were slow to believe that ‘chemical
manures” could ever do more than stimulate the crop, and
declared they must ultimately exhaust the ground. The
Rothamsted plots falsified this prediction; manured year after
year with the same substances and sown always with the same
crops, they even now, after sixty years of chemical manuring,
continue to produce good crops, although secondary effects
have sometimes set in. In France the great missionary was
Georges Ville, whose lectures were given at the experimental
farm at Vincennes during 1867 and 1874-5 (288). He went
even further than Lawes and Gilbert, and maintained that
artificial manures were not only more remunerative than dung,
but were the only way of keeping up fertility. In recom-
mending mixtures of salts for manure he was not gujded by
ash analysis but by field trials. For each crop one of the
four constituents, nitrogen compounds, phosphates, lime, and
potassium compounds (he did not consider it necessary to
add any others to his manures) was found by trial to be more
wanted than the others and was therefore called the
“dominant” constituent. Thus for wheat he obtained the
following results, and therefore concluded that on his soil
wheat required a good supply of nitrogen, less phosphate, and
still less potassium :—

20 SOIL CONDITIONS AND PLANT GROWTH

Crop per acre.

Bushels.
Normal manure. ; : ‘ : : ; : . 43
Manure without lime. ; : ; ; ‘ N eT
Pp » potash . : , ; : F : er
A »» phosphate . ; : ; ‘ . - 264
Pa RS nitrogen : : " ‘ . ee
Soil without manure. ° ; ; : . 5 Fgh)

Other experiments of the same kind showed that nitrogen
was the dominant for all cereals and beetroot, potassium for
potatoes and vines, phosphates for the sugar cane. An excess
of the dominant constituent was always added to the crop
manure. The composition of the soil had to be taken into
account, but soil analysis was no good for the purpose. In-
stead he drew up a simple scheme of plot trials to enable
farmers to determine for themselves just what nutrient was
Jacking in their soil. His method was: thus essentially
empirical, but it still remains the best we have; his view that
chemical manures are always better and cheaper than dung is,
however, too narrow and has not survived.

The second controversy dealt with the source of nitrogen
in plants. Priestley had stated that a plant of Epzlobium
hirsutum placed in a small vessel absorbed during the course
of the month seven-eighths of the air present. De Saussure,
however, denied that plants assimilated gaseous nitrogen.
Boussingault’s pot-experiments showed that peas and clover
could get nitrogen from the air while wheat could not (45),
and his rotation experiments emphasised this distinction.
He himself did not make as much of this discovery as he
might have done, but Dumas (88) fully realised its importance.

Liebig, as we have seen, maintained that ammonia, but
not gaseous nitrogen, was taken up by plants, a view con-
firmed by Lawes, Gilbert, and Pugh (164) in the most rigid
demonstration that had yet been attempted. Plants of several
natural orders, including the leguminose, were grown in
surroundings free from ammonia or any other nitrogen com-
pound. The soil was burnt to remove all trace of nitrogen
compounds while the plants were kept throughout the ex-

HISTORICAL AND INTRODUCTORY 21

periment under glass shades, but supplied with washed and
purified air and with pure water. In spite of the ample
supply of mineral food the plants languished and died: the
conclusion seemed irresistible that plants could not utilise
gaseous nitrogen. For all non-leguminous crops this con-
clusion agreed with the results of field trials. But there re-
mained the very troublesome fact that leguminous crops
required no nitrogenous manure and yet they contained large
quantities of nitrogen, and also enriched the soil considerably
in this element. Where had the nitrogen come from? The
amount of combined nitrogen brought down by the rain was
found to be far too small to account for the result. For years
experiments were carried on, but the problem remained un-
solved. Looking back over the papers! one can see how very
_ close some of the older investigators were to the discovery of
the cause of the mystery: in particular Lachmann (158) in
1858 and Bretschneider (54) in 1861. Lachmann showed
that the nodules invariably present on the roots contained
“-vibrionenartige” organisms, while Bretschneider showed
that the nitrogen fixation which occurred in normal soil did
not take place in ignited soil. But these papers were both
_ published in obscure journals and attracted little attention,
and once again an investigation in agricultural chemistry had
been brought to a standstill for want of new methods of
attack.

The Beginnings of Soil Bacteriology.

It had been a maxim with the older agricultural chemists
that “corruption is the mother of vegetation”. Animal and
vegetable matter had long been known to decompose with
formation of nitrates: indeed nitre beds made up from such
decaying matter were the recognised source of nitrates for
the manufacture of gunpowder during the European Wars of
the seventeenth and eighteenth centuries.2 No satisfactory

1A good summary’ of the voluminous literature is contained in Léhnis
Handbuch der Landw. Bakteriologie, pp. 646 et seq.

2“Tnstructions sur |’établissement des nitriéres, publié par les Régisseurs
généraux des Poudres et Salpétre. Paris, 1777.

32 SOIL CONDITIONS AND PLANT GROWTH

explanation of the process had been offered, although the
discussion of rival hypotheses continued up till 1860, but the
conditions under which it worked were known and on the
whole fairly accurately described.

No connection was at first observed between nitrate
formation and soil productiveness. Liebig rather diverted
attention from the possibility of tracing what now seems an
obvious relationship by regarding ammonia as the essential
nitrogenous plant nutrient, though he admitted the possible
suitability of nitrates (174c). Way came much nearer to the’
truth. In 1856 (2980) he showed that nitrates were formed
in soils to which nitrogenous fertilisers were added. Un-
fortunately he failed to realise the significance of this discovery.
He was still obsessed with the idea that ammonia was essential
to the plant, and he believed that ammonia, unlike other
nitrogen compounds, could not change to nitrate in the soil,
but was absorbed by the soil by the change he had already
described (p. 17). But he only narrowly missed making an
important advance in the subject, for after pointing out that
nitrates are comparable with ammonium salts as fertilisers he
writes: ‘ Indeed the French chemists 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 myself at present coincide: and it is
sufficient here to admit that nitric acid in te form of nitrates
has at least a very high value as a manure.’

It was not till ten years later, and as a result of work by
plant physiologists, that the French view prevailed over
Liebig’s and agricultural investigators recognised the im-
portance of nitrates to the plant and of nitrification to soil
fertility. It then became necessary to discover the cause of
nitrification.

During the sixties and seventies great advances were
being made in bacteriology, and it was definitely established
that bacteria bring about putrefaction, decomposition and
other changes; it was therefore conceivable that they were
the active agents in the soil and that the process of decom-

HISTORICAL AND INTRODUCTORY "23

position there taking place was not the purely chemical
“eremacausis” Liebig had postulated. Pasteur himself had
expressed the opinion that nitrification was a bacterial pro-
cess. The new knowledge was first brought to bear on
agricultural problems by Schloesing and Mintz (245) in 1877
during a study of the purification of sewage water by land
filters. A continuous stream of sewage was allowed to trickle
_down a column of sand and limestone so slowly that it took
eight days to pass. For the first twenty days the ammonia
in the sewage was not affected, then it began to be converted
into nitrate; finally all the ammonia was converted during its
passage through the column, and nitrates alone were found in
the issuing liquid. Why, asked the authors, was there a
delay of twenty days before nitrification began? If the pro-
cess were simply chemical, oxidation should begin at once.
They therefore examined the possibility of bacterial action
and found that the process was entirely stopped by a little
chloroform vapour, but could be started again after the
_ chloroform was removed by adding a little turbid extract of
dry soil. Nitrification was thus shown to be due to micro-
organisms—“‘ organised ferments,” to use their own expression.

Warington (295-6) had been investigating the nitrates in
the Rothamsted soils, and at once applied the new discovery
to soil processes. He showed that nitrification in the soil is
stopped by chloroform and carbon disulphide; further, that
solutions of ammonium salts could be nitrified by adding a
trace of soil. By a careful series of experiments described in
his four papers to the Chemical Society he found that there
were two stages in the process and two distinct organisms:
the ammonia was first converted into nitrite and then to
nitrate. But he failed altogether to obtain the organisms, in
spite of some years of study, by the gelatin plate methods then
in vogue. The reason was discovered later: the organisms
will not grow in presence of nitrogenous organic matter.
Not till 1890 did Winogradsky (311) succeed in isolating
them, and thus complete the evidence.

Warington established definitely the. fact that nitrogen

24 SOIL CONDITIONS AND PLANT GROWTH

compounds rapidly change to nitrates in the soil, so that what-
ever compound is supplied as manure plants get practically
nothing but nitrate as food. This closed the long discussion
as to the nitrogenous food of non-leguminous plants; in
natural conditions they take up nitrates only (or at any rate
chiefly), because the activities of the nitrifying organisms
leave them no option. The view that plants assimilate
gaseous nitrogen has from time to time been revived,’ 1 but has
not been taken apmowely

TaBLE III.—RELATION BETWEEN NITROGEN SUPPLY AND PLANT GROWTH.
HELLRIEGEL AND WILFARTH (1300).

cium nitrate sup-;| mone; ‘056 *II2 *168 "224 *336
plied per pot, grams
Weight of oats ob-

: 59024 ‘ ;
tained (grain and { 3605 {5 8sr0 ‘o g8r4 15°GQ74 fe +o" 30°I750

Nitrogen in the a0}

21°4409

straw) . “4191 | (5 +2867 | | 10°9413
Weight of peas ob- 551 °9776|( 4°9146 ,
tained (grain and 3°496 {3047 { 9°7671| 5°6185 Bae 113520
4°1283 | | 8-4969 45°.

straw) . - | 05°233

The apparently hopeless problem of the nitrogen nutrition
of leguminous plants was soon to be solved. In a striking
series of sand cultures Hellriegel and Wilfarth (130) showed
that the growth of non-leguminous plants, barley, oats, etc.,
was directly proportional to the amount of nitrate supplied,
_ the duplicate pots agreeing satisfactorily ; while in the case of
leguminous plants no sort of relationship existed and duplicate
pots failed to agree. After the seedling stage was passed the
leguminous plants grown without nitrate made no further
progress for a time, then some of them started to grow and
did well, while others failed. This stagnant period was
not seen where nitrate was supplied. Two of their experi-
ments are given in Table III.

Analysis showed that the nitrogen contained in the oat
crop and sand at the end of the experiment was always a little
less than that originally supplied, but was distinctly greater
in the case of peas; the gain in three cases amounted to ‘910,

leg. see (224b).

HISTORICAL AND INTRODUCTORY 25

-1'242 and ‘789 grm. per pot respectively. They drew two

conclusions: (1) the peas took their nitrogen from the air ;

(2) the process of nitrogen assimilation was conditioned by
some factor that did not come into their experiment except
_bychance. In trying to frame an explanation they connected
two facts that were already known. Berthelot (26) had made
experiments to show that certain micro-organisms in the soil
can assimilate gaseous nitrogen. It was known to botanists
that the nodules on the roots of leguminose contained bacteria.’
Hellriegel and Wilfarth, therefore, supposed that the bacteria
in the nodules assimilated gaseous nitrogen, and then handed
on some of the resulting nitrogenous compounds to the plant.
This hypothesis was shown to be well founded by the follow-
ing facts :—

1. In absence of nitrates peas made only small growth
and developed no nodules in sterilised sand; when calcium
nitrate was added they behaved like oats and barley, giving
regular increases in crop for each increment of nitrates (the
discordant results. of Table II. were obtained on unsterilised
sand).

2. They grew well and developed nodules in sterilised
_ sand watered with an extract of arable soil. .

3. They sometimes did well and sometimes failed when
grown without soil extract and without nitrate in umsterilised
sand, which might or might not contain the necessary organ-
isms. An extract that worked well for peas might be without
effect on lupins or serradella. In other words, the organism
is specific.

Hellriegel and Wilfarth read their paper and exhibited
some of their plants at the Naturforscher-Versammlung at
Berlin in 1886. Gilbert was present at the meeting, and on
returning to Rothamsted repeated and confirmed the experi-
ments (165). At a later date Schloesing fils and Laurent

1This had been demonstrated by Lachmann in 1858 (158) (p. 21) and by
Woronin in 1866 (322). Eriksson in 1874 (Doctor’s dissertation, abs. in Botan.
Ztg., 1874, 32, 381-384) carried on the investigation, while Brunchorst in 1885
Ber. d. Deutsch, Bot, Ges, iii., 241-257, gave the name “ bacteroids”.

26 SOIL CONDITIONS AND PLANT GROWTH

(247) showed that the weight of nitrogen absorbed from the
air was approximately equal to the gain by the plant and the
soil, and thus finally clinched the evidence.

| Control.| Peas. |Mustard.| Cress. | Spurge.

134°6 | —2°6 | —3°8 | —2"4

rie)
4°0 | 142"4 | —2°5 2°0 3°2

|
'| Nitrogen lost from the‘air, mgm. : |
Nitrogen gained by crop and soil, mgm. |

The organism was isolated by Beijerinck (p. 204) and called
Bacterium radicicola. '

Thus another great controversy came to an end, and the
discrepancy between the field trials and the laboratory experi-
ments of Lawes, Gilbert, and Pugh was cleared up. The
laboratory experiments gave the correct conclusion that legu-
minous plants, like non-leguminous plants, have themselves
no power of assimilating gaseous nitrogen ; this power belongs
to the bacteria associated with them. But so carefully was
all organic matter removed from the soil, the apparatus, and
the air in endeavouring to exclude all trace of ammonia,
that theré was no chance of infection with the necessary
bacteria. Hence no assimilation could go on. In the field
trials the bacteria were active, and here there was a gain of
nitrogen.

The general conclusion that bacteria are the real makers
of plant food in the soil, and are, therefore, essential to the
growth of all plants, was developed by Wollny (3170) and
Berthelot (28). It was supposed to be proved by Laurent’s
experiments (160, see also 87). He grew buckwheat on
humus obtained from well-rotted dung, and found that plants
grew well on the untreated humus, but only badly on the
humus sterilised by heat. When, however, soil bacteria were
added to the sterilised humus (by adding an aqueous extract
of unsterilised soil) good growth took place. The experi-
ment looks convincing, but is really unsound. When a rich
soil is heated some substance is formed toxic to plants. The
failure of the plants on the sterilised humus was, therefore,
not due to absence of bacteria, but to the presence of a toxin.

HISTORICAL AND INTRODUCTORY 27

No one has yet succeeded in carrying out this fundamental
experiment of growing plants in two soils differing only in
that one contains bacteria while the other does not.

Similarly Caron! thought he had direct evidence of the
beneficial effect of bacteria in plant growth, but in reality’ the
evidence is unsatisfactory.

The close connection between bacterial activity and the
nutrition of plants is, however, fully justified by many experi-
ments, and forms a considerable part of our modern concep-
tion of the soil as a producer of crops, as will appear in the
following chapters.

The Rise of Modern Knowledge of the Soil: the Search
for Fresh Factors and for Mathematical Expressions.

Further investigation of soil problems has shown that they
are more complex than was at first supposed. The older
_workers had thought of soil fertility as a simple chemical
problem ; the early bacteriologists thought of it as bacterio-
logical. It was demonstrated by F. H. King at Wisconsin
(147a) that physical considerations must also be taken into
account. Van Bemellen showed that soil has colloidal pro-
perties and present-day workers have reproduced in the soil
many of the phenomena investigated in laboratories devoted
to the study of colloids. Whitney and Cameron at Washing-
ton greatly widened the subject by revealing the importance
of the soil solution and introducing the methods and principles
of physical chemistry. Russell and Hutchinson at Rothamsted
showed that bacterial action alone would not account for the
biological phenomena in the soil, but that other organisms
are also concerned, and subsequent work in the Rothamsted
laboratories has revealed the presence of a complex soil
population, the various members of which react on one another
and on the growing plant. Fresh advances are continually
being made in the vigorous experiment stations in the United
States, the British Empire, Japan, and Europe.

In the main the work is analytical and involves a search

1 Landw. Versuchs. Stat., 1895, 45, 401-418.

28 SOIL CONDITIONS AND PLANT GROWTH

for new factors: synthesis is hardly attempted as yet. As
the factors are discovered attempts are made to give them
mathematical expression. Thus Liebig’s Law of the Minimum
and F. F, Blackman’s Limiting Factors are expressed mathe-
matically by Mitscherlich (p. 32): V. H. Blackman’ ex-
presses plant growth by the “compound interest law”: Miyake
(pp. 185 and 188) brings ammonification and nitrification within
the equation for autocatalytic actions ; and the modern agricul-
tural chemist is acquiring a taste for mathematical formule
and constants unknown to the older generation of workers.

This attempt to find mathematical expressions has been
resisted on two grounds: some suppose that phenomena
associated with life cannot in any case be expressed mathe-
matically and that nothing but a hollow appearance of agree-
ment can be obtained ; others consider that the mathematical
formula, if it is to hold at all, must be expressed in such
general terms as to become meaningless, eg. many of the
actions going on in Nature can be expressed by exponential
equations if the terms are chosen with sufficient ingenuity.
The soil investigator, however, will be wise to secure all the
assistance he can, as the subject is complex, and it cuts across
the conventional divisions of science.

In modern Experimental Stations the tendency is towards
team work. As an instance chosen because it is best known
to the writer: at the Rothamsted Experimental Station,
,instead of a number of isolated individuals, there is a body
of workers investigating the subject, each from his own special
point of view, but each fully cognisant of the work of the
others, and periodically submitting his results to discussion
by them, Separate workers investigate respectively the
bacteria, protozoa, fungi, alge, helminths, and insects of the
soil; in addition physical and organic chemists are studying
the soil conditions, while others are concerned in the study of
the growing plant. A body of workers by harmonious co-
operation is able to make advances that would be impossible
for any single individual, however brilliant.

1 Annals of Botany, 1919, 33) 353-

HISTORICAL AND INTRODUCTORY 29

The nature of the subject necessitates a further departure
from the usual procedure. In purely laboratory investigations.
it is customary to adopt the Baconian method in which factors
are studied one at a time, all others being kept constant
except the particular one under investigation. In dealing
with soils in natural conditions, however, it is impossible to
proceed in this way: climatic factors will not be kept constant,
and however careful the effort to ensure equality of conditions
there is always the probability, and sometimes the certainty,
that the variable factor under investigation is interacting with
climatic factors and exerting indirect effects which modify or
even obscure the direct effects it is desired to study.

Of recent years statisticians have devised methods for
dealing with cases where several factors are varying simul-
taneously. The data obtained by the various workers at
Rothamsted are therefore examined by a statistician who
endeavours to disentangle the effects of various factors and
to state a number of probable relationships which can then
be investigated in the laboratory by the ordinary single factor
method.

The modern methods as applied at Rothamsted include
three distinct processes :—

1. Observations or experiments in the field by a group of
specialists working independently, but with full cognisance of
each other’s results.

2. Examination of the data by modern statistical methods
so as to ascertain the probable effects of the known factors
and to indicate where known factors are insufficient to account
for the results, and where, therefore, new factors must be
sought.

3. Laboratory studies by the specialist staff of the relation- ©
ships indicated by the statistical examination, these being
reduced to single factor problems.

CHAPTER II.
SOIL CONDITIONS AFFECTING PLANT GROWTH.

IN order to put limits to our discussion, it is necessary at the
outset to state what soil factors influence plant growth and
how, in general, the influence is manifested. We shall, there-
fore, in the present chapter bring together certain of the results
obtained by plant physiologists which are indispensable for the
proper study of the subject.

It has been shown that the five following soil factors pro-
foundly affect the growth of plants :—

Water supply.

Air supply.

Temperature.

Supply of plant nutrients.
Various injurious factors.

So ae

The plant may be affected in two general ways—in the
amount of growth (ze. the total amount of dry matter formed),
. or in the habit or other characteristic of growth. The former
is susceptible of quantitative investigation, the latter is not, or
only with difficulty ; it has, therefore, proved less attractive to
investigators.

The Effect on the Amount of Growth.
1. The Study of Single Factors.

Of the five factors concerned plant nutrients are, on the
whole, the easiest to investigate quantitatively, The general
relationship between the supply of a given plant nutrient and
the amount of dry matter formed was demonstrated by Hell-
riegel at Dhame in the eighties of the last century. Barley

30

of.

7 SOIL CONDITIONS AFFECTING PLANT GROWTH 31

was grown in pots of sand, all necessary factors were amply
provided, excepting only potassium salts, the amount of which
varied in the different pots, The weights of dry matter formed
are shown in Table IV.

Tas_e 1V.—Errect of PoTassium SALTS ON GROWTH OF BARLEY
HELLRIEGEL (130d).

Mgms, of K,O per pot.|o ~/|23°5 | 47 70°5 194 188 282
Dry matter formed when
KCI was given -|2°271| 5°414| 9°024} 9°963|15°322| 21°246| 24°47
K,SO, » -|2°549| 5°140| 5°283 | 13°363|14°768| 21°593| 23°774
KNO, a -| — | 4°552| 6°621| 9°949|14°576| 21°499| 24°206
KH,PO, _,, -| — | 4687) 6°346| 9°931|12°377| I7°171| —
K,HPO, ,, | — 6°684; — |11°736| 20°255| —
4 Average 2°410| 4°948) 6°79I | Io‘80I | 13°755| 20°357| 24°132

These results are plotted in Fig. 1.

_Mgms. |

N

a

Dry matter produced

al

o-395° 47 70594 188 28?
KCl. used (Mgms K20)

Fic. 1.—Relation between potash supply and growth of barley (Hellriegel).

Similar curves are obtained when the varying nutrient is
nitrogen or phosphorus.

The smoothness of the curve suggests that it can be ex-
pressed by a mathematical equation, and E. A. Mitscherlich
has attempted to do this, proceeding in the following manner :
If all the conditions were ideal, a certain maximum yield would
be obtained, but in so far as any essential factor is deficient

32 SOIL CONDITIONS AND PLANT GROWTH

there is a corresponding shortage in the yield. The yield
rises if some of the lacking factor is added, and goes up all the
further the lower it had previously fallen. Mitscherlich put
this as follows: the increase of crop produced by unit incre-
ment of the lacking factor is proportional to the decrement

100; Log (94-6-y) = 1,9613-1-23 x?
90F x

80

eS]
BN w my SN)
S) =) ° °

Yield of Oat
S$

20;

A L

5” ic 25 50 100

Calcium Monophosphate.

Fic. 2.—Mitscherlich’s curve showing relation between yield of oats and amount
of phosphate supplied.

from the maximum. The advantages of this form is that it
can be expressed mathematically :—

a
(A — y)k or log(A - y= )e- kx,

where y is the yield obtained when x = the amount of the

SOIL CONDITIONS AFFECTING PLANT GROWTH 33

factor present and A is the maximum yield obtainable if the
factor were present in excess, this being calculated from the
equation.
___ Mitscherlich’s own experiments were made with oats grown
in sand cultures supplied with excess of all nutrients except-
ing phosphate. This constituted the variable +: the. yields
actually attained when monocalcium phosphate was used and
those calculated from the equation shown in Table V. (Fig.
2).

Experiments were also made with di- and tri-calcic phos-
phates and constants were calculated corresponding to &.

The ratio of these constants

k,(di-calcic phosphate)
&,(mono-calcic phosphate)

is a measure of the relative nutrient efficiency of the two salts ;
& is therefore called the efficiency value (Wirkungswert).
There are some very attractive possibilities about this method
of treatment, since it gives a constant independent of the yield
and having a definite mathematical meaning.’

TABLE V.—YIELD oF OATS WITH DIFFERENT DRESSINGS OF PHOSPHATES.
; MITSCHERLICH (2018).

: . i x
PaOs in Manure. | "produced | “from Formula. | Difference. | proabie Error
Grams. Grams. Grams.

0°00 g°8 + 0°50 9°80 — 0°39 — 08
0°05 19°3 + 0°52 18°gI — 0°56 — 0°3
oro 27°2 + 2'00 26°64 — 2°37 — 2°38
0°20 4I°0 + 0°85 38°63 + 3°22 + 2°9
0°30 43°9 £112 47°12 + 2°49 + 0°7
0°50 54°9 + 3°66 57°39 + 6°64 + 3°0
2°00 6r°o + 2°24 67°64 — no

The method has given rise to considerable controversy in

1 For an interesting application see Zur Frage der Wurzelausscheidungen der
Pflanze (Landw. Versuchs-Stat., 1913, \xxxi., 467-474), in which Mitscherlich
argues that the root excretions of clover cannot differ from those of oats.

3

34 SOIL CONDITIONS AND PLANT GROWTH

Germany. Pfeiffer! insists that the curves must be profoundly
modified by other limiting factors, while Frohlich takes excep-
tion to the method of calculation.? Mitscherlich apparently
recognises the force of these criticisms, for he now admits
that Liebig’s Law of the Minimum is not a correct expression
of the facts: the yield is determined not by the one factor
which is lacking, but by all the factors. Baule (9) has deve-
loped an equation on these lines which, he claims, agrees
satisfactorily with the experimental results.

Curves of similar type have been obtained for variations in
water supply and in temperature, but owing to greater experi-
mental difficulties the data are less reliable, and, therefore, not
susceptible to mathematical treatment. The temperature re-
sults are of interest, in that they bring out an important differ-

1 Th. Pfeiffer, E. Blanck, and M. Fliigel, Wasser und Licht als Vegetations-
Factoren und ihre Beziehungen zum Gesetze von Minimum (Landw. Versuchs-Stat.,
1912, Ixxvi., 169-236. See also 224¢).

2The method of’ calculation is as follows: Obtain two equations by sub-
stituting two of the numerical values of x and y obtained experimentally. Calling
these numbers +, xg, etc., the equations are

loge (A -—y,)=c-—hke, . + Gee = os i
loge (A — yo) =c— kt. . : : - (2)
Then by subtraction log (A — y,) — log (A — y.) = k (% —- a ; 5 - (3)

Obtain another equation like (3) but select the numerical values so that

tg — %g = %qy— %

loge (A — yy) — loge(A — yx) = (%3- 4%) - + + (4)
By subtracting (4) from (3) loge (A — y,) + loge (A — ys) = 2 loge (A — yz),
(A = ys) (A - pe
1.€. (A - 7) . ° . - (5)

Since y,, v9, and y, are all known, the value of A is easily calculated.
The value of k is then found from equation (3)

p — 108A — 91) = loge(A — yo)
Xo Tr, x) :

As all the quantities on the right-hand side are known the value of & is
readily obtained. A difficulty is that different values for A are, in fact, obtained
when other equations like (3) are worked out. Ifthere were a very large number
of points, a probable value for A could be obtained: with a small number, such
as almost necessarily are obtained in practice, some selection apparently has to
be made, which is objectionable. This was done by Mitscherlich in the case
quoted in Table V.

:
iy
A!
4
¢
a
a
~

SOIL CONDITIONS AFFECTING PLANT GROWTH 35

ence between sustained growth and the individual processes of
assimilation, etc. In the classical experiments of F. F. Black-
man and of Miss Matthaei (now A, G. L. Howard) (193) the
effect of temperature on assimilation, all other factors being
eliminated, was precisely that obtaining in an ordinary chemi-
cal reaction ; so also for respiration. Miss Matthaei found that
the Paibunits of carbon dioxide assimilated by a cherry laurel
leaf per 30 sq. cms. (about 8 sq. ins.) per hour at various
temperatures were :—

Temperature, deg. C. | — 6° |+ 8°8°| 11°4°| 15° |23°7°| 30°5° | 37°5° | 40°5° | 43°)

Weight of CO, as-
similated, grams . | ‘0002 | *0038 | 0048 |*0070|"or02! 0157 | ‘0238 | ‘or4g | “o102

By interpolation, the values at 0°, 10°, 20°, etc., can be found,
and the rate of assimilation is thus seen approximately to
double for every increase of 10°, the usual order of increase in

_ chemical reactions :-—?

Temperature . 0° 10° 20° 30° 37°
Amount of CO, assimilated per hour I7°5 42 89 157 238
Increased rate “for to° C, 4 2°4 2°I 18 1°8

But on the other hand, the effect of temperature on the rate
of growth of a plant as a whole is in no wise like its effect in
accelerating chemical change. Bialoblocki’s (37) results with
barley were as follows :—

.

Temperature . (0) 10° 20° 30° 40°
Dry matter formed, grams - | (nil) 7°64 8°22 3°85 0°93

The two curves are shown in Fig. 3; the difference between

1The rates were maintained only for a short time at the higher tempera-
tures.

2 A list of the papers dealing with the temperature co-efficient for cell growth
is given in Science, 6th November, 1908. See also Zaleski, W., Ber. Deut. Bot.
Ges., 1909, 27, 56-62.

2%

36 SOIL CONDITIONS AND PLANT GROWTH

them is of fundamental importance to our subject and must be
discussed in some detail.
Each of the separate processes—assimilation, respiration,

3 280
S

240

PS)
Ss
S

~~
is)
S

~
)
=)

Qo
S

40

—> Assimilation; Tenths of Mgrams of

A
\
\
We ,
Hk pee
x%
ae .
10° O° +/0° +20° +30° +40° +50°

—> Temperature

Fic. 3.—Relation between temperature and assimilation.

(Miss Matthaei.)

etc., gives, so far as is known, curves like Fig. 3, continuous
over the whole range of temperature nearly up to the death’
point. At higher temperatures it is necessary to work fora

“A
S:

——

™

Ra

-™

-Grams of Dry Matter formed
Is © A DD @

—> Temperature

+70° +20

°

+30° +40° +50°

Fic. 3a.—Relation between temperature and plant growth. (Bialoblocki.)

short period only, so as to reduce the injurious effects then
produced, but there is no break in the curve.

For the growth of the plant, however, it is necessary that
all the processes should work harmoniously together, and that

SOIL CONDITIONS AFFECTING PLANT GROWTH 37

the protoplasm should remain healthy and vigorous. Now
the. temperature range over which protoplasm lives and the
somewhat delicate adjustment of the processes holds together
is very restricted; beyond a certain point, which varies with
different plants, further temperature increases do not cause
more growth, but throw the adjustment out of gear. Thus
the curve begins to\bend over.

The student will observe a close similarity between this

TEMPERATURE

Fic. 4.—Influence of temperature on enzyme action, showing fall in quantity
but increase in activity as temperature rises. (Duclaux.)

curve and that obtained by Duclaux ' for the relation between
enzyme action and temperature. In Fig. 4 AB shows the
relation between enzyme action and temperature so long as
the activity remains unimpaired; CD shows the relation
between temperature and quantity of enzyme, the enzyme
being destroyed as the temperature rises; AOE is the resul-
tant curve showing the relation between temperature and the
activity of a given initial quantity of enzyme.

1E, Duclaux, Tvaité de Microbiologie, Tome 2, Paris, 1899.

38 SOIL CONDITIONS AND PLANT GROWTH

In this case the falling off in activity at higher tempera-
tures (OE) is due to the destruction of the enzyme; in the
case of the plant it is attributable to two factors, disadjust-
ment of processes and injury to protoplasm.

(2) Variation of Two or more Factors.

The simple case presented by single factor variation is
unusual in natural conditions: more generally two or more
factors are present in quantities insufficient for perfect growth.
This case has been discussed by F. F. Blackman who has in-
troduced the very happy phrase “ limiting factor” to express
a conception previously used by H. T. Brown under the name
“throttle valve”. Generally speaking the effect of each
separate factor is expressible by the single factor curve up to
the point where a second factor begins to be insufficient, and
then the curve alters considerably: instead of going on con-
tinuously the increase of growth falls off considerably or even
is brought to an end, A factor that thus proves insufficient
and stops or greatly retards what ought to be a continuous
process is called a “limiting factor”. Growth is once more
resumed when the amount of the limiting factor is increased
until again this factor proves insufficient, or some new factor
comes into play. .

These phenomena are illustrated by von Seelhorst’s inves-
tigations on the effect of water supply on plant growth. In
one of their investigations Tucker and von Seelhorst (256) put
up three series of soil pots in which the water was kept at a
definite amount; one was just moist, another was moister, and
a third still moister. ‘These were then each subdivided into
three others, one receiving no nitrogen compounds, another
one dose, and the third two doses. Oats were sown in all
nine sets with results that are given in Table VI.

When only little water is present the added 0°5 grm. of
nitrogen is without effect, the supply in the soil being suf-
ficient for the crop needs: the water and not the nitrogen is
the limiting factor. When more.water is added the plant can

SOIL CONDITIONS AFFECTING PLANT GROWTH 39

make more growth, and can therefore utilise more nitrogen :
the added 0°5 grm. now raises the crop by lo grms. Again,
however, the water supply sets a limit, and the second 0°5 grm.
of nitrogen is without effect. When a liberal supply of water
is added the first 0°5 grm. of nitrogen gives 20 grms. of crop,
double the previous increment; but even this does not repre-
sent the whole possibility, for the second 0°5 grm. of nitrogen
gives a still further increase of 15°5 grms.
TaBLe VI.—INFLUENCE OF WATER SUPPLY ON THE EFFECTIVENESS OF
Manures. VON SEELHORST AND TUCKER (256).

Dry Weight of Oat Crop.

Nitrogen Series, Increased Crop for
Manuring.
First Increment | Second Increment
KP. KPN. KPgN. of Nitrogen. of Nitrogen.
I. Moist soil ! 67°5 68°5 68°5 ro fe)
II. Moister soil 83°6 93°4 94°0 9°8 6
III. Wettest soil} 99°5 IIgQ‘5 135°0 20°0 15°5

K = r gram of K,O as K,CO, per pot; P = 1 gram of P,O, as Ca(H,PO,),
per pot; N = °5 gram of N as NaNO, per pot.

Phosphate Series. P ; a ay —— for
. irst - -| Complete
Manarlog. ment of P.| ment of P. | Manure.
None.| KN. | KNP. | KNoP. KNP.
I. Moist soil! | 41°5 | 38°5 | 68°5 | 79°2 30°0 10°7 27
II. Moister soil | 47°2 | 40°0 | 93°4 | 108°0 | 53°4 14°6 46°2
III. Wettest soil | 68°5 | 63°5 | 119°5 | 127°5 56°0 8 51

The results of the phosphate series are somewhat different
in detail, but not in principle. The first dose of P,O, in the
dry soil gives an increased crop, and so does the second, the
first not having been large enough ; in the wetter soil, however,
the increase is much larger. There is a still further increase in
the wettest soil, but less than before, some other limiting

1The moist soil contained 14°35 per cent. of water (41°6 per cent, of satura-
tion), the moister soil 15°41 per cent. at the beginning, increasing to 18°43 (51°7

per cent. of saturation) as the experiment proceeded, and the wettest soil, 16°44
per cent. at the beginning, increasing to 22°59 (63°7 per cent. of saturation).

40 SOIL CONDITIONS AND PLANT GROWTH

factor now coming in. These relations are shown in the
curves of Fig. 5. ,

A further illustration is afforded by experiments made by
the writer on the growth of tomatoes in pots of sand in which
supplies both of water and of nitrate were varied, The results

‘are shown in the curves of Fig. 6. The series of curves is
expressible by a surface, which is the proper way of represent-
ing the effect of two varying factors on plant growth. No
account was taken of temperature variations : to do this would

Mast Water.(22-6 %)
Q, 120
&
~
S
S
Hig
a More Water.(18-4%)
2 .
we 90
= Sole:
&
=
4
Q Little Water.(14'3%)
65 |

—>Nitrogen added over and above supply in Soil.
Fic, 5.—Influence of water supply on the effectiveness of manures. (Von
Seelhorst and Tucker.)
necessitate the construction of a series of surfaces each valid
for a particular temperature, or to adopt some mathematical
device equivalent to projection in a fourth dimension.

In all these experiments it has been assumed that the ob-
served effect is caused by the direct action of the added sub-
stance on the plant, and the assumption is justifiable because
the plant is grown in sand which is very inert. When, how-
ever, a step nearer to natural conditions is taken, and the ex-
periment is carried out in soil it is no longer safe to assume
that the soil remains unaltered while the conditions are varied,

Experiments on the growth of tomatoes in soil made

*

SOIL CONDITIONS AFFECTING PLANT GROWTH 41

grams.

Sand,
| 02

to
°o

Weight of dry matter produced,
iS

EE

3 10 Water.
Z:
=
Q -
3 or)
S 307 Soil.
2
Soe
Q.
Ol
== 20}
6
ae
Or
So
—
eg
2
Y 1 1 2 4
3 5 10 Water

Fic. 6.—Effect of varying water and nitrate supply on the growth of tomatoes
in pots of sand and of soil. (E. J. Russell.)

42 SOIL CONDITIONS AND PLANT GROWTH

simultaneously with those in sand gave a very different result
Owing to interactions in the soil which will be discussed later.
It is evident, however, that the soil must be studied dynami-
cally and not statically. While the curve is still of the
general type it differs from those obtained in sand in that
there are now a number of factors concerned and the final —
result cannot be expressed by a surface or by a series of
surfaces, but would involve something equivalent to projection
into an wth dimension, an operation not within the scope of
ordinary simple mathematics.

The general result of the quantitative work is that the

/ Luniting Factor.

: :

© &

S ny

> %

pea S

S & -
s|

S 3

a XN

Increment of Factor.

Fic. 7.—General relation between any particular factor and plant growth.
An increment in the factor causes increases in growth up to the point when
some second factor sets a limit; further increases then have no effect,
Finally, excess of the factor may cause positive injury.

numerous soil factors involved in plant growth can be dis-
entangled and separately studied. The operation of each
factor is expressed by a curve which would probably be like
Figs. I or 3, flattening out and finally bending over when
larger amounts of the factor exert a harmful effect. In actual —
natural conditions the case is complicated by the fact that two
or more factors are almost always concerned, one of which
acts as a limiting factor. The curves are of the general type
of Fig. 7.

Any possibility of simple mathematical treatment of the
amount of growth in soil is ruled out by the circumstance that |
the various factors act not only on the plant, but also on the

SOIL CONDITIONS AFFECTING PLANT GROWTH 43

soil, which in turn, reacts on the plant growth. If mathe-
matical treatment is to be attempted it must be on statistical
lines.

'The Effect on the Habit of Growth or Other Plant Char-
acteristics. :

Hitherto, we have dealt only with the actual weight of the
plant. Observations in the plant culture house or in the
field, however, show that two plants may have the same weight
and yet differ considerably in appearance, in proportion of
root or of seed to leaf and stem, in degree of maturation and
in other respects. These differences are often of considerable
technical importance, profoundly affecting the value of the
crop, and they are of great interest as indicators of soil condi-
tions. It cannot, of course, beassumed that a certain appear-
ance or character in the plant is always and necessarily
produced by the same soil conditions, but the appearances are
often symptomatic and serve to narrow the problem. The
detailed discussion of these differences is the province of the
modern science of Ecology, but the general results are of great
‘importance to the soil student... They will be dealt with
under the headings of the separate factors.

Effect of Water Supply.

Water.—The relationship between the amount of growth
and the supply of water is shown by Hellriegel’s experiments
(130a, Table VII.) with barley grown under favourable con-
ditions in sand cultures. ;

The yield rises as the water increases up to a certain point,
and then falls off because the excess of water reduces the air
supply for the roots. |

The grain suffers sooner than the leaves and stems. When
a series of plants is grown in this way, with varying water
supply, certain important qualitative differences are revealed.

1 For an example of a survey carried out on these lines, see T. H. Kearney ;
L. J. Briggs, H. L. Shantz, J. W. McLane, and R. L. Piemeisel, Indicator

Significance of Vegetation in Tooele Valley, Utah, Fourn. Ag. Research, 1914,
I, 365-417.

44 SOIL CONDITIONS AND PLANT GROWTH

Those receiving only small amounts of water, have small
glaucous leaves and tend early to form seed. As the water
supply increases, the root system increases rapidly both in
extent and in fineness ; with further supplies, the leaves be-—
come successively larger and greener in colour, ripening be-
comes delayed, but the root system becomes restricted and
alters in character, finally consisting of a few stout roots only.
Von Seelhorst (2574) has made quantitative determinations of
the proportions of root system to entire plant under conditions
of varying water supply.
TaBLE VII.—GrowTH oF BARLEY WITH VARYING SupPLY OF WATER.
HELLRIEGEL (1304).

Amount of water . -1 5 to 20 30 40 60 80
Dry matter in grain, gms. | nil 0°72 | 7°75 | 9°73 | t0°5r | 9°96] 8°77
Dry matter in straw, gms.| ‘12 | 1°80 | 5°50 | 8°20] 9°64|1r'00| 9°47
I grain weighed, mgms. — | 23 35 | 36 34 32 32

100 represents the amount of water required to saturate the sand.

These morphological differences are accompanied by dif-
ferences in chemical composition of the plant, Even the seed
is affected, a phenomenon which does not usually occur when
other factors such as supplies of plant nutrients are varied.
Thus the composition of the wheat grain is hardly affected by
manuring, but it is profoundly altered by variations in water
supply! (Shutt, 265). Other instances have been recorded

1C, H. Bailey, Minnesota Expt. Sta. Bull., 131, 1913, has drawn up the
following table, showing the protein content of the wheat and flour of the
hard spring wheat grown in sixteen counties in Minnesota (rst April to rst
September, 1911).

Rainfall. Protein Protein

per cent. per cent.

: Wheat. Flour.

Between 12-13 ins. 14°93 13°47
” 14-15 95 13°73 12°61

‘ 16-17 +, 12°21 12°56

» 18-19 ,, 13°42 12°29

Ay 20-21 ,, 12°88 11°87

aly 22-24 4, Ir'63 10°65

Stewart and Hirst, Utah Expt. Sta. Bull., 125, p. 145, obtained the follow-
ing results at Utah, showing the effects of irrigation. a

SOIL CONDITIONS AFFECTING PLANT GROWTH 45

by von Seelhorst (257d), and by Pfeiffer, Blanck, and Friske
(224e).

In soil, other. and indirect factors come into play ; too much
water causes the exclusion of air; certain reduction products
are then formed by bacteria which have a direct toxic effect
on the plant.

These relationships are constantly recurring in field obser-
vations. On moist soils—clays and loams—the plants usually
have large wide leaves and grow to a considerable size, whilst
on the drier sands the vegetation is narrow leaved and more
stunted, but the root system is well developed.t A copious
water supply leads to a more protracted growth and to a re-
tardation of the ripening processes ; indeed, in very wet dis-
tricts, wheat and barley are grown only with difficulty, if at all,
because ripening may be so long delayed that frosts supervene
and damage the crop. Oats are less affected, as they are
usually cut before they are ripe.

Water supply and temperature are the two chief factors
determining the distribution of crops. In the warm, dry,
eastern counties of Great Britain crops are grown for seed ;
great quantities of wheat and barley are grown in Norfolk,
Suffolk, and the Isle of Thanet; mangold seed and turnip
seed is produced in East Kent. Wetter districts are more
favourable for swedes and oats; very wet districts for grass.
The warm, moist, south-west of Cornwall is very favourable
for early vegetables, cabbage, cauliflower, etc., whilst the cooler
Lincolnshire and Cheshire are well suited to potatoes. It is
possible by suitable operations to modify somewhat both the
temperature and the water content of the soil, and so to make

Footnote continued from opposite page—

Irrigation. Protein per cent.
25 ins. 16°23
5» | 12°92

No irrigation. 13°62

For the small effect of soil conditions see Washington Ag. Expt. Sta. Bull.,
trr, and A. D. Hall, Fourn, Bd. Agric., 1904, 2, 321. Sodium hydrate and
magnesium oxide affect the composition (pp. 73, 75).

1See p. 310 et seq.

46 SOIL CONDITIONS AND PLANT GROWTH

the soil conditions rather more favourable for any particular
crop.

Some interesting results are obtained in glass house practice.
Tomato growers have learned to regulate water supply and
temperature in such a way as to produce compact bushy
plants, which they know by experience give more fruit than
the softer, larger plants, obtainable under other conditions.
Until the blossom is fertilised or has “ set,” therefore, vigorous
growth is not encouraged, and, in many cases, while the atmos-
phere is artificially damped, water is actually withheld from the
roots until, in the picturesque language of the grower, ‘ the
plants cry for it”. After ‘‘ setting,” water is liberally supplied
and top deisides of manure are given.

It is impossible at present to make a complete analysis of
these phenomena. The osmotic pressure of the plant cell is
known to alter with changes in moisture content of the soil,
and it may, and probably does, react on other characteristics

of the plant. Some of Iljin’s results (141) are given in
Table VIIL

TaBLE VIII.—Cuance in Osmotic PRESSURE OF PLANT CELL WITH
CHANGES IN MoIsTtuRE CONTENT OF SOIL.

Ratio of Soil Moisture Osmotic Pressure in Normal Solution of NaCl.1
to Absolute Water Helianthus annuus. Zea Mays.
Capacity of Soil.

80 per cent. 0°I4-0°16 O°L7
OO 5 0°25-0°28 o°Ig
3° ” O°4I-0°45 0°49

Effect of Soil Conditions on Consumption of Water by
the Plant. |

In the experimental work just described the water content
of the soil is kept artificially constant, thus eliminating the
effect of the rate of consumption by the plant. In natural
conditions, however, the supply is not maintained constant
and the rate of consumption therefore becomes an important
factor. Many determinations have been made of the weight
of the water transpired per gram of dry matter formed, the figure
being called, the transpiration coefficient. This mode of

10°r normal NaCl has an osmotic pressure of approximately 4 atmospheres.

SOIL CONDITIONS AFFECTING PLANT GROWTH 47

expression is convenient and it gives useful information to the
practical grower in irrigated districts, but from the strictly
scientific point of view it suffers from the disadvantage that
it implies a causal relationship between transpiration and as-
similation when in reality there is none.

The transpiration coefficient is not constant but varies
with the plant and the conditions, increasing with the tempera-
ture and to some extent with the water supply, but decreasing
as the food supply increases.

TaBLe IX.—TRANsPIRATION COEFFICIENTS, i.e. AMOUNT OF WATER TRANS-

PIRED DURING THE PRODUCTION oF ONE Part oF Dry MATTER. BrIGGS
AND SHANTZ (55c).

Crop Extreme Values for Mean Value
i Different Varieties. for Genus.
Proso . - f x ‘ ‘ 268 to 341 293
Millet . 3 F r : ; 261 to 444 3I0
Sorghum. 4 ; 4 A 285 to 467 322
Maize . ‘ ; is : 315 to 413 368
Wheat . Fi A ; ‘ 4 473 to 559 513
Barley . ‘ : ; ‘ ‘ 502 to 556 534
ats ***; - F é d ; 559 to 622 507
Flax. i f é 4 ‘ —— 905
Sugar beet . 3 ‘ ‘ ; — 307
Potato . 4 : pF; . . — 636
Cow pea ; ‘ 4 . — 571
Clover . ; $ : = ; 789 to 805 797
Lucerne P ‘ 5 ; ; 651 to 963 831
Grasses } . ; — 861
Various native plants (i.c. weeds) 277 to 1076 —

With the plant variations we are not concerned. It is
demonstrated, however, that a difference exists between
different varieties of the same crop and that there are con-
siderable prospects for breeding or selecting varieties specially
suited for dry conditions. This work is already in hand
with good results in Australia, the Western States and else-
where. Unfortunately no correlation has been traced between
water requirements and plant structure, so that the breeder
has no guide in his selection except actual and tedious trials.

The effect of soil conditions has been studied at the Be-
sentchuk Agricultural Experiment Station, Samara, Russia,
situated in a district which suffers from prolonged summer
drought and excessive variations in crops. The transpiration

48 SOIL CONDITIONS AND PLANT GROWTH

coefficient is found to vary from year to year with the external

meteorological conditions, being greatest in dry years and

lowest in wet years; it is also higher in wet soils than in dry

ones. Toulaikoff considers that it is these conditions, rather

than the biological character of the plant, that determines the

magnitude of the coefficient (284).
Some of his results are :—

IgIl. Igi2. 1913. 1914.

Wheat, var. Polltawka, . 628°4 444°5 338°6 387°6
» » Bieloturka’ 756°3 475°9 316°5 - 397°E
Oats sa aodant, ih, 655°1 510°3 347°4 369°9
Barley ,, Moravian ; 617°9 461°6 230°3 413°3

The year 1911 was excessively dry, 1913 was very rainy :
1912 was an average year and 1914 was rather dry.

The effect of variations in water and food supply on the
water requirements of plants was studied by Hellriegel, and
subsequently by von Seelhorst at Géttingen (256-258), who
has worked extensively at the various water relationships of
plants. His results with oats are given in Table X.

TaBLE X.—ErFrect oF VARYING WaTER SuppLy! anp Foop SupPLY ON THE
WATER REQUIREMENTS OF OaTs. VON SEELHORST (257@).

Dry Matter Produced Total Water Transpired,{ Water Transpired per

Grams, : Grams. Gram of Dry Matter,
F : Soil : A Soil . . Soil
Soil Soil still Soil Soil still Soil Soil still

Moist. |Moister. Motiter Moist. |Moister. Moister | Moist. |Moister. Mojater.

No manure | 39°6 | 48°8 | 52°6 | ro°215 |15°245 |16°290 | 259°9 | 312°9 | 307°1

Complete
manure. | 49°9 | 86°7 | g5‘r | 11°r70 |20°490 |23'030 | 225°1 | 236°8 | 231°6

1 The variations in water supply were :—

5 to 12 May. | 12 May tor June. | 1 June to x July.

Soil moist . ; 54°4 54°4 44°8
Soil moister . 3 59°2 64°0 59°2
Soil still moister . 64/0 | 73°9 73°6

Where t00 = saturation of the soil.

SOIL CONDITIONS AFFECTING PLANT GROWTH 49

Similar results have been obtained by Wilfarth (Table XL.),
5 (309a) with sugar beets grown in pots of soil containing known
but varying amounts of nitrate.

- Tasce XI.—Errect or VaryinGc Foop Suppty oN THE WATER REQUIRE-
MENTS OF SuGAR BEET. WILFARTH.

Oe ee ee ee! COC
ee Cae ee Emte :

aig supplied, grams . "42 1°26 2°10 2°94 3°36 3°78
Weight of dry matter pro-
duced, grams . ; -| 23°0 | 73°99 | 96°5 | 132°4 | 167°6 | 188°8
Water transpired, grams. | 13,100 | 34,570 | 39,420 | 55,190 | 62,600 | 72,280
Stated as inches ofrain ./ 3°6 9°4 | I10°7 15°38 | I7°0 19°6
Water used per Pisa of dry
E matter formed 569 468 409 417 374 383
j s
j 1S Tons
- 77 5 tows j
“1 ane pr De se ty
. Sr.” s
: F
4 "si
» 5}
i B i orpece. |
i a Serpent
a. |
< 3}
°
-
2}
I 5
0 5 10 20 30 40
: Water applied.

Fic. 8.—Effect of water supply on the effectiveness of farmyard manure.
Yield of maize (stover and grain) in tons per acre for o to 40 inches irrigation
water. (Harris and Butt, Utah, 123.)

Two deductions may be drawn: (1) water is economised
by increasing the food supply ; (2) the total amount of water
required during the growing season may be greater than is
supplied by the rain, in which case the balance must be
otherwise provided, or the food cannot be utilised,

4

*

50 SOIL CONDITIONS AND PLANT GROWTH

Over large areas of the world the rainfall is insufficient,
and recourse is had to irrigation. In endeavouring to ascer-
tain the best way of irrigating crops two considerations have
to be kept in view: (1) excessive watering has secondary
injurious effects on the soil, such as the deterioration of the
physical condition, the accumulation of alkali salts, or the
formation of toxic reduction products; (2) the requirements
of the plant are not always the same, more water being needed
during the period of active growth than during germination
or ripening.

Some of the results obtained in Utah are set out in
Table XII. and in Fig. 8 :—

TaBLE XII.—AvERAGE YIELD oF Dry MaTTeR AND NITROGEN FROM THE
EXPERIMENTAL PLOTS ON GREENVILLE Farm, UTAH. GREAVES, STEWART,
AND Hirst (1136).

(Expressed as lbs. per Acre.)

Lucerne. Potatoes.
Water Applied.
Hay. Nitrogen. Tubers. Nitrogen.
ins.
37°5 10,464 282°5 1,464 20°4
25°0 9,963 265°0 1,540 24°8
15°0 9,779 259°L 1,759 _ 33°2
None 6,808 I70°L 1,075 Ig‘t
i Oats. Basted Maize.
Water Applied. :
Grain. Straw. Nitrogen. Grain. Stover. Nitrogen.
f ins.
37°5 2,273 | 2,989 89°5 2,080 | 3,316 66°3
25°0 2,093 | 2,582 83°9 1,995 | 35332 69°6
15°0 1,885 1,821 9t°7 2,179 3,605 76°6
None 1,560 1,928 64°0 1,600 3,280 62°3

Field experiments like those conducted by the Punjab
Irrigation Department! have shown that the cultivator every-

1 These and similar experiments are discussed by A. and G. L. C. Howard
in Wheat in India: Its Production, Varieties, and Improvement (Imperial Depart-
ment of Agriculture, India, 1909). German experience is recorded in Ervfahrung
bei der Ackerbewdsserung (Fahrb. Deutsch. Landw. Gesell., 1913, 28, 76).

SOIL CONDITIONS AFFECTING PLANT GROWTH 51

where tends to take too much water, with loss not only to
others on the same irrigation system, but also to himself.

Air Supply.

It is well known among farmers and gardeners that soil
aeration is essential to fertility but exact measurements are
difficult to obtain. The phenomena are more complex than
appears at first sight, involving two wholly distinct factors :—

_ I. The necessity of a supply of oxygen to the plant root.

2. The toxic effect of the carbon dioxide which invariably
accumulates in a non-aerated soil or other medium.

‘Moreover, plants vary considerably in their sensitiveness to
these factors.

_The simplest case is seen in water cultures where aeration
produces marked effects. In the experiments of Hall,
Brenchley, and Underwood (12Ic) the amounts of dry matter
produced were as follows :—

Barley, Lupins,
Grams per Grams per
Plant. _ Plant.
Non-aerated a ‘ a I°314 0°83
Continuously aerated 5 y 2°I22 1°53

The more recent experiments of E. E. Free (100a@) show
that plants do not all stand in equal need of oxygen. Buck-
wheat was grown in water cultures through one set of which
was blown air, and through others nitrogen, oxygen, and
_ carbon dioxide respectively. No difference was observable
between the plants supplied with nitrogen and those supplied
with air or oxygen: they all grew normally to maturity. In
this case, therefore, the root apparently can dispense with
gaseous oxygen. When, however, carbon dioxide was given
the plants sickened and wilted within a few hours, and died
in a few days.

Stiles and Jorgensen (272) have confirmed this difference
between barley and buckwheat.

Soil experiments are more difficult to carry out. B. E.

4°

52 SOIL CONDITIONS AND PLANT GROWTH

Livingstone and E. E. Free (178c) grew plants in soil so
sealed that its atmosphere could be controlled. Different
plants varied in their susceptibility to the exclusion of
oxygen ; Coleus blumet and Meliotropium peruvianum were
the most sensitive, the intake of water in their roots ceasing
within 12 to 24 hours owing to death of the roots and the
entire plants ultimately died when oxygen was replaced by
nitrogen. Salix nigra, on the other hand, successfully en-
dured the exclusion of oxygen.

Application of these general results to field conditions
have been made in India by A. and G. L. C. Howard (139),
who have shown that increased soil aeration resulting from
lessened irrigation or addition of potsherds to the soil leads
to increased plant growth. Some of their results are given in
Table XIII. :—

TaBLE XIII.—Tue Errect or DILUTING THE PusA SoIL WITH POTSHERDS
oR Sand. A. AND G. L. C. Howarp (139).

3 Yield per Acre
Yield per Acre : : Increase Percentage
Crop. of Control Plot. yee ee f per Acre. Increase.
Ib. lb. lb.
Oats . 1954 2312 358 18
Wheat 1316 1580 264 rae
Tobacco 1680 1846 166 Io

EFrectT OF REDUCING IRRIGATION AND THEREBY INCREASING AIR SUPPLY.

WHEAT.

Three Waterings.

Two Waterings.

One Watering.

Yield of wheat:
Grain lb.
Straw, etc. Ib.

1222
1764

1302
2004

788
1714

Hall, Brenchley, and Underwood (121@) found considerable
differences in the amount of growth of lupins in sand and
kaolin on the one hand, where ample aeration was possible,
and fine sand, silt, or water culture on the other where there

was much less aeration. Their results are :—

I oe

r

‘(pizx) sauidny jo yuaudojaaap 3001 uo wNIpaw'yo aouanyuy—"6 *or,7

SOIL CONDITIONS AFFECTING PLANT GROWTH 53

Weight of Dry Matter
Formed, Grams.

Material allowing diffusion of air: Coarse sand . 2°474
: Kaolin . 1°833
~ Material allowing less diffusion of air: Fine sand I'r3r
Silt . F I°393
Water . I'942

The results are illustrated in Fig. 9.!

Temperature.

It is difficult to separate soil temperature effects from
temperature effects in general, and in practice the distinction
is unnecessary since the temperature of the air is largely de-
termined by that of the soil.

The effect of temperature shows itself in three ways :-—

I. It profoundly affects the rate of growth of plants, and
thus determines what plants can be grown and what. cannot
be grown in a given area.

If the temperature is too low a yellow or purplish colour
May appear in the leaf, and the plant grows so slowly that
it is liable in its early stages to succumb to insect pests, such
as wireworms, and in its later stages to be cut down by
autumn frosts before it has time to ripen. If, on the other
hand, the temperature is too high, the plant becomes taller
than usual, less robust and, when much water is also supplied,
liable to fungoid pests that prove very troublesome in com-
mercial glasshouses. Only over a comparatively restricted
range of temperature is it possible to obtain. the compact
sturdy habit aimed at by the grower. This favourable range
has not yet been correlated with other properties of the plant
and has to be discovered empirically; it is, on the whole,
lower for the seedling than for the growing plant, but it is
highest for the period of maturation. It varies for different
crops ; wheat requires a cool time for sowing but a hot time
for ripening, barley requires a cool, and oats a still cooler

1 For other papers see C. A. Shull, Bot. Gaz., 1911, 52, 453-477, and W, A,
Cannon and E, E, Free, Science, 1917, 45, 178-180.

54 _ SOIL CONDITIONS AND PLANT GROWTH

time throughout. It varies even for different varieties of the
same crop; plant breeders are continually trying to evolve
strains suited to particular ranges, e.g. wheats have been bred
at Ottawa to ripen in the northern parts of Canada.

2. It affects the lengths of the periods of vegetative
growth and of maturation and therefore causes certain modi-
fications in the plant itself.

A long ripening period gives wheat a plump kernel with
a low percentage of protein, while a short ripening period
gives an increased protein content (Lawes and Gilbert in
1857, 166c). |

Turnips in the south of England not only make less growth
than in the north, but have a somewhat different composition.
Oat straw in Scotland differs in composition from that in
England, the translocation of material to the grain being
apparently less complete.

3. The temperature at the time of ripening profoundly
affects the germination capacity of the seed.*

Light.

Although light is not a soil factor it nevertheless indirectly
affects the soil by modifying the flora which, as we shall

1¥For a discussion of the physiological effects produced by temperature see—

Action de la Chaleur et du froid sur Vactivité des étres vivants: par Georges
Matisse. Paris (Larose), 1919.

Bull. Internat. Inst. Ag. Rome, 1917, 8, 340.

Darwin, Francis, On the growth of the fruit of ‘‘ Cucurbita” (Annals of
Botany, 1893, 28, 459-487).

Leitch, I., Some experiments on the influence of the rate of growth of ‘* Pisum
sativum” (Ann. Botany, 1916, 30, 25-46).

Lehenbauer, P. A., Growth of maize seedlings in relation to temperature
(Physiol. Researches, 1, 247-288, 1914).

Lepeschkin, W. W., Zur Erkenntnis der Einwirkung supramaximaler
Temperaturen auf die Pflanze (Ber. Deutsch. Bot. Gesell., 30, 703-714,,
Igi2).

Maximow N. A., Chemische Schutzmittel der Pflanzen gegen Erfrieren (Ber.
Deutsch. Bot. Gesell., 30, 52-65, 293-305, 504-916, 1912).

Groves, J. F., Temperature and life duration of seeds (Bot. Gazette, 63,
169-189, 1917).

Livingstone, B. E., and Livingston, Grace J., Temperature Coefficients in
Plant Geography and Climatology (Bot. Gazette, 1913, 56, 349-375. Abs,
in Fourn. Ecol., 1914, 2, 179).

SOIL CONDITIONS AFFECTING PLANT GROWTH 55

subsequently see, largely determines the nature and amount
of the organic matter of the soil. Tall vegetation keeps off
light from the lower growing plants and more or less sup-
presses them. Thus on the Rothamsted grass plots clover is
seriously reduced in amount by nitrate of soda which causes
tall growing grasses to flourish: Lathyrus, on the other hand,
in consequence of its tall growing habit is not adversely
affected but grows vigorously. Numerous other instances are
recorded in the Journal of Ecology.. The extreme case is
seen in wood-land where there is very little undergrowth and
where, therefore, organic matter has not accumulated in the
soil. Adjacent pieces of land at Rothamsted, both untreated
and differing only in the flora, showed the following differences
in composition :—

Long Established. Established 40 Years.

Open Land.
Flora of Grass and
Clover.

Open Land.
Flora of Grass and
Clover.

Wood-land.
No Green Plants.

Wood-land.
No Green Plants.

Organic matter—

o-— gins. 85 6°7 79 8°r

g-18 » 4°8 48 6°7 5'2
Nitrogen—

o- gins. 0°256 o°185 o'182 0°L73

g-18 ,, 0°097 ©7093 0°084 o*081

One of the most effective ways of suppressing weeds is to
grow a heavy crop which, in the farmer’s language ‘‘ smothers ”
them by excluding light and by exerting certain root effects.

Food,

The nutrition of plants is complicated by the fact that
plants synthesise their own food from various substances
taken out of the air and the soil. It is common in farmers’
lectures to speak of these as the actual foods, but the student
must be perfectly clear in his own mind that they are only
the raw materials out of which the food is made. It is

1See e.g. E. Farrow, ¥ourn. Ecology, 1916, 4.

56 SOIL CONDITIONS AND PLANT GROWTH

convenient to make a distinction between the elements neces-
sary in large quantities, and those of which mere traces suffice :
the effect of the former can readily be demonstrated in water and
sand cultures ; the latter are more difficult to study, as traces
are always present in the seed, and often also in the nutritive
medium, or the vessel in which the plant is grown.

The substances needed in quantity are carbon ducal
water, oxygen, and suitable compounds of nitrogen, phos-
phorus, sulphur, potassium, calcium, magnesium, and iron. Of
these nitrogen, phosphorus, and potassium compounds are re-
quired. in such large amounts that they usually have to be
added to soils as artificial fertilisers in order to obtain maxi-
mum yields in agricultural practice. The other nutrient ele-
ments are generally present in air, in soils or in rain, in sufficient
amounts to exert their full effect.

In addition to the above substances, small amounts of
manganese and silicon are known to be beneficial: recent
evidence suggests that others may be also. The list of ele-
ments of which traces only are needed has been much extended
in an important investigation by Mazé (197), who includes
boron, fluorine, iodine, chlorine, aluminium, and zinc. Evidence
is steadily accumulating that these substances, in suitable traces,
are beneficial to plant growth, as will be shown below.

Between nutritive effects and toxicity the margin appears
to be narrow, and almost all the elements essential to plant
nutrition are capable of producing toxic effects under other
conditions. Inthe case of the major nutritive elements toxi-
city occurs only when a sufficiently large excess is present
to alter considerably the osmotic relationships or the proper
physiological balance of the nutrient medium : the transition
from beneficial to harmful effects is very gradual and proceeds
through the long inert stage shown in Fig. 7. In the case of
elements of which only small amounts are needed the transition
is much sharper: the limits are easily overstepped and toxic
effects set in when the minute beneficial amount is exceeded.

Carbon.—It is generally assumed that plants derive all
their carbon from the air, but the French investigators have

\

SOIL CONDITIONS AFFECTING PLANT GROWTH 357

persistently held the view that some may come from the soil
(p. 15). The physiological work has usually been done in
water cultures, Knudson! finds that saccharose, glucose,
maltose, and fructose are directly absorbed and utilised by
green maize, Canada field pea, timothy, radish, vetch, etc.,
giving rise to a characteristic branched root system. While
these substances do not occur in the soil other soluble carbon
compounds are present, especially in glasshouse soils, and may
exert important effects. 4

There is considerable evidence, however, that by far the
larger portion of the carbon of the plant is taken up from the
atmosphere and not from the soil, but the phenomena are not
wholly independent of the soil. The amount of carbon dioxide
in the atmosphere is subject to slight variations which may
arise from variations in biochemical activity in the soil, and
may be a factor of importance in crop production. Brown and
Escombe (59a) found that the amount varied at Kew from 2°43
to 3°60” volumes per 10,000 volumes of air, the average being
2:94. Taking the month of July as an example, the average
values were :—

1898. 1899. 1900. IgOI.
' CO, in 10,000 volumes of air . 2°83 2°88 2°86 3°11

It is probable that the plant asa whole would respond to
variations of this order, making greater or less growth as the
amount of carbon dioxide rises or falls.*

Nitrogen.—Of all the nitrogen compounds yet investigated
nitrates are the best, and, in natural conditions, probably the
only nitrogenous food for non-leguminous plants. The seed-
ling, still drawing its sustenance from the seed, lives on other
compounds: H. T. Brown (59) found that asparagine was the
most effective nutrient for the detached embryo of barley,
followed by other relatively simple substances like nitrates,
glutamic and aspartic acids, ammonium sulphate, etc., the more
complex substances being less useful. The experimental

1Cornell Repts. (Ithaca, N.Y.), 1917, 747-813 (Memoir g of 1916).
2 Only on one occasion was so high a number obtained.
See E. Demoussy (83) and Otto Warburg, Biochem. Zeitsch., 1919, 100, 230,

58 SOIL CONDITIONS AND PLANT GROWTH

study of the nitrogen nutrition of adult plants is complicated
by the difficulty of growing plants under sterile conditions in
which the decompositions effected by bacteria are obviated ;
much of the earlier work is vitiated by this circumstance.
Later work has satisfactorily shown that ammonia is readily
assimilated from solutions of ammonium sulphate if the con-
centration is not too high; but even o'1 per cent. was found
injurious by Mazé (196). Kriiger (1564) concludes that am-
monium sulphate is less beneficial than sodium nitrate for
mangolds ; both compounds are equally useful for oats, barley,
and mustard, while ammonium sulphate is better for potatoes.
Brigham? maintains that maize also thrives better on am-
monium sulphate than on sodium nitrate. Hutchinson and
Miller (140@) found that peas assimilate nitrates and ammon-
ium salts equally well, while wheat showed a decided preference
for nitrates. Séderbaum? found that the effect varied with
the salt: ammonium phosphate was more beneficial than the
sulphate, while the chloride was harmful.

An interesting attempt has been made by Prianichnikow .
(2290) to elucidate the phenomena of ammonia assimilation by
plants. He supposes that the ammonia taken up by the roots
is transformed in the plant into asparagine, which is then con-
verted into protein. Some plants, e.g. barley, maize, pumpkin,
etc., readily take up ammonia and effect the conversion ;
others, such as peas and vetches, do so only in presence of
calcium carbonate ; whilst others, such as lupin, will not nor-
mally take up ammonia at all. He suggests that this difference
is due to the different quantities of carbohydrates at the dis-
posal of the plant ; by increasing or diminishing the amount
of carbohydrate it is possible to pass from one type of assimi-
lation to another. Ina state of inanition the power of forming
asparagine is lost; with plentiful supply of carbohydrate, on
the other hand, even plants of the lupin type could absorb
ammonia and convert it into asparagine. Asparagine can
accumulate in the plant without detriment, and can be built

1 Soil Sct., 1917, 3, 155-
2 Kungl. Landt. Handlingar, 1917, 56, 537-561.

SOIL CONDITIONS AFFECTING PLANT GROWTH 59

up into protein when sufficient carbohydrate is present. This
accumulation would account for the fact on which all experi-
menters agree, that plants fed on ammonium salts contain a
higher percentage of nitrogen than those fed on nitrates (Table
XIV.).

TaBLeE XIV.—PeERcENTAGE oF NITROGEN IN Dry MarTer oF Pants.

Fed on Ammonium

Fed on Nitrates. alts.

Observer.

Maize . ‘ 3°17 3°43 Mazé (196)

Mustard ‘ 2°87 3°48 Kriiger (156)

Oats . ‘ 1°80 2°05 ie a

Wheat . ‘ I'gl 2°17 Hutchinson and Miller (140a)

Nitrites are also assimilated so long as the solution is not
too concentrated or too acid.’

In spite of a considerable amount of work it is not known
whether nitrogen compounds other than nitrates and am-.
monia are assimilated by plants. That many other com-
pounds serve as nitrogen nutrients, even without the inter-
vention of bacteria, seems to be certain (140d), but it has
never been shown whether assimilation of the compound
as a whole takes place, or whether there is decomposition at
the surface of the root. Many of the supposed assimilated
compounds are as a matter of fact more or less easily hydrolys-
able, or otherwise decomposable, with formation of ammonia,
and the decomposition will obviously proceed as fast as the
ammonia is removed by the plant. Two factors that determine
how far a given compound serves as a nitrogen nutrient are:
(1) the ease with which it splits off ammonia, (2) the effect on
the plant of the other decomposition products : if these happen
to be toxic the whole process stops as soon as they have suf-
ficiently accumulated.

The normal nitrogenous food of plants is, however, a
nitrate, and there is a close connection between the amount
supplied and the amount of plant growth, which is well shown
in Hellriegel and Wilfarth’s (130c) experiments (Table XV.).

1See Perciabosco and Rosso, Staz. Speriment. Agrar. ital., 1909, xlii., 5,

\

60 SOIL CONDITIONS AND PLANT GROWTH

TABLE XV.—EFFEcT oF NitrrRoGENouS Foop Supply oN THE GROWTH OF
BarLey IN SAND CULTURES. HELLRIEGEL.

Milligrams of nitrogen supplied | 56 II2 168 280 420
Dry matter in crop, grams. | *742 | 4°856 | 10°803 | 177528 | 21-289 28°727
Increased yield for each extra *
56mgms. nitrogen. — 4°II4| 5°947| 6°725| 1°880| 2°975
Grain, per cent, of aad matter
in crop x -| II°9 | 37°9 38 42°6 38°6 43°4
Weight of one grain, mgms. - | 19°5 | 30 33 32 21 30

The figures are plotted in Fig 10. Similar results are ob-
tained on the field plots at Rothamsted (Table XVL.).

a
§
§ *
¢ B
~
S oe
3 76
ss
S
~
RQ
ee
S
=
= A
3 oO
@) W2 224 336 “. 448

M.gms. N supplied as Ca (N03).

Fic. 10.—Effect of nitrogenous food supply on the growth of barley.
(Hellriegel.)

TABLE XVI.—BROADBALK WHEATFIELD, AVERAGE YIELDS, FIFTY-SIxX YEARS,

1852-1907.
Plot 5. | Plot 6. Plot 7. Plot 8.
Nitrogen supplied in manure, lb. per acre . ° 43 86 129
Total produce (straw and grain), lb. per acre | 2315 39048 | 5833 7005
Increase for each 43 lb. nitrogen : ; — 1633 1885 I172

The increasing effects produced up to a certain point by
successive increments of nitrogen may be due to the circum-
stance that the additional nitrate not only increases the con-

SOIL CONDITIONS AFFECTING PLANT GROWTH 61

centration of nitrogenous food in the soil, but also increases
the amount of root, ze. of absorbing surface, and of leaf, ze.
assimilating surface. The process thus resembles autocatalysis,
where one of the products of the reaction acts as a catalyser
and hastens the reaction. The increase does not go on in-
definitely because some limiting factor steps in. |

The effect of nitrogen supply on the grain is very marked.
In Table XV. it is seen that the grain formed, when nitro-
genous food is wholly withheld, is only two-thirds of the normal
weight per, ‘individual. The first addition of nitrate causes a
marked rise in the weight per grain and the proportion of
grain ‘to total produce, but successive additions cause no further
rise, Indeed other experiments prove that excess of nitrogen-
ous food « causes the proportion of grain to fall off somewhat.
The leaf and the general character of growth are affected to a
much greater extent. Nitrogen starvation causes yellowing of
the leaf, especially in cold spring weather, absence of growth,
and a poor starved appearance generally: a moderate supply
of nitrogen leads to more rapid growth, very useful in cold
weather or in case of attacks by insect pests. Abundance of
_ nitrogen, on the other hand, leads to the development of large
dark green leaves which are often crinkled, and usually soft,
_sappy, and liable to insect and fungoid pests (apparently
because of the thinning of the walls and some change in com-
position of the sap) and to retarded ripening: the effects
resemble those produced by abundant water supply. A series
of plants receiving varying amounts of nitrate are thus at
somewhat different stages of their development at any given
time, even though they were all sown on the same day, those
supplied with large quantities of nitrate being less advanced
than the rest. If they could all be kept under constant con-
ditions till they had ripened this difference might finally
disappear, but in crop production it is not possible much to

delay the harvest owing to the fear of damage by autumn.

frosts, so that the retardation is of great practical importance.
Seed crops like barley that are cut dead ripe are not supplied
with much nitrate, but oats, which are cut before being quite

62 SOIL CONDITIONS AND PLANT GROWTH ~

ripe, can receive larger quantities. All cereal crops, however,
produce too much straw if the nitrate supply is excessive, and the
straw does not commonly stand up well, but fs beaten down
or “lodged” by wind and rain. Swede and potato crops also
produce more leaf, but not proportionately more root or tuber,
as the nitrogen supply increases ; no doubt the increased root
would follow, but the whole process is sooner or later stopped
by the advancing season—the increased root does, in fact, follow
in the case of the late-growing mangold. Tomatoes, again,
produce too much leaf and too little fruit if they receive excess
of nitrate. -At the Cheshunt Experiment Station ! the omission
of nitrogen compounds from the fertiliser mixture has caused
the yield of fruit to increase 11 per cent. With the variety
Comet the following quantities of fruit have been obtained :—

Lb. per plant. Tons per acre, " |Relative Weights,
Average, :
1916-1919.
1916.| 1917. | 1918. | 1919. | 1916. |. 1917. | 1918. | 199.
Complete artificials | 4°9 | 5°11 | 3°32 | 5°57 | 38°7 | 35°8 | 25°8 | 42°2 100
No nitrogen . - | 5°7| 5°60 | 3°62 | 5°98! 45°0 | 39°2 | 28°2 | 47°4 IIL

On the other hand, crops grown solely for the sake of their
leaves are wholly improved by increased nitrate supply : growers
of cabbages have learned that they can not only improve the.
size of their crops by judicious applications of nitrates, but
they can also impart the tenderness and bright green colour
desired by purchasers. Unfortunately the softness of the
tissues prevents the cabbage standing the rough handling of
the market. These qualitative differences are of great import-
ance in agriculture and horticulture. .

Three cases are illustrated in Table XVII.; as the nitio-
gen supply is increased wheat shows increases in straw greater
than those in grain; white turnips show increases in leaf
greater than those in root, but mangolds show substantially
the same increase both in leaf and root, because their growing

1 Annual Reports for 1917 e¢ seq.

SOIL CONDITIONS AFFECTING PLANT GROWTH 63

period i is so much longer than that of the other crops, con-
tinuing until the end of October.

TaBLE XVII.—Errect oF VARYING SupPLY OF NITROGENOUS MANURE ON
THE GROWTH OF Crops. ROTHAMSTED.

Wheat 1000 Ib. White Turnips, |Mangolds, 1000 lb.
3 - |- per acre * :y | 1000 lb. per acre | n; . per acre.
Manore th, | (852186). | Muncse tb, | (2845-1848) [Manure sib, | (1906-1910).
per acre. Den Are, | ae PEL ACTS.
Grain. | Straw. Roots. | Leaves. Roots. | Leaves.
none 1°06 | 1°86 none 18°37 | 6°05 none Tr°84 | 2°55

43 1°68 | 3°03 47 22°18 | 9°63 86 4o°r2 | 851
86 2°18 | 4°28 137 22°96 | 13°78 134 65°67 | 13°88
129 2°27 | 4°78 — — — — — _-
172 2°29 | 5°22 — — — —_— * — —

The actual increase of growth brought about by successive
increments of nitrogenous food depends on the amount of
water and other nutrients, on the temperature, and so on; any
of these may act as limiting factors. Table XVIII. shows the
crops obtained on some of the Rothamsted mangold plots; in
one case the supply of potassium is so small that it becomes
the limiting factor, in the other sufficient potassium is supplied.

TasLe XVIII.—INFLUENCE orf PotrasstuM SALTS ON THE ACTION OF
NITROGENOUS MANURES. ROTHAMSTED.

Average Weights, Mangolds, 1906-1910.

Roots, 1ooo Ib, per acre. Caiabdaie tooo lb. per acre.

Insufficient potassium (Series 5) | 11°97 | 14°68 | 18°62 | 2°59 | 7°25 | 7°75!
Sufficient potassium (Series 4) . | 11°84 | 40°12 | 65°67 | 2°55 | 85x | 13°88

Nitrogen supplied in ern Ib.
per acre i ; -| — 86! 1847 | — | 86! | 184?

The effect of varying water supply is more conveniently
studied in pot experiments than in the field, since any com-
parison between yields in wet and dry seasons is complicated

1 From 400 Ib. ammonium salts.
? From 400 lb. ammonium salts and 200 Ib, rape cake.

64 SOIL CONDITIONS AND PLANT GROWTH

by the great differences in temperature conditions. Tucker
and von Seelhorst’s experiments have already been described
(pp. 38 and 44).

From the practical point of view the important result is
that a given increase in the food supply may produce no in-
creased growth, small increase, or a larger increase, according
to the extent of the water supply.

Phosphorus.—Phosphates are by far the most efficient
phosphorus foods known for plants. The relationship between
phosphorus supply and growth has been measured by E. A.
Mitscherlich (p. 33) in a series of experiments on oats grown
in sand with each of the three calcium phosphates. For equal
weights of the three salts the relative efficiencies corresponded
with the basicity ; for equal weights of P,O,, however, the
values were 2°66 : 2°31: 1°65. This was in sand cultures; in
soils different efficiencies were found: thus for the mono-
phosphate the values were :—

Sand. Soil 1. Soil 2. Soil 3.
por POG 1°80 1°74 2°40

The effect of a phosphate on the crop is twofold. In the.
early stages of growth it promotes root formation in a re-
markable way. So long ago as 1847 Lawes (161) wrote:
‘“‘Whether or not superphosphate of lime owes much of its
effect to its chemical actions in the soil, it is certainly true
that it causes a much enhanced development of the wmder-
ground collective apparatus of the plant, especially of lateral
and fibrous root, distributing a complete network to a con-
siderable distance around the plant, and throwing innumerable
mouths to the surface”. Dressings of phosphates are par-
ticularly effective wherever greater root development is re-
quired than the soil conditions normally bring about. They
are invaluable on clay soils, where roots do not naturally form
well, but, on the other hand, they are less needed on sands,
because great root growth takes place on these soils in any
case. They are used for all root crops like swedes, turnips,
potatoes, and mangolds; in their absence swedes and turnip
roots will not swell but remain permanently dwarfed like

(379q]15 pue somv’q)
“IOSI[IIJaz 9}9]dwWI09 pue £ srasip13103 d1ssejod pue sayeydsoydiodns ‘ pamnuewun : yyeqpeorg ‘PIPY [Ppsy wos sapamg— ‘tr

2) 0 |
‘IaSI[NIJej d1ssvjod pue a3eydsoydiedns usZonIN "Jast[IjJ9y d1ssejod pue oyeydsoydiodng

‘sINUBU ON,

SOIL CONDITIONS AFFECTING PLANT GROWTH 65

radishes (Fig. 11 shows one of Lawes and Gilbert's photographs) :
the introduction of superphosphate as a fertiliser revolutionised
agriculture on some of the heavier soils by allowing, better
growth of these crops. Phosphates are needed also for
shallow-rooted crops with a short period of growth, like
barley. Further, they are beneficial wherever drought is
likely to set in, because they induce the young roots to grow
rapidly into the moister layers of soil below the surface;
probably, as Hall has suggested, this explains the marked
effect of poe oaehate on wheat in the dry regions’ of
Australia.

_Later on in the life of the plant phosphates hasten the
ripening processes, thus producing the same effect as a de-
ficiency of water, but to a less extent; for this reason they
are applied to the wheat crop in some of the northern districts
of England, and the oat crop in the west, to bring on the
harvest a few days earlier and obviate risk of loss by bad
weather, The northern limit of growth of several crops may
in like manner be extended. This ripening effect is well
shown on the barley plots at Rothamsted; crops receiving
phosphates are golden yellow in colour while the others are
still green.

But these effects, important as they are, are nothing like
as striking as those shown by nitrogen compounds. There is
no obvious change in the appearance of the plant announcing
deficiency or excess of phosphate’ like those changes showing
nitrogen starvation or excess; the hastening of maturity is
seen only when there is a control plot unsupplied with phos-
phates: it leads to no increase in the proportion of grain
borne by the plant. On the Rothamsted plots supplied with
nitrogen and potassium compounds, but no phosphate, the
grain formed 44°9 per cent. of the total produce during the
first ten years of the experiment (1852-1861), and almost
exactly the same proportion (44°7 per cent.) during the fifth
ten years (1892-1901) when phosphate starvation was very

1 Barley grown in water cultures without phosphorus compounds acquires a
red colour in the stem, but this is not commonly seen in the field.

5

66 SOIL CONDITIONS AND PLANT GROWTH

pronounced ; it fell a little to 41°3 per cent. in the sixth ten
years (1902-1911), but rose to 46°8 per cent. in the period
1913-1919. Even in sand cultures the difference is not very
marked: Hellriegel (130d) grew barley with varying supplies
of phosphate with results given in Table XIX. In absence
of phosphate no grain was formed; when a little was added
grain formation proceeded normally, and the resulting grain.
was nearly full weight per individual ; as the phosphate supply
increased the percentage of grain increased, but soon reached
a maximum beyond which it would not go.

TasBLE XIX.—EFFECT OF VARYING PHOSPHATE SUPPLY ON THE GROWTH OF
BarLey IN SAND CULTURES. HELLRIEGEL (130d).

Weight of P,O,
supplied, mgms.
per pot . :
Weight of dry
matter in crop,
grams per pot . |1°856) 8°254|12°613|19°505|19°549| 20°195| 18°667| 17°785| 31°306
Grain per cent. of

°

14'2 |28°4 |56°8 |85'2 |r13°6 |142 213 284

dry matter -| — |22%4 |31°8 138°4 |41°6 | 43°38 | 413 | 40° | 43°4
Weight of one
grain, mgms. .| — |27 29 38 34 41 38 30 34

It is in the total growth of straw and of grain that the
effect of phosphate is manifested as shown in Table XX.

The Rothamsted results are plotted in Fig. 12. The
effect of phosphate starvation shows itself in depressing the
yield of straw and of grain, the straw being the first to suffer.
Potash starvation takes longer to set in, not because potassium
is less necessary but because the soil contains a larger quan-
tity ; it also affects the straw first. Nitrogen starvation sets
in at once, rapidly bringing both grain and straw down toa
very low level.

It is difficult to get behind these effects and ascertain
their causes. The function of phosphoric acid in the cell is
not easy to discover ; even when the problem is reduced to
its simplest state by experimenting with Spirogyra in culture |
solutions little more has been ascertained than that phosphates
are essential for mitotic cell division, doubtless because phos-

SOIL CONDITIONS AFFECTING PLANT GROWTH 67

phorus is a constituent of the nucleus, and also for the
normal transformations of starch. Loew (1800) found that fat
and albumin accumulated in absence of phosphates, but the
colour was yellow and there was no cell division ; as soon as
a trace of potassium phosphate was added, however, energetic
cell division took place. Reed (236) showed that: starch was
formed in absence of phosphorus, but did not change to sugars ;
erythrodextrin was formed instead and also cellulose.

TasLe XX.—REsvuLTS OF WITHHOLDING PHOSPHATES, PoTass1um ComPpounpDs,
AND NirRoGEN CompouNDS FROM BarLey. Hoos Figr_tp ExXpEerIMENTs,

ROTHAMSTED.
; Yield of Grain, rooo lb. per acre.
lot.
5 years,|5 years,|I0 years,|Io years,|10 years,| Io years, |10 years,|7 years,
1852-56.|1857-61.| 1862-71. | 1872-81. | 1882-91. |1892-1901.| 1902-11. |1913-19.
7| Dung. ‘ - | 2°31 | 2°78 | 3°00 | 2°88 | 2°66 | 2°56 | 2°50 | 2°35

A 4| Complete manure
(salts of NH,,
K and P) - | 2°47 | 2°71 | 2°67 | 2°34 | 2°24 | 2:02 | 2°25 | 2°06
A 3| No phosphates .| 2°27 | 1°7z | 1°99 | 1°68 | 1°38 £°26°} 2°33 1-38
A2|No potassium .| 2°42 | 2°70 | 2°76 | 2°29 | 2°01 1°63 | 18x | 1°88
O 4| No nitrogen . | 1°86 | 1°57 | 1°39 *98 "92 "74 "94 | 1°31

Yield of Straw, 1000 Ib. per acre.

Plot.
5 years,|5 years,|IO years,|ro years,|10 years,| Lo years, |I0 years,|7 years.
1852-56.|1857-61.| 1862-71. |, 1872-81. | 1882-91. |1892-1901.| 1902-11. |1913-19,

Dung. .-— «| 2°82 | 3°15 | 3°35 | 3°37 | 3°28 | 3°35 | 3°54 | 2°73
A 4| Complete manure
(salts of NH,,
K and P) - | 3°29 | 3°17 | 3°14 | 2°63 | 2°61 2°36 | 2°83 | 2°17
A 3)|No phosphates .| 2°86 | 2°03 | 2°20 | 1°75 | 1°64 1°56 | 1°75 | 1°57
A2|No potassium .| 3°21 | 3°03 | 3°07 | 2°30 | 2°20 I*g0 | 2°16 | 1°78
O 4| No nitrogen .| 2°03 | 1°58 | 1°42 "95 "94 "90 | 1°39 | 1°46

The effects of phosphates in raising the quality and feeding
value of the crop are very great. The most nutritious pastures
in England and the best dairy pastures in France are those
richest in phosphates. Soils deficient in phosphates are nearly
alway unsatisfactory. Paturel’ has also shown that the best

1 Bull, Soc. Nat. Agric., 1911, p. 977.
5 *

68 SOIL CONDITIONS AND PLANT GROWTH

wines contain most P,O, (about 0-3 grm. per litre), the second
and lower qualities containing successively less. Further,
when the vintages for different years were arranged in order
of their P,O; content a list was obtained almost identical with
the order assigned by the wine merchants. Davis (78a) has
emphasised the importance of phosphate supply for the indigo ©
crop.

35r
) 2G ae en ae 422 Cmyarg] manueey
i ant as mci
(>) ies an
6 "yee, apenas le
s. 30} oe We,
e ‘ *. “ey
2. 4 Ce
a HN Manure
= \ fe,

25 7
‘5 \ iy)
” A Beh ti ais A aati «eG
Bo | \ “ \ “}Mo
me eee oe f ff
520 4:
. 1 j .

— -—T™~
— ee) N -
a i Ba sag ey as i: as
' : Sees
ys 15 BP ag
3
2
> 10} NoN
Qa.
o
i=
i)
5 ‘ ’ '
52'56 5761 62'71 72'8\ 82'91 92 01 0211 1319

Fic. 12a.—Effect on yield of grain of withholding various nutrients from
barley. (Hoos field, Rothamsted.)

The close connection between cell division and phosphate

‘ supply may account for the large amount of phosphorus com-

pounds stored up in the seed for the use of the young plant,
and also the relatively large amounts of phosphate taken
from the soil during the early life of the plant.

Potasstum.— Hellriegel has shown (Table IV., p. 31) that

SOIL CONDITIONS AFFECTING PLANT GROWTH 69

equivalent amounts of the soluble compounds of potassium have
practically all the same nutritive value.

_ The effect of potassium compounds is more localised than
that of phosphates, so that potash starvation can be more
readily detected. The colour of the leaf becomes abnormal ;
the potash-starved grass plots at Rothamsted have a poor,
dull colour, as also have the mangold plots ; the leaves also

acre.

bs. per

B

Hundreds of |
3

a

So

Crop Yields -

=

$256 57°61 6271 72'3) 82’91 92'01 02°11 1319

Fic. 128.—Effect on yield of straw of withholding various nutrients from
barley. (Hoos field, Rothamsted.)

tend to die early at the tips. The stem is weaker so that
the plant does not stand up well; this is apparently a tur-
gidity effect, although anatomical differences were observed
by Miss O. N. Purvis! The most striking effect, however, is
the loss of efficiency in making starch, pointed out long ago
by Nobbe (2152) ; either photosynthesis or translocation—it is
not yet clear which—is so dependent on potassium salts that

1 Yourn, Agric. Sci., 1919, 9, 360.

70 SOIL CONDITIONS AND PLANT GROWTH

the whole process comes abruptly to an end without them.
Mangolds, sugar beets, potatoes, and other sugar- and starch-
forming crops reduce their production of sugar with decreasing
potassium supply even before the leaf area has been dimin-
ished. Thus, in the mangold experiments of Table XVIII.
(p. 63), 7255 lb. of leaf give rise to 14,684 lb. of root where
potash food is deficient, while very little more leaf, 8508 |b.,
give rise to nearly three times as much root, 40,128 lb., where
more potassium salts are supplied. The harmful effect of
potash starvation on carbohydrate production does not seem
to be the result of a pathological condition of the chloro-
plastids. Reed found that they remained normal for two
months and even increased in numbers in potash-starved
alge.

A second effect is on the formation of grain; unlike phos-
phates and nitrates, potassium compounds have a very marked
effect on the weight of the individual grains, as may be seen
by comparing Table XXI. with the corresponding Tables XV-
(p. 60) and XIX. (p. 66); indeed, to withhold potash is the
surest way of producing stunted grain. At Rothanisted the
average weights per bushel of wheat for the ten years I910-
I9IQ were :—

Farmyard Complete Artificials without | Artificials without
ai seri Manure. Artificials. Nitrogen. Potash.
3 Plot 2. Plot 7. Plot 5. Plot rr.
61°5 62°3 62°2 61°8 60°7

Taste XXI.—ErFFeEct oF Potassium SALTS ON THE DEVELOPMENT OF BARLEY.
HELLRIEGEL (130d).

K,O supplied, mgs. . -|O 23°5 |47 7o°5 |94 188 282
Dry matter in crop, grams . |2°271| 5°414| 9'024|11°636|15°302| 20°946) 29°766
Grain, per cent of dry matter | — | 4°8 |2t5 |27°2 |30°r | 38°5 | 42°7
Weight of one grain, mgs. — 15 O5 153 17 26 34

Lastly, the vigour and healthiness of the plant are very
dependent on the potassium supply; potash-starved plants
are the first to suffer in a bad season, or to succumb to

SOIL CONDITIONS AFFECTING PLANT GROWTH 71

_ disease. The Broadbalk’ wheat plots receiving potassium
salts give conspicuously better results than the others when-
ever the year is unfavourable to plant growth; taking the
yield on the unmanured plot as an index of the character of
the season, we obtain the following results for a series of
good and of bad years respectively :—

TaBLeE XXII.—YieLp oF WHEAT IN THOUSAND POUNDS PER ACRE.

ROTHAMSTED.
eS In Nine Bad Seasons.1 ln Nine Good Seasons.1
ot
No.
Grain. Straw. Grain. Straw.
Unmanured R f 3 uh ihe 55 87 *88 1°08
Insufficient potash. A .| II 106 1°86 1°51 2°20
Sufficient potash ‘ ‘ .| 13 1°70 3°02 1°98 3°16
Percentage increase due to potash | — | 60°3 62°3 31r°r 43°6

In the bad years the average rainfall was 32°55 inches
(harvest years, September-August), while in the good years
it was 27°10 inches; the badness of the season may be
connected with the high rainfall and corresponding low tem-
perature. Similar results are obtained, however, if other un-
favourable conditions set in.

The improvement in healthiness is well exemplified by the
power of resisting disease. At Rothamsted the potash-starved
wheat and mangolds are liable to be attacked by disease,
especially where there is excess of nitrogen, while the sur-
rounding plots, equally liable to infection, remain healthy.
Flax growers in the north of Ireland have found that potassic
fertilisers increase the resistance of the plant to the attacks of
the wilt organism.

At the Cheshunt Experimental Station liberal treatment
with potassic fertilisers makes the tomato plant more resistant

1The bad years were 1867, "71, 72, '75, 76, ‘77, ’79, °86, ’88; the good
years were 1868, ’69, ’70, ’81, ’83, ’85, 87, ’89, gr.

72 SOIL CONDITIONS AND PLANT GROWTH

to the bacterial stripe disease : the numbers of plants affected -
out of a total of 120 in each plot were :—!

Complete Fertiliser. | No Potassic Fertiliser.

Var. Comet . . ; ; : 40 78
Var. Kondine Red . ‘ ; 13 33

Potassic fertilisers often afford the simplest method of dealing
with fungoid diseases and they are usually more effective than
other fertilisers under glass.

Next to the sugar-producing plants, the leguminosz seem
to stand most in need of potassium salts. The potash-starved
grass plots at Rothamsted contain notably less clover than
those fully manured, the actual depression fluctuating accord- —
ing to the season. Some of the weeds, especially the sorrel,
require a good supply of potash.

There is some controversy as to whether potassium plays
any important part in protein synthesis in plants.’

In absence of potassium salts mitotic cell division does
not go to completion ; Reed observed that the cell and nucleus
both elongate, but actual division does not occur (236).

It is not at present possible to say whether all these
phenomena are different manifestations of one and the same
specific action of potassium in the plant, or whether there
are several different causes at work. Zwaardemaker* puts
forward the interesting suggestion that the potassium ion
(which is somewhat radioactive) may be replaced by any
other radioactive element, light or heavy, or by free radio-
active radiation, provided the doses are equi-radioactive.

Sodium does not appear to be essential even to salt marsh
plants, although salicornia grew better in presence of salt
than in its absence.* It can partially, but not completely,

1S. G. Paine and W. F. Bewley, Annals of Applied Biology, 1919, 6, 185.

2 J. Stoklasa, Biochem. Zeitsch., 1917, 82, 310-323 ; T. Weevers, ibid., 1917,
78, 354.

5 ¥. Physiol., 1920, 53, 273.

4A. C. Halket, Annals of Botany, 1915, 29, 143-154.

SOIL CONDITIONS AFFECTING PLANT.GROWTH 73

replace potassium as a plant nutrient; it thus delays the
setting in of potash starvation, but will not keep it off alto-
gether. Hellriegel (130¢@) found that sodium salts always
gave increases in crop even when potassium salts were present
in quantity.

Tas_e XXIII.—Errecr or Soprum SALTs wiTH SMALL AND WITH LARGE

Amounts oF Potassium SALTS ON THE GROWTH OF BARLEY.
HELLRIEGEL (130d).

K,O supplied, mgs. . : ‘ a8 i 04 188 282 376

Dry matter produced when sodium

saltsadded . - | 4°925 | 23°0Fg | 32°278 | 36°535 | 38°270
matter produced when | no sodium
saltsadded . é . ¢ . | 2°658 | 15°638 | 20°724 | 34°897 | 36°28

Difference due to sodium salts . . | 2°267 | 7°381 | 2°554| 1638] 1'989

Breazeale (514) has more recently obtained similar results

in water cultures. It is well ascertained in farming practice
_ that sodium salts can be used with great effect as manures

wherever there is any deficiency of potash in the soil."
J. A. Voelcker (290) has made the interesting observation

that sodium hydrate and sodium carbonate, unlike most other

salts, cause an increase in the percentage of nitrogen in the
wheat grain, besides increasing the yield of crop. The sulphate
and the chloride increased the crop, but beyond a relatively
low concentration limit further increases in amount of sodium
chloride proved toxic.

Lithium salts, on the other hand, have a toxic action on
plants. Gaunersdorper’s older experiments * have been con-
firmed by J. A. Voelcker (290, 1912), who found that amounts
of the chloride, sulphate, or nitrate, corresponding to ‘003
per cent. of the metal were distinctly injurious to wheat;

1¥For fuller details see Kriiger, Zeitschr. Ver. Deut. Zuckerindus., 1914, 694-
702; B. Schulze, Beitrag zur Frage der Diingung mit Natronsalzen (Landw.
Versuchs-Stat,, 1913, ‘79-80, 431, and 1915, 86, 323-330); E. J. Russell, hiatus
Bd. Agric., 1915, 22, 393-406.

2 Landw. Versuchs-Stat., 1887, 34, 171-206,

74 SOIL CONDITIONS AND PLANT GROWTH .,

smaller amounts, however, appeared to cause an increased
growth.

Cesium salts are less harmful (290).

Calcium is an essential plant food, the function of which
was first carefully studied by von Raumer (234), but has not
yet been satisfactorily cleared up. Little has been inferred
from the fact that, like potassium, it occurs more in the leaf —
than in the seed~ It certainly gives tone and vigour to the
plant; gypsum is used in alkali regions to counteract the
harmful effects of excessive amounts of saline matter in the
soil. It also appears to stimulate root production : if calcium
is withheld from water cultures the size of the root is much
reduced. Maquenne and Demoussy! show that the amount
present in the seed of the pea is insufficient for root develop-
ment, which therefore ceased on the third or fourth day of
germination in pure water. Addition even of traces of calcium
sulphate was followed by further root growth: o-o1 mg. of
calcium sulphate per seed, representing an addition of calcium
equal to 1/40,000 the weight of the dry seed, led to formation
of root hairs and a 40 per cent. increase in root length.

The close relationship between calcium and nitrogen
content suggests that the calcium may be associated with
protein metabolism, perhaps combining with the acids to
which such metabolism gives rise. Plants fall into two groups
so far as calcium is concerned :—

(a) Those (including calcifuges) with low content of
calcium and low calcium-nitrogen ratio; (4) those (including
calcicolous plants) in which these quantities are high.”

Barium and strontium cannot replace calcium in the
nutrition of plants. McHague (187) has shown that the
carbonates are toxic, though in the presence of calcium car-
bonate they cause an increase in plant growth, strontium being
more effective than barium. In Voelcker’s experiments (290)
the addition to the soil of even o'1 per cent. of strontium
sulphate, hydrate, or carbonate was without effect, but the

' Compt. Rend., 1917, 164, 979-985; and 165, 45-51.
2F, W. Parker and E. Truog (Soil Sci., 1920, 10, 49).

SOIL CONDITIONS AFFECTING PLANT GROWTH 75

chloride was distinctly toxic. In Loew’s experiments on alge
(180e) strontium salts injuriously affected the chlorophyll
bodies, causing loss of starch-making power and finally death.

Magnesium, \ike phosphorus, finally moves to the seed,
and is thus in contrast with calcium and potassium, which re-
main behind in the leaf or the straw. Willstatter has shown
(310) chlorophyll to be a magnesium compound, an observa-
tion that accounts for the unhealthy condition of the chloro-
phyll bodies, and the final etiolation of magnesium-starved
plants. Further, magnesium seems to be necessary for the
formation of oil, the globules being absent from alge growing
in solutions free from magnesium salts; oil seeds are richer
in magnesium than starch seeds. An excess of magnesium salts
produces harmful effects which, as we shall see (p. 78), can be
lessened by addition of calcium salts; Loew indeed considers

(1 80a) that plants require a definite ane fe ratio in their food,
but neither Géssel! nor Lemmermann? could obtain evidence
of any such necessity.

In J. A. Voelcker’s experiments* magnesium oxide, car-
bonate, and chloride had, like sodium hydroxide, the unusual
effect of causing an increase in the nitrogen content of the
wheat grain. The sulphate did not act in this way, although
in suitable small amounts it caused increases in yield of grain
and of straw. The chloride proved toxic at higher concentra-
tions.

Aluminium compounds have been found beneficial by
Stoklasa.*

Ivon.—For some reason difficult to explain the formation
of chlorophyll is absolutely dependent on the presence of a
trace of some ferric salt, although iron does not enter into the
composition of chlorophyll.~ So little is wanted that iron salts
never need be Gied as manures, excepting for water or sand
cultures,

1 Bied. Zentr., 1904, xxxiii., 226.
2 Landw. Fahrbuch, 1911, xl., 175 and 255.

3 ¥. Roy. Agric. Soc., 1915, 76, 354; 1916, 77, 260.
4 Biochem, Zeitsch., 1918, 91, 137-

76 SOTL CONDITIONS AND PLANT GROWTH

Manganese is considered by Bertrand to be a constituent
of oxidases, and, therefore, necessary to the plant; minute
traces only are required, larger quantities being harmful.
Dr. Brenchley’s water cultures (54a) show that barley benefits
by small doses of manganese salts, and a number of field ex-
periments in Japan and in Italy } have indicated some manurial
value. Bertrand regards manganese. salts as ‘‘engrais com-
plémentaires” (35). Field trials at Rothamsted, however,
gave negative results.

Chlorine does not appear to be necessary to the plant in
large quantity ; indeed, Knop grew even the halophytes without
it. Mazé finds that small amounts are necessary, which, how-
ever, would not need to be added in manure, as rainwater in-
variably contains chlorides: at Rothamsted the amount of
chlorine brought down per acre ‘averages 16 lb. per annum,
the annual fluctuations varying with the rainfall between 10°3
and 24°4 lb.? Voelcker finds that, on the whole, chlorides
are more toxic than sulphates at equivalent concentrations
(290).

In small quantities both fluorine and zodine appear to in-
crease plant growth: this was first shown in Japan by Loew
(180c) and Suzuki (277a): it is also accepted in France by
Mazé (197). Gautier and Clausmann (102) go even further
and claim that a dressing of 5 kgms. of amorphous calcium
fluoride per acre was followed by increases in cereal crops of
5 to 18 per cent., and sometimes considerably more in the
case of root crops.

Sulphur is an essential food constituent, and occurs in
plants, especially in cabbages and swedes, to a greater extent
thanis usually recognised, the’ older analytical methods giving
low results (Hart and Peterson (127), Peterson (223)). Sul-
phates are present in rain and in soil, but further additions in

1The Japanese experiments are recorded in the Bull. Coll. Agric., Tokyo,
1906 et seg. (210), and the Italian experiments in the Studi e Ricerche di
Chimica Agraria, Pisa, 1906-8; pot experiments have also been made by J. A.
Voelcker at the Woburn Experiment Station (fourn. Roy. Ag. Soc., 1903, 64,

348-359). See also E. P. Deatrick, Cornell Mem., 1919, 19, 371.
2E, J. Russell and E. H. Richards, fourn. Ag. Sci., 1919, 9, 309.

SOIL CONDITIONS AFFECTING PLANT GROWTH 77

4

manure were found by Bogdanow to be helpful! Dymond (92)
showed that sulphates increased the yield of heavy crops rich
in protein, although they were not needed for cereals or per-
manent pastures. A number of recent investigations in the
United States by Pitz,? H. G. Miller, C. B. Lipman and W.
F, Gericke (175@) and others have confirmed and extended
these observations: the last-named authors found that sulphate
of ammonia was superior to nitrate of soda for barley on
certain Californian soils, though it was no better than a mix-
ture of nitrate and sulphate of soda.

Silicon does not seem to be essential in any quantity, but
it occurs to so large an extent in some plants that it is not
likely to be wholly useless. Wolff and Kreutzhage (315)
found that soluble silicates increased the yield of oats in water
cultures and also the proportion of grain, behaving, in their
opinion, much like phosphates. On some of the phosphate-
starved plots at Rothamsted marked crop increases are ob-
tained by addition of sodium silicate (Table XXIV.). Hall
-and Morison (120c) conclude that silicates act by causing an

TaBLeE XXIV.—Errect or SILICATES ON THE GROWTH'OF BARLEY, 1864-1904.
ROTHAMSTED,

Yield of Dressed | Yield of Straw, | pati, otal Grain
Grain, bushels. cwts. Straw

Without} With | Without}; With ]| Without | With
Silicate | Silicate. | Silicate. | Silicate. | Silicate. | Silicate.

Nitrate only ‘ Ph Haley hak. a UO, Wt) 16°2 19°8 85°1 86°6
Nitrate+phosphate . | 42°2 43°5 24°6 25'8 87°2 85°8
Nitrate+ potassium salts .| 28°6 36°4 17°9 21°7 80°6 85°0
Nitrate + phosphate + uso
sium salts A 41°2 44°5 25°3 27°6 82°7 82°

increased assimilation of phosphoric acid by the plant, the
seat of action being in the plant and not in the soil. Bene-
ficial results were likewise obtained by Jennings.?*

1 Expt. Stat. Record, 1900, 11, 723, and 1903, 15, 565.

2Pitz, Fourn. Ag. Research, 1916, 5, 771-780; H. G, Miller, Fourn. Ag.

Research, 1919, 17, 87-102.
3 Soil Sci., 1919, 7, 201.

+78 SOIL CONDITIONS AND PLANT GROWTH

Boron, given as boric acid, was found by Dr. Brenchley (536)
to increase the growth of peas, but not of barley, in water
cultures at concentrations of about 1/100,000; above this
point harmful effects were produced. In Voelcker’s pot ex-
periments (290) even I part in 200,000 of soil proved toxic~
for barley, but at lower concentrations there was a slight
stimulating effect. More recent experiments are recorded by
Cook and Wilson.*

Physiological Balance.

It is not only necessary to supply all the essential nutrient
substances to the plant, in addition there must be maintained
some kind of proportion between the various salts; this is
spoken of as the physiological balance. Plant physiologists
have long recognised that single salts of potassium, magnesium,
sodium, etc., are toxic to plants, while a mixture of salts is
not. Calcium salts are by much the most powerful reducers
of this toxic effect. Thus Kearney and Cameron (145a) found
that a root of Lupinus albus was just killed when immersed in
‘00125 N magnesium sulphate solution (7 parts per 100,000),
but the effect was modified by added salts, as shown in
Table XXV.

TABLE XXV.—EFFECT ON VARIOUS SALTS IN REDUCING THE TOXICITY OF
MgSO,. KEARNEY AND CAMERON (1454).

+ MgClg. |+ NagCOy/+ NaoSO, NaCl |+ CaCl. CaSO,
Alone. | (‘oo28 Nj. | Coozs N)-| (orN)."| (ors N).| Ca N).- (Saturated),
Strength of
MgSO, that
just kills the
root . . |°00125 N}000625 N|‘oor25 N}00375 Nj‘0075 N/ *2 N ‘ON

Hansteen found that the toxic effect of potassium salts
used ‘singly was overcome even when so little lime was

CaO I
Ro bac Osterhout found (220a)

that Vaucheria sessilis lived for three weeks in distilled water,

added that the ratio

1F. C. Cook and J. B. Wilson, fourn. Ag. Research, 1918, 13, 451-

SOIL CONDITIONS AFFECTING PLANT GROWTH 79

but was killed in a few minutes by = N NaCl, and ina few

days by ‘ooot N NaCl; yet the toxic effect even of the
Stronger solution disappeared on adding one gram-molecule
of CaCl, for every 100 gram-molecules of NaCl. It does not
appear that calcium prevents the entrance of the sodium or
other ion into the plant: apparently it gives greater vigour to
the plant... Magnesium chloride and sulphate, potassium
chloride and calcium chloride were also toxic when used
singly, but in admixture they formed a nutrient medium in
which the plant grew normally and developed fruit even

when 8 N NaCl was also present. This action is called

antagonism of ions.” Osterhout shows (2200) that the phen-
omena hold generally both for land and water plants.

Other facts are less easy to explain, such as Grafe and
Portheim’s observation that the toxic effects of a single salt
fail to appear, or are much delayed, when sugar is supplied.*

These and many other experiments all indicate that a
complex equilibrium normally exists in the cell between
colloids and electrolytes which can be maintained only when
the external medium has an appropriate composition.

Even when all the nutrient salts are present and the total
osmotic concentration is maintained constant at a suitable
level it is still possible to produce the most diverse effects on
the growing plant, from violent injury to excellent growth,
by varying the proportions in which the salts occur. This
problem has been much investigated by Tottingham (283) and
by Shive (2622). Tottingham studied the effects of eighty-four
mixtures of KH,PO,, KNO,, MgSO, and Ca(NO,),, plus a trace
of an iron salt, all of which had the same total osmotic con-
centration. Shive simplified the investigation by reducing the
salts to three, KH2PO,, Ca(NO,), and MgSOQ,, and using

1J. A. Le Clerc and J. F. Breazeale, ¥ourn. Ag. Research, 1920, 18, 347;
L. Maquenne and E, Demoussy, Compt. Rend., 1920, 1'70, 420.

? For sand culture experiments see Wolkoff, Soil Sci., 1918, 5, 123-150.

® Bied. Zentr., 1908, xxxvii., 571.

80 SOIL CONDITIONS AND PLANT GROWTH

thirty-six different combinations of equal total concentra-
tion. .

The extent of the injurious effect depends on the concen-
tration: at Ol atmosphere osmotic pressure none of the
solutions proved injurious to seedlings; at a higher concen-
tration seven caused severe and four slight injury; fifteen
caused various degrees of productiveness. without injury.

The specific injury caused in these circumstances by mono-
potassic phosphate and by sulphate of ammonia to soy beans
was studied by Shive (262c) and by Wolkoff (316) respectively.

The amount of growth produced by well-balanced solutions
also depends on the total concentration, increasing up to a
certain point.

_ The optimum ratio of nutrients for a given stage of plant
development alters with the concentration ; it is not the same
at O'I, 1°75, and 4 atmospheres. But it is not affected by
the nature of the medium; it is the same in sand as in water
culture. So also it is independent of variations in the
moisture content of the sand, being the same for degrees of
moistness varying from 40, 60, to 80 per cent. of the water-
retaining capacity of the sand. But it is not constant for
the whole range of growth of the plant, being different in
seedling and ripening stages and different for the growth of
“top” and of roots.

It must not be supposed, however, that the physiological
balance is a rigid ratio: Hoagland and Sharp (136¢) obtained
satisfactory growth with a wide range of mixtures so long
as the total supply and concentration of essential elements
was adequate. Mazé has adduced evidence that physiological
balance is an important factor in soils and is affected by
calcium carbonate and sometimes by humus (196).

Absence of Injurious Substances.

We have seen that many salts have a toxic effect if given
alone to the plant, but for our purpose we need consider only
those causing injury in presence of other compounds. Two
cases arise in practice: some substances are injurious even in
small quantities, others only in excess.

SOIL CONDITIONS AFFECTING PLANT GROWTH 31

Substances Injurious in Small Quantities: Acids and Al-
kalis.—H and OH ions. It has long been known that plants do
not grow well on acid soils, and the conclusion has been drawn
that the acidity is harmful to plants. As a general statement
this is true: additions of a strong acid, such as HCl or H,SO,
to a culture solution or to soil soon kills the plant. But the
change in reaction thus induced is vastly greater than is found
in soils, and from our point of view it is necessary to know the
effect of changes of the same magnitude as occur in nature.
The proper basis of comparison is the hydrogen-ion concen-
tration (see p. 113). Hoagland has shown (136a) that an acid
condition up to o0’7 x 10-°H ion (Py = 5°16) is favourable
to the growth of barley seedlings, while stronger acidity is
harmful. Alkaline solutions stronger than 1°8 x 10°° OH ion
(Py = 8°26) caused injury, and those stronger than 2°5 x 107°
(Py = 9°40) were extremely toxic. Salter and Mcllvaine
(243) obtained the best growth in slightly acid conditions ;
for wheat, soy beans, and lucerne Py = 5°94, and for maize
Py = 5°16; 2°96 was harmful, and 2°16 fatal. Alkalinity
was decidedly more harmful than acidity * (Fig. 13).

It will be shown later that the Py value of soils varies
between 3°7 and 9°7, values beyond 4°5 and 8°5 being, how-
ever, unusual. The acidity is often less than that of cell sap,
the Py value of which varies between 4°0 and 6:0,” Thus,

1 See also J. S. Joffe, Soil Sci., 1920, 10, 301.

*Truog and Meacham, Soil Sci., 1918, 5,177. For further data showing
variation with stage of growth and conditions see A. R. C. Haas, Soil Sci., 1920,
9, 341. For acidity of root sap (which is generally less than that of the aerial

portion), H. Kappen (Landw. Versuchs-Stat. 1918, 91, 1-40) obtained the follow-
ing Py values :—

Wheat . : a , , . . ; : ; she
Barley . 4 ‘ 4 ; ; i ! : 4 6°85
Oats. : < : P : ‘ : : : 6°7-6°8
Rye: 0 A ; ‘ ; d 3 : 4 ; 6°6
Mustard ‘ : A ; ‘ F , 2 ‘ 6°2
Horse beans : ; , j , ; : 6°0
Lupins . ‘ ; 4 ‘ : ‘ ; : ; 5°6-5°8
Buckwheat . : ‘ ‘ ‘ 4 : ; \ 5°0-5°3

For other determinations see C. B. Clevenger, Soil Sci., 1919, 8, 217 (values
5°8-6).
6

82 SOIL CONDITIONS AND PLANT GROWTH

direct injury to the plant caused by true acidity of the soil is
probably not very frequent in nature: harmful effects of
alkalinity, however, do occur.

The unsuitability of the atmosphere of industrial towns
has been traced in part to the presence of acids, which affect
the leaves as well as the roots. Wieler’ found that assimila-
tion of carbon dioxide was profoundly modified by sulphur

100

Relative Growth.

10 F Alfalfa ————

Oo lL 1 = i i

7 J 4 5 6 > 8
. H
Reaction as FP

Fic. 13.—Relation between hydrogen-ion concentration (Px value) and growth
of crops. (Salter and Mcllvaine, 243.)

dioxide, most injury being done in moist weather when the
stomata were more widely opened and the gas could readily
enter the leaf tissues. Crowther and Ruston (71) obtained
the following yields from pots of Timothy, showing that acid
water gradually kills the plant :—

1 Bied. Zentr., 1908, xxxvii., 572.

SOIL CONDITIONS AFFECTING PLANT GROWTH 83

TABLE XXVI.—Errect or Acid RAIN-WATER ON THE GROWTH OF TIMOTHY
Grass. CROWTHER AND RusToN (71).

Weight of dry matter obtained when plants were regularly watered with :—

Solution of Sulphuric Acid, Parts per
Country Rain | Leeds Rain 100,000 of Water.
Neutralised. (Acid).

I 2 4 8. 16 32
Ist crop, 1908 .| 28°0 gms. | 23°8 gms. | 30°5 | 28°7 | 28°8 | 24°8 | 23°38 | 14°1
2nd crop, 1909 .| 24°9 ,, Ey ft Ga 18°2|17°8| ro°o0| 8:2] 18] o
3rd crop, I910 .| 14°7 ,, G6: 43) Faro;| SO): -3:g | 3391 oO °

\

Metallic Salts.—Complaints are sometimes made by
farmers in mining districts that their crops suffer damage from
the waste products—generally metallic salts—turned into the
streams from the works, especially where the water is wanted
for irrigation, or where, as in Japan, rice is grown in the
marshes. The damage done to pastures by the lead mines of
Cardiganshire has been investigated by J. J. Griffith (115) at
Aberystwyth. Clover is particularly susceptible. A heavy
dressing of lime proved a useful remedy.

Zinc also causes injury in parts of Wales.!

Traces of zinc are regarded as essential by Mazé (197).
Working with larger quantities Dr. Brenchley (534) was un-
able to find definite indications of stimulating action in water
culture, although Javillier (143) claimed to obtain increases
in soil. Ehrenberg (93a) concludes that zinc salts are always
toxic when the action is simply on the plant, but they may
lead to increased growth through some indirect action on the
soil itself (see p. 281).

A vast number of experiments have shown that copper
salts are extraordinarily toxic in water cultures or where they
‘actually come into contact with the plant, even the minute
trace sometimes present in distilled water being harmful.
This property finds useful application in removing alge from
water and in killing weeds. For example, a 3 per cent. solu-
tion of copper sulphate is sprayed over cornfields in early

1 The soil of an Anglesey garden examined at Rothamsted contained 0°78

per cent. of zinc, It proved, as might be expected, highly infertile.
6 *

84 SOIL CONDITIONS AND PLANT GROWTH

spring at the rate of fifty gallons per acre to destroy charlock
(Brassica sinapis), one of the most troublesome weeds on light
soils. The solution adheres to the rough horizontal leaves of
the charlock and kills the plant, but runs off the smooth vertical
leaves of the wheat without doing much damage. Even the
insoluble complex copper salt present in Bordeaux mixture
and sprayed on to fruit trees to kill fungoid pests, was found
by Amos’ to retard assimilation of carbog dioxide by the
leaf.

Copper salts do not appear to be anything like so toxic in
the soil as in water culture.

It is often asserted that any toxic substance must, at
proper dilutions, act as a stimulant to plants; with copper
sulphate, however, Dr. Brenchley (53a) could obtain no evi-
dence of increased growth in water cultures at any dilution,
even down to I part in 10,000,000 of water, although the
toxic effect was dtways shown. The pot experiments of
Russell and Darbishire lead to the same conclusion (24148).

Ferrous salts are toxic and are commonly regarded as one
cause of the sterility of badly aerated soils.

Aluminium salts have received attention, to them having
been attributed much of the injury formerly ascribed to acidity
in acid soils (see p. 112). While in low concentrations they
are apparently beneficial (p. 75), at higher concentrations they
are harmful, especially in acid soils,

Salts of arsenious acid were found by Dr. Brenchley to be
much more toxic than those of arsenic acid; they are used
as weed-killers. Greaves, however, claims to have obtained
evidence that arsenic compounds stimulate the nitrogen fixing
organisms in the soil;* while Green has isolated bacteria ~
which are capable of converting arsenites into arsenates, and
vice versa.

Most metallic salts appear to be toxic, except those of the

1Fourn. Ag. Science, 1907, ii., 257-266.

2 Fourn. Ag. Research, 1916, 6, 389-416.

3H. H. Green, 5th and 6th Report, Veterinary Research, South Africa, 1918, _
593-624.

‘SOIL CONDITIONS AFFECTING PLANT GROWTH 85

_ few metals required for nutrition. No unexceptionable evid-
ence of a stimulating effect on the plant has yet been obtained,
although certain effects may be produced in the soil leading
to increased growth (see p. 280). The literature of the sub-
ject is summarised by Dr. Brenchley in her monograph (53d).

Whenever infertility is traced to any of these metallic
salts a good dressing of lime is usually found to be an
effective. antidote.

Various Other Substances—Sulphuretted hydrogen is ex-
tremely toxic, so also is ammonium sulpho-cyanide which, in
the early days, used to cause trouble as an impurity in
ammonium sulphate made from gas liquor. It is rarely, if
ever, found now. Toxic nitrogen compounds include nitrites,
which have to be removed from synthetical calcium nitrate
used for manure, the dicyano-diamide associated with com-
mercial cyanamide, and ammonium salts at too high a con-
centration. None of these, however, are for long harmful
in the soil, since all are ultimately converted into nitrates.
Perchlorates are harmful and used sometimes to occur in
sodium nitrate, but they are now carefully removed.

Substances Injurious in Large Quantities : Carbon Dioxide.—
In an interesting series of investigations Kidd (147) has shown
that CO, exerts a marked inhibiting effect on the germination
of seeds, even though all other conditions are favourable. The
seed is not permanently affected, and it will germinate freely,
though not always immediately, when the CO, is removed.
He suggests that this is the cause of the remarkable dormancy
of certain seeds, especially weed seeds, buried below a certain
depth in the soil ; some of these will survive for years, and
will produce a copious and vigorous crop of weeds when
brought to the surface by deeper ploughing or breaking up of
grass land.,*

Soluble Salts——In many arid districts the soil contains
such large quantities of sodium and potassium salts that the
soil water is too concentrated to permit of plant growth.
Sodium carbonate not infrequently occurs and directly poisons

1W, E, Brenchley, Fourn. Ag. Science, 1918, 9, I-31.

86 SOIL CONDITIONS AND PLANT GROWTH

the plant. Such soils are called alkali soils: they may be
treated with gypsum, or, still better, carefully washed with
irrigation water, adequate provision being simultaneously made
for drainage. PAN pirate

Calcium Carbonate is sometimes considered harmful be-
cause plants are liable to chlorosis on chalky soils. It is
equally probable, however, that the general soil conditions are
responsible for the disease (see p. 304).

Magnesium Salts.—The toxicity of magnesium salts was
discovered by Tennant in the eighteenth century in studying
the alleged harmful effects of certain limestones found near
Doncaster (280). Modern investigations’ on magnesian
limestone, however, have failed to show any harmful effect ;
indeed, in the Woburn experiments (290) Voelcker has ob-
tained an actual benefit both on wheat and on mangolds by
using magnesia (MgO). But the soluble salts, the sulphate
and especially the chloride, are harmful. Cases are reported
by Loew where excess of magnesia in the soil has caused
infertility ; none, however, have fallen under the writer's
observation in this country. The soil of the Greenville Ex-
perimental Farm, Utah, is rich in magnesia—containing over
6 per cent. of MgO—and is remarkably fertile. It also con-
tains, however, 17 per cent. of CaO and 20 per cent. CO,.’
As already stated, any injurious effect can be overcome by
treatment with lime.

Effects of Salts on Germination.—Salts generally cause a
retardation in the rate of germination; some of Guthrie and
Helms’ (117) results are given-in Table XXVII. Sigmund
has studied the effects of a very large number of substances
(267).. The technical interest in the work lies in the fact that
seeds are sometimes treated with antiseptics before sowing in
order to kill any spores of disease organisms, and, moreover,
certain soluble salts—artificial manures—are often put into
the soil about the same time as the seeds are sown.

1See, e.g., New Fersey Bull., 267, 914, and on the other side Durham
Coll. Bull., 12, 1915.

* J. E. Greaves, R. Stewart, and C. T. Hirst, fourn. Ag. Research, 1917, 9,
301.

SOIL CONDITIONS AFFECTING PLANT GROWTH 87

TasLe XXVII.—EFFect oF SOLUBLE SALTS ON GERMINATION. GUTHRIE
‘AND HeELMs (117).

Sodium Sodium Sodium Arsenic
Chloride, Carbonate, Chlorate, Trioxide,
per cent. per cent. per cent. per cent.

Barley. | Rye. | Barley.| Rye. | Barley. | Rye. | Barley.| Rye.

Germination affected .| o*r0 | otro | 0°25 | 0°25 | 0°005 | 0°004| 0°6 | 0°20

és prevented| 0°25 | 0°40 | 0°60 | 0°50/ 0°007 | 07006| — | 0%4
Growth affected -| O°FO | O°15 | O*F5 | 0°25 | 0°003 | 07002} 0°05 | O°15
as prevented .| 0°20 | 020 | 0*40 | 0°40 | 0°006 | 0°004 | o'ro | 0°30

When a solution comes in contact with a seed it does not
necessarily enter as a whole. Adrian Brown (58) has shown
that the barley seed is surrounded by a membrane which ‘has
the remarkable property of keeping out many dissolved sub-
stances and allowing the water only to pass in, so that the
solution loses water and becomes more concentrated. A
number of substances can, however, pass through the mem-
brane, and to these H. E. and E. F. Armstrong (5) have
applied the term Hormones. In general they have no great
affinity for water; in the Armstrongs’ nomenclature they are
anhydrophilic : they pass into the cell and there disturb the
normal course of events. Ammonia, tolyeng,. ett ether, chloro-
form, are all highly effective hormones readily ce the
cells of seeds, leaves, etc., and hastening the normal sequence
of processes.

Supposed Stimulation of Plants by Electricity, Heat, and
Radium.

The Electric Discharge.—I\t has often been stated that an
electric discharge increases the rate of growth of plants either
by direct action on the plant, or by indirect action in the soil.
As far back as 1783 the Abbé Bertholon (34) constructed his
electro-végétometre, a kind of lightning conductor that collected
atmospheric electricity and then discharged it from a series of
points over the plant. The view that atmospheric electricity
is an important factor in crop growth has always found

88 SOIL CONDITIONS AND PLANT GROWTH

supporters in France. Grandeau (1120) stated that plants
protected from atmospheric electricity by a wire cage made
less growth than control plants outside. Lesage’ confirmed
this observation, but found that silk thread caused as much
retardation as wire, so that the effect is not necessarily
electrical: in point of fact the rate of evaporation was con-
siderably less under the cage than in the open.

Instead of relying on atmospheric electricity Lemstrom
(171) generated electricity on a large scale and discharged it
from a series of points fixed on wires over the plant. This
method has been used at Bitton, near Bristol, and studied on
the electrical side by Sir Oliver Lodge, on the botanical side
by J. H. Priestley (231), and on the practical side by J. E.
Newman, J. H. Priestley, I. Jorgensen, and Miss Dudgeon.
Numerous field experiments are recorded, but there is usually
some uncertainty about the check plots. The Bromberg
experiments (104a) gave negative results. Further studies
are in hand by V. H. Blackman who has put the whole
subject on a sound basis for investigation.

Various Rays.—Experiments by Miss Dudgeon suggest
that the rays of the Cooper-Hewitt mercury vapour lamp” may
have a stimulating effect, accelerating germination and in-
creasing growth. Priestley found that the rays from a quartz
mercury vapour lamp were harmful at close range, whilst
farther off they stimulated growth. There is much scope for
work in this direction; the problem is of great economic
importance, because of the enhanced market value of early
crops.

Effect of Heat.—Molisch* has shown that perennial plants
steeped in hot water towards the close of their deepest
period of rest come at once into activity. His hypothesis is
that the “rest” required by plants is of two kinds, the
freiwillig rest due to external conditions and therefore cap-
able of being shortened, and the unfrecwillig rest inherent in

1 Compt. Rend., 1913, 157, 784. 2A glass envelope was used.
3 Das Warmbad als Mittel zum Treiben der Pflanzen, 1909, Prague.

SOIL CONDITIONS AFFECTING PLANT GROWTH . 8&9

the nature of the plant. Parkinson’ has tested the method
with satisfactory results; spirea, rhubarb, seakale, etc.,
steeped for twelve hours in water at 95°, at the end of
November, or early in December, made rapid growth when
subsequently forced.

Effect of Radium—R. J. Strutt has shown? that the
typical soil deposits contain measurable amounts of radio-
active substance equivalent to 0°25 (but more usually 1) to
5°38 x 10°" germs. of radium per gram. Zircon and apatite
are much richer, the figures being 75 to 865 x 1o-™ and II
to 30 x 10°” respectively. Joly*® finds that the amount of
radium emanation in soil air is many thousand times greater
than in the atmosphere; J. Satterly * at Cambridge gives a
lower estimate, v7z. 2 x 107” curie per litre, or 2000 times
the usual amount in the atmosphere ; while J. C. Sanderson ®
in America estimates the amount in I c.c. of soil air as the
- quantity in equilibrium with 2-4 x 107'* grms. of radium.
Among the many remarkable properties of radium it was
perhaps natural to expect that it might have some definite
effect on plants or micro-organisms. The suggestion has
even been made that radium emanations might, under suitable
conditions, cause sufficient increase in the amount of growth
to justify its use in horticulture and agriculture. The early
observations of Dixon and Wigham ° at Dublin, however, did
not seem very promising; 100 seeds of cress (Lepidium
sativum) were uniformly distributed on an even surface of
moist quartz sand, and after germination had taken place, a
sealed tube containing 5 mgms. of radium bromide was set
r cm. above the central seed. The seedlings grew up, but
without any curvature indicating positive or negative ‘‘ radio-
tropism,” and the only noticeable effect was a slight depression

1 ¥ourn. South-Eastern Agric. Coll., 1909, 19, 245-257.

. *Proc. Roy. Soc., 1906, '77a, 472, and 1907, '78a, 150.

8 Sct. Proc. Roy. Soc., Dublin, rg11, 13, 148.

- 4Proc, Camb. Phil. Soc., 1912, 16, 514. The soil air was taken at a depth
of r00-150 cms. For his estimates of the amount in the atmosphere see Phil.
Mag., 1910 (vi.), 20, 1.

5 Amer. F. Sci., 1911 (iv.), 32, 169.
§ Proc. Roy. Soc., Dublin, 1904, 10, 178-192.

go ' SOIL CONDITIONS AND PLANT GROWTH

of growth in those within 1 cm. radius of the tube. As
stronger preparations of radium became available more
definite retardations and inhibitions were observed; thus
Gager, in an elaborate report,' noted a more or less complete
inhibition in cell activities in younger and especially embryonic
tissues, with a few exceptions. The action of radium through.
the soil, however, was different; germination and growth
were both accelerated, and the plants farthest away were .
stimulated most. Acqua? found that different plants, and
even different organs of the same plant, were differently
affected, the root system in general responding more markedly
than the aerial parts, and in his experiments being arrested in
their development. The intensity of the radiation, however,
is important, and G. Fabre,* using Linum catharticum as a
test plant, was able to obtain increased development and
germination of seedlings by working with emanations up to
I°5 microcuries per 2 litres of air, and to retard development
by using emanations of 40 microcuries per litre of air. H.
Molisch* obtained a like result; young plants of vetches,
beans, sunflower, etc., were stimulated in growth by weak
emanations, but checked, or entirely stopped, by stronger
ones. He further claimed that the “rest period” could be
broken by the radium emanation, and he forced lilac into bloom
in November by attaching pipettes containing small quantities
of radium chloride to the terminal buds.° In his earlier
experiments he, like Dixon and Wigham, failed to detect any
radiotropism, but later on he found indications in the case of
certain heliotropically-sensitive plants, e.g. oats and vetches.®

These, and similar results, naturally suggested that the
residues left after the extraction of radium, but still containing
radio-active material, might have definite manurial value, and
it was not long before definite statements were forthcoming.
Baker’? claimed that increased yields of wheat and radishes

1Mem. New York Bot. Gard., 1908.

2 Ann. Bot. (Rome), 1910, 8, 223-238.

3 Compt. Rend. Soc. Biol. (Paris), 1911, '70, 187-188.

4 Umschau, 1913, 17, 95-98. 5 Ocesterr. Gart. Ztg., 1912, '7, 197-202,

6 Sitzber. K. Akad. Wiss. (Vienna), 1911, 120, 305-318,
7 Fourn, Roy. Soc, Arts, 1913, 62, 70-78.

SOIL CONDITIONS AFFECTING PLANT GROWTH 91

had been obtained by mixing 1 part of radio-active material
(2 mgs. Ra per ton) with 10 of soil. It isjtrue that Stoklasa’s*
results were negative (although in his other experiments radium
emanations increased growth to a marked extent), but this did
not prevent the introduction of radio-active fertilisers, and the
enterprising syndicates and companies concerned were by no
means loth to push their wares. These were investigated by
Martin H. F. Sutton,? the experiments being made with
radishes, tomatoes, potatoes, onions, carrots, and marrows,
some grown in pots, others in plots out of doors. Eight
different radium residues were used, in addition to pure radium
bromide; the dressings were so arranged that equivalent
quantities of radium were given in each case (,3, grm. radium
bromide to 15 lb. of soil: 24 times this amount per sq. yard
to the plots).

In no case was there any clear evidence of increased
growth, even the pure radium bromide seemed to be without
_ action,

We are therefore left with the apparent discrepancy already
observed on p. 35. The work of the physiologists, assuming
it to be sound, indicates that radium emanation is capable of
stimulating certain cell activities. Sutton’s results show that
such stimulus, if it exists, does not affect the final growth of
the plant: This discrepancy is periodically confronting the
agricultural investigator. Thus, Dr. Winifred Brenchley at
Rothamsted has failed to obtain increases in growth by supply-
ing plants with inorganic poisons which have been supposed
to stimulate certain cell functions in suitable dilutions. The
result opens up the prospect of an interesting discussion, but
it also shows the danger of arguing from a simple physiological
observation to a complex phenomenon like the growth of a
plant in soil.

Zwaardemaker * has obtained some interesting results in
the case of animal organs which deserve close study by plant
physiologists.

1 Chem. Ztg., 1914, 38, 841-844. 2 Messrs. Sutton’s Bull., No. 6, 1916.
* Fourn. Physiology, 1920, 53, 273, discussed by V. H, Blackman in Annals
of Botany, 1920, 34, 299.

CHAPTER. Hi,
THE COMPOSITION OF THE SOIL.

IT is well known that only the top six or eight inches of the
soil is suited to plant life, and that the lower part, or subsoil,
plays only an indirect part in plant nutrition. We shall,

therefore, confine our attention «)».ost exclusively to the
surface layer.
The soil was in the first ins‘x:.c’ ierived from rocks, partly

by disintegration and partly by decomposition. In most
cases the fragments split off were sooner or later carried away
by water and deposited at the bottom of a river or sea. There
they mingled with residues of living organisms which have
subsequently played an important part in the history of the
soil as its chief source of calcium carbonate and calcium phos-
phate. In course of time the material accumulated to con-
siderable depths ; then, as the result of some earth change, the
water retreated leaving the deposited material as dry land or
rock. No sooner was this exposed to the air than it began
once again to undergo disintegration and erosion. Air, water,
and frost all played a part in the disintegration process ; water
and sometimes ice have acted as transporting agents. For
immense ages the particles have been subjected to these
actions, and the fact that they have survived shows them to be
very resistant and practically unalterable during any period of
time that interests us. Reference to Table LXXXVIL, p. 332,
shows that the particles in the surface soil which have been
exposed to weathering ever since the soil was laid down, and
in some cases, to cultivation for some hundreds of years, are
almost indistinguishable in size from those in the subsoil which
have been protected from all these changes,
92

THE COMPOSITION OF THE SOIL 93

However, the soil particles are not wholly unalterable.
The rain water and its dissolved carbonic acid exert a slight
solvent action, and the soil water always contains small
amounts of calcium and magnesium compounds, silica, and
other substances in solution. Each individual particle only
loses a very minute amount of substance to the soil water,.and
its life is extraordinarily long; nevertheless dissolution is
perpetually taking place. The surface soil contains less of the
smallest, and, therefore, most easily attacked, particles than
the subsoil.

In any region where the rainfall and temperature conditions
are favourable, soil rapidly covers itself with vegetation ; even
a bare rock surface is not without its flora. The first vegeta-
tion must obviously have obtained its mineral food from the
dissolved material of the soil particles, but when it died and
decayed all the substances taken up were returned to the soil,
so that subsequent vegetation has food from two sources : from
the substances dissolved direct out of the soil particles during
the life of the plant, and from those dissolved out at earlier
times and taken up by previous races of plants. Thus in the
natural state, and where the vegetation is not removed, the
mineral plant food can be used over and over again and indeed
tends to accumulate as fast as it is extracted from the soil
particles by the rain water.

The plant, however, returns to the soil more than it takes
away ; during its life it has been synthesising starch, cellulose,
protein, and other complex unstable and endothermic material,
much of which falls back on the soil when it is dead. This
added organic matter introduces a fundamental change because
it contains stored-up energy ; the difference between the soil
as it now stands and the original heap of mineral matter is
that the soil contains sources of energy while the mineral
matter does not. Hence it soon becomes the abode of a
varied set of organisms, drawing their sustenance and their
energy from the accumuluted residues, and bringing about
certain changes to be studied later; some, as we shall see, are
capable of fixing gaseous nitrogen and so increasing the supply

94 SOIL CONDITIONS AND PLANT GROWTH

of protein-like compounds, whilst others can assimilate carbon
dioxide.

Thus the complex that we speak of as the soil consists of —
four parts :—

1. The mineral matter derived from rock material, which
constitutes the frame-work of the soil and is in the main
unalterable, but it contains some _ reactive ee
products.

2. The calcium carbonate and phosphate (the latter bets
usually in much smaller amount), and organic matter derived
from marine or other organisms deposited simultaneously with
the soil. :

3. The soil water, a dilute solution of carbonic acid con-
taining small quantities of any soluble soil constituent.

4. The residues of plants that have grown since the soil
occupied its present position, consisting of the mineral plant
_ food taken up from the soil water and of part of the complex
organic matter synthesised during their life period. As the
source of energy for the soil population this may be regarded
as the distinguishing characteristic of soils.

These four constituents are invariably present, but not in
the same proportion; their relative abundance affords the
basis on which soils are classified. From the agricultural
point of view we thus have: (a) mineral soils consisting mainly
of rock material, subdivided into sands, loams, and clays;
(4) calcareous soils containing notable amounts of chalk or
limestone ; (¢) alkali soils rich in soluble, saline matter; (d)
acid humus or peat soils where much organic matter has ac-
cumulated in absence of calcium carbonate ; (e) neutral humus
soils where much organic matter has also accumulated, but in
presence of sufficient calcium carbonate to prevent acidity.

By far the greater proportion of agricultural soils beldng to
the first group.

The Mineral Portion’ of the Soil.

It is usual to divide the soil into a number of fractions by
a sedimentation process, and this method has been used for a

-

THE COMPOSITION OF THE SOIL 9s

large number of analyses, which have given information of
considerable value to the student and the cultivator. The
method is fundamentally defective in that the grouping is quite
arbitrary, sharp lines being drawn where none exist in Nature,
and the soil is represented as a mixture of five or six different
substances when in point of fact the number of components is
indefinitely large. It is very difficult to use the resulting
figures for further investigations: they cannot be plotted on
curves or reduced to simple factors. They can, it is true, be
set out in columns which are very convenient for lecture
purposes. Some limited success has followed the attempts to
correlate the figures with other soil properties, as will be shown
later; but in the main the actual figures obtained have not
proved very fruitful to investigators, though the broad general
results have been useful.

Very much better results might be expected if a distribu-
tion curve could be obtained, showing how the particles are
distributed according to size.

A certain amount of artificiality seems inevitable, and one
must still keep to the conventional “ effective radii’’ and deal
with the soil particles, not as they are, but as they would be
if they were perfect spheres. A distribution curve on these
lines, while not perfectly expressing the soil conditions, would
be a great advance on anything that we have at present.

A serious effort to obtain such a curve has been made by
Sven Odén, whose elegant method of mechanical analysis
deserves serious attention from soil investigators. Instead of
ascertaining the weights of the fractions of soil falling between
certain limits of size, Odén proceeds in the reverse order, and
ascertains the time taken for small successive equal weights to
fall through a column of water. A suspension of soil in water
is poured into a cylinder near the bottom of which is a large
flat plate attached to one arm of a balance, the other arm
being counterpoised and containing in its pan a small weight.
As soon as the weight of the soil particles settling on the plate
begins to exceed the weight in the pan the balance moves:
this makes an electric connection which registers the time on

96 SOIL CONDITIONS AND PLANT GROWTH

a chronograph and causes a second weight to fall into the pan,
thereby restoring the balance to its original position (Fig. 14).
The process is repeated when the weight of sediment again
just exceeds the weight on the pan.

_ The second step consists in calculating from the known

O
Q
: Ty} 4
7 4)
= 6B
nTiT o
— wa
~ yl € R = R y

=

Fic, 14.—Sven Odén’s apparatus for mechanical analysis of soil diagrammatically
represented (218c).

laws of motion of solids in fluids the mass or number of
particles of any given radius present in the soil: but as the
actual particles are very irregular in shape it is necessary to
adopt an “equivalent” or “effective” radius, z.e. the radius of
a sphere which would fall with the same velocity as the particle.

THE COMPOSITION OF THE SOIL 97

Odén gives a formula for effecting the calculation, but its use
requires some mathematical training.’

The type of curve finally obtained is shown in Fig. 15.
This is a mass distribution curve: not a frequency curve.
On the horizontal axis are plotted successive values of the
radii. The axis of y represents a complex function which
gives the percentage weight of particles comprised between
successive integral values of u. Thus the percentage weight
of particles between 1 and 2y in diameter is the area bounded
by ordinates drawn at points 1 and 2 on the axis of x: without

5

a

fonda + 4. |

—

8

&

b

1
'
'
!
'
'
!
a
'
'
!
‘
'
1
'
!
7
U

PERCENTAGE WEIGHT

1 i i i i. i

° te} 1o iS 20 25 30 35

EQUIVALENT RADII uw

Fic. 15.—Mass distribution curve obtained by Odén’s method of soil analysis.
sensible error it is represented by the ordinate drawn from the
point 1°5.
_ It is improbable that this could ever become a working
analytical method ; but as a method for investigation it sur-
passes any other at present available, because for the first
time it affords the possibility of representing the soil fractions
by a distribution curve of the type familiar to physicists and
mathematicians.

1A full description of the method and the mode of calculating the results

will be given in B. A. Keen’s monograph in this series. A simpler method is

described by O, Wiegner, Landw. Versuchs-Stat., 1918, 91, 41-80.
7

98

SOIL CONDITIONS AND PLANT GROWTH

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THE COMPOSITION OF THE SOIL " 99

For practical purposes it is necessary to have resort to
simpler but less exact methods.

By the method of mechanical analysis described in the
appendix the particles of soil can be sorted out into fractions,
each falling within certain specified limits of diameter ; but
unfortunately there is no international agreement as to the
limits, so that the same words are used in different countries
with different meanings. Some of the commoner units are
given in Table XXVIII.

The justification of the ‘oo2 mm. limit for clay adopted in
Britain is that a tolerably sharp change in the physical pro-
perties of bodies does occur at or about this point.’

The difference between the British and the American units
can be considerably reduced by splitting the fine silt fraction
into two, vz. O’O1 to 07005 mm. diameter, and 0'005 to
0002 mm. diameter. This does not greatly lengthen the
analysis, and it should always be done? when there is any
likelihood of comparison with American results. There still
remain small differences in the fine sand and silt which in many
cases would not greatly affect the discussion.

If none but pulverising forces had been at work during
soil formation the soil particles would be identical in composi-
tion with the original rock. But weathering and leaching have
always wrought changes, and in extreme cases only the most
resistant minerals have survived unaltered. A succession of
grades may therefore be expected, shading off imperceptibly
one into the other : from the extreme grade where the changes
have been at a minimum and the soil particles, both coarse
and fine, are complex in composition : through the intermedi-
ate grades where more change has occurred, but a number of
minerals can still be found in the coarse particles : to the other
extreme where the coarse particles copsist almost wholly of
silica, everything else having gone. The case of minimum
change is exemplified in arid soils. The intermediate case is
seen in soils derived direct from igneous or old rocks and

1R. Zsigmondy, Colloidal Chemistry, trs. E. B. Blair, 1917.
2 See p. 355 for method.
7*

100 SOIL CONDITIONS AND PLANT GROWTH

either left 2% sztu or transported only a short distance, such as
the soils investigated mineralogically by Lagatu (82) in France,
those studied chemically by Hendrick and Ogg (132) in Scot-
land, and by Robinson (240) in N. Wales, and various glacial
and loessial soils of the United States. The case of extreme
change is shown in the secondary and tertiary soils of the
south-eastern part of England and in the soils of the Atlantic
coastal plains of the United States. "4

These grades are illustrated by the following summary by
McCaughey and William (186) of the minerals present in the
sand and silt fractions separated from various United States
soils :—

Sand Fractions. Silt Fractions.
No. of
Soils. Minerals Minerals :
other than| Quartz. | other than| Quartz.
Quartz. Quartz.

Case 1 | Arid ; ‘ x 3 37 62 42 58
Case 2} Residual . : P I2 15 85 21 79
Glacial and Loessial 6 12 88 re 85
Case3| Marine . .. 4 5 95 iil 92

The clay fraction cannot be dealt with by mineralogical
methods: this indeed is their weakness, they break down for
the finest particles where they would be most helpful.’

Failyer, Smith, and Wade (97) have made numerous chemi-
cal analyses which also show the successive eliminations of
minerals other than quartz in these grades of soil :-—

1 An improvement in technique has been effected by Victor M. Mosséri
(Les dépéts Nilotiques des Gazayer et Saouahel d’ Egypte. Bull. de V'Inst. d’ Egypte,
IgIQ, I, 151-180).

THE COMPOSITION OF THE SOIL IOI

Percentage of KgO in | Percentage of CaO in casas -* of P2Os5

Sand. | Silt. | Clay. } Sand. | Silt. | Clay. [Sand.| Silt. | Clay.

Type 1| Arid . «1 3°05 | 4°15 | 5°06 | 4°09 | 9°22 | 8°03 | *19 | *24 | °45
Type 2| Glacial .] 1°72 | 2°30 | 3°07 | 1°28 | 1°30 | 2°69 | “15 | *23 | ‘86
Residual .| 1°60 | 2°37 | 2°86] *50| °82)| ‘94 | °07 | ‘22 | °7o

Type 3| Coastal .| °37 | 1°33 | 1°62] °07 | ‘I9 | °55 | °03 | *I0 | °34

Table XXIX. shows the difference in composition between
the fractions obtained by Hendrick and Ogg from the Aberdeen
soil, belonging to the intermediate grade, and the more com-
pletely washed and weathered soils studied by Hall and
Russell, where practically nothing but quartz had survived in
the coarse particles.

So far as is known all the coarser particles are chemically
inert. The clay fraction, on the other hand, stands out in
sharp contrast both in composition and in chemical and physi-
cal properties.'_ At least two groups of clay were recognised
by Hall and Russell, one associated with fertile soils, the other
with less fertile soils. The high proportion of iron and alumina
in the latter case is not causally connected with infertility as’
the clay from fertile soils in North Wales contained even more
(Robinson (240)). The analytical figures throw very little light
on the constitution of the clay beyond showing that it is nota
simple silicate expressible by a definite chemical formula.

By successive extraction with acids of increasing concen-
tration van Bemmelen found (22) two distinct groups of silicates
in the Dutch alluvial soils, one soluble in dilute hydrochloric
molecules of SiO,
molecules of Al,O, ~
soluble only in hot, strong sulphuric acid in which the ratio is

acid in which the ratio 3 to 5,2 the other

1 For some comparable German analyses of clay see E. Blanck, Landw.
Versuchs-Stat., 1918, 91, 85-92.

2The higher numbers were obtained from sandy clays and the lower from
heavy clays. As the silica was insoluble in the acid it was extracted by digest-
ing the residue for a few minutes at 55° with dilute alkali of sp. gr. 1°04.

SOIL CONDITIONS AND PLANT GROWTH

102

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.

THE COMPOSITION OF THE SOIL 103

approximately equal to 2. Other soils of volcanic origin from
Java gave up larger amounts of base relative to the silica, but
in no case were the ratios constant whole numbers; the
alkaline bases showed the same lack of constancy in the ratios
to pee

MoLEcULES oF SiO
TABLE XXX.—RatTI0 2 EXTRACTED FROM VARIOUS
MOLECULES oF AI,O,

Soirs. VAN BEMMELEN (22).

Soktent Temperature and Time (Alluvial Soils} Volcanic jLaterite Soils,
a of Extraction. Holland. | Soils, Java. | Surinam.

HCI of sp. gr. 1°03 | 15 mins. at 55° SSO Wi aed I°r
HCl of sp. gr. r°2 | 1 hour boiling temp. | 3°4 | 4°6 | 2°2 | 2°7 1°6
H,SO, conc. . 4 — 2°0. | 2°4 9'3*2 | 2°O 1°6

Alkaline bases extracted from a heavy clay, Surinam.

Mols. of Bases Extracted
Tempera- AlgO3 Ratio for 1 Mol. of AlgOz3.
Solvent. ture of | Dissolved, | Mols. SiOg
Extraction.| per cent. | Mols. Al,O3°

CaO. | MgO.| K,0.| NagO.

HCl of sp. gr. 1°03 55° I'2 Ya *33. | .°83 | *z0

HCl of sp. gr. 1°r. I00° ate 2°7 "05 «| °32 | °08

HCI of sp. gr. 1°2. | boiling 4°6 2°7 03. «| °I4 | ‘09 | > *or
HCl of sp. gr. r2. is 2°5 ary "03. | ‘Io | *I0

HCI of sp. gr. 1°2. Ni I°9 2°7 03 "08 | ‘12

Conc. H,SO, 4 oo 8°8 2°0 7005 | 06 | ‘17 | 02

Different soils gave up different proportions of alkaline
bases, but again without showing definite simple ratios one to
another. Detailed studies of clay revealed the presence of
chemically unchanged crystals of the original silicates and
also of easily soluble substances including a fusible group with
molecules of SiO,
molecules of Al,O,
resembling kaolin with ratios varying between 2 and 3. The
easily soluble material represents the products of weathering
since it does not occur in rocks. If it were a definite chemical
compound the ratio of its constituents should be constant
whole numbers, but this is not the case. It behaves, however,

ratio varying from 3 to 6, and a silicate

104 SOIL CONDITIONS AND PLANT GROWTH

precisely like a solid solution and is therefore regarded by
van Bemmelen as an ‘absorption compound,” SiO,, mAl,O;,
nFe,O, . . . pH,O, in which the constituents are not chemic-
ally united but are held by the feebler forces manifested by
colloids in their attractions one for the other.'

The Physical Properties of the Various Fractions.—
Serious studies of the soil by competent physicists have
scarcely been attempted as yet, and the work hitherto done
can only be regarded as preliminary. The fundamental
difficulty in applying the ordinary physical methods is to
synthesise the soil; numerous studies have been made of the
physical properties of sand, silt, clay, etc., considered as
separate entities, but no one has worke: © + the resultant
when all the varying grades of sand, silt, ard clay are inti-
mately mingled, or drawn up a scheme or formula to express
the properties of the soil in terms of the mechanical analysis.
More useful results are obtained by the method of correlation ;
soils of known properties are analysed and the results are
correlated so far as is possible with the properties ; even this
method, however, can only be used very crudely, because the
physical properties of the soil as a whole cannot at present be
expressed by definite numbers. Only a very general summary
will therefore be attempted.

The Clay Fraction—The word “clay” is unfortunately
used in many senses; the soil worker and the ceramic chemist
in particular attach widely different meanings to it. In soil
investigations clay is the material of less than ‘oo2 (or in the
U.S.A. ‘005) mm. diameter: in ceramic work it is the
material of o°l mm. diameter downwards. Clay in the soil
sense may be regarded as a plastic colloid, but its special
properties are seen only when a certain amount of water is
present.? If it is well rubbed with water it becomes very
sticky and absolutely impervious to air or water; it is also

1For Gedroiz’ method of estimating the zeolitic silicic acids see Bull.
Internat. Instit. (Rome), 1917, 8, I1g0.

2 Older work on the constitution of clay is summarised by Rohland in
Abegg’s Handbuch der Anorganischen Chemie, 1906, 3, 97-119.

THE COMPOSITION OF THE SOIL a oes

highly plastic, and can be moulded into shapes which remain
permanent on drying and baking. It shrinks very much on
drying and absorbs heat; on moistening again, however, there
is a considerable swelling and evolution of heat. The re-
versibility of the process has been studied by van Bemmelen
(20, 25), who has also shown that the rate at which water is
lost on drying over sulphuric acid is not essentially different
from the rate at which evaporation takes place from a pure
water surface under the same conditions. The separate
particles of clay are so small that, when placed in water, they
assume a state of Brownian movement and sink only very
slowly in spite of their high specific gravity. Traces of
electrolytes have a profound effect on these properties ; small
quantities of acids or salts cause the temporary loss of
plasticity, impermeability, and the property of remaining long
suspended in water without settling ; the clay is now said to
be flocculated. The change can be watched if a small quantity
of any flocculating substance is added to the turbid liquid
obtained by shaking clay with water; the minute particles
are then seen to unite to larger aggregates which settle,
leaving the liquid clear. There is, however, no permanent
change, deflocculation takes place and the original properties
return as soon as the flocculating agent is washed away.
Alkalis (caustic soda, caustic potash, ammonia and their car-
bonates) produce the reverse effect: they deflocculate clay,
intensifying its stickiness and impermeability and causing it
to remain suspended in water for long periods! (see p. 161).

Clay is thus an electro-negative colloid, its reaction prob-
ably being conditioned by a trace of potash liberated by
hydrolysis.

Further, it appears to act as a sistidbeicebie membrane
in relation to the movement of water:? this property might

1Leoncini and Masoni (172) were unable to find that the modification in
permeability in soil caused by saline solutions had any relation to their powers
of flocculating clay.

2Lynde, ¥. Phys. Chem., 1912, 16, 759-778, and ¥. Amer. Soc. Agron., 1913,
5, 102-106. \

106 SOIL CONDITIONS AND PLANT GROWTH

cause some peculiarities of behaviour on sandy soils, such as
the Bagshot beds, where there are thin partings of clay.

A remarkable change sets in when clay is sufficiently

heated, and it permanently loses all its special properties.

Several theories have been put forward to account for
‘the special properties of clay, and in particular its plasticity.
It has often been assumed that these properties are the
necessary result of the smallness of the particles, which brings
into prominence the surface forces. Rohland! attributes
plasticity to hydrated colloidal substances forming a gelatin-
ous film round the clay particles, and Le Chatelier* to minute
flake-like particles. At present the evidence is insufficient
to allow of discrimination.’

These clay properties are of great importance to the fertility
of the soil, and no constituent is more necessary in proper
proportions, or more harmful in excess. Clay impedes the
movement of water in the soil and keeps it in the surface
layers within reach of the plant roots, thus making the soil
retentive of water. Excess of clay, however, interferes too
much with the water movements, making the soil water-
logged in wet weather and parched in dry seasons even though
the permanent water level is near the surface; it-also impedes
the movement of air to the roots and lowers the temperature
of the soil. The adhesive properties of clay cause the soil
particles to bind together into those aggregates on which
‘€tilth ” depends ; soil without clay would be very like a
sand heap. Here also, however, excess of clay does harm
and makes the soil so adhesive that it sticks to the tillage
implements and retards their movements; it also tends to
form large clods unfavourable to vegetation. These effects
are intensified in wet weather; the soil becomes sticky or
“poached” and must not be worked or the tilth is injured for
along time. Another effect of a large amount of clay is to
make the soil shrink very much on drying, so that large
cracks appear in the fields in summer time. These harmful

1P. Rohland, Die Tone (Vienna), 1910.

2 La silice et les silicates. Paris, 1914.
®For discussion see A. B, Searle, British Clays, Shales, and Sauk Igi2

THE COMPOSITION OF THE SOIL 107

effects are reduced by flocculation effected by dressings of
lime or chalk (which become converted into calcium bicarbon-
ate in the soil) and by organic matter; on the other hand,
they are intensified by the deflocculation resulting from the
use of alkaline manures like liquid manure, or by sodium
nitrate, which leaves a residue of sodium carbonate in 'the soil.
Further, as pointed out above, clay “fixes” and retains the
ammonia and potash supplied as manure. In general 8 to
16 per cent. is a satisfactory proportion of clay in a soil where
the rainfall is 20 to 30 inches per annum,

Fine silt (0:01 to 0‘002 mm. in diameter) has also great
water-holding power, and in excessive amounts (above 10 to
I5 per cent.) it increases the difficulty of working the soil,
especially if much clay is present. It does not possess the
marked plastic and colloidal properties of clay, it behaves
differently towards electrolytes (see p. 162) and is less altered
by lime; indeed no method is known for making it tractable.
It is usually less in amount than the clay; certain peculiarities
in cultivation are in some cases manifested where the reverse
obtains, e.g. in the Lower Wealden strata, the Upper Green-
sand and the Lincolnshire warp lands; in North Wales, how-
ever, many soils possess more silt than clay without any
apparent disadvantage.

The coarser grade of sz/¢ (0'04 to O‘OI mm. in diameter)
appears to be very valuable, and constitutes 30 to 40 per
cent. of many of the loams most famous in the south-east of
England for carrying their crops well and not drying out.
The fertile loess soils of the United States are also rich in
silt, containing 55 or more per cent. of material of ‘05 to
‘005 mm. diameter.' Light, sandy loams, on the other hand,
may contain only 10 to 20 per cent.; some of these are
highly fertile, but as a rule they require large dressings of
dung, or a situation favourable for water-supply. Probably
silt plays a very important part in maintaining the even con-
ditions of moisture so desirable for plant growth. It is fine

1J. G. Mosier and A. F, Gustafson, Soil Physics and Management, 1917
p- 64.

108 SOIL CONDITIONS AND PLANT GROWTH

enough to retard, but not to prevent, percolation, and it
facilitates capillary movement of water.

Fine sand (0'2 to 0'04 mm. in diameter) forms a consider-
able fraction—usually 10 to 30 per cent. or more—of nearly
all soils. Although its dimensions are relatively large, it still
possesses cohesiveness and a tendency to cake together; it
has not, however, so great an effect as silt in maintaining a
good moist condition. Soils containing 40 per cent. or more
of fine sand tend to form, after rain, a hard crust on the,
surface, through which young plants can only make their way
with difficulty until it has been broken by a roller. But they
have no great water-holding capacity or retentive power, and
are not infrequently described by their cultivators as hungry
soils that cannot stand drought. The notoriously infertile
Bagshot sands and the barren Hythe beds in West Surrey
‘are largely composed of this fraction, as much as 70 per cent.
being sometimes present. In all these cases, however, clay
is deficient and the situation is dry ; better results are obtained
when the clay exceeds 8 or 9 per cent., or when the water
table is near the surface, especially if the amounts of coarse
sand and gravel are not too high.

Coarse sand (1 to o'2 mm. in diameter) is perhaps the most
variable of all soil constituents in amount, and, as its pro-
perties are in many ways the reverse of those of clay, it
exercises a very great effect in determining fertility. Through
its lack of cohesion it keeps the soil open and friable; in
moderate amounts it facilitates working, but in excess it
increases drainage and evaporation so much as to interfere
seriously with the water-holding capacity of the soil. Many
good loams contain less than 4 per cent., and in general
strong or tenacious soils contain less coarse sand than one-half
the quantity of clay present. When the coarse sand exceeds
the clay in amount the soil becomes light, unless of course
the clay is above 20 per cent., when the soil must always
remain heavy. Not being a colloid, it possesses no power of
absorbing water or soluble salts. Soils containing 40 per
cent. or more of coarse sand and less than 3 per cent. of clay

THE COMPOSITION OF THE SOIL 109

are cultivated only where large quantities of dung are avail-
able, or where the water-supply is exceptionally good. As
the amount of coarse sand increases, the soils become less
and less suited to cultivation, till finally the sand dune con-
dition may be reached, though in adequately moist conditions
cultivation may still continue even when 90 per cent. of coarse
sand and no clay are present (e.g. part of Anglesea).

Fine gravel is not usually present to any great extent, and
is of importance only when the coarse sand is already danger- °
ously high. Stones cannot be determined quantitatively by
any method of sampling in use, and their effect must be
judged by a visit to the field. If they are uniformly scattered:
through a stiff soil, as in the Clay-with-Flints, they are on the
whole beneficial, because they facilitate tillage. Where they
form a bed underlying the soil they may do harm by causing
over-drainage. Some typical examples are discussed in
Chapter VII.

Reactive Inorganic Constituents.

It was formerly supposed that zeolites occurred in the
soil and accounted for many of the soil reactions and absorp-
tions. The direct evidence is so slight that this view is now
generally given up. It seems necessary, however, to-assume
that some of the inorganic constituents of the soil are very
reactive, since certain chemical changes are brought about
which can hardly be attributed to micro-organisms. Thus,
calcium cyanamide is decomposed by soil with formation of
urea, and the change proceeds whether the soil be fresh or
ignited ; this change can also be effected by certain minerals,
e.g. prehnite. So phenol reacts instantly with soil to form
some insoluble compound not decomposable by steam. These
obscure reactions are being studied at Rothamsted (p. 215).

Soil Acidity.

It has long been known that many soils are acid to litmus
paper but become neutral on addition of lime or calcium

/

110 SOIL CONDITIONS AND PLANT GROWTH

carbonate. Many cultivated plants, notably clover and its
allies, fail to grow well on such soils, and they succeed only
after lime has been added: Azotobacter and other organisms
are also adversely affected (p. 239).

The older chemists took the simple and obvious view that
these soils contained an acid or acids, and, as “high moor”
peat showed the same property, they concluded that the acid
was of the same general nature in both cases. It was assumed
that plant residues at a certain stage in their decomposition
formed some acid substance which accumulated in circum-
stances where decomposition became very slow, eg. in badly
drained and badly aerated soils. ;

But instances \vere vserved of acid soils, well drained
and therefore not s| ing from slowness of decomposition,
containing so little organie matter that it was difficult to
attribute acidity to organic compounds. It was therefore
necessary to assume the presence of acid mineral substances
in the soil, and a number of investigations were made showing
that kaolin and similar silicates, which might be expected to
occur in soil, become more and more acid to litmus paper on
treatment with CO, solution.!

Chemical hypotheses, satisfactory as they might otherwise
have been, suffered from the serious drawback that no one
was ever able to isolate an undeniable acid from soils in
any significant quantity. A wholly different hypothesis was
therefore put forward by Cameron. Setting out from van
Bemmelen’s demonstration that humus is a colloid he showed
that all the phenomena of soil acidity could be explained
as simple colloidal manifestations and did not require the
assumption of soil acids at all. It was only necessary to
suppose that the soil colloids absorbed the base more readily
than the acid from blue litmus and the whole phenomena
are explained. In support of this view Cameron showed that
cotton and other absorbents behaved exactly like “acid” soils,
slowly turning blue litmus red ; the phenomenon was therefore
a general property of a class of absorbents.

1 See Gans (101) for a review of the literature of this problem.

~

THE COMPOSITION OF THE SOIL III

Baumann and Gully (100) applied this idea to the case of
peat and showed that it fully explained all the facts then
known.

In the first instance they pointed out that it was not
necessary to assume that the “acid” was a decomposition
product because the original sphagnum was almost as “acid”
as the peat.

Secondly, the acid if it exists must be insoluble because
the water extract of the peat is practically neutral to
litmus.

It must, however, be very potent, because solutions of neutral
salts such as calcium chloride, sodium nitrate, etc., are decom-
posed with liberation of free hydrochloric and nitric acids when
treated with peat or sphagnum.

Baumann and Gully argue that no acid of this character
is known to chemists, and it involves less strain to conceive of
a physical absorption of the base from the dissolved salt with
liberation of the acid than to imagine an insoluble organic
acid capable of decomposing simple salts in solution.

Further reasons for supposing that the phenomena are due
to absorption and not to chemical action are :—

1. The amounts of acid liberated from equivalent quantities
of different salts of the same base are not equal as they should
be in a chemical action.

2. The amounts of base absorbed are not equivalent, e.g.
potassium is absorbed to a greater extent than sodium.

3. The amount of action varies with the concentration of
the solution and the mass of the sphagnum, but not in the way
that would be expected of a chemical change.

4. The electrical conductivity of peat is very low, much
less than that of an acid having the same solvent action on
tricalcic phosphate.

_ The view that acidity of the mineral acid soils is due to
preferential absorption of the base was developed by Harris
(124) in an investigation of Michigan soils. The phenomena
are substantially the same as for peat: the soil turns blue
litmus red: an aqueous extract is neutral: but an extract

112 SOIL CONDITIONS AND PLANT GROWTH

made with a solution of a salt, eg. calcium nitrate, is acid.
We must therefore assume either an insoluble but very potent
mineral acid, or a preferential absorption of the base over the
acid. The latter is indicated because, as in the case of peat,
the amount of acid liberated from equivalent quantities of
different salts is not the same, as it should be in a chemical
reaction.

Daikuhara (744) has applied this view to the case of the
acid mineral soils of Japan and Korea, but he has modified
the explanation and made it more easily intelligible to the
chemist, who finds it difficult to understand why an unparal-
leled physical decomposition of a simple salt should be ac-
cepted, and the assumption of a difficultly soluble but potent
acid rejected. Daikuhara shows that the development of
acidity in the salt solution is due to an exchange of bases and
not to simple absorption of the base from the salt. If the
acid solution is analysed it is found to be really a solution of
an aluminium salt: aluminium being given up from the soil
in amount approximately equivalent to the base absorbed,
Aluminium salts, as is well known, turn blue litmus red and ©
therefore are indicated as acids. The phenomenon is still
essentially an absorption, but the seat of the reaction is
located.

This view is supported by Rice’s experiments (238) which
have demonstrated the substantial identity in hydrogen ion
concentration of a solution of aluminium nitrate and the solution
obtained by treating an “acid’’ soil with potassium nitrate
solution. Hartwell and Pember (128a) also support this view
by an ingenious line ofargument. True acids added to nutrient’
solutions affect barley and rye in water culture similarly,
while extracts of acid soils affect them differently. Therefore
acid soil extracts contain something not present in acid
solutions ; on testing they were found to contain aluminium.
The effect of aluminium salts on plant growth was examined *

1J. B. Abbott, S. D. Conner, and H. R. Smalley had previously attributed »
the infertility of an Indiana soil to the presence of aluminium salts (Ind. Expt. .
Sta. Bul., 170, 1913).

THE COMPOSITION OF THE SOIL 113

and found to resemble that of the acid soil extract. Mirasol!
has confirmed and extended these observations.

Ramann also adopts this physical hypothesis and gives
up the expression “acid soils,” using instead “ absorptiv
ungesittigte Boden”. Kappen? confirms the observations
without entirely accepting the explanation.

Recent work shows that both views have a foundation of
truth ; there are at least two causes at work: lack of bases in
the soil brings about the absorption phenomena discussed
above, but there are also true acids present in soil under
certain conditions. Rindall of Helsingfors (239), Sven Odén
of Upsala (218a), Tacke (278), and Ehrenberg and Bahr (93@)
_have each argued in favour of definite humic acids in peat (see
p. 139). Truog (285) finds, in the case of mineral soils, that
equivalent amounts of different bases are required to neutralise
the acid properties of the soil—which if generally true would
be easier to explain by assuming an acid than an adsorption.

The view that soil acidity is caused by actual acids has
gained support from recent work ® on the hydrogen ion con-
centration in soils.

Chemists study acids in two ways :— |

1. By measuring the hydrogen ion concentration, a value
based on the assumption that an acid on solution in water
dissociates into two parts, called ions—one being hydrogen,
and the other the rest of the molecule.

2. By determining the titration value, z.e. the number of
c.c, of standard alkali solution which a given volume of the
acid solution will neutralise. In the language of the dis-
sociation hypothesis this value measures the total quantity
of hydrogen ions producible under the conditions of the
experiment, supposing them to be neutralised or linked up
with — OH ions as quickly as they are liberated.

1 Soil Sci., 1920, 10, 153.

2H. Kappen, Studien an saurem Mineralbiden aus der Nahe von Fena
(Landw. Versuchs-Stat., 1916, 88, 13-104).

8 For a critical summary of recent work see E. A. Fisher, ¥. Ag. Sci., 1920,

II, 19.
: 8

114 SOIL CONDITIONS AND PLANT GROWTH

The titration value measures the quantity of the acid but
it makes no distinction between a strong acid, such as sulphuric
acid, and a weak acid, like acetic acid: the great difference
in action on plant life exhibited by the two acids is missed
altogether. Moreover, the titration value is not an absolute
constant ; polybasic acids, which are by far the most numer-.
ous, have several titration values, according to whether one,
two or more of the hydrogen atoms are affected. Different
indicators, therefore, give different numerical results. So long
as the constitution of the acid is known this does not matter,
but it causes complications in dealing with a mixture of un-
known acids,

Many of the methods suggested fog measuring soil acidity,
including all the “lime requirement” methods, afford a more
or less rough measure of the titration value’: the differences in
results are partly due to absorptions and partly to differences
in the number of hydrogen atoms concerned.

The determination of the hydrogen ion concentration, 2.2.
of the ions actually present, as distinct from those that would
finally be liberated on neutralisation, is an attempt to measure
the intensity of the acid as distinct from its quantity. The
principle of the method is simple in dealing with solutions
of pure acids: it is based on the ordinary dissociation law :
Poe X!

HX
the dissociation of the acid HX, and K is a constant. The
number or the concentration of the ions is measured by the
electrical conductivity of the solution, but once this has been
determined for a given acid it can become a standard against
which the hydrogen ion concentrations of other acids can be
rapidly compared by means of a set of indicators.” ;

The problem becomes more complex when salts are
present, because the additional ions thus introduced affect the

= K where H* and X’ are the ions produced on

1 Probably all the results are too high, as they include the absorptions. See
H. R. Christensen (67)), also ro5c.

2A useful account of the measurement of the hydrogen ion concentration is
given by J. F. McClendon in Physical Chemistry of Vital Phenomena (1917),
which was written for biological students.

THE COMPOSITION OF THE SOIL 115

amount of dissociation: in particular they make it difficult
for the hydrogen ion concentration to increase when another
acid is added. Colloidal substances act in the same way.
This effect, known as “ buffer action,” is of great importance
in vital phenomena and is well marked in the case of soils.
Thus if an acid is added to soil the increase in the hydrogen
ion concentration is not nearly so great as if the same quantity
of acid were added to water.

The numerical values of the hydrogen ion concentrations
are rather unmanageable and a convention has therefore been
adopted in dealing with them.

In pure water or in a neutral solution

H* = OH’ = 1 x 1077 gram-ions per litre.

Acidity means a hydrogen ion concentration in excess of
that of water, z.c. the index instead of being - 7:0 is greater:
as the quantity is negative this means that the number itself
is less: it may fall to zero. Simple trial will show that it is
very difficult to plot on the same curve numbers ranging from
I to 1 x 10°". A further complication began to arise but
fortunately was checked : investigators expressed their results
in terms not of 1, but of some other number, multiplied by
10 to some negative power, eg. one solution might be repre-
sented by the value 4:4 x 107°, and another by 6°8 x 1077,
and it was rather difficult to make a comparison.

To overcome the first of these difficulties Sérensen ! adopted
the familiar device of plotting the logarithms of the numbers
instead of the numbers themselves ;. and to overcome the
second he reduced all to terms of I x some power of I0:
thus the two cases just quoted become 1 x 10°°** and
I x 10°°!7 respectively. He calls the numbers 5°36 and
6:17 the P,, values of the solutions: in reality they are the
values for the — log (H’). On this notation the Py value for
neutrality is 7:07; that for acidity is anything less—down to
nothing—that for alkalinity is anything greater—up to 14°14.

1S. P. L. Sérensen, Biochem. Zeitsch., 1909, 21, 130, and 22, 352; see also
Ergebnisse d. Physiologie (Asher and Spiro), 1912, 12, 393.
8 *

116 SOIL CONDITIONS AND PLANT GROWTH

In consequence of the buffer action already mentioned
these values are not liable to disturbance by small additions
of substances to the soil.

A large number of determinations of the Py or — log (H’)
values for American soils have been made by Gillespie (1052@)
and G. Sharp and D. R. Hoagland (260), while for British
soils a beginning has been made in the Rothamsted laboratories.
Some of the values are as follows :—

American Soils. oid ;
_| Sharp and Hoagland, | British Soils.

Acid soils, extreme value . s x 3°7 4°6
Fertile soils ‘ : : : F : 7°04-7°52
Alkaline soils, extreme value ‘ "i 9°7

The acidity of root sap is of the order of 5°5-6°8 (p. 81).

The relatively small range of variation in comparison with
the large variation in amount of titratable acidity is explained
by the buffer action described above.

In addition to and probably distinct from this natural
acidity it is possible to induce acidity in soils deficient in
calcium carbonate by the long-continued application of am-
monium sulphate. This was first observed by Wheeler in
Rhode Island (303) and it is demonstrated in a remarkable
manner at the Royal Agricultural Society’s Experiment Station
at Woburn. Apparently this is not the same as “acidity”
liberated by solutions of neutral salts in acids, because am-
monium sulphate appears to act specifically, no other fertiliser
behaving in this manner at Woburn."

Hall supposes, with considerable probability, that the

ammonia is taken up by the plant, leaving the sulphuric acid
in the soil, and this view seems justified by the fact that
nitrate of soda behaves altogether differently, leaving an alkaline
residue in the soil.

1 There are a few cases on record where potassium salts are said to have.

reduced yields, and here it is possible that acid substances have been liberated
by the dissolved salt.

}

THE COMPOSITION OF THE SOIL 117

Taste XXXI.—Errect or Catcium CARBONATE ON THE TEXTURE OF SOILS,

Hamsey Gisan. Rothamsted.
Fi F Too Sticky
r Too Stick Arable Soil,
Arable Soil. for Aeabte. Barnfield. oe
Fine gravel . IF 16 2°4 1°8
Coarse sand : =| 5°3 9°5 5°5 4°9
Fine sand 28°7 22°3 20°3 27°83
Silt 26°3 25°4 24°4 25°4
Fine Silt 10'2 9°9 12°7 10°6
Clay 16*4 160 22°0 I9’0
Loss on ignition . r 4°8 5°2 4°7 5°1
Calcium carbonate x I'02 48 3°0 *16

The present position may be summarised as follows :—

The existence of a hydrogen ion concentration greater
than that of water is satisfactory evidence of the presence of
true acids in soils. The measurements at present throw no
light on the nature or quantity of the acids. The acids may
be— .

(a) Organic,

(4) Siliceous,

(c) Formed by hydrolysis of iron or aluminium salts.
They may adversely affect plants or micro-organisms by
reason of their strength or their quantity. On the other hand,
the acids themselves may be without serious action (see p. 81).
The special properties of an “acid soil” may result from a
lack ov basicity, whereby—

. Special absorption eli ionshine appear ;

2. Lack of calcium may affect plants or micro-organisms ;

3. Certain toxic substances, eg. metallic salts, may remain
effective, which in presence of lime would be thrown out of
action;

4. The clay may become deflocculated and therefore as-
sume a sticky condition unfavourable to plant growth.

Table XXXI. shows pairs of soils similar in constitution
and in general external conditions, temperature, water-supply,

118 SOIL CONDITIONS AND PLANT GROWTH

etc., but very different in agricultural value because of their
different content of calcium carbonate, one being readily
cultivated while the other is wet and sticky, and suitable only
for pasture land.

“Sourness’’ of Soil: Its Relation to Acidity and Calcium
Carbonate.

Soils which are infertile from lack of calcium carbonate
are called “sour” by farmers. The older chemists substituted
the word “acid,” but, as shown above, the effect is not
necessarily in all cases attributed to acids, and therefore it
seems desirable to retain the farmers’ term as the broad one
and distinguish between ‘“sourness” due' to acidity and that
due to lack of basicity.?

Whatever its cause in a given case sourness can be over-
come by addition of lime or calcium carbonate ; soils containing
calcium carbonate are never sour, and they are generally
fertile.

It by no means follows, however, that soils devoid of
calcium carbonate are infertile. Hendrick and Ogg at Aberdeen
(132) and Robinson in N. Wales (240) have both described
soils free from calcium carbonate but fertile and indeed
neutral in reaction: Hoagland observes? that certain Cali-
fornian soils with low Py value, ze. considerable intensity of
acidity, and large “lime requirements,” are nevertheless able
to produce excellent crops of many types.

In the present state of our knowledge it is hardly possible
to say from chemical examination of the soil alone whether
it is “sour” or not, z.e. whether it is or is not infertile through
lack of calcium carbonate. A combination of chemical analysis
and field observation, however, enables something to be done:
soils known by vegetation observations to be comparable can

1 This seems preferable to the distinction made by some of the investigators
between “ positive acidity” caused by actual acids, and ‘‘ negative acidity ”’
caused by adsorption of base (cf. Lyon, Soils and Fertilisers, p. 112. See also
H. R. Christensen, Soil Sci., 1917, 4, 115, and C. J. Schollenberger, Soil Sci.,
1917, 3; 279).

2 Private communication to the author.

THE COMPOSITION OF THE SOIL 119

be set out in order of their calcium carbonate content by the ~
usual analytical methods, or of their lack of calcium carbonate
by one of the lime requirement methods.’ Before any indi-
cation can be given of the amount of lime required for cultiva-
tion it is necessary to make field trials (p. 335).

In general, sandy soils require only sufficient calcium
carbonate to prevent sourness, while clay soils need in addi-
tion enough to keep thé*texture good. Sands well supplied
with calcareous water and under ordinary arable cultivation
may get along with o'! per cent. or even less calcium carbon-
ate. Many light soils that are intensively farmed respond to
dressings of chalk or of ground limestone, even though 0-2
or 0°3 per cent. is already present. 0°5 per cent. of calcium
carbonate commonly proves insufficient for clay soils, and
even IO per cent. may not be enough in highly-farmed
districts, especially where cattle are fed on the land and tread
the soil into a somewhat sticky state. Further increases in
calcium carbonate over and above the proper amount are not
known to have any effect except to provide a margin of safety.
Considerable work has been done on the effect of lime on peat
soils, which seems to be more than a neutralisation of acidity.
Odén (2182) suggests that the calcium humate produced may
be directly beneficial to plants.

Calcium carbonate is not a permanent constituent of the
soil, but changes into the soluble bicarbonate and washes out
into the drainage water ; the average loss per acre per annum
throughout England and Wales has been estimated at 500 Ib.,
and at Rothamsted on the arable land at 800 to 1000 lb.
(1206). The rate of loss is influenced by the treatment, being
increased by the use of ammonium sulphate and decreased by
dung and by a growing crop; it is much less on pasture than
on arable land. Repeated additions of calcium carbonate to

1 Such as Veitch’s (287) or Hutchinson and McLennan’s (140d). L. J. Wild,
F. Ag. Sci., 1917, 8, 154, shows that the latter when modified by a correction
factor of o*r per cent. proves helpful in the study of New Zealand soils. For
a critical account of the various methods see Ames and Schollenberger (4) and

E, A. Fisher (99). It is hardly necessary to point out that the “lime require-
ment” measures something entirely distinct from the Py value (p. 115).

120 SOIL CONDITIONS AND PLANT GROWTH

the soil are, therefore, necessary : indeed, chalk and lime are
among the oldest of manures. Soils lying immediately above
chalk and limestone are no exceptions, and in wet regions they
may become thoroughly decalcified.

On chalk soils the percentage of calcium carbonate may
rise very high, and then a wholly new set of properties comes ©
in. It is impossible to draw any exact line showing where
these properties begin to appear, but they entirely mask the
effects of the silica and silicate particles and obliterate the
distinctions between sands, loams, clays. Chalk soils, there-
fore, form a class by themselves to which the ordinary
laboratory methods of analysis and investigation do not apply :
unfortunately, appropriate methods ha. not yet been worked
out.

The Phosphorus, Potassium, and Calcium Compounds
of the Soil.

Part of the phosphorus in the soil is probably present in
-organic combinations derived’ from plant or animal resi-
dues : this has been investigated by Vincent,’ by Potter and
Snyder” and by Schollenberger ;* none of whom, however,
were able to effect identification of the compounds. Part,
however, is in inorganic combination and Bassett has shown *
that it most probably occurs as hydroxyapatite—

(Ca,P,0,),Ca(OH),,
this being the solid phase stable over a range extending from
faintly acid to alkaline conditions: any phosphate such as
superphosphate or basic slag added to the soil as fertiliser
would tend to be converted into this substance.

The potassium compounds in the soil are probably silicates,
Their distribution has been studied by Dumont® who found
that they may occur in the finer or coarser portions of the

1 Compt. Rend., 1917, 164, 409. 2 Soil Sci., 1918, 6, 321-332.

3 Ibid., 365-395 ; 1920, 10, 127. In this case about one-third of the phos-
phorus occurred in organic combination.

4 Trans. Chen. Soc., 1917, 111, 620-642.

® Compt. Rend., 1904, 138, 215-217.

THE COMPOSITION OF THE SOIL 121

soil with very different effects in either case. He instances
two soils of nearly equal potash content, one of which from
la Creuse responds to potassic fertilisers, while the other from
Grignon does not: in the former case the potash is present
mainly in the coarser material, in the latter aad in the fine
(Table XXXII).

Tarn_e XXXII.—DisrrisuTion oF Potash AMONG Sort ParticLes (DUMONT).

Percentage Distribution
Response to Per of Potash in Soil.

Per cent. of Per cent. of} cent.

. : Potassic i 2
K;O in Soil. *13 * Argile”. | KoO in
Fertilisers :
: Argile | Sabl Sable :
Greesier. Fin. Argile.
Grignon . 0°85 Nil 16°38 0°94 166 | 65°38 | 17°7
La Creuse 0°89 Good 4°5 0°51 70°9 |-26%4 2°7

The calcium compounds of a number of soils have been
studied by E. C. Shorey, Fry and Hazen (263). The follow-
ing were detected by petrographic methods :—

Frequently— ~
Hornblende, chiefly Ca(MgFe),Si,O,, with Na, Al,Si,O,,
and (MgFe),(AlFe),Si,O,..
Plagioclase, isomorphous mixtures of CaAISi,O, and
NaAlSi,QO,.
Epidote, Ca,H(AlFe),Si,O,3.
Occasionally—
Calcite, CaCO .
Titanite, CaTiSiO,.
Garnet (Ca, Mg, Fe, Mn),(Al, Fe, Cr, Ti),(SiO,)s.
Rarely—
Dolomite (Ca, Mg)COs.
Augite, CaMgSi,O, with (Mg, Fe)(Al, Fe),SiO,.
Gypsum, CaSO, + 2H,0O.

Chemical analysis showed that pairs of soils containing the
same total CaO might differ considerably in their content of
the various calcium compounds (Table XX XIII.) :—

122 SOIL CONDITIONS AND PLANT GROWTH

TaBLE XXXIII.—Catcium Compounps In Soir (SHorzy, Fry & Hazen (263)).

CaO as :—

Soil No. | Total CaO. | CaCOz.
Easily Decom- Difficultly De- Humus
posable Silicates. | composable Silicates. | Combination.

58 5°37 2°50 0°54 2°33 Nil
61 4°64 0°06 O°IL 4°34 0°13
24 1°98 oO°12 0°02 1°84 Nil
36 I'92 0°49 0°34 0°98 OvrL

The Soil Water or Soil Solution.

The soil retains |, .isorption and surface attractions some
10 to 20 per cent. of its weight of water, distributed as films
over its particles. This water is of obvious importance as the
medium through which plants and micro-organisms derive
their food ; indeed, it may be regarded as the culture solution
for.the plant. Its relationship to the mineral matter is dis-
. cussed by Cameron (65¢, d, and e) and the, displacements of
equilibrium under the influence of climate, plant roots, etc., by
Nolte (216). Several methods have been used for extracting
the solution in order to determine its composition.

As often happens the pioneer work was done in France:
Schloesing in 1866 (245e) devised a method which is still
often used—displacement of the soil solution by means of
water.

Displacement Methods.—Schloesing placed 30-35 kgms,
of freshly taken soil in a large inverted tubulated bell jar and
poured on it water coloured with carmine, this being done to
simulate the action of rain. The added water at once dis-
placed the soil water and caused it to descend so that it could
be collected: a sharp horizontal line of demarcation between
the added and the original water persisted throughout the ex-
periment even when 8 days were occupied in the descent. A
typical analysis of the displaced liquid in milligrams per litre
was :—

THE COMPOSITION OF THE SOIL 123

SiO,. | “Nitric | Carbonic] cao, | mgo. | Ka0. |NagO. | SMPBUTIC | Chiorine, | Qreanic

29°1 305 118 | 264 | 13°5 | 69 | 7°8 57°9 7°4 37°5

and in addition traces of phosphorus and of ammonia. This
soil contained 19°I per cent. of water.

The total concentration is seen to be about ‘08 per cent.

Gola (108) also adopted the artificial rain device but the
addition of water ceased as soon as a regular flow from the
soil had set up: the soil was then allowed to drain for some
36 hours ; the solution thus obtained was called the “ pedolytic ”
solution. Finally, the water was squeezed out by a screw
press ; this was called the “ pedopiezic” solution. The total
concentration varied in different soils from 2 per cent. down
to 0-2 per cent. or less, the lower amounts being from soils
corresponding to normal agricultural soils (see p. 237). The
weak point in the method is that the added water may upset
some of the adsorption relationships of the soil and thus
vitiate the results. Itscherekov! substituted methyl or ethyl
alcohol; van Suchtelen? used paraffin oil; J. F. Morgan (203)
improved on this by using paraffin oil under pressure and
obtained a solution containing 0°04 to o'13 per cent. of total
solids. Some of his analyses are given in Table XXXIV. :—

TABLE XXXIV.—ComposiTIon or Sort SOLUTION (O1L DISPLACEMENT).
J. F. Morean (203).

Parts per millionin Soil - Parts per million in
Mature of Méidstute Solution. 2 Oven-Dried Soil.
Soil. in Soil.
K. PO4. Ca. N. K, PO,. Ca. N.

Fine sand maga, | 24°r | 6°21. 30°}. g°r 72 | 2's g'r | o'9
Loam . 37°8 Orr) E392) 68°32) 3°2: | 27°0°| 4°6 25'°9 ¥a
Clay : 24°5 44°38} 4°6.| 42°9 | 6°6 | 1r°0 | i'r 1o°6 | 1°6
Peat .| 132°9 50°r | 2°5 | 183°8 | 17°t | 94°9 | 3°4 | 244°3 | 22°7

1 Russian ¥. Expt. Ag., 1907, 8, 147-165.
2. fiir Landw., 1912, 60, 369.

124 SOIL CONDITIONS AND PLANT GROWTH

The clay also contained 32°6 parts of magnesium in the
dried soil and 132°3 in the solution.

A more rapid, but less accurate method of obtaining a soil
solution has been adopted by the United States Bureau of
Soils. The soil is stirred up with water and filtered under
pressure through a Chamberland filter. “The results are taken
to indicate the following average composition of the soil
water :-—

PO; OND, Ca K .

704 547 11167 22°74 per million of dry soil?

A centrifugal method was used by Whitney and Cameron
(305a) with the results given in Table XXXV. :—

TaBLE XXXV.—CompositTion oF Sor, SOLUTION (CENTRIFUGAL METHOD),
WHITNEY AND CAMERON (305a).

Soil Solution, parts Dry Soil, parts
per million. per million.

PO4. | NO3.| Ca. | K. | PO,g. | NOg.|Ca.| K.

Sassafras loam, New Jersey—

Wheat, good . : i - | 7°20 | 7°20 |44°40/33"60} 1°35 | 1°35 |8°34/6°31

$i SPOOL: 7°00 | °40 |26°g0/24°4c] 1°40 | *08 |5°38)4°88
Leonardtown loam, Maryland—

Wheat, good . : 5 - | 6°30 | 1°44 |16°20/21°6o} 1°38 | +32 13°5614°75

‘3 Seales . ‘ - | 8°40 | 4°08 |21°60/38°40] 1°48 | *72 |3°80/6°75

igh | POOF. . : - | 9°75 | 4°80 | 8°50)19°25} 2°45 | r°2z |2°12/5°10

The method, however, is difficult in application and has not
come into general use.

Pressure methods have been adopted by several investi-
gators. In Ramann’s (233c) laboratory 3 kilos of soil were!
subjected to a pressure of 300 kilos per sq. cm. The amounts
of calcium and of potassium were found to vary considerably
in different extracts obtained from the surface soil, the pro-
portions relative to the other constituents increasing as the
solution became concentrated by dry weather and falling as
the solution became diluted by rain. In extracts prepared

1 The Bureau of Soils prefers to express the compositions in terms of dry soil
rather than of solution.

THE COMPOSITION OF THE SOIL 125

from the subsoil, on. the other hand, the amount of calcium
showed less variation, except only for a rise at midsummer.
There was evidence of a transportation of calcium and
potassium from the subsoil to the surface during a prolonged
period of drought. No indication was obtained, however,
that soil adsorption exercised any regulating effect on the
concentration of the soil solution; an exchange of bases took
place only when the proportions between the dissolved sub-
stances were altered.

In van Zyl’s experiments (323) the concentration of the
solution obtained also varied according to manurial treatment
and season of the year, but the percentage composition of the
_ ignited dry matter of the extract remained constant; this
being the reverse of what Ramann obtained.

A steel plunger was used by C. B. Lipman.*

An absorption method used by Briggs and McCall consists
in driving a Pasteur Chamberland filter into the soil and con-
necting it with an exhausted 2-litre bottle.?

A totally different method is adopted by Bouyoucos who
studies the changes in the soil solution zm s¢tu by determining
the changes in the freezing-point (49c). He finds the osmotic
pressure varies between 0-2 to I atmospheres in moist soils,
and 4°5 to 16°5 at low percentages of moisture. The con-
centration of the extracted solution was 0°04 to 0°18 per cent.
in moist soils and o’9 to 3°0 per cent. in drier soils.

Some of his results are given in Table XXXVI.

Hoagland and Sharp (1364) have applied the method to
Californian soils with interesting results.? They find no evi-
dence that the soil water is a saturated solution: on the con-
trary, it appears to be dilute, probably containing less total
dissolved matter than the soil extract, and it varies in con-
centration with the conditions: in their experiments the range

1 Univ. California Pubn. Ag. Sci., 1918, 3, 131.

2L. J. Briggs and J. R. McCall, ‘An Artificial Root,” Science, 1904, 20,
566-569; Bull. 31, Bureau of Soils, rgor.

3 For further results see J. C. Martin and A. W. Christie, Journ. Ag. Res.,
Ig1Q, 18, 1390.

SOIL CONDITIONS AND PLANT GROWTH

126

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THE COMPOSITION OF THE SOIL 127

was generally from ol to I atmosphere, The seasonal varia-
tions are shown in Fig. 16.

These various methods naturally give different results.
The general conclusion, however, seems to be that the solution
in a normal agricultural soil contains mainly calcium nitrate
and bicarbonate; with some organic matter, sodium, mag-
nesium, silica, chlorine, sulphuric acid, less potassium, a trace
of ammonia, and only little phosphate. In ordinarily moist

025
CROP NONE

act

010+ ia Roe BA

vad soil) WS
ae Baslis of 177 moisture
3

-005

OrowasldWas 16 Wns 24Wks. 32 Wks. 40Wks. SIWxs
Juv JO. Jui 24. Aue 21. 0ct23. Dec.18. Fes.l7 Mav |
Time from planting.

Fic. 16.—Variations in concentration of soil solution with crop and season, show-
ing that the barley crop much lowers the concentration even after its
removal in August. On the vertical axis one-quarter of each freezing-point
depression is plotted: an approximate estimate of the corresponding osmotic
pressure is obtained by multiplying the plotted value by 50 (Hoagland and
Sharp, 136d).

, 7

soil it has a concentration of the order of 0-1 to 1°o per cent.,
or some 0°5 to 5 atmospheres osmotic pressure; this varies
however, with the rainfall and the manuring. The propor-
tions of the components also varies, again changing with the
season and the manuring. The nitrates are perhaps the most
variable constituents, but the calcium and potassium also vary
both relatively and absolutely—excepting in van Zyl’s ex-
periments.

The numerous analyses of land drainage water that have

128 SOIL CONDITIONS AND PLANT GROWTH

been made in this country and on the Continent throw some —
light on the composition of the soil solution. As might be
expected from the known absorptive properties of clay and of
humus, drainage water contains mere traces of NH, and PO,,
and only little K; it contains chiefly carbonic acid, SiO,, Cl,
SO,, NO;, Ca with some Fe, Mg, and Na. Typical analyses
are given in Table XX XVII.

TABLE XXXVII.—ANALYsSIS OF DRAINAGE WATERS FROM CULTIVATED FIELDS :
PARTS PER MILLION OF SOLUTION.

Rothamsted: Broadbalk Field.1 {Field at Gottingen.?
| ‘
N 1 C lete | Highest | Li t
Manure: Dung. Artificials, Result. Result.
Plots 3and4.| Plot 2. Plot 6.

CaO ; ‘ , é 98° 147°4 143°9 184 157
PEO he i Pee 51 4°9 79 46°4 | 31°3
K,O Sr Sy ae 1°7 5°4 4°4 3°7 1'7
Na,O ‘ 6°o 13°F I0°7 — —
FeO; . 5°7 2°6 2°7 Meas ~~
Cl q 10°7 20°7 20°7 _ a
SO; . 24°7 To6°I 73°3 59°2 | 43°5
P.O, : ; _ 1°54 _—
sid, ‘ $ 10°9 35°7 24°7 —_— a
N as NH, - : 14 *20 _ *24 — --
N as Nitrate é I5'0 62°0 32°9 8:2 I‘o
Organic matter, CO,, etc. 67°7 77°3 84°6 — —
Total solids . . ‘ 246°4 4760 407°6 —_ _

It will be observed that the total concentration of the
Rothamsted drainage water varies from ‘02 to ‘05 per cent.

Organic Matter. ~

The distinguishing characteristic of soil is that it contains.

part of the complex material synthesised by plants. This

material affords energy to numerous micro-organisms, and is

gradually converted by them into simple substances appro-

1A, Voelcker’s analyses of five samples collected between 1866 and 1869.
(280¢). :

2 Von Seelhorst’s analyses of samples collected weekly, or fortnightly, from
a field between August, 1899, and August, 1900 (258)

THE COMPOSITION OF THE SOIL 129

priate for plant nutrition. We may look upon its constituents
as taking part in a perpetual cycle: in one stage nourishing
the growing plant and storing up the energy of sunlight, in
the other stage nourishing micro-organisms and liberating
energy. In addition, it has important physical effects on the
soil. Unfortunately, not much is known of the highly com-
plex components of the plant and even less is known about
the important organic substances of the soil. The difficulty
of working with insoluble, unstable bodies mingled with
twenty times or more their weight of sand, silt, and clay has
hitherto proved almost insuperable. The ideas current in the
textbooks go back to the time before organic chemistry arose,
and have come down direct from C. Sprengel (2702), Mulder
(205) and Detmer (84).

We can thus only speak in the most general terms about
what is admittedly the characteristic component of soil. Two
great groups are to be carefully distinguished: one furnished
by recent generations of plants; the other deposited with the
soil during its formation, and therefore as old as the soil itself.
Unfortunately, no actual method of separation is known, but
some idea of the amount and properties of the original organic
matter can be obtained from a study of the subsoil at depths
below the root-range of plants. Ten feet or more below the
surface, sandy subsoils usually contain less than ‘oI per cent.
of nitrogen and clays less than ‘05 per cent., but shales con-
tain more than ol per cent. The percentage of carbon
fluctuates, but is usually five to ten times that of nitrogen
(20042). Now these values are about one-tenth to one-fifth of
those obtained in the surface soil, so that at the very outside,
and assuming there has been no decomposition, not more than
10 to 20 per cent. of the surface organic matter is original.

The organic matter furnished by recent vegetation may
roughly be classified as: (1) material that has not yet had
time to decompose and still retains its definite cell structure ;
(2) partially decomposed and still decomposing material ; (3)
simple soluble decomposition products ; (4) plant or animal
constituents not decomposable in the soil.

9

130 SOIL CONDITIONS AND PLANT GROWTH

The undecomposed material is important as the reserve
supply for the entire chain of reactions to be considered later.
It also has a certain mechanical effect in opening up the soil
and facilitating aeration and drainage, an effect useful on clays
but often harmful on sands where these processes already
tend to go too far.

The partially decomposed material forms a particularly
vague and indefinite group containing all the non-volatile
products of bacterial, fungal, enzymic and other actions on
the plant residues. It shades off in one direction into the
simple soluble decomposition products, and in the other into
undecomposed plant fragments, so that it cannot be sharply
defined or accurately estimated. A detailed study of the
group being thus out of the question, we must ascertain in the
first instance what part it plays in determining those relation-
ships between the soil and the living plant that it is our
business to study, and then, when we know what to look for,
try to discover what constituents are important from our point
of view and fix attention on them. For the preliminary
inquiries recourse is had to the indirect method of correlation
already used in ascertaining the properties of the mineral
fractions of the soil. Numerous studies on these lines have
proved that this group (or some component) possesses at least
six properties not usually shown by the undecomposed plant
residues.

1. It gives a dark brown or black colour to the soil.

2. It can withdraw various ions—NH,, K, PO,—from
their solutions. The experiments of van Bemmelen (10; 21)
indicate a complete parallelism with clay in this respect.

Baumann and Gully (10) and Odén (2184) show that un-
decomposed sphagnum can absorb ions from solutions, but the
phenomena differ in detail from those shown by humus.

3. It causes the soil to puff up, or in the expressive phrase
of the German farmer, to “ferment’’ (Bodengdérung), and so
leads to an increase in the pore space (see p. 220). From
this results a marked improvement in the tilth and general
mechanical condition. The Rothamsted mangold plots re-

THE COMPOSITION OF THE SOIL 131

ceiving no organic manure, and therefore poor in partially
decomposed organic matter, get into so sticky and “un-
kindly” a state that the young plants have some difficulty

ie
rane

May

Apr.

Mar

Feb.

(Broadbalk field, Rothamsted.)

Jan
i914

Novy.
17.—Curves showing the percentage of water in two soils on adjacent plots, one of which

annually receives farmyard manure while the other does not.

Dec

Ko
‘72 2a
Aug Sep Oct

QO
a

|
ly

June Ju
1913

_ aera ones aa

-

aun siow Jo abequaovay

Fic.

in surviving however much food is supplied, and may fail

altogether if bad weather intervenes in the spring (as in 1908

and 1911); the dunged plots which are rich in this group

are much more favourable to the plant and never fail to give
9 *

132 SOIL CONDITIONS AND PLANT GROWTH

a crop. But the puffing up or “ lightening” may go too far,

and sometimes causes much trouble in old gardens that have

long been heavily dunged.

| 4. It increases the water-holding capacity of the soil. The
amounts of moisture present in adjacent plots at Rothamsted
are shown in Fig. 17, from which it appears that the plot
‘annually receiving farmyard manure contains normally 3 or 4
| per cent. more water than the adjoining plot receiving no
| organic manure.

The variations in water content follow very closely the
variation in the amount of organic matter present. So marked
are these physical effects that if 15 or 20 per cent. of organic
matter is present in a soil the operation of other factors ceases.
to count for much, and the distinctions between sands, loams,
and clays are obliterated. Thus, much of the famous Red
River prairie soil of Manitoba is identical in mineral composi,
tion with certain poor infertile Wealden soils, but the presence
of 26 per cent. of organic matter completely masks the harm-
ful effect of the clay and fine silt. A similar pair of soils,
owing their difference in agricultural properties to their different
organic matter content, have been analysed by C. T. Giming-
ham (106) (Table XXXVIILI.).

TaBLE XXXVIII.—EFFEcT or OrGAnic MATTER! ON THE TEXTURE OF
SoILs.

Good Texture. | Poor Texture. | Good Texture. | Poor Texture.

Manitoban Prairies. | Weald Clay. |(Reported by C. T. Gimingham.)}

Fine gravel . ‘ -- ‘es —_ —
Coarse sand . ; r°6 Ito2 6 5
Fine sand . . 3°8 ote pee © 4°3 tie
Silt : . é I7°I 20 ,, 30 Ir‘2 13°8
Fine silt : H 28°2 25 5, 30 28°7 26°5
Clay . ; K 23°3 20 4, 25 23°8 250
Loss on ignition . 26°3 Bye 19°8 14°5

1 Measured by the loss on ignition.

THE COMPOSITION OF THE SOIL 133

5. It swells when wetted.'

6. Although the group is essentially transitional it has
a certain degree of permanency and only slowly disappears
from the soil. It disappears more rapidly from chalky and
sandy soils than from loams and clays.

These properties greatly enhance the fertility of the soil,
and in most schemes of husbandry definite arrangements are
made to keep up or even increase the supply of organic matter,
while in forests the removal of leaves and other decomposable
material has led to such bad effects that in all state forests of
France, Belgium, Germany, etc., the practice is absolutely
forbidden.

The group of substances possessing these properties is
generally called “humus,” and so long as the word is used in
a collective sense as a convenient label it may be retained.
But the practice has been responsible for a good deal of loose
thinking, because it obscures the fact that the group is an in-
definite and complex mixture, and gives instead the impression
that it is a single definite substance.

From these half-dozen general properties we may infer that
humus is a brown, slowly oxidisable colloid. Careful ex-
amination of a number of soils in their vegetation relationships
has shown that there must be several distinct types of humus
but the laboratory methods are not yet as sensitive as the
growing plant and fail to explain some of the differences ob-
served by ecologists. We have to look to field observations
for the facts on which to base a scheme of classification. \

Peats.—Most of the recorded investigations are on peats

1 Peat shows this phenomenon in a marked degree ; indeed, after heavy rain-
fall inadequately-drained peat bogs may swell so much as to overflow into valleys |
with disastrous results. After drainage, however, drying and shrinkage set in,
followed by a slow but steady erosion as air penetrates into the newly-formed
spaces and starts the oxidation processes. When Whittlesey Mere was drained
in 1851 a pillar was driven through the peat into the underlying gault, and the top
of the pillar was made flush with the surface of the soil. So great has been the
subsequent shrinkage that over ro feet of the pillar is now out of the ground
and the process has not yet reached its limit, for a perceptible shrinkage took
place during the dry season of rgrr.

134 SOIL CONDITIONS AND PLANT GROWTH

of various kinds, and in this country the work has been done
largely by Dr. Moss and other members of the British Vege-
tation Committee (279). At least three great classes and
another two that may be transition forms were recognised :—

1. Dry peat (the German 7vockentorf) found on heaths in
relatively dry regions and on poor sandy soils. It is often
only a fraction of an inch in thickness, and is largely formed
by lichens and mosses (e.g. Cladonia rangiferina, Polytrichum
piliferum, and others). The dominant plant is Ca//una. Much
of the organic matter of heath soils, however, often consists
of undecomposed vegetation, e.g. bracken fronds, etc.

1a. In wetter districts the layer of peat becomes thicker,
and no doubt changes in composition, but it still carries
essentially ‘‘ heath” vegetation, although it shows resemblances
le aa .

2. Wet peat (the German Hochmoor) formed in wet tracts
or regions of high rainfall, and accumulating to so great a
depth that it entirely determines the character of the vegetation
whatever the underlying rock. It receives no supplies of
spring or underground water, and, therefore, no dissolved
salts; the drainage water is acid and poor in soluble mineral
matter. Two great divisions are recognised : lowland moors
or mosses, formed in low-lying wet places largely from sphag-
num, cotton grass (Eriophorum), and Calluna; and upland
moors, formed mainly from Eriophorum spp. and Scirpus
cespitosus in elevated districts of high rainfall.

3. Fen (Miedermoor in German, see (299)) formed ras a
calcicolous vegetation (Phragmites, Cladium, Scirpus, Carex,
etc.), in presence of calcium carbonate and soluble mineral
salts, showing no acid properties’ and giving alkaline drainage
waters.

3a. Carr, genetically related to the fen, containing much
decaying tree residues, and formed in what at one time was a
marshy wood.

Between fen and peat several transition forms have been
described by Weber (299) and also recognised in England.
Some of our moors are built up on older fens.

THE COMPOSITION OF THE SOIL 135

Forbes! recognises three classes of peat in Ireland :—

1. Mountain peat, corresponding with the wet peat low-
land moss of the British Committee, which originates wherever
the conditions are too sterile or the subsoil too impervious or
water-logged to allow deep-rooted vegetation to flourish, and
where, therefore, shallow rooted plants come in and, on dying,
form a layer of organic matter on which sphagnum, cotton
grass, etc., begin to develop. This occurs above the 800 feet
level in most parts of Ireland, but in the west it often covers
the entire surface down to the sea-level.

2. Marsh peat, corresponding with the British fen, which
arises from reeds, sedges, rushes, etc., and which, so long as
the water contains lime and nutrient salts, is as favourable a
medium for plant growth as ordinary soil, though it affords no
root-hold for trees, so that they are liable to be overturned in
strong gales. This kind of peat forms the basis of all the low-
land bogs in Ireland and of many of the small bogs in mountain
districts,

3. This marsh peat finally becomes so consolidated with
time and pressure that it loses connection with the water table,
and a surface swamp forms on which a sphagnum bog of the
“mountain type” arises. This, therefore, becomes similar in
character to the first group: it differs, however, in its uniform-
ity of growth, being higher in the centre than at the margins
where soil water can get in and where, therefore, decomposition
is more rapid.

The Scotch peats have been described by Lewis? and the
Yorkshire moors by Elgee.*

Within each of the great classes described above several
subdivisions are recognised, but how far they arise from differ-
ences in the organic matter, or from other differences, cannot
yet be ascertained.

1A, C, Forbes, Clare Island Survey, 1914, 9 (Proc. Roy. Ivish Acad.,
1914, 31).

2 Trans. Roy. Soc., Edinburgh, 1905, 41, 699-724; 1906, 45, 335-360; 1907,
46, 33-70; 1911, 48, 793-833.

3 F, Elgee, The Moorland of North-Eastern Yorkshire. London, 1912.

>.

136 SOIL CONDITIONS AND PLANT GROWTH

Numerous chemical analyses of the peats have been made
by Tacke at Bremen (278), Gully (116), Michelet and Sebelien
(199) and others."

As a general rule, though with many exceptions, the per-
- centage of nitrogen varies with that of the lime, and the high
moor contains least of these, the low moor a larger quantity,
and the fens a still larger quantity (Table XX XIX.).

TABLE XXXIX.—NITROGEN AND CaLciuM CONTENT OF VARIOUS PEATS.

Nitrogen CaO
r cent. in er cent. in Observer.
ry Matter. ty Matter.
High moor | Bremen I'r4 0°44 Tacke
Lancashire 0°85 OvrIL Russell and Prescott
Low moor Bremen 1°62 1°24 Tacke
Cheshire roe o'r6 Russell and Prescott
Fen Norfolk 2°85 7°5 Ae a

The investigations on the cause of the acid properties of
peat are dealt with on p. III.

Humus of Forest Sotls—An admirable series of studies has
been made by P. E. Miller (206) of the types of humus oc-
curring in the Danish forests.? In beech forests he found two
types, which he called mul/ and Zorf, our nearest equivalents
being mould and peat. On mu// the characteristic plants were
Asperula odorata with its associated Mercurialis perennis, Mi-
lium effusum, Melica unifiora, Stellaria nemorum, and others,
moss being absent. The mul itself was only a few inches
thick, and was underlain by 1 to 5 feet of loose soil, lighter in
colour than mu//, but almost equally rich in organic matter ;
still lower came a compact but porous layer of soil. The,
surface of the soil was covered by a layer of leaves, twigs, etc.
Earthworms were numerous throughout ; their potent influence
in the soil had recently been shown by Darwin (76). Detailed
chemical examination was not made: it was shown, however,
that mu// was free from acid and contained about 5 to Io per

1See ¥ahresber. Agrik. Chem., 1878, p. 29; 1904, pp. 87, 88, etc.

2 Other investigations on forest humus are dealt with by Ramann, Forstliche
Bodenkunde u. Standortslehre, 1893.

THE COMPOSITION OF THE SOIL 137

cent. of organic matter completely disintegrated and most
intimately mingled with the mineral matter.

Torf differed completely. The characteristic plant was
Trientalis europea with the associated Azra flexuosa and moss,
but surface vegetation was not very common. The loose layer
of leaves was absent, and the “orf itself was so tough and
compact that rain water could not readily penetrate. Below
it was a layer of loose, greyish sand (d/ezsand), and lower still
a layer of reddish soil (voterde), or else a pan (ortstein). Prac-
tically no earthworms were found in the sorvf, but there were
numerous moulds and fungi, Cladosporium humifaciens Rostrup
and Sorocybe Resing Fr. being perhaps the commonest.

Torf was acid, contained about 30 per cent. of organic
matter not completely disintegrated, nor well mixed with the
mineral matter. It was not very favourable to the growth of
young trees, and the forest tended to become an open heath
as the old trees died.

The distribution of mud/ and torf did not seem to be deter-
mined by the nature of the soil, or by the amounts of soluble
alkali salts or calcium carbonate present, but rather by the
nature of the living organisms in the soil. Animals, especially
earthworms, gave rise to mu//, fungi produced ‘orf. If the
conditions were favourable to earthworms mu// was therefore
found, if not, zorf was produced. The nature of the vegetation
was also a factor: oak only rarely formed torf but commonly
' gave rise to mu//, at least two varieties of which were observed ;
pine, like beech, could form either forf or mull, while Calluna
vulgaris and Vaccinium myrtillus generally produced forf.

Humus of Field Soils.—It is commonly assumed that the
humus of field soils is of the same nature as that of peat, fen,
or forest. There are undoubtedly certain properties in common.
Alway and Neller (3@) find that the differences in moisture
content of adjacent plots of varying organic matter content can
be explained on the assumption that soil organic matter has
the same water-holding capacity as the most absorbent peats,
z.é. three to four times its own weight. The difference in
organic matter was 1°37 per cent.; the difference in moisture

138 SOIL CONDITIONS AND PLANT GROWTH

was up to 4 per cent. A close study jointly by a botanist and
a chemist would carry the problem much nearer to a solu-
tion.

‘The observations indicate that the mixture we have agreed
to call humus does not vary erratically from field to field, but
produces much the same effects over any tract where similar
soil and climatic conditions prevail. The mixture changes
when a new set of conditions occurs, but its general character
persists over a certain range and then it merges into another
type. Two great divisions of the types can already be recog-
nised : a neutral group commonly spoken of as neutral humus
or “mild humus,” and a group reacting like an acid and called
“sour humus,” acid humus, or by German writers Rohhumus.

TaBLE XL.—ANALYSES OF THE ORGANIC MATERIAL EXTRACTED BY ALKALIS
FROM SOIL (OFTEN CALLED Humus, SoLuBLE Humus, AcTIVE Humus, ~
MaTizRE Noire, ETC.). :

Source. Carbon.|Hydrogen.|Oxygen.|Nitrogen.| Ash. Observer.
Arable land . -| 56°3] 4°4 36°0 3°3 3 9 | Mulder (205)
Garden soil . -| 56°38]. 4°9 34°8 3°55 I 2 a Bs
Pasture land . = .| 56°r| 5°3 325 | 6r lOse “i
Pest. : -| 59°0| 4°7 32°7 3°6 os a

Pe : ; -| 57°74] 4°6 — |<a° — |F. Fuchs!
Rich prairie soil .| 45°r| 3°7 28°6 | 10%4 12°2 | Snyder (268)
Soil never cultivated| 44°1| 6°0 35°2 81 6°6 We
Cultivated subsoil(a)| 48°2| 5°4 33°2 g'I 4°2 ”

” 9 5o'r| 4°8 3710S 4°9 ”
‘* Humic acid ” from ' | Berthelot and

sugar . 66°4 | 4°6 29°0 — —_ André (30-32)
“Humic acid” from Berthelot and

compost 53°3| 5°6 37°5 3°6 — André (30-32)
“ Humic acid” from

sugar through K Robertson, Irvine

salt. 64°70| 4°55 | 30°71; — — and Dobson ?
‘* Humic acid ” from :

sugar irousgh NH, Robertson, Irvine

salt’ 3%. 64°74| 4°69 | 29°81] 0°76 oo and Dobson ?
*¢ Humic acid” » from Robertson, Irvine

soil through K salt|56°67| 5°16 | 35°68| 2°49 — and Dobson ?
“* Humic acid” from "

soil diab NH, Robertson, Irvine

Salt es -|54°29| 4°94 | 38°16] 2°61 - and Dobson ?

1 Chem. Zeit., 1920, 44, 551. The molecular weight was about 680.
2 Biochemical Yourn., 1907, 2, 458. For other studies see Mary Cunning-
ham and Chas. Dorée, Trans. Chem. Soc., 1917, 111, 589-608.

THE COMPOSITION OF THE SOIL 139

| In order to understand much of the chemical work that
has been done, it is necessary to remember that the older
chemists regarded humus as a single definite substance, or as
a mixture of two or three definite substances. The favourite
view was to consider neutral humus as the calcium salt of
“humic acid,” which could be extracted from the soil by dilute
alkalis after preliminary treatment with hydrochloric acid. On
acidifying this alkaline extract the “ humic acid” came down
as a brown colloidal precipitate. Acid humus was the actual
humic acid itself. It was further supposed that humic acid
could be synthesised by boiling sugar with hydrochloric acid,
on the singularly inadequate ground that the product thus
obtained is also a brown colloid, soluble in alkalis and pre-
cipitated by acids. Numerous analyses have been made both
of the natural and the synthetic “ humic acid,” some of which

are given in Table XL.
_ Robértson, Irvine, and Dobson give the formula C,,H,,0,,
to humus from sugar and C,,H,,0,,N to the soil humus ob-
tained through the potassium salt ; the latter may be related
to the acid prepared from Dopplerite by Mayer,’ containing
1 Many partial analyses have been made. Cameron and Breazeale (65a) in

nineteen samples obtained percentages of carbon varying from 33 3 to 50°I,
whilst Hilgard (1335) found the nitrogen content to be :—

Percentage of | Percentage of Nitrogen in the
Rigt = a mt Hanis Acid.
Soils of the arid regions (decom-
position rapid) 0°20 to 3°0 | 8°7 to 22°0 (average 15°2)
Soil of the sub- irrigated arid
regions . B30 20 1 4 ROS C.., 84)
Soil of the humid regions (de-
composition slow) . WOO Ke HOLE Ge HO EO 5 4°2)

C. B, Lipman, however, is convinced that there is an error here : he has not
found more than 7 or 8 per cent, of N in the humus from arid soils : this is no
more than occurs in the humus from humid soils (Soil Science, 1916, 1, 285-290).
He also finds no difference in general nitrifying power (¥ourn. Ag. Research, 1916,
7,47). Westermann (302) has analysed humus from the Danish moors, and Gully
(rx6) has studied humus from South Bavarian moors. Many of the older analyses
have been collected by Wollny (318).

2 Landw. Versuchs, 1883, 29, 313.

140 SOIL CONDITIONS AND PLANT GROWTH

58°'2 per cent. carbon and 5:0 per cent. hydrogen. In
view, however, of Hoppe-Seyler’s discovery, confirmed by
Odén,' that soil humus is partly soluble and partly in-
soluble in hot alcohol, it is obvious that at least two sub-
stances must have been present in the material analysed in the
above cases and the validity of any formula becomes doubtful.?

The purification of humus for investigation in the labora-
tory is rendered extraordinarily difficult by its colloidal nature,
which was demonstrated by van Bemmelen in a remarkable
paper in 1888 (19). Baumann’s researches (10) have carried
the subject a good deal farther, and it is now known that
“humus,” freshly precipitated by acids from an alkaline ex-
tract of soil, compost, etc., possesses the following colloidal
properties :—

(1) Very high capacity for retaining water.

(2) Extraordinary shrinkage on drying.

(3) Is coagulated by acids and salts, the electric current
and frost.

(4) Reversibility, ze. the freshly precipitated material re-
dissolves when the precipitant is washed away.

(5) Decomposes salts—calcium carbonate, calcium phos-
phate, etc.

(6) Forms difficultly soluble and easily decomposable col-
loidal mixtures with other colloids.

(7) Masks certain ion reactions (e.g. Fe cannot be detected
by potassium ferrocyanide, etc.).

(8) Forms absorption compounds.

The colloidal nature of the humus accounts for much of
the failure of the earlier chemical work, and it adds to the
difficulties of those modern chemists who have had the courage
to tackle the problem. In spite of these difficulties several
good investigations have been made.

1 Ber., 1912, 45, 651.

2 Miklauz (Zeit. f. Moorkultur u, Torfverwertung, 1908, 285) and Mayer
both showed that variations in the time or the method of extraction cause differ-
ences in the results even from the same soil, Sostegni (Landw. Versuchs-Stat., °
1886, 32, 9) had shown that humus is readily fractionated.

THE COMPOSITION OF THE SOIL 14!

Schreiner and Shorey (250) have attempted a resolution
of the “humic acid” and of the “crenic acid” (the part not
precipitated by HCl) and have obtained the following sub-

stances from the alkaline or alcoholic extract :—

Substances Precipitated by Acids (the so-called
Humic and Ulmic Acids).

Substances not Precipitated by Acids (the so-
called Crenie and A pocrenic Acids).

Resin acids,

Resin esters.

Glycerides.

Paraffinic acid, C.4H30., m.pt. 45°-48°,
probably identical with the acid
formed on treating paraffin with fum-
ing nitric acid.

Lignoceric acid, C.4H4,0,, m.pt. 80°-81°,
isomeric with above.

Agroceric acid, CaHlesOs» m.pt. 72°-73°,
a hydroxy fatty acid

riba CogH,,0, m. pt. 237°.
tosterol, Cog,H,,O . H,O, m.pt. 135°.
Both of the cholesterol group.

Dihydroxystearic acid, C,gH;,0,, m.pt.
98°-g9°, identical with the acid formed
on oxidising elaidic acid.

a-Picoline y-carboxylic acid, C,H,0.N,
m.pt. 239°, identical with the acid
formed on heating uvitonic acid to
274°.

Xanthine, C;H,O.N,.

Hypoxanthine, C,H,ON,.

Cytosine, CHH,ON,. H,O.

Histidine, C,H,O,N3.

Arginine, C,H,,0,.N,.

A pentosan.

None of these, however, is

the black substance which is

the real characteristic of “humus,” and until recently this had
eluded investigation and was vaguely described as a “ melanoid

body”.

It can be split up by solvents into the following sub-

stances which are no doubt further resolvable :—

Soluble in NH,HO

Insoluble in NH,HO

|
Soluble in acids
(Mulder’s apocrenic acid)

|
Pptd. by acids
Humus

Humin

Insoluble in alcohol
Humic acid

Soluble in alcohol
Hymatomelanic acid!
(see Schreiner and Shorey)

Insoluble in pyridine

|
Soluble in pyridine

The substances are probably related: at any rate they occur
together. There is good evidence that soil humus is formed
mainly from cellulose: no great amount is produced from
protein. Addition to the soil of wool, silk waste, flour, and
lucerne meal causes no gain, but on the contrary loss of

1 For Hoppe-Seyler’s hymatomelanic and fulvic acids see Odén, 2rge.

142 SOIL CONDITIONS AND PLANT GROWTH

humus.’ This formation from cellulose suggests an interest-
ing possibility as to its origin. It has already been stated
that. a substance resembling humus is formed on’ heating
sugar with hydrochloric acid (eg. 250 grms. dextrose with
I litre 24 per cent. HCl for 12 hours), Maillard has shown
(1894) that the reaction is general and not confined to strong
acids.

He was led by his investigations on the function of alcohol
in the synthesis of albuminoids to study the action of a typical
_amino acid, glycocoll, on d-glucose. When 1 part of glycocoll
and 4 parts of glucose were added to 3 to 4 parts of water
and heated on the water-bath to facilitate dissolution the re-
action mixture rapidly changed to yellow and finally to dark
brown, CO, being given off. The action was a true condensa-
tion and not an oxidation, for practically no oxygen was
absorbed. The black material had the properties of soil
humus: it was insoluble in water, dissolved in alkalis, but was
reprecipitated on adding acids, and contained 4:4 to 6 per
cent. nitrogen: this question is discussed at length in the later
paper (189¢@), The reaction turned out to be general, and
was given by all the amino acids tried (glycocoll, sarcosine,
alanine, valine, leucine, tyrosine, glutamic acid and also the
polypeptides) and all the sugars. Xylose and arabinose reacted
especially quickly, fructose, galactose, glucose, and mannose
less quickly, and lactose and maltose still less, while saccharose
acted only slowly. The reaction proceeds at ordinary tem-
peratures, but more slowly.

Now it is certain that both amino acids and pentose com-
pounds are formed during the decompositions in the soil, the
former from the proteins and the latter from the celluloses,
and it is highly probable that the black substance is formed by
their interaction. A detailed study of the reaction was there-
fore made at Rothamsted by V. A. Beckley (12) ; setting out
from an observation by Fenton, he showed that sugars, on treat-
ment with acids, give rise to hydroxymethylfurfuraldehyde,
which readily condenses to form a substance closely re-

1 Gortner, Soil Sci., 1917, 3, 1-8.

THE COMPOSITION OF THE SOIL 143

sembling humus. He also found indications of hydroxy-
methylfurfuraldehyde in a dunged soil and in rotting straw in
which humus was being produced. He suggests, therefore,
that the formation of humus in the soil proceeds in two
stages :—

Carbohydrate (Cellulose, etc.) (+ Amino acid) >
Hydroxymethylfurfuraldehyde.
Hydroxymethylfurfuraldehyde + amino acid > humus +
furfural + CO, by condensation.

Eller and Koch,! on the other hand, suppose that humus
is formed by the oxidation of quinones which arise by the
elimination of water from hexoses ;—

C,H,,0, = 4H,O + C,H,O,

Beijerinck (17) has already found quinone among the products
of certain soil organisms working in culture solution.

As to the constitution of humus little is known. Sven
Odén of Upsala (218a@) has adduced evidence from the con-
ductivity of the solution in ammonia that the alcohol insoluble
portion is a true acid, as was supposed by the older chemists
and by Tacke (278). Ehrenberg and Bahr (93d@) also agree
with this view ;: they show that it neutralises sodium hydroxide,
the point of neutralisation being sharply indicated by con-
ductivity measurements. On dilution the sodium compound
behaves as a salt ora tri- or tetra-basic acid. Further evi-
dence of true acidic nature is afforded by the fact that the
alkaline solution behaves as a true solution, and shows none
of the properties of colloids (Odén, 2184). There is, however,
another view (see p. 111) that the acidic properties are the
result of the colloidal condition and not of a true acid con-
stitution, but it is improbable that this is a complete explana-
tion of the phenomena.

For a long time the humus soluble in alkalis was supposed
to play a great part in determining fertility: Grandeau (112a)
and Hilgard (133) especially considered it to be the most

? Wilhelm Eller and Kate Koch, Ber., 1920, 53 B, 1469-1476.

144 SOIL CONDITIONS AND PLANT GROWTH

useful material for making plant food. Methods for deter-
mining its amount in soils were elaborated and many estima-
tions were made. But nothing came of them: on the
Rothamsted plots about one-half of the total nitrogen is soluble
and one-half insoluble in alkalis whatever the manurial treat-
ment. The idea rested on no experimental basis and, in the
only recorded tests, Weir (300a@) found that the removal of
this soluble nitrogen caused no diminution in the productive-
ness of the soil. If this turned out to be general it would
show that the really important nitrogen reserves are in the
insoluble part, as is not unlikely in view of the circumstance
that they probably consist of protein and similar bodies which
do not dissolve in alkalis,

An indirectly beneficial effect on the plant is suggested by
Briggs, Jensen, and McLane’s observations that the percentage
of mottling in citrus trees of S. California is inversely related
to the amount of soluble humus in the soil: they therefore
recommend mulching with straw so that the decomposition
products can be washed into the soil.!

Kaserer* suggests that an important function of humus in
the soil, so far as micro-organisms are concerned, is to act as a
carrier of the numerous inorganic constituents they require.

Wax-like Constituents.

Some of the soil organic matter is wax-like in properties,
interfering very much with the wetting of the soil and the
movement of the water. As it decomposes only slowly, it
tends to accumulate in rich soils and to become rather trouble-
some. It can be extracted by organic solvents, eg. toluene,
and obtained as a yellowish-brown mass containing appreciable
quantities of nitrogen (a soil yielded ‘003 per cent. of a
substance containing 3 per cent. of nitrogen in one of the
writer’s analyses).

1 F¥ourn. Ag. Research, 1916, 6, 721; and 1917, 9, 253.
2H. Kaserer, Internat. Mitt. 7. Bodenkunde, 1911, 1, 367-75.

TAE COMPOSITION OF THE SOIL 145

The Nitrogen Compounds in the Sotl.

It is convenient to collect together the main data connected
with the nitrogen compounds of the soil. The total nitrogen
in arable soils is usually about 0°15 per cent., in pasture soils

°

° °
“ @ Nn - ° ‘
a L 1 J -
~ dazjey JO Sayau/ naa al . S
mL ial tee
Oo
o| =
aS
a n
18
a] 2
Z| 2
-—s
4s
| 8
»| oO
~ jo} Se
paybnoj-- O| 3A
ie Be
al 2 y
1 38
M Bg
o 3
sis
a & 90
5 4
| ~
Tas
o
eS
peybnosg.-- Q b Be
an
3} 24
x | aed
0 a Ba
. 3) ~
c| § ~
3} 2
357) 6
ml
wg
ac
©
=| 8
&
il
1 |i
o °o o °o o < -
4 x 2 2 oO + ~) iS)
"sul Of dog ‘asap sad-gg7 ajeszipy se vabosziVN om

about 0°3 per cent.; higher amounts are present in chalk soils.
and still higher in fen, moorland, and black prairie soils.
About half of the nitrogen in arable soils is contained in
compounds soluble in alkalis, and a small proportion in un-

stable compounds readily breaking down to ammonia. The
fe)

146 SOIL CONDITIONS AND PLANT GROWTH

amount of nitrogen present as free or combined ammonia is
about ‘Ooo! per cent. (ze. I part per 1,000,000) in arable soils
not rich in organic matter,’ and some ten times this quantity
in pasture or heavily dunged arable soils ; much larger quan-
tities occasionally occur in special cases, as in heated soil, but
abnormal growth effects are then produced.? There is con-
siderable variation in the amount of nitrogen present as
nitrate; rich garden soils may contain 60 or more parts per
1,000,000 (‘006 per cent.), arable soils 2 to 20 parts (‘0002 to
*002 per cent.), pasture soils rather less and woodland soils
still less.* No soil constituent fluctuates more,in amount
than nitrates; plants and rain rapidly remove them and
bacterial action rapidly forms them. The producing agencies
are active in spring, and work throughout summer and autumn,
while the removal agencies are active in summer and winter.
Thus the amount of nitrate actually present in arable soil is
usually highest in spring, falls in summer, often rises some-
what in autumn, and falls again in winter as shown in Fig. 18
(p. 145) (see also Table LXVL., p. 249).*

The nitrate and ammonia together rarely account for more
than I per cent. of the nitrogen in the soil; the remainder is
in more complex forms. The part soluble in alkalis® prob-
ably includes the same type of compound as was obtained
by Maillard in the condensation of sugars and amino acids
(p. 142). The part “insoluble in alkalis may be of protein
nature. On boiling with strong HCl! soluble nitrogen com-
pounds are formed, and it has been supposed ® that the re-
action qualitatively and quantitatively resembles protein
hydrolysis. This may be true, but exact proof is difficult.

* André (Compt. Rend., 1903, 136, 820) obtained higher results in early spring

which he attributed to the cessation of nitrification, but not of ammonification,
during winter.

* See E, J. Russell and F. R. Petherbridge, ¥ourn. Ag. Sci., 1913, 5, 248.

* It is sometimes stated that woodland soils do not contain nitrates and are,
unsuited for nitrification, but Weis (301) has shown this to be incorrect.

*For American data see R. Stewart and J. E. Greaves, Centr. Bakt. Par.,
IQ17, 34, 115.

» After preliminary treatment with dilute acid (see p. 139).

5 E.g. Jodidi, Iowa Research Bull., No. 1, rgtt.

THE COMPOSITION OF THE SOIL 147

Morrow’ has shown that the figures for the distribution of
nitrogen given by the van Slyke method are unreliable in
presence of ferric chloride, which always occurs in soil ex-
tracts. Potter and Snyder have studied the amino-nitrogen
in soil.?

1 Soil Sci., 1918, 5, 163.

2 Fourn. Amer. Chem. Soc., 1915, 37, 2219, and F¥ourn. Ag. Res., 1916, 6, 61.

Io *

CHAPTER IV.
THE COLLOIDAL PROPERTIES OF SOIL.

GRAHAM in 1861 introduced the term “colloids,” to denote
those bodies which, like glue, tend to form jellies rather than
crystals, and which possess certain properties distinguishing
them from crystalloids, such as high power of absorbing water
and also dissolved substances from their solution. The idea
subsequently arose that colloids were a distinct group of
bodies, and when clay was observed to possess colloidal
properties chemists went to much trouble to try and isolate
the colloidal constituent: Schloesing (2450) described a pro-
cess that required several weeks for its performance. Later
on, however, it was recognised that colloids are not a group
of substances but a state into which most solid substances
can be brought: their properties are attributed to their highly
extended surface. Any substance, therefore, that can be.
brought into a sufficiently finely divided state, or can be got
into. a “web” structure so as to make its surface sufficiently
large, will show colloidal properties.

Van Bemmelen was the first to show how completely the
clay and humus in the soil behave like colloids. And, as both
these substances dominate the soil to a considerable extent,
it is not remarkable that the soil as a whole possesses colloidal
properties.

It is not my purpose to write a chapter on the properties
of colloids. The soil student must, however, make himself
thoroughly familiar with them’ because he will find at every

1 Convenient accounts of the properties of colloids are given by Hatschek in
An Introduction to the Physics and Chemistry of Colloids, London, 1913; by We.
Ostwald, in Colloid Chemistry, translated by Fischer, London,.1916; and by R.
Zsigmondy, Chemistry of Colloids, translated by E. B. Spear, New York, 1917.

For a good discussion see W. D. Bancroft, fourn. Phys. Chem., 1914, 18, 552;

and 1916, 20, 85.
148

THE COLLOIDAL PROPERTIES OF SOIL 149

oe

turn that the colloidal nature of the soil modifies or even deter-
mines its behaviour and leads to all sorts of unexpected
results.

So far as the soil is concerned the most striking colloidal
properties are :—

_ (1) The power of absorbing substances from their solu-
tions.

The absorption is in effect a precipitation: it may be re-
garded as a concentration of the absorbed body on the surface
of the colloid and in this sense it is called an “adsorption”.
It differs from a simple chemical precipitation in that it does
not follow the ordinary laws of chemical reaction but special
laws of its own. In general, but not invariably, the adsorption

by colloids can be expressed by the equation Y _ Keo where
m

¢ = the concentration of the dissolved substance when
equilibrium is attained ;

y = the amount absorbed by a quantity m of the ad-
sorbent ;

(this can readily be expressed as (a - ¢c) where a = the
initial concentration) ;

K and ~ = constants depending on the nature of the solu-
tion and adsorbent.

As we shall see, this equation fits a large number of soil
reactions. ;

(2) The power of absorbing water in considerable amount
and holding it rather loosely: parting with it again by
evaporation in a continuous manner without any critical
points or regions.

(3) The change that can be brought about in some of the
finer mineral particles from the flocculated to the defloc-
culated state, and vice versa. This is particularly shown by
clay and has been discussed on p. 105.

(4) The property whereby some of the soil components
can enter into solution in pure water (not, however, a true
solution) and be thrown out again on addition of small

150 SOIL CONDITIONS AND PLANT GROWTH

quantities of electrolytes. The former state is called a ‘‘sol,”
the latter a ‘“‘gel ”.

The effect of these properties has not yet been fully studied.
It will, therefore, be most convenient to deal with a few direc-
tions in which work has been done, and this plan will have
the double advantage of showing how the new ideas have
developed and how far reaching is likely to be their effect in
soil chemistry.

Absorption by Soil.’

The facts of absorption by soil have long been known :
they were established, indeed, by the end of the sixties.
Soluble salts, such as ammonium or potassium sulphate,
which might be expected to wash out of the soil with rain,
do not, as a matter of fact, do so, but are kept -back in such
form that the plant can get them. In becoming absorbed,
however, these substances displace something else, and con-
versely they can themselves be displaced by another salt.
Hence there arise a series of interchanges which profoundly
affect the nutrition of the plant.

The first quantitative investigation was made by Thompson
(282) who showed that ammonium sulphate is decomposed
when dissolved and shaken with soil, ammonia being fixed
and calcium going into solution. . The problem was taken up
by Way (298) who found that the quantities of ammonia ab-
sorbed and of calcium displaced were equivalent. Further
experiments by A. Voelcker (289a and 4) and others, showed
that the same action takes place in the soil itself when am-
monium sulphate is added in the ordinary way as manure, an
insoluble nitrogen compound being formed which remains in
the soil, while the calcium sulphate washes out in the drainage
water. Potassium sulphate reacts in the same way, the potas-
sium being precipitated and an equivalent amount of calcium
going into solution. Potassium phosphate undergoes a more

1 The literature of this subject has been summarised by J. A. Prescott in
Fourn. Ag. Sci., 1916, 8, 111-130.

THE COLLOIDAL PROPERTIES OF SOIL 15r

complete precipitation, since calcium phosphate is insoluble.
The precipitated potassium compound dissolves somewhat in
water, but it has no definite solution pressure, instead the
amount of potassium dissolving increases with the amount.
present. It can also be decomposed by sodium salts; hence
addition of sodium sulphate to the soil increases the amount
of soluble potassium ions and to this extent acts like a dress-
ing of potassic manure. Magnesium salts have a similar
effect, and, like sodium salts, lead to an increase in the amount
of potassium available for the crop. Some of Lawes and
Gilbert’s results (1662) are given in Table XLI.

TaBLE XLI.—Errect or SopIuM AND MAGNESIUM SULPHATES IN INCREASING
THE SUPPLY OF PoTAsH TO THE PLANT, LaWwES AND GILBERT (166a).

b go gs ;3 wees
a6 Ze gS 5 Bn So BNSS | BHoR ae
Sa Sam co+ag £402 S468 | Siayzes
2 ete | oases | tse | tes | 8230 be
eg | S22 | E2a% | G22 | esas | 5ee5e
< a <84 a+e e+e ate g
1852-1861. Plot ro. Plot 11. Plot 12. Plot 14. Plot 13. Plot 7.
| K,O in ash of straw,
percent. . 18°8 14°8 20°1 22°0 24°I 23°7
K,O in‘ash of grain,
percent. . 33°9 31°7 32°8 32°6 32°9 32°9
Weight of K,O in
ten whole crops,
Ib. f -| 300 309 454 498 532 560
1862-1871. r
K,O in ash of iia
percent, . 14°5 14°I 17°2 18°5 25°0 24°6
K,O in ash of grain,
percent. . 34°I 32°I 33°3. | 33°t 33°5 33°4
Weight of K,O in|:
ten whole crops,
Ib. 240 260 378 391 552 530
Total amount of K,O
taken by crop dur-
ing the sama
years, lb. . 540 569 832 889 1084 I0go

In the twenty years the sodium sulphate has enabled the
plant to take up an additional 263 lb. of K,O, whilst the

152 SOIL CONDITIONS AND PLANT GROWTH

magnesium sulphate has furnished it with an extra 320 Ib.
over and above what the crop on Plot 11 can get.

On the other hand, gypsum has no such effect. The super-
phosphate applied to Plot 11 contains a considerable propor-
tion of calcium sulphate, but it does not increase the weight
of potassium in the crop. This appears to be the general
rule: Briggs and Breazeale’ obtained the same result on
Californian soils. The result is at variance with a statement
commonly made that potassium is dissolved out when soil is
shaken with calcium sulphate solution.”

Some ions are not precipitated in the soil, including CO,,
SO,, NO,, Cl, Mg, Ca, Na;* these are, therefore, the chief
constituents of drainage water (see p. 128).

Organic substances, particularly those of high molecular
weight, are also withdrawn from their solutions, but the
reaction is apparently of a different type, since nothing appears
to be given up from the soil in exchange. The result is of
extreme importance; practically the whole of the organic
matter added to the soil by plant residues or manure remains
near the surface unless carried down mechanically by some
agency such as earthworms. Even when heavy dressings of
dung are annually supplied at Rothamsted there is after fifty
years no appreciable enrichment of the subsoil in nitrogen
(Table XLII.). The purification of sewage by land treat-
ment affords further illustrations of the absorptive power of
soil for organic matter. In English experience a sewage farm
on a good loam can deal with 30,000 to 40,000 gallons of
sewage per acre per day (z.e. 1°3 to 1°8 inches per day).

1 F¥ourn. Ag. Res., 1917, 8, 21.

2E.g. Hilgard, Soils, 1906,-p. 43; probably based on experiments by
E. Heiden, $ahresber. Agrik, Chem., 1868, p. 59; Annal. Landw, Preussen.,
1868, 50, 29; Kalmann and Bocker, Landw. Versuchs-Stat., 1878, 21, 349. See
also G. André, Compt. Rend., 1913, 157, 856.

3’ From the time of Aristotle it has been known that sea water could be
‘*desalted” by filtering through sand or soil, But it has been shown by Von
Lipman and Erdmann (Chem. Zeit., 1911, xxxv., 629) that the water first
running through the sand filter is not desalted sea water, but displaced water.
When this has all gone the salt water runs through unchanged.

THE COLLOIDAL PROPERTIES OF SOIL 153

TaBLeE XLII.—NITROGEN IN BROADBALK WHEAT SOILS, 1893.

Per cent of dry soil.

Annual Dressing of Manure.

Minerals Minerals Minerals

* IMinerals' + 200 Ib. + 400 Ib. oa 600 Ib.

Unmanured.| (200 Ib. 1 Ammonium/Ammonium | Ammonium
NS se dy Salts Salts Salts

(43 Ib. N). | (86 1b..N). | (129 Ib. N).

Topgin. . -| *0992 | *2207 | 1013} °II0O7 "1222 1188

gtor8in, . ky 80730 0767 | '0739 "0720 ‘0681 "0752
18 to 27 in. . -| ‘065% | *0656 | *0645 0628 0583 0630

Lb. per acre.

Topgin. . 3 2572 5150 | 2630 2870 3170 3080
gtorin. . y Tg50 2050 | 1970 Ig20 1820 2010
18 to 27 in. . j 1820 1830 | 1800 1750 1630 1760

Nitrogen supplied
in manure in
the 50 years .| None 10,000 | None 2150 4300 6450

| ‘i

The Mechanism of Absorption.—In a classical investigation
Way (298@) argued that the absorption is purely chemical: the
ammonia and the calcium simply changed places as usual in
double decompositions or precipitations. He then proceeded

to discover the particular constituent of the soil with which the ~

reaction took place; he found it was neither the calcium
carbonate, the sand, the undecomposed rock, however finely
ground, nor the organic matter.!_ The active constituent was
in the clay, but it formed only part of the clay, and, moreover,
it lost its power on ignition. No known simple silicates
showed these properties, but he prepared a number of ‘“ double
silicates ” of lime and alumina, of soda and alumina, etc., that
did; thus they reacted, like clay, with ammonium salts to

1It was subsequently shown by Konig (153) that soil organic matter has a
marked power of absorbing ammonia from ammonium sulphate.

154 SOIL CONDITIONS AND PLANT GROWTH

form an almost insoluble double ammonium silicate and a
soluble calcium salt, and also, like clay, they lost this property
after ignition. Although he did not establish the existence
of such double silicates in soil, their resemblance to the
reactive constituent in the soil was so close that he considered
himself justified in assuming their presence.

This chemical view was generally accepted in England,
but it was controverted by Liebig, who held that the whole
phenomenon was physical and comparable with absorption
by charcoal. Plant food constituents occur in the soil in two
states: chemically combined and physically retained: the
latter being the looser is the more suited for the purposes of
plant nutrition. ‘‘ The power of the soil to nourish cultivated
plants,” he writes, “is therefore in exact proportion to the
quantity of nutritive substances which it contains in a state of —
physical saturation. The quantity of other elements in a state
of chemical combination distributed through the ground is
also highly important, as serving to restore the state of
saturation, when the nutritive substances in physical combina-
tion have been withdrawn from the soil by a series of crops
reaped from it” (174d, pp. 67-69),

Knop (150) combined both chemical and physical hy-
potheses. The absorption of acid radicles he attributed to
precipitation by the iron and aluminium hydroxides supposed
to be present in soil: phosphoric acid, however, reacted
first with the calcium compounds to form calcium phosphate
and then with the iron compounds. With bases the action
was rather more complex: the absorption in the first instance
was due to a surface attraction, which was followed by a
combination with silica or aluminium silicates: there was,
however, invariably an equilibrium, the whole of the base
never being removed, no matter how dilute the solution.

Liebig’s proposition which we have quoted above was
expressed more tersely by Knop as follows: Soils of great
fertility have a high content of easily replaceable bases ; and
he measured this by determining the ammonia absorbed from
a O'5 per cent. solution of ammonium chloride, assuming that,

THE COLLOIDAL PROPERTIES OF SOIL 155

as absorption was only a substitution, the greater the amount
of replaceable base the greater would be the absorption of
ammonia. The method was applied to a number of soils and
gave results in fair agreement with their agricultural history.
It was somewhat modified by Kellner,| who measured the
quantities of potassium and calcium displaced and found that
they agreed exactly with the amounts taken up by plants in
pot culture. The method was still furthur improved by
Ramann: a 5 per cent. solution of ammonium nitrate is
allowed to percolate through the soil and the displaced potas-
sium and calcium are estimated.2 A detailed study has
been made by Prianischnikow (2292).

Van Bemmelen (20a) began by accepting Way’s chemical
hypothesis, and showed that soils with a high power of ab-
sorption usually contained a large quantity of easily decom-
posable silicates (19a@): Way’s double silicates would pre-
sumably be of this nature. Further, absorption of bases
always involved displacement of other bases from the soil, a
strong indication of chemical change. Later on, however, he
made extensive studies of absorption by simple gels: silica,
alumina, ferric hydroxide, tin hydroxide, etc., and found it
closely to resemble absorption by soils: other studies of
colloids were made and in each case the similarity to soil
phenomena was so close as to leave no doubt that soil was
essentially a colloid and soil absorption simply a manifesta-
tion of the colloidal properties.

This new idea was soon found to explain many of the old
discrepancies. Chemists had several times attempted to bring
the phenomena of absorption equilibrium into line with those
of chemical equilibrium, but the equations would not fit except
for a narrow range of concentrations. Boedecker in 1859?
had fitted an expression to Henneberg and Stohmann’s results
for ammonia absorbed and calcium displaced in the interaction

10, Kellner, Landw. Versuchs-Stat., 1886, 33, 349.

2 Recorded by J. A. Hanley in Nature, 1914, 93, 598. See also Kiillenberg,
Fahresber. Agric. Chem., 1865, 8, 15.

3 Fourn, f. Landw., 1859, 48.

156 SOIL CONDITIONS AND PLANT GROWTH

‘between soil and ammonium sulphate, and Hall and Giming-
ham (120e), dealing with the same reaction, showed that the
ordinary formula for chemical equilibrium held over a limited
range of concentrations ; but Cameron and Patten (654, see
also 300) found that it did not hold over a wider range.
The absorption of potassium could not be fitted at all by the
formula.

When, however, the adsorption formula is used a complete
fit can be obtained: Wiegner (308) has gone over the re-
corded data and shown that they all fit the equation evens on

page 149, vzz.,

fe ae PD

m
the constants having the values given in Table XLIII. The
constants vary with changes in experimental conditions, and
they are by no means absolute quantities. But for a given
set of conditions they remain unchanged.

TasLe XLIII.—Va.tue or “ ConsTaNTs”’ OBTAINED IN ADSORPTIONS BY
Sor. WIEGNER (308).

Absorbent. Solute. K. = Worker.
Garden soil. ; . | NH,Cl 070948 | 0'039 } Henneberg and
# = ; ; . | NH,Cl o°131 0°424 Stohmann

Nile sediment ; . | NH,Cl 0°489 0°399 Armsby
Permutite NH,Cl 2°823 0°398 Wiegner
Sodium zeolite ae CaCl, 2°487 0°317 Armsby
Zeolite LiCl 24°419 O°414 Campbell
Soil i ; .| NH,OH 0°0994 | 0°434

: : : . | NH,OH 0°47 0°461 } Brustlein

3 - ; ‘ . | NH,OH 0°054 0°386

We still, however, have to account for the fact that the
absorbed bases displace an equivalent amount of some other
bases from the soil—a procedure which would be wholly un-
necessary if nothing but adsorption were involved. This is
done by supposing that only the hydroxide is absorbed: the
acid radicle in general is not: it therefore dissolves out some >
of the bases from the soil. As this is a purely chemical re-

THE COLLOIDAL PROPERTIES OF SOIL 157

action the amount of base brought out is equivalent to the
acid set free, z.e. to the amount of base adsorbed by the soil.

Thus the modern position is essentially that of Knop, but
the idea has been expanded, and, above all, the phenomena have
been connected up with a wide range of others.

-Miyake’s experiments (202a) indicate that the rate of ab-
- sorption of the ammonium ion by soil follows the ordinary time
rate for the diffusion of liquids into absorbing substances, w7z,
x = Ki" where x = the amount absorbed, ¢ = time and
K and m are constants. The rate, however, is affected by the
presence of other ions.

The absorption of potassium by soil has been studied by
Schreiner and Failyer,! by Patten and Waggamann? and re-
cently by McCall, Hildebrandt and Johnston.®

The Action of Dilute Acids on Soils.

It will be shown later that the reaction between dilute
acids and soil is of very great importance to the soil chemist
in enabling him to form some estimate of the amounts of
the mineral plant nutrients present. The reaction ought in
principle to be simple, but numerous investigations by Hall
and Amos (120a), de Sigmund and others, have shown that
it is not, and that it falls quite out of line with the ordinary
chemical reactions. In particular different acids, even at
equivalent concentrations, do not dissolve out the same amount
of material, nor can any connection be traced between the
“strength” of the acid and the amount of its action.

Russell and Prescott (241%) have studied the reaction
between dilute acids and the phosphates in the soil and find
that it can be interpreted as a simple solvent action followed
by an adsorption.

When a soil is acted upon by a dilute acid the amount of
P,O, dissolved is found to increase continuously with the
concentration of the acid. The curves obtained for different

1 Bull. 32, U.S. Bureau of Soils, 1906.

2 Bull. 52, U.S, Bureau of Soils, 1908.
3 Fourn. Phys. Chem., 1916, 20, 51-63.

158 SOIL CONDITIONS AND PLANT GROWTH

acids are all of the same type, but they show two remarkable
peculiarities: (1) some of the strong acids, such as hydro-
chloric and nitric, bring out less P,O,; than the weaker citric
and oxalic acids at equivalent concentration ; (2) the amount
of action is not always proportional to the time, and in the
case of some acids, eg. hydrochloric and _ nitric, there is.
actually less P,O, dissolved after 24 hours’ action than after
20 minutes (Fig. 19).

Amount of P,O,extracted from soil by HNO,
of varying concentration acting for

different times.

: 0°33 Hr
r)
a oF
wo 2 Hr.
er
D
24 hin
©
oO 4F
4
a
Tey
Mm ab
=
wn
oe TS
vi mN
ee]
i i . i i i i |
‘O04 "06 08 40 412 14 16 48 20

Initial acidily, equivalents HNO; per litre.

Fic, 19.—Amount of P,O, extracted from soil by HNO, of varying concentra-
tion acting for different times.

This second result indicates that a reverse action is coming
into play, proceeding more slowly than the direct action: so
that after 20 minutes the effect is determined largely by the
direct action, while after 24 hours it is determined by the
reverse action.

_The reverse action was eliminated by a diffusion method.
When this was done all dilute acids were found to act in a very

THE COLLOIDAL PROPERTIES OF SOIL 159

similar manner and the peculiarities disappeared. The direct
action of dilute acids on soil phosphates, therefore, appears to.
be a simple chemical reaction of the ordinary kind.

The reverse action was investigated by swamping the
direct action by adding sodium phosphate to the solution : it was

60r : € 5

50

nm
2)

P,O, mgs. per 100 gms. of soil.

1 i 1 L A

° id 20 30 40 50

RO, mgs. per \O0O0 ces C

Fic. 20.—Showing that the experimental points obtained in the action of dilute
acid on soils lie on adsorption curves.

found that P,O, was absorbed from the solution notwith-
standing the presence of acid, and the phenomenon was of
the ordinary type fitted by the equation given on page 149.
Absorption was not confined to phosphoric acid: it also

160 SOIL CONDITIONS AND PLANT GROWTH

occurred with oxalic acid and citric acids but not with hydro-
chloric and nitric acids. .

Thus the reaction of the soil phosphorus compounds with
dilute acids may be resolved into two separate actions: a
direct action of the acid on the phosphorus compound, and an
adsorption of the dissolved P,O; by the soil. In high acid
concentrations the former action predominates, but both
actions always goon. ‘The solvent action is practically the
same for nitric, hydrochloric, and citric acids of equivalent
strengths, and appears to be the normal action of an acid on
a phosphate. The reverse action, the adsorption, can be ex-
pressed by the equation which has been found to fit so many
others. It is considerably influenced by the acid being greater
in the presence of the mineral acids than of the organic acids.
The amount of phosphorus compound actually brought out is —
the difference between the direct and the reverse action. Thus
hydrochloric acid dissolves out a certain amount of phosphate,
but considerable adsorption takes place, so that the net amount
left in solution becomes small. Citric acid dissolves out the
same amount of phosphate, but there is much less adsorption,
and therefore the amount left in solution is markedly greater,
The difference between the various dilute acids lies, therefore,
not so much in their solvent power, which is very similar for
all, but in their influence on the adsorption process.

Flocculation of Clay and Silt.

It is customary to divide colloids in a state of suspension
in a liquid into two groups according as they do or do not
combine with the liquid ; those which do not combine but may
be supposed to possess a surface sharply separating them from
the medium are often called suspensoids, while those that do
combine and possess no sharp surface of separation ‘are called
emulsoids. This nomenclature is not always used, even the
distinction is not universally recognised, and is probably not
always valid, but it covers a good many cases.’

1 E.g. it is not accepted by R. Zsigmondy, Chemistry of Colloids, translated.
by E. B. Spear, 1917, p. 27.

THE COLLOIDAL PROPERTIES OF SOIL 161

As a rule the suspensoids are readily precipitated by
traces of electrolytes; emulsoids, on the other hand, are not,
but usually require salting out. The phenomena have been
explained by W. B. Hardy (122) on an electrical hypothesis
and by Freundlich as the result of selective adsorption of the
coagulating ion.' .

Hardy (122, p. 238) supposes that the particles of sus-
pensoids remain suspended so long as they carry an electric
charge, z.¢. so long as there is a difference in electric potential
between them and the liquid in which they are immersed.
But they precipitate immediately this charge is neutralised by
an opposite charge, or to be more precise, as soon as they
attain the same potential as the liquid. This point where the
difference in electric potential has disappeared is called the
isoelectric point.

The flocculation of clay has been much investigated.
Schloesing in 1870 (245d) discovered the main facts and de-
monstrated the reversibility of the process. References to sub-
sequent work will be found in Wolkoff’s paper.2 The
general result is that clay behaves like an electro-negative
colloid; it is deposited at the anode on the passage of an
electric current through its suspension,’ and it is flocculated
by positively charged ions, but beyond a certain point it is
deflocculated or rendered still more disperse by negatively
charged hydroxyl ions. The flocculating effect depends more
on the valency than the atomic weight of the cation; univalent
cations are less effective than bivalent, which in turn are less
effective than trivalent ions;* there is, however, no simple
proportionality between valency and flocculation (Hall and
Morison, 120¢). In some cases, if not all;some absorption of the
flocculating ions occurs and the phenomena are therefore more

‘For a discussion of these views see W. C. McC. Lewis, A System of
Physical Chemistry, Vol. I., p. 362 (Longmans).

2 Soil Sci., 1916, 1, 585-601.

% Advantage is taken of this property to purify clay from admixed coarser
grains of silica. The process is known as electro-osmosis.

4 Among recent papers see Leoncini (172); Wo. Ostwald, Kolloid Zeitsch.,
1920, 26, 69-81; N. Bach, ¥. Chim. Phys., 1920, 18, 46-64.

II

162 . SOIL CONDITIONS AND PLANT GROWTH

nearly expressible by an absorption curve.’ Pickering (226a)
has demonstrated an absorption in the case of clay.

Comber (69) has observed an interesting difference in the
flocculation of silt and of clay. Silt behaves like other sus-
pensions of particles insoluble in water; it is flocculated by
small quantities of electrolytes, and the flocculation proceeds
most rapidly at the iso-electric point. Flocculation is hindered
by the presence of hydroxy] ions which increase the negative
charge, and therefore calcium nitrate is a more potent floccu-
lator than lime. This is the normal behaviour of such sus-
pensions under the conditions of the experiment.

Clay, on the other hand, behaves differently ; flocculation
by small quantities of electrolytes is most rapid in slightly
alkaline solutions, away from the iso-electric point. Lime is
a better flocculator than calcium nitrate. In order to account
for the difference Comber assumes that the clay particles are
coated or protected with a layer of emulsoid silica which
impresses emulsoid characters on the whole system. On
this view the OH ion “ peptises” the protecting emulsoid and
the Ca ion then flocculates the clay. Support for this view
was found in the fact that addition of colloidal silica to a
ferric oxide suspension made the system behave like clay,
flocculated more easily in alkaline than in neutral solutions.

These observations throw important light on the well-
known difference between clay and silt? (p. 107).

Odén discusses the protective effect of humic acid on the
coagulation of clay.*

Pan Formation.

A pan is a layer of hard impermeable rock that gradually
forms at the usual water level below the surface of the soil
under certain conditions. Its effect is to cut off the soil above
from the material below and therefore to modify profoundly

1 Linder and Picton, Trans. Chem. Soc., 1895, 67, 63. For further work see
H. Freundlich, Zeit. f. phys. chemie, 1910, '73, 385-423.
2 See also Otto M. Smith, ¥. Amer. Chem. Soc., 1920, 42, 460-472.
3 Sven Odén, ¥. Landw., 1919, 67, 177-208.

THE COLLOIDAL PROPERTIES OF SOIL 163

the movements of water and of air, leading often to swamp
conditions. The effect on vegetation becomes so marked that
in agricultural practice the pan has usually to be removéd,
often at considerable trouble and expense.

The conditions determining the formation of pan seem to
be a supply of organic matter, permeability of soil, low content
of soluble mineral matter, and absence of calcium carbonate.
These conditions occur most frequently on light sandy soils
_ where for some reason the water is held sufficiently near the
surface.

Pans are best seen when the sand is overlain by a deposit
of peat. The sand is then bleached to a depth of 5 to 60 cms.
Suddenly there comes a change: a coloured layer of solid
rock occurs which may vary in colour from yellow to black
and in thickness from 10 to 60 cms.: on closer-inspection this
is seen to consist of particles of sand cemented firmly together.
This is the pan:! underneath it lies the sand proper. But
pans are by no means confined to peat: they often occur in
forests, on heaths and on certain cultivated soils.

Chemical analysis shows that the pan is much richer in
- Organic matter, iron, alumina, and especially material dis-
placeable by ammonium nitrate solution than either the sand
above or the'sand below. Table XLIV. gives typical results
showing the amounts of material soluble in HCl.

The process of pan formation, therefore, involves a con-
centration of these substances in a certain layer of the soil.

On the older view humic acid was supposed to be formed
from the organic matter present, and to dissolve iron, alumina,
and other substances, forming soluble humates which were
washed down into the soil and precipitated at the point where
pan formation occurred.

Various hypothesis were advanced to account for these
changes. —

+
1In German the pan is called “ Ortstein ” and the bleached sand “ Bleisand,”
“« Bleichsand,’’ or occasionally ‘‘Grausand’’, When the formation, instead of
being sand, is clay, the white soils are called ‘‘ Molkenbéden”. Some of these
are described in Internat. Mitt. Bodenkunde, 1914, 4, 105-137. See also (24).
33.7

164 SOIL CONDITIONS AND PLANT GROWTH

TaBLE XLIV.—ANALYSES SHOWING CONCENTRATION OF IRON, ALUMINA,
AND ORGANIC MATTER IN THE PAN.

Pan from Cesar’s Camp Pan from Freudenstadt,
(Morison and Sothers) (204). Black Forest (Miinst).
l
f Soil Soil
Mande | Pan. | Below | PEGS! Pan, | below
Hygroscopic moisture Ase ae iy | 3°06 “90 *300 | 3°79 | *768
Loss on ignition . 1°84 7°22 | 1°36 _ — —_
Material soluble in n NH.NO, —- — — I°577 | Io'92 | 1°946
Fe,O, . “lf. i : "253 | 1857) *
Al,O; «. ; : F f L493 4°066 | Scars "180 | 4°946| 1°268
a0" 3 ; : ; “080 *350| ‘106 | trace ‘OIQ| ‘o2I
MgO . ‘ ‘ ‘ ' 063 °084| ‘IIo O15 ‘118 | °*088
5 0 es 4 : % 5 087 "155 | °I52 035 ‘I72| *052
Na,O . , : é Fe — _— ‘OI "041 | *or6
P.O; .. ‘ % : . | trace 037 | ‘018 "029 *059| *043
SiO, . i i : . — as —— *3I2 | 2°698 | 1°527

Mayer (194), extending the earlier work of Emeis,’ sup-
posed that anzrobic conditions arose during part of the year
when the land was waterlogged, and humic acid was then
formed and the iron reduced to the ferrous state. Ferrous
humate is soluble in water and therefore washes downwards :
even aluminium silicate becomes partly soluble. At a certain
depth this ceases ; various causes may come into play: the
water table may be reached, or there may be an accumulation
of washed-down clay on which the humates are precipitated.
Then in the dry part of the year oxygen can gain access to
_ this depth, converting the ferrous humate to ferric humate,
which being insoluble is protected against further washing.

Ramann modified this somewhat by assuming that pre-
cipitation occurred when the solution of iron humate reached
the zone, intermediate between the surface soil and the sub-
soil, where he supposes weathering to be still proceeding and
where, therefore, there is a larger proportion than usual of
soluble reactive mineral salts.

As an alternative Hall sugge$ted that the solvent is CO,:
the iron is reduced to the ferrous state by the organic matter

C, Emeis, Waldbauliche Forschungen und Betrachtungen, Berlin, 1875.

THE COLLOIDAL PROPERTIES OF SOIL 165

and is then dissolved as ferrous bicarbonate. At the lower
level the CO, escapes and the iron carbonate is precipitated
and then oxidised.

These explanations seem very simple but they present
several difficulties. In the first place, Morison and Sothers
could find no evidence that peat or humic acid can reduce
ferric oxide, the first stage in the process, according to Mayer.
Ferric chloride is easily reduced: so easily, indeed, that ferrous
iron is produced during the process of testing a mixture of
ferric oxide and peat if one begins by extracting with HCl;
but ferric oxide is not. Secondly, it is difficult to understand
why the deposition should be so local and so sharply defined.

Recent workers, therefore (Miinst,! Ramann,? Morison, and
Sothers (204), regard the whole process as a formation first
of a “sol” and then of a “gel,” and Morison and Sothers
suggest the following as the most probable course of events.

It is well known that “sols” change to “gels” in presence
of small quantities of electrolytes, and conversely “gels” often
change to “‘sols” when electrolytes are removed. In normal
soils the conditions are favourable to gel formation, but when
_ in these particular soils the upper layer of sand becomes de-
nuded of its soluble material by the persistent washing of rain
water, the conditions become favourable for the formation of
sols of ferric hydroxide and of humus—or ferric humate, if
one likes to put it that way. Morison and Sothers actually
obtained such sols* by persistent washing of ferric-humus
gels.

As the sol is washed down some is deposited on the
bleached sand, but the bulk of it passes to the permanent
water level where it remains and accumulates, diffusion being
practically non-existent. During the dry months a certain
amount of desiccation takes place, involving a deposition of
the sol as a gel: there is also a certain amount of transformation

1 Mist, Bied. Zentr. Agrik. Chem., 1912, 41, 3-10.

* Ramann, Bodenkunde, Berlin, 1911, p. 204. See also H. Stremme, Kolloid
Zeitsch., 1917, 20, 161-168,

* As might be expected these did not give the ordinary iron reactions.

166 SOIL CONDITIONS AND PLANT GROWTH

of sol to gel through the presence of electrolytes in the
ground water.! Some of the humus gel becomes oxidised,
some of both humus and ferric gels change their colloidal
properties in other ways. When wet weather comes on again
it is no longer possible for the whole of the deposited gel to
change back to a sol: some will no doubt change, and there °
will be a certain washing down of the gel deposited on the
bleached sand. But where the bulk had accumulated deposi-
tion has begun, and the place where this happened serves as
a seat of further action.

This view seems more in accordance with the facts than the
older one and it does not involve any unproved assumptions—
such as reduction of ferric to ferrous iron and presence of ferrous
iron in the pan,

There is another type of pan formation which must not be
confused with this. On clay soils the continued ploughing to
a uniform depth with heavy ploughs often leads to the consoli-
dation of the underlying soil and the formation of a compact
layer of clay or ploughsole which behaves like rock, This effect
is purely physical, it can be overcome by periodically sending

a subsoiler behind the plough to break through the mass.

The Retention of Water by Soil and the Rate of
Evaporation.

The older chemists and physicists divided the soil moisture
into two kinds: free water, which was supposed to be suspended
on the particles just as sea water is suspended on grains of sand
or as oil on leaden bullets; and hygroscopic water, which was
retained in a closer sort of way so that it could not be taken up
by plants or micro-organisms. Methods for discriminating be-
tween these were devised, and pot experiments were made to
ascertain directly the amount of free water available for the
plant.

The first experiments made on the rate of evaporation of
water from soil gave broken curves which could be explained —

1 Ramann laysstresson this; Morison and Sothers do not, because their sols
were very stable in presence of electrolytes.

THE COLLOIDAL PROPERTIES OF SOIL 167

on the assumption that these two kinds of soil water existed.
Shull’s results (see p. 224) presented more difficulty and led him
to regard the state of the soil moisture as unbroken and con-
tinuous. Keen (146a) furnished rigid proof: he found that the
relationship of water to soil differed fundamentally from its re-
lationship to sand. The evaporation of water from sand, silt,
china clay, and ignited soil proved to be relatively simple and
could be explained by the known laws of evaporation and
diffusion. But the evaporation of water from soil could not:
it was more complex. Instead of the simple proportionality
between water content and rate of evaporation observed in the
case of sand, the curves for soil were more exponential in type-
The difference was traced to the soil colloids and it disappeared
when the soil was ignited and the colloidal properties lost: the
curve then became identical with that obtained for sand.
The influence of the colloids has so far only been expressed
empirically, but it is probably connected with the relation
between vapour pressure and moisture content. But there is
clearly something else at work, for the curve is not of a simple
exponential type. It is necessary to allow for another factor :
the effect on the rate of evaporation of the decreasing water
surface in the soil, the surface obviously diminishing in area
as evaporation continues.
The equation finally developed by Keen is :—

3 / WS
AT = ( ar 1)[2"303 log, 9(w + K) - log.K},

where “ = rate of evaporation.
w = percentage of water present by weight.
Ss = specific gravity of the soil.
A and K = constants.

This relationship holds without any break, proving that the
water in a normally moist soil is all held in the same way
without any break in physical state (Fig. 21). At one end of
the curve the water is more easily given up than at the other,
and in the competition for water between soil colloids and

168 SOIL CONDITIONS AND PLANT GROWTH

plants or micro-organisms some kind of equilibrium may be
attained under definite conditions: this equilibrium is the
“ wilting-point” of the physiologist, On this view the other
constants and critical points that have been indicated by
various investigators are all equilibrium points and do not re-
present breaks in the condition of water in the soil. Further
discussion will be found on p. 218.

fie ;

u

Per cent. of water.

Fig, 21.—Curves showing rate o1 evaporation of water from: I. Ignited garden soil
over conc. H,SO,; II. Arable soil (Hoos field, dunged) over conc. H,SO,; III.
Arable soil (Hoos field, dunged) over 55°4 per cent. H,SO,.

The Influence of Colloids on the Reactions Taking Place
in the Soil.

It is obvious that a reaction taking place in a colloidal
medium possessing properties like selective adsorption must

THE COLLOIDAL PROPERTIES OF SOIL 169

differ from one going on in aqueous solution such as most
chemists are accustomed to think about. We must therefore
be prepared for unexpected results: it is possible that some
of the changes formerly attributed to bacteria may in part be
due to colloids (see p. 215).

Further, the colloids influence both micro-organisms and
plants (p. 239) and therefore indirectly affect the reactions in
the soil. ;

Again, the remarkable changes observed in heated soils * and
soils stored in a dry condition, such as the increase in amount
of soluble matter,” in the rate of nitrification,* and in the pro-
ductiveness, are probably much influenced by changes in col-
loids.* Gedroiz’s*® experiments with oats grown in soils kept
dry for a number of years are given in Table XLV.

TaBLE XLV.—EFFEcT oF STORAGE IN A Dry STATE ON THE PRODUCTIVE-
NESS OF SorLs. (Oats: Gedroix, 1908.)

No. of Years of Complete without | Manure without
Storage. No Manure. | Complete Manure. Nitroges Phosphate.
ce) I0°3 83°5 13°5 rr
r 17°8 83°9 32°3 19
3 24°6 go"9 23°6 35°4

5 25'0 102°8 32°2 42

The Estimation of Soil Colloids.

Efforts have been made to determine the amount of col-
loidal material in the soil by studying the absorption of dye-
stuffs or of water vapour. So far the results are not easy to
interpret nor are they always related to the other colloidal
phenomena. It would therefore be premature to attempt any

1S. U. Pickering, ¥ourn. Agric. Sci., 1910, 3, 32 and 258.

2U.S, Department of Agric. Bureau of Soils Bull. 8, p. 13; Bull. 22, p. 41;
also fourn. Agric. Res., 1918, 12, 383.

* Buddin, Yourn. Agric. Sci., 1914, 6, 452-455.

4 For study of the biological changes in soil during storage see F. E. Allison,
Soil Sci., 1917, 3, 37, and W. Giltner and H. V. Langworthy, ¥ourn. Ag. Res.,
1916, 5, 927.

> Gedroiz, Bull. Internat. Instit. Agric. Rome, 1915, p. 37.

170 SOIL CONDITIONS AND PLANT GROWTH

summary as yet: the student wishing full information may be
referred to the papers on the absorption of dye-stuffs by
Sjollema,! Endell,? Ashley,* Konig, Hasenbaumer and Hassler,*
Hanley,° and Tadokoro,’ the last-named giving other references
also, and to Mitscherlich’s papers on the absorption of water
vapour.’ The general discussion of Leeden and Schneider *®
may also be consulted.

The Constitution of the Soil.

The components of the soil do not form a mere casual
mixture. A much more intimate mingling prevails, amount-
ing almost to a loose state of combination, from which the
separate substances are only extracted by drastic mechanical
means, or gentle chemical treatment. The soil colloids and
the calcium carbonate appear to be responsible for the forma-
tion of the compound particles, and as soon as they are altered
by treatment, first with acid and then with alkali, the particles
fall to pieces and the silt, clay, etc., can be readily separated
by sedimentation processes. No method has been devised for
measuring the size of the compound particles. Their exist-
ence can be shown in a clay soil by making two analyses of
the same soil, one after the usual treatment with acid and alkali
to break up the compound particles completely, the other on
the untreated soil where the breaking up is only partial. The
demonstration, however, does not work so well for loams
(Table XLVI).

The existence of these compound particles puts out of the
question any complete quantitative interpretation of a mechani-
cal analysis. The properties of a soil are not the sum of the
properties of the separate fractions—clay, fine silt, etc.—be-

1 Fourn. f. Landw., 1905, 53, 67. 2 Kolloid Zeitsch., 1909, 4, 246.

3H. E. Ashley, The Colloidal Matter of Clay and its Measurement (U.S.
Geol. Sur. Bull., 388, 1909).

4Landw. Versuchs-Stat., 1911, '75, 377:

5), A. Hanley, f¥ourn. Agric. Sci., 1914, 6, 58.

6 T, Tadokoro, ¥ourn. Tohoku Imp. Univ. Sapporo (Japan), 1914, 6, 27.

7 These are summarised in Internat. Mitt. f. Bodenkunde, 1912, 1, 463-480.

8 Tbid., 2, 81.

THE COLLOIDAL PROPERTIES OF SOIL 171

cause in a normal soil these fractions, which we may regard
as the ultimate particles, are largely bound together into com-
pound particles. How far the properties of the ultimate par-
ticles are modified by this union we cannot say, but no very
profound alteration seems to take place in the sands and silts
because the properties of the separate fractions, deduced by
correlation methods from studies of numerous soils, agree
tolerably well with the properties revealed by direct experi-
ments on the fractions themselves. The finer particles are
more changed, the result being to minimise the effects of
their smallness. Thus, while the limits within which the
properties of a soil fall are determined by the ultimate par-
ticles, a considerable variation is possible within these limits
through the formation of compound particles.

TaBLE XLVI.—EFFECT oF COMPLETE DISINTEGRATION OF SoIL By ACID AND
ALKALI ON THE RESULTS OF MECHANICAL ANALYSIS OF A CLAY AND A

Loam.
Clay Soil, Woburn. Loam, Rothamsted.
Disintegration | Disintegration | Disintegration | Disintegration
Complete. Incomplete. Complete. Incomplete.

Fine gravel . ‘ ‘9 *2 I'°3 I°3
Coarse sand . ‘ 8°3 8°6 9°5 8°5
Fine sand ; ; 3°2 2°9 22°7 26°9
Silt F ‘ 8°6 I2°I 32°3 33°6
Fine Silt . 3 Ir‘2 13°3 8°9 9°3
Clay s ; ‘ '43°2 36°3 12'9 IIL

It is unfortunate that so little is known about the com-
pound particles, because they play a great part in determining
the relationships between soil and plant growth. They can
be disintegrated by various cultivation methods, such as plough-
ing the soil when wet, or by allowing the stock of organic
matter and calcium carbonate to fall too low, and when this
has happened the “clay” properties become emphasised, so
that the soil loses its fine crumbly state and is very apt to be-
come sticky when wet, and to dryinto a hard cake through
which young plants can only force their way with difficulty.

172 SOIL CONDITIONS AND PLANT GROWTH

The compound particles can be reformed by careful cultiva-
tion and by adequate additions of organic matter and calcium
carbonate, but the process may take years, and it cannot be
hastened until it is better understood.

In the preceding pages we have shown how many of the
important soil properties are due to colloids. The formation
of these compound particles, the absorption of soluble manures,
the retention of water (in part), the swelling of the soil when
wet and its shrinkage when dry, are all. colloidal phenomena.
-If we regard the mineral particles as the skeleton of the soil
we must look upon the colloids as clothing it in many of its
essential attributes. How the colloids are arranged in the
soil is not known, but the simplest view, and one in accord-
ance with all the facts, is that the mineral particles, especially
the fine silicate particles, are coated! with a colloidal complex
containing silica, alumina, ferric oxide, alkaline bases, and phos-
phoric acid derived from the weathering of the rock material
and the so-called humus. These various components are not
in true chemical combination, but in a state of absorption, or
solid solution. The complex is decomposable by changes in
temperature, concentration of the soil solution, etc., but it de-
composes continuously and not in the fer saltem manner of
ordinary chemical reactions.- It can interact with various solu-
tions, absorbing certain substances as a whole—e.g. organic
dye-stuffs—or simply giving up to the solution an amount of
base equivalent to what it has absorbed.

A wholly different conception of the constitution of the
soil has been put forward by Whitney of the Bureau of Soils,
United States Department of Agriculture, Washington (304-6).
Soil particles are supposed to arise by disintegration and to
_consist of the original minerals of which the rock’ was com-
posed ; little importance is attached to the weathered silicates
that play so large a part in the view just set out. Colloidal
properties and the special clay properties begin to appear when
the disintegration has gone so far that the particles become
very minute: these properties are not associated with any par-

1See also 89a and b.

THE COLLOIDAL PROPERTIES OF SOIL 173

ticular complex, but are supposed to. be exhibited by any sub-
stance that is sufficiently finely divided. Most agricultural
soils arise from the same minerals and are therefore of similar
chemical constitution : in consequence, the solution in contact
with the particles, z.e. the soil moisture, is of similar composi-
tion and concentration for all soils. It is further supposed that,
under similar climatic conditions, the concentration of any par-
ticular ion in the soil solution is not materially altered by addi-
tion of soluble salts, any such addition only forcing out of the
solution a number of the ions already there. Special import-
ance is attached to this soil solution and it is regarded as the
food of plants! and the source of fertility of the soil; indeed,
the function of the mineral part of the soil iS mainly to hold
up and distribute this solution. Soluble fertilisers, like potas-
sium salts, are not supposed to increase the amount of food
available to the plant, but to owe their beneficial effects to in-
direct actions in the soil, such as the precipitation of toxic
substances, facilitation of movements of soil water, etc.

Controversy has arisen in three directions. It was at first
supposed that the physical properties of the soil moisture
were in the main deducible from the hypothesis that the
water was suspended as on grains of sand or like oil on
bullets (see p. 166). This has been shown to be inadequate ;
it is necessary to take the soil colloids into account.

The view that the soil solution is unalterable in. composi-
tion led to considerable discussion. Hall, Brenchley, and
Underwood (1212) and F. H. King (1474) obtained diametri-
cally opposite results to those of Whitney and Cameron.’ It
is now generally recognised that the composition of the soil
moisture varies with the soil and the treatment, and, indeed,
that the changes in composition are on the whole related to
the fertility of the soil (see p. 125). Bouyoucos (49c) finds

1It is interesting to note that a controversy on this point was going on sixty
years ago when agricultural chemists first began to use water cultures. See
Schumacher, Landw. Versuchs-Stat., 1863, v., 270-307.

*For bibliography of this discussion see Stewart, ¥ourn. Ag. Res.,.1918,
I2, 311-368.

174 SOIL CONDITIONS AND PLANT GROWTH

that the addition of soluble salts other than phosphates in-
creases the concentration of the soil solution. Much dis-
cussion has also arisen in regard to the supposed toxins: that
is dealt with on p. 244.

As developed more recently by Cameron (65c), however,
the views of the American Soil Bureau have more in common
with those of the British workers. The solid particles of the
soil are supposed to consist of disintegrated rock minerals,
along with adsorption complexes, solid solutions such as the
so-called basic phosphates, and indeterminate substances in
an extremely fine state of division apparently containing
“humus,” oxides of iron and aluminium, etc. The solubility
of these constituents in water is influenced by three circum-
stances: their fine state of division, the presence of CO, in the ©
soil water, and the fact that, as regards the solid solutions and
adsorption complexes, some sort of distribution coefficient
comes into play. Since the soil minerals are salts of strong
bases with weak and almost insoluble acids they become more
or less completely hydrolysed in solution, so that the con-
centration of the base would have to be very high before
equilibrium was attained. It is therefore improbable in
humid areas that equilibrium ever is attained. Moreover,
and this is a central part of the thesis, soil phenomena are
dynamic and not static; the soil moisture, the soil solution,
even the very particles of the soil itself are in continual
motion and state of change. Hence the composition of the
soil solution must be continually changing.

But it is considered that the amount of change in com-
position is small in comparison with the changes in the soil,
and such changes are not correlated with changes in the
productiveness of the soil. All the factors in soil fertility are
interdependent, and it is a mistake to confine attention ex-
clusively to any one aspect of their action. Thus fertilisers
should not be regarded exclusively as plant foods: they affect
more or less every soil factor influencing crop production, and
the problem can be satisfactorily solved only by discovering
the nature and extent of these interrelations.

CHAPTER V.
THE CARBON AND NITROGEN CYCLES IN THE SOIL.

THE organic matter added to the soil by plants, etc., rapidly
undergoes a number of changes in presence of air. Oxygen
is slowly but continuously absorbed, and an almost equal
volume of carbon dioxide is evolved, indicating that the main.
change is of the type—

C,HymOn + 20, = nCO, + mH,O.
Thus the carbon in the soil tends to fall off relatively to the

Be ag Ay ;
_ nitrogen, and the ratio N which, in the original plant material,

2g. the stubble, is about 40,! becomes reduced in the soil to
10 (Table XLVII.). Other products are formed as well,
including ammonia and the dark-coloured humus _ bodies
already described, but the details of these changes are un-
known. Investigations with the individual plant constituents,
cellulose, fats, various organic acids, proteins, have so far
brought out little beyond the fact that they all oxidise to CO,
in the soil, while the calcium salts of organic acids change to
CaCO,

The absorption of oxygen by soil was demonstrated by
de Saussure about 1800, studied by Boussingault (48), and
definitely attributed by Schloesing to micro-organisms in
1883.7, Wollny developed this thesis in 1884 (317) and
showed that the rate of oxidation is much diminished by

1 For leguminous crops, however, it is about 25.

2 Lecons de chimie Agricole, 1883, p. 277: ‘ C’etait la, pensait on alors, un fait
purement chimique. On sait aujourd’hui que c’est principalement un fait biolo-
gique, c’est-a-dire que la combustion observée est le resultat de la vie de nom-
breux organismes, tel par exemple que le ferment nitrique, lequel est chargé de
transporter l’oxygéne sur l’azote”’.

175

176 SOIL CONDITIONS AND PLANT GROWTH

traces of antiseptics. It shows a general increase up to a
certain point with the amounts of moisture, organic matter
and calcium carbonate present, although no sharp proportion-—
ality exists. It is related to productiveness; in a series of ©
similar soils under similar climatic and other external cir-
cumstances the respective rates of oxidation were found to.
vary in the same way as the values for productiveness (Table
XLVII.).

TasBLe XLVII.—RatEes oF OXIDATION, ORDER OF PRODUCTIVENESS, AND
ANALYTICAL Data FOR CERTAIN WoBURN SOILS. RuSSELL (241a).

- 3 Analytical Data.
3| 28>]
—o, of Agricultural History. 35 3 iS: S g I g gS:
B5/ 8s i
Os} ws £ | |as/83
ffR 15/16 |Abids
. mi eet i eB
Stiff Oxford | Wheat stubble I | 23°2 [°252/2°53|8°73|'o21
clay
Road Piece | Wheat stubble preceded by mangolds
fedonthe land . 2| 18°7 ['172|1°76|5°31|072
Lansome | Barley following mustard ploughed
field in; mineral manure 3 | 14°t ['122|r°r9|4°17|"027
Lansome Barley following tares ploughed i in;
field mineral manure . 4 | Lo'2 [°132/1°24/3°22/"051
Lansome | Barley following tares ploughed i in;
field no mineral manure 5 8:2 ['I09|1°18|3°46|"008
Stackyard | Wheat unmanured 6 | 82 [060|1°39|4"07|"004
field
Stackyard. | Wheat, ammonium salts only 7 7°8 ['102|1°29|4°58} nil
field

The reasons for the connection between oxidation and
fertility will become more evident as we proceed.

In so far

as oxidation is due to micro-organisms, its velocity obviously
affords a measure of their activity; Wollny and later van
Suchtelen! used the CO, evolution for the same purpose.
But there is a more fundamental relationship. Oxidation
affords, so far as is known, the chief source of energy for the
numerous micro-organisms of the soil. Gillespie” has sug-

1 Van Suchtelen, Cent. Bakt. Par., 1910, 28, 45-89. See also F. G. Merkle,

¥. Am. Soc. Agron., 1918, 10, 281.
2 Soil Sci., 1920, 9, 199.

CARBON AND NITROGEN CYCLES IN THE SOIL 177

gested an interesting distinction between high and low
potentials in oxidation which may prove of value in in-
vestigations.

Stoklasa and Ernest (273a@) found about o'05 grm. CO,
evolved per kg. of soil in 24 hours at 20° C. under aerobic
conditions. Under anaerobic conditions the quantities were
less.

The Decomposition of the Carbon Compounds.

Cellulose forms the chief constituent of the plant residues
added to the soil, and its decomposition is probably the largest
single process occurring in the soil, accounting for the chief
part of the “humus” found. The classical investigations of
Omelianski showed that both marsh gas and hydrogen arise
under anaerobic conditions, such as obtain in swamps
and ponds, but in normal soils the decomposition proceeds
aerobically and without the evolution of these gases.

It is generally supposed that fungi, and particularly
actinomycetes, play an important part in cellulose decomposi-
tion: McBeth (1854, c,) has also described bacteria to which he
attributes this change. The only organism that has been
studied in detail is Spzrocheta cytophaga (Hutchinson and
Clayton, 140/). This organism is distributed in soils; it is
aerobic and very selective in its action, cellulose being the
only compound with which growth was secured: it is indeed
inhibited by many carbohydrates, especially those containing
reducing groups. Its nitrogen requirements are met by simple
compounds such as ammonium salts, nitrates, amino-acids,
but not by higher compounds such as peptone (except in
weak solution), gelatine, etc. The products of decomposition
of cellulose include small quantities of volatile acids ; mucilage
soluble in ammonia but insoluble in acids, and which does not.
give rise to optically active compounds on hydrolysis; and a
pigment apparently related to the carotin group: there is no
obvious gas in the cultures.

The organism possesses certain morphological character-

istics which explain some of the difficulties of previous investi--
12

178 SOIL CONDITIONS AND PLANT GROWTH

gators: it passes through phases in its life history that give

the appearance of wholly distinct organisms. ‘

The Nitrogen Cycle.

The chemical investigations of the nitrogen cycle in soils
have usually been confined to changes in the percentage of
nitrogen and in the amount of nitrate present, and conse-
quently they throw little light on the actual reactions taking
place. Incomplete as they are, however, they have served a
useful purpose by indicating the nature of the problem and
furnishing material that has helped in unravelling the rather
complex changes going on. Four cases have been studied :—

1. The simplest is that of an ordinary arable loam kept
moist, aerated, and at 10° to 15° C.—these being normal con-
ditions—free from vegetation and from the washing action
of rain—these being abnormal conditions. A -considerable
formation of nitrate then takes place, about 3 per cent. per
annum of the nitrogen being converted, and generally there is
a small loss of nitrogen, presumably in the free state. How
far the accumulation of nitrate would go under these circum-
stances has never been ascertained, because the experiment is
necessarily very slow. Boussingault (484) stated that in eleven
years one-third of the nitrogen of a rich soil changed to nitrate,
and about one-half of the carbon to carbon dioxide.

2. If the conditions are made more normal by exposing
the soil (still kept free from vegetation) out-of-doors to the ©
action of rain and weather generally, the nitrates do not
accumulate but wash out, and can be detected in the drain-
age water. The soil thus loses nitrogen compounds, and in
course of time the loss becomes very considerable. At
Rothamsted a little plot of arable land z9'55 acre in extent has
been kept free from vegetation by hoeing, but not otherwise
disturbed, since 1870; it has now lost one-third of its original
stock of nitrogen. The plot has been converted into a lysimeter
by isolating it from the surrounding ground by cement
partitions and then underdraining: the drainage water is all
collected and analysed. At the end of forty-seven years the

CARBON AND NITROGEN CYCLES IN THE SOIL 179

amounts of nitrogen found as nitrate in the drainage waters
were added up and found approximately to equal the total
loss of nitrogen from the soil (Table XLVIII.). |

The obvious uncertainty attaching to so prolonged an
experiment is reduced in this case by the fact that the
determinations were for the last twenty-eight years of the
period made by the same analyst. Miller found that the rate
of loss of nitrogen (estimated by the quantities of nitrates in
the drainage water) was about 40 |b. per annum in the earlier
years, and fell below 30 lb. and finally below 25 Ib. per
annum in the later years. The experiment is not fine enough
to justify any discussion of the small balance, but it shows
that the loss of nitrogen is maznly due to leaching out of
nitrates.

Taste XLVIII.—Cuances in NITROGEN CONTENT OF A SoIL Kept FREE
FROM VEGETATION FOR FORTY-SEVEN YEARS, BUT EXPOSED TO RAIN AND
WEATHER. MILLER (200b), RUSSELL AND RICHARDS (241).

Per cent. of Nitrogen in Lb. of Nitrogen per Acre, Nitrogen Recovered
Soil, top 9 inches. top 9 inches. as Nitrate, 1870-1917.
In 1870. In 1917. In 1870. In 1917. | Loss in 35 Years. Lb, per Acre,
: "099 2376 1124 1247}
146 "097 3500 2328 1172 1200

It is unfortunate that this highly important experiment has
not been repeated with other types of soil, because there is
evidence that a richer soil would lose more nitrogen than is
accounted for by the nitrates formed, the rest presumably
escaping as gas.

Consideration of the curves for the rate of washing out of
nitrate led Russell and Richards (241%) to suppose that there
must be a nitrate immobiliser, probably certain organisms,
functioning in soils even when uncropped, taking up a part of
the nitrate formed and only slowly liberating it.

1 After deduction of the amount brought down in the rain. The upper line

of figures refers to the 20 inch and the lower to the 60 inch gauge. Some nitrate
is no doubt contributed by the sub-soil.

ta?

180 SOIL CONDITIONS AND PLANT GROWTH

3. When the conditions are made wholly normal by allow-
ing vegetation to grow, some of the nitrate is taken up by the
plant and only a part is washed away, the division depending
on the favourableness of the conditions for plant growth.
The absorption of nitrate by the plant is much greater, and
the amount of nitrate in the drainage water is therefore much
less, on the Rothamsted wheat plots where ample supplies of —
potassium salts and phosphates are present, than on the plots
where these nutrients are less abundant and the crops smaller
(Table XLIX.).

TABLE XLIX.—ErrectT oF PHOSPHATES AND POTASSIUM SALTS ON THE
UTILISATION OF NITRATES BY PLANTS.

Crop Yield per | y; Nitrogen Present
Acre per Annum. Kaas as Nitrate in | pe. Cent _—
; Drainage Water | * & Cent. :
Treatment. ei ane ARRON, nag of N in Soil,
fsiead Vee Ib. per a. Pp Soil. Ib. per
rain, Taw, e Annum.
Bushels. | Cwts, fees Million. sans
Ammonium salts con-
taining 86 lb. N +
No P or K salts -| 160 | 14°75|. 33°5 17°8 *106 67°5
Abundant supplies of
P and K salts. -| 26°7 130°75| 45 8°5 "116 51

Part of the absorbed nitrate remains in the root and
stubble, and is again added to the soil when the plant dies.
Hence the percentage of nitrogen in the soil is higher where
the conditions are favourable for the growth of plants than
where, by the operating of some limiting factor, plants cannot
make full growth and therefore leave untouched much of the
nitrate to be washed away. It is this that accounts for the
losses in fallow ground—losses that have been discussed by
Lawes, Gilbert, and Warington (1664), by Russell (241/) and
by von Seelhorst (258). .

Most of the data hitherto accumulated are incomplete,
because they refer only to crop results and take no account of
nitrates washed out in the drainage water: fuller data have,
however, been obtained in the lysimeter experiments of Gerlach

CARBON AND NITROGEN CYCLES IN THE SOIL 181

at Bromberg (104c), and they fully confirm the results set out
above.

4. There is no reason to suppose that the amount of
nitrogen in an uncultivated soil alters appreciably from year to
year so long as the landis untouched. But directly ploughing
and cultivation operations begin great losses of nitrogen set
in, as shown by Shutt’s analyses of the Indian Head soil,
Saskatchewan (Table L.). In this particular case there is
practically no drainage water, and therefore little or no wash-
ing away of nitrates, yet only one-third of the lost nitrogen is
recovered in the crop. Snyder (2684) has given similar results
for Minnesota soils, and Swanson for Kansas soils.!

_ Taste L.—Losses or NirroGen CONSEQUENT ON BREAKING UP OF PRAIRIE
Lanp, Tor 8 Inches. Suutt (265).

| Per Cent. | Lb. per Acre,
Nitrogen present in unbroken prairie - | *371 | 6940
= » after 22 years’ cultivation . "254 4750
Loss from soil . "i ; A ‘ $ 7 . 2190
Recovered in crop d 4 : f ; é 12 Oo
Deficit, being dead loss : i ; 4 : . 1490
Annual dead loss . : 4 ; 5 ‘ x 4 68

The exhaustion of the soil is due, therefore, not to the
removal of the crop, but to the cultivation.

Similar losses take place when heavy dressings of farmyard
manure are repeatedly applied to land. One of the Broadbalk
wheat plots receives annually 14 tons of farmyard manure per
acre, containing 200 Ib. of nitrogen. Only little drainage can
be detected” though considerable leaching out of nitrates may
occur, but the loss of nitrogen is enormous, amounting to nearly
70 per cent. of the added quantity. Alongside is a plot
receiving no farmyard manure, from which, in spite of drain-
age, there is no apparent loss (Table LI.).

1 Kansas Bulls., 199, 1914; 220, 1918. See also Yourn. Ind. Eng. Chem.,
1915, '7, 529. Summaries of other results are given by A. W. Blair and H. C.
McLean, Soil Sci., 1917, 4, 283-293.

182 SOIL CONDITIONS AND PLANT GROWTH

TasLe LI.—Losses or NITROGEN FROM CULTIVATED SoILs, BROADBALK |
WHEAT FIELD, ROTHAMSTED, FortTy-SEVEN YEARS, 1865-1912.

Rich Soil, Plot 2, Poor Soil, Plot 3,
Ib, per Acre. lb. per Acre.
Nitrogen in soil in 1865 . . | 'I75 percent. = 4,340 |*105 percent. = 2,720
Nitrogen added in manure, rain
(5 lb. per annum) and seed (2
Ib. perannum) . A . 9,739 330
Nitrogen expected in 1912 i 14,070 3,050
Nitrogen found in1g12 . .|°245 percent. = 5,730 | ‘103 percent, = 2,510
Loss.from soil . . . 8,340 540
Nitrogen accounted for in crops 2,550 750
Balance, being dead loss . 5,790 — 210
Annual dead loss : ; : : 123 -5

Experiments of this kind have led to the conclusion that some
gaseous product is formed in addition to nitrates, and, as no
sufficient amount of ammonia can be detected, it is supposed
that gaseous nitrogen is given off. The conditions for this
decomposition appear to be copious aeration, such as is
produced by cultivation, and the presence of large quantities
of easily decomposable organic matter. Now these are
precisely the conditions of intense farming in old countries
and of pioneer farming in new lands, and the result is that the
reserves of soil and manurial nitrogen are everywhere being
depleted at an appalling rate. Fortunately there are recupera-
tive actions, but one of the most pressing problems at the
present time is to learn how to suppress this gaseous de-
composition and to direct the process wholly into the nitrate
channel.

It is evident that there must be some recuperative agency
or the stock of soil nitrogen, which is never very great, would
long ago have disappeared in old countries. Experiment has
shown that soil gaims nitrogen when it is allowed to remain
undisturbed and covered with unharvested vegetation as in
natural conditions. On the Broadbalk field a third plot
adjacent to the two already mentioned was in 1882 allowed
to go out of cultivation and has not been touched since; it

CARBON AND NITROGEN CYCLES IN THE SOIL 183

soon covered itself with vegetation, the leaves and stems of
which go to enrich the soil in organic matter. The gain in
nitrogen is very marked, as shown in’ Table LII. The gain
is much influenced by the amount of calcium carbonate in the
soil, and is considerably less on another plot in Geescroft field

TasBLe LII.—Gains IN NITROGEN IN SOILS PERMANENTLY COVERED WITH
VEGETATION—ROTHAMSTED Soi_s Lert To Run WILD FOR 22-24 YEARS,
HALL (r20f).

Broadbalk : CaCQOs, 3°32 per Cent. | Geescroft: CaCOsz, 0°16 per Cent.

Carbon, Nitrogen, Carbon, Nitrogen,
per Cent. per Cent. per Cent. per Cent,

1881, 1904. 1881. 1904. J 1883. 1904. 1883. 1904.

Ist g inches Pt Stas) 2°23, f.08 | 245 } rr |. t*49 | *x08' | *13z

2nd g inches . *62 *70 | ‘070 | ‘095 ‘60 *63 | '074 | '083

3rd g inches 4 *46 *55 | °058 | 084] 45 *44 | *060 | 065
- Approximate gain in mneetes Ib.

peracre. . 2200 : : : . 1400

Lb. per acre per annum . ‘ sehgr7 i : 60

Lanp Larp Down To Grass IN 1856 AND Mown ANNUALLY (DR, GILBERT’S
MEADOW, ROTHAMSTED).

1856. 1879. 1888, 1912.
Per cent. of N in top g inches . « | ['152]2 | +205 "235 "338

oe

where only little calcium carbonate is present ; whether this is
due to any specific action, or to the changed physical con-
ditions brought out by decalcifying a soil, is not clear. Gains
of nitrogen also take place on land covered with perennial
grasses and clovers even when the crop is mown or grazed.
On clay pastures dressings of basic slag have been found to
increase thé nitrogen content of the soil, whilst potassium
salts, such as kainit, have had the same effect on sandy soil.

In all these cases leguminous plants are present in greatest
extent where the gains in nitrogen are greatest, but they are
not necessarily the only nitrogen fixers.

1 Estimated.

184 SOIL CONDITIONS AND PLANT GROWTH

Advantage is taken of this recuperative effect in all
rotations by alternating the periods of arable cultivation with
periods of “rest” in grass and leguminous crops. In the old
Norfolk rotations one year in four was given up to clover,!
in modern rotations the clover or “seeds” mixture is some-
times left for two or three years before it is ploughed up, so —
that the enrichment may become more marked. Mr. Mason
at Eynsham Hall? considerably enriched in nitrogen some
poor Oxford clay by the growth of lucerne. But the gain in
nitrogen does not go on indefinitely ; in course of time a point
of equilibrium is reached, higher or lower according to the soil
conditions, where further gains are balanced by losses, so that
the nitrogen content remains constant.

Thus there is an upper as well as a lower limit to the
_ nitrogen content of the soil, the actual values depending on
the soil conditions. Between these limits the nitrogen con-
, tent may be maintained at any desired level, high when the
ground is left in grass and leguminous crops, low when the
ground is continuously cultivated, Unfortunately, on our
_ present knowledge it is impossible to maintain a high content
_ of nitrogen on cultivated land except at a wasteful expenditure
of nitrogenous manure.

Tentative determinations of some of these limits are :—

Black Organic —
(containing more than :
to per Cent. of Organic Chalk Soils.3
Matter).

Loams.3 Sands.3

Upper limit : ; I "42 *25 *20
Lower limit ‘ ‘ "25 23 09 03

The reactions involved in all,these changes are obviously
complex, but they have been partially disentangled, and we
can now pass on to a more detailed consideration of the
separate changes.

1It was known to the Romans that vetches were a good preparation for
wheat (cf. Virgil, Georgics, Book I., lines 73 et seq.).

2 Fourn. Roy. Agric. Soc., 1904, Ixv., 106-124.

3 Containing less than ro per cent. of organic matter

CARBON AND NITROGEN CYCLES IN THE SOIL 185

The Formation of Ammonia.

Ammonia is in all probability an intermediate product in
the formation of nitrates. It is formed in the soil from the
proteins of plant residues or manures, and the process is
effected mainly by micro-organisms, but not entirely, for it
still continues at a diminished rate in presence of antiseptics.
The reaction has not yet been completely elucidated.

There is evidence of the production of amino acids which
subsequently hydrolyse, or oxidise.' Although amino acids
are in general fairly stable, several reactions are known
whereby they may be decomposed with production of
ammonia :—

R-‘CH‘NH,°COOH + H, = R‘CH,COOH + NH,.
R‘CH'NH,-COOH + O, = RCOOH + CO, +NH,.
R-CH-‘NH,‘COOH + H,O = RCH‘OH ‘COOH + NH,.
R-CH-‘NH,‘COOH + H,O = RCH,‘OH + CO, + NH;

It is not, however, known how they break down in the soil.

Miyake (2020) has shown that the rate of production of
ammonia can be expressed by the equation for autocatalytic
reactions, which means that it is much affected by the ac-
cumulation of the products. Both Gainey * and Neller® have
studied the connection between the production of ammonia
in soil and the evolution of CO,.

The investigations by Marchal (190) in 1893 of the method
of ammonia production in the soil are so complete that little
has since been added to the facts he ascertained. Miintz and
Coudon (207) had established the micro-organic nature of
the process by showing that it was stopped by sterilisation.
Marchal, therefore, made systematic bacteriological and myco-
logical analyses of soils, and studied the action of the organ-
isms thus obtained on solutions of albumin. Of the dozen

*See R. H. Robinson and H. V. Tartar, ¥. Biol. Chem., 1917, 30, 135-144.

2 Dakin, four. Biolog. Chem., 1908, iv., 63 ; Oxidation and Reduction in the
Animal Body (Longmans, 1912).

’Ehrlich, Zeitsch. Verein. Ribenzucker Ind., 1905, 539-567.

4 Gainey, Soil Sci., 1919, 7, 293.
. R. Neller, Soil Sci., 1918, 5, 225.

186 SOIL CONDITIONS AND PLANT GROWTH

or so varieties that invariably occurred, practically all de-
composed the albumin and formed ammonia. One of the
mycotdes group proved very vigorous and was studied in some
detail. The process was considered to be a simple oxidation
necessary to the life of the organism; oxygen was absorbed

and carbon dioxide evolved, the ratio NH,;:CO, produced —

being 1:89. For complete oxidation of the carbon, hydro-
gen, and sulphur of the albumin molecule the ratio would be
I:10°3; but the change was known to be incomplete, and
small quantities of leucine, tyrosine, and fatty acids could
also be detected. Free oxygen, however, was not essential.
When grown ina culture solution containing sugar and nitrate
the organism took its oxygen from the nitrate, but it still
produced ammonia.

The energy relationships thus indicated were wholly over-
looked by investigators for nearly a quarter of a century: it
was not till 1916 that Doryland (86) showed their significance.
The organism produces ammonia, not because it must, but
because it can do so, and can leave ammonia in excess of its
needs: its prime requirement is energy. If sources of energy
other than proteins are supplied, e.g. carbohydrates, these may
be used and then there is a decrease in ammonia production
which may finally fall to nothing or become negative: the
ammonia producers then become ammonia absorbers just
like the higher plants, and indeed they compete with growing
crops. It follows that no simple numerical expression can
be given for the ammonification of organic matter: the true
constant is the requirement of energy. This might be ex-
pressed in terms of protein if no other carbon compounds
were present, but it is improbable that any equivalent could
be worked out for the complex mixture in ordinary soils.’

1 For other experiments on the effect of carbohydrate in reducing ammonia
production see H. M. Jones, ¥. Infect. Dis., 1919, 19, 33, showing that Bac-
proteus does not produce a proteolytic enzyme in presence of available carbo-
hydrate; J, G. Lipman and Blair, New $ersey Bull., 247, 1912, and Annual Reft.,
1914, p. 220, ammonia producing power of soil is diminished by carbohydrates ;
S. A. Waksman, ¥. Amer. Chem. Soc., 1917, 39, 1503, similar effects are produced

in cultures of aspergillus ; I. J. Kliger, four. Bact., 1, 663, and Berman and Rettger,
Four. Bact., 1918, 3, 389.

CARBON AND NITROGEN CYCLES IN THE SOIL 187

Subsequent developments have been entirely on the
bacteriological side. A number of organisms are now known
to produce ammonia from complex nitrogen compounds, but
soil bacteriologists, notably J. G. Lipman and his school at
New Jersey, have generally preferred to study the group as
a whole, rather than isolate and study individual members.
The method consists in inoculating soil into various arbitrary
culture media, each designed to favour one group only of
organisms. Some of the results obtained are discussed in
Chapter VII. ; they show the method has value as a bacterio-
logical test, but it has thrown little or no light on the processes
going on in the soil. Indeed, so dependent is bacterial activity
on temperature, concentration, reaction of medium (whether
acid or alkaline), and other conditions, that it may be doubted
whether any method of study, except in the actual soil itself,
will further our knowledge of the reaction very much.

Of the few attempts to study the individual species of
organisms concerned in ammonification H. J. Conn’s is per-
haps the most notable. Contrary to the accepted view he
claims that ammonia formation is mainly brought about by
_ non-spore formers: 3. mycoides,’ generally regarded as one of
the most common ammonia producers in the soil, he dismisses
as ineffective. He maintains that of the eight ammonifiers
studied by Marchal only one, B. fluorescens lig. (a non-spore
former), is a typical soil organism. He describes in detail
two organisms, Ps. fluorescens and Ps. caudatus, which, while
not very numerous in unmanured soil, multiply vigorously
on addition of farmyard manure and also produce ammonia
(70c). Waksman has adduced evidence (see p. 259) that fungi,
and especially actinomycetes, are active ammonia producers
in soil,

Nitrification.

The ammonia formed by the action of soil bacteria, or
added in manures, is changed to carbonate, which is then
rapidly converted by WVitrosomonas into nitrite, and this by

1 Really a group, not a single organism.

188 SOIL CONDITIONS AND PLANT GROWTH

,Nitrobacter into nitrate, the changes proceeding so rapidly
that only traces of ammonia or nitrite are ever found in normal
arable soils (241/). We may, therefore, infer that the production
of nitrates is the quickest of the three reactions, the production
of nitrites is slower, while the formation of ammonia is the
slowest of all and sets a limit to the speed at which they
can take place. Thus a measure of the speed at which nitrates
are formed in soil does not measure the rate of nitrification,
as is sometimes assumed, but the rate of ammonia production.?

The essential facts of nitrification are readily demonstrated
by putting a small quantity of soil—-2 to °5 grm.—into 5oc.c.
of a dilute solution of ammonium sulphate containing nutrient.
inorganic salts and some calcium or magnesium carbonate,
but no other carbon compound.? After three or four weeks
at 25° the ammonia has all gone and its place is taken by
nitrates. The conversion is almost quantitative, only an in-
significant quantity of nitrogen being retained by the organisms.

The course of the oxidation is unknown, and nothing
intermediate between ammonia and nitrous acid has been.
detected. On general grounds one would look for hydroxy-
lamine. A. Bonazzi* has made the significant observation
that nitrification in culture solutions is intensified by thorough
zeration or continuous motion of the culture medium.

Miyake has shown (2020) that the reaction resembles
autocatalysis in its course. Omelianski could obtain no
evidence of an oxidase in WVztrosomonas (219c). The action
of both organisms seems to be entirely specific. Nitrosomonas
oxidises ammonium carbonate and nothing else; it will not
touch nitrates, urea, or the substituted ammonias. Even

1See also P. L. Gainey, Soil Sci., 1917, 3, 399-416.

2 Omelianski (2196) used 2 grms, each (NH,),SO, and NaCl, 1 grm,
KH,PO,, +5 MgSO,, -4 FeSO, in 1 litre of water, and added *5:grm. MgCO, for
each 50 c.c. of solution used. Nitrite formation goes on in this solution. For
nitrate production he used 1 grm. each NaNO, and Na,CO,, *5 each KH,PO, and
NaCl, -4 FeSO, and *3 MgSO, in 1 litre of water. Ashby (7a and 6) found that
both processes went on simultaneously when he diluted the first of these solutions
to one quarter the strength.

3A. Bonazzi, ¥. Bact., 1919, 4, 58.

CARBON AND NITROGEN CYCLES IN THE SOIL 189

ammonium salts are nitrified only in presence of a carbonate
that can change them into ammonium carbonate (296), Nitro-
bacter is equally specific, oxidising nitrites only and not
ammonia.

Addition to the solution of almost any carbon compound
other than calcium or magnesium carbonates retards the rate
of nitrification, glucose and peptone being particularly harm-
ful (3114). Carbon dioxide suffices as the source of carbon
for the growth of the organism. Godlewski’ showed that
nitrification proceeds in solutions free from organic matter so
long as the air supplied contained carbon dioxide, but: stops
as soon as the carbon dioxide is removed by passage over
caustic potash. But the synthesis of complex cell substances
from carbon dioxide is an endothermic process requiring a
supply of energy. In the case of the green plant—the only
other living thing known to utilise carbon. dioxide—the energy
comes from light, the transformer being chlorophyll. Here,
however, light is out of the question, and is even fatal to the
organism. Winogradsky (311@) suggested that the necessary
energy is afforded by the oxidation of ammonia and of the
nitrite, and he traced a definite relationship between the

amount of ammonia oxidised and the carbon assimilated. -

Experiment 1. Experiment 2.| Experiment 3. | Experiment 4.
Ammonia oxidised (ex-
pressed as nitrogen) .| 722°0mg. | 5061 mg. | 928°3 mg. | 8154 mg.
Carbon assimilated 19°7 35 de ee 20%4. ,, 224.45.
.N
Ratio & 36°6 ,, 33°3 » 35°2 55 36°4 ,,

In these experiments mixed cultures were used, the nitrate
producers predominating. More recently Coleman (68), using
pure cultures of nitrate producers, obtained ratios varying
from 40 to 44 for the second stage of the process.

No useful hypothesis has yet been put forward to account

1 Quoted in Lafar, Tech. Mykologie, 1906, Bd. 3, 165.

eR rere >
————

190 SOIL CONDITIONS AND PLANT GROWTH

for these remarkable facts. The whole subject deserves serious
attention from some competent chemist.

It was somewhat hastily inferred that organic matter
would have a retarding effect in the soil just as it has in culture
solutions. From the outset, however, certain facts were
known to be against this view: thus, there was a good déal
of organic matter in the old nitre beds (235) and also in
rich gardens, and yet nitrification went on vigorously in both
cases. An exception was therefore made in favour of “ humus ”
(208). Later on Adeney (2), and again Miss Chick (66), found
another exception : the organic matter of the filter beds used
in sewage purification. Richards finds that nitrification pro-
ceeds very vigorously during the activation of sludge by
eration. Coleman has now shown (68), and Stevens and
Withers (2710) have confirmed it, that only in culture solutions
is organic matter injurious: in the soil it does no harm, and
may even help the process. Thus quantities of dextrose that
stopped nitrification entirely in Winogradsky and Omelianski’s
culture solutions were found to act beneficially in soil under
normal conditions of temperature and moisture content. The
discrepancy cannot yet be explained. Sucrose, lactose, and
certain other non-nitrogenous compounds had no effect, but
nitrogenous compounds were distinctly injurious.

The organisms will not tolerate an acid medium ; a suf-
ficient excess of calcium carbonate is therefore necessary both
in culture solutions and in soils. Nor will they tolerate free am-
monia. In culture solutions the nitrate producer is somewhat
sensitive even to ammonium salts, indeed. both Warington
(296) and Omelianski (2194) suppressed it by maintaining a
sufficient concentration of ammonium sulphate; Léhnis has
shown, however (181a@), that it is more tolerant in the soil.
Some substance toxic to them is produced when soil is heated
to 98° C. or more, and in such soils they cease to act. Neither
nitrosomonas nor nitrobacter has been observed to form spores,
or to survive temperatures above 45° C., or treatment with
mild antiseptics like carbon disulphide and toluene. But
so widely distributed are they and so readily can they spread

CARBON AND NITROGEN CYCLES IN THE SOIL 191

in the soil, if the conditions are at all favourable, that they
may reappear unless special precautions are taken to prevent
infection. Thus, it is commonly stated that treatment of the
soil with carbon disulphide merely depresses without killing
the organisms. Russell and Hutchinson found, however, that
the organisms did not reappear if the soil was kept carefully
free from re-infection (241¢).

In pure cultures the organisms cannot tolerate absence
of moisture, but die at once. In soil, however, they are more
resistant. Absence of air puts an end to their activity.

There is some evidence that nitrobacter is more sensitive
to adverse circumstances than nitrosomonas; it is also more
rapid in action. Otherwise the two sets of organisms show
very similar behaviour to external influences, their main

difference being the fundamental one that nitrosomonas |

oxidises ammonia, but not nitrites, while nitrobacter oxidises
nitrites, but not ammonia. There are also certain morpho-
logical differences. Nitrosomonas, or coccus, occurs in several
forms, mostly oval in shape, 0°5 to 1 w« wide and up to 2 yw long,
but whether these are really distinct varieties is not known ;
a zooglea stage is also found; nitrobacter is rod-shaped, I uw
_ long and about o°3 yw thick; only one variety has been re-
cognised. No other organisms are known with certainty to
produce nitrates in the soil, nor can any other compound
except ammonia be nitrified (2192).

During the course of nitrification some calcium is dissolved,
but apparently no phosphate, except in culture solution.!

The Evolution of Gaseous Nitrogen.

It has long been known that losses of nitrogen may occur
during the bacterial decomposition of organic matter which
cannot be attributed to the volatilisation of ammonia, and
which, therefore, are put down to an evolution of gaseous
nitrogen: instances are afforded by manure heaps, by rich

1 Kelley, fourn. Agric. Res., 1918, 12, 682. But see Hopkins and Whiting

(Ill. Bul., 190, 1916), who maintain that phosphate is rendered soluble during
nitrification.

con

192 SOIL CONDITIONS AND PLANT GROWTH

soils, and by sewage beds. The observations go back to the
time when the sources of nitrogen for vegetation were being
investigated, and attempts were made to set up a balance
sheet showing the relation between the amounts of nitrogen
in the plant and the soil at the beginning and at the end
of its growth respectively. Reiset,! Ville (288), and Boussin-
gault (45) sometimes found less nitrogen in soil + plant at —
the end of an experiment than in soil + seed at the beginning,
and attributed the difference to an evolution of free nitrogen.
Some of these early observations were probably faulty by
reason of the crudeness of the analytical methods, but Lawes,
Gilbert, and Pugh (164) showed that loses of nitrogen un-
doubtedly took place sometimes, though not always, when
nitrogenous organic matter, wheat-meal, barley-meal, or bone-
meal, was made into an “ agglutinated condition” with water,
and allowed to decompose in presence of air. Practically no
ammonia could be detected. Lawes and Gilbert suggested
three possible reactions, a suggestion that still holds the field.

1. An oxidation analogous to that of the action of chlorine
on ammonia, by which free nitrogen is evolved.

2. A reduction similar to that of a great number of sub-
stances upon the oxygen compounds of nitrogen, by which
the oxygen is appropriated and the nitrogen set free.

3. These two actions may operate in succession the one
to the other.

Little attention was paid to these results at the time, but
later on losses of nitrogen were found to occur in the purification
of water and of sewage. . Angus Smith? in 1863 observed
an evolution of gaseous nitrogen from a dilute solution of
putrefying blood, and showed that nitrates gave off nitrogen
under certain circumstances. The earlier sewage workers,
Frankland,’? and others, did not actually mention any loss of

1 Reiset, ¥ahresbericht der Chemie, 1856.
2 Angus Smith, Memoirs Manchester Lit. and Phil. Soc., 1863, 1867-8, Vol.
IV.; also Report to the Local Govt. Board, 1882. ,
3 Frankland, Denison, and Chalmers Morton, Royal Commission, Pollution.
of Rivers, 1868, Vols. 1-4.

CARBON AND NITROGEN CYCLES IN THE SOIL 193

nitrogen during sewage purification though the published re-
sults show that it occurred. Later sewage workers recognised
the loss, and Letts, indeed, made measurements of the evolved
nitrogen, special gasimetric methods being devised for the
purpose.!

A serious attempt to grapple with the problem was made
in 1896 and 1897 at some of the German Experiment Stations,
notably Jena, in consequence of the request made by the
German Agricultural Society for an investigation into the
losses of nitrogen from farmyard manure. The first hypoth-
esis, set up by Wagner, was a reduction hypothesis to the effect
that nitrates are present in the manure, and decompose in ab-
sence of air, giving rise tonitrogen. This view, however, soon
proved to be erroneous, for it supposed that the loss only
occurred in absence of air, whereas, in point of fact, it only
occurs in presence of air.

Immendorf (141@) recognised this fact and favoured the
oxidation hypothesis, considering that evolution of nitrogen
was the result of direct oxidation or combustion of the nitro-
gen compounds to gaseous nitrogen. No reduction of nitrate
- was assumed. Pfeiffer and his assistants at Jena (224@) began
on the reduction hypothesis, and supposed that nitrates were
formed on the outside of the heap and then diffused inside,
where they were denitrified. But during the course of their
experiments they changed their view, and ended by accepting
the oxidation hypothesis.

Two lots of cow manure (made with peat) were put up:
air was blown through one, and over the other. The losses
of nitrogen were :—

Grams. Per Cent.
Air blown through . ; ; « 4°26 42°6
MIA ays OVEF : ‘ : oh 296 27°6

They argue that blowing air over the dung would be
favourable to their supposed nitrification and denitrification
process, while blowing air through would be unfavourable

1 Letts, Fifth Report, Sewage Commission, Appendix 6, 171-194 ; also Report
to the Corporation of Belfast on the Purification of Belfast Sewage.
13

194 SOIL CONDITIONS AND PLANT GROWTH

to it since denitrification requires an absence of air. Yet
blowing air through the dung causes greater loss. Again,
nitrogen may be lost where no nitrification is observed, and
further, the effect of antiseptics on the extént of the loss is
not what might be expected from a_ knowledge of their effect
on denitrifying organisms. In consequence of these results -
they gave up the alternate oxidation and reduction hypothesis —
and accepted the direct oxidation hypothesis. Some support
was lent to this direct oxidation view by the announcement
that Wood and Wilcox! had isolated from bran infusions
an organism able to liberate nitrogen direct from nitrogenous
compounds: apparently, however, this has not been confirmed.

On the other hand, Miintz and Lainé (208c), working on
sewage purification, and dealing with percolation filters, reject
the oxidation hypothesis. In the first instance they note that
loss occurs only in presence of organic matter: it is not seen
when a solution of ammonium salts is run on to the filter.
Secondly, they find that the addition of nitrate to the liquor
increases the loss of nitrogen. Laboratory experiments
showed that it caused an evolution of nitrogen, and by working
in a vacuum a complete balance was made as follows :—

At Start. At End.

Nitrogen as ammonia. : i : + Ors 15°2 mgs.
Br in organic compounds j ‘ Pe 6 ae I3°1
te as nitrite . . f A : pays 0°5
3 as nitrate . 3 4 ‘ c - 30°6 8:0
Total: \s.. ; ; » 603 36°8
Lose.’ 5 ; _ = 23°5

But they found dissolved in the liquid :—

Free nitrogen 38°1 c.c. at N.T.P. = 23'9 mgs., which thus
almost exactly balances the loss.

This experiment shows that denitrification can.take place
and cause loss of nitrogen, though it does not necessarily ex-
clude the hypothesis of a direct oxidation of ammonia.

Adeney’s (2) results are also inconsistent with the direct
oxidation hypothesis. When albuminose, asparagin, etc., are

1¥ourn,. Soc. Chem. Ind., 1897, 16, 510.

CARBON AND NITROGEN CYCLES IN THE SOIL 195

decomposed by bacteria in dilute solutions saturated with
oxygen there is no loss of nitrogen. Russell and Richards
(241k) were also unable to find any loss of nitrogen in the
aerobic fermentation of urine or faces, even though in the
case of urine considerable nitrification occurred.

The present position of the problem, therefore, is that this
loss of nitrogen does not occur under anaerobic conditions,
nor under aerobic conditions ; it requires a combination of both,
such as obtains in manure heaps, in rich soils and in the system
of beds and filters in a sewage works.

These facts are inconsistent with either a direct oxidation
or a direct reduction process; both Wagner’s and Immendorf’s
hypotheses, therefore, are ruled out. They are consistent with
Miintz’s alternate nitrification and denitrification hypothesis,
but they do not prove it. Russell and Richards have given a
more general explanation, which seems to be free from some
of the mechanical objections that can be urged against alter-
nate nitrification and denitrification. They suppose that
molecular groupings arise under anaerobic conditions which
become unstable as soon as aerobic conditions set in, and
_ decompose, splitting off nitrogen. The change is parallel to
the shortening of the propionic acid chain with formation of
an acetic acid derivative, which is known to take place in these
conditions.

Although the investigations have been mainly on manure
heaps and sewage beds there is evidence that the same process
occurs in heavily manured soils and in virgin soils rich in
organic matter brought into arable cultivation, and that it
causes the losses already discussed on p.. 181.

The Fixation of Nitrogen.

The first systematic search for a recuperative agency to
make good the losses of nitrogen from the soil was started
thirty years ago by Berthelot. He found that certain organic

1See Barger, The Simple Natural Bases (Longmans’ Biochemical Mono-
graphs), where various instances are given.
137

196 SOIL CONDITIONS AND PLANT GROWTH

compounds could absorb free nitrogen under the influence of
silent electric discharges, and at first attributed the natural
recuperation to this cause. He also examined the possibility —
of bacterial action, as micro-organisms at that time were play-
ing a large part in French science under Pasteur’s influence.
Accordingly he exposed sterilised and unsterilised sands and.
clays poor in nitrogen (‘oI per cent. or less) to air in large
closed flasks for five months, and found distinct gains in
nitrogen in the unsterilised, but not in the sterilised soils.
Fixation is, therefore, not due to any external physical cause,
which would operate equally in both cases, but to micro-
organisms (26). This research was at once fruitful of results
because it gave Hellreigel and Wilfarth the key to the clover
problem (p. 25), and led Winogradsky (312) to search for the
actual organism.

No investigator of our subject has shown greatering enuity
than Winogradsky in devising methods at once simple, direct,
and effective. In looking for the nitrogen-fixing organisms
he inoculated soil into a medium containing every nutrient
except nitrogen compounds: only bacteria capable of assimi-
lating gaseous nitrogen could therefore develop, and these had
a clear field. But he further recognised that the process was
endothermic and required some source of energy, hence he
added sugar to the solution. The method (known as the

| elective method) thus consists in making the conditions as

favourable as possible for the group of organisms under in-
vestigation, and as unfavourable as possible for all others; it
has proved extremely valuable in the subsequent development
of soil bacteriology.

Winogradsky’s solution contained 2 to 4 per cent. dextrose,
a little freshly washed chalk, o°1 per cent. K,HPO,, 0:05 of
MgSO, and traces of NaCl, FeSO,, and MnSO,, together with
a little soil. Under aerobic conditions nitrogen was assimi-
lated and the sugar was decomposed with evolution of carbon
dioxide and hydrogen and formation of n-butyric and acetic
acids in the proportion of three or four molecules of the
former to one molecule of the latter, the two acids together

CARBON AND NITROGEN CYCLES IN THE SOIL 197

accounting for nearly half the sugar. A little alcohol was
found, but practically no non-Wolatile acid. There was a dis-
tinct relationship between the amount of nitrogen assimilation
and the sugar decomposed, each milligram of nitrogen fixed
requiring the oxidation of about 500 milligrams of sugar.

‘Three organisms were present, a clostridium and two
bacteria, and they obstinately refused to be separated by the
method of successive cultures. Not until recourse was had to
anaerobic conditions were the two bacteria suppressed and the
clostridium obtained pure. The bacteria having been isolated,
it appeared that the clostridium alone possessed the power of
fixing nitrogen, but a fresh difficulty now arose because in
pure cultures the organism would work only under anaerobic
conditions. Only when the protective bacteria were simul-
taneously present did fixation go on in presence of air. The
organism was called Clostridium pasteurianum:' it formed
rods 1°2 w thick and 1°5 to 2 uw long and also spores (312).

In order to simplify the bacterial flora Winogradsky had
heated his soil to 75°, thereby killing non-spore formers, but
later on Beijerinck (14 and 15), working with unheated soil
_ discovered three other nitrogen-fixing organisms ; Azotobacter
chroococcum (so called because, as it ages, it turns brown and
finally almost black), Granulobacter and Radiobacter.? Of
these azotobacter is the most active ; it forms large cocci, or
rods, 4 to 6 win thickness.’ It differs in two important respects
from clostridium : (1) it is aerobic; (2) it produces practically
no butyric acid. Its effects can be studied by inoculating o°!
to 0-2 grm. of soil into 100 c.c. of tap water containing 2 per
cent. mannitol, ‘o2 per cent. K,HPO,, and sufficient CaCO,,
and keeping for some weeks at 27° to 30° C. in a thin, well-
aerated layer* in an Erlenmeyer flask. Azotobacter fixed

1 For further investigation of clostridium see Omelianski, Rome Bull., 1917,

, ITgo.
: * Since shown by Stoklasa (2756) to possess only slight nitrogen-fixing
power.

8 Léhnis and Smith describe cyclical changes through which azotobacter is
considered to pass (¥ourn. Ag. Res., 1916, 6, 675).

4 Later on Beijerinck used calcium malate in place of sugar, and showed also
how to make plate cultures of the organisms (16).

198 SOIL CONDITIONS AND PLANT GROWTH

more nitrogen than clostridium per grm. of sugar decom-
posed, ‘ °

The nature of the compound is also important. Table
LIII. gives the amounts of nitrogen fixed per grm. of com-
pound decomposed,

TasBLeE LIII.—Mems. or NITROGEN FIXED BY AZOTOBACTER PER GRM. OF
SuBsTaNce DecomposED.! LOHNIS AND PILLAI (181d).

Mg. of Nitrogen
Fixed.

7°5 to ro Mannitol, xylose, lactose, levulose, inulin, galactose, maltose,
dextrin, sucrose + calcium carbonate.
5 to 7°5 Sucrose alone, dextrose, sodium tartrate + calcium carbneaiee
glycerol + calcium carbonate.
2°5 to 5 Starch, sodium tartrate, sodium succinate, calcium lactate.
I to 2°5 Sodium propionate, sodium citrate, glycerol alone, .
Nil Calcium butyrate, potassium oxalate.

GERLACH AND VOGEL (1046).

Glucose decom-
posed, mgs. . |1,000|2,000}3,000|4,000|5,000|6,000|7,000| 10,000/12 ,000/15,0007

Nitrogen fixed,
mgs, . «| 7°4 | 13°5| 17°8| 34°4 | 30°4 | 45°9.|59°9| 9T°4 | 127°9| 62°9

Nitrogen _ fixed,

per grm. of

sugar. -1 74.1 68 | 5°38 | 7°83] zol 76) 85 | of) 107) —

Mean 8°g mgs. nitrogen fixed for 1 grm. sugar.

Little is known of the chemistry of the process, even the
fate of the sugar has not been definitely ascertained. The
only obvious product is carbon dioxide, fatty acids being
formed only in small quantities, in sharp contrast with
clostridium.

Starting with 15:9 grms. of dextrose Stoklasa (2750) re-
‘covered 7:9 as carbon dioxide, 0°3 as ethyl alcohol, 0:2 as
formic acid, 0°7 as acetic acid, 0'2 as lactic acid, but could not
trace the remaining 6°5 grms.

1 For list of other substances see F. A. Mockeridge, Biochem. Fourn., 1915

9, 272-283.
2 The sugar was not all used up in this experiment.

CARBON AND NITROGEN CYCLES IN THE SOIL 199

The energy relationships are equally little known: it is
concluded, however, that only about 1 per cent. of the total
available energy is utilised in the fixation of nitrogen.!

Beijerinck’s solution works satisfactorily for crude cultures
but not for pure cultures. Various hypotheses have been put
forward in explanation; it was supposed that azotobacter re-
quired the presence of some other organism, or that it lost its
efficiency on cultivation. Krzemieniewski (157) found that
neither of these views is correct, and in an important investi-
gation showed that the determining factor is the presence of
a little soil; so long as this is added pure cultures retain their
effectiveness. The active agent is the humus, but its effect is
not to furnish carbon or nitrogen to the organism; further,
the humus loses its power after treatment with hydrochloric
acid. Remy and Rosing (2370) frankly call it a stimulating
action and attribute it to the iron invariably present.” Allen,®
on the other hand, supposes that the colloids of the humus
prevent complete precipitation of the phosphate and thus
facilitate phosphorus nutrition of the organism.

The nitrogen is found partly in compounds dissolved in
_ the liquid, but mostly in the bacterial mass. The organism is
remarkably active, one grm. weight evolving no less than 1°3
grms. of CO, in twenty-four hours (273a@). An adequate
supply of phosphate, calcium carbonate and other mineral
nutrients is essential, any deficiency limiting the amount of
fixation. Traces of nitrogen compounds are helpful in the
early stages, but larger quantities reduce the amount of fixa-
tion, and may themselves suffer some change: thus sodium
nitrate is partially reduced to nitrite even under aerobic
conditions (Stoklasa (2754)). Hills (134) finds that the bene-
ficial effect of small quantities of nitrate of soda and nitrate of
potash is not confined to the growth of the organism, but ex-
tends (though to a less extent) to fixation also, and it is

2G. A. Linhart, ¥ourn, Gen. Physiol., 1920, 2, 247-251.
But see also Reed and Williams, Centr. Bakt. Par., 1915, 43, 166.
E. R. Allen, Ann. Mis. Bot. Garden, 1919, 6, 1.

200 SOIL CONDITIONS AND PLANT GROWTH

shown both in culture solutions and in soil.’ Several forms
of azotobacter are now known: A. agilis, A. vinelandit, etc.,
and also various less efficient nitrogen fixers that more re-
semble clostridium, such as amylobacter (52) and granulo-
bacter, some of which are aerobic and others anaerobic.? The
great distinction between the two groups is that the azotobacter
give carbon dioxide as the chief product from the sugar, while
the others, even the aerobic organisms, form butyric acid in
considerable amount and fix smaller quantities of nitrogen.
Amylobacter also makes and stores glycogen, a property pos-
sessed by few other micro-organisms.

Kossowitch (154@) has shown that a mixture of azotabaaine
and algze, especially nostoc, can work together very well, the
alge furnishing the necessary carbon compounds, while the
azotobacter fixes nitrogen, an observation that has been con-
firmed by Bouillac (43 and 44), Sand has been found to gain
nitrogen where the growth of alge was possible and the
proper bacteria were present.2 Evidence is now being ad-
duced that some of the algz can fix nitrogen by themselves,
and without the co-operation of azotobacter* (p. 255).

How far azotobacter is active in the soil in natural condi-
tions has not been definitely ascertained, partly because of the
analytical difficulties of measuring small gains of nitrogen,
and partly because of the losses of nitrogen that, as we have
seen, go on in presence of organic matter. The mere occur-
rence of azotobacter in the soil is no proof that it is actually
fixing nitrogen, the only satisfactory evidence would be a
demonstrated gain in nitrogen effected by azotobacter, all.
other possibilities being ruled out by the experimental condi-
tions. The usual method of investigation has been to add
sugar, or other carbohydrates, ‘to the soil and measure the
change in nitrogen content after various intervals of time.

1D. A. Coleman obtained less fixation in presence of nitrate (Soil Sci.,
1917, 4) 345):

2A list is given by C. B. Lipman in ¥ourn. Biol. Chem., 1g11, x., 169-182.

3 A. Koch has collected instances in Lafar, Tech. Mykologie, Bd. iii., p. 15.

4B. Moore and T. A. Webster, Proc. Roy. Soc., 1920, B gi, 201.

CARBON AND NITROGEN CYCLES IN THE SOIL 201

Generally, there is a gain of nitrogen; losses are, however,
often recorded (248, 151, etc.), whilst a certain loss of nitrate
invariably occurs (p. 266). A. Koch (1514) added successive
small doses of dextrose to 500 grms. of loam, mixed with
sand and spread on plates to secure copious aeration, kept
uniformly moist and at 20°C. Nitrogen fixation began very
soon and reached its maximum after eighteen weeks, when
losses set in; the results are given in Table LIV.

Tasite LIV.—NirTRoGEN FIxeD IN Soit By BACTERIAL ACTION IN PRESENCE
oF Dextrose. Kocn (151d).

Total Dextrose Supplied in grms. per Mgs. N fixed per 100 grms. of
Increments of 100 grms. of Soil after : Soil after
Dextrose per

100 “— of
oil. —_|s Weeks.|8 Weeks.|18 Weeks.|26 Weeks.f5 Weeks.|8 Weeks. 18 Weeks.|26 Weeks.

June 26. | July 20.| Oct. 3. | Nov. 30. [June 26. | July 20.| Oct.3. | Nov. 30.

as page) r°6 3°6 5'2 8°3 I4°9 17°8 18°9
“5 2°5 4°0 g°0 I3'0 | 20°L 32°5 36°38 31°6
I’o 50 8-0 18°0 26:0 | 35°8 B72 58°7 52°7
a a 75 I2°0 27°O 37°5 | 40°5 66°7 | 68°5 66°38
2'0 8'0 I4°0 26-0 36°0 | 43°9 | 78°8 80°0 78°8

For each grm. of dextrose supplied in the small doses
‘about 8 milligrams of nitrogen were fixed during the first
eight weeks; but only 4 or 5 milligrams later on. In larger
dozes the sugar was less effective, only 5 to 6 milligrams of
nitrogen being fixed per grm. of sugar at first and 3 milligrams
later.

Pot experiments showed that the nitrogen thus added to
the soil became available for plant food. Dextrose and sucrose
first depressed the crop, then caused an increase, and finally
left the soil richer in nitrogen at the end of the experiment
than at the beginning (Table LV.).

But if the soil temperature fell too low nitrogen fixation
ceased: it was not observed at 7° C. although it appeared to
goon at 15°C. The optimum temperature lies between 25°
and 30° C,

Pfeiffer and Blank (224c), however, were unable to obtain
any beneficial results from sugar. The Rothamsted trials

202 SOIL CONDITIONS AND PLANT GROWTH

showed increases for autumn applications but decreases for
spring dressings (140¢).

TaBLeE LV.—Errect of DExTROSE AND SUCROSE ON THE PRODUCTIVENESS
AND NITROGEN CONTENT OF THE Sort, Kocn (1510).

Crops Obtained.
° Total N
aiiaaevaal Nitrogen Left in Soil,
i ring, 1906.
Sugar added per Oats, 1905. Sugar Beets, | in Crop. pring, 1906.
roo grms. of Soil. 1906,

i : N as Nitrates
Dry | Yield] Dr Yield Total N :
Matter. | of N. |Matter. | of N.| Gt™s- per Cent. pr ae
roo | 100 | 100 | 100 | o*5914 | .093 Io
2 per cent. dextrose .} 32°8 | 62°5| 186 | 190 | 0°6814 | ‘105 17
2 5, 4, Canesugar] 33°3 | 58°7| 179 | 195 | 0°680 *IO5 15
4» 9». » 9» | 37°7 | 78°r| 283 | 339] r’o0g2 | -119 37

Increased yields of sugar cane have followed the applica-
tion of molasses to soils at the Station Agronomique and on
Mr. Ebbels’ estate’ in Mauritius, where the residual effect is
well shown, and also in Antigua Peck in Hawaii, on the
other hand, observed marked losses of nitrate, as also did
Harrison in British Guiana.*

An increase in crop following the application of sugar or
starch to the soil is not evidence of nitrogen fixation, but
might equally well be adduced to show that sugar and its
decomposition products are direct plant nutrients. Only
when an actual gain in nitrogen is demonstrated by analysis
does the proof become satisfactory. As a practicable scheme
the addition of sugar to the soil would be out of the question
for field work. ~Beijerinck (15) has shown, however, that
certain compounds producible in the decomposition of cellulose
also serve as sources of energy to azotobacter, and Pringsheim
(232) found that the same holds true for clostridium also.

Hutchinson (140e) has shown that leaves, stubble, etc.,

See The Agricultural News, 1908, vii., 227; 1910, ix., 339; and rgITt, x.,
179.
2 See Manurial Experiments with Sugar Cane in the Leeward Islands, 1908-09
and 1gog-10. (Pamphlets 64 and 68, West Indian Department of Agriculture.)

3 West India Bull., 1913, 13, 136. This contains an interesting discussion
on the losses of nitrogen from soil.

CARBON AND NITROGEN CYCLES IN THE SOIL 203

serve to increase nitrogen fixation, though under other
conditions they can, like sugar, depress the nitrate content
of the soil. Doryland! has discussed the possibility of
using various waste substances as energy supply, and Emer-
son? has discussed the prospect of soil inoculation with
azotobacter. ;

Richards (238a) has made the interesting observation that
animal feces also serve for the organism both. in culture and
in more natural conditions, but there is a sharp connection
between the diet and the effect. Horses fed on oats gave
faeces which induced the greatest fixation: horses in grass
came next: cattle receiving cake were next, while the faeces
from cattle fed on grass proved unsuitable.

The difficulty of material might therefore be overcome
because large quantities of cellulose are available on the farm
in the form of straw. But there still remains the question of
temperature. Azotobacter, as we have seen, requires more
warmth than many other organisms, and according to Koch’s
experiments ceases to work at 7° C. Thiele read tempera-
tures daily for three years of arable and grass soils at different
depths at Breslau (281), and concluded that only rarely were
they favourable for azotobacter. But it is impossible to argue
from a culture solution to the soil, and, indeed, Lohnis has |
shown that the mixed cultures of the soil are almost as
effective at 10° as at 20°.® ;

It seems legitimate to conclude that azotobacter fixes}
nitrogen in well-aerated soils sufficiently provided with }
calcium carbonate, potassium salts and phosphates, carbon-
aceous material of the right kind and moisture, so long as
the temperature is high enough. Where the air supply is
diminished owing to the close texture of the soil there is still
the possibility of fixation by clostridium. Ashby (7c) found

1 Sci. Proc. Soc. Amer. Bacteriologists, 1918.
2 Towa Research Bul., 45, 1918.
3 Mitt. Landw. Inst., Leipsic, 1905, vii., 94.
to°-12° C, 20°-22° C, 30°-32° C.
3°15 mg. 4°55 mg. 4°27 mg. nitrogen fixed.

204 SOIL CONDITIONS AND PLANT GROWTH

that the relative distribution of azotobacter and clostridium at
Rothamsted depended on the amount of calcium carbonate
in the soil ; wherever any notable quantity was present, azoto-
bacter invariably occurred: otherwise clostridium alone was
found. This result appears to be general.' Remy suggested
in 1906 that azotobacter was a good organism for the bacterial
diagnosis of soils—absence showing some harmful factors and
a rich development showing favourable conditions. Christensen
(67a) gave definite form by designing a workable method (see
p. 242), while Gainey? has ascertained that azotobacter occurs
in soils with a P, value 60 or more, but not in those with Py
value 5°9 or less.

Nitrogen Fixation by Bacteria in Symbiosis with
Leguminose.

After Hellriegel and Wilfarth’s great discovery of the
relationship between bacteria and leguminosz (p. 24) many
unsuccessful attempts were made to isolate and study the
organisms by the methods then in vogue. In 1888 Beijerinck
(13) broke away from the ordinary meat-bouillon-gelatin plate
and substituted a slightly acid medium made up of infusion
of pea leaves, gelatin (7 per cent.), asparagine (‘25 per cent.)
and sucrose (*5 per cent.). Growth readily took place and
the colonies yielded rods 1 w wide and 4 to 5 w long, some
of which showed signs of bacteriod formation, and “ swarmers ”
o'9 w long and 0°18 pw wide, these being among the smallest
soil organisms known.?

The life cycle has been shown by Bewley and Hutchinson
(36) to include non-motile and motile stages: conditions
were ascertained under which one passed into the other.

1 Hugo Fischer (1905) found azotobacter on the limed plots at Bonn-
Poppelsdorf but not on the unlimed. Burri (1904) found it in only one-third of
the Swiss soils examined.

2 Fourn. Agric. Res., 1918, 14, 265. E. B. Fred and A. Davenport found the
limits in culture solutions to lie between 6°5 and 8°6 (¥ourn. Agric. Res., 1918,
14, 317).

3 Golding has shown that they will even pass through a porcelain filter and
has prepared pure cultures in this way.

CARBON AND NITROGEN CYCLES IN THE SOIL 205

C. B. Lipman and L. W. Fowler! have succeeded in isolat-
ing the organism from soil, and T. F. Manns and Goheen?
also claim to have done so.

The mode of entry into the pea was studied by Prazmow-
ski (227), and later by Nobbe and Hiltner (2154). The organ-
ism—presumably in the swarmer stage—attacks the root hair,
secreting a substance which causes the root tip to curl up;
the membrane of the hair becomes swollen and the bacteria
then penetrate.? A filament, known as the infection thread,
and shown to be formed of rapidly multiplying bacteria,
gradually extends up into the root where the nodule begins
to form; beyond this, the organisms do not penetrate. The
morphological changes have been described by Marshall Ward
(294), Miss Dawson (80) and others. Soon the organisms sur-
round themselves with slime and appear as bacterial rods, which
may then change to the characteristic branched or Y-shaped
bacteriods and assimilate free nitrogen. There appear to be
some 6 or seven different kinds of nodule organisms which
can be distinguished by the agglutination test* and which
show characteristic discrimination between the various legumin-
ous plants.

Hiltner (135) regards them as parasites attracted chemo-
tactically to the root hair by root excretions, but prevented
from getting too far into the plant by excess of the attract-
ing material, which now becomes a deterrent. He grades
them according to their virulence, the less virulent either
being unable to enter the plant, or, if they do enter, being
quickly resorbed, or only fixing little nitrogen; the more
virulent, on the other hand, bring about energetic fixation.
As evidence he adduces the well-known fact that infection
proceeds best in plants weakened by nitrogen starvation, and
scarcely takes place at all in plants growing vigorously on

1 Science, 1915, 41, 256 and 725, where earlier isolations by Nobbe, etc., are
discussed,
2 Delaware Ag. Expt. Sta. Bull., 115, 1916.
3L. Hiltner (1352).
’ 4M, Klimmer and R. Kriiger, Cent. Bakt. Par., 1914, 40, 256-265.

pA ne A A at OLE AACN

Peace centt ett
\

206 SOIL CONDITIONS AND PLANT GROWTH

rich soils. The parasitism is beneficial to both parties: the
plant gains nitrogen and the organism gains carbohydrates.

In its general outlines the process has been reproduced ©

artificially. Leguminous plants can be fed with nitrogen
compounds and made to grow perfectly without the organism.

On the other hand, the organism can be grown on artificial —
media containing carbohydrates,’ made to pass through all its |

stages from swarmers to bacteroids, and to fix nitrogen.2 The
change to bacteroids is conditioned by the presence of carbo-
hydrates or of small quantities of various acids, such as are
known to occur in the plant (276). The fixation of nitrogen
rapidly comes to a stop unless the resulting compound is
removed, as in the plant. Golding has attained this end by
an ingenious filtering device, and has thus succeeded in fixing
considerable quantities of nitrogen. He has also shown that
the reaction of the medium during actual fixation is alkaline,
but changes to acid when fixation is stopped by the accumula-
tion of nitrogen compounds. An actual loss then seems to
set in (109).

The chemistry of the process is unknown; even the
changes in the carbohydrates of the culture medium have not

been worked out. Nitrogen fixation is known to take place

in the nodule, which thus becomes richer in nitrogen than the
rest of the root,’ and its final product is supposed to be a
soluble protein which is passed on to the plant. Phosphates,
calcium compounds, and carbon compounds, such as sugars,

1 Harrison and Barlow (125) used maltose: other observers have used an
infusion of the host plant. Neumann’suggests pentosans (213).

2 See also (13).

3 Stoklasa’s analytical results with yellow lupines (Landw. ¥ahrb., 1895,
xxiv., 827) are:—

"

Seed Beginning :
Blossom Formed. toRowh: Seed Ripe.

2°6 iy

Nitrogen in nodule, per cent.
r°8 I"4

in rest of root, per cent.

HY
an

”

Whiting (Illinois Bull., 179, 1915) has discussed this question also.

‘

CARBON AND NITROGEN CYCLES IN THE SOIL 207

organic acids, etc., and according to Olaru,' manganese com-
pounds, have a marked effect in stimulating nodule formation
in soil cultures.’

The amount of nitrogen fixed in this way is so large
that it is easily measured on the field. When the host plant
dies, or is ploughed into the ground, the nitrogen compounds
speedily change into nitrates. A uniform piece of ground
at Rothamsted was divided into two parts; on one a crop of
clover was taken, on the other barley was grown. After the
crops were removed samples of soil were taken for analysis,
and then barley was grown in both plots. The analytical
results were :-—

Plot where Clover | Plot where no Clover
was Grown, was Grown.
Nitrogen in crop (1873), lb. per acre . 151°3 37°3
(in clover) (in barley)
Nitrogen left in soil after crop was
removed (1873), percent. . *1566 "1416
Nitrogen in crop (1874), lb. per acre . 69°4 39°1
(in barley) (in barley)

These facts are well known to the practical man, and are
utilised for increasing the nitrogen supply of cultivated soils
and for reclaiming barren sands and clays (pp. 314 and 184).
Leguminosz are among our commonest plants, both wild and
cultivated. Wherever they grow they lead to enrichment of
the soil in organic nitrogen compounds through the operation
of the nodule organisms. The difference between the action
of this organism and that of azotobacter is that it gets its
carbohydrates from the plant, and is, therefore, independent
of soil organic matter. Thus, it operates perfectly well in the
poorest soils, provided potassium salts, phosphates and calcium
carbonate are present in sufficient quantity for the host plant,

1 Compt. Rend., 1915, 160, 280.

2J. K. Wilson, N.Y., Cornell, Agric. Expt. Sta. Bull., 386, 1917. W. A.
Albrecht, Soil Sci., 1920, 9, 275, and various papers from A. L. Whiting’s labora-
tory, Illinois. This may explain the action of farmyard manure on the clover
crop. (E. J. Russell, fourn. Bd. Agric., 1919, 26, 124.) "

208 SOIL CONDITIONS AND PLANT GROWTH

while azotobacter (except where it is associated with alge, a
case that requires further investigation) requires a supply of
organic matter in the soil, and therefore works only in fairly
rich soils where its effects are more difficult to measure.

Few improvements in agriculture have produced more

marked effects than the extension of leguminous cropping.

Where a leguminous crop has not hitherto been commonly
grown it may be necessary to introduce the appropriate
organism, as has been successfully done in Canada by
Harrison and Barlow (126), in the United States by the
Bureau of Plant Industry, and on the North German moors
by Hiltner (135; see also 215 c and @). In Great Britain
these inoculations have not proved useful, and they have never
come into farming practice: the high hopes sometimes enter-
tained that the whole problem of nitrogenous manuring—the
most costly item in the farmer’s fertiliser bill—might reduce
itself solely to bacterial inoculation have never been realised.

Other Nitrogen Fixing Organisms.—Other organisms have
been described which have the power of fixing gaseous
nitrogen: among them Phoma, which in culture solutions
proved about half as effective as azotobacter!; this organism
belongs to the mycorrhiza group and may pie a part in
plant nutrition,

Denitrification.

If the air supply of the soil is cut off by water-logging,
or in the laboratory by means of an air-pump, the nitrates
rapidly disappear, whilst nitrites, ammonia, or gaseous
nitrogen are formed. The conditions can be so arranged
that the decomposition of nitrate-bouillon by soil shall give

*

rise to notable quantities of gaseous nitrogen, nitrous oxide —

(18), or nitric oxide (168 and 2776).
The reduction of nitrates to nitrite has long been known.
As early as 1867 Schénbein? stated that it could be brought

1B, M. Duggar and Davis, Annals Missouri Botanical Gardens, 1916, 3,
413-437-

2C, F. Schonbein, Beitrage zur Physiologischen Chemie, Zeitsch. f. Biologie,
1867, 3) 325-340.

CARBON AND NITROGEN CYCLES IN THE SOIL 209

about by “frische Conferven, wie sie so hiufig in stehendem
Wasser vorkommen”; after 10 to 15 minutes’ boiling, how-
ever, the property was lost. Meusel in 1875 (198) showed
that it was bacterial, and could be stopped by antiseptics.
The property appears to be generally possessed by bacteria
and was shown by no fewer than 85 out of 109 kinds investi-
gated by Maassen (184).

The formation of gaseous products is effected by a smaller
but still considerable number of organisms ; these were first
investigated by Gayon and Dupetit (103), and by Déhérain
and Maquenne (81a).

The physiological significance of the reduction appears to
be that nitrates can supply oxygen to the organisms when
free oxygen is no longer obtainable. It is not simply a re-
action between the organism and the nitrate: easily oxidisable
organic matter must be present at the same time. The
partially decomposed organic matter of the soil—the “‘ humus ”
—does not seem to be very serviceable (274).

There is a very sharp contrast between the bacterial pro-
duction and the bacterial destruction of nitrates. Nitrate
_ production is the work of one organism only at each stage,
and the end result is a single product quantitatively equivalent
to the original ammonia; no single chemical process oxidises
ammonia in this complete manner. The bacterial reduction
of nitrates, on the other hand, gives no single product, but a
number of products not in any simple ratio, whilst the chemical
reduction can readily be made to go quantitatively to ammonia.

Whether denitrification goes on to any extent in properly
drained agricultural soils is doubtful, because the three essential
conditions, lack of air, presence of much easily decomposable |
organic matter and of nitrate are rarely obtained. In 1895
Wagner and Maercker startled the agricultural world by an-
nouncing that unrotted dung destroys the nitrates in the soil
and reduces the crop yield (291). Their experiments were
criticised by Warington (297) who pointed out that their dress-
ings of dung were enormous and their results would not apply
to ordinary farm practice. But it may well occur in rich soils

‘ 14

210 SOIL CONDITIONS AND PLANT GROWTH ‘%

and it goes on to a marked extent in wet soils. Nagaoka
(211, see also 74a) has shown that nitrate of soda frequently
depresses, instead of increasing, the yield of rice, sagittaria and
juncus on the swamp soils of Japan, an action which he
attributes to the formation of poisonous nitrites. Organic
manures or sulphate of ammonia are always used on such
soils. Kelley has obtained similar results in Hawaii.?

Denitrification will also take place in peaty soils, and this
has led to a very interesting observation as to the effect of
lime. So long as these soils are left in their natural acid state
nitrification.cannot take place and therefore there is no denitri-
fication. But as soon as lime is added in sufficient quantities
to give a neutral reaction nitrification begins: part of the
nitrate is then reduced to nitrite by microbic activity, then a
chemical reaction sets in between the nitrite and the peat
whereby some nitrogen is lost and some transformed into in-
soluble compounds. Thus large doses of lime may produce
injurious effects on peat soils (Arnd (6)).

Assimilation of Ammonia and Nitrates by Bacteria and
other Micro-organisms.

Probably most of the bacteria and moulds occurring in soil

are capable under suitable conditions of assimilating ammonia,’
_ The process has not been observed in ordinary arable soils
rather poor in organic matter; Schldsing pére (245c) recovered
as nitrate 98 per cent. of the added ammonium compounds,
so also did Russell and Hutchinson. In peaty soils, however,
the assimilation of added ammonia appears to be more pro-
nounced, amounting to nearly 30 per cent. in Lemmermann’s
“experiments (170).

Certain organisms are capable of taking up nitrates: there
is evidence (p. 179) that the action normally occurs in soils.
Algz would be expected to behave in this way, and certain
bacteria are known to do so in presence of easily decomposed
organic matter (38) and air, in which respect the action differs

1 Hawaii Bull., No. 24.
2 Observed by Bierema (38) in 1909, and much investigated subsequently.

CARBON AND NITROGEN CYCLES IN THE SOIL 211

markedly from denitrification proper; it apparently goes on
when sugar is added to the soil (151¢), But such assimilation
does not necessarily involve any loss of nitrogen, for as the
organisms die they are probably decomposed with formation
of ammonia and nitrates once again.

Doryland (86) has shown that this assimilation of am-
monia and nitrates depends on the amount of energy material
available, and may become considerable in favourable circum-
stances (pp. 186, 266).

The Sulphur Cycle.

Sulphur enters into the composition of several plant con-
stituents, and it appears to undergo a series of changes in the
soil whereby it is converted into sulphate, in which form it is
readily taken up once more by plants. The conditions of this
change have been investigated by Brown and his co-workers
(60 4 and ¢c) who call the process “ sulfofication ”.1

Effect of Bacteria on Soil Phosphates.

It is frequently suggested that the phosphates in the soil
_are made more soluble through the activity of bacteria, but
there is no sufficient proof. British agriculturists in the
eighteenth century recognised that bones acted better as
manure after fermentation than before, but the older chemists
attributed the action to the decomposition of the organic
matter and consequent greater accessibility of the phosphate,
More recent bacteriological investigations by Stoklasa? and by
Sackett, Patten, and Brown,’ however, show that certain bac-
teria have the power of dissolving both bone and mineral
phosphates in culture solution, but the mechanism of the pro-
cess is not clear. It is uncertain whether this action goes on
in the soil: direct analysis has failed to demonstrate it and
the evidence from field experiments is conflicting (p. 191).

1See also Kappen and Quensell, Landw. Versuchs-Stat., 1915, 86, and
J. W. Ames and T. E. Richmond, Soil Sci., 1918, 5, 311.

2 Stoklasa, Duchacek, and Pitra, Centr. Bakt. Par., 1900, 6, 526 and 554.

3Sackett, Patten, and Brown, Michigan Special Bull., 43, 1908.

an

14

212 SOIL CONDITIONS AND PLANT GROWTH

Hartwell and Pember,' and Tottingham and Hoffman,’ failed
to find any increased solubility even when rock phosphate was
composted with stable manure.?®

Decomposition of Organic Comipounds.

Many organic compounds suffer decomposition in the soil,
including some toxic to the higher forms of life and others
usually regarded as very stable. Among those readily de-
composed are phenol‘ potassium sulphocyanide, naphthalene,
pyridine and vanillin®: apparently the decomposition is
brought about in each case by micro-organisms. :

Some of these toxic substances may arise during the
ordinary decomposition processes in the soil,® and if so it is
obvious that the micro-organisms which further break them
down to innocuous compounds are playing an important part
in soil fertility.

Swamp and Paddy Soils.

In the East—in India, Japan, etc.—considerable quantities
of rice are grown on Swamp soils, and the biochemical changes
differ considerably from those in normal soils.

As already pointed out, nitrification does not go on but
the converse process, denitrification, occurs, so that if nitro-
genous artificial manures are to be used nitrates are out of
the question and organic manures and ammonium salts only
are possible (p. 210).

1 Hartwell and Pember, Rhode Island Bull., 151, 1912.

2 Tottingham and Hoffman, Wisconsin Research Bull., 29, 1913.

3 For a summary of the recent work see Soil Sci., 1919, 7, 141. Solution
of the phosphate proceeded when sulphur was added (J. C. Lipman and McLean,
Soil Sci., 1918, 5, 243 and 533; also O. M. Shedd, Fourn. Ag. Res., 1919, 18, 329).

4N. Sen Gupta, F¥ourn. Ag. Science, 1921, 11. For the organisms see
G. J. Fowler, Ardern, and Lockett, Proc. Roy. Soc., 1911, 83, 149: -156, and
Wagner, Bied. Zentr. Agrik. Chem., 1915, 44, 212.

5 W. J. Robbins and A. B. Manes? | Soil Sci., 1920, 10, 237, also Ala. Expt.
St. Bul., 195, 1917, and 196, 1917 (M. J. Funchess).

6 F.g. Liechti and Mooser (Land. Fahrb. Schweiz, 1906, 1) estimate that
34-83 kilo of phenol per hectare (30-74 lb. per acre) are formed during the de-
composition of an ordinary dressing of liquid manure. Of benzoic acid no less
than 400-500 kilos are supposed to be formed.

CARBON AND NITROGEN CYCLES IN THE SOIL 213

Green manuring is, however, commonly adopted and con-
siderably benefits the crop: the decompositions taking place
_when the green crop is ploughed in have been studied by
W. H. Harrison and Aiyer (126). In the body of the soil marsh
gas, hydrogen, and CO, are evolved, as would be expected
from the anaerobic decomposition of cellulose. But at the
surface of the soil the change is entirely different and the
gases consist of oxygen and nitrogen only. The difference
was traced to a film of organisms which have the power of
converting the marsh gas into CO,' and this into oxygen.
The oxygen is directly beneficial to the plant by providing
for the aeration of the root. The production of oxygen was
suppressed when the film was killed by adding copper sul-
phate: marsh gas and hydrogen then appeared at the surface.

It is only so long as the film is working that green manur-
ing is beneficial to the crop. The plant roots must have
oxygen and the film supplies it. The green manure, there-
fore, not only supplies ammonia, but also dissolved oxygen.

It would be interesting to have studies of the conditions
obtaining in water cress-beds.

The Effect of Adding Plant Residues to the Soil.

The addition of plant residues to the soil is a normal oc-
currence and a recognised method of manuring land. The
effects produced depend on the proportions of carbohydrate
and protein present in the residues.’

If the conditions are favourable to the activity of micro-
organisms the result of the addition is to cause an increase in
numbers and in activity of the micro-organisms: this is shown -
by increases in oxygen absorption and CO, evolution. The
increased numbers may have any of the following effects :—

+ The biological oxidation of marsh gas has been described by Kaserer and
' Séhngen and by I. Giglioli and G. Masoni (Pisa: Chim. Agric. Stud. e. Ri-
cherche, 1909-14, Part 22, 76-108).

®For further discussion see H, B, Hutchinson (140e); P. E. Brown and
F. E. Allison (60e); P, Felber, Mitt. Landw. Hochschule f. Bodenkulture in
Wien, 1916, 3, 23.

\
214 SOIL CONDITIONS AND PLANT GROWTH

1. If the material is rich in energy supply but not in
nitrogen (¢g. carbohydrates) the organisms may assimilate
nitrates or ammonia already existing in the soil and thus re-
duce the amounts of these substances present (p. 266). These
circumstances are favourable to the nitrogen-fixing bacteria,
and if the temperature is sufficiently high there may be an
increase in the amount of nitrogen fixed (p. 203). The as-
similation effects are temporarily harmful but the fixation is
ultimately beneficial to the growing plant.

2. If the material is rich in nitrogen (eg. protein), the
organisms will produce considerable quantities of ammonia in
obtaining their energy supplies. This effect is wholly benefi-
cial to the plant (p. 185).

3. If the air supply is insufficient—either generally or
locally—but other conditions are favourable, the organisms
will obtain some of their oxygen from the nitrates present in
the soil, and gaseous nitrogen will be evolved. This effect is
wholly harmful to the plant (p. 208).

4. If, however, conditions are unfavourable (e.g. if there
is acidity or too lowa temperature), the organisms will not
act; the material will only partly decompose and it will ac-
cumulate as peat (p. 133).

Summary of the Changes Taking Place and the Agencies
Involved.

It is unfortunate that no synthesis of a soil has yet been
effected, and consequently the preceding analysis of the changes
taking place cannot be tested by reconstructing the whole pro-
cess out of its constituent parts. On the whole the evidence
is satisfactory as to the general course of the changes, but in-
sufficient for sorting them out quantitatively and precisely.
The following scheme summarises them as completely as is
possible at present :—!

1It will be noticed that these processes show certain resemblances to those
of sewage purification beds, as worked out by Adeney (2) and Fowler (Bacterio-
logical and Enzyme Chemistry, London, 1911). The decomposition in the septic
tank, however, and especially in the percolating filter and the contact bed, ap-

CARBON AND NITROGEN CYCLES IN THE SOIL 215

Carbohydrales ;
ie Cellulose Oils Waxes
ice bide other Compounds ee undecomposea
Humus
NA3 Hydroxy Acids Caleuun Salts
GC a Vv y y
ry Nitrates la; 00, Q, a, >

There is good reason to suppose that all of these changes are
effected by micro-organisms, but the evidence is by no means
conclusive. The obvious test of working with sterilised soil is
out of the question, because soil can only be sterilised by
drastic methods that wholly change its character. The fact
that antiseptics put an end to most of the reactions always
used to be regarded as sufficient proof of their biological nature,
but this argument has lost much of its force since Bredig and
others! have shown that indisputably dead materials like
spongy platinum partially lose their power of bringing about
chemical changes when treated with antiseptics. Now the
soil is a spongy mass, measurably radio-active (p. 89), con-
taining numerous colloidal bodies conceivably capable of act-
ing as catalysts, and it is possible to imagine a series of
catalysts that would bring about all the known changes and
be put out of action by antiseptics. Hypotheses of this nature
have indeed been put forward from time to time and reactions
have been discovered which cannot be attributed to micro-
organisms. Cowie has shown? that the first stage of the de-
composition of cyanamide in the soil, the formation of urea,
is not due to organisms, but to some mineral constituent.*

pears extremely rapid to the agricultural chemist; changes such as nitrification,
for which he is accustomed to allow days or even weeks, being brought about
in two or three hours even at the low temperature.

For the decomposition of fats, see O. Rahn, Centr. Bak. Par., Abt. II., 1906,
15, 52-61; 422-429. j

1Cf. Bredig and Ikeda, Zeit. Physikal Chem., 1901, xxxvii., 1-68.

2 Yourn. Agric. Sci., 1920, 10: see also Liéhnis, Zeitsch. f. Gdrungsphysiol,
1914, 5, 16-25.

3 For a study of solid catalysts see E, F. Armstrong and Hilditch, Proc, Roy,
Soc., 1919, 96 A, 137,

% ) i ) p 4
216 SOIL CONDITIONS AND PLANT GROWTH

Soil rapidly decomposes hydrogen peroxide. Some decom-
position of protein and of phenol and hydrolysis of cane
sugar also occurs under conditions where living organisms
seem excluded. There is some evidence that soil can bring
about most of the chemical actions that a large number of
organisms can effect, such as ammonification, but not changes
peculiar to one or two organisms only like nitrification and
nitrogen fixation. Sestini,! indeed, supposed that ammonia is
oxidised catalytically by the ferric oxide always present to
nitrites and nitrates, while Loew (180d) states that nitrogen
can be catalytically ‘‘fixed” and converted into nitrates;
Russell and Smith,? however, failed to reproduce these
changes catalytically. Indeed, the chief argument in favour
of the bacterial hypothesis is that all known soil processes
can be reproduced in the laboratory by soil bacteria acting
under conditions comparable with those known to obtain in
nature, whilst they have not been produced by catalysts. The
biological hypothesis, therefore, remains the simplest and most
satisfying, but there is room for more evidence before it can
be regarded as positively established.?

It is certain that living bacteria occur in the soil in addi-
tion to those present as spores. Some idea of the relative pro-
portions of these two forms was obtained by making gelatin
plate cultures of soil before and after treatment with toluene, |
which destroys living forms but not spores, or at any rate not
all spores (Table LVI.). Spores form only about 25 to 30 per
cent. of the total numbers, and for some unknown reason do
not accumulate. Conn (see p. 187) does not consider, however,
that they represent organisms of importance in the soil. The
bacterial numbers are seen to be very high, but even these
figures do not represent the true totals, and no medium has
yet been devised that allows of the growth of all the organisms
known to occur in the soil.

1 Landw,. Versuchs-Stat., 1904, 60, 103-112.

2 Fourn. Ag. Sci., 1905, I, 444-453-

3 For a recent discussion see The Proof of Microbial Agency in the Chemical
Transformation of Soil (H. J. Conn, Science, 1917, 46, 252).

CARBON AND NITROGEN CYCLES IN THE SOIL 217

TasLle LVI.—NumpBers or AcTIVE BACTERIA AND SPORES OCCURRING IN
SoIts AND CAPABLE OF GROWTH ON GELATIN PLATES. RUSSELL AND
HuTcHINSON (241c). MILLIONS PER GRAM OF Dry SolL,

: Rich Green-

Arable Soil. house Soils.
Forms killed by toluene (i.e, active forms) . 7°4 39°6
Spores surviving toluene ‘ : : 2°9 13°3
Total growing on gelatin | A I0°3 52°9

The active forms must be held responsible for some at
least of the oxygen absorption, carbon dioxide evolution and
decomposition going on. Under comparable soil conditions a
distinct relationship exists between the productiveness of the
soil and the amount of bacterial activity, although it cannot
be expressed in any definite form. Counts of the numbers of
bacteria by any particular method fail to give results sharply
connected with productiveness (although there is a general
relationship) because the organisms are of the most varied
description (111), and of widely different efficiency as food
makers. Nor, on the other hand, have the methods of physi-

ological grouping helped much, since they necessitate growth
in culture media wholly different from the soil under tempera-
ture and water conditions that never obtain in nature. Not
until the fundamental difficulty has been overcome of syn-
thesising a soil identical with natural soil will it be possible
fully to interpret the many interesting observations that soil
bacteriologists are now accumulating. The subject is further
discussed in Chap. VII.

CHAPTER VI.
THE BIOLOGICAL CONDITIONS IN THE SOIL.

IT is now necessary to study the soil conditions that determine
the growth not only of plants but also of the micro-organisms
that, as we have seen, make new plant food out of old plant
residues, and render possible the continuance of vegetable life
on the earth. These cond’ ~ are water supply, air supply,
temperature, food supply, «: asence of injurious factors.

The Water S uply of the Soil.

The water content of the soil is the difference between the
amount supplied and the amount lost. The supply may
come from rain, from irrigation, and from the subsoil: the loss
is by evaporation or drainage from the soil or transpiration
through the plant. The broad general facts are that a soil
tends to maintain its water supply within fairly definite —
seasonal limits, any excess draining rapidly away (unless
there is some mechanical hindrance), and any deficiency below
the lower limit setting in only slowly. This is well shown in
the curves of Fig. 17 (p. 131), where the winter level is never
greatly exceeded and the summer level is fairly well maintained.
The fluctuations, however, are of great importance.

The moisture range depends on the structure of the soil.
The balance retained in the soil is obviously determined by
the resistance the soil can offer to the forces involved in
drainage or evaporation, which depends on its composition,
its texture and its colloidal matter. To a considerable degree
evaporation can be controlled by cultivation: this forms the
basis of the so-called Dry Farming.'

1 See British Ass. Repts., 1914, p. 645.
218

THE BIOLOGICAE CONDITIONS IN THE SOIL 219

The main facts in regard to water supply are as follows :—
1. A sandy soil is liable to great fluctuations in moisture
content, easily becoming very wet or nearly dry. Movement
_ from underground sources of supply is rapid, but except where
it is gravitational, ze. due to seepage from higher ground,
it does not appear to extend far. A chalk soil behaves
similarly. The fluctuations are reduced and the tendency
to become dry is considerably lowered when organic matter
or clay is added to the sand.

2. A heavy clay soil fluctuates proportionately much less in
water content, and its maximum and minimum contents are
both high. Water travels only very slowly: it is common to
see the soil of a field cracking with drought almost up to the
edge of a stream.

3. Peat soils like clay soils have a high maximum anda
high minimum water content. They show in an exaggerated
degree a tendency seen in all soils—a difficulty in becoming
remoistened after the soil has become dry. ©

These relationships are well shown in Table LVIL.

TasBLe LVII.—Moisturre Content! or SAanpy, Loamy, AND CxLay SoILs AT
Wosurn, Lyinc not Far APART, AND UNDER APPROXIMATELY EQuAL
RAINFALL CONDITIONS. RUSSELL.

Sandy Soil Loamy Soil Clay Soil

(Clay=5"0 (Clay=9°3 (Clay =43°0

per Cent.). per Cent.). per Cent)
Highest observed . I4’0 16°5 35'0
Lowest observed . ; I'l 6°0 15°8
Mean of all observations g'0 12'0 27°

For biological purposes a better idea of the meaning of
these results is obtained by translating them into volumes.
The soil is a porous mass and a large part of it is not occu-
pied by solid matter at all but by air and water. Comparison
of the specific gravity of the soil particles determined by the
specific gravity bottle—the so-called “true specific gravity ”—

1 This determination is made by drying at 40° C,

220 SOIL CONDITIONS AND PLANT GROWTH

with the specific gravity of the mass—called the apparent den-
sity —obtained by weighing a block of soil of which the volume
in sttu is known, shows that the solid matter forms 50 to 65
per cent., leaving 50 to 35 per cent. of pore space. Organic
matter increases the pore space in consequence of its ‘‘ lighten-
ing’ action (p. 130). This is shown in Table LVIIL.

TaBLE LVIII.1—Pore Space, WATER CONTENT, AND AIR CONTENT OF CERTAIN
Sorts. Russe.

Volume Occu-

Specific Gravity] _-~,: Volume of Volume of
: pied in Natural F
of Dry Soil. State by Water. Air.
hee
oOo . “4, vay 3
re “3 | eg | 82 1a | 8s
a a zs 6S = "to £ | Sw
o o a a nD” a) BM =]
] 3 he} on o mS Cha [Var
& & | os | de | 42 | ek 128) a
s 5 | ace | £2 Tee | eh
b*| ot aS <0 mS | a6

Poor heavy loam
(Rothamsteg), loss
on ignition, 4°3 per
cent. $ . a

Heavily dunged ar-
able soil (Rotham-
sted), loss on igni-
tion, roo per cent. | 1°46 | 2°31 | 62°8 | 38°2 | 30°3 | 20°0 | 7°9! 182

Pasture soil, loss on
ignition, 13°0 per
cent, .

1°57 | 2°36 | 65’9 | 34°r | 23°2 | I7°O | 10°G| 17°1

1°r7 | 2°22 | 52°7 | 47°3 | 40° | 22°3 | 7°3 | 25°0

In attempting to elucidate the phenomena of the water
supply of the soil the most significant fact is that the soil
has a certain, but not an indefinite, capacity for holding water.
Two consequences follow. The plant is never able to obtain
all the water present; it wilts and dies when it has taken all
that its root cells can absorb, in spite of the fact that some
water is left behind in the soil. At the other extreme, a soil
is not able to hold more than a certain amount of water
against the force of gravity, and any excess rapidly drains
away. It does not appear that water once passed into the

1 Driest periods of 1909 and 1910, During the abnormal drought of 1911
the numbers fell to 6 and 8 for the first two soils,

THE BIOLOGICAL CONDITIONS IN THE SOIL 221

subsoil ever comes back again sufficiently quickly to be an
important factor in plant growth."

The facts have been studied by two sets of investigators.
Plant physiologists and ecologists have divided up the water
into three kinds :— i

1. The unavailable or useless water, being all below a
certain minimum amount which the plant cannot absorb ;

2. The available water, being the portion absorbable by
the plant, but not including the excess water;

3. The excess water, being the amount which causes
water-logging of the soil and exclusion of air, with conse-
quent injury to the plant.

The unavailable water is marked off from the available
‘ supply by the point at which wilting just begins? The per-
centage of moisture in the soil at this point, calculated on the
dry weight, is called the wilting coefficient: it has been in-
vestigated by Heinrich (129) and by Briggs (554, c). The
experimental method consists in growing plants in large
pots of soil, which is protected from evaporation and not
supplied with water; then when the plants wilt and die, de-
termining the amount of water left in the soil.

It must be admitted that the wilting point is not a definite
constant. For any given soil and plant it varies in a regular
and predictable way with the evaporating power of the air:®
wilting occurs as soon as the rate of loss of water by trans-
piration from the leaves exceeds the rate of entry through
the roots. The loss is determined by atmospheric conditions
and the entry by resistance of the soil to the passage of
water: the wilting coefficient, therefore, includes not only the
water which is too firmly held by the soil to pass into the

1For a review of the literature see F. J. Alway and G. R. McDole, Fourn,
Agric. Research, 1917, 9, 27; and for a calculation of the theoretical maximum
height of rise see B. A. Keen, Fourn. Agric. Sci., 1919, 9, 396. Leather
(167a) obtained but little evidence of movement at Pusa,
2 J.e. permanent wilting, when the leaves do not recover, even ina saturated
atmosphere, without the addition of water to the soil.
8 J.S. Caldwell, The Relation of Environment Conditions to the Phenomenon
of Permanent Wilting (Physiol. Res., 1913, 1, 1-36).

222 SOIL2CONDITIONS AND PLANT GROWTH

plant, but also the water which would enter the roots if only
it could: move quickly enough. Shull has shown by a method
described later that the water at the wilting coefficient is held
by the soil with a force of about 4 atmospheres only, while
the pull exerted by the plant root is approximately equa
to its osmotic pressure, which, according to Hannig’s*
measurements, is about 7 or 8 atmospheres, and according
to Dixon and Atkins it may exceed 20 atmospheres.?”

Other investigators adopt more rapid methods. W. B.
Crump (72) assumes that water left in soil air-dried at 15° C.
is unavailable: he deducts this from the total moisture lost on
drying at 100° C. in order to obtain the available water. This -
method is open to criticism in that air drying at 15°C. isa
very indefinite process, and that water left in the soil at 15°
may, as Shull has shown for water at the wilting coefficient, be
obtainable by the plant.

Much less work has been done on the available water.
Crump in studying peat soils, found large variations in
the total water content of different layers, although the root
development failed to indicate any corresponding difference.

water
humus ®
obtained a much less variable result: he therefore assumed
that it is not the total water which must be regarded as free,
but the water per unit weight of humus :—

Soil No. 13 (September 1905) Vaccinium myetilien and
Deschampsia flexuosa, growing ina shallow hollow on a Calluna
Moor, Yorks :—

But when instead of taking total water he took

1E, Hannig, Ber. Deut, bot. Gesell., 1912, 30, 194-204.

2 Proc. Roy. Dublin Soc., 1912, 13, 229. Some of their results are: Roots
of Beta vulgaris, up to 21°18 atmos. ; of Ilex aquifolium, 7°64 to 10°38 atmos. ; .
of Helianthus tuberosus, 12°76 to 18°67 atmos,

3 The water being weight when sampled minus weight after drying at 15°:
and the humus being loss on ignition of oven-dry soil.

THE BIOLOGICAL CONDITIONS IN THE SOIL 223

sandy Bey ‘i Water
ter atI . umus, .
Per Cent. Per Cent. Ely al
I. Loose fibrous peat $ to Zinch . 1764 77°8 2°23
II. Compact peat # to finch . ; 6r°0 17"4 3°52
III. Sandy ‘‘ sub-peat ” 1 to 2 inches 20°r 3° 2°59

Rootlets penetrated II. and entered III.

No similar studies of agricultural soils seem to have been
made,

Soil physicists, on the other hand, have divided the soil
water under the following headings :—

1. Hygroscopic moisture, being the moisture that a dry
soil can absorb from a moist atmosphere.

2. Capillary water, being water held by capillary or sur-
face forces which, being much stronger than gravity, prevent
_ it being lost by drainage.

3. Gravitational water, being the excess water which is
capable of draining away.

The hygroscopic moisture is marked off from the capillary
water by the point called the hygroscopic coefficient, the per-
centage of moisture in a soil which, initially dry, has been
placed in a saturated atmosphere and absorbed water till it is
in equilibrium therewith (Hilgard (133@); Alway (3)). In
most soils this falls between 14 and 3.

The upper limit for the capillary water is determined by
allowing a column of soil saturated with water to stand ina
cylinder with a perforated base till all drainage has ceased, the
soil being meanwhile protected from evaporation (Hilgard
_(133@)). The percentage of water finally retained is called the
water-holding capacity under the conditions of the experiment.
It is difficult to determine exactly, but it usually varies between
I5 and 40.

These distinctions were drawn up in the first instance
when soil was supposed to have a sand grain structure and
when considerable importance was attached to the capillary
films surrounding the particles. Some modification seems

' 224 SOIL CONDITIONS AND PLANT GROWTH

desirable. now that the soil is recognised to possess marked
colloidal properties. The modification is rather in the con-
ception than in the grouping, but it is important because of
the necessity for clear ideas on the subject.

We start from the fact recognised by Briggs and Shull,
and demonstrated by Keen (146a, 0) that the state of the water
in the soil is the same whether we are considering the first
per cent. or the last: there is no break over the whole range
of moisture content from dryness to saturation.

It follows, therefore, that such divisions as the foregoing
must indicate changes in the force by which the water is
retained, not changes in the water itself. Broadly speaking,
there are three types of forces which come into play :—

1. Intimate surface forces of very high magnitude (eg.
1000 atmospheres or more) which, however, are capable of
acting effectively only on a small amount of water ;

2. The ordinary surface forces commonly spoken of as
capillarity, capable of acting on considerably more, but not on
an indefinite amount of water ;

3. Gravity, which acts on all the water independent of its
amount.

All these forces act, but the relative importance and
effectiveness of each depends on the amount of water present.
While no sharp points of demarcation exist there are, for any
soil, percentages of moisture for which one force exerts a
predominating effect.

The first attempt to divide soil water on the basis of the
forces retaining it was made by Briggs and Shantz in 1912
(55¢): a powerful centrifuge was employed which threw out
all water held by forces of less than 1000 times that of
gravity, which is equivalent to about one atmosphere. The
percentage of water held at this point is called the smozsture
equivalent. :

A considerable advance was made by Shull (264). He
used the attractive pull by seeds as a means of estimating the
force with which water was held by the soil, and he calculated
the pull of the seeds by studying their absorption of water

THE BIOLOGICAL CONDITIONS IN THE SOIL (225

vapour from solutions of knowin osmotic pressure and their
absorption of water vapour when suspended over dilute
solutions of sulphuric acid. Fig. 22 shows the force with
which the water is held in a heavy loam where the wilting
coefficient is high: between 80 per cent. and 20 per cent. of
moisture the hold is but slight, then it increases rapidly and
continuously till at 5 per cent. of moisture it amounts to 1000
atmospheres. But there was no break at the wilting coefficient

80
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wc
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te
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°
40-
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cu]
<
at
Y 4
ul 20°
a
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. AN
SAND
° 200 400 600 800 1000
ATMOSPHERES.

Fic. 22,—Magnitude of force with which water is held by soil, showing
change with decreasing water content. (C. A, Shull, 264.)

(4 atmospheres) : indeed, seeds absorbed nearly as much water
from soil at this point as they did from pure water.

With sand the hold is slight almost all the way: only
near the end does it become great.

Bouyoucos (49c¢) has made a division by a somewhat
different method. He measured the depression of the freezing-
point of the soil water as the soil became successively dryer
and drew the qualitative conclusion that in soil some of the:

15

°

226 SOIL CONDITIONS AND PLANT GROWTH

water was “unfree” in the sense that it took no part in the
process. Keen has shown (146c) that Bouyoucos’ figures are
capable of quantitative interpretation, the amount of “unfree
water” at any total moisture content being a function of the
amounts of “free” and total water. .

These various divisions do not correspond with one another.
Briggs and Shantz (55c) have worked out the relationships
between some of them. They find that the hygroscopic co-
efficient is on the average about 0°68 times the wilting
coefficient. Alway, on the other hand (36), finds that the
wilting coefficient and the hygroscopic coefficient are ap-
proximately equal.

Further, Briggs and Shantz (55¢) have shown that the
following relationship exists between the hygroscopic co-
efficient and the composition of the soil :—

Hygroscopic coefficient = 0'007 sand + 0082 silt + 0°39
clay, where sand, silt, and clay are expresses in American
units.

Alway (3a) has modified the formula to make it fit the
semi-arid or transition soils of Nebraska :—

Hygroscopic coefficient = 0°005 coarser fractions + 0°07
very fine sand + 0'082 silt + 0°39 clay.

Briggs’ “moisture equivalent” is rather different and is
about 1°8 times the hygroscopic coefficient. Briggs and
Shantz in their paper show its relationship to the other
quantities.

General agricultural experience shows that the excess
water of the plant physiologist is approximately the same as
the gravitational water of the physicist.

The fact that these divisions do not exactly correspond is
a strong argument against the reality of their existence. It
is inconceivable that water could exist in a dozen different
statesin the soil. Briggs and Shull both regarded the condition
of the water as continuous, and Keen has afforded rigid proof
that this is the case. In the course of its range from o to 20
or 40 per cent. by weight of the soil the water is held by forces
varying in magnitude from 1000 atmospheres or more down

Arable, no dung for 12 | 19-20] 0’9 — — [Boussingault and
months . - ° Léwy. (48a).
_|Pastureland . . | 18-20 | *5-1°5] To-20 | *5-11°5 [Schloesing fils (246).

°

THE BIOLOGICAL CONDITIONS IN THE SOIL 227

to a fraction of an atmosphere, and finally so weak that gravity
easily overcomes them. Over such a wide range it is con-
venient for the investigator to make divisions, and he is quite
at liberty to do this so long as he realises that his divisions
are arbitrary and not real. The various coefficients referred
to in the preceding pages have their uses, but they become
dangerous when they are taken to imply a break or an abrupt
change in the condition of the water. There appears to be
neither break nor division in the state of the soil water.

Air Supply.

The figures given in Table LVIII. show that about Io
per cent. of the volume of a normally moist soil is occupied
by air, but this volume is perpetually varying inversely as the
amount of water varies. These changes alone would lead to

TasLe LIX.—ComposiTIoN OF THE AIR OF SoILs, PER CENT. BY VOLUME.

Usual Extreme Limits
Composition. Observed.
Soil. Analyst.
’|Carbon-| Carbon-

Oxygen.| dioxide.) O*Y8E™- | dioxide.

Arable, sandy soil | 20°6 | *16 }20°4-20°8}°05- *30]Lau (159) mean of de-
uncropped, + loam soil | 20°6 *23, |20°0-20'9] ‘07- °55] terminations made
no manure, | moor soil | 20°0 *65 |19°2-20'5| °28-1°40] frequently during a
Sandy soil, dunged and period of 12 months.

cropped. - etme Values at depths of

oy Cin, 20°3 “61 419°8-21"0| "09- *94} 15 cm., 30 cm., and
Seradella, 15 cm. . | 20°7 "18 }20°4-20"9] *12- *38] 60 cm. not widely

different, (30 cm.
values given here.)

Arable land unmanured | 20°4 | 0°2 [18°0-22°3| o’o1-1°4 [Russell and Appleyard
a » dunged . | 20°3 0°4 [15°7-21°2| 0°03-3'2] (241g).
Grass land . z - | 18°4 I°6 4$16°7-20°5| 0°3-3°3

(Atmospheric air contains 21 per cent, of oxygen and ‘o3 per cent. of CO,.)
I5*

228 SOIL CONDITIONS AND PLANT GROWTH

a renewal of the air supply in the soil, but other factors,
diffusion, changes in pressure, air movements, etc., come in,
making the gaseous interchange still more complete. At soil
depths reached by plant roots—some 6 to 12 inches—the soil
air presents no abnormal features : there is some accumulation
of carbon dioxide, because this gas diffuses out more slowly
than water vapour, oxygen, or nitrogen, but the percentage
volumes of oxygen and nitrogen are nearly the same as those
of the atmosphere. Of course, if the air supply is cut off by
an accumulation of water on the surface, the oxygen may fall
considerably in volume, but this case is exceptional in England
on cultivated land. At still lower depths the volume of carbon
dioxide may rise above I per cent.!

As might be expected, the carbon dioxide is higher in
amount in summer than in winter, and higher in grass land
than in arable. It may rise considerably in grass land, or in
land recently dunged. .

Russell and Appleyard (241g) obtained evidence of a dis-
solved atmosphere in soil, composed mainly of CO, with some
nitrogen. E. H. Richards has shown? that rain brings down
an appreciable amount of dissolved oxygen :—

DISSOLVED OxyYGEN BrouGHut Down IN RAIN. RICHARDS.

Dissolved Oxygen.
Average Rainfall (Inches)
at Rothamsted
(28 Years). Parts Per Lb. Per

Million. Acre.

Summer : 13°32 g'0 2a7°I2
Winter. j 15°50 II‘2 39°27
Year. ‘ 28°82 a 66°39

Leather has obtained much higher CO, and lower oxygen
values in India under monsoon conditions when the soil is
virtually water-logged and at high temperature; in land

1 Pettenkofer’s determinations at Munich at depths 14-4 m. below the sur-
ace are published in N. Rep. Pharm., 1873, xxi., 677-702, and abstracted in
Fourn. Chem. Soc., 1873, 361, and 1874, 36.

2 ¥ourn. Ag. Sci., 1917, 8, 331-337.

THE BIOLOGICAL CONDITIONS IN THE SOIL 229

carrying Sanai (Crotalaria juncea) and other crops during the
monsoon the CO, rose to 16 to 20 per cent. and the oxygen
fell to 2 to 4 per cent. only.!

The Temperature of the Soil.

The temperature of the surface layer of soil, which in turn
determines the temperature of the lower layers, is the resultant
of several different effects. The actual amount of heat reach-
ing the surface is that portion of the sun’s rays that passes
unabsorbed through the atmosphere, and is therefore dependent
on the climate. The intensity of distribution of the heat over
the surface depends on the slope of the land, and is greater
the more nearly the land lies at right angles to the mid-day
rays: thus, in our latitudes a south slope is warmer than a
north slope, so much as often to produce marked vegetation
differences. Many of the rays may be intercepted by vegeta-
tion, consequently land densely covered by plants is cooler
and moister than bare land ; advantage is often taken of this
fact in tropical countries to protect soil from intense evapora-
tion by the growth of “‘shade” crops. Of the rays that do
finally reach the surface not all are absorbed, an unknown
fraction being reflected back again into space: although no
actual measurements have been made, the loss from this cause
is probably greater on a white chalky soil than on a black
humus soil.

The extent to which a given quantity of absorbed heat
raises the temperature of a soil depends on its specific heat
and this again on its water content. Dry soil has a specific
heat of 0°2, while wet soil has a specific heat approximating
more closely to 1, hence under equal sunshine conditions a dry
soil will attain a higher temperature than a moist one. Not
infrequently the surface layer of the soil is hotter than the air,
especially on a sunny day.’

1J. W. Leather, Pusa Chem. Memoirs, 1914, 4, 85-134. The effect on
germination is discussed by A. Howard in Ag. ¥ourn., India, 1915, 10, 106.

2 This is especially the case in hot climates. Leather (Pusa Chem. Memoirs,

1914, 4, I-49) states that the maximum temperature at Pusa (India) is 20° C,
above the maximum air temperature in the shade,

230 SOIL CONDITIONS AND PLANT GROWTH

The temperature of the body of the soil depends on two
factors—the specific heat of the various layers of soil, and the
rate of propagation of the heat wave. At Rothamsted the
maximum temperature is commonly attained about 2.15 p.m.
in the air, but at 5.30 p.m. at 6 inches depth in the soil. The
ease of propagation is not constant, ¢.g. it is much increased
by an increase in moisture content. This, however, much in-
creases the specific heat; and the result of the two factors is
a considerable damping of the temperature wave as it pro-
gresses through the soil. In consequence, fluctuations in air
temperature of less than 2° C. are often inappreciable at
6 inches depth.

The temperature curve of the soil at a depth of 6 inches
below the surface somewhat resembles that of the air in
summer, but it lacks the sharp peaks and depressions, The
soil minimum is always greater than that of the air, especially
in summer; the maximum is also usually greater in winter,
although it is sometimes below in summer. In winter time,
however, the curve is often flat all the twenty-four hours and —
sometimes shows practically no variation for two or three days
together (146d, Fig. 23).

As a result of the interaction of these factors marked
temperature variations occur over comparatively small areas
of soil, being produced by differences in aspect, moistness
vegetation cover, looseness, ‘and so on. Illustrations are
afforded in Table LX.

For ordinary working purposes the following summary
will be found useful :—

(1) A south slope is warmer than a north slope.

(2) Bare land is warmer than land covered with vegetation,
excepting during winter months.

(3) Soil exposed to the sun’s rays is often hotter than the
air, and is subject to considerable temperature variations,
which, however, only slowly affect layers three or more inches
deep. :

(4) Moist soil, being a better conductor than dry soil, is
much more uniform in temperature.

231

THE BIOLOGICAL CONDITIONS IN THE SOIL

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232 SOIL CONDITIONS AND PLANT GROWTH

(5) The top 6 inches of soil has a higher mean tempera-
ture than the air both in summer and in winter. At 6 inches
the warmer part of the day centres round 5.30 p.m., and the
cooler part round 9°30 a.m.

(6) The warming of the soil in spring is facilitated by
drying ; the cooling in autumn is increased by clear nights
and diminished by rain (Keen and Russell, 1462).

TaBLE LX.—TEMPERATURES OF SOIL AT DIFFERENT DEPTHS UNDER
VARYING CONDITIONS. RUSSELL.

Effect of Weather.
Temperature of Bare Soil.
Air
Temp.

Untouched. Surface Stirred by Hoes.

: 4 inch. 3 inch. 6 inch. 4 inch. |3 inch. |6 inch.
Hot sunny day, 20th June, rgro| 30° | 35° | 30°5° | 27° | 31°5° | 29°8° | 26°5°
Cold cloudy day, 27th June, rg10| ‘18° | 17°5° 16°97?) 15°8° |.59°. "| 26°37 ee

E ffect of Vegetation.

Warm Weather, 5th Oct.,J Cold Weather, 4th Jan.,
1910, Air Temperature, 17°.f1g11, Air Temperature, 3°5°.

4 inch. | 3 inch. | 6 inch. | } inch. | 3 inch, | 6 inch,
| Bare soil . 7°) 16°77 see Ee 25° | 2°5°
Soil covered with living vegeta-
tion (grass land) . 15°5° | 15° | 14°5°f 3° ps 3°
Soil covered with dead vegeta.
tion (mulched land) . «| 1§°5°:| 5°. } rasg? fas" ae

Records extending over periods of some months have been
published by Wollny (318) and by Thiele (281). British data
generally refer only to 6 inch or 12 inch readings ; they have
been collected and worked up by Mawley, by Mellish,’ and by
Franklin.? Systematic readings are taken at the Radcliffe
Observatory, Oxford,* at Kew, and also at Rothamsted, where
a continuous self-recording thermometer is installed. Detailed

1 See Quart. Fourn. Roy. Meteor. Soc., 1899, xxv., 238-265.

2 Proc. Roy. Soc., Edin., 1920, 40, 10, 56.

3See A. A. Rambaut, Radcliffe Observations, 1901, 48, 1-245, and r1grt, 51,
103-204; also Phil. Trans., 1901, 195 A, 235-8.

THE BIOLOGICAL CONDITIONS IN THE SOIL 233

records of soil temperatures at East Lansing, Mich., have
been taken by Bouyoucos (49a), who has also discussed their
effect on the physical properties of the soil.

Food Supply.

In spite of numerous investigations our knowledge of the
plant food in the soil is very limited. On physiological
grounds it is supposed that the whole of the nutrient material
coming from the soil enters the plant in the dissolved state,
but whether the actual soil solution is all the plant gets, as
Whitney and Cameron suppose (see p. 173), or whether the
carbon dioxide respired from the roots! effects the solution of
more material than is already dissolved, has not been ascer-
tained, The soil solution may safely be regarded as the mini-
mum food supply, which is reinforced to an unknown extent
by the soluble substances in the soil.

Nitrogen nutrition presents a tolerably simple case because
plants growing on cultivated soils probably absorb all their
nitrogen as nitrates, which are readily and completely dissolved
by water, whilst plants in undisturbed soil—grass land, etc.—
probably utilise ammonium compounds as well. Potassium
and phosphorus nutrition present greater difficulties because
very little is known about the compounds of these elements
in the soil. This particular problem is of such technical im-
portance that it has been necessary to do something empirically,
and by common agreement the small fraction of the phosphorus
and potassium compounds soluble in dilute acids is called
“available” food material, while the rest is said to be

1 At one time it was supposed that special acids were excreted by plant roots
to dissolve insoluble food materials in the soil. This idea, which was a sur-
vival of the medizval view that plants are wholly analogous to animals, persisted
into our own times, but has been shown to be untenable by Czapek (Biochemie
der Pflanzen, Bd, 2, pp. 872 et seq.). So far as is known CO, is the only acid ex-
creted. The evidence is of the negative kind and is therefore not entirely satis-
fying, so that the problem is periodically brought up again; recently, for
instance, Pfeiffer and Blanck stated that other acids also are given off (224d)
Cf. (275), and footnote, p. 33. For a recent discussion, with references, see H.
Kappen, Untersuchungen an Wurzelsdften (Landw. Versuchs-Stat., 1918, 91, I-40).

234 SOIL CONDITIONS AND PLANT GROWTH

“ unavailable ”.1* Here, however, the agreement ends, for no
two dilute acids give the same results and no two Associations
of Agricultural Chemists recommend the same dilute acid.
Dyer’s 1 per cent. citric acid (91) is adopted in Great Britain,
and its use has been justified by Wood’s investigations (319)
and by those of Hall and Plymen (120a).2_ N/200 hydro-
chloric acid has been recommended in the United States, 2 per
cent. hydrochloric acid by Nilson in Sweden, aspartic acid in
Hawaii, and so on. Mitscherlich (201c) uses a saturated solu-
tion of CO, as being the nearest approach to natural con-
ditions. Some of the younger workers in the United States
are using the water extract as the nearest feasible approach to
the soil solution.* A 2 per cent. citric acid solution has been
‘suggested by Bergu.* Bogdanow® in his investigations of
Russian soils used 2 per cent. acetic acid. The German
“Verband landwirtschaftlicher Versuchsstationen” recom-
mends both 25 per cent. and 10 per cent. hydrochloric acid
(see 2247). Lastly Ramann proceeds in a different manner
altogether and adopts a method depending on the interchange
of bases. The results obtained by different acids are shown in
Table LXI. Similar results have been obtained by Engels
(95) (see also p. 344).

An empirical method can be retained only so long as it
justifies itself by results. Few agricultural chemists of repute
would be prepared to draw up a scheme of manuring on the
basis of soil analysis alone, though probably most would like
to have some analytical data before them.

Even strong hydrochloric acid dissolves only a part of the
potash and phosphoric acid, the remainder not coming out till
after treatment with hydrofluoric acid.

The different action of the various dilute acids has been

1 Daubeny (78) originated this distinction, using the terms ‘‘ active” and
‘¢dormant’”’.

2 Also those of O. Lemmermann, A. Einecke, and L, Fresenius in Germany
(Landw. Versuchs-Stat., 1916, 89, 81-195).

3J. S. Burd (62a).

4 Landw. Versuchs-Stat., 1901, 55, 19; also by Engels (95).

5 Expt. Station Record, 1900, II, 130; 1901, 12, 725.

THE BIOLOGICAL CONDITIONS IN THE SOIL 235

adduced as evidence of the existence in soils of a considerable
number of phosphorus and potassium compounds of varying
degrees of solubility, but no such assumption is necessary. It
more probably represents the division of these compounds be-
tween two solvents, the weak acid and the colloidal complex in
which they are present in the soil (see p. 172).

Taste LXI.—Amounts or K,O anp P,O, ExrracTEp By ACIDS FROM ROTH-
AMSTED Soizs, Per Cent. oF Dry Soir. Hau AnD PLYMEN (120a).1

Dilute Acids Equivalent to 1 per Cent. Citric Acid.

K,O Strong
3M HCl.
Citric Acid.| HCl. | Acetic Acid. | Carbonic Acid.
Broadbalk, unmanured . |} 0°380 | 0°0043 | 0°0147 | 0°0082 o°OIII
pe minerals only | 0°463 070458 | 070522 0°0307 0°02T5
a dung . . | 0°453 | O*0400 | 070684 | 0°0451 0°0380
P2O;5.
Broadbalk, unmanured. | o*114 *o:0080 | oroo2t | ovoorr 0°0005
is minerals only | 0°228 | o*o510 | 0°0360 0°0098 0°0058
es dung . . | 0°209 | 0°0477 | 00224 | 00166 0°0095

More definite information can be obtained about the
nitrogen compounds. The amount of ammonia and of
nitrate can be ascertained in any desired depth of soil. On
cultivated land the amount is not generally more than enough
for one year’s crop, any balance being liable to be washed out
in winter, so that the plant depends in spring on the activities
of the decomposition processes for a regular supply of nitro-
genous food. This is one of the factors that produce the
marked retardation of plant growth in spring when the soil is
wet and cold, especially after a wet winter when the washing-
out process is complete, and it further accounts for the re-
markable benefits produced by even small additions of nitrate
of soda or sulphate of ammonia to the soil at this period.

In dry regions the accumulation of plant food and other
soluble decomposition products in the soil may be too great
to admit of plant growth, and bare patches or regions arise

1 For other results see F. Miinter, Zur chemischen Bodenanalyse (Landw.
Versuchs-Stat., 1919, 91, 181-189).

236 SOIL CONDITIONS AND PLANT GROWTH

known as “alkali soils” from the circumstance that sodium
and potassium carbonates are often present. In wetter
climates the soluble substances tend to be washed out more
completely, but notable quantities often persist in heavy clay
soils, especially where the drainage is bad, and may produce
injurious effects on vegetation.

In discussions of the'plant food in soils it is often assumed
that the only significant plant nutrients are nitrates, phos-
phates, sulphates and salts of potassium, calcium, magnesium
and iron, commonly called the “essential” plant foods.
Perfect plants can be raised in water cultures containing only
these substances. Reference to Fig. 12 (p. 68), however,
shows that productiveness is not always maintained indefinitely
simply by supplying these salts, and not nearly so well as
when the complex farmyard manure—excreta, litter, etc.—
is used. It is known also that certain other compounds
besides the ‘‘essential” foods may increase plant growth
(p. 56). Further, Russell and Petherbridge have shown
that on heating soil to 100° something is formed that
stimulates root production to a remarkable degree. Are these
effects the results of nutrition or of stimulation? Are the
manganese, fluorine, etc., compounds and the beneficial soil
constituents indispensable nutrients of which only traces are
required, or are they, as Armstrong expresses it, “condi-
mental ”’ foods?

It would be attractive to think that some of the vague
physiological conditions that trouble the grower are to the
plant what beri-beri and similar diseases are to the animal—
the result of withholding some essential or useful ‘“ accessory
substance”. Bottomley’ considers that certain substances
obtained in the bacterial decomposition of peat are of this
nature. Mazé has published some remarkable results? show-

1W. B. Bottomley, Proc. Roy. Soc., 1914, 88, 237-247, and 1920, B or,
83; Florence A. Mockeridge, Biochem. fourn., 1920, 14, 432. The substances
are called auximones. See also J. F. Breazeale, fourn. Ag. Research, 1919,
18, 267, who found that citrus seedlings grew better in peat extract than in

distilled water.
2Mazé, Compt. Rend., 1915, 160, 211.

THE BIOLOGICAL CONDITIONS IN THE SOIL 237

ing that maize fails to complete its growth in water cultures |
containing all the recognised nutrient salts if these are chemi-
cally pure, but it grows normally as soon as tap water is intro-
duced. No combination of added salts has as good an effect
as the tap water (Table LXII.).

TaBLE LXII.—GrowrTu or Maize IN WATER CuLTuRES. Mazé, 1915.

Culture solution containing compounds of boron and aluminium (tap water) 43
” ” ” ” ” ” ” ” (distilled water) 24
», aluminium andarsenic . 16°5

= ¥ is “ a 4 » arsenic andiodine 36°6

A considerable amount of work has obviously to be done
before the problem can even be clearly stated. And against
all this is the fact, abundantly demonstrated by Dr. Brenchley,
that barley and peas make full growth in nutrient solutions
containing only the ordinary “ essential” foods (54d).

The Concentration of the Soil Solution.

It is well-known that plants cannot tolerate too con-
centrated a solution, otherwise their osmotic relationships are
seriously disturbed. The cell sap of the plant root exerts an
osmotic pressure of 7-20 atmospheres (p. 222). In ordinary
agricultural soils the soil moisture has a concentration of the
order of o°I to 0'2 per cent. and an osmotic pressure of about
0’2 to I atmospheres (p. 125). In some soils, however, the con-
centration and the osmotic pressure of the soil solution rise
much higher, and in the alkali soils it becomes too high
for plant growth. Gola (108) has proposed to classify
habitats on the basis of concentration of soil solution, his
groups being—over 2 per cent. ; 2 too’5 per cent. ; 0°5 to o'2
per cent. ; below 0’2 percent.!. The scheme is interesting and
promising: it would be improved by taking into account the
composition of the solution which at higher concentrations
is of great importance (p. 78) and by stating the limits in
terms of osmotic pressures.

1 The total concentration of spring and river waters is of the order of o'r to
o°3 per cent.

238 SOIL CONDITIONS AND PLANT GROWTH

There has been much controversy as to the effect of con-
centration of the nutrient solution on plant growth. The
relationship is curiously elusive: at first sight it appears a
most simple thing to ascertain. The difficulty is to change |
the solutions sufficiently often to prevent complications due to
exhaustion of essential nutrient substances.

Breazeale (51a) concluded from his experiments that
small variations in concentration of the nutrient medium are
without effect on plant growth; this view is accepted by
Whitney and Cameron and in this country by Stiles. It is
vigorously controverted by Hall; Brenchley, and Underwood
(12I¢c: see also §4c), who maintain that within certain limits
the plant growth is affected by and increases with the con-
centration, although recognising that the differences become
less as the solutions are more frequently changed. Hoagland
and Sharp (136c) also find that growth increases with increas-
ing concentration up to a point, beyond which no further
growth is obtained : absorption of nutrient salts still continues,
however. Some of their results with barley, after two days
growth, are given in Table LXIII.

TaBLe LXIII.—TRANSPIRATION AND GROWTH OF BARLEY IN SOLUTIONS OF:
DIFFERENT CONCENTRATIONS. HOAGLAND AND SHARP, 136¢c.

re Teenaperetion ae grm.
Concentration .. | Average Dry ty Weight of Tops. | Average Absorption
of Solution, | ,2smotic , | Weight of of Electrolytes from
iy gta WP ale 8 Tops. each Jar, Parts per
Million. piste Grms. 1st Day. and Day. Million.
Grms. Grms.
200 o°ro 0°43 50 94 — 25
800 0°32 0°46 46 QI 72
2500 0°85 0°50 34 64. 288
6000 2°07 0°50 33 48 402

Adequate concentration of nutrients is of great importance
in the early stages of plant growth: in later stages high
concentration is less necessary and may be undesirable.?

1 Annals of Botany, 1915, 29, 89.
2See also J. S. Burd, ¥ourn. Ag. Research, 1919, 18, 51-72.

THE BIOLOGICAL CONDITIONS IN THE SOIL 239

Rate of Renewal of Plant Nutrients.

Whatever the effect of variations in concentration there is
no question that the rate of renewal of the supply of plant
nutrients to the roots is an important factor in soil fertility.
Some of the most fertile soils in England are not particularly
rich in plant nutrients, but they are able to keep the roots
well supplied.

The Nature of the Medium on which the Soil Life
goes on.

It is a mistake to suppose—and this point cannot be too
strongly emphasised—that the medium on which the soil
organisms live and which is in contact with the plant roots, is
the inert mineral matter that forms the bulk of the soil.
Instead, the medium is the colloidal complex of organic and
inorganic compounds, usually more or less saturated with
water, that envelops the mineral particles; it is, therefore,
analogous to the plate of nutrient jelly used by bacteriologists,
while the mineral particles serve mainly to support the
medium and control the supply of air and water and, to some
extent, the temperature. As yet our knowledge of the
detailed composition of this medium is slight but it is steadily
improving (see Chap. IV.); and we shall get a very false
idea of the conditions of life in the soil unless we recognise
the main fact of its existence and fundamental significance.
Gola (1084) has discussed the influence of the colloidal com-
plex in determining plant habitats, and Sdohngen (269) its
effect on the activities of micro-organisms.

Sourness, or Reaction of the Soil.

Many soils, including some of the most fertile, are neutral
or feebly alkaline to litmus paper and contain so much basic
material that interaction with dissolved salts leads only to an
interchange of bases without turning the solution acid.

But other soils, including many infertile ones, are acid to

240 SOIL CONDITIONS AND PLANT GROWTH

litmus paper, and so poor in basic material that on interaction
with a dissolved salt they give an acid solution.

Such soils are called “sour” or ‘‘acid”: the word “ sour”
is at present more suitable, as it is uncertain whether the effects
produced on plants and micro-organisms are really due to
acidity or to lack of bases (see p. 117). Whatever the cause,
however, the effects are very striking. Some of the plants
and organisms are very intolerant and rapidly disappear from
sour soil; few can tolerate any large degree of sourness. The
simplest case is seen on cultivated land where single crops
are grown; experience in England and in America is set
out in Table LXIV.,

TaBLE LXIV.—DEGREE OF TOLERANCE OF SINGLE CROPS TO SOURNESS OR

ACIDITY.
England. : Rhode Island (Hartwell and Damon, 1286).
Less Tolerant. More Tolerant. Less Tolerant. More Tolerant.
White clover Lupines ! Red clover Beans
Red clover Alsyke Alopecurus prat. | Maize
Foxtail (Alopecurus | Oats Lucerne Potatoes
pratensis) Potatoes Poa pratensis Tomato
Barley Sweet vernal grass | Dactylis glomerata | Agrostis canina
(Anthoxanthum) | Barley '| Agrostis vulgaris
Sheep’s fescue
Yorkshire fog
(Holcus lanatus)
Sorrel (Rumex
acetosa)
Rhubarb

It would obviously be feasible to draw up a system of
cropping suitable to sour soils containing only crops tolerant
of acid conditions, and Corville? has suggested that this would
often prove more economical than attempts to neutralise
acidity by carrying lime from long distances.

The phenomena are very striking in natural conditions
where there is a mixed flora. On the grass plots of neutral
reaction at Rothamsted there are about 45 species of plants:
on the somewhat acid plots there are fewer, and finally on the

1See p. 305. 2U.S. Dept. Agric. Bull., 6, 1913.

THE BIOLOGICAL CONDITIONS IN THE SOIL 241

most acid plots there are only 7 species. Many instances
are found in England on the loams overlying the chalk
Downs in the southern counties, some of which are sour, while
others are not. Thus on the Wiltshire Downs, the so-called
_ “bake” land is sour, but it is surrounded by neutral or chalky
soils: on it is found the Sheep’s sorrel (Rumex acetosella),
Scarlet Pimpernel (Anagal/is), and Knawel (Scleranthus
annuus), while the Toadflax (Linaria vulgaris), white and
bladder campion (Lychnis vespertina and Silene inflata) are
conspicuously absent, although common on the chalk in the
district." The influence of sourness on the wild vegetation of
uncultivated land (Harpenden Common) has been studied by
Hutchinson and McLennan (140d) and on woodland vegeta-
tion by E. J. Salisbury (242), in both cases the “lime require-
ment” was used as the measure of sourness. Their results are
given in Table LXV. Salisbury further finds Quercus sessilt-
fora on the more sour and Q. robur on the less sour soils.
Attempts have often been made to draw up lists of plants in-
dicating sourness, but the problem is not altogether simple.
A plant may be eliminated from the natural flora not because
it cannot tolerate the degree of sourness in the soil, but be-
cause it tolerates this sourness less well than its competitors.
Another plant may flourish in sour soil, not because condi-
tions of sourness are suitable to it, but because of the absence
of effective competition. Thus sorrel is often described as an
indicator of sourness, but this is not entirely correct: it is not
the presence of sorrel that is symptomatic, for sorrel will
grow quite well on chalk soils: it is rather the presence of
sorrel, sweet vernal grass, etc., combined with absence of
clover. The effect of sourness, like that of any other adverse
factor, is to alter the balance in competition somewhat
against a particular set of plants, which tend therefore to be
eliminated in time. We shall find other cases (p. 301) where
even a small factor can profoundly affect the natural flora if
its action continues long enough.

1W. E. Brenchley, Annals of Botany, 1912, 26, 95-109.
16

242 SOIL CONDITIONS AND PLANT GROWTH

TaBLE LXV.—LIME REQUIREMENT AS RELATED TO VEGETATION,

Harpenden Common. Hutchinson and McLennan (140d). Woods. Salisbury, 242.
Average Lime Requirement of Soil. Dominant Flora. Rane
Approx. 0°22 per cent, CaCO, | Wild white clover 0°24 | Mercurialis
(Trifolium repens)
seit0'26 af e Fescues (F. ovina and
rubra
1 hegt 4 3 Mixed, Yarrow, wood-
tush and moss
i - O59 as »» | Gorse
Holcus,
pein: OAR - Yorkshire fog. 0°52 { Anthoxantium,
Cnicus
9 19°53 5 »» | Sorrel 0°62 y wii
nenome

Similarly there is a close relation between the micro-
organic flora and the sourness of the soil. Christensen (672)
found that azotobacter was especially sensitive, its occurrence
depending sharply on a sufficiency of base: so much so,
indeed, that he uses the presence or absence of azotobacter as _
a test of the need of adding lime to soil. The test is generally
adopted in Denmark. Some of his results are as follo ws :—

Percentage of
CaO soluble ‘
in NH,Cl |Below 0'05|0'06-0'10/0"11-0°15|0°16-0°20/0'20-0°25|0"26-0°30|0°31-0°35|0°36-0"40| A bove 0*40
Percentage of
cases in which
azotobacter :
develop 1 ° ° 5 II 44 56 70 gi 84

Nitrifying organisms are also sensitive: they still occur
even in acid soils, but their activity becomes much more
marked after adding lime. Hall, Miller, and Gimingham? found
nitrification going on slowly in the Rothamsted grass plots
receiving 600 Ib. of ammonium salts annually, and now become
so acid that only a few species of plants survive.

1 The culture solution contains no CaCO, but is inoculated with the soil
only. Hence the experiment demonstrates the sufficiency or otherwise of the
CaCO, in the soil for the needs of the organism. A parallel test is made using
the full culture solution so as to show whether the organism is present in cases
where negative results are obtained.

2 Proc. Roy. Soc., 1908, 80 B, 196-212.

THE BIOLOGICAL CONDITIONS IN THE SOIL 243

The general mixed flora developing on nutrient gelatine
plates is also much less numerous on sour than on a neutral
soil: Fischer (98) and Bear (11) have both shown marked
increases in bacterial numbers after applying lime to soils,!
The rate of ammonia production does not appear to be so
much affected as the numbers of bacteria, but this is explained
by assuming that fungi play an increasingly important part
in ammonia production as the acidity of the soil increases.”
Nevertheless there is on the whole a marked increase in the
rates of decomposition of organic matter and of production of
nitrate * when lime is added to soil.

Organisms causing plant diseases also vary in their degrees
‘of tolerance of acidity: in some cases the true acidity was
determined. Pythium, which produces damping off in seedlings,
is somewhat intolerant ; consequently no good, and sometimes
harm, is done by adding lime to affected soils. Also the
organism producing potato scab (Spongospora subterranea) is
somewhat intolerant, a fact which permits the control of the
disease since the potato itself tolerates a higher degree of
acidity. Actual measurements by Gillespie and Hurst (1050)
show that scab is common on the Washburn loams of North
Maine where the Py value is 5:9, but rare on the Caribou
loam where the Py value is 5-2, indicating a higher acidity.

On the other hand, some organisms are more tolerant of
acid conditions or even grow better in them. P/lasmodiophora
brassica, producing finger and toe, does so; it is common on
acid soils, but is usually kept in check by addition of lime.
Further, it is commonly stated that fungi predominate in acid
soils, and this may be the case,* but the evidence is rather
@ prior in character, since no satisfactory method of enumera-
tion is known (p. 257).

1See also data in Christensen’s and Hutchinson’s papers.
2Lyon and Bizzell, Cornell Memoir, 1, 1913; N. Kopeloff, Soil Sci., 1916,
571.

‘4 ’For quantitative measurements see Blair and McLean, Soil Sci., 1916, 1,
489-504.

4It has been noticed at sewage works, ¢.g. at Huddersfield, that fungi
develop when the sewage becomes acid through trade effluents

76

244 SOIL CONDITIONS AND. PLANT GROWTH

There is the possibility of much useful work in analysing
the phenomena of “ sourness,” and if acidity be the effective
agent, in determining the limits of tolerance of acidity by the
common crops and disease organisms. The old recommenda-
tion to add sufficient lime to ensure neutrality breaks down in
the case of potatoes and of certain market garden crops,
rhubarb, and in America blueberry,’ while in other cases it
involves an unnecessarily large addition of lime. The object
to be aimed at is to keep within certain limits.

Potato growers, whose scheme of cropping includes clover .
and potatoes, are frequently in an awkward dilemma: if they
withhold lime from the soil the clover fails, if they add lime
the potatoes become affected with ‘‘scab”. The ideal
arrangement would be to alter the reaction of the soil with
each course in the rotation, making it just neutral in the clover
year, and in the potato year just sufficiently acid to suppress
the “scab” erganism. Unfortunately, while it is easy to move
in the direction of less acidity, it is difficult to effect the con-
verse change and increase the acidity.”

Are Toxins Present in the Soil?

A persistent idea that one crop may poison another is
current among practical men. Early in the last century De
Candolle formulated the hypothesis that plants excrete from
their roots toxins that remain in the soil for some time and
injure other plants of the same species, but not necessarily
plants of different species. He thus explained the well-known
fact that a rotation of crops is more effective than a system
of continuous cropping; in a rotation the toxin excreted by
a particular crop is innocuous to the succeeding crop and
disappears from the soil before the same plant is sown again.

The hypothesis was tested in a classical research by
Daubeny at Oxford (78), but could not be justified. Eighteen

1F. V. Colville, U.S. Dept. Agric. Bull, 334, 1915.
2 J.C. Lipman has made the interesting suggestion that sulphur could be
added to the soil to increase the acidity (Soil Sci., 1919, '7, 181).

THE BIOLOGICAL CONDITIONS IN THE SOIL 245

different crops were grown continuously on the same plots,
and the yields compared with those obtained when the same
crops were shifted from one plot to another, so that no crop
ever followed another of the same kind. No manure was
supplied. The results showed a gradual decrease in the yield
in almost every instance, and the decrease was generally
greater when the crop was repeated year after year on the
‘same plot than where it was shifted from one to another.
_ Nevertheless the difference between the yields in the two cases
was not sufficient to justify any assumption of the existence
of a toxin, except perhaps in the case of Euphorbia lathyris ;
in the other seventeen cases it was attributed to the more
rapid removal from the continuous plots of the mineral
nutrients required by the plant. This explanation was
supported by analyses of the plant ash and of the soil—
analyses which led to the important distinction between
“available” and “unavailable” plant food.

Pot experiments made by the writer at Rothamsted have
led to the same conclusions. Six crops of rye were grown in
succession in sand to which only nutrient salts were added so
as to maintain the food material at a constant amount. A
seventh crop was then taken and at the same time a crop was
grown on perfectly fresh sand, on which no crop had ever
grown before, but which was supplied with an equal amount
of the same nutrient salts. There was no significant difference
in the two crop yields. A similar experiment was made with
buckwheat, another with spinach, and a parallel series was
made in soil cultures. In all cases but one the result was the
same; the 1910 weights were as follows :—

Sand Cultures. Soil Cultures.

Weight of dry matter found, Weight of dry matter found,
grams (mean of 4 pots). grams (mean of 4 pots).

Rye. | Buckwheat. | Spinach.] Rye. | Buckwheat. | Spinach.
Cropped six times pre-

viously . : - | 30°4 5°4 33°3 | 26°4 23°9 20°0
Fresh sand or soil. . | 31°3 13°5 29°5 | 27°I 25°2 20°38

Both sand and soil contained 2 per cent. of calcium carbonate,

246 SOIL CONDITIONS AND PLANT GROWTH

If either the rye, buckwheat, or spinach excreted any toxin the.
amount accumulating during the growth of six successive
crops was insufficient to cause any appreciable depression in
yield in the next crop; the exceptional result given by buck-
wheat in sand could not be confirmed.

These and similar experiments show that no lasting toxic
effect is produced by any of the crops studied, and they rule
out any toxin hypothesis as an explanation of the advantages
of rotations’ where there is always a lengthy interval between
the crops. They do not, however, show that there is no tran-
sient effect, and they are thus quite consistent with some re-
markable observations by Pickering on the effect of grass on
apple-trees (225a). It was found at Woburn—and the observa-
tion has since been confirmed elsewhere—that the effect of
growing grass round apple-trees is to arrest all healthy growth
and absolutely stunt the tree. The leaves become unhealthy
and light coloured, the bark also becomes light coloured, while
the fruit loses its green matter and becomes waxy yellow, or
brilliant red. Where the grassing was done gradually the trees
accommodated themselves somewhat to the altering conditions,
but never grew so well as when grass was absent.

This effect might have been due to various causes: changes
in aeration, temperature, water supply, food supply, or physical
condition of the soil, but careful experiments failed to show
that any of these factors came into play. Covering the soil
with cement excluded air at least as thoroughly as grass, and
yet did not produce the grass effect, nor was it suppressed by
wet seasons, liberal watering, or a supply (in pot experiments)
of sufficient water or nutrient solution to keep the soils of
grassed and ungrassed trees equally moist, or equally well
supplied with food. On the other hand, the grass effect was
produced when perforated trays of sand containing growing
grass were placed on the surface of the soil in which trees
were growing, so that the washings from the grass went
straight down to the tree roots. There seemed no possibility

1Some curious problems are thus left unsolved, some of which are discussed
_ more fully by the author in Science Progress, July, 1911, p. 135.

THE BIOLOGICAL CONDITIONS IN THE SOIL 247

_of the grass roots in the trays abstracting anything from the
soil, and the only explanation appears to be that a toxin is
excreted by the grass. Such a toxin, however, must be very
readily decomposed, because no toxic properties could be dis-
covered by laboratory tests either in soil that had been re-
moved from the grass roots or in the washings from the
above-mentioned trays. Pickering has since shown that the
phenomena are general and hold whatever crops are grown in
the pots and trays. In consequence we must be prepared to
consider possible toxic effects of one plant on another growing
alongside of it, and the part such effects may play in determin-
ing natural plant associations and in explaining some of the
bad effects of weeds.

Pickering’s results agree with the hypothesis already put
forward by Whitney, and developed by him and Cameron,
Schreiner (306, 65¢, 249, etc.) and their colleagues at the
Bureau of Soils, Washington. Certain soils are supposed to
contain toxins, which are not necessarily plant excretions, but
may arise by decomposition of organic matter in the soil.
The genesis of this hypothesis is interesting. Reference has
already been made to Whitney’s view that the soil solution
furnishes the food of plants and is of the same composition
and concentration in all soils, from which it follows that in-
fertility of any soil cannot be due to lack of food. But in
certain cases this infertility is transmitted to the aqueous ex-
tract of the soil, and must, therefore, arise from some soluble
toxin. As an example Whitney and Cameron (305) selected
two Cecil clays of very different productiveness but of identical
chemical and physical constitution, prepared aqueous extracts
and used them as culture solutions for wheat seedlings. The
extracts contained in parts per million :—

NOs. PO,. K. Ca.
Good soil . ‘ 3°2 16 3°6 3°2
Poor soil . : 3°5 1°6 2°0 2°8

and were thus identical in their content of plant nutrients ;

~

j

248 SOIL CONDITIONS AND PLANT GROWTH

they were also both neutral, Yet they produced very dif-
ferent effects on the wheat seedlings: the ‘‘ good soil” extract
caused a larger and healthier development of root and a some-
what better development of leaves. In other cases it has been
found that growth in extracts of poor soils is even worse than
in distilled water. The productiveness of the extract could be
raised, according to Livingstone (179), by dilution, shaking
with calcium carbonate, precipitated ferric or aluminium oxide,
animal charcoal, or soil; results which are explained by sup-
posing that these agents precipitate a toxin. Addition of
fertilisers, and especially of an aqueous extract of farmyard
manure, improved the solution; these substances also were
supposed to precipitate the toxin.

A double set of experiments was therefore begun by
Schreiner and his colleagues; a careful search was made in
the soil for such organic compounds as could be identified
(see p. 141); and the effect of these and similar compounds
on plant growth was studied by elaborate water cultures,
Considerable attention has been devoted to dihydroxy-stearic
acid. This substance is toxic to plants in water culture, and
is almost invariably present in infertile soils, especially such
as are badly drained, badly aerated, too compact, and deficient
in lime (251); soils, in short, that in England are called
*‘sour”’.

On the other hand, Russell and Petherbridge could obtain
no aqueous extract toxic to plants from greenhouse “sick”
soils. These soils, however, are rich in organic matter, in
plant food, and in calcium carbonate.

The present position may briefly be summed up as follows:
There is no evidence of the presence of soluble toxins in nor-
mally aerated soils sufficiently supplied with plant food and
with calcium carbonate, but toxins may occur on “sour”
soils badly aerated and lacking in calcium carbonate, or on
other exhausted soils. There is no evidence of any plant
excretions conferring toxic properties on the soil, but the
Woburn fruit-tree results show that a growing plant may
poison its neighbour. The effect does not appear to be speci-

THE BIOLOGICAL CONDITIONS IN THE SOIL 249

fic; any plant will be injured by any other within its range,
_ and it may suffer less from one of another kind than from one
of its own kind.?

Bacterio-toxins.—Several observers, including Greig-Smith
(114), Bottomley (42), and others, have claimed to find soluble
bacterio-toxins in soils. Hutchinson and Thaysen (140g), on
the other hand, obtained wholly negative results, and con-
cluded that soluble bacterio-toxins are not normal constitu-
ents of soils, but must represent unusual conditions wherever
they occur. But the possibility of the existence of toxins in-
soluble in water still remains.

Normal Conditions on a Heavy Soil.

Table LX VI. summarises many of the Rothamsted results
and shows the conditions normally obtaining on a heavy soil
in Hertfordshire under a rainfall of 28-30 inches.

TasBLE LXVI.—ConpITIONS NORMALLY OBTAINING IN THE SoIL AT ROTHAMSTED.

Soil Total Nitrogen as zg s
Moisture.* Nitrogen. Nitrate. = <
; : ose] =
sapien} oe Per Cent by he Pe oe az S26 Pr
‘apse sa | oS keh Se | Be PSEET 8
2 a as # aw 23 & 36 a
be bee baa hoger aaa s LS
= eo
S : 2 a0 ~ : jee)
(top 9°’)
No manure . . | Cropped | 23-12 | 15-8 \ 990 2500] { 5-12] 12-30] 7-12 | 0°2-0°6?
Condition poor . Fallow 23 | 15 8-15] 20-36] 7-12] 0°2-0'4
Farmyard manure | Cropped | 30-15} 17-8 } 2200] 5000 I0-20| 25-50 | 15-25 } 0°5-1°0°
Condition good . | Fallow 30 17 20-35) 50-84 | 15-25 | 0°2-0°6
Ordinary arable
field 4

> Cropped | 25-12| 15-8] 1500] 3700} 10-15/ 25-36 | 15-25 } 0°5-r‘o*

Thus Burmeister (Fihl. Landw. Zeit., 1914, 63, 547-556; see Rome Bull.,
Ig14, 1691) found that couch (Triticum or Agropyron repens) increased the yield of
oats, and Dr. Brenchley found that certain weeds had the same effect on the yield
of wheat per plant (New Phytologist, 1917).

2 Running on occasions up to 1°8. 3 Occasionally up to 2°3.

4 Occasionally up to 2°5.

* The concentration of the dissolved matter is of the order of o°2 per cent.
and the osmotic pressure about 1 atmosphere.

CHAPTER VII.

THE MICRO-ORGANIC POPULATION OF THE SOIL AND ITS
RELATION TO THE GROWTH OF PLANTS,

THE soil is inhabited by a great variety of micro-organisms,
but their precise relationship to the growing plant is difficult
to determine because we. know so little about them. The
micro-organic population is certainly highly complex: it is
known to contain many kinds of bacteria, moulds, protozoa
and other organisms, and new members are discovered almost
every month.

Usually they are picked out by some culture media: and
their physiological effect is studied in an arbitrary culture
solution: sometimes the results are applied straightway to
the soil without further ado. The method is defective for
two reasons. Firstly, micro-organisms are considerably in-
fluenced by the medium in which they happen to find them-
selves, and may effect one change under one set of circumstances
but quite another change under other circumstances. Secondly,
most micro-organisms exist in two states: an active or trophic
state, and a resting state, and it is reasonable to suppose that
the resting forms are comparatively unimportant. Probably
in many cases no sharp line exists between the two, the active
forms changing to the resting stage or back again as the soil
conditions alter; but it is never safe to assume without proof
that any organism discovered by culture methods is active in
the soil. The main difficulty in applying the results is that
soil cannot be sterilised’ because of its chemical instability,
nor can it be made up artificially ; in consequence one cannot
begin with a sterile soil and inoculate into it a particular set

1Intermittent sterilisation at 80° causes less decomposition, but it does pro-

duce change (D. A. Coleman, H. C. Lint, and N. Kopeloff, Soil Sci., 1916, 1, 259).
250

THE MICRO-ORGANIC POPULATION OF THE SOIL 251

of organisms so as to observe their behaviour under natural
conditions. These difficulties have not proved insuperable,
and a number of organisms have been discovered which nor-
mally lead a trophic life in the soil. These organisms may be
regarded as the micro-organic population of soil.

The essential conditions for the life of a micro-organic
population are adequate supplies of food and energy materials,
sufficient air and water, a proper range of temperature, and
absence of harmful factors. With the exception of the
energy supply these requirements are the same as for higher
plants ; and as many of the nutrients are common to the plant
and to micro-organisms it is not surprising that soils well
suited to the growth of plants often carry a large population
of micro-organisms also.

Investigators have long realised that soils contain numbers
of bacteria, and in Chap. V. some of the changes they bring
about are described. Evidence has been accumulated that
other groups of micro-organisms are normally present also
—fungi, algz, protozoa, nematodes, etc.

In previous editions of this book the organisms were
classified according as they were or were not useful to plants.
This scheme is useful as a first approximation, but it does
not admit of fuller development: it is therefore discarded in
the present. edition. The members of the soil population
must be regarded as leading their own lives and, with few
exceptions, possessing powers of adaptation which enable
them to draw sometimes on one compound and sometimes
on another for food and for energy, and therefore not necessarily
always producing the same substances. The mycoides, for ex-
ample, where they are active, can produce ammonia from
protein and would then be helpful to plant growth ; under
other circumstances, however, they assimilate ammonia and

therefore compete with growing plants.
: Certain broad relationships have been established between
the various members of the soil population, and between the
population as a whole and the growing plant: there is evidence
for the following propositions :—

252 SOIL CONDITIONS AND PLANT GROWTH

I. The population at any given moment is as large as the
conditions allow, and is limited by presence of a harmful factor
or by lack of some essential factor such as energy supply, food
supply, air, water, or temperature. Improvement in the supply
of the limiting factor, or removal of the harmful factor, allows
an increase in numbers of the populaton.

2. Competition’ therefore is an important factor in de-
termining the relative numbers of the various groups: any
increase in the numbers of one group may lead to a decrease
in the numbers of others having similar requirements. Changes
in numbers of any one group, therefore, are not necessarily
sharply related to changes in external conditions, and when
the external conditions are brought back to their original level
the numbers of any one group may not return to what they
were before.

3. Owing to the great differences in size the numbers of
the various groups do not allow a ready basis of comparison.
The most suitable basis for quantitative comparisons between
one group and another is their respective energy requirements :
_ these are comparable. Bacteria, fungi, protozoa, and nematodes
are all consumers: algz only are producers of energy materials.

The relationships of the soil population to the growing
plant are as follows :—

1. The energy supplies in the soil come from the residues
of green plants. Since energy supply is probably the most
important factor limiting the numbers of the soil population
it follows that the soil population is to this extent dependent
on the plant.

2. But the plant is also dependent on the soil population
since the soil population in its search for energy supplies
decomposes so much protein substance with formation of
ammonia and of nitrate as to leave over a surplus for the
plant. Further, certain members of the population also de-
compose materials such as straw, etc., which possess definite
structure and have undesirable physical effects on the soil,
while other members decompose toxins such as phenol, which

they accumulated would be harmful to the plant.

THE MICRO-ORGANIC POPULATION OF THE SOIL 253

3. During the growth of the plant some at any rate of
the soil organisms appear to be adversely affected : some of the
activities of the growing plant seem-to be detrimental to the
micro-organic population.

4. While sharp distinctions cannot be‘drawn some group-
ings of the micro-organic population are more conducive to the
production of plant nutrients than others, and some organisms
are definitely harmful to the plant.

Organisms Acting Directly on the Plant.

(1) Parasitic and Disease Organisms.

The study of these organisms has developed into a special
branch of Economic Biology, and we need therefore only briefly
refer to them here. The commonest are the eel-worms, the
myxomycete Plasmodiophora, some of the “ wilts,’ and certain
organisms that attack potatoes.

Of the numerous kinds of eel-worms occurring in the soil,
about six are known to attack and enter the plant, and do
considerable direct injury, besides opening the way for the
entrance of fungi, bacteria, etc. The commonest are Heterodera
vadicicola, which causes swellings or “knots” on the roots of
tomatoes, cucumbers and other plants, and 7 ylenchus dipsact
(syn. devastatrix) which attacks oats, causing tulip root, and
clover, bringing on one form of clover sickness. In some soils,
especially those short of lime, another pest is common: the
myxomycete Plasmodiophora, which enters the roots of swedes,
turnips and other plants of the Arassica tribe, causing the
disease known as finger-and-toe.

(2) Symbiotic Organisms.—In normal conditions legu-
minous plants possess nodules on their roots which contain
numbers of bacteroids living in association with the plant.
This organism, Bactllus radicicola, enters the plant roots
at an early stage and, having brought about the formation
of the nodule, proceeds to manufacture nitrogen compounds
for the plant from the gaseous nitrogen of the air (see p. 204)

Certain trees and shrubs (notably beech, heather, etc.)
become associated with mycorrhiza, fungi which grow on their

254 SOIL CONDITIONS AND PLANT GROWTH

roots and aid in the nutrition of the plant. These were first
investigated by Frank and have received considerable attention
from mycologists,!

Organisms Capable of Bringing About Changes of Im-:
portance to the Plant.

1. Alge.

All soils hitherto examined contain alge including diatoms,
green and blue-green algz, and the flora seems to be par-
ticularly well-developed in cultivated soils. F. Esmarch?
studied the soil flora of the German African colonies and of
Schleswig-Holstein, W. W. Robbins*® that of the Colorado,
and J. B. Petersen‘ the diatoms of the Danish soils; Dr. B.
Muriel Bristol of Rothamsted® the flora of certain English
soils. The alge occur to a depth of 12 inches and in some
cases even 2 feet: the species in the lower layers are almost
identical with those at the surface.

It is now known that certain algz can develop in darkness
provided suitable nutrients are present, and assuming these
conditions there is nothing against the possibility of a trophic
alga-flora in the soil. Esmarch attempted to obtain positive
evidence by burying algz in the soil and making microscopic
examinations after given intervals of time; in these circum-
stances the blue-green algz retained their colour for some
weeks, but ultimately became yellowish, and the filaments
disintegrated.

1 B, Frank’s paper is in Botan. Zig., 1891, 9, 244-253, where references to his

earlier papers are given. Recent investigations have been made by M. C. Rayner,
Annals of Botany, 1915, 29, 97-133.

2F, Esmarch, Beitrag zur Cyanophyceen-Flora unserer Kolonien (fahrbuch
der Hamburgischcn wissensch, Anstalten, xxviii., 3 Beiheft, 62-82, 1910). Unter-
suchungen tiber die Verbreitung der Cyanophyceen auf und in verschiedenen Boden
(Hedwigia, Band lv., Heft 4-5, September, 1914).

3°W. W. Robbins, Alg@ in some Colorado Soils (Bulletin 184, Agri. Exp.
Sta., Colorado, June, 1912).

4J. B. Petersen, Danske aerofile alger (D. Kgl. Danske Vidensk. Selsk.
Skrifter, 7, Raekke, Naturv. og Mathem., Afd. xii, 7, 1915).

5B. M. Bristol, On the Alga-Flora of some Desiccated English Soils (Annals
of Botany, 1920, 34, 35-80).

THE MICRO-ORGANIC POPULATION OF THE SOIL 255

Considerable numbers of species have been found, but
estimates of total numbers are rendered uncertain by the
breaking up of the filaments during the laboratory opera-
tions.

There has been some speculation as to the possible function
of algz in the soil. Assuming that they continue to assimilate
CO, they are, of course, accumulators of energy and of carbo-
hydrate material, and while they may thus function at the
surface in the light it is difficult to see how they can be other
than consumers in the dark recesses of thesoil. The possible
effects of this accumulation of organic matter in newly formed
soils is discussed by Treub ! in his account of the recolonisation
of Krakatoa after the eruption of 1886, and by Fritsch.”

There is also the possibility that alge may take part in
nitrogen fixation in the soil.. It was at first thought by Frank ®
and by Schloesing and Laurent * that algz could themselves
fix nitrogen, and this view has recently been revived by Moore
and Webster.’ Kossowitsch,® however, maintained that they can-
not, bacteria alone having this power. He observed, however,
that the presence of algz facilitated the process, and concluded
that symbiosis occurs between algz and nitrogen-fixing bacteria,
the alge supplying the necessary carbohydrate energy material
and the bacteria furnishing the nitrogen required for growth.
Nakano ‘ shows that the relationship with azotobacter holds
not only for blue-green but also for certain green alge.

1 Treub, Notice sur la Nouvelle Flore de Krakatau (Ann. Fard. bot. Buiten-
zorg, Vol. 7, 1888, p. 213).
; 2F, E. Fritsch, The Role of Algal Growth in the Colonisation of New

Ground and in the Determination of Scenery (Georg. fourn., November, 1907).

$B. Frank, Ueber den experimentallen Nachweis der Assimilation freien
Stickstoffs durch erdbodenbewohnende Algen (Ber. der D. Bot. Ges., vii., 1889,
PP- 34-42).

4Schloesing, fils, and E. Laurent, Recherches sur la fixation de l’azote libre
par les plantes (Ann. de l’Institut Pasteur, vi., 1892, p. 65-115).

5 Proc. Roy. Soc., 1920, B. gt, 201.

6P. Kossowitsch, Untersuchungen tiber die Frage, ob die Algen freien
Stickstoff fixren (Bot. Zeit., 1894, Heft 5, pp. 98-116).

7H. Nakano, Untersuchungen tiber die Entwicklungs- und Ernahrungs-
physiologie einiger Chlorophyceen (four. of Coll. of Science, Imperial Univ.,
Tokyo, vol. xl., 1917, Art. 2, p. 66, etc.).

256 SOIL CONDITIONS AND PLANT GROWTH

Bouilhac’ and Giustiniani? showed that in sand free from
organic matter and nitrogen compounds, soil bacteria and
algz not only develop normally, but also enrich the soil
with nitrogen sufficiently to support the growth of higher
plants ; while still more recently Pringsheim * has shown that
the ability of bacteria to fix nitrogen is closely dependent
upon the presence of blue-green alge (see p. 200).

A third possibility discussed by Gautier and Drouin * is
that algze may assimilate ammonia or nitrate in the soil and
convert it into complex organic substances. The existence
of an action of this kind in normal soils has been indicated
by Russell and Richards (2412).

Harrison and Aiyer (126) maintain that alge serve a
special purpose in swamp soils, taking in CO, and giving
out oxygen, which is then available for the plant roots.

2. Fungt.

It has long been known that fungi occur in the soil, and as ~
long ago as 1886 no less than eleven different species were
isolated by Adametz.® Subsequent workers have greatly en-
larged the list and more than 200 species have now been de-
scribed as soil fungi. x

The investigations have fallen into several groups. Some -
workers, as Butler ® in India, Hagem’ in Norway, and Lendner®

1R. Bouilhac, Sur la fixation de Vazote atmosphérique par l association
des algues et des bactéries (Compt. Rend., cxxiii., 1896, pp. 828-830).

2R. Bouilhac and Giustiniani, Sur une culture de sarrasin en présence d'un
mélange d’algues et de bactéries (Compt. Rend., cxxxvii., 1903, pp. 1274-6).

3E, Pringsheim, Kulturversuche mit chlorophyllfihrenden Mikro-organ-
ismen., III. Zur Physiologie der Schizophyceen (Cohns, Beitrage 2. Biol. d.
Pflanzen, Bd. xii., pp. 99-107).

*Gautier and Drouin, Recherches sur la fixation de l’azote par le sol et les ~
végétaux (Compt. Rend., cvi., 1888, pp. 754, 863, 944, 1098, 1174, 1232, 1605).

5L. Adametz, Untarsuckiangen iiber die niederen Pilze der Ackerkrume
(Inaug. Diss., Leipzig, 1886).

®E, J. Butler, An Account of the Genus Pythium and some Chytridiacee
(Mem. Deft. Agric., India, Bot. Ser., 5, 1, 1-160, 1907).

70, Hagem, Untersuchungen iiber norwegische Mucorineen (Videns. Skrift,
I., Math, Nat. Kl., 1907, No. 7; Part II., rgro, No. 4).

8 A, Lendner, Les Mucorinées de la Suisse, Berne, 1908.

THE MICRO-ORGANIC POPULATION OF THE SOIL 257

in Switzerland, have confined themselves mainly to particular
types, and have not attempted a comprehensive survey of the
whole field. Others have tried to ascertain how far facultative
parasitic fungi could live saprophytically in the soil: apparently
certain disease-producing Fusaria and Pythium can do this.
_ Pratt’ has isolated fungi which cause disease in potatoes from
virgin desert lands and also from Idaho soils that have never
been cropped with potatoes.

The more serious problem of studying the fungus flora of
the soil as a whole has been attempted in Holland by Oude-
mans and Koning (221), in the United States by Waksman
(292c), and in England by Miss E. Dale (75). It is now
under investigation at Rothamsted by W. B. Brierley and
Miss S. T. Jewson. |

The great difficulty is the lack of suitable methods of in-
vestigation: neither the morphologists nor the physiologists
have yet developed precise, simple methods. The fungi are
invariably isolated from the soil by means of cultural media,
but there is no means of ensuring that the medium used is
suitable for the development of all forms that may be present ;
many of the parasitic forms and others in addition are doubt-
less missed. Further, the morphological characters vary
with different media:? hence it is impossible to describe the
organisms as they exist in the soil. Nor are the physiological
properties any more definite: they depend on the condition of
the organism—the newly germinated fungi behaving differently
from the older mycelium.

In these circumstances rigid identification is a matter of
great difficulty and it is impossible to reprehend too strongly
the practice, so tempting to pioneers, to describe forms as new
species, unless careful examination of their behaviour on
different media has-shown that they really are new.

Again, there is no method of estimating even approxi-

10. A, Pratt, fourn, Ag. Res., 1918, 13, 73-100.
2 For a discussion of the phenomena see W. B. Brierley, Some Concepts in
Mycology—An Attempt at Synthesis (Trans. Brit. Mycol. Soc., VI., part 2,

1gI9).
17

258 SOIL CONDITIONS AND PLANT GROWTH

mately the numbers of various fungi in the soil. When used
for fungi, the plating methods devised for bacteria and for
protozoa suffer from the drawback that they involve the break-
ing up of pieces of mycelium and the scattering of clusters of
spores, with the result that a single fragment of fungus in the
soil might appear as hundreds or even thousands of individuals
on the plates.

Finally, there is great difficulty in ascertaining the physio-
logical effects of the soil fungi. Because a fungus brings
about a particular reaction in a culture medium it by no means
follows that the same reaction will occur in the soil where the
conditions and the compounds are wholly different.

In these circumstances the only safe method is to ac-
cumulate observations and interpret them cautiously. The
existence of a fungus flora is undoubted, and it is largely con-
fined to the top 6 inches of soil. Formerly it was supposed
(and the view was emphasised by Ramann, 233@) that fungi
predominated in acid soils, and bacteria in neutral soils, but
the evidence is not beyond criticism: fungi can, however,
certainly produce and tolerate acidity. Little can be stated
as to, relative numbers in grass and arable land, or in poor and
rich soils, The species most frequently found in temperate
regions, and therefore presumed to be commonest, are the
Penicillia and Mucors: in addition Fusarium, Aspergillus,
Trichoderma and Cladosporium are common. In the warmer
Southern States, however (e.g. Texas), Waksman (292d) found
Aspergillus more frequently than Penicillium. In the absence
of quantitative methods of estimating numbers or of physio-
logical methods of estimating activity it is impossible to say
how fungal activity varies with soil conditions.

Like all other living organisms fungi require sources of
energy and of nitrogen. All species can obtain their energy -
supply from various carbohydrates, and most species can
obtain it from cellulose, in which respect fungi differ from
many bacteria. McBeth and Scales (185c) found the most
active cellulose decomposers to be the Penicillia, Aspergilli,
Trichodermze and Verticillia : on the other hand, the Mucorales

THE MICRO-ORGANIC POPULATION OF THE SOIL 259

apparently effect little or no decomposition of cellulose (292d).

The chemical changes involved are unknown, but it is unlikely
that the reaction would proceed by itself in the soil; in all
probability the soil bacteria would participate.

When ample supplies of carbohydrate (including cellulose)
are present, the most economical nitrogen nutrients are the
amino-acids (Czapek).1 In absence of sufficient supplies of
carbohydrate, Waksman shows that the fungi can obtain both
energy and nitrogen from soil protein compounds, but in this
case more degradation occurs than is necessary to supply the
nitrogen required, and consequently ammonia remains over
(292d). Under suitable conditions certain fungi (eg. Tricho-
derma Koningt) can produce ammonia more rapidly than
bacteria (McLean and Wilson, 188). Fungi have no power
of oxidising ammonia to nitrates. They can, however, readily
assimilate both ammonia and nitrates, and they have been
supposed by Ehrenberg (93c) to cause considerable locking
up of these compounds in the soil (see p. 210).

It was at one time thought that fungi could fix gaseous
nitrogen, but Duggar and Davis,’ in a careful series of ex-
periments, were unable to obtain evidence that any fungus
excepting Phoma Betae possessed this property.?*

It is impossible on present knowledge to assess the im-
portance of fungi in soil fertility. The decomposition of cellu-
lose is undoubtedly beneficial and probably justifies the old
view that fungi are the humus formers of the soil (Ramann,
233): on the other hand, the locking up of nitrogen compounds
is a disadvantage which, however, would be considerably
counterbalanced if the substance of the mycelium were readily
decomposable by bacteria on the death of the organism.

3. Actinomyces.

This group of organisms, sometimes included in the fungi
and sometimes in the bacteria, is of frequent occurrence in the

1 Beit. Chem. Physiol. u. Path., 1901-2, 1, 538; 2, 5573 3) 47-
2 Ann. Mo. Bot. Gard., 1916, 3, 413-437.
* According to Ternetz (F¥ahrb. f. wiss. Bot., 1907, 44, 353-408) Phoma
radicis can also assimilate gaseous nitrogen.
7."

260 SOIL CONDITIONS AND PLANT GROWTH

soil. Conn (700) estimated that they were one-quarter as
numerous as the bacteria. in arable soil and one-half as
numerous in grassland. It is very doubtful, however, whether
any counting method is reliable. Waksman and Curtis (292¢)
have made an extensive study of this group’ and have de-
scribed thirty or forty soil species: Apparently the actinomyces
decompose cellulose though they do not readily produce

ammonia from protein. It is said also that they can reduce
nitrates to nitrites.

.

4. Bacteria.

From time to time indications have been obtained that
some of the soil bacteria bring about changes harmful to the
plant, but the evidence is insufficient to justify any detailed
discussion. It has been supposed. that plant toxins are
produced (p. 247), that soil nitrates are assimilated (p. 210), x:
and that the food, air, and water which should otherwise be~ ‘
available to the plant are taken up by the micro-organisms.’ ‘

The subject that has been most frequently investigated is™
the part played by soil bacteria in the decomposition of the /
organic matter of the soil and the production of nitrates.
Both changes are brought about by bacteria and they are of
such obvious advantage to the plant as to suggest that re-
lationships ought to exist between bacterial activity and the
growth of plants. Several methods have been adopted to
trace such relationships.

The direct method consists in picking out definite
organisms and studying them in conditions calculated to
throw light on their action in the soil. This has proved very
difficult, and has been successfully achieved only by a few of
the best bacteriologists ; instances are afforded by the work of
Winogradsky (pp. 187, 196), Beijerinck (p. 197) and others.

Three indirect methods have therefore been used :—

1 For a morphological study see Charles Drechsler (Bot. Gaz., 1919, 67;
65-83, 147-168).
2 See, ¢.g., Dachnowski (Ohio), Eapt. Sta. Record, 1910, 23, 122.

THE MICRO-ORGANIC POPULATION OF THE SO/L 261

(1) Soil is inoculated into various media each arranged to
bring out one group of organisms, and the amount of decom-
position is taken as a measure of the number and vigour of
the members of the group. This is often called the method
of physiological grouping.

(2) Platings on gelatin or agar media are made of soil
suspensions suitably diluted, and the colonies which develop
are counted. The results are expressed as millions of bacteria
per grm. of soil.

(3) Chemical analyses are made at stated intervals to

‘determine the rate of progress of the various changes going
on—the absorption of oxygen, the evolution of carbon dioxide,
the production of nitrates, etc.

The difficulty with the first or direct ackhant is to imitate
the soil conditions, and the history of the subject affords
many instances of the danger of getting away from them:
for example, Krzemieniewski’s work on the nitrogen-fixing
organisms may be quoted (p. 199). It is impracticable, for
reasons already given, to keep to the soil as the medium for
work, and most investigators have therefore used the indirect
methods,

Physiological Grouping.— This method was introduced
by Remy (237a@) and developed by Léhnis (182); it has
become very popular. Four distinct media are in use,
arranged respectively to favour nitrification, ammonia pro-
duction, nitrogen fixation, and denitrification. The experi-
ments are easy to carry out, but they require skilful
interpretation, and the results may prove treacherous unless
carefully handled. The fundamental objection to the method
is that the reaction goes on in a medium very different from
ordinary soil, so that it throws no light on the relationships
obtaining in the soil itself. The results really only prove
that the bacteria from one soil will flourish better in a certain
artificial medium than those from another.

The medium for studying nitrification is usually that sug-
gested by Omelianski or Ashby’s modification (p. 188); it is
inoculated witha definite weight of soil and incubated: the

Fe

262 SOIL CONDITIONS AND PLANT GROWTH

nitrates produced after a certain time are determined. By
working under uniform conditions (which each investigator
fixes for himself) the results obtained are comparable for the
series of soils under investigation The amount of nitrate
produced by unit weight of soil is called the “nitrifying
power’’. The actual figure is obviously arbitrary, depending
on the conditions selected, and it has meaning only in relation
to the other soils in the same set of experiments. Several
investigators, however, have found that nitrifying power shows
some relation to plant growth, the soils most favourable to
plants having, on the whole, the highest nitrifying power.
This result has been obtained by Gainey in Kansas?; C. B.
Lipman in California*; G. P. Given in Pennsylvania *; Percy
Brown in Iowa (60a); P. S. Burgess in Hawaii (63); at
Rothamsted by Ashby (7a) ; and at Fallon, Nevada, by Keller-
man and Allen ®; some of the data are given in Table LX VIL.

TaBLE LXVII.—NuitTRIFYING Powers oF Various SoILs o—r KNOwN

PRODUCTIVENESS.
se ape <9) and pate Ba cteria 1
Ashby, itrifyin, itrifyin um i
Rothamsted (7a). Powsr. . abe not et Pues Millions
Productiveness). poy
Agdell Field, A. 3, Very productive 54 0°02
most productive 93 Pro- Plot 40 20 0°21
Agdell Field, A. 2, \ 8 ductive { Plot 190 36 0°003
intermediate f 3 plots Plot 290 30 0°27
Agdell Field, A. 1, a Plot ro 4 0°44
poorest Poor Plot 30 3 0°16
Plot 180 5 0°06

Withers and Fraps (313) have modified the method, and
use sterilised soil as the medium for the growth of, the

1¥For a statistical study of the magnitude of the error see D. D. Waynick,
Univ. Calif. Pub. Ag. Sct., 1918, 3, 243.

2 Soil Sci., 1917, 3, 339-416.

3 Proc. Soc. Prom. Agr. Sct., 1914, 33-39, and Cal. Bull., 260, 107-127.

4 Penn. Rept., 1912-13, 204-206.

5 Bacteriological Studies of the Soil of the Truckee-Carson Irrigation Project,
Karl F, Kellerman and E. R. Allen (U.S. Dept. of Agric. Bureau of Plant In-
dustry, Bull. No. 211, 1911). See also Ehrenberg (936).

THE MICRO-ORGANIC POPULATION OF THE SOIL 263

organisms. A careful distinction must be made between the
nitrifying power ascertained from culture media and the rate
at which nitrates accumulate in the ‘soil. The experiments
in culture media measure the rate of nitrification, under the
circumstances of the experiment: the accumulation of nitrate
in the soil, on the other hand, measures the rate of ammonia
production (p. 188).

The “ammonifying power” or “ putrefactive power’’ is
determined by inoculating soil into a I per cent. peptone
solution, and determining the ammonia formed after incubation
at 20°. Remy found that certain soils known to give good
crop returns for organic manures also possessed high putre-
factive power. He incubated for four days, but Russell and
Hutchinson (241c) obtained better results by taking definite
intervals and plotting curves showing the respective rates of
ammonia production by the different soils. Léhnis has used
this method a good deal (1814) as also has J. G. Lipman, who,
however, modifies it considerably, and among other things
uses sterilised soil as the medium and substitutes dried blood
or cotton-seed meal! for peptone (176). Percy Brown (60)
used a similar modification in his studies of Iowa soils, and
found that the “ammonifying power” ran along with the
“nitrifying power” and, in four out of the six plots, with the
crop-producing power also (Table LX VIIL.).

Other workers have observed a general similarity between
ammonifying power and productiveness which, however,
frequently breaks down in individual cases. Further, the
differences between good and poor soils are not particularly
marked and would often be considered to lie within the error
of the experiment. The relationship with productiveness is
therefore less definite than in the case of nitrification.”

1 Sackett (Colorado Bull., 184, 1912) has shown that these cannot be used
indiscriminately ; in some soils dried blood is ammonified more rapidly than
cotton-seed meal while in others the reverse holds.

2For evidence to this effect see P. S. Burgess (63), Gainey, Soil Sci.,
1917, 3, 399; Kelley, Science, 43, 30-33, and Hawaii Bull., 37, 1915, p. 52;
Stevens and Withers (271); J.G. Temple, Ga. Bull., 126, 1919.

264 SOIL CONDITIONS AND PLANT GROWTH

TasLe LXVIII.—Bacreriav Activity IN SoILs oF KNown Paonveriy ae
P. E. Brown (60a). Iowa SoIts.

Yield of Nive Bacterial
Plot | Maize. Histor Ammonifying] Nitrifying Finn, 1 Numbers.
No. | Bushels y Power. Power. hice Millions
per Acre. “| per grm.

607 | 52°7 |2 year rotation, maize,
oats, clover ploughed

i é ‘ 4 175 oie 14°3 2°8
604 | 50°7 |3 year rotation, maize,

oats, and clover - 189 12°6 20°6 3°3
602 | 46:0 | 2 year rotation, maize

and oats . x « 178 81 175 2°6
gor | 43°2 | 2 year rotation, maize,

oats, rye ploughed in 175 67 14°3 2°5
601 | 35°5 |Continuous maize . I7I 5°0 Q°5 ay

609 | 32°55 |2 year rotation,
maize, oats, cowpeas
ploughedin . : 180 II‘9 18°2 27

P. S. Burcess (63). Hawa Soits. (Plots Grouped in Order of Merit.) —

Best. | Very Good—not Placed in Order. Poorer. Least.

Yield ofsugar: Plot No.| 2 I 8 4 6 3 5 7 9

Ammonification: dried
blood: Plot No. Ry Fare: I 9 4 6 8 3 7 5
Megrms.- of added
nitrogen ammoni-
fied after 10 days|100°2 |76°4 75°99 74°5 68°6 57°7 |57°I 47°6 | 26°6
Alfalfa meal . 2 9 3 6 4 8 I — 5
Mgrms. of added
nitrogen an moni-

fied after 10 days| 14°38 |14°0 129 «Ir2 g% og'5 | 67° — I"4
Nitrification: dried
blood 2°23 4&8) te

Megrms. of added ni-
trogen __nitrified
after 30 days .| 20°38 |20°0 180 17°2 16°38 15°2 |13% 4°0 | 4°0
Alfalfa meal. : 2 7 4 3 8 6 I 5 9
Megrms, of added
nitrogen nitrified
after 30 days .| 152 | — 128 12:0 ro70)§6©99'6 | go 72 | 4°5
Nitrogen rendered water
soluble: dried blood| 9 2 6 3 4 8 7 I 5
Mgrms, of nitrogen re-
covered after 20 days} 31°I |29°4 27°0 24°4 23°9 22°38 |22°3 18°3 | 10°3
Nitrogen fixation (man-
nite solution) y 2 8 6 4 3 I 7 9 5
Mgrms. nitrogen fixed
per grm. of mannite | 11°20] 10°64 7°28 7°00 6°44 5°60] 3°92 3°60} 2°80

No
Azoto-
bacter.

Good Azotobacter surface membrane, Doubtful
Azotobacter.

THE MICRO-ORGANIC POPULATION OF THE SOIL 265

“ Nitrogen-fixing power” is measured by inoculating soil
into Beijerinck’s or some similar solution (p. 197) and in-
cubating for a definite time. This reaction proceeds only
slowly. The results usually show a general resemblance to
those given by nitrification experiments.’

Bacterial Counts.—The method of counting the number of
colonies that develop on gelatin or agar plates is admittedly
faulty, but it has the advantage of showing whether the
numbers are high or low and whether they are increasing or
decreasing. It has, unfortunately, three serious defects. No
medium is known that brings out all the soil organisms, so
that the results are invariably low,? and their quantitative
appearance is wholly illusory. No medium even distantly

resembles the soil in composition or in structure, so that thé — ld

flora developing on the plates does not necessarily reflect the
_ flora active in the soil; in particular it is impossible to tell
which of the forms developing on the plate are active and
which are spores in the soil. Account is seldom taken of the
kinds of bacteria on the plates; in practice it proves far too
laborious to attempt any but the simplest identifications.
This disregard of the nature of the bacteria constitutes a
fundamental distinction from the method dependent on physi-
ological grouping, and the two methods do not always give
similar results. ‘The counts show fairly correctly whether
any given treatment of the soil has raised or has lowered the
number of bacteria, but unless the change has been drastic
they do not show whether all varieties have been equally
affected. Thus they have always to be combined with deter-
minations of the amounts of ammonia and nitrate in the soil.
Chemical Analysis in Conjunction with Bacterial Counts.—
The third method of making bacteriological counts in con-
junction with chemical analysis, has been largely used in the
Rothamsted laboratories. Increases in bacterial numbers
are so often associated with increased production of nitrate as

1E.g. see papers by P. E. Brown; also P. S. Burgess.
2 Comparisons of various media have been made by Cook (Soil Sci., 1916,
I, 153-161), The more uniform the results the better the medium.

266 SOIL CONDITIONS AND PLANT GROWTH

to justify the assumption that the two phenomena are causally
connected. Two cases have, however, been studied where no
such relationship exists,

(1) The soil treatment, while raising the total numbers,
has either acted differentially on the organisms and did not
encourage the ammonia producers to develop, or it has caused
them to effect some decomposition that does not give rise to
ammonia. (See p. 186.) The addition of certain organic
compounds to the soil has this effect (Table LXIX.).

TaBLeE LXIX.—ErFrect oF CERTAIN ORGANIC SUBSTANCES ON BACTERIAL
NUMBERS AND ON NITRATE PRODUCTION.

Bacterial Numbers} Ammonia and
After 50 Days Nitrate Present
(millions per grm.).| After 50 Days.

Substance Added. . 2 Observers.
In In In In ¢
Control |} Treated | Control | Treated
Soil. Soil. Soil. Soil.

Russell and
Cane sugar (0°25 per cent.)| 21 51 32 20 |Hutchinson (240a)
Amylalcohol (o'r per cent.)| 30 85 37 35 Buddin (64a)
Phenol (M/200 per kilo)! .| 27 ror 30 33 :;
Hydroquinone (M/200 per
kilo). ‘ ‘ ;

16 55 35 44 ”

(2) Even when the ammonia-producing organisms are
caused to multiply they do not increase the stock of ammonia
and nitrates in the soil beyond a certain limiting amount.
Thus partial sterilisation increases bacterial numbers and
usually increases the amount of ammonia and nitrate also, but
it fails to do this after a certain quantity has accumulated ~
(Fig. 24).

On looking over the figures in Tables LX VII. and LX VIII.
(p. 262) it is evident that the numbers of bacteria revealed by
this method bear no relationship to the amount of crop growth.
Other field experiments have given similar results. Yet in
laboratory and pot experiments bacterial counts have often

1 See p. 212.

THE MICRO-ORGANIC POPULATION OF THE SOIL 267
NH, and Nitrate.

Bacterial Numbers.
Limit
200+ 200r
oll
“ solk ng ene? is ae
ene” segue Tou abe
mere" cc UNTREATED SOIL
100F prc 100 re "
U a
| UNTREATED Sorc.
Mf
/
UG
4 n j j J i L 1
19 40 70 130 19 40 70 130
Days

Case 1.—Small amounts of NH, and nitrate initially present. A relation-
ship is indicated between bacterial numbers and the rate of production of NH,

and nitrate.
Bacterial Numbers, NH, and Nitrate.
250+ TOLUENED Soit re Limit
“ie AN ES al come 500 soll
' Pi,
! aS
' at
!
/
150} | une
;
!
| UNTREATED Soi
i
bas uy —s-100F
Yio 14 110
Days
No

4
Case 2.—Large amounts of ammonia and nitrate initially present.

relationship like that in Case 1 is indicated.
Fic. 24.—Relation between bacterial numbers and amounts of nitrate and

ammonia formed.

268 SOIL CONDITIONS AND. PLANT GROWTH

proved most valuable; numerous instances are given in the
~ Rothamsted papers on soil sterilisation (241).

This discrepancy between field and laboratory experience
is cleared up by a closer examination of the nature of the
relationship between bacterial activity and plant growth.
The connection, as already pointed out, lies in the fact that
bacteria decompose the organic matter of the soil and make
new plant food out of old plant residues. If the factor
limiting plant growth happens to be the supply of nitrogenous
plant food or the rate of decomposition of the plant residues
we may expect to find a close connection between bacterial
activity and soil fertility; iff on the contrary, the limiting
factor is something else—such as water supply, lack of phos-
phates, etc.—no such connection is necessary. Even here,
however, aconnection may exist, for bacteria-are living things,
affected by the same circumstances that influence plants.

Three distinct cases, therefore, arise :—

(1) Bacterial activity may show no sort of relationship with
soil fertility, because fertility is limited by some factor other
than the nitrogen supply or rate of decomposition of plant
residues.

(2) Bacterial activity may be directly related to soil
fertility but the relationship is only accidental, both bacteria
and plants being affected by the same limiting factor.

(3) Bacterial activity may be directly related to soil
fertility and the relationship is causal, fertility being limited
by the amount of ammonia and nitrate produced by the
bacteria or by other decompositions which they effect.

Instances of the first are common in arid and semi-arid
districts.

The second case is not unfrequent. An admirable illustra-
tion is afforded by the experiments of Crowther and Ruston
on the effect of acid rain-water on plant growth (71). The
pots were watered with solutions of sulphuric acid, some being
of the same order of concentration as the Leeds rain-water. The
acid depressed the growth not only of plants but of bacteria
also, and the effect is very similar in both cases (Table LXX.).

THE MICRO-ORGANIC POPULATION OF THE SOIL 269

Taste LXX.—ErFrect oF ACIDULATED WATER ON THE GROWTH OF PLANTS

. AND BAcTERIA. CROWTHER AND RusTON (71).

a bo a. wee

Bele |y |e. lee.| fe GEES

= ime SS & au ho

S. | 32 | SE | gf | $225) <& [3285

om} £2 5O 0° bESu ag Bom &

rs Gel eee | em ee SS Bl) So |Sere

aS § Zz 5 az ogee “638
§2 < 3 5 Een
<i

Soil watered with Garforth

rain-water, neutralised 14°7 \|) £339: | 1°02} 4°6, | ‘5°2 I2 9°3
I part H,SO, per 100,000 | 12°0 | 12°I | 0°80 | 3°3 | 1°3 8 6°2
2 99 ” ” 8'0 112 0°85 3°0 ivr 6 82
4 » ” ” 3°9 | 10°5 | 0°52 | 2°38 | 07 4 8°1
8 4 ”» ” 3°7 | 10°3 | 0°36.) 2°4 | or 4 8°4
Gi 55 + i nil | 10°3 | 0°28 | I°9 | 0°04 3 8°1
Bai ss ‘a es nil S°E: | Of13> 1} PS. O°015 o 9°4

At first sight this looks like a close relationship between
bacterial activity and plant growth. But the figures in the
* last column show that the failure of the crop is not due to the
failure of the bacteria to produce ammonia and nitrate, for
relatively large amounts of these substances are left at the end
of the experiment.

In similar manner the growth both of bacteria and of
plants may be helped by the same cause. This is more fully
discussed in the following section. Speaking generally, it is
found that the bacterial numbers increase as the intensity of
the farming increases. Thus Stoklasa and Ernest (273a)
found only 1 to 2 million organisms per grm. in their barley
land, 3 to 5 millions on the better treated sugar-beet land,
and 7 to8 millions on the clover land. Again, the addition
of plant residues or of farmyard manure to the soil increases
the bacterial numbers by furnishing the organisms with addi-
tional food;* it also increases the crop. Moorland soils
contain only few bacteria and are very unsuited to the growth
of most plants. But after cultivation and treatment with
lime and manures they become much better media both for
plants and bacteria. Fabricius and von Feilitzen (974) found

1 Examples are given by P. E. Brown, Jowa Research Bull., No. 13, 1913,
and by Heinze, Landw. Fahrbiicher, 1910, 39; Ergdnz. Bd. 3, 314-343.

270 SOIL CONDITIONS AND PLANT GROWTH ©

o'r million of bacteria per grm. in the raw moorland soil,
but 7 millions in similar soil that had been cultivated and
manured.
So close is the similarity between ordinary crops and
azotobacter in relation to soil acidity that an azotobacter test
\ ; aie i
is used in Denmark for determining the need of lime (p. 242).
It may often be difficult in practice to determine whether
the relationship between the bacterial numbers and plant
growth is causal or accidental, but the principle is perfectly
clear; the relationship is causal only when the plant growth is
limited by the supply of compounds produced by bacterial
activity, or the rate at which plant residues or harmful
substances such as phenol, thiocyanates, etc., are decomposed
by bacteria. The recognition of this central principle greatly
facilitates investigation, for it shows the futility of haphazard
attempts to correlate bacterial activity and plant growth over
a set of soils that are not strictly comparable. The better
course is to narrow down the problem and confine it to the
elucidation of the connection between bacterial activity and
nitrate production.

Effect of Soil Conditions on Bacterial Numbers and on
Nitrate Production.

On general grounds it might be expected that the soil
bacteria would be affected by external conditions in much
the same way as plants, and to a considerable extent this
happens. Most of the effects observed with growing plants
have been paralleled in the case of the soil bacteria. There are,
however, nearly always differences between the effects produced
on the bacterial numbers and on the amount of work done:
one may be increased but not the other. Discussion of the
effects of external conditions on bacterial numbers is much
hampered by the paucity of data. Observers have often been
content to make counts once in twenty or thirty days, and
they have usually assumed that changes in numbers are slow.
Cutler and Crump have shown (73c) that this supposition
is unfounded: changes proceed rapidly from day to day.

Soil + Ammonium Sulphate.

100r Temp. 15°C.
023 8
i wi acs 29 20%

bes]
i=)

mS
Pr} 64:5
= 60F
“3 90995%
ie
=
~ 40F SON ox
ia 2 )
fe?)
=
Y
= ; 23:8
E 20
oO
od
© Yosye 26 66 a sa
Additional Nitrate found in Soil alone.
| Loam Gdéttingen)
80, Temp.13°C.
6ot
40} EAE Segue ed Cm aye iC Be
20 %
+ an ee 25%
10%
3% 5%

Days 26 66 100

Fic. 25.—Effect of moisture on nitrification process in the soil (Traaen).

272 SOIL CONDITIONS AND PLANT GROWTH

Inspection of their curves shows that reliable conclusions
cannot be drawn when counts are taken at long intervals,

Effect of Temperature.—Bacteria being living organisms it
is natural to expect that their activity increases with the
temperature up to a certain point. The amount of nitrate
does show this expected increase but the bacterial numbers do
not, there being no steady rise as the temperature of storage
increases (Table LXXI.). Field observations lead to the same
conclusion (p. 275).

TABLE LXXI.—Errecr oF TEMPERATURE OF STORAGE ON, BACTERIAL
NUMBERS AND NirraTE PropucTIon. RussELL AND HuTCHINSON (2414).

Bacteria, Millions per Grm. Nitrate and Ammonia, Paris
of Dry Soil. per Million of Dry Soil.
Temperature
of Storage.
At -After After At After After
Start. | 10 Days. | 50days. | Start. | ro Days. | 50 days.
7°-12° 16 16 16 17 18 22
20° 12 21 16 30
30° 15 14 24 36
40° 9 14 55 76

Effect of Moisture.—Increasing moisture supply causes in-
creases in bacterial numbers, but they are not regular (2410).
Field observations give the same result (p. 275). The rate
of nitrate production increases up to a certain point, beyond
which it decreases, presumably because of the lack of air.
Traaen’s results are plotted in Fig. 25.1 On the practical
side it is found that application of irrigation water in arid
regions has a distinctly beneficial effect on the ammonifying
and nitrifying powers, both of cropped and fallow soils, and
on the numbers of organisms on fallow soils. Excess of water,
however, washes out the resulting nitrate from the soil and so
deprives the plant of the advantage it would otherwise gain.
(J. E. Greaves, R. Stewart, and C. T. Hirst (1130).)

Effect of Added Organic Matter.—As shown on p. 252 the

1 Centr. Bakt. Par., 1916, 45, 119. Other results are plotted in Greaves and
Carter’s paper (113d), where also a full bibliography is given.

nn

«

THE MICRO-ORGANIC POPULATION OF THE SOIL 273

effect of added organic matter is to increase supplies of energy
and therefore to increase bacterial numbers. The question
whether nitrate supplies will be increased depends on the pro-
portion of nitrogen present in the added matter: if it is rich
in nitrogen a considerable amount of nitrate may be formed ;1

if it contains no nitrogen there arises an actual loss of nitrate

though there may be a fixation of gaseous nitrogen, The
special case of farmyard manure has been much studied.’
Effects of Lime, Calcium Carbonate, and Magnesium Car-
bonate.—Numerous papers have been written on this subject
and the results are at first sight somewhat contradictory.
Much of the work has been done on acid soils, where, as
might be expected, benefit has been derived from neutralisa-
tion. F. E. Bear® finds that calcium carbonate greatly in-

‘creases the numbers of bacteria in acid soils once the neutral

point is passed, but not before: it much increases ammonifi-
cation and nitrification up to the neutral point, but to a less
extent afterwards. In neutral soils, however, less concordant
results have been obtained; some investigators have observed
detrimental effects from further addition of calcium carbonate,*

while others have obtained only beneficial results,® bacterial

numbers, ammonifying power and nitrifying power all being
increased. Magnesium carbonate may be more effective than
calcium carbonate in small quantities, but it is toxic in larger
amounts. Lime in excess of a certain amount acts as a
sterilising agent (139c).

1 For the effect of the N:C ratio see P. E. Brown and F. E. Allison, Soil
Sci., 1916, 1, 49-75; and also H. B. Hutchinson and J. Clayton (139/).

2P. 193, also T. G. Temple, Georgia Rpt., 1916; Greaves and Carter,
Fourn. Ag. Res., 1916, 6, 889.

® Soil Sci., 1917, 4, 433-

4E.g. J. G. Lipman, P. E. Brown, and I. L. Owen, Centr. Bakt. Par., 1911,
30, 156-181.

5S.S. Peck, Hawaiian Sugar Planters’ Chem. Bull., 34,1911; P. E. Brown,
Iowa Research Bull., 2, 1911; Bull., 44, 1918; J. E. Greaves, Soil Sci., 1916,
2, 443-480; H. L. Fulmer, ¥ourn. Agric. Res., 1918, 12, 463-504, although he
obtained harmful effects on nitrification.

$j. G, Lipman and P. E. Brown, N.}. Agric. Expt. Sta. 28th Ann. Reft.,

141-204: C. B, Lipman and P. S. Burgess, fourn. Agric. Sci., 1914, 6, 484-498 ;
H. L. Fulmer, fourn. Ag. Res., 1918, 12, 463.

18

274 SOIL CONDITIONS AND PLANT GROWTH

Effect of Salts!—Alkali salts adversely affect bacteria
somewhat as they do green plants: in India a sufficient de-
gree of similarity exists to allow the wheat-yielding power
of an alkali soil to be estimated from such bacterial ©
activities as CO, production, ammonification, etc.? The prob-
lem has been much investigated in the Western United
States where alkali salts are apt to cause trouble. System-
atic investigations have been made by C. B. Lipman in
California (1750) and by J. E. Greaves and his co-workers in
Utah (113). Typical results are given in Table LXXII. The
ammonifying organisms are usually stimulated by small con-
centrations of ‘‘alkali salts” but adversely affected by larger
ones, though they are less susceptible than wheat seedlings
The effect is not constant but varies with differences in soil
and conditions.* As a general rule chlorides are the most
toxic salts, while nitrates, sulphates, and carbonates are suc-
céssively less toxic; it is suggested that the electronegative
ion plays the more important part (113). Nitrifying organ-
isms, however, are more susceptible than ammonia producers
and the effects are determined by the specific compound
rather than by one ion. There is also a well-marked anta-
gonism of ions (eg. Ca and K; Mg and Na; K and Na;
(Na,)CO, and (Na)Cl)* as in the case of green plants (p. 79).
Salts of arsenic,’ copper,® lead, zinc, and iron appear to be
capable, in suitable concentration, of stimulating nitrifying
organisms but not ammonification. Nutritive salts have a
marked effect. Phosphates notably increase all types of bac-
terial activity in the soil:’ potassium salts have acted well in

1For a summary of the extensive literature see J. E. Greaves, Soil Sci.,
1916, 2, 443-480.

2 J. H. Barnes and Barkat Ali, Ag. ¥ourn. India, 1917, 12, 368.

3 W. P. Kelley, fourn, Agric. Research, 1916, 7, 417-437.

‘C. B. Lipman, Bot, Gaz., 1909, 48, 106, and Centr. Bakt. Par., 1914, 41,
430-444 (with P. S. Burgess); see also J. E. Greaves, Soil Sci., 1920, 10, 77-102.

5J. E. Greaves, Science, 1917, p. 204.

°C, B. Lipman and P.'S. Burgess, Univ. Cal. Pub. Agric. Sci., 1914, 1,
127-139.

7E. B. Fred and E. B. Hart, Wisconsin Research Bull., 35, 1915; and
Centr, Bakt. Par., I1., 1916, 45, 379; G. P. Koch, ¥ourn. Biol. Chem., 1917, 31,
411; see also p. 203.

THE MICRO-ORGANIC POPULATION OF THE SOIL 275

certain soils,! but not on all. Nitrates increase the amount of

ammonification,? the growth of the organisms causing decay

(176a), the nitrogen fixing azotobacter (p. 199) and of nitrate

assimilating organisms (113c), though in higher quantities

they become toxic.

TaBLE LXXII.—PeERcENTAGES oF VaRIoUS SALTS IN LOAM SOIL WHICH ARE
NECESSARY TO REDUCE AMMONIFICATION, GERMINATION AND Dry MATTER

PRODUCED IN WHEAT TO aBouUT HaLF Norma. J. E. Greaves, E. G.
CarTER, AND H, C, GoLDTHORPE (II3a AND ¢).

Wheat Seedling | Ammonification Nitrification
to Half Normal. | to Half Normal. | to Half Normal.
Sodium chloride . . 0°20 O°1L7 0°234
Calcium sd ‘ ; *30 222 —
Potassium ¥ ; ! "25 *298 *298
Magnesium _,, , f *40 “381 006
Potassium nitrate . 3 *40 607 *IOL
Sodium Pe p : *30 "850 ‘I70
ee sulphate. °. 55 "852 568
Magnesium ‘s ; ‘ *70 "963 "361
Sodium carbonate . i *30 1°166 "212
Magnesium nitrate . 5 "45 1°187 074
Potassium sulphate . : “60 I°394 "349
4 carbonate . : "70 1'520 138

Effect of Dissolved Oxygen Supply.—Russell and Apple-
yard (241g) found that rainfall has a more definite effect than
moisture in increasing bacterial numbers and_ biochemical
changes in the soil. They attribute this action to the oxygen
dissolved in the rain-water which renews the dissolved atmos-
phere in the soil and gives the organisms a new lease of
activity.

Effect of Previous Treatment of the Soil.—Prolonged drought
affects the soil even after it has passed away and the soil has
become moistened. The rate of production of nitrate and the
bacterial numbers both increase (24c¢ and also Prescott *).

Field Observations.—The general phenomena observed in
the laboratory can be seen also in the field, but it is less easy
to disentangle the various factors.

1J. Dumont, Compt. Rend., 1897, 125, 469-472.

2D. A, Coleman, Soil Sci., 1917, 4, 345. .

* A note on the Sheraqi soils of Egypt (Fourn. Ag. Sci., 1920, 10, 177.)
18 *

276 SOIL CONDITIONS AND PLANT GROWTH

As in the laboratory experiments (p. 272) the bacterial
numbers show no relation to the temperature and moisture
content of the soil. Numerous counts were made by Hiltner
and Stormer (135c¢) from plots of cropped ground, and of un-
manured and dunged fallows. Some of these are recorded in

Table LXXIIL.

TaBLE LXXIII.—Bacreria IN CRopPED AND FaLLow Solis, MILLIONS PER
GraM. HILTNER AND STORMER (135¢).

10 May. | 27 Aug.| 18 Oct. | 1 Feb. | 12 June. | 18 Aug.

Cropped land, grass and clover | 8°3 ER 6°4 | 6°6 81 4°9
Cultivated fallow, unmanured . 8°0 4°2 4°0 | 4°I 5°7 4°I
* 3 dungedt.. 1 2 E0 8, ross Ere) gs 7°2 84

The only marked effect is that of the dung; the net result
of the clover and grass has only been small in spite of the
organic residues shed by the roots. On no plot has the
warm summer weather increased the bacterial numbers.

Later on Engberding (94) made a more extensive series of
counts of the bacteria in plots of ground under known treat-
ment and published his results in very complete form, giving
details of temperature, moisture content, etc. Here again no
connection could be traced between temperature or moisture
content and bacterial numbers. A very similar result was
obtained by W. E. King and Doryland (148). Neither tem-.
perature nor moisture changes explained the observed fluc-
tuations in bacterial numbers.”

Some of Waksman’s results (292a) with a garden soil at
New Jersey are given in Table LXXIV.

Russell and Appleyard (241g) have studied the problem
in some detail at Rothamsted, and have determined the
bacterial numbers, and the nitrate in the soil, and the amount

1 Dung applied in July at the rate of 130 to 140 Centner pro Morgen (10 to
II tons per acre).

2In these exptriments, however, one cubic millimetre only of the soil sus-
pension was used for the plate cultures.

THE MICRO-ORGANIC POPULATION OF THE SOIL 277

of CO, in the soil air, at frequent intervals for a period of two
seasons on several of the plots. The experimental data do not
represent the amounts of production but only of accumulation ;
nevertheless the curves are found to give useful information as
to production.

TaBLeE LXXIV.—BacTERIA IN GARDEN SoiIL, NEw JERSEY, MILLIONS PER
GraM. WAKSMAN (292a).

Bette ay e)sl|etsl@l | «| Blas
Date of Sampling as < & S o > 8 2 w]e 2 EIS E
S| & = |s < = =/ei1<1/316 y &
Nos. of bacteria,
millions per grm.| 8°7 | 6°0 | 88 | 4°3 | 10°7 | 4°8 | 69 | 7°8 | 5°70 | 88 | 5°9 | 69 | ov2
Moisture content,
per cent. 15 | 14 13 8 9 9 9 9 12 Io 9 9 | 19

These figures are for the top inch: samples were taken at six different
depths with similar results.

In the first instance it is observed that the curves are all
sufficiently alike to justify the view that in the soil, as in the
laboratory, the phenomena of nitrate production and CO,
evolution are closely related to the numbers of bacteria. When
the bacterial numbers rise there is a rise in the amount of COg
and of nitrate. But the nitrate curve does not sharply agree
with the others: it is displaced, showing a lag of two or three
weeks.

The general results are illustrated by the curves in Fig. 26.
During the winter months there is very little activity. But
as soon as the temperature rises above 5° C. change sets in:
bacterial numbers, CO,,' and nitrates all increase. The rise,
however, is not long sustained; it is followed by a fall, not-
withstanding the continuance of favourable temperature con-
ditions. To some extent this is due to lack of moisture, for
the curves now begin to resemble the soil moisture curves.
It is also due to lack of something supplied by rain—presum-
ably dissolved oxygen—for the rainfall curves more closely fit
the CO, and bacterial number curves. This period of summer

1 The very sharp rise of CO, in May and August appears to be associated
with the crop (241g).

oP Pe:

(‘S1bz ‘prefajddy pur jjassny) ‘“1ea4 ay} Jo
_ SUOSEAS JUDIOYIP 3¥ [0S UT Saquinu [¥1I9}0"q PUL ‘[IOS UT 238x31U JO ‘IIe []OS U} OpIxorpuogi¥d Jo SjuNOUTY—"9z “OI

‘elayoeg Wey vad suonpiyy

SOIL CONDITIONS AND PLANT GROWTH

278

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THE MICRO-ORGANIC POPULATION OF THE SOIL 279

sluggishness is followed by one of considerable activity in
autumn and this in turn by the period of winter inertness,
The growing crop appears to affect the biochemical changes
(p. 296). . :

These periods of spring activity, summer sluggishness, and
autumn activity seem to be fairly general, and they have been
recorded elsewhere.

Using Remy’s method of physiological grouping Léhnis
and Sabaschnikoff at Leipzig (181e) obtained a curious and
wholly unexpected set of curves suggesting some remarkable
seasonal relationships. The urea-decomposing power, nitrify-

.ing power, nitrogen-fixing power, and to a less extent the

denitrifying power, all reached a maximum in spring, a
minimum in summer and a maximum again in September,
Miintz and Gaudechon (209) also showed by a somewhat
different method that the nitrifying power is at a maxi-
mum in spring. Conn (70) obtained a similar curve for
the bacterial numbers in his plots: the numbers of bacteria
being high in February when the land was frozen, they fell
in summer but rose again in autumn (Fig. 27). Brown and
Smith at lowa (60d) also found the highest numbers in frozen
soils, but Wojtkiewicz (314), on the other hand, found the
maximum later on in spring, the winter numbers being much
lower.

Leather (1674) and Jensen (144) have obtained parallel
results for the amount of nitrate in the soil.

Reviewing the whole of the preceding results, it seems
clear that in normal soils we are dealing with something more

than a bacterial population. The seasonal fluctuations may

represent some deep-seated phenomenon: seasonal fluctua-
tions also occur in the plankton of the sea.'| But the erratic
results obtained with changes in temperature and moisture
suggest that other organisms are also present interfering with
the activity of the soil bacteria. This view arose out of the
work on partial sterilisation which revealed the presence of

1See W, A, Herdman, Pres. Address Brit, Ass., 1920.

25

20

280 SOIL CONDITIONS AND PLANT GROWTH

another great group of organisms, and which we must now
proceed to discuss.
Investigation on the Partial Sterilisation of the Soil.

The earliest observations that soil is altered by an ap-
parently inert antiseptic arose out of attempts to kill insect

Le Réveil de la Terre (Miintz and Gaudechon)
Rate of Nitrification.

1600
1200
800
Ul
400 ;
ae | | | |
Feb Apri) May Feb. April
“ Terre 9 Terreau
April | May | June | July | Aug.| Sept.) Oct. | Nov.| Dec.| Jan.| FebA,Mar. | April
/\ \
A fa
} \
wi Bd \ J \
N 4 Mi J ‘
. n Bh \ | 4B
tee 4 at 7 \
Bet et Shoat NESE Cee Ea V4
. Sie Og
~,
ee ‘IB

Bacterial numbers in soil: plots 18 and 48. Cropped with millet: unmanured.
(The curve showing moisture content is very similar to that for 48.) (Conn.)

Fic. 27.—Bacterial activity in soils at different seasons of the year.

pests in the soil by means of carbon disulphide. This sub-
stance, which for fifty years has been known as an insecticide,
was used in 1877 by Oberlin,! an Alsatian vine-grower,

1 Boden-Miidigkeit und Schwefelkohlenstoff, Mainz, 1894.

THE MICRO-ORGANIC POPULATION OF THE SOIL 281

to kill phylloxera, and by Girard! in 1887 to clear a
piece of sugar-beet ground badly infested with nematodes,
In both cases the subsequent crops- showed that the pro-
ductiveness of the soil had been increased by the treatment.

The first piece of scientific work came from A. Koch
in 1899 (151a@), who, working with varying quantities of carbon
disulphide, concluded that it stimulates the plant root to in-
creased growth. Four years later Hiltner and Stormer (135c)
showed that the bacterial flora of the soil undergoes a change.
The immediate effect of the antiseptic was to decrease by
about 75 per cent. the number of organisms capable of
developing on gelatin plates; then as soon as the antiseptic
had evaporated, the numbers rose far higher than before, and
there was also some change in the type of flora. It was
argued that the increased numbers of bacteria must result in
an increased food supply for the plant, and it was claimed that
the new type of flora was actually better than the old, in that
denitrifying organisms were killed, nitrogen-fixing organisms
increased, and nitrification only suspended during a period
when nitrates were not wanted and might undergo loss by
drainage. In a later publication Hiltner (135@) shows that
other volatile or easily decomposable antiseptics produce the
same effect. The important part of this work is unquestion-
ably the discovery that the organisms in the treated soils
ultimately outnumber those in the original soil.” ‘ The hy-
pothesis that the new type of flora is actually more efficient
than the old rests on less trustworthy evidence, and has indeed
been modified in some of its details by Hiltner himself.

The effect of heat on the productiveness of the soil was
first noticed by the early bacteriologists. It had been assumed
that heat simply sterilised the soil and produced no other
change, until Frank (100) in 1888 showed that it increased

1 Bull. Soc. Nat. d’Agric., 1894, 54, 356.

2 Sewage investigators have found that the disinfection of sand filters leads
to increases in the numbers of bacteria. This was observed in the typhoid
epidemic at Lincoln in 1904 and also in the experimental sewage filter at Guild-

ford in 1907 (Houston and McGowan, Fifth Report, Sewage Commission»
Appendix 4, Cd. 4282, rgro, p. rrr).

282 SOIL CONDITIONS AND PLANT GROWTH

the soluble mineral and organic matter and also the produc-
tiveness. Later work by Pfeiffer and Franke (2246) and by
Kriiger and Schneidewind (1562) showed that plants actually
take more food from a heated than from an unheated soil.
Heat undoubtedly causes decomposition of some of the soil
constituents, quite apart from its effect on the soil flora; it
also produces physical effects; all these actions probably
play a part in determining the increased productiveness of
heated soils.

Other explanations of the effects of partial sterilisation
have been put forward. Pickering (2264) finds that the
amount of soluble matter in the soil is increased by treatment
with volatile antiseptics. Greig Smith supposes that soils
contain a harmful waxy material which he calls “ agricere”
and which he supposes to be removed during partial sterilisa-
tion. Russell and Hutchinson were unable to confirm this
at Rothamsted, nor could J. P, du Buisson at Cornell, who
found that extraction with volatile antiseptics gave no better
results than addition of liquid and subsequent evaporation.?
Bolley supposes that soils normally contain parasitic fungi
which are destroyed by partial sterilisation. Russell and
Hutchinson (241¢) consider that the soil population is complex
and that some of its numbers act detrimentally on the bacteria
which produce- plant nutrients: these detrimental forms are
more readily killed than the useful bacteria, with the result
that the new population produces more ammonia and nitrate
than the old one. The detrimental organisms are provision-
ally identified with protozoa (p. 288).

It is agreed by all who have seriously studied the subject
that the effects produced by partial sterilisation are complex :
several factors operate simultaneously. In consequence, a
particular factor cannot be regarded as established until its
operation has been proved by some wholly independent

1For chemical effects see 2416 and also Pickering, fourn. Ag. Sct., iii.,
171-178; Seaver and Clark, Biochem. Bull., 1912, 1, 413; and Schreiner and
Lathrop, U.S. Dept. of Ag. Bureau of Soils, Bull. 89, 1912. For physical
effects see Czermak, Landw. Versuchs-Stat., 1912, '76, 75.

2 Soil Sct., 3, 353-392.

THE MICRO-ORGANIC POPULATION OF THE SOIL 283

method. There has been much discussion of the view
expressed by Russell and Hutchinson, and many investigators,
eg. J. M. Sherman (261), do not admit the presence of any
biological factor in soils detrimental to bacteria. The evidence
in favour of the view will therefore be set out in some detail.

The writer’s investigation on this subject began in the first
instance as the result of an accident. In virtue of its large
population of micro-organisms soil absorbs a considerable
quantity of oxygen, and evolves a corresponding amount of
carbon dioxide. An experiment had been arranged to de-
-monstrate the well-known fact that soil heated to 130° C.,,
and therefore completely devoid of micro-organisms, lost much
of its power of absorbing oxygen. By an accident the auto-
clave was not available; the soil was therefore heated only in
_ the steam oven, and it gave the remarkable result that its
power of absorbing oxygen instead of falling, as was expected,
considerably increased. Now the steam did not kill all the
organisms, but spared those capable of forming spores; ze.
sterilisation was only partial. Partial sterilisation by means
of volatile antiseptics gave the same result. The conclusion
was drawn that partial sterilisation increased the bacterial
activity, and consequently the amount of decomposition.
The increased quantity of plant food thus formed is shown by
the amounts taken up by the plant. Table LX XV. contains a
typical series of results.

Further investigations by Russell and Hutchinson (240c)
led to the following conclusions :—

(1) Partial sterilisation of soil, ze. heating to a tempera-
ture of 60° C. or more, or treatment for a short time with
vapours of antiseptics, such as toluene, causes first a fall then
a rise in bacterial numbers. The rise sets in soon after the
antiseptic has been removed and the soil conditions are once
more favourable for bacterial development ; it goes on till the
numbers considerably exceed those present in the original
soil.

(2) Simultaneously there is a marked increase in the rate
of accumulation of ammonia, This sets in as soon as the

284 SOIL CONDITIONS AND PLANT GROWTH

bacterial numbers begin to rise, and the connection between
the two quantities is normally so close as to indicate a causal
relationship ; the increased ammonia production is, therefore,
attributed to the increased numbers of bacteria. There is no
disappearance of nitrate; the ammonia is formed from
organic nitrogen compounds.

TABLE LXXV.—WEIGHT AND ComposITION OF Crops GROWN ON PARTIALLY
STERILISED Soits, RusseL_ AND DARBISHIRE (2408).

a
Dry Weight | Percentage Composition}Weight of Nutrients Taken by

of Crop. of Dry Matter. the Plant from Soil, grms.
Grms. N. P,O;5. K2O. N. P2035. K20.
Buckwheat.
Untreated soil A 18°14 2°75 | 1°87 5°62 "499 | °339 I°o1g

Soil treated with
carbon disulphide} 23°27 3°15 | 2°34 | 5°97] °733 | ‘544 | 173890

Mustard. ’
Untreated soil “ 15°88 2°30 | I°00 | 4:20 | *367 | *159 *€68
Heated soil . ; 24°33 4°43 | 2°08 | 5°02 | 1°077 | *506 | 1°221

These conclusions are generally accepted.

TaBLE LXXVI.—AmMoniIA AND NITRATE ACCUMULATING IN A Sort KEpT
TWENTY-THREE Days aT ABouT 15° C. In A Moist CONDITION, PARTS PER
MILLION OF Dry SolL.

: ‘
Nitrogen Present|Nitrogen Present Lr otal Nitrogen Present as
as Ammonia. as Nitrates. Ammonia and Nitrate.

At Be-| After | At Be-| After | At Be-| After | Gain in
ginning.|23 Days.|ginning. |23 Days.fginning.|23 Days.|23 Days.

Untreated soil ; ; hae 0 -) 7 I2 16 E3'S: |) DHF 3°90
Soil heated 2 hours to 98° C.| 6°5 43°8 13 12 19’5 | 55°38 | 363
Soil treated with toluene,

which was then evapor-

ated: ; ; | 5°0 27°8 12 I2 17°70 | 39°8 | 22°38
Soil treated with toluene,
which was not removed] 7°2 14°5 II Io 18°2 | 24°5 6°3

(3) The increase in bacterial numbers is the result of
improvement in the soil as a medium for bacterial growth

THE MICRO-ORGANIC POPULATION OF THE SOIL 285

and not an improvement in the bacterial flora. Indeed, the
new flora fer se is less able to attain high numbers than the
old. This is shown by the fact that the old flora when re-
introduced into partially sterilised soil attains higher numbers
and effects more decomposition than the new flora. Partially
sterilised soil plus 0°5 per cent. of untreated soil soon contains
higher bacterial numbers per grm. and accumulates ammonia
at a faster rate than partially sterilised soil alone.

(4) The improvement in the soil brought about by partial
sterilisation is permanent, the high bacterial numbers being
kept up even for 200 days or more.’ The improvement, there-
fore, did not consist in the removal of the products of bacterial
activity, because there is much more activity in partially steri-
lised soil than in untreated soil. Further evidence is afforded
_ by the fact that a second treatment of the soil some months
after the first produces little or no effect.

It is evident from (3) and (4) that the factor limiting
bacterial numbers in ordinary soils is not bacterial, nor is it
any product of bacterial activity, nor does it arise spontaneously
in soils.

(5) But if some of the untreated soil is introduced into
partially sterilised soil, the bacterial numbers, after the initial rise
(see (3)), begin to fall. The effect is rather variable, but is
usually most marked in moist soils that have been well sup-
plied with organic manures; e.g. in dunged soils, greenhouse
soils, sewage farm soils, etc. Thus the limiting factor can be
reintroduced from untreated soils.

This is controverted by J. M. Sherman at Wisconsin (261),
who failed to find any evidence that the introduction of un-
treated soil brought down the bacterial numbers in partially
sterilised soil.

(6) Evidence of the action of the limiting factor in untreated
soils is obtained by studying the effect of temperature on

1 Subsequent work indicates that a rapid and considerable rise followed by a
fall takes place in the first few days after treatment with certain antiseptics.
This is attributed to the utilisation by certain organisms of the trace of anti-

septic left in the soil; it is regarded by the writer as distinct from the sustained
but lower rise referred to here.

«(one

A
286 SOIL CONDITIONS AND PLANT GROWTH ~

bacterial numbers. Untreated soils were maintained at 10°,
20°, 30° C., etc., in a well-moistened aerated condition, and
periodical counts were made of the numbers of bacteria per
grm. Rise in temperature rarely caused any increase in
bacterial numbers ; sometimes it had no action, often it caused
a fall. But after the soil was partially sterilised the bacterial
numbers showed the normal increase with increasing tempera-
tures. Similar results were obtained by varying the amount

TaBLE LXXVII.—Errect oF TEMPERATURE OF STORAGE ON BACTERIAL
NUMBERS IN SOILS, MILLIONS PER GRAM.

Untreated Soil. Soil Treated with Toluene.

Tomperatins of :
torage °C. After | After After After | After | After
At Start. 13 Days.i25 Days.|7o. Days. ‘At Start. 13 Days.|25 Days.|7o Days.

5°-12° 65 63 41 32 8°5 73 IOL 137
20° 65 41 22 23 8°5 187 128 182
30° 65 27 50 16 8°5 | 197 | 145 51
40° 65 14 9 33 85 | 148 52 | 100

of moisture but keeping the temperature constant (20° C.).
The bacterial numbers in untreated soil behave erratically and
tended rather to fall than to rise when the conditions were
made more favourable to trophic life; on the other hand, in
partially sterilised soil, the bacterial numbers steadily increased
with increasing moisture content. Again, when untreated
soils are stored in the laboratory or glass-house under varying
- conditions of temperature and of moisture content the bacterial
numbers fluctuate erratically; when partially sterilised soils
are thus stored the fluctuations are regular.

- (7) When the curves obtained in (6) are examined it be-
comes evident that the limiting factor in the untreated soils is
not the lack of anything! but the presence of something.
active.

(8) This factor, as already shown, is put out of action by
antiseptics and by heating the soil to 60° C., and once out of

1 The soils included fertile loams well supplied with organic matter, calcium
carbonate, phosphates, etc.

THE MICRO-ORGANIC POPULATION OF THE SOIL 287

action it does not reappear. Less drastic methods of treating
the soil put it out for a time, but not permanently: e.g.
heating to 50°, rapid drying at 35°, treatment with organic
vapours less toxic than toluene (eg. hexane), incomplete
treatment with toluene. In all these cases the rise induced
in the bacterial numbers per grm. is less in amount than after
toluene treatment and is not permanent; the factor sets up
again. As a general rule, if the nitrifying organisms are
killed the limiting factor is also extinguished ; if they are only
temporarily suppressed the factor also is only put out for a
time.

(9) The properties of the limiting factor are :—

(a) It is active and not a lack of something (see (7)).

(6) It is not bacterial (see (3) and (4)).

(c) It is extinguished by heat or poisons, and does not
reappear if the treatment has sufficed to kill sensitive and
non-spore-forming organisms; it may appear, however, if the
treatment has not been sufficient to do this.

(dz) It can be reintroduced into soils from which it has
been permanently extinguished by the addition of a little un-
treated soil.

(e) It develops more slowly than bacteria, and for some
time may show little or no effect, then it causes a marked re-
duction in the numbers of bacteria, and its final effect is out
of all proportion to the amount introduced.

(f) It is favoured by conditions favourable to trophic life
in soil, and finally becomes so active that the bacteria become
unduly depressed. This is one of the conditions obtaining in
glass-house “ sick” soils.

It is difficult to see what agent other than a living organism
can fulfil these conditions. Search was therefore made for
larger organisms capable of destroying bacteria, and consider-
able numbers of protozoa were found. Active ciliates and
amoebe are killed by partial sterilisation. Whenever they are
killed the detrimental factor is found to be put out of action,

1This is dealt with fully in Yourn. Agric. Sci., vol. 5, pp. 27-47, 86-111
(1912).

288 SOIL CONDITIONS AND PLANT GROWTH

the bacterial numbers rise and maintain a high level. When-
ever the detrimental factor is not put out of action the protozoa
are not killed, To these rules we have found no exception.

TaBLE LXXVIII.—NumpBeErs or BACTERIA AND AMMONIA PRODUCTION IN
PARTIALLY STERILISED SOILS. R

Numbers of Organisms per grm. of Dry Soil, in

Millions, Gelatin Plate Cultures. |Ammonia and
Soil Treatment. Protozoa Nitrate
Found. Produced

At After 7| After 21 | After 68| After 142 after 68 Days.

Beginning.| Days. | Days. | Days. Days.

Untreated ; II Io 12 II 4 C.A.M. 14
Heated to 40° 7 9 BK) 8 3 C.A.M. 15
oon SO 2 14 16 38 48 M. 60
hs SN cas 2 he WE 24 | 27 M. 38
pi. yy ROO Qptor 17 22 ro ro _- 44.

C = ciliates. A= amcebe. M = monads.

Further, intermediate effects are obtained when a series of
organic liquids of varying degrees of toxicity is used in
quantities gradually increasing from small ineffective up to
completely effective doses. The detrimental factor is not
completely suppressed but sets up again after a time, so that
the rise in bacterial numbers is not sustained. But the -
parallelism with ciliates and amcebe is still preserved: they
are completely killed when the detrimental factor is completely
put out of action; they are not completely killed, but only
suppressed to a greater or less degree, when the detrimental
factor is only partly put out of action.

Now this parallelism between the properties of the detri-
mental factor and the protozoa is not proof that the protozoa
constitute the limiting factor, but it affords sufficient presump-
tive evidence to justify further examination.

Soil Protozoa.

The preceding experiments show that protozoa commonly
occur in soils but they give no information as to the kinds,
the probable numbers, or the state in which the protozoa exist,
whether active or simply as cysts.

THE MICRO-ORGANIC POPULATION OF THE SOIL 289

A protozoological survey of the soil has been begun, and
. in order to give it as permanent a value as possible the inves-
tigations are not confined to the narrow issue whether soil
protozoa do or do not interfere with soil bacteria, but they
are put on the broader and safer lines of ascertaining whether
a trophic protozoan fauna normally occurs in soil, and if so,
how the protozoa live, and what is their relation to other soil
inhabitants.

The first experiments by Goodey dealt mainly with ciliates
(110a) and indicated that these protozoa were present only as
cysts. Subsequent investigations, however, by Martin and
Lewin (191) established the following conclusions :—

1. A protozoan fauna in a trophic state normally occurs in
soils,

2. The trophic fauna found in the soil differs from that
developing when soil. is inoculated into hay infusions,’ and
vice versa the forms predominating in the hay infusions do
not necessarily figure largely in the soil.

3. The trophic fauna is most readily demonstrated, and is
therefore presumably most numerous, in moist soils well sup-
plied with organic fhanures, e.g. dunged soils, sewage soils
and especially glass-house “sick ” soils.

The enumeration of the protozoa in the soil is carried out
by means of a dilution method somewhat similar to that
adopted for soil bacteria. It is already clear that amcebe
and flagellates are present in at least thousands per grm. of
soil, while ciliates can be found only in hundreds. Some of
the organisms appear to be new to science, and many of them
are of considerable interest.

The presence of protozoa is not peculiar to British soils :
they have been found in Egypt,? “rance,’ Italy, Germany,®
the United States, and elsewhere. Methods have been devised. _

1 This is not necessarily true of all artificial media.

2Ronald Ross and D. Thomson, Egyptian Sand Amebe (Proc. Soc. Med.
Sect. Epidem., 1916, 9, 33), and H. Sandon (Rothamsted).

3 In the Rothamsted laboratory.

4A, Cauda and C. Sangiorgi (Turin), Cent. Bakt. Par., 2 Abt., 1914, 42, 393-
5R. Oehler, Arch. f. Protistenkunde, 1916, 3'7, 175; 1919, 40, 16,

19

290 SOIL CONDITIONS AND PLANT GROWTH

by D. W. Cutler and Miss L. M. Crump at Rothamsted, by
Waksman at the Rutgers College (2924), by Kopeloff, Lint
and Coleman at the New Jersey Experiment Station (152),
and by others, for estimating their numbers and activities.

There has been much discussion as to whether protozoa
are normally active in the soil or whether they are simply
there as cysts. Waksman (2924) recognises a trophic fauna.
Sherman (261), Koch,? Moore,* Fellers and Allison,* consider
the protozoa are not active because they cannot be seen when
soil is examined under a microscope.

D. W. Cutler (73a) has, however, found a satisfactory
explanation of the difficulty of seeing living protozoa in the soil.
He finds that the organisms rigidly adhere to the soil particles
and indeed up to a certain limit they can be completely
removed from a suspension by shaking for a few minutes with
soil. The saturation capacity of the soil is high ; at Rotham-
sted it is 1,500,000 or 2,000,000 per grm., a figure consider-
ably in excess of the numbers present, and only in exceptional
cases can organisms be dislodged sufficiently readily to be
recognised under the microscope.

At Rothamsted frequent estimatioris are made of the
number of protozoa found in certain arable soils. The method
(730) discriminates between cysts and active forms, and 9 species
of flagellates, 5 of amcebz, 2 of thecamcebe, and 3 of ciliates
are enumerated. Table LXXIX. shows >the order of values
frequently obtained. It is difficult to explain these high
values if they simply represented cysts blown in from ponds.

The numbers are considerably higher in autumn and spring
than in summer, and they are higher in summer than in winter,
and they are much higher on the plot receiving dung than on
that continuously unmanuféd.

1See N. Kopeloff and D. A. Coleman for a review of the investigations
up to 1917 (Soil Sci., 1917, 3, 197-269).

2G. P. Koch, ¥ourn. Agric. Research, 1915, 4, 511; Soil Sci., 1916, 2,
163.
; 3G. T. Moore, Science, N.S., Nov. 8th, 1912; see also E, J. Russell, Science,
N.S., April 4th, 1913.

4C. R. Fellers and F. E, Allison, Soil Sct., 1920, 9, 1.

THE MICRO-ORGANIC POPULATION OF THE SOIL 291

TasLe LXXIX.—NumBERS oF PROTOZOA AND OF BACTERIA PER GRM. OF SOIL
FREQUENTLY FOUND ON Two BroapBaLk PLoTs. CUTLER AND CRUMP,

Broadbalk Dunged Broadbalk Continu-
Plot. ously Unmanured.
Active Approximate
Forms. iameter.
Winter Summer Winter | Summer
Total. Total. Total. Total.
Flagellates . 150,000} 600,000 5,500, 15,000]15 to 95 pet] 7°5-15 mw
Amcebze +} -« 5,000 15,000 40 2,000} cent. of 8-22 w
Thecamcebze oo I,00o) )=—— — total in all 15 pw
Ciliates ‘ 50 2008 — — cases, 20-40 pw
Bacteria . |10,000,000}24,000,000}4,000,000) 5,000,000 I-4 me -

20.000

y 18,000

16,000

14,000

ITI
Th] AVY:
os WAVIVIVAV EA Ty

fyeVer lt

1 7 14 21 28

Fic. 28.—Daily counts of active flagellates (three species) in Broadbalk Plot 2 :
(dunged) (Cutler and Crump, 73c). The numbers are in thousands per
grm., and show a remarkable periodicity.

10,

: |

12,000 —* | |
|

|

D
8, ¥

6.000

The investigations have further shown that the numbers,
while fairly regular on a uniform plot of land at a given time,
do not remain constant, but vary from day to day. The pro-
portion of active forms varies considerably, but on 50 per cent.
of the days it exceeds 90 per cent. of the total number. The
daily variations of three species of active flagellates on the
Broadbalk dunged plot are shown in Fig. 28, and those of the

Ig *

292 SOIL CONDITIONS AND PLANT GROWTH

amoebe in Fig. 29. These variations are not accidental and
their cause is being investigated.

The rapid daily fluctuations in numbers apparently bear
no relationship to external conditions, such as moisture and
temperature. In order to obtain further information daily
counts are being made at Rothamsted of the protozoa and
bacteria in a field soil for a period of 365 consecutive days.

Numbers of Bacteria & Protozoa,

Broadbalk.

38007
36004
3400
3200
30004 ; “14 y
0° ‘ ‘
Q 28 | ‘ \3 Oo
Plas’ 2600 / we oa 12 —_| .
i] —_
= 2400 ] Ne “tt pia
S 2200 4 \ We -10 >
ee 2000 : i ‘ pr. -9
\e- N
1800 \ ( ay" : -3 ©
\ ey —
S 1600 : 1 a ve oe na Pa >
es 14900 +} ay my -6 ~~
YS 1200 \ Bs Y
<x \ 0)
1000 “eo - ©
800-4 e a
< 9
6004 P f
400°
<
2004
Feb.23 Mar8&

Fic. 29.— Daily counts of active amcebe and of bacteria in Broadbalk Plot 2
(dunged) (Cutler and Crump, 73c). The active amoebe are given in
thousands, and the bacteria in millions, per grm. of soil.

These observations when completed will also give im-
portant information as to the relationship between numbers
of bacteria and protozoa. The result of fourteen consecutive
daily counts made in February, 1920, is shown in Fig. 29;
these show that the numbers of active amcebe and of bac-

THE MICRO-ORGANIC POPULATION OF THE SOIL 293

teria are in inverse relationship—when one is high the other
is low.*

This relationship appears always to hold when counts are
taken at short intervals.. Since amcebe normally feed on bac-
teria and do not live saprophytically they seem indicated as
suppressors of bacterial numbers; they are, however, probably
selective in their food, and it is possible that all soil bacteria
are not equally affected by them.

The effect of flagellates in the soil is quite unknown. They
do not necessarily feed solely on bacteria, but can and frequently
do utilise other substances, including amino-acids.” Thus it
would appear that they must effect some changes in the soil.
Their large numbers and rapid fluctuations suggest an im-
portant part in soil economy which at present is unknown.

If protozoa are detrimental to soil bacteria it ought to be
possible to reduce bacterial numbers in protozoa-free soil by
the introduction of a protozoan fauna. Considerable import-
ance is attached by Sherman and others to the fact that this
fundamental experiment has not yet been satisfactorily carried
out, or at any rate not more than once or twice, only A. Cun-
ningham * and T. Goody (1104) having reported decreases in
numbers.

The experiment is admittedly crucial, but it is very diffi-
cult and is not likely to succeed until more is known of the
life histories and physiology of the particular organisms con-
cerned, and of the conditions necessary for their growth in
soil. Cutler's results (Fig. 29) are difficult to explain in any
other way except by supposing that active amoebz are detri-
mental to bacterial numbers.

The present position in the controversy that has arisen may

1 The relationship thus resembles that obtaining in water: see Otto Hunte-
miller, Vernichtung der Bakterien in Wasser durch Protozoen (Arch. f. Hyg., 1905,
liv., 89-100); Fehrs, Die Beeinflussung der Lebensdauer von Krankheitskeimen in
Wasser durch Protozoen (Hyg. Rundschau, 1906, xvi.); P. Th. Miiller, Ueber die

Rolle der Protozoen bei der Selbst-reinigung stehenden Wassers (Arch. f. Hyg.,
IgI2, 75).

2See H. G. Thornton and Geoffrey Smith, On Certain Soil Flagellates
(Proc. Roy. Soc., 1914, 88 B, 151-165).

3 Fourn. Agric. Sci., 1915, 7, 49-74.

294 SOIL CONDITIONS AND PLANT GROWTH

be briefly summed up. Most workers now recognise protozoa
as a normal component of the soil population. Many admit
the existence of a trophic protozoan fauna. Some, ag.
Waksman (2926), recognise that certain of the trophic fauna
act detrimentally on soil bacteria, but few are prepared
to admit that they act detrimentally on the decomposition
processes, such as ammonification, nitrification, etc. In the
writer’s opinion the evidence is in favour of the existence of
the trophic protozoan population acting detrimentally on soil
bacteria, but as yet there is no direct evidence to show whether,
or to what extent, the decomposition processes in the soil are
affected.

The Effect of the Complexity of the Soil Population on
Productiveness.

Apart from the effect of partial sterilisation there is other
evidence that simplification of the soil population may lead to
increased productiveness, and conversely that a more complex
population may be relatively less effective as producers of
plant nutrients. It has been observed by practical men in
various countries that certain soil conditions harmful to the
growth of organisms were ultimately beneficial to productive-
ness ; such are long continued and severe frosts, long drought
(especially if associated with hot weather), sufficient heat,
treatment with appropriate dressings of lime, gas lime, carbon
disulphide, etc. :

Further, it has been observed that conditions which are
undoubtedly favourable to life, such as the combination of
warmth, moisture, and organic manures found in glass-houses,
lead to reduced productiveness aftera time. This phenomenon
is spoken of as “sickness”? by the practical man.

1 For references see E. J. Russell, ¥ourn, Roy. Ag. Soc., 1910, 71, 9; also
J. A. Prescott (228).

2 « Sickness ’’ is a very vague term and no doubt includes many dissimilar
cases. Other instances of ‘‘sickness,’”” which may refer to something quite dif-
ferent, are recorded by Hudig (Landw. Fahrb., 1911, 40, 613-644) and Loew

Porto Rico Report, 1910). Some soil in Orkney was said to be ‘“‘ oat-sick” but
experiment revealed nothing (Aberdeen Leaflet 15, 1911).

THE MICRO-ORGANIC POPULATION OF THE SOIL 295

It is difficult to account for this result on the old view that
the useful plant-food making bacteria are the only active
micro-organisms in the soil. On the other hand, the view
that detrimental organisms are also present readily explains
the observed facts. ,

The ‘“‘sickness” that affects the soils of glass-houses run
at a high pitch (such as cucumber-houses) and less slowly
those run at a lower pitch (such as tomato-houses) has been
investigated in some detail owing to its great technical im-
portance (Russell and Petherbridge, 241¢). It was traced to
two causes: an accumulation of various pests, and an ab-
normal development, especially in cucumber-houses, of the
factor detrimental to bacteria. The properties of this factor
show that it is identical in character with that present in
normal soil, and strongly indicate its biological nature. No
evidence of a soluble toxin could be obtained but the pos-
sibility of insoluble toxins was not excluded. On the other
hand, some remarkably interesting protozoa and allied organ-
isms have been picked out from these sick soils and described
by Martin and Lewin (191) and T. Goodey. Finally it has
been shown that the whole trouble can be cured by partial
sterilisation, and methods suitable for large scale work have
been investigated and are now in use in practice. Steam
heat at present proves most convenient, but the suitability
and detailed effects of lime have been studied by Hutchinson
and McLennan (140c) and of various antiseptics by Buddin
(61), Russell, and others.’

Again, it should be stated that the views set out here are
not generally accepted by American workers (see J. M. Sher-
man, 261). Bolley attributes the whole trouble in North
Dakota to plant diseases,” which, of course, may be the pre-
dominating factor there. It is possible that there are im-
portant differences in soil population in British and American

1See F¥ourn. Roy. Hort. Soc.,; 1920, and Reports of the Nursery and
Be 45) P J,

- Market Garden Expt. Station, Cheshunt, Herts, 1917, onwards.

2H. L. Bolley, North Dakota Bull., 107, 1913; Science, 1910, 32, 529-541,
and 1913, 38, 249-259.

}
296 SOIL CONDITIONS AND PLANT GROWTH

soils, but it is highly probable that much of the apparent
discrepancy will disappear with further investigation.

The Action of the Plant on the Micro-organic Popula-
‘ tion of the Soil.

The whole existence of the soil population is intimately
bound up with the growing plant: from this source it obtains
its supplies of energy and of those nitrogen compounds the
changes in which form the chief theme of soil bacteriology.
These effects are mainly exercised by the residues of dead
vegetation ; there are, however, others hardly less potent pro-
duced by the living plant.

A growing plant removes a considerable amount of soluble
material from the soil,! and thus modifies the composition of
the soil solution which serves as part of the medium in which
the organisms live. Further, the plant roots are continuall
giving off carbonic acid. It might be supposed that this
would cause the surrounding medium to become acid, but
as a matter of fact the contrary happens and the medium be-
comes alkaline. This has been known for some time in the
case of water cultures; the explanation offered is that the
plant takes up the acid radicle of the sodium nitrate and leaves
behind the base, which immediately appears as the carbonate.
Hall and Miller (1204), and subsequently Maschhaupt (192)
have obtained evidence of a similar action in the soil, the
calcium nitrate formed during nitrification being converted into
calcium carbonate while the nitrate radicle is taken by the
plant. These effects, involving as they do an accumulation
of calcium compounds in the surface soil, are favourable to
micro-organisms ; others are unfavourable, such as the removal
of moisture by the plant and the evolution of carbon dioxide
from the roots. Some investigators have supposed that the
plant secretes part of the mineral matter which it has taken up
but no longer needs, and if so this would affect the micro-
organisms. ;

Burd, in California (624), recognises three periods in the

For amounts see Appendix II., p. 357.

THE MICRO-ORGANIC POPULATION OF THE SOIL 297

growth of barley: up to the time of formation of the head
the rate of absorption progressively increases till finally the
amounts of nitrogen and potassium in the plant reach a
maximum. During the period of translocation of material
into the developing heads, however, there is not only a
decreased rate of absorption from the soil but a substantial
loss of nitrogen and potassium from the aerial parts of the
plant. Towards the end of this period some of the lost
materials are regained. In the third period ripening is com-
pleted, absorption ceases and losses are resumed. These
losses are experimentally traced from the aerial parts alone,
and it is impossible to assert that they take place from the
whole plant owing to the difficulty of completely extracting
the roots from the soil. The presumption is, however, that
such losses do take place.’

It is interesting, and may be significant, that Russell and
Appleyard (241g) observed a marked increase in CO, content
of the atmosphere of cropped soils during the period of ripen-
ing, when these losses from the plant are presumably taking
place.

We have now to ascertain how these various effects react
on the soil micro-organisms. Several attempts have been
made to correlate bacterial numbers with the nature of the
crop, but the data hitherto obtained are inadequate for satis-
factory discussion. There is, however, considerable evidence
to show that nitrate accumulates more readily on uncropped
than on cropped soils, even after allowance is made for the
quantity absorbed by the plant. Table LXXX. gives some
of the results obtained at Rothamsted.

To some extent climatic factors come into play, the cropped
land frequently being somewhat cooler and drier than the fal-
low. But this does not hold universally, and the phenomenon
has been observed under such widely different conditions that

1Le Clerc, U.S. Dept. of Agric. Year Book, 1908, 389-402; also André,
Compt. Rend., 1910, 151, 1378-82, support the view that the losses occur by
washing from the leaf rather than from the root. For mass of data suggesting
root losses see Wilfarth, Rémer, and Wimmer, 3098.

298 SOIL CONDITIONS AND PLANT GROWTH

TABLE LXXX.—NITRATES IN CROPPED AND UNcRopPED Soits at RoTHAM-
' STED EXPRESSED AS N,, Ib. PER ACRE. RussELL (241f).

June, 1911. July, 1912,

Fallow | Cropped | Fallow | Cro
Land. Land. Land. ae

N. as nitrate in top 18 in, of soil (June) 54 15 46 13
Nitrogen in crop, lb. per acre “ — 23 — 6
Total lb, per acre 54 38 46 19.

Deficit in cropped land, lb. per acre — 16 _ 27

Expressed as N., parts per million.

N. as nitrate o to g in. depth 12 4 8 2
g to 18 in. depth 9 2 Io 3

climatic factors seem to be ruled out. In 1905 Waring-
ton showed that the amount of nitrate in the drainage waters
from Broadbalk field was considerably less than was ex-
pected from the manure supplied and the crop reaped. He
thought that denitrification might account for some of the
discrepancy but not for all, as it could hardly be supposed to
act in dry summer weather; he further suggested that the
nitrate might be taken up by the plant and then somehow
lost before harvest. More recently Lyon and Bizzell (183)
found more nitrate on land cropped with maize (after allowing
for the nitrogen present in the crop) than on fallow land of
similar history, and concluded that the growing maize plant
in some way stimulated nitrification. During the latter part
of the life of the plant less nitrate was found in the cropped
than in the fallow land, and the further conclusion is drawn
that nitrification is, inhibited by the conditions accompanying
the decreasing activities of the roots. On the other hand,
where oats and potatoes were grown the nitrates were never
so high in the cropped as in the uncropped land, again,
apparently, after allowing for what has been absorbed by the
crop. The following amounts of nitrogen as nitrate occurred
in parts per million of soils are given in Table LXXXI. :—

THE MICRO-ORGANIC POPULATION OF THE SOIL 299

TABLE LXXXI.—NITROGEN Aas NITRATE IN CROPPED AND UNCROPPED SOILS,

Irnaca, N.Y. (Lyon anp Bizze tt).

PaRTS PER MILLION.

Fallow rand Fallow Land
ica dgd italia MN 2" Ma lg
May to . 4°9 3°9 April 22 . Ig'0 10’9
June 22 . 10'9 9°3 June 24 . 12°6 2°5
July 6 14°5 14°2 July 12 I2°5 Io
July 27 42°I 43°2 August 7. 18°4 0°8
August Io 40°3 37°3 — — —

It is interesting to observe that the figures are generally of
the same order as at Rothamsted excepting only in July and
August, 1908. We have never observed, however, any in-
crease in nitrate on cropped land such as is recorded in their
Maize experiments; our results with wheat and barley have
always shown a decrease, like theirs with oats. Leather’s
experiments in India (1674) and Prescott’s in Egypt (228)
also show a decrease. The nitrate in the drainage water
from the fallow gauges at Pusa contained respectively 261°5
and 209°6 lb. per acre during the period 1907-9, while that in
the drainage water and crops of the gauges cropped with grass
accounted only for 128°4 and 115°6 lb. per acre over the same
period. The final rainfall before the account was made up was
so heavy as to deplete the gauges of nitrate, so that no error
arises through the retention of nitrate in the soil.

Deherain’s experiments made at Grignon, near Paris (814),
between 1892 and 1897, also showed much more nitrate
coming from the fallow lysimeters than from those covered
with crops, even after allowing for what was absorbed by the
crop. In this case, however, it is uncertain how much nitrate
was left in the soil, the rainfall probably being insufficient to
wash it all out.

Thus it seems to be an established fact that less nitrate
usually accumulates in cropped land than in fallow land, even
after allowing for what is absorbed by the crop. The wide
range of climatic conditions under which the result is obtained
seems to preclude any assumption that it is due to the effect

300 SOIL CONDITIONS AND PLANT GROWTH

of the crop on the temperature or moisture content of the
soil.

Two hypotheses are possible: there may be a diminished
production of nitrate in cropped land, indicating an adverse
effect of the growing crop on bacterial activity, or there may
be some process destructive of nitrates (possibly by conversion
into protein)’ at work in the cropped soil that does not go on
in fallow soil.

Destruction of nitrates is at least as probable as a dimin-
ished bacterial activity. It may quite well happen that
portions of the roots and rootlets are decomposed during the
life of the plant, and if their nitrogen content is insufficient,
the organisms might assimilate some of the soil nitrates
(p. 210). This view is consistent with certain observations
which are otherwise difficult to explain, and which seem to
indicate an increased rather than diminished bacterial activity
in the neighbourhood of the growing plant. Working with
the irrigated soils of Utah, McBeth, and Smith (185a) found
that the “ nitrifying power’’ (z.e. the power to produce nitrate
when incubated under optimum conditions with dilute am-
monium sulphate solution) was highet on cropped than on
fallow land. Again, Le Clerc found higher bacterial numbers
under cow-peas than in the fallow plots. A similar result
was obtained in Joshi’s pot experiments with other crops.’

On the other hand, the alternative view that plant growth
depresses bacterial activity seems to be supported by Harri-
son and Aiyer’s investigations in paddy soils already quoted
(p. 213). The production of methane and hydrogen was
markedly less on the cropped than on the fallow land. There
is no question here of deficient water supply or temperature,
and the conclusion of the authors is that the crop reduced
bacterial action either by giving out some bacteriotoxin or by
absorbing some of the products of the early stages of decom-
position. ;

1 Burd (62a, p. 304) suggests that this change occurs in Californian soils,

2 Fourn. Ag. Res., 1916, 5, 439-447.
3 Memoirs Dept. of India, Bact. Ser., 1920, I, 247-276.

CHAPTER VIII.

THE SOIL IN RELATION TO PLANT GROWTH.

WE are now in a position to summarise the effect of the vari-
ous soil conditions on the growth of plants. The soil serves
several functions ; it affords a more or less continuous supply
of food and water, it regulates the temperature and provides
anchorage for the roots without interfering too much with the
air supply. The extent to which any of these things is done
depends on the nature of the particles and, therefore, varies
from soil to soil. As the requirements of plants also vary in
' degree, the different soil types have come to possess their own |
flora made up of those plants that tolerate the conditions
better than other possible competitors, and on cultivation they |
show different agricultural characteristics. And since the
natural flora is profoundly affected by competition, a small
factor operating for a sufficient length of time may produce
extraordinary differences in vegetation, as is well seen on the
Rothamsted grass plots.

At the same time the plant is not rigidly unalterable but
possesses considerable power of adaptation to its surroundings :
in rooting depth, size of leaves, glaucous characters, etc.

In discussing the relationship between plant growth and
the soil it is necessary to take into account not only the
intrinsic properties of the soil due to the nature of its con-
stituent parts, but the ertrzusic properties impressed by topo-
graphical and climatic factors. A certain indefiniteness thus
becomes unavoidable, because none of the latter can be ex-
pressed in exact measurements. This point is well illustrated

by the water supply. The amount of water in the soil at
301

302 SOIL CONDITIONS AND PLANT GROWTH

any time is the balance of gains over losses. The gains de-
pend on the rainfall (ze. climate), and on the amounts of water
derived by drainage from higher land (z.e. topographical posi-
tion), or by surface tension from the subsoil; the losses depend
on the extent to which the subsoil facilitates drainage and on
the rate of evaporation, which in turn depends on the tempera-
ture, the exposure of the soil, the velocity of the wind and the
mode of cultivation. Five factors have, therefore, to be con-
sidered: (1) the nature of the soil particles, (2) the amount
and distribution of the rainfall, (3) the position of the soil in
relation to the land round about it, its aspect, shade, and any
other factors affecting its relative temperature and water supply,
(4) the depth of the soil, (5) the nature of the subsoil, especially
its perviousness to air, water, and plant roots. Any of
these factors may, within certain limits, dominate the rest and
profoundly affect the flora and the agricultural value; thus
a sandy soil may, without any change in type, be a dry and
barren heath if underlain near the surface with rock or gravel,
a highly fertile fruit or market-garden soil if sufficiently deep,
or a stagnant marsh giving rise to peat if so situated that
water accumulates and cannot drain away.

No sharp division can be drawn between the intrinsic and
extrinsic properties. The significant units in the soil that
determine its intrinsic properties are the compound particles
made up of the ultimate mineral particles—clay, silt, sand,
etc.—together with calcium carbonate and organic matter de-
rived from plant remains. Now the nature and amount of the’
organic matter are greatly influenced by the extrinsic condi-
tions—the temperature and water supply—that have obtained
in the past. Moisture, warmth, and aeration favour the de-
velopment of a succulent vegetation which sheds easily decom-
posable leaves and stems on to the soil; earth-worms and
bacteria can now flourish and produce the normal decomposi-
tion products that go to make up “mild humus” and a
fertile soil. Dryness necessitates a narrow-leaved xerophytic
vegetation, the leathery fragments of which mingle with the
soil, but afford a very indifferent medium for the growth of

THE SOIL IN RELATION TO PLANT GROWTA = 303

earth-worms and bacteria, so that little decomposition goes
on, and a barren sand results. Wetness and lack of aeration
necessitate special vegetation and decomposition agents, and
there may result a ‘mild humus” if sufficient calcium car-
bonate is present to determine a calcicolous flora, or an “acid
humus” in the contrary event. Even such small differences
as affect the liability to late frosts may affect the native
vegetation and therefore the humus: for example, bracken is

“more readily killed than heather and is therefore less likely to
survive at levels where late frosts are common.

Thus the soil is very much the result of circumstances ; its
character is determined in part by the rock from which it was
derived, and in part by subsequent events, particularly the
temperature and water supply it happened to obtain, in other
words, its climate. A given set of mineral particles may,
under different climatic conditions, give rise to soils wholly
different in agricultural value and in natural flora. This thesis
was first set up by Hilgard, but it has been greatly developed
by the Russian investigators and, indeed, has been one of their
most striking contributions to the study of the soil (85, 266).

Further, the farmer has discovered how to build up these
compound particles by cultivation and thus change to a very
great extent the relation of the soil to the plant: the process
consists in adding dung, or ploughing in green crops, adding
lime, exposure to frost, and skilful (but wholly empirical)
cultivation, and, although not very rapid, it takes only a few
seasons to accomplish.

But while the ultimate mineral particles do not entirely
control the relationships of the soil to vegetation they fix the
limits within which these relationships may vary and beyond
which they cannot pass. Farmers recognise five great divi-
sions: clays, loams, sands, calcareous soils and soils rich in
organic matter, all shading off into one another and without
sharp lines of demarcation, but representing classes of soil
that cannot be changed one into the other by any cultivator’s’
artifice.

Calcareous Soils.—The simplest case is presented by soils

304 . SOIL CONDITIONS AND PLANT GROWTH

where the calcium carbonate exceeds about Io per cent. and
dominates every other constituent, becoming the controlling
factor in determining the soil properties. The conditions
here seem to be extraordinarily well suited to plant and animal
life. Bacteria are numerous and active, rapidly oxidising
organic matter. Hosts of animals, wireworms, earthworms,
and others live in the grass land, and even get into the arable
land, honeycombing the soil with their passages, puffing it
up or “lightening” it considerably, and encouraging the
multiplication of moles. Rabbits abound in dry places.
Vegetation is restricted on thin exposed soils, but becomes
astonishingly varied where there is sufficient depth of soil and
shelter to maintain an adequate water supply. Ash is the
characteristic tree in the north and west, and beech in the
south of England, and there is a great profusion of shrubs—
buck thorn, spindle, guelder rose, dogwood, hawthorn, hazel,
maple ; and especially of flowering plants—scabious (S. Colwm-
baria), the bedstraws, vetches, ragwort, yellow wort (Chlora
perfoliata), salad burnet (Poterium sangutsorba), lady's fingers
(Anthyllis), Linum cartharticum, Bromus erectus. Still more
remarkable, perhaps, is the fact that a few plants—the so-
called calcifuges—do not occur. Where the amount of calcium
carbonate is too high plants tend to become chlorotic ;
Chauzit showed that vines suffered badly when 35 per cent.
or more was present, but not when the amounts fell to 3 per
cent.!. Mazé (197) attributes part of the action to the render-
ing insoluble of zinc, manganese, etc., necessary for complete
growth (p. 56).

Investigations in Porto Rico (105), where a considerable
portion of the arable land is sufficiently calcareous to produce
nutritional disturbances in crops, show that bush beans (Phase-
olus nanus) and radishes are unaffected by even 35 per cent. of
CaCO, ; sunflowers, soy beans, and sugar canes are somewhat
depressed ; while sweet cassava (Manzhot palmata), rice and
pine-apples were considerably depressed by this amount. The

1 See Revue de Viticulture, 1902, xviii., 15, and also Molz, Centr. Bakt. Par.,
Abt. ii., 1907, xix., 475. f

THE SOIL IN RELATION TO PLANT GROWTH = 395

amount of nitrogen, potash, and phosphoric acid in the various
crops was apparently unaffected by the carbonate, but the iron
was notably depressed.

The reason for these special features is not clear, but is
probably not to be found in any one factor. The plants do
not require such high amounts of calcium carbonate, because
they will wander on to an adjacent loam ; even the absence of

__ the calcifuges cannot always be attributed to a supposed toxic
effect of calcium carbonate, because in other regions, or in pot
experiments, some at any rate of them may be found growing
in its presence. The lupin affords an example. It is much
grown in Germany and is supposed to suffer in presence of
chalk (Kalkempfindlichkeit): Hiltner proposed to use ferrous
sulphate to counteract this trouble. But in experimental
plots Pfeiffer and Blanck? obtained even better growth in
presence of chalk than in its absence; possibly the beneficial
effect was on the nodule organism rather than on the plant.
Rhododendrons and azaleas in this country are said to be
intolerant: even of traces of chalk, for which reason hard
water is not used for them; this case deserves further in-
vestigation. In a prolonged investigation near Karlstadt
Kraus (155) found no plant occurring exclusively on soils
with even approximately equal content of calcium carbonate,
although some preferred more, e.g. Festuca glauca, Teucrium
montanum, and Melica ciliata, while others preferred less, e.g.
Brachypodium pinnatum, Keleria cristata, and Hieracium
pilosella. True chalk plants were found on the adjoining
sand, especially when some calcium carbonate was present,
although the true sand plants did not wander on to the chalk.
In such cases of displacement or “heterotopy” it was shown
that the general physical conditions of the two locations were
similar in spite of their wide difference in chemical composi-
tion. Kraus, therefore, argues that the true chalk plants.
inhabit chalk soils not because they need much calcium car-.
bonate, but because they find there the general physical and
chemical conditions they require. Dr. Salisbury’s view (242),

1 Mitt. Land, Inst., Breslau, 1914, 7.
20

306 SOIL CONDITIONS AND PLANT GROWTH

is that two conditions come in: wetness and acidity—from
both of which chalk soils are free; the so-called calcicolus
plants will, however, grow elsewhere if these conditions are
absent; thus he finds beech on non-calcareous soils on the
continent so long as these soils are dry. Dr. Brenchley finds
that the calcifuges of the West of England are not all calci-
fuges in the Eastern counties, while Massart’ showed that
the typical calcifuge, Ca/luna vulgaris, grows on the “ lime-
stone pavements” of the West of Ireland.

The fact that a calcareous rock lies beneath is no proof
that the soil itself is calcareous: on the contrary, the soil may
often contain practically no calcium carbonate, either because
it has become decalcified by rain, or because it really represents
some deposit of wholly extraneous origin.

The agricultural value of chalk soils depends very largely
on their depth, and is much greater in valleys where the soil
and water collect than on the higher ground where the soil is
thinner. The two defects most in need of remedy are the lack
of organic matter and the tendency to become light; these
are met by additions of dung or other organic manures, by
rolling and. cultivating with heavy instruments, and above all
by feeding animals on the land with the crops actually growing
there and with purchased food, a process known as “folding”.
Heavy wooden ploughs are still in use, and until recently
were worked in many places by large teams of heavy oxen.
Sheep are by far the most suitable animals to be fed on the
land, and they form the centre round which the husbandry of
chalk districts has developed ; indeed, so important are they
that each chalk region has evolved its own breed of sheep
—South Downs, Hampshires, etc. As fertilisers potassic
manures, especially kainit, are generally profitable ; superphos-
phate is needed for turnips, and in wet districts basic slag is
useful on the grass land. Skilful cultivation is always neces-
sary, or the soil dries into hard, steely lumps that will not

1 Brit, Assoc. Report, 1911, 564. Thecase of Calluna is discussed by M. C.
Rayner in Annals of Botany, 1915, 29, 97-133, and Sphagnum by M. Skene,
ibid., 1915, 29, 65-37.

THE SOIL IN RELATION TO PLANT GROWTH 307

break down. And, lastly, the pre-eminent suitability of the
chalk to plant and animal life has its disadvantages ; no soils
are more prone to carry weeds, turnip “ fly,” or wireworm.
Skilful management is the keynote of success and it generally
obtains, the bad farmers not usually surviving many seasons.

Black Soils or Humus Sotls.—In these the organic matter
dominates all other factors, but the case is more complex than
the preceding, because several varieties of organic matter occur,
giving rise to several types of soil.

There is a sharp distinction between :—

I. True peat, such as is found in moorlands, acid in
nature, and commonly occurring in wet districts and regions
of high rainfall or low temperature. This again is subdivided
into high moor and low moor. The organic matter shows
definite structure.

2. Fen, neutral in nature, occurring more locally: in
England mainly in the Eastern counties. The organic matter
usually shows no structure.

Fen land when drained has a high agricultural value, being
worth some 42 per acre annual rent and producing consider-
able crops of potatoes, celery, and wheat.

True peat, on the other hand, has little agricultural value
in its natural state and where high-lying is largely waste land,
being unsuited to the growth of many plants and micro-
organisms‘ (p. 269). In this country it may have a certain
sporting value, but elsewhere it lacks even this.

_ Peat Sotls.—A number of schemes have been projected for
utilising the great peat areas, nai they may roughly be grouped
into two classes :—

1. Ameliorating substances (such as lime, artificial
manures, etc.) are added, and the peat is cultivated as if it
were normal soil. This is possible only when the deposits do
not lie too high.

1 For studies of nitrification and other bacterial actions in moorland soils see
E. Gully, Das Nitritzerstérungs und Nitritbildungvermigen der Moorboden

(Landw. Fahrb. Bayern, 1916, 6, 1-81); Th. Arnd (6), Zur Kenntnis der Nitri-
fication in Moorboden (Centr. Bakt. Par., I1., 1919, 49, 1-51).

20 *

308 SOIL CONDITIONS AND PLANT GROWTH

2. The peat is removed and sold, and if the climate allows
the underlying formation is :—

(a) Ploughed up, if it is clay or sand.

(6) Covered with town refuse and then cultivated, as at
Chat Moss.

(c) Warped, z.e. systematically flooded with tidal water
carrying silt till several feet of soil have been formed ;
this is only possible in a few areas, e.g. in Lincoln-
shire, lying below high water-level.

The first of these methods is in use in Ireland! and on
the continent; it is much investigated at the experiment
stations at Jonkoping (Sweden) by H. von Feilitzen, at
Bremen (Prussia), Munich, and Arnheim (Holland). The
second has been the subject of many experiments in England.

The removal and sale of peat is perfectly sound in
principle. Peat is an asset-of considerable value, but it is es-
sentially a wasting asset; it disappears at a measurable rate
after the drainage that is nowadays necessary. Under modern
conditions peat cannot be conserved for future generations,
and we are therefore fully justified in using it ourselves, even
if the process be somewhat wasteful.

Peat may be made to serve three purposes :—

(a) Fuel, after it has been cut and dried.

(4) Litter, if it is sufficiently fibrous, or

(c) It may be decomposed by heat, and made to yield
valuable products, such as sulphate of ammonia,
power gas, tar, etc.

Fen Sotls.—The fen soils form an interesting group in the
low-lying areas adjoining the river Ouse and its tributaries in
Norfolk, Cambridge, etc. As already stated, the soil water is
not acid but contains calcium carbonate: the vegetation is not.
of the acid-soil type and crops do not respond to lime. The
main characteristics of these soils are their richness in lime
and their very high content of nitrogen, of which no less than
3 per cent. is found in some cases (see p. 136).

The region was not much in cultivation till the great

1See Duncan for experiments in Ireland, Sourn. Deft. Agric., Ireland, 1915.

THE SOIL IN RELATION TO PLANT GROWTH 309

reclamation schemes of the seventeenth century, when Vermuy-
den, the engineer, and the Duke of Bedford and other enter-
prising capitalists took the matter in hand and erected the
great dykes, drains, and pumps that alone keep the land dry
and usable. The water is thus artificially kept down and it
can be maintained at any desirable level within the capacity
of the pumps: the land is thus subirrigated, a feature which
contributes no little to its extraordinary fertility.

Farmers distinguish two types of fen: the clay fen on the
western side and the sandy fen on the eastern side of the
region ; the difference lies probably in the ‘subjacent material,
there being little evidence of any difference in the actual fen.
The Kimeridge clay on the west lies about 5 feet below, but
occasionally it comes above the surface and forms the rising
ground on: which alone dwellings could be built before the
reclamation—the “eys” or islands of the old days. On the
east the fen is underlain by sand.

The most suitable crops are oats, wheat, and above all
potatoes, the introduction of which some thirty years ago
completely revolutionised fen husbandry. In smaller quanti-
ties mangolds, celery, mustard seed, cole seed, rye grass seed,
buckwheat, and other seeds are grown. Corn crops, however,
do not finish well: they start well but do not “corn out”.
But where clay lies underneath a complete remedy lies in bring-
ing up the clay and spreading it: this is done about once in
twenty years. The soils shrink very much on drying, forming
large cracks dangerous to animals and sometimes destructive
to cart wheels. Oxidation is continually proceeding at a rapid
rate and within living memory the fen has shrunk several feet :
in many cases it only has another 5 or 6 feet to fall before
disappearing altogether,

Fen soils do not require lime or nitrogenous manures, or,
as a rule, potash. But they do respond in a marked degree to
superphosphate, as shown by the following experiments made
by the Cambridge Department of Agriculture :—'

+ Cambridge Farmers’ Bull. No. 6. Two of the soils overlay clay, the third
was over gravel,

310 SOIL CONDITIONS AND PLANT GROWTH

With Mangolds. With Potatoes.
Tons, | Cwts. Tons. | Cwts,
No manure : Ph a 2% | No manure . : «:) hanes I2
3 cwts. superphosphate | 16 2 | 4 cwts. superphosphate | 7 15
O55 *§ 21 13 a ” 8 ro

Bones also give good results, but basic slag appears to be
without effect. Summer fallow has been observed to have a
bad effect on fen soils,

In practice the.clay fen soils after their periodical claying
receive nothing but superphosphate: they are extraordinarily
fertile, commonly yielding 8 tons of potatoes, 90 bushels
of oats and 56 bushels of wheat per acre. The sandy fen
soils are less fertile because they cannot be clayed except at
great expense: the chief need again is for phosphate, but
potash also is wanted.

The wonderful black earths of Russia, Canada, etc., are
probably akin to the fen soils. The black soils of the Canadian
prairies have been described by Shutt (265): under wheat
cultivation they require no fertiliser ; the similar Tchernozem
of Russia and Hungary also carry practically nothing but
wheat and receive little or no manure (85, 1546).

The “muck land” of the United States appears to be of
the same type: it is underlain by shell deposits containing
lime, and no structure can be detected in the organic matter :
it differs from peat, however, in that it responds to lime and
potassic fertilisers,

Clay Soztls.—Clay soils are characterised by the presence of
20 to 50 per cent. of “clay” and similar quantities of silt and
fine silt ; in consequence of this ‘excess of fine particles the size
of the pores is ‘so diminished that neither air nor water can
move freely. Clay soils, therefore, readily become water-
logged and in time of drought may not sufficiently quickly
supply the plant with water; in our climate, however, they are
usually moist or wet. The high content of colloidal matter
impresses marked colloidal properties: (1) the soil shrinks on

a

THE SOIL IN RELATION TO PLANT GROWTH © 311

drying and forms large gaping cracks which may be several
inches wide and more than a foot deep; (2) it absorbs much
water, a good deal still being held even when the soil appears
to be dry; (3) the water, however, moves only slowly, and it
is not uncommon to find a fairly sharp line of demarcation
between moist and dry soil, and for land to crack badly
within a few feet of a ditch full of water; (4) the soil readily
absorbs certain soluble salts and organic substances. In
addition the special clay properties are shown: plasticity and
adhesiveness when wet, and a tendency to form very hard
clods when dry. All these properties are much modified by
calcium carbonate and intensified by alkalis; the chalky
boulder clay is usually fertile: on the other hand, liquid
manure (which contains ammonium carbonate) and nitrate of
soda (which gives rise to sodium carbonate in the soil) are
both to be avoided.

Clay soils have had rather a chequered agricultural history.
Originally covered with oak forest and hazel undergrowth
they were early reclaimed for agriculture purposes by draining,
applications of lime,' and, later, of ground bones. Wheat and
beans were the great clay crops, and in the early part of the
last century, under the combined influence of high prices, large
drainage schemes and artificial stimulus to enclosure, great
areas came into cultivation so that now only little unreclaimed
clay remains, excepting where the forest was preserved for
hunting. Crops grew well but ripened late; a wet harvest
was a terrible calamity. Bare fallowing was always necessary
once in four years, and any of the intervening years might, if
wet, be lost by the difficulty of getting on the land to sow the
crop. When the price of wheat fell in the eighties many of
these soils went out of cultivation and became covered with a
mixed growth of grass and weed, which was grazed by stock
and gradually deteriorated as the old drains choked up and
the land became more and more waterlogged. Azra cespitosa,
“bent” grass (Agrostis vulgaris), yellow rattle (RKhinanthus

1 E.g. see Gervase Markham, Inrichment of the Weald of Kent, 1683.

312 SOIL CONDITIONS AND PLANT GROWTH

Crista-gallt), and in drier places the quaking grass (Briza
media) and ox-eyed daisy (Chrysanthemum Leucanthemum) are
among the more obvious plants on these neglected fields ; the
only relics of the past are the field names and the high ridges
or ‘“‘lands” made years ago to facilitate drainage. But
recently marked improvement has set in. Drainage is
gradually being attended to, improved implements allow of
better cultivation: mangolds, kohlrabi, and other fodder crops
suitable for heavy soils have been grown, whilst additions of
lime and phosphates (as basic slag) have markedly improved
the herbage, favouring the development of white clover (77-
folium repens) and the pasture grasses, and crowding out the
weeds. Fig. 30A and B shows specimens of her':age from some
of Robertson’s plots at Horndon-on-the-hill, '.ssex (London
clay). Potassic fertilisers are not usually needed. Only in
the dry eastern counties has the old arable cultivation survived.

It is commonly observed that plants growing on clay
soils tend to have larger leaves, and to make shorter-jointed
growth, than plants on sandy soils.

Many of the ecological and agricultural observations on
clay need revising in view of the distinction which has recently
been set up between the silty clays and the true clays. The
former owe their heaviness to the presence of much fine silt
which differs in colloidal properties from clay (p. 107), and is
not flocculated by lime, frost, etc. Indeed, no way is known
for ameliorating these soils and they are generally left as
rather poor pasture. The true clays are often indistinguish-
able on casual inspection, but they behave differently on
cultivation and respond to lime and good treatment whenever
it is deemed worth while to improve them.

Sandy sotls are formed of large silica particles deficient
in colloidal matter, and therefore possessing little power of
cohesion, or of retaining water or soluble salts. Hence they
tend to be dry, loose, and poor in soluble substances—
“hungry,” the practical man calls them. Their behaviour
towards vegetation depends very largely on their position,
their depth, and the nature of the subsoil, these being the

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THE SOIL IN RELATION TO PLANT GROWTH 313

factors that determine the water supply to the crop. The
water supply is usually satisfactory when the surface soil
contains sufficient clay and not too much coarse sand and
gravel, and rests on a deep subsoil containing rather more of
the finer particles. It is a further advantage if other land lies

higher and furnishes a supply of underground water. In such
_ cases the land is nearly always cultivated ; it yields early crops
of high quality rather than heavy crops, the tendency to
drought inducing early maturation, while the absence of
stickiness makes sowing an easy matter at any time. Fruit,
nursery stocks, potatoes, and market-garden produce are often
raised, and high quality barley is also grown. The winter
feeding of sheep on the land is a common way of fertilising,
but crops must be sown early, or the fertilising material is
washed out unused, and the young roots have no time to
strike into the subsoil before the surface layer dries out. High
farming is the only profitable way of dealing with these soils ;
any carelessness in cultivation lets in hosts of weeds, such as
poppies, knot-weed (Polygonum aviculare), spurrey (Spergula
arvensis), sorrel, horsetail, convolvulus, creeping buttercup, and
others. Crops should follow each other in rapid succession,
any interval being a period of loss; under good management
two or even three market-garden crops can be secured in the
year, while in purely farming districts catch crops should
always be taken. Organic manures are very necessary to
increase the water-holding capacity : sheep-folding or green-
manuring are, therefore, very desirable. Frequent repetition
is necessary, as the organic matter speedily disappears.
Calcium carbonate is often needed and is better applied as
ground chalk or limestone than as lime. Potassium salts are
beneficial and may be added as kainit; nitrates often give
remarkable results, but phosphates are not usually needed
because the soil conditions already tend to promote good root
development. Only small quantities of manure must be added
at the time, as the soil has little retentive power. Above all,
no very costly scheme of manuring should be recommended till
preliminary trials have shown its profitableness.

314 SOIL CONDITIONS AND PLANT GROWTH

In reclaiming or fixing sand dunes the first step is to grow
grass, eg. Marram grass (Psamma), which will stabilise the
_ form and then allow a more varied vegetation, eg. sand
binders (Carex arenaria, Festuca rubra, var. arenaria) and
ephemerals, to stabilise the surface, thus leading to the de-
velopment of a festuca sward or, in other conditions, to heath
and scrub.

A soil underlain at a short distance below the surface by a
bed of gravel, a layer of rock, or a “ pan,” is liable to be either
parched or waterlogged, and its water supply is usually so
unsatisfactory that cultivation is unprofitable. Under low
rainfall the land becomes a steppe, under rather higher rainfall
a heath, but the vegetation is always xerophytic, consisting
of heather, ragwort, broom, etc., the trees being birch and
conifers—the latter often planted in recent times. No method
of cultivating these soils has ever been devised, and most of
them still remain barren wastes, defying all attempts at
reclamation. Two special cases have, however, yielded to
treatment :-—

1. When the layer of rock or the pan is only thin and is,
in turn, underlain by a rather fine-grained sand, its removal
brings about continuity in the soil mass and thus effects a
great improvement in the water supply. The soil now
resembles the fertile sands, and should be treated in the same
way. A good example is afforded by Cox Heath, Maidstone
(p. 333).

2. Where the gravel or rock is not too near the surface,
systematic green manuring with lupines and other crops
fertilised by potassium salts and calcium carbonate will often
effect sufficient improvement to make cultivation profitable.
Examples are afforded by the Schultz-Lupitz estate, Germany
(255), and Dr. Edwards’ experiments at Capel St. Andrews,
Suffolk, and at Methwold. On such land an industrious
cultivator may make a living but not a fortune.

Under favourable conditions recourse may be had to

1See F. W. Oliver and E. J. Salisbury, Topography and Vegetation of
Blakeney Point (Trans. Norfolk and Norwich Nat. Soc., 1913, 9, 485-543)

ies

THE SOIL IN RELATION TO PLANT GROWTH 315

dressings of clay (as in Lincolnshire) or to warping (in the
Fens).

Barren conditions also result when, by reason of a thin
parting of clay or its low situation, water cannot run away but
accumulates and forms a marsh. Reclamation in such cases is
possible as soon as a way out has been found for the water.’

Loams.—As the proportion of fine material in the soil
increases and that of coarse material falls off, a gradual change
in the character of the soil sets in, till finally, but without any
sharp transition, a new type is reached known asa loam. The
increase of fine material somewhat retards the movements
both of air and of water, so that loams are characterised by a
more uniform water content throughout the mass than sands.
On the other hand, loams show less tendency to become
water-logged or to allow plants to become parched in very
dry weather than clays, The soil decompositions proceed
normally, rapidly producing plant food, with little tendency
to ‘‘sour”’? or other abnormal conditions so long as sufficient
calcium carbonate is present. In consequence, most plants
will grow on loams, even some of those supposed to be specially
associated with some other soil type. Thus, where a chalk

.and a loam soil meet, it is not uncommon to find the chalk —

plants, e.g. traveller’s joy (Clematis Vitalba), guelder rose, etc.,
wandering on to the loam, and it is much more difficult to
find the line of separation of the soils than where the chalk
abuts on to a sand or a clay. For the same reason loams
allow of very wide choice in the systems of husbandry, and,
as they become very fertile under good management, they are
usually in this country all cultivated. Closer observation over
a limited area shows, however, that a given class of loam is
more suited to one crop than to another; the ecologist
recognises differences in the sub-associations or facies, and the
practical man will distinguish between a potato soil, a barley
soil, a wheat soil, etc. : distinctions due, no doubt, to water

1 For a fuller discussion of land reclamation see E. J. Russell, fourn. Roy.

Ag. Soc., 1919, 80, 112-132.
2See p. 118.

316 SOIL CONDITIONS AND PLANT GROWTH

and air relationships, and arising from differences in the com-
pound particles. Unfortunately no method of investigating
the compound particles has yet been devised, and a study of
the ultimate particles by a mechanical analysis is alone
possible. But even though the differences are thus attenuated
they can still be traced, as shown by the analysis in Table
LXXXII. of soils in Kent, Surrey, and Sussex, known to be
well suited to the particular crops.

Low amounts of clay and fine silt, and high amounts of
coarse sand, whenever the clay begins to approach 12 per
cent., characterise the potato soil; these are the most porous
of the series, allowing free drainage and aeration. Barley
tolerates heavier and shallower soils, Fruit and hops both

TaBLE LXXXII.—MEcHANICAL ANALYSES OF SoILs WELL SuITED TO CER-
TAIN CROPS IN THE SouTH-EasTERN Counties; LIMITS OF VARIATION,
HALL AND RUSSELL (122a),

Potatoes. | Barley. Fruit. Hops. | Wheat. | Waste Land.
Fine gravel -| O*I-3 | 0'2-2°5 | 0°3-2°3 | 0°3-2°3 | 04-6 I-7
Coarse sand -| 2-47 I-53. | 0°8-9°5 | 0°7-9°5| 0-13 16-69
Fine sand . - | 23-68 20-45 30-55 25-39 | 15-31 18-64:
Silt . . «| 3°5-2°4| 5-33 13-44 | 20-45 | I1-35°5 2-7
Fine silt . : 5-9 | 3°5-16"4 6-11 6-1r | 9°5-24 2
Clay . : - | 5°5-12°6 | 4-Ig | 10°5-14°6 | 11°5-15 | 13-24 | 1 or less

require deep soils, and seem to find their most favourable circum-
stances only in a restricted class of soils: the fruit soils gener-
ally contain rather more sand and less silt than the hop soils.
But the fruits differ among themselves ; the best nursery stock
is raised on soils of the potato class, where the conditions are
for some unknown reason very favourable to fibrous root
development ; strawberries prefer the lighter and apples the
heavier kinds of fruit soil. Even different varieties of the
same plant show distinct preferences for one class of soil over
another: the finest varieties of hops are found only on the
typical hop soils, and have to be replaced by coarser varieties
directly it is desired to grow hops on heavier soils. Prefer-
ences for certain soil conditions are also shown by varieties of

THE SOIL IN RELATION TO PLANT GROWTH 317

' the common crops, oats, barley, wheat, etc.; unfortunately
these can only be discovered by direct field trials, and even
then the results only hold so long as similar conditions
prevail and may often be reversed in a different climate or
season.

Still more subtle differences may be observed: one and the
same variety of a crop will acquire one habit of growth on one
soil and a different habit on another. Wheat growing under
the best soil conditions will produce stiff straw and ears well

, set with corn, so that a crop of 50 or 60 bushels per acre may
be raised without difficulty ; on soil rather different in type,
and especially under somewhat different climatic conditions,
only 30 or 40 bushels can be raised, because the ears are less
thickly set and the straw is too weak to carry a heavier crop,
becoming “ laid” directly an attempt is made to increase pro-
duction by increased manuring.1 Whether some unknown
nutrient is absent from these soils, or whether the adjustment
of the air and water supply is wrong, is not known; but the
limitation of yield arising from this unsuitability of soil con-
ditions is one of the most serious problems of our time.
Another instance may be given: in Romney Marsh pastures
commonly occur carrying a vegetation of rye grass and white
clover, with crested dog’s-tail and agrostis, easily capable of
fattening sheep in summer without any other food. All
round these pastures are others, with he same type of vegetation,
but the plants grow more slowly, produce more stem and less
leaf, are less nutritious and zucapadble of fattening sheep. The
soils are identical in mechanical analysis and in general water
and temperature relationships, although certain differences
have been detected (12I1c). Again: grass grown on Lower
Lias pastures in Somersetshire and Warwickshire causes acute
diarrhoea (“scouring”) in cattle, whilst grass on adjoining
alluvial pastures does not (106). Harmful effects of a wholly
different nature are recorded from certain Swiss pastures.”

1 Further illustrations are given by the author in Science Progress, 1910, v.,
286.
2 Fahrb. Schweiz., 1898, 104-5.

318 SOIL CONDITIONS AND PLANT GROWTH

Some of the South African grazing lands tend to give the
animals a serious disease, Lamziekte, which is attributed to a
harmful quality in the herbage and not to any particular
plant or organism.’ Lastly: potatoes grown in the Dunbar
district are remarkable for their quality, they will stand boiling
and subsequent warming-up without going black. The same
varieties of potatoes grown in the same*way in the Fens
blacken badly under the same treatment, and consequently
command a much lower price in the market (7d). Instances
might be multiplied; enough have been given to show that
the plant responds in a remarkable degree to variations in
soil conditions. Our knowledge of these variations is frag-
mentary and wholly empirical, and would be much furthered
by close and detailed study, jointly by a botanist and a
chemist, of the factors causing differences in plant associations
in two nearly similar habitats.

The agricultural treatment of loams, as already indicated, . —
admits of considerable variety. The old plan was to apply a
good dressing of dung every third or fourth year and a smaller
intermediate dressing; clover was also grown every fourth
year, and, on light loams, the root crop was eaten by animals
on the land. At long intervals lime was applied and some-
times bones. ~The modern movement is towards specialisation,
each man producing the crops he can best grow and manag-
ing them in the way he finds most profitable, but the system
usually involves feeding a good deal of imported food to sheep
and cattle, either on the land, or in yards, and utilising the
excretions as manure, buying nitrate of soda, sulphate of
ammonia, and manufacturers’ waste products (generally those
derived from imported animal or vegetable products) to
supply more nitrogen, and buying also imported phosphates
and potassium salts, Thus the fertility of highly-farmed
countries like England tends. to increase at the expense of
new countries that export large amounts of animal and
vegetable produce. But the transfer is prodigiously wasteful ;

1See Ingle, ¥ourn. Agric. Sci., 1908, 3, 22-31; A. Theiler, also H. H.
Green, 5th and 6th Veterinary Reports, Union of South Africa, 1918.

THE SOIL IN RELATION TO PLANT GROWTH 319

enormous losses arise in virgin countries through continuous

cultivation (p. 181), and at this end in making dung (p. 193),
and especially through our methods of sewage disposal. It
seems inevitable that these losses must make themselves felt
some day, unless the movement for the conservation of natural
resources ever becomes a potent factor in international
life.

Soil Fertility and Soil Exhaustion.

_ From the preceding paragraphs it is clear that fertility is
not an absolute property of soils, but has meaning only in
relation to particular plants, Plant requirements vary ; a soil
may be fertile for one plant and not for another; every soil
might conceivably prove fertile for something. But in practice
the agriculturist can find use only for a very limited number
of plants; he, therefore, has to select those combining the
double features of saleability in his markets and suitability to
his conditions of soil and climate. To a certain extent. it is
possible to bridge the gap between plant requirements and
soil conditions; the former may be permanently altered by
breeding if suitable plants cannot be found by selection, and
the latter may be changed by such processes as draining,
liming, etc. When all has been done that is economically
possible there may still remain a divergency between the
conditions ideal for the plant and those it finds in the soil;
this divergency is the measure of the infertility of the soil for
the crop.

The problem has to be simplified by restricting attention
to the common agricultural crops and interpreting fertility to
mean the capacity for producing heavy crops regardless of any
subtle distinctions of quality. Four factors then come into
play: an adequate supply of air and water to the roots, a
sufficiently rapid production or solution of food material, a
sufficiently rapid movement of nutrient substances to the plant
roots, and absence of harmful agencies. These have already
been discussed in Chapters III. and VI., where also it is
shown that the three are not independent, but related to one

320 SOIL CONDITIONS AND PLANT GROWTH

another, inasmuch as they are all directly bound up with the
nature of the compound particles, and, therefore, with the
ultimate particles as revealed by mechanical analysis, and
with the amounts of calcium carbonate and of organic
matter.

We have seen that the compound particles can be altered
considerably by human efforts, within limits fixed by the
properties of the unalterable ultimate particles. In trying to
improve a soil, therefore, four courses are open :—

1, The water supply may be increased by deepening the
soil, e.g. by breaking a “ pan,” by enriching the lower spit, or
other device, while the air supply can be increased by drain-
age. :
2. The compound particles may be built up by proper
cultivation and the addition of organic matter (eg. dung,
green manuring, etc.) and of calcium carbonate.

3. Sufficient calcium carbonate must be added for the
needs of the crop and the micro-organisms—nothing but a
field trial can determine what this is.

4. The food supply can be increased by the addition of
fertilisers, the ploughing-in of green leguminous crops, feeding
cake on the land, etc.

Conversely the “exhaustion” of soil is limited in our
climate to the removal of organic matter, calcium carbonate,
and some of the food (often the nitrogen compounds), and the
destruction of the desirable compound particles ; the ultimate
particles, and all the possibilities. they stand for, remain
untouched. A distinction is therefore made between the
temporary fertility or “condition” within the cultivator’s
control, and the “inherent” fertility that depends on the
unalterable ultimate particles. Of course, the distinction is
very indefinite and, in practice, wholly empirical, no proper
methods of estimation having yet been worked out, but it is
of importance in compensation and valuation cases.

Serious soil exhaustion did not arise under the old
agricultural conditions where the people practically lived on
the land and no great amount of material had to be sold

THE SOIL IN RELATION TO PLANT GROWTH 321

away from the farm. Phosphate exhaustion was the most
serious occurrence because there was no way of meeting it,
and as the original supplies were not as a rule very great, it
must have become acute by the end of the eighteenth century
in England, for remarkable improvements were, and still are,
effected all over the country by adding phosphates. Then
began a process, which has gone on to an increasing extent
ever since, of ransacking the whole world for phosphates ; at
first the search was for bones, even the old battle-fields not
being spared, if we may believe some of the accounts that
have come down; later on (in 1842) Henslow discovered
large deposits of mineral phosphates, to which more and more
attention has been paid. Phosphate supplies may yet become
the factor that will determine the course of history.

The crowding of the population into cities, and the
enormous cheapening of transport rates, led during the
nineteenth century to the adoption in new countries, particu-
larly in North America, of what is perhaps the most wasteful
method of farming known: continuous arable cultivation
without periodical spells of leguminous and grass crops. The
organic matter was rapidly oxidised away, leaching and
erosion increased considerably when the cover of vegetation
was removed, while the compound particles that had slowly
been forming through the ages soon broke down. Nothing
was returned to the soil, the grain and other portable products
were sold and the straw burnt. The result has been a rate of
exhaustion unparalleled in older countries, and wholly beyond
the farmer’s power to remedy, consequently he left the land -
and moved on, The excellent experimental studies of Hop-
kins (138) at the Illinois Experimental Station, of Whitson
(307) at Wisconsin, and other American investigators, have
shown that additions of lime, of phosphates, and sometimes of
potassium salts, with the introduction of rotations including
grass and leguminous crops, and proper cultivations will
slowly bring about a very marked improvement,

21

CHAPTER IX.
SOIL ANALYSIS AND ITS INTERPRETATION.

WHEN an agricultural chemist is asked to analyse a soil he is
expected to give some information about the crops to which
it is best suited, and the manures that must be applied; he
has, therefore, a much more complex problem than the
mineralogical or geological chemist who simply has to report
on the actual constituents he finds. It has been shown in the
last chapter that the vegetation relationships of soils are not
determined solely by the nature of the soil, but also by its
position, subsoil, climate, and other circumstances, so that it
is manifestly impossible for the chemist to make a satisfactory
report on a sample of soil on the basis of analytical data only.
He cannot even give a complete account of the soil itself,
since he has no method of estimating the compound particles
on which the water and air supply, the temperature and the
cultivation properties depend, but he can only get at the ulti-
mate particles out of which they are built.

Soil analysis is, therefore, restricted to: (1) comparisons
between soils, showing which are fundamentally identical and
in what respects others differ; (2) the tracing of such corre-
lations as exist between the chemical and physical properties
of the soils of a given area and the crops and agricultural
methods generally associated with them. In order to carry
out either of these satisfactorily it is necessary to make a
systematic soil survey, a task that is certainly laborious but
by no means impossible, since each agricultural chemist usually
confines his attention to a definite region—a county or so—
and is not called upon to deal with outside soils.

In planning a soil survey it must be remembered that the
322

————— ee

— Ss

SOIL ANALYSIS AND ITS INTERPRETATION 323

basis of the whole work is empirical: the agricultural and
vegetation characteristics have first to be ascertained by field
trials, and then systematised and amplified by aid of the
laboratory data. It is necessary to begin, therefore, by going
over the whole region very carefully and dividing it up in
areas within which similar agricultural or vegetation character-
istics prevail. In general the areas differentiated in the
geological drift maps will be found identical with the vegeta-
tion areas, especially if the drift map is interpreted in the
light of the Wemozrs of the Geological Survey. But as agri-
culture is influenced by altitude and rainfall, the investigator
must also use an ordinary contour map and a rainfall map,
which, unfortunately, he must construct for himself from data
in British Rainfall. Within each vegetation area a number

_ of soil samples must then be taken from points representing

the area as closely as possible: situated, for example, on level
regions or long gentle slopes, and not on made ground, steep
slopes, or places of local disturbance, etc.

In seeking for typical places the investigator is likely to
meet with a good deal of discouragement; farmers will tell
him that half a dozen or more different kinds of soil occur on
their particular farms, while the vegetation of a wild area may
show considerable changes. But these differences often arise
from differences in the compound particles and not in the
ultimate particles; small variations in the amount of calcium
carbonate or of organic matter, or differences in the water
supply, or management, may considerably affect the ease of
cultivation and the vegetation relationships and give the im-
pression of a wholly different type of soil. So little does
cropping, cultivation, etc., affect the ultimate particles that it
is quite immaterial for the purpose of a soil survey whether the
sample is taken from pasture land or arable land, but it is well
to take a number of samples from both. With a little practice
abnormal places are easily avoided. It is necessary to take a
large number of samples, not only to get at the type, but also
to trace out the causes of the phenomena noted by the farmer,

and to discover the main factors determining the cultivation
ar.*

324 SOIL CONDITIONS AND PLANT GROWTH

and vegetation relationships of the soil. Very full inquiries
must be made on the spot as to the agricultural value of the
land, the crops and manures most suitable, its behaviour
during drought and wet weather, and any special points to be
observed during cultivation. Information is also wanted
about the most troublesome weeds, the native vegetation,
hedgerow and other timber, etc., and note must be taken of
the position of the soil in regard to water supply, the nature
of the strata down to the permanent water table, etc. The
most reliable information is obtained only by properly con-
ducted manurial trials.

It is usual to take: the sample to a depth of 9 inches and
a lower sample to a depth of 18 inches, but if any marked
change occurs in the soil the sample should only be taken to
the point where it sets in. The subsoil sample does not char-
acterise the formation any better than the surface sample, but
it affords a useful check and helps in detecting abnormalities.

The vegetation areas correspond with the geological forma-
tions only so long as the lithological characters remain con-
stant. Some formations are very uniform, eg. the Folkestone
beds of the Lower Greensand, but, in general, certain changes
are observed. Where the formation has been laid down in an
estuary of no very great size, the coarser particles are deposited
near the old shore and the finer particles farther out, so that
a gradual change from finer to. coarser soil is observed in
travelling along the formation, necessitating a soil division
into two or three vegetation areas; the Hythe beds of the
Lower Greensand and the London clay afford illustrations.
Considerable trouble often arises when the formation consists
of a number of strata of sands and clays, and the outcrops are
so narrow that no one type persists over any large area. The
simplest method of procedure is to map out any uniform areas
that possess sufficient agricultural importance, and then group
the remaining less important soils simply into gravels, sands,
loams, and clays. In dealing with drift soils, it is well first to
map the uniform areas and then look out for lines of uniformity
and make up regions within which the agricultural character-

SOIL ANALYSIS AND ITS INTERPRETATION — 325

istics vary between a higher and a lower limit. The investi-
gator must be guided by the importance of the region from
the particular point of view in deciding how closely he is to
map out the country.

Absolute uniformity cannot be expected over any con-
siderable area, and even such uniformity as existed has often
been upset by subsequent rearrangements of the soil which
result in the original surface being covered up with a later
deposit or being washed away, leaving the original subsoil to
become the new soil. Such changes are readily detected by
mechanical analysis of the surface and subsoils ; examples are
given in Table LXXXIII. At Merton the subsoil on the lower
ground appears to have been the original surface soil, because
of its identity with the surface soil and its marked difference
from the subsoil of the land higher up; it has been covered
with a deposit identical with and presumably derived from the
higher lying soil. The same thing has happened at Hamsey
Green. At Woodchurch, however, it appears that the old
surface of 69, now the subsoil, has been covered with rather
a lighter soil; it is equally possible, however, in this particular
case that 70 has lost its original surface soil, the present

TaBLeE LXXXIII.—VariaTION IN Soin DUE TO WASHING OR FLOODING.

Formation . London Clay. Clay-with-Flints. Weald.
Locality ‘ Merton, Surrey, Hamsey Green, Surrey. Woodchurch, Kent.
- Lower On the ; Soil 110, Z
Ground. Hill. Soil rog. 200 yards Soil 69. Soil 70,
¥ away.
g mB $ FI Sad | ie eal | 9g = 9. \ a
SS 3 ss 2 3s $ % S 8 ° Ss °
Bis isleie sitet sleials
n n n n n n n n n n n n
Fine gravel} 1°7| 1°3| 1°5| o°3] 1°6| 3°r| 1°7| 1°4] 0°5| 0°6} o’9| 0°7
Coarse sand 18*4 | 23°6|16°9| 8:4] 9°5| 6°7| 5°3] 7°I] 2°5| Ig] Ir] rx
Fine sand .| 12°7 | 11°3 | 12°4 | 12°7 | 22°3 | 28-0 | 28°7 | 25°1| 14°7| 13°0| 9°3|/ g'0
Silt . = .; 16°6 | 18°0 | 16°6 | 13°4 | 25°4 | 22°5 | 26°3 | 17°6 | 24°2 | 27°8 | 25°9 | 18°8
Fine silt .| rr°r|11°4| 10°r| 9°8} 9°9| 12°6| 10°2| 9°5 | 23°7 | 23°3 | 24°4 | 26°5
Clay . -| 24°6 | 24°9 | 26°7| 41°7] 16°0 | 16°4 | 164 | 28°3 | 20°r | 28°9 | 28°6 | 37°38

326 SOIL CONDITIONS AND PLANT GROWTH

surface being the bared subsoil. This kind of variation is
common on clay soils and often leads to differences in agri-
cultural value that are fairly marked, but not sufficient to
affect the type. Normally the subsoil contains more clay than
the surface soil (as in 110, 69, and 70), and any deviation
should be carefully investigated.

The characterisation of soil types is usually effected by
mechanical analysis and determinations of calcium carbonate
and organic matter. A representative set of soils should,
however, be subjected to chemical analysis, the clay fractions
being, if possible, broken up by ammonium fluoride and

analysed completely. ‘ils about which precise information
has been obtainec i. «. :\urial and other trials should be very
completely examir . order that they may serve as standards

in the analysis ot _.ner soils from the same area.

The problem set by the farmer is wholly different. He
does not want to know to what type his soil belongs, but how
he must manure it, etc. Ifthe analyst has an adequate know-
ledge of the soil type and the locality he can readily ascertain
in what respects the soil differs from the type, and then, from
the known results of manurial and other trials on that type,
he can give the information wanted with a reasonable degree
of probability ; otherwise his report can only be a matter of
guesswork. In short, the farmer's problem can be satisfac-
torily solved, and the manurial trials fully interpreted, only
when a complete soil survey has been made.

The analyst must consider the soil from three points of
view > (1) its physical properties, especially those relating to
the ease of movement of the soil water; (2) its store of plant
food, actual and potential; (3) the rate at which potential,
food can be converted into actual food.

The Interpretation of Mechanical Analyses.’

The properties of the various fractions have already been
given in Chapter III., but some little practice is necessary

1 See Appendix for Methods of Analysis and PP. 98 and ro2 for details as to
dimensions and composition of fractions.

SOIL ANALYSIS AND ITS INTERPRETATION 327

before they can be used for the interpretation of an analysis.
A few illustrations are therefore given from Hall and Russell’s
survey of Kent, Surrey, and Sussex (121@): the data are set
out in Table LXX XIV.

The Chilworth soil contains so little clay and fine silt and
so much coarse sand that it has very little power of retaining
water. As it lies too high to obtain any seepage water from
the neighbouring formations it is dependent on the immediate
rainfall, and is therefore not in cultivation but has always been
heath land. Owing to its bad constitution and its high situa-’
tion it could not by any known method be made suitable for
farming. .

TaBLE LXXXIV.—MEcHANICAL ANALYSES OF SOILS AND THEIR INTER-

PRETATIONS,
Formation. 7 > Higa Thanet Beds. | Brick Earth., af rn Alluvium.
P Chil- | Shal-| Gold- | Bar- | Ick- ; Tol- |Shaddox-
Locality . * | worth.) ford. | stone. | ton. | ham. Oving. worth.| hurst. Ewhurst.
Gravel aemee bh BS | 0°21 O82.) 03) OG 0°4. | O'S) (| OFF | OF

Coarse sand | 65°9 | 52°6 | 15°3 | 2°3 | o°7 | 1°3 | 12°8 TA Ps
Fine sand . | 23°7 | 26°2| 44°9 |34°7 |24°7 |16°0 | 25°5 | 110 |19°8 |19°3

Silt. -} 24 | 4°8127°3 |36°2 | 44°8 | 35°5 | I1°3 | 19°6 |28°4 |13°0
Fine silt .| 2°0 | 3°5| 63 | 6°3 | 86 |13°3 | Ir°r | 26°3 |r2*r |20°0
Clay . a hero.) 9°3-|: Sg | 1x95 | t4°7 | 15:9 | 23°7 | 22°r | |19°7. |26°9
Calcium car-

bonate .| nil | 0°3] o°08| 0°18] 0°4 | 0°75| 2°0 | o°r6 | 0°05! 0°28
Loss on igni-

tion . e}. 20) |. -3°3,)° B°r-| 4°3 | 4°6. | 65 56 | 98 {ror2 |x1r°3

The Shalford soil lies lower down and has a better water-
supply, less coarse sand, and more clay and fine silt. But its
water-holding capacity and its retentive power for manures are
still very low; artificial manures are of much less value than
organic manures, and the best treatment of the land is to grow
green crops and fold them off to sheep. It is better suited to
special purposes like the production of malting barley or
market-garden crops than to ordinary mixed farming. —

The Goldstone soil contains more clay and fine silt, and

328 SOIL CONDITIONS AND PLANT GROWTH

has therefore better power of retaining water and manures,
and is more productive and more generally useful. But as
the coarse sand exceeds the clay in amount it is still distinctly
light ; it responds better to organic than to artificial manures
and suffers rather in droughty weather in spite of lying not far
above the marshes. It contains 45 per cent. of fine sand and
therefore tends to cake on the surface after rain and to form
steely lumps if worked when wet. Under proper management,
however, it produces good crops and is equally suited for
ordinary arable and for fruit or potato cultivation.

The next three soils may be taken as illustrations of the
very best loams in the three counties. Silt forms the largest
fraction and therefore the soils possess sufficient, but not too —
great, a power of retaining water. The fine silt is always
lower than the clay; the latter varies between 12 and 16 per
cent., a very satisfactory amount where the rainfall is not too
high. As there is a considerable amount of fine sand and no
excess of fine silt and clay, the absence of coarse sand is no
disadvantage.

The Tolworth soil is highly productive arable land but
almost too heavy for profitable cultivation ; only by dressings
of dung (fortunately obtainable cheaply from town) can it be
kept workable: It contains rather too much clay and would
no doubt have gone down to grass had there not been so
much coarse sand present.

The Shaddoxhurst soil is bad. It contains much clay and
still more fine silt, consequently its texture is not improved as
much as might be expected by liming. There is practically
no coarse sand and not much fine sand to keep the soil open,
it has always and deservedly been in bad repute. It is best
as pasture land, and, after drainage and treatment with basic
slag, it may be made useful but never very good.

The Ewhurst soils are both in pasture, being too heavy
for arable cultivation on account of their high clay and low
coarse sand content. The first has the better constitution ;
silt is the predominant feature, the clay is not too high, nor is
the fine silt. It has all the characteristics of a good, heavy,

*

SOIL ANALYSIS AND ITS INTERPRETATION 329»

soil, and is indeed known to be an excellent bullock pasture.
The second is not so good; it contains too much clay and
fine silt, and too little silt and coarse sand. It has no great
agricultural value.

Factors Modifying the Interpretation of a Mechanical
Analysis.

The Amount of Organic Matter.—Organic matter at the
proper stage of decomposition has the effect of binding a loose
soil and lightening a heavy one; thus it reduces the difference
between a light sand and a heavy clay, bringing them both
closer to the loams. When Io to 15 per cent. of organic

-- matter is present it so impresses its properties on the soil that

the mechanical analysis loses much of its significance, and all
the analyst can do is to point out what the soil would become
if by persistence in certain methods of management the organic
matter were reduced below a certain point.

It is, however, essential that the organic matter should be
properly decomposed. Barren sandy wastes not infrequently
contain 5 to 10 or even I5 per cent. of organic matter, but
much of it is simply dried bracken or other vegetation that
has not broken down and has no value, but rather the reverse,
in improving the physical conditions. Ifthe drainage is bad
a good deal of peat may form; further, the water fills up the
soil, making its condition bad whatever its composition may be.

The Amount of Calcium Carbonate.—In interpreting a
mechanical analysis it must be remembered, as shown on
p. 117, that 1 or 2 per cent. of calcium carbonate may greatly
modify the clay properties and give a considerable degree of
friability to a soil which otherwise would be very intractable.
When the percentage rises to much higher amounts the soil
becomes very chalky and the mechanical analysis loses its
meaning, just as when much organic matter is present.

Water-supply and the Interpretation of Mechanical Analysis.

It has already been pointed out that a mechanical analy-
sis can be interpreted and discussed with any degree of

330 SOIL CONDITIONS AND PLANT GROWTH

completeness only in terms of the water-supply ; the rainfall, -
the coolness of the climate, the presence of moving under-
ground water, and the nature of the subsoil all have to be
taken into account. | Wes

Effect of the Rainfall.—The effect of a high rainfall is to
bring into prominence the “sticky” properties of the fine
fractions, and to put into the background their water-holding
capacity. Thus a light soil under a high rainfall behaves
like a heavier soil under a low rainfall; it is as well supplied
with water and on the whole behaves in the same kind of
way on cultivation. For example, the Stedham soil (Table
LXXXV.) is rather lighter than the Swanley soil, and yet in
virtue of its extra rainfall is more useful for farming purposes ; *
indeed, the Swanley soil is essentially a market-garden soil,
requiring large dressings of dung for successful cultivation.
The North Chapel soil is physically as good as the East Far-
leigh soil but agriculturally much inferior ; owing to the higher
rainfall it becomes somewhat too sticky to cultivate profitably
and so is in rather poor grass; the East Farleigh soil, on the -
other hand, is from a highly fertile hop garden.

TaBLE LXXXV.—WATER-SUPPLY AND INTERPRETATION OF MECHANICAL

ANALYSIS.
seinen hdueierr Chepel. Farleigh. worth. | fed. | Dunbar.

Fine gravel . 7 2 I°4 tobe) 2°3 0°6 2°9 3°0 1'0
Coarse sand 162 9°3 II"4 9°5 |37°8 | 46°6 | 33°53 | 23°7
Fine sand . -| 58°6 | 68°5 | 43°2 | 30°6 | 33°r | 22°9 | 280 | 38°2
ht : sr DM a: 3°0: =. |-13°0 || 10°F y iy 4 3°5 555 6°8
Fine silt . E 5°L 56°) roe.) rae 4°7 8:8 | r0°S | 1x8
Clay . r : 5°55 5°5 I1o’"9. | 13°3 7°6 6'9 6°6 9°5
Loss on ignition . 2°9 3°4 5'I 56 3°6 | 3°6 69 | 6:2
Calcium carbonate "02 03 "80 |] I°0 °277 "aI I5 *31
Rainfall in inches

(approximate) .| 24 33 30 24 28 27°5 | 25 25

Coolness of Climate.—Soils containing so much coarse sand
or fine sand that they would scorch or burn ina dry warm

1 An example is given by G. W. Robinson, Yourn. Ag. Sci., 1917, 8, 370.

— se eee eo

ae

nea

SOIL ANALYSIS AND ITS INTERPRETATION — 331

district may prove very suitable for cultivation in a cooler
district where evaporation is lessened. Potato soils afford
some good illustrations; potatoes require a light soil, but it
must be cool and moist. The Nutfield soil (Table LXXXV.)
fulfils these conditions ; it is ona slope facing northwards not

very far above a stream, and, therefore, does not quickly dry

out, hence it is very good for main crop potatoes. The
Tolworth soil, on the other hand, although similar in com-
position, is so placed that it quickly dries and is of much less
value. Some of the potato soils of Dunbar, analysed by
S. F. Ashby (7d), have all the appearance of soils readily
drying out, but in their cool climate this property does not
show itself to an injurious extent.

Effect of Underground Water.—When the underground
water is near the surface, but sufficiently far below to allow
of proper root development, the most important property of
the soil becomes its power of lifting the water by surface
action up to the roots. The silt and sands are in such cases
the useful constituents, the clay and fine silt being less neces-
sary. The Weybridge soil (Table LXXXVI.), at about 3 feet
below its surface, has a current of underground water which is
brought to the roots by the fine and coarse sand. It therefore
grows excellent wheat crops. The Bagshot sands, however,
although similar in physical type, have in general no such

water-supplies and are sterile because they lack the clay

which, in their circumstances, could alone confer an adequate
power of holding water from one shower to the next.

TaBLE LXXXVI.—UNDERGROUND WATER AND MECHANICAL ANALYSIS.

Weybridge. (Bagshot Beds.| ° Shalford. Lydd.
Fine gravel . 1°3 "r to °6 2°5 o°L
Coarse sand. a 38°4 20 4, 30 52°6 0"9
Fine sand . E A 39°9 45 5, 65 26°2 - 66°7
ae , ; ‘ 5°6 5 », 10 4°8 By:
Fine silt 5°1 5 5, 10 3°5 II"4
Clay 3°8 3» 7 3°8 3°9

The Shalford soil is a light sand with too little power of

332 SOIL CONDITIONS AND PLANT GROWTH

retaining water for pastures to last through a hot summer,
consequently the grass land, except near the brooks, is parched
and scorched. The Lydd soil is certainly somewhat finer
grained, but not so very different that one would expect to
find it much better for pasture purposes, yet it produces one
of the best pastures in Romney Marsh, not only carrying but
fattening sheep throughout the summer. It has, however, a
constant supply of water 3 or 4 feet below the surface, while
at Shalford the water level is much lower down.

This underground flow is one of the factors concerned in
the proverbial fertility of valleys. Soils lying towards the
bottom of a long slope receive not only the rainfall but also
the water steadily drifting downwards to the stream or marsh
at the bottom, and this advantage is further enhanced by the
gradual transport of soil down the slope which increases the
depth through which the plant roots can range.

Effect of the Subsotl.—In general the subsoil is vislens
heavier in type than the surface soil, especially in the case of
clays; examples are given in Table LXXXVII. The excep-
tions to this rule may arise through periodical flooding with
water containing much clay in suspension, or through the oc-
currence of a bed of sand just below the surface.

TABLE LXXXVII.—NATURE OF THE SUBSOIL.

Loddington. Harting. Dicker. Shopwyke. Wye.

Sur- Sub- | Sur- | Sub- Sur- | Sub- | Sur- | Sub- | Sur- | Sub-
face. soil. | face. | soil. | face. soil. | face. | soil. | face. | soil.

Fine gravel] 3°5 SO OG) Oe T’o | '.0°6!] 06! o'r} 26a
Coarse sand | t0°2 | 98 | 33] 3°2] 20] xr] o8| 04 | 3°0| Ig
Fine sand .| 33°5 | 30°2 | 31°6 | 33°9 | 26°6 | 23°2 | 25°0 | 21g | 27°2 | 25°3

Silt . -| 14°6 | 17°5 | 17°3 | 21°3 | 23°0 | 15°r | 27°3 | 38°0 | 4oro | 41°4
Fine silt .]| r4°9 | 15°5 | 14°5 | 13°4 | 17°38 | 21°g | 16°4| 152 | 8:9] 9°6
Clay . » | £2°2 .| 15°3 | 12°3"| £6°O || T7°9 | °25°7'| RTs) Gey | eee

Two cases described on page 314 may be illustrated here.
The bad effect of a layer of impermeable material near the
surface is shown by the Loddington soil (Table LXXXVIL.),

en ee ee es

yo

SOIL ANALYSIS AND ITS INTERPRETATION — 333

typical of an area near Maidstone (Cox Heath), much of which
was waste land. Its sterility was due to no fault in the soil,
which is obviously of excellent type, but to a thin layer of
rock lying near the surface. When this was removed a very
good soil was obtained. The Harting soil lies on the Upper
Greensand in West Sussex; the rock comes close to the
surface, restricting both the root range of the plant and the
water supply where it lies horizontally, but proving much less
harmful where it dips at any considerable angle. The soil
itself is good, although it has rather too much fine silt, and
it becomes very productive when the effect of the rock is
counteracted. The Dicker soil, while not of the best type, as
its fine silt is too high, is far from being hopeless, but it
unfortunately lies on a deep bed of stiff clay which keeps it
wet in winter and parched in summer. It is therefore very
poor, and even with the best management never gives great
results.

The second case, over-drainage, is illustrated by the
Shopwyke soil in the same table. It is a fair soil, containing
too much fine silt to be in the first rank, but it is spoiled by
lying on a deep bed of gravel only nine inches or a foot below
the surface—the subsoil sample could only be taken in one or
two instances. Consequently it dries out badly in summer

and does not repay much expenditure in the way of manures, -

The Wye soil is given as an instance of the normal case
where a soil becomes rather heavier in its lower depths, with
the result that the movement of water is somewhat impeded
without being stopped. Thus the subsoil furnishes a reserve
of water for the surface, yet even in wet weather it does not
hold up too much water. The Wye soil contrasts with the
Dicker soil, the usual case in a clay, where the subsoil contains
much more true clay than the surface.

Chemical Analysis of Soils.

Recourse is had to chemical analysis to discover the
amounts of potential and actual plant food in the soil, and the

1 See Appendix for methods of analysis.

334 SOIL CONDITIONS AND PLANT GROWTH

rate at which potential food is likely to become available.
But as the problem is vague, so the methods are empirical and
the interpretation of the results often very difficult.

The following discussion is confined to English soils and
English methods. No claim is made that these methods are
superior to those of other countries or that they give absolute
information about the amounts of plant nutrients in soils,
They are intended only to facilitate comparison of given soils
with a standard soil, and they are probably neither better nor
worse than other analytical methods which might be pro-

posed.
| In the United States chemical analysis is not in much
favour with the younger school of investigators, though
determinations are made of the components of the water
extracts of soils.

For a discussion of the chemical and physical vaca
of soils formed direct from granite in Aberdeen, or from the
paleozoic soils of North Wales, the reader is referred to the
papers by Hendrick and Ogg (132) and G. W. Robinson
(240).

Organic Matter—The analyst should note whether the
organic matter is fairly well decomposed, whether it still shows
definite plant structure, and, whether or not it is acid to litmus
paper. Hecan then interpret his observations as shown on
“pages 128 ef seq.

Nitrogen.—Unlike the other soil constituents nitrogen
and carbonates are determined absolutely, The amount of
nitrogen is closely related to the loss on ignition, of which in
a large proportion of cases it is about 3 per cent. Asa guide
to fertility it is therefore subject to the same limitations; a
high nitrogen content may be associated either with a rich
soil containing abundance of valuable non-acid organic matter,

1 See, e.g., J. S. Burd, Fourn. Agric. Res., 1918, 12, 297-310, and Soil Sci.,
Igt8, 5, 405-419, who finds water more useful than citric acid and holds hydro-
chloric acid worthless as a means of assessing the crop producing power of a
soil (see p. 234).

2 See also Aberdeen Bulls., 1, 3, and 10, where the relationship between soil
analysis and manurial results is discussed.

————-

M seageed

tia pte Sd

forms accumulations of peat.

SOIL ANALYSIS AND ITS INTERPRETATION — 335

or with a soil where the conditions are so unfavourable that
organic debris does not decompose (e.g. acid soils), or only
On the other hand, some of the
best loams, where the conditions are most favourable to rapid
decomposition and nitrification, contain but little nitrogen.
A few typical examples are given in Table LXXXVIII.

ms TasBLeE LXXXVIII.—NITROGEN AND Loss ON IGNITION.

hasan erin rr ne

3
=

x

Fertile Arable Soils.

Poor Arable Soils.

Barren Wastes.

Loss on ignition |4°65
Nitrogen . | *I20
Loss on ignition
in subsoil .|3*00
Nitrogen in sub-
soil : .| °078

6°58 |3°70
*220| 133

4°94 (2°81
*139| ‘081

4°65 14°13
*I41] ‘128

3°29 13°74

*og7] “II2

6°23 |3°60

5°50 |2°58

"104| ‘o61| 096

5°14
*143| *182| 152

4°I4

"94 |7°00 |5°SI
*130| *195| ‘167
— | — |2°70
<1 = } 058

open downland.

Soils containing much calcium carbonate are as a rule
rich in nitrogen, partly no doubt because of the rather high
nitrogen content of the rock and partly also because they are
folded, green manured, cropped with leguminous plants like
sainfoin, lucerne, etc., all of which tend to increase the nitrogen
supply. The nitrogen in some chalk soils is given in Table
LXXXIX.; all are arable soils, excepting the last, which is

TaBLE LXXXIX.—NITROGEN CONTENT OF CERTAIN CHALK SOILS.

Nitrogen in surface soil . *25 "194
BA subsoil . "128 *130
Calcium carbonate in sur-
| face soil ‘ -| r8°r 3°70
Calcium carbonate in sub-
soil ; F 11°37 | 14°9

*331
*162

49°7
61°3

258
*Ig2

66°0

55°2

249 “419
*196 "180

65°6 44°0

54°8 716

Carbonates.—The analyst is often asked whether or not

a particular soil contains sufficient calcium carbonate, and in
endeavouring to answer this question he must bear in mind
the twofold function of this substance, to prevent ‘“‘ sourness”

(p. 118), and to flocculate the clay. Where only a small

_. amount of clay—say 8 per cent. or less—is present the floccu-
lating action is less needed and a smaller amount of calcium

336 SOIL CONDITIONS AND PLANT GROWTH

carbonate suffices. The Stedham soil (Table LXXXV.) is an
example; it is near the bottom of a slope along which water
containing calcium bicarbonate in solution is drifting, and
therefore shows no tendency to become sour. The 0°03 per
cent. of calcium carbonate present, hopelessly inadequate as
it appears, suffices for its needs and no increase in crop is ob-
tained by applying lime. The Lydd soil (Table LXXXVL)
contains only ‘o2 per cent., but is also well supplied with cal-
careous water from below and shows no sign of sourness.
Similar soils that have not this advantage of position stand in
great need of lime even when 0’! per cent. is present. As the
amount of clay increases, the need for lime becomes greater
because flocculation is now wanted ; soils with 20 per cent. or
more of clay may need two or three times as much lime as
sandy soils. It is impossible to fix limits that shall hold —
universally. Some soils free from lime appear to need none:
Hendrick and Ogg’s Craibstone soil has already been men-
tioned (p. 118). Grégoire! also found soils practically free
from carbonate and yet not acid, and presumably not needing
lime. Before an analyst recommends lime or chalk on a sandy
soil he should satisfy himself that the need is indicated by the
vegetation, and before he states that lime is not necessary on ~
a clay he should be quite sure that further additions would
have no beneficial flocculating effect. The need of lime is
indicated :—

1. If clover fails to start well, or to stand the winter, or
if it looks bad in spring.

Cases have been examined by the writer where clover
or lucerne were failing in patches in the field and weeds
were consequently getting a firm hold. The amounts of
lime in the soil were :—

On the good parts: Suffolk lucerne, 0°8, Norfolk lucerne, 0°6,
Herts clover, o*2 per cent. calcium carbonate.

On the bad patches: Suffolk lucerne, 0°07, Norfolk lucerne, o*2,
Herts clover, o’or per cent. calcium carbonate.

Ann, Stat. Agron., Gembloux, 1913, 2, 87.

SOIL ANALYSIS AND ITS INTERPRETATION 437

2. If swedes, turnips, or cabbages get finger-and-toe
rather badly.

The Armstrong College experiments have shown that

2 tons per acre of ground lime, or 34 tons per acre of

ground limestone, afford suitable dressings in this case.’

_3. If mayweed springs up vigorously among the wheat,
or if spurrey, sorrel, or bent grass become prevalent.

Land that has been wet through the winter ought to
have lime in the spring: otherwise uneven patches may arise
in the field on which weeds develop and the crop ripens
unevenly.

Neither lime nor limestone, however, should be applied to
potatoes or oats unless actual trials have shown that benefit
will be obtained ; as a general rule these two crops respond
_ less than others: and in the Kilmarnock trials,’ lasting over
_ eight years, potatoes were actually injured by lime, though
oats benefited by it (p. 244).

a Basic slag reduces the need for lime, but superphosphate
does not. Sulphate of ammonia increases the need: for lime.
Instances of soils known to respond to lime are given in Table

XC.

Taste XC.—Catcium CARBONATE CONTENT OF SoiLs KNOWN TO RESPOND

TO Lime.
Sandy Soils. Loams and Clays.
No.of /|Percentage Percentage of No. of Percentage Percentage of
Soil.3 of Clay. | Calcium Carbonate. Soil.3 of Clay. Calcium Carbonate.
126 73 | 04 207 II*r ‘02
675 89 08 I19 10"4 03
193 60 18 118 II’5 18
189 3°8 95 412 152 x2°2 +26
215 13°0 45
127 13°3 I*0o

1 Armstrong Coll. Bull., No. 12, 1915.

2 West of Scotland Agric. Coll. Bull., No. 55,-1911 (pp. 193-222).

*The numbers are those used in Soils and Agriculture of Kent, Surrey, and
Sussex (Hall and Russell).

22

338 SOIL CONDITIONS AND PLANT GROWTH

Alumina.—In general, in the much weathered soils of
the south of England the alumina is approximately equal in
amount to one-third of the clay fraction, indicating that the
acid treatment breaks down some definite group of silicates
associated mainly with the clay fraction in the soil? Ex-
amples are given in Table XCI.

TasBLeE XCI.—ALUMINA AND CLAY CONTENTS OF VARIOUS SOILS.

Sand- |Folke-

y Lond Thanet Weald

Formation. Bagshot Sands. Clay. 4 Beds. Fp Boa Clay.
Percentage of clay

in soil -13°6 |4°9 |7°r | 36°8 | 21°3 | 1I°5 |7°I |15°3 | 6°9 | 33°8
Percentage of Al,O,

in soil ‘ -| °92] 1°43 | 1°94] 11°75] 6°78] 3°46) 2°66} 5°14 | 1°99 | 10°45

R ti Al,O, 2 "2 2 *2T °2T *2y . . * 8 "QI

OH lay fit . 5 | 29 7 3 2 31) °37| °33| °2 3

Exceptions to the rule occur when much fine silt is
present, the alumina then being markedly less than one-third
of the clay (Table XCII.). °

TaBLE XCII.—ALUMINA AND CLAY CONTENTS OF SILTY SOILS.

Formation. Weald Clay. Se Pe 20 Gault.

Percentage of fine silt in soil |27°4 |35°8 |25°9 |15°8 |2r°s |14°3 |15°9 14°5 |14°0
i clay in soil .|2r°5 |22°r |19°4 | 5°4 |12°5 | o°7 |13°r |12°3 |11°8

% Al,O, . -} 5°02] 5°42] 5°68! °r7| 1°66) 2°38] 2°48) 2°39 5°11
Rati Al,O3 . . ° ~ . . *r8| - .
atio Giny ; ‘ 23| °25| °29), 203). .* 03) faa ee Ig °43

Iron Oxide.—The iron oxide is present in quantities com-
parable with those of alumina, but no close relationship is
observable, nor does the amount of iron oxide afford any
indication of the fertility of the soil. F. V. Dutton finds no
more iron in the fertile red soils of Devon than in the infertile
grey soils. Light soils, good or bad, contain about I to 2°5

1 The percentage of Al,O; given here represents the amounts extracted by
HCl and dre not comparable with those on p. 102 obtained after treatment with
ammonium fluoride.

re ee ee en, ee Bey eee
> * :

a pe eee oe ee a

SOIL ANALYSIS AND ITS INTERPRETATION 339

per cent., good loams and poor clays contain 3'5 to 5 per
cent. Larger amounts of iron oxide are not common. Soils
containing ferrous compounds are generally infertile.’

Lime and Magnesia.—About ‘I to ‘5 per cent. of magnesia
is found in the soils we have examined, and in general the

oe — ratio falls between I and 3, but ratios of 4 and 5 are
magnesia
not uncommon, while on chalk soils they may rise very high,

lime
magnesia
and the productiveness of the soil ; indeed, Table XCIII. shows
that very good and very poor soils may have practically
identical ratios.

No connection could be traced between the

LIME
See RAF s
TasLe XCIII. Lt peneqnarg ATIO IN VARIOUS SOILS
Barren Wastes. Poor Cultivated Soils. Fertile Soils.
Ratio Ratio Ratio
No. of | cao. | MgO. ee. No. of | cao. | MgO. | C20 | N:9f| cao. |MgO.| Cad
gO ? MgO : MgVU

170 | °05 | °06 | I°’o 45 “4% F223 Ig | 183 *56| *40 | 1°4
toa} 13/900 | 1:6 | 242 *30 | °I3 pba 222 "46 | *28 | 16
168 | ‘2x | *13 | 1r°6 | 106 4. take paaigs Bouse xto2 |.°4m: Peete
50 | 15 | 708 | IQ | 255 bho fas Nah 17 2a le I22 “OO! f S22 ho ee
107° | 2% |} °0S | 2°6 196 “43° “E25 3°6 2II | 1°79 | “40 | 4°5
Gio ae: | 03 | 2°71 287 | 1*r9 | *29 4'0 72 | 1°94 | *42 | 4°6
24I | °22 | ‘07 | 3°1 127 | 2°t4 | ‘40 | 574
13 | 58 | ‘14 | 4°12

Potash.—In the south of England soils the amount of
potash is closely associated with that of alumina, being
commonly about one-tenth ; it is, therefore, about one-thirtieth
of the clay. Some examples are given in Table XCIV.

The “available” potash? shows no kind of regularity,
but varies between 5 and 50 per cent. of the quantity ex.
tracted by strong acids. In deciding whether or not sufficient
is present, attention must be paid to the soil, the crop and the

1For the effect of soil iron compounds on soluble phosphates see A. C.
de Jongh, Int. Mitt, Bodenkunde, 1914, 4, 32-45.
2 T.e. extracted by 1 per cent. citric acid.
22:*

340 SOIL CONDITIONS AND PLANT GROWTH

TaBLE XCIV.—PotasH aNnp CLay CONTENTS OF VARIOUS SOILS.

‘ Percentage : KgO. Ala) 4
No. of Soil. | * oF AlLOn. ghia yk RO aro oben: Ratig oe |
112 4°07 *45 “Tr I3°I 034
120 2°34 *31 ‘IT 104 “029
Bere) 3°83 "40 *IO II‘2 "035
133 3°67 “44 “12 Tr*7 37,
I03 3°66 *30 “08 Ir'g "025
161 7°97 r'08 *I4 27°7 039
67 11°75 I"44 ‘12 36°8 *039
118 3°46 "404 *I2 II'5 "035
79 5°14 "40 "08 15°3 *026
43 10°45 *76 "07 33°8 *022
I47 7°88 *96 ‘52 22°5 043

rainfall. Thin chalky soils, sandy soils, and soils rich in
organic matter are peculiarly responsive to potassic manures,
whilst clay soils generally are not. Carbohydrate-making
crops, like sugar-beet, mangolds, and* potatoes also invariably
want more potash than they find in the soil or in dung.
Potassic manures also tend to prolong the life of the plant,
and, therefore, to increase the yield in dry districts where the
conditions all tend to early stoppage of growth. Illustrations
are afforded in Table XCV. where soils in dry districts, known
to respond profitably to potassic manures, are compared with
soils in places of much higher rainfall where potassic manures
do not prove profitable.

TaBLE XCV,—‘‘ AVAILABLE ’’ POTASH IN SOILS OF KNOWN BEHAVIOUR
Towarps Potassic MANURES,

Soils Responding to Potassic Soils not Responding to Potassic
Manures. Manures.
East Kent. Surrey. | Sussex. West Sussex. Kent.

Newing-| Barton. | Redhill. agg Oving. | Rogate. | Stedham. | Yalding.

Available K,0| ‘or3 °0I5| ‘oro| ‘oo7] ‘or4) ‘024 ‘oro 044
K,O extracted

by conc, HCl *200 *404| ‘181 | ‘260 *43 18 ‘I4 “59
Clay . .| Go |II°5 78 |25°5 |15°9 | 6°7 5°5 gi

Rainfall «| 22°5 23 27°7. |28°6 [28 33 33 24

t

SOIL ANALYSIS AND ITS INTERPRETATION 341

All are arable soils. The chalk pastures on the South
Downs usually contain less than ‘o1 per cent. of available
potash (e.g. the Patching soil), and they respond to potassic
manures. It will be observed that ‘o15 per cent. is insuffi-
cient in East Kent where the rainfall is 23 inches, whilst -o10
per cent. suffices in West Sussex under Io inches higher rain-
fall and generally better water-supply in the soil.

Phosphoric Actd.—Generally speaking, the largest amount
of phosphoric acid is found in chalk soils, 0-2 to 0°25 per cent.
being present ; about o°15 to o’2 per cent. is found in good
loams; sandy loams contain about o'1 per cent. while poor
clay pastures and poor sands contain still less, Little if any
direct connection can be traced between the phosphoric acid
and the productiveness ; in general it tends to increase as the
clay, fine silt, and silt increase—the poor clay pastures form
a readily explained exception—but it does not appear to be
closely associated with any one fraction like the potash. The
amounts of “available” phosphoric acid vary enormously ;
Kentish hop gardens commonly contain from ‘o5 to ‘18 per
cent. ; well-farmed arable soils contain some ‘O15 per cent.,
while in poor worn-out pastures the quantity may sink as low
as ‘002 per cent. In most cases. these quantities are insuffi-
cient for some of the crops grown, especially where high
quality or feeding value is aimed at; hop growers regularly,
and, they maintain, profitably, apply phosphates to gardens

_ already containing ‘o5 per cent. of available phosphoric acid,

whilst arable farmers use them for swedes when ‘O15 per cent.,
or sometimes even more, is present. The exceptions to this
rule are the light soils sufficiently provided with moisture and
a forward climate ; on these the need for phosphates appears
to be less. But in all cases where much purchased food is fed
on the land phosphates appear to be of advantage to the suc-
ceeding crop.

Rainfall does not appear to have so marked an effect in
controlling the need for phosphates as it has for potassic
manures. The explanation is to be found in the fact that
phosphates are useful both in dry and in moist situations:

342 SOIL CONDITIONS AND PLANT GROWTA :

they tend to promote root development, an obvious advantage
in a dry soil where the plant will fail unless the roots strike
into the deeper, moister layers; they also stimulate the vital
processes going on at the end of the season and are thus
valuable in wet, cold districts. But rainfall and water-supply
are important factors in determining the choice of phosphates
on arable land; basic slag proves less useful than super-
phosphates on dry soils, but it is sometimes nearly as useful
on moister soils or under higher rainfall. The amount of chalk
in the soil is not the determining factor, but the moistness ;
if, as often happens, a chalky soil is dry, superphosphates will
prove the more useful; where the soil is moister, baste slag is
as good, and of course cheaper. Evidence is accumulating
that mineral phosphates are often of value. On grass land
basic slag is often more effective than superphosphate.

The Relative Advantages of Mechanical and Chemical
Analysis.

The fundamental distinction between mechanical and
chemical analysis is that the former deals with the whole of
the soil, which it sorts out into fractions of varying sizes,
while the latter deals only with the part that is readily dis-
solved by acids. Mechanical analysis, therefore, gives a
picture of the whole (albeit very incomplete), while chemical
analysis does not; it is in consequence. eminently suited for
the purpose of a soil survey, the chief object of which is to
classify and describe the soils. Further, it enables the in-
vestigator to explain to some degree the observed water re-
lationships of the soil when sufficient is known about the
water-supply, and also to account for many of the peculiarities
observed in cultivation. It enables him to say, as far as can
be said on our present knowledge, whether any observed de-
fects are due to defects in the soil or its situation, or to the
system of management that has been adopted. As it cannot
be interpreted fully without a knowledge of the amounts of
organic matter and calcium carbonate present, these two quan-
tities must be determined in every sample.

m

ees 9

SOIL ANALYSIS AND ITS INTERPRETATION — 343

_ In the preceding discussion the soils have all belonged to
one type—the much-weathered soils of the South of England.
We have seen that in this case there is a close correlation be-
tween the potash, the alumina and the clay. For purposes
of a survey it seems superfluous to determine these two bases
in every sample taken. The iron oxide shows a general but
by no means a close correlation with the others; but no con-
nection could be traced between iron oxide and fertility in
the soils examined by the author, the iron oxide being almost
always less than 5 percent. in amount. Nor did it appear
that the ratio of lime to magnesia in these soils was significant.
The nitrogen is closely correlated with the organic matter, ze.
the loss on ignition. The total phosphoric acid shows no great
variations in the different soils, but the available phosphoric
acid, like the available potash, varies greatly with the manage-
ment of the soil. Thus the figures obtained by chemical
analysis, apart from the loss on ignition and the calcium car-
bonate, fall into two groups: the nitrogen, potash and alumina,
which are so closely correlated with quantities already deter-
mined in the mechanical analysis that their separate deter-
- mination is almost superfluous ; and the iron oxide, magnesia,
lime, etc., which do not give sufficiently useful indications to
be worth determining in every case. Since chemical analysis
fails to describe the soil with sufficient completeness for
agricultural purposes Hall and Russell recommend that for
purposes of a survey a large number of soils should be sub-
mitted to mechanical analysis, including the determination of
organic matter and of calcium carbonate, and then a carefully
chosen representative set should be analysed chemically so as.
to characterise the type ; these can further serve as standards
with which farmers’ samples can be compared by the citric acid
method. They agree with Whitney that mechanical analysis
should form the basis of the survey, because it alone takes
account of those physical functions—the regulation of the
water-supply and therefore of the temperature, of the air
supply, ease of cultivation, etc.—that play so large a part in
determining the value of a soil,

344 SOIL CONDITIONS AND PLANT GROWTH

But, on the other hand, mechanical analysis is restricted
in its application and breaks down altogether on chalk soils,
acid humus or peat soils, and neutral humus soils, while it
gives useful indications only on the mineral soils, ze. sands,
loams and clays. Agricultural soils belong so largely to this
group that the method is really applicable in by far the great
majority of cases.

Among the mineral soils the chemical grouping cuts across
the mechanical classification. When the clays are separated
out and subjected to hydrofluoric acid treatment or fusion with
alkalis and then analysed, they are seen to fall into two or
more types, as shown in Table XXIX. (p. 102). Further
evidence of dissimilarity among the clays is obtained by a
study of the results of the acid extraction of the soil; in
general the alumina is about one-third of the clay in amount
and the potash is roughly one-tenth of the alumina ; in other
cases, however, very different relationships obtain.

For the comparison or characterisation of types chemical
analysis becomes of considerable importance. For this pur-
pose Hissink (136) and von ’Sigmond (2662) have each em-
phasised the value of extraction by strong HCl, though they -
do not agree as to the interpretation of the results. Hissink
claims to be able, by suitable modification of the analytical
process, to differentiate van Bemmelen’s three groups :—

(1) “Complex A,” the ‘weathered silicates” capable of

absorption and exchange of bases.

(2) ‘‘Complex B,” the less active “kaolin silicates,” '

(3) Inert “unweathered silicates” incapable of absorption

and base exchangeieffects.
The advantages of such discrimination are manifest, but
von ’Sigmond is not prepared to admit that it can be done.
He considers, however, that strong hydrochloric acid has a
practical value for describing soils. Other chemists do not
altogether agree: E. A. Mitscherlich sees no advantage in the
method ? (see p. 234).

1 For a discussion of these see R. Gans, Int. Mitt. Bodenkunde, 1913, 3, 546.
2 [bid., 1914, 4, 327-

SOIL ANALYSIS AND ITS INTERPRETATION 345

SOIL SURVEYS.

Te Soil surveys of the following counties have been pub-
lished :-—
ENGLAND.

a -Beprorp.—
a Rigg, Th. Zhe Soils and Crops of the Market-garden District
a | Of Biggleswade. Journ. Ag. Sci., 1916, vii., 385.
CAMBRIDGE.—
' Foreman, F. W. Sozls of Cambridgeshire. Journ. Ag. Sci.,
1907, ii., 161.
DorsET.—
Gilchrist, D. A., and Luxmoore, C. M. Ze Soils of Dorset.
Reading Coll. Dorset C. Council, 1907.
a Kent, SURREY, AND SUSSEX.—
_ Hall, A. D., and Russell, E. J. Agriculture and Soils of Kent,
Surrey, and Sussex. Bd. Ag. and Fisheries, tgtt.
NotTrincHamM.—
( Goodwin, Wm. TZ%e Soils of Nottinghamshire. Mid. Ag. and
) Dairy Coll. Kingston, Derby.
NoRFOLK.—
Newman, L. F. Soils and ether of Norfolk. Trans.
Norfolk and Norwich Nat. Soc., 1912, 1X., 349-393:

i SHROPSHIRE.—
Robinson, G. W. A Survey of the Sotls and Agriculture of
Shropshire. County of Salop Higher Education Com., n.d.

WALES.

Robinson, G. W. Studies on the Paleozoic Soils of North Wales.
Tourn. Ag. Sci., 1917, Vill., 338.

- Robinson, G. W., and Hili, C. F. Further Studies on the
Soils of North Wales. Journ. Ag. Sci., 1919, ix.. 259.

Griffith, J. J. Jnfluence of Mines upon Land and Livestock in
Cardiganshire. Journ. Ag. Sci., 1919, 1X., 366.

346 SOIL CONDITIONS AND PLANT GROWTH

SCOTLAND.

Hendrick, James, and Ogg, W. G. Studies of a Scottish
Drift Soil. I. The Composition of the Soil and of the Mineral
Particles which compose tt. Journ. Ag. Sci., 1916, vil., 458.

IRELAND.

Kilroe, J. R. Soil Geology of Ireland. Dept. Ag. and Tech.
Instruction for Ireland, 1907.

See also publications of Dept. of Ag. and Tech. Inst. for Ireland.

In France the surveys have been made on a petrogeological
basis as initiated by M. Risler in 1856. The work is particularly
associated with the Station Agronomique de |’Aisne et Laon. Many
of the maps are beautifully drawn: for an example see Journ. Min.
Ag., 1920, 27, 57. In the United States an extensive soil survey is
organised by the Dept. of Agriculture and in many of the individual
States by the Colleges or Agricultural Experiment Stations. For the
soil map of Germany see /nt. Mitt. Bodenkunde, 1917, ‘7; 1.

ER Re ote a Week aay Te
- : = 3 ia s

- ha

ate ts

APPENDIX I.

THE METHODS OF SOIL ANALYSIS.

How to Take the Sample of Soil.—Owing to the variation in
composition of the soil at different depths it is particularly necessary
that the sample should always be taken to the same depth and with
a tool making a clean vertical cut. Samples taken with a spade are
of very doubtful value and do not justify any lengthy examination.
A suitable tool consists of a steel tube 2 ins. in diameter and 12 ins.
long, with a #-in. slit cut lengthwise and all
its edges sharpened fixed: on to a vertical
steel rod, bent at the end to a ring 2 ins.
in diameter, through which passes a stout
wooden handle (Fig. 31). A mark is made
9 ins. from the bottom so that the boring
process can be stopped as soon as this ES
depth is reached. On withdrawing the tool
the core of soil is removed by a pointed iron
rod. Five or six samples should be taken
along lines crossing the field so as to get as
representative a sample as possible; the
whole bulk must then be sent to the labora- WU
tory. Samples should not be taken from Fic. 31.—Tool for taking
freshly ploughed or recently manured Jand. Boe Remriee
; In very stony soils it is easier to use a 2-in. auger, but this does
not, in our experience, yield as satisfactory a sample as the tool shown
here.

i. A spud is useful for rapid preliminary inspection of waste land
to ensure that the sample is normal; much disturbance of these soils
is sometimes caused by rabbits.

For precautions to be taken in drawing the sample see Russell,
| Journ. Bd. of Agric., 1916, 23, 342, and for a discussion of the
_ magnitude of the experimental error see Robinson and Lloyd, Journ,
347

¢ a

bi aia

SE ON ig STN ee TO Be

348 SOIL CONDITIONS AND PLANT GROWTH

Agric. Sct., 1915, '7, 144-153, and Leather, Zrans. Chem. Soc., 1902,
81, 883- 886.

For American methods of sampling see Bot. Gazette, 1919, Feb.,
p. 173. C. B. Lipman and D. E. Martin’ show that no further or
unusual precautions need be taken when the sample is required ior
bacteriological purposes.

The Analysis.—On arrival at the laboratory the soil is spread
out to dry, and is then pounded with a wooden pestle and passed
through a 3 mm. sieve. The stones that do not pass through, and
the fine earth that does, are separately weighed, and the proportion
of stones to 100 of fine earth is calculated. Subsequent analytical
operations are made on the fine earth.

Moisture.—Four or five grms. of the soil are dried at 100° C.
till there is no further change in weight. -

Organic Matter.—No accurate method of estimation has yet
been devised. It is usual to ignite at low redness the sample dried
as above. The loss includes organic matter, water not given off at
too° C., and carbon dioxide from the carbonates ; allowance may be ~
made for the latter, but not for the combined water. The carbon is
sometimes determined either by the ordinary combustion or by some
wet combustion method. Methods have also been described for
determining ‘‘ humus,” but they have not come into general use.
For ordinary purposes it is sufficient to take the loss on ignition as
organic matter.

Total Nitrogen.—Kjeldahl’s method is almost invariably adopted.
About 25 to 30 grms. of soil are ground up finely in an iron mortar ;
10 to 15 grms. are heated in a Kjeldahl flask with 20 to 25 c.c. of
strong sulphuric acid for three-quarters of an hour; 5 grms. of
potassium sulphate are added, and then a crystal of copper sulphate.
The heating is continued till all the black colour has gone. Then
cool and dilute the mixture, transfer the fluid part to a distillation |
flask, but leave as much as possible of the sand behind, wash well to
remove all the adhering liquid. Add saturated soda solution till the
liquid is strongly alkaline, distil, and collect the ammonia in standard
acid.

LVitrates must be determined in a sample taken direct from the
field and dried without any delay at 55° C.; 200 to 500 grms. of the
dried soil are pressed firmly on to a Buchner funnel fitted to a filter
flask, and distilled water is poured on. The. first 300 c.c. of water

1 Soil Sci., 1918, 6, 131-36.

APPENDIX I 349

passing contains practically all the nitrates, but it is safer to wash
more fully. The solution is poured into a flask covered by an
inverted porcelain crucible-lid, ro c.c. of 4.per cent. caustic soda
and 1 or 2 cc. of 3 per cent. potassium permanganate are
added, and the whole is then boiled down to some 75 c.c. and

_ kept just boiling for about two hours. If the permanganate is com-

pletely decolorised a little more is added until there is no appreci-
able change in half an hour. The excess is then destroyed by
cautious addition of sodium sulphite solution and the solution is
diluted to 200 c.c. and distilled down to 50 c.c. with the addition
of 1 grm. powdered Devarda alloy, 10 c.c. more of 4 per cent.
caustic soda and half a grm. of recently ignited lime. The condenser
should be of pure. tin with a short length of hard glass tubing at its

lower end to dip into the N/so sulphuric acid in the receiver.

Shortly before the end of the distillation the cooling water is emptied
out so that steam passes through. A large volume of hydrogen and
spray is given off at the beginning of the operation, so special
attention must be given to trapping. The titration is carried out as
described under Ammonia.

In the United States the more rapid phenol sulphonic acid

method is used. (See C. B. Lipman and L. T. Sharp, Univ. Cad.
_ Pub. Ag. Sct., 1912, I, 23-37-)

For statistical discussion of errors see D. D. Waynick, Univ.
Cal. Pub. Ag. Sci., 1918, 3, 243-270.

Ammonia is determined by aeration. The apparatus consists of
a glass tube sloping at an angle of about 40°; its dimensions are
about 83 cm. by 2°2 cm. and it has a pear-shaped bulb blown on it
at about one-third of its length from the upper end, the small end
of the bulb being downwards. It is closed at the lower end bya
rubber stopper, through which a glass inlet tube for air passes close
to the wall of the larger tube at the under side. The upper end of
the large tube is connected by a rubber stopper with a short tube
packed with cotton-wool to stop spray, and this in its turn is con-
nected with an absorber containing from 5 to 10 cc. of N/s50
sulphuric acid diluted with water and a few drops of a 0’o5 per cent.
solution of methyl red. The freshly taken soil is passed as quickly
as possible through a 3 mm. sieve, stones above this size being
rejected; 25 grms. are placed at the lower end of the tube on a

_ loose cotton-wool plug, and 50 c.c. of a solution containing 108

grms. of sodium carbonate crystals and 150 grms. of sodium chloride

350 SOIL CONDITIONS AND PLANT GROWTH

per litre are added, together with a few drops of paraffin lamp-oil
to stop foaming. The upper stopper is replaced and a current of
purified air is sucked through the apparatus until the whole of the
ammonia has been carried over into the absorber. With the
amounts given a total of 1000 litres of air is sufficient ; a good filter
pump should aspirate this amount in from 34 to 4 hours. The
current should not fall below 250 litres per hour. The acid is then
poured into a hard glass flask, titrated with N/s50 caustic soda till
nearly neutral, boiled to expel carbon dioxide, cooled, and titrated
to a clear yellow! (Fig. 32).

10. r REL 20 36 46 50 6p 70
i 2 J

Scale of Centrimetres.

Fic. 32.

Carbonates are determined by treating a weighed quantity of the
soil with dilute sulphuric acid and estimating the carbon dioxide
evolved. Collins’ calcimeter is a satisfactory instrument.’

Lime Requirement.—Hutchinson and McLennan’s method for
the determination of the lime requirement of the soil is as follows :

1See D. J. Matthews, ¥ourn. Agric. Sci., 1920, 10, 72.
2 F¥ourn, Chem. Soc. Ind., 1906, 25, 518. The apparatus is made by Messrs.
Brady and Martin, Newcastle-on-Tyne.

APPENDIX 1 351

Io to 20 grms. o/ the soil are placed in a bottle of 500 to 1000 c.c.
capacity, together with 200 to 300 c.c. of approximately N/50 solu-
tion of calcium bicarbonate, and the air in the bottle is displaced
by a current of carbon dioxide in order to avoid possible precipita-
tion of calcium carbonate during the period of the determination.
The bottle is then placed in a shaking machine for three hours, after
which time the solution is filtered and an aliquot portion of the
filtrate is titrated against N/1o acid, using methyl orange as indicator.
The difference in strength of this filtrate and that of the initial
solution represents the amount of calcium carbonate absorbed, each
cubic centimetre of N/1o acid being equal to 5 mgs. calcium
carbonate.

For a criticism of the method see E. A. Fisher (gg) and for a
modification see F. J. Warth and M. P. Saw, Chemical Mem. Deft.
Ag. India, 1919, 5, 157-172. For an account of the numerous other
methods see H. R. Christensen (674).

Lydrogen Ion Concentration, Py value (see p. 115, and E. A.
Fisher (99)).

Mineral Substances—Complete analysis of a soil after the
silicates have been decomposed and the silica volatilised by treat-
ment with hydrofluoric acid is only rarely attempted. The British
method, adopted by the Agricultural Education Association, is thus
described by Hall: ‘‘ 20 grms. of the powdered soil are placed ina
flask of Jena glass, covered with about 70 c.c. of strong hydrochloric
acid, and boiled for a short time over a naked flame to bring it to
constant strength. The acid will now contain about 20°2 per cent.
of pure hydrogen chloride. The flask is loosely stoppered, placed
on the water bath, and the contents allowed to digest for about forty-
eight hours. The solution is then cooled, diluted, and filtered.
The washed residue is dried and weighed as the material insoluble
in acids. The solution is made up to 250 c.c., and aliquot portions
are taken for the various determinations. ‘The analytical operations
are carried out in the usual manner, but special care must be taken
to free the solution from silica or organic matter” (Zhe Soz?). Asa
rule only potash and phosphoric acid are determined, but where
necessary other bases are estimated in the usual way.

Hissink * has discussed the value of strong HCl as a solvent in
soil analysis. American methods are described by F. E. Bear and

1 Internat. Mitt. Bodenkunde, 1915, 5, 1-24. For determination of iron see
Morison and Doyne, ¥ourn. Agric. Sci., 1914, 6, 97.

352 SOIL CONDITIONS AND PLANT GROWTH

M. Salter! and Continental methods by von ’Sigmond (2662).
See also p. 344.

Potash.—5o0 to 100 c.c. of the solisicin are evaporated to dry-
ness, after addition of 0'5 grm. of pure CaCO, if the original soil did
not effervesce on addition of HCl. Two courses are then open :—

(az) The residue is gently ignited over a Bunsen burner until
completely charred, and is then extracted with water until all the
potassium chloride has dissolved (Neubauer’s method? (212)). To
the clear filtrate 5 c.c. of platinum chloride (containing ‘oo5 grm.
Pt per c.c.) are added and the mixture slowly concentrated on the
water bath to a very small bulk. The potassium platino-chloride is
filtered off in a Gooch crucible, washed with 80 per cent. alcohol,
dried and weighed.

or (6) Add 10 cc. of 5 per cent. baryta solution, evaporate, to
dryness, ignite and take up with water as in (a), add 2°5 c.c. per-
chloric acid (sp. gr. 1°12), concentrate until dense fumes are given
off, allow to cool, add 20 c.c. 95 per cent. alcohol, and stir. Decant
off the clear alcohol, add 40 c.c. alcohol containing 0*2 per cent.
perchloric acid, transfer to a tared filter paper, wash with 50 to 100
c.c. of 95 per cent. alcohol till the runnings are no longer acid, dry
at 100° and weigh as KClO,.

Phosphoric Acid.—The charred residue from which the potassium
chloride has been removed is digested for half an hour on a sand
bath with 50 c.c. of ro per cent. HeSO, and filtered; the filtrate is
treated with 25 c.c. conc. NH,NO; solution and warmed to 55° C. ;
25 c.c. ammonium molybdate, previously warmed to 55° C., is added
and the whole allowed to stand for two hours and filtered. Wash
with 2 per cent. NaNO, till the washings are neutral; transfer the
precipitate and filter paper to the beaker used for the precipitation,
and add a known volume of standard alkali so that the precipitate
completely dissolves. Measure the excess by titration, using phenol-

phthalein as indicator. 1 c.c. of - alkali = ‘0003004 grm. P,O;.8

1West Virginia Bul., 159, 1916.

2 The older method due to Tatlock is still sometimes used. It is described
by Dyer (91).

3 This volumetric method was originally described in Bull. 46 (revised),
United States Division of Chemistry (Washington, 1898). A careful examination
has been made by Prescott and the conditions laid down under which it gives
satisfactory results (Fourn. Agric. Sci., 1914, 6, 111-120). Prescott’s modifica-
tion is given here. The method is applicable for the ‘‘ available” P,O,, but in

APPENDIX I 353

Available Potash and Phosphoric Acid.—Dyer’s directions are as
follows: 200 grms. dry soil are placed in a Winchester quart bottle
with 2 litres of distilled water, in which are dissolved 20 grms. of
pure citric acid. ‘The soil is allowed to remain in contact with the
solution at ordinary temperatures for seven days, and is shaken a
number of times each day. The solution is then filtered, and 500
c.c. taken for each determination ; this is evaporated to dryness, and
gently incinerated at a low temperature. The residue is dissolved
in hydrochloric acid, evaporated to dryness, redissolved, and filtered ;
in the filtrate the potash is determined. For the phosphoric acid
determination the last solution is made, as before, with nitric acid.

Mechanical Analysis.—The object is to obtain information about
the size of the ultimate particles of which the soil is composed; the
compound particles are therefore broken down by treatment with
hydrochloric acid, and afterwards with ammonia. Direct measure-
ment of the ultimate particles is found to be impracticable; indirect
methods have to be adopted, depending on the time taken to fall
through a column of water of given height. When a body falls

_ through a vacuum the time taken is independent of its size or weight,

but if air or any other fluid is present the case becomes more com-
plicated and the proper mathematical relationship has been found

Stag)
by Stokes to be v = 2g ot) where v = velocity of the falling

particle, o its density, a its radius (assuming it to be a sphere), and
p the density and » the coefficient of viscosity of the medium (Z7ans.
Camb. Phil. Soc., 1851, vol. ix., p. 8).

The numerical values at 16°C. are: g = 981, o = 2°5, p = I,
n = ‘ort, and the equation therefore reduces to v = a? x 29730,

Jv
ora = “cm.
HERS YE

The calculated and observed values are found to agree fairly
well, differences being due to the fact that the particles are not true
spheres, and to the existence of convection currents produced by
changes of temperature. ‘The effect of variations in temperature is
discussed by Robinson in Journ. Agric. Sci., 1914, 7, 142.

The method adopted by the Agricultural Education Association
(see Journ. Agric. Sci., 1906, i., 470) is as follows :—
this case the residue from the citric acid extraction has first to be heated two

hours at 120° to 160° to render the silica insoluble. The older method is de-
scribed by Dyer (91).

‘ 23

354 SOIL CONDITIONS AND PLANT GROWTH

(1) Ten grms. of the air-dry earth, which has passed a 3 mm.
sieve, are weighed out into a porcelain basin and triturated with
too c.c. of N/5 hydrochloric acid, further acid being added if much
calcium carbonate is present. After standing in contact with the
acid for one hour, the mixture is transferred to a dried, tared filter
which is washed until acid-free, dried and weighed. The loss repre-
sents hygroscopic moisture and material dissolved by the acid.

(2) The soil is washed off the filter with ammoniacal water on
to a small sieve of 100 meshes to the linear inch, the portion passing
through being collected in a beaker marked at 10, 8°5, and 7°5 cm.
respectively from the bottom. The portion which remains upon the
sieve is dried, weighed, and divided into “fine gravel” and ‘coarse
sand” by means of a sieve with round holes of 1 mm. diameter.
The portion which does not pass this sieve is the “fine gravel ”.
This is dried and weighed. The difference gives the “‘coarse sand ”.
If required, both these fractions can also be weighed after ignition.

(3) The portion which passed the 100 mesh sieve is triturated
with a rubber pestle (made by inserting a glass rod as handle into an
inverted rubber stopper), and the beaker filled up to the 8$°5 cm.
mark and allowed to stand for twenty-four hours, ‘The ammoniacal
liquid which contains the “clay” is then decanted off into a Win-
chester quart bottle, the operation being repeated as long as any
matter remains in suspension for twenty-four hours. The liquid
containing the ‘“‘clay ” is either evaporated in bulk or measured, and, |
after being well shaken, an aliquot portion taken and evaporated.
In either case the dried residue consists of ‘‘ clay” and ‘soluble
humus”. After ignition the residue gives the “ clay,” and the loss
on ignition the “soluble humus”. Here minimum value of
v = o’o0001 cm. per second, and the minimum diameter of the
particles = ‘oo13 mm.

(4) The sediment from which the “clay” has been removed is
triturated as. before in the beaker, which is filled to the to cm. mark
and allowed to stand for 100 seconds. The operation is repeated.
until the “fine sand” settled in 100 seconds is clean, when it is
collected, dried, and weighed.

Here minimum value of v = o'1 cm. per second; the calculated
minimum diameter = ‘037 mm. :

(5) The turbid liquid decanted from the “ fine sand ”’ is collected
in a suitable bottle, allowed to settle, and the clear liquid syphoned
or decanted off. The sediment is then washed into the marked

APPENDIX I 355

beaker and made up to the 7°5.cm. mark. After stirring, it is allowed
to settle for twelve and a half minutes, and the liquid decanted off.
The operation is then repeated as before till all the sediment sinks
in twelve and a half minutes, leaving the liquid quite clear. The
sediment obtained is the “silt,” which is dried and weighed as usual.
_ The liquid contains the “fine silt,” which, when it has settled down,
can be separated by decanting off the clear liquid, and dried and
weighed.

For silt minimum value of v = o’or cm. per second, minimum
- diameter of particles = o‘or2 mm. For fine silt the diameter ob-
viously lies between this value and the one found for clay.

When it is desired to compare the results with American data
the fine silt can be divided into two groups: settling for two hours
five minutes brings out a group oor to o'005 mm. diameter, and
the remainder lies between o’005 and o’ooz2 mm. diameter. The
fractions can then be made to correspond fairly closely with those
adopted in the United States.

(6) Determinations are made of the ‘‘moisture” and “loss on
ignition ” of another ro grms. of the air-dry earth. The sum of the
weights of the fractions after ignition + loss on ignition + moisture
+ material dissolved in weak acid should approximate to 1o grms.

(7) It is advisable to make a control determination of the “fine
gravel” in a portion of fifty grms. of the air-dry earth. The soil
should be treated with acid, as in (1), and after that is removed by
decantation may be at once treated with dilute ammonia and washed
on the sieve with 1 mm. round holes. The “fine gravel” left on
the sieve is then dried and weighed.

The American method is somewhat different. ‘The breaking
down of the aggregates is brought about by physical means—e.g.
violent shaking—and sedimentation is hastened by a centrifugal
apparatus which is a distinct improvement on the British method :
the centrifuge was also used by Beam in Khartoum.' Hilgard does not
adopt a sedimentation method but proceeds in the converse manner ;
he collects and weighs the particles carried off by successive streams
of water of varying velocity. Full details are given in Bull. 24,
Bureau of Soils, 1904, and in Wiley’s Agricultural Analysis, vol. i.,
where the continental methods are also described. An entirely new
method has been described by Odén (218c).

1W. Beam, The Mechanical Analysis of Arid Soils (Cairo Scientific fournal,
IgII, 5, 107-119). Pub. Wellcome Tropical Research Lab,, Khartoum.
23°"

356 SOIL CONDITIONS AND PLANT GROWTH

Methods of Sand and Soil Cultures.—See Hellriegel (1306) and—

Sand, McCall, A. G., Journ. Amer. Soc. Agron., 1915, "7, 249-
282; Soil Sct, 1916, 2, 207-253; Journ. Amer. Soc. Agron., 1918,
10, 127-134. Hoagland, D. R., Journ, Ag. Res., 1919, 18, 177.

Comparison of Sand and Water Cultures, Hoagland, D. R., Journ.
Ag. Res., 1919, 18, 11.

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A SELECTED BIBLIOGRAPHY.

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1 Most of the papers and books quoted here can be seen in the Rothamsted
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359

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x
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Pa Nt ng

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Bull, 22, 1903. ; oo L2Ay Lan eey
(4) ‘Investigations in Soil Fertility,” ibea., Bull. 23, 1904 . Mame sasS

A SELECTED BIBLIOGRAPHY 385

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reference
is made,
06. Whitney, M., “Soil Fertility,” U.S. Dept. cae Bureau of Soils,
Farmers’ Bull., No. 257 . : ve R72 Bae

307. Whitson, A. R., and Stoddart, C. W., is The Cédisecvution of Phos-
phates on Wisconsin Farms,” Univ. of Wisconsin, Bull. 174 Tg09 ;

see also Bulls. 202, 204 and 205, all 1911 : 321
308. Wiegner, Georg, ‘‘ Zum Basenaustausch in der Aetocendle? KG Four. f
Landw., 1912, 1x., 110-50; 197-222 . : 156

309. Wilfarth, (a) “ Veber den Nahrstoffverbrauch det Giiekerribe ulidl din
Beziehungen desselben zur Wasseraufnahme,” Bied. Zent. Agric.
Chem., 1905, xxxiv., 167-69 5 c ; ¢ ‘ P OAS.

(6) Wilfarth, H., Romer, H., and Wimmer, G. “ Ueber die Nahr-
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stums,”’ Landw. Versuchs-Stat., 1905, Ixiii., 1-70. 297

310. Willstatter, Richard, and Benz, Max, ‘‘ Untersuchungen iiber Chior:
phyll, vi., Ueber Sarasa saan in * Bex rc Annalen,
1908, ccclviii., 267-87 P 75

311. Winogradsky, S., (a) “ beeches sur fen: ‘abi cadin de +? niteitia
tion,” Ann. de V Inst. Pasteur, 1890, iv., 1@ Memoire, 213-31; 2¢
Memoire, 257-75 ; 3° Memoire, 760-71. A ays SRiTSO

(6) Winogradsky, S., and Omelianski, ‘‘ Ueber des Bintiuee der
organischen Substanz auf die Arbeit der nitrifizierenden Mikrobien,”’
Centr. Bakt. Par., I1., 1899, v., 329, 429 . : i . 189

312. Winogradsky, S., (a) “ Gi tieertives sur l’assimilation de Vasc libre
de Patmosphére par les microbes,” Archives des Sci. Biol., St.
Petersburg, 1895, iii., 297-352 . 3 196

(6) “ Clostridium Pastorianum, seine Méinhologie wtid seine Pinen.
schaften als Buttersdure ferment,’ Centr. Bakt. Par., I1., re ib oe

43-54, 107-12. . 197
313. Withers, W. A., and Fraps, youre dee: a Soe. ay 1901, Xxiii., 38

and 1902, xxiv., 528 . 262
314. Wojtkiewicz, A., “ Beitrige zu Nalcticiakogiechen Baden - fae

suchungen,” Centr. Bakt. Par., 1914, xlii., 254-61. ; z +296

315- Wolff, E., and Kreutzhage, C., ‘‘ Bedeutung der Kieselsaure fiir die
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Landw. Versuchs-Stat., 1884, xxx., 161-97 ‘ ; ; , Oi 4

316. Wolkoff, M. I., “Effect of Ammonium Sulphate in Nutrient Solu-
tion on the Growth of wid Beans in Sand biased Soil Sci., mote

V., 123-50 . : ' 80

317. Wollny, E., (a) “ idsiiingen fiber den Koblensturcechatt ve
Bodenluft, » Landw. Versuchs-Stat., 1880, xxv., 373-91. ‘ 175

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318. Wollny, E., Die Zersetzung der Organischen Stoffe, 1897 . 139

(A summary of Wollny’s work. For the papers by himself and his
students see his Journal Forschungen auf dem Gebiete der agricultur
Physik, 1878 et seq.)

319. Wood, T. B., ‘‘ The Available Potash and eo Acid in Soils,”
Trans. Chem. Soc., 1896, Ixix., 287 . Z 234
320. Wood, T. B., and Berry, R. A., ‘‘Soil Aisiyeid as a Guide t to Th
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25

386 SOIL CONDITIONS AND PLANT GROWTH

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reference
is made,

321. Woodward, J., “ Thoughts and is eon: on Vege Phil.
Trans., 1699, xxi., 382-92 ; 4

322. Woronin, M., ‘‘ Ueber die bei der Geliwaeceste (Alnus giuciaeil sad
der gewéhnlichen Garten-Lupine (Lupinus mutabilis) auftretenden
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mensetzung und Anwendung bei der pt F. fir
Landw., 1916, Ixiv., 201-75 ; ; : : 8 “ as 273

ee ee ee

AUTHOR INDEX.

The numbers in square brackets marked B refer to numbers in the bibliography
to which the reader should turn for fuller reference.

Asse, Cleveland, [B 1].

Abbott, J. B., rr2.

Acqua, C., go.

Adametz, L., 256.

Adeney, W. E. [B 2], 190, 194, 214.
Ageton, C. N. [B 105], 304.

Aiyer, S. [B 126], 213, 256, 300.

Albrecht, W. A., 207.

Ali, Barkat, 274.
Allen, E. R., 199, 262.
Allison, F. E. [B 60], 169, 213, 273,

290.
Alves, A. [B 1516], 201.
Alway, F. J. [B 3], 137, 221, 223, 226.
Ames, J. W. [B 4], 119, 211.
Amos, “ 84, 157-
André, G. [B 30, 31], 138, 146, 152,

297-

Appleyard, A. [B 241g], 227, 228, 275,
276, 297.

Ardern, E., 212.

Aristotle, 152.

Armsby, H. P., 156.

Armstrong, E. F. [B 5], 87, 215.

— H. E. [B 5], 87, 236.

Arnd, Th. [B 6, 278], 210, 307.

Ashby, S. F. [B 7], 188, 203, 261, 262,
318, 331.

Ashley, H. E., 170.

Aso, K. [B 277].

Atkins, W. R. G.,

Atterberg, A., 98.

272.

Bacu, N., 161.

Bacon, Francis, [B 8], 2.
Bahr, F. [B 93], 113, 143.
Bailey, C. H., 44.

Baker, T. T., go.

Bancroft, W. D., 148.
Barger, G., 195.

Barlow, B. [B 125], 206, 208.
Barnes, J. H., 274.

Bassett, H., 120.

387

Bauer, H. [B 233c], 124.

Baule, B. [B 9], 34.

Baumann, A. [B ro], 111, 130, 140.

Beam, W., 355.

Bear, F. E. [B 11], 243, 273, 351.

Beckley, V. A. [B 12], 141, 142.

Bedford, Duke of, [B 225].

Beijerinck, Martinus W. [B 13-18], 26,
143, 199, 202, 204, 208, 260.

Bell, J. M. [B 65d].

Bemmelen, Jakob M. van, [B 19-25],
27, IOI, 105, 130, 140, 148, 155,

344:

Benz, Max, [B 310].

Bergu, G., 234.

Berman, N., 186¢

Berry, R. A. [B 320], 234.

Berthelot, Marcellin, [B 26-33], 25, 26,
138, 195.

Bertholon, Abbé, [B 34], 87.

Bertrand, Gabriel, [B 35], 76.

Berzelius, J. J., 12.

Bewley, W. F. [B 36], 72, 204.

Bialoblocki, J. [B 37], 35.

Bierema, S. [B 38], 210.

Birner, H. [B 39].

Bizzell, James A. [B 183], 243, 298,

299.
Blackman, F. F. [B 40], 28, 35, 38.
— V. H., 28, 88, gt.
Blanck E. [B 170, 224c-f], 34, 45, 101,
2OI, 210, 233, 234, 305.
Blair, A. W., 181, 186, 243.
Bocker, F., 152.
Boedecker, C., 155.
Boerhaave, H. [B 41], 5
Bogdanow, S., 77, 234.
Bolley, H. L., 282, 295.
Bonazzi, A., 188.
Bottomley, W. B. [B 42], 236, 249.
Bouilhac, R. [B 43, 44], 200, 256.
Boussingault, J. B. [B 45-48, 88], 12,
15, 20, 175, 178, 192, 227.
he

388

Bouyoucos, G. J. [B 49], 125, 173, 225,

233.

Boyle, Robert, [B 50], 3, 4.

Breazeale, J. F. [B 51, 65a, 179], 73,
79, 139, 152, 236, 238, 248.

Bredemann, G. [B 52], 200.

Bredig, G., 215.

Brenchley, Winifred E. [B 53, 121d],
51, 52, 76, 78, 83, 84, 85, QI, 173;
237, 238, 241, 249, 306.

Bretschneider, P. [B 54], 21.

Brierley, W. B., 257.

Brigham, R. O., 58.

Briggs, L. J. [B 55], 43, 47, 125, 144;
152, 221, 224, 226.

Bristol, B. Muriel, [B 56], 254.

Broounoff, P. [B 57].

Brown, C., E., 211.

— Adrian J. [B 58], 87.

— Horace, T. [B 59], 38, 57.

— Percy E. [B 60, 176], 211, 213, 262,
263, 265, 269, 273, 275, 279.

Brunchorst, J., 25.

Brustlein, F., 156.

Buddin, Walter, [B 61], 169, 266, 295.

Buisson, J. P. du, 282.

Burd, John S. [B 62], 234, 238, 296,
300, 334.

Burgess, P. S. [B 63], 262, 263, 265,
273, 274.

Burmeister, H., 249.

Burri, R., 204.

Butler, E. J., 256.

Butt, N. I. [B 123], 49.

CaILLETET, L. [B 64], 15.

Caldwell, J. S., 221.

Cameron, Frank K.[B 65, 145, 305], 27,
78, 110, 122, 124, 139, 156, 173,
174, 233, 238, 247.

Campbell, F. H., 156.

Candolle, A. de, 244.

Cannon, W. A., 53.

Caron, A., 27,

Carter, Es'G:'[B" 113° c,d, 272, 273;

275.

Cauda, A., 289.

Chauzit, B., 304.

Chick, Hariette, [B 66], rgo.

Christensen, Harald R.[B 67], 114, 118,
204, 242, 243, 351.

Christie, A. W., 125.

Clark, E. D., 282.

Clausmann, P. [B 102], 76.

Clayton, J. [B r40f], 177, 273.

Clevenger, C. B

Cobbett, William, 6

Coleman, D. A. [B 152], 200, 250, 275,
290.

SOIL CONDITIONS AND PLANT GROWTH

Coleman, Leslie C. [B 68], 189, 190.
Collins, S. H., 350.

Colville, F. V., 244.

Comber, N. [B 69], 162.

Conn, H. J. [B 70], 187, 216, 260, 279.
Conner, S. D., 112.

Cook, R. C., 78, 265.

Coville, F. V., 240.

Coudon, H. [B 2076], 185.

Cowie, G. A., 215.

Crescentius, Petrus, 1.

Crowther, Charles, [B 71], 82, 268.
Crump, L. M. [B 73c], 270, 290, 29t.
— W. B. [B 72], 222.

Cunningham, A., 293.

— Mary, 138.

Curtis, R. E. [B 292e], 260.

Cutler, D. W. [B 73], 270, 290, 291.
Czapek, F., 233, 259.

Czermak, W., 282.

DacuNowskI, A., 260.

Daikuhara, G. (B 74], 112, 210.

Dakin, H. D., 185.

Dale, E. [B 75], 257.

Damon, S. C. [B 1280], 240.

Darbishire, F. V. [B 2416], 84, 282, 284.

Darwin, C. [B 76], 136.

— Erasmus, [B 77], 8

— Francis, 54.

Daubeny, C. G. B. [B 78], 234, 244.

Davenport, A., 204.

Davis, W. A. [B 78a], 68.

— A. R., 208, 259.

Davy, Humphry, [B 79], 11.

Dawson, Maria, bs 80], 205.

Deatrick, E. P.,

Déhérain, P. P. as 81], 209, 299.

Delage, A. [B 82], roo.

Delden, A. van, [B 15].

Demoussy, E. [B 83], 57; 74, 79.

Denison, W., 192.

Densch, A. [B 278c].

Detmer, W. [B 84], 129.

Dixon, H. H., 89, 222.

Dobson, Mildred E., 138, 139.

Dokoutchoew, W. [B 85], 303, 310.

Dorée, Chas., 138.

Doryland, C. J. T. [B 86, 148], 186, 203,
211, 276.

Doyne, H. C., 351.

Drechsler, Chas., 260.

Drouin, R., 256.

Duchacek, F., 211.

Duclaux, E. [B 87], 37.

Dudgeon, E. C., 88.

Duggar, B. M., 208, 259.

Dumas, J. B. A., [B 88], 14, 20.

Dumont, J. [B 89], 120, 275.

AUTHOR INDEX

Duncan, J. L., 308.
Dundonald, Bail of, [B go], 7
Dupetit, G. [B 103), 209.
Dutton, F. V., 338.

Dyer, Bernard, [B 91], 352.
Dymond, T. S. [B 92], 77.

Esse s, W. P., ves
Edwards, C. S., 314.
eee P. [B 93], 83, 113, 143, 259,

Ehrlich” F,, 185.
Einecke, A. 234.
Elgee, F., 135.
Eller, W., 143.
Emeis, C., 164.
Endell, K., 170.
Engberding, D. [B 94], 276.
Engels, O. [B 95], 234.
ann, E., 152.
— R. [B 215a], 69.
Eriksson, 25.
Erlwein, [B 104], 88. .
Ernest, Adolf [B 273a, 274-5], 177, 197,
198, 199, 209, 269
Escombe, F. [B 59], 57.
Esmarch, F., 254.
Evelyn, J. [B 96], 7

Fasrr, G., 84.

Fabricius, O. [B 975], 269.
Fahrenholtz, F. [B 257c].

Failyer, G. H. [B 97], 100, 157.
Farrow, E., 55.

Fehrs, 293.

Feilitzen, H. von, [B 97a], 269, 308.
Felber, P., 213.

Fellers, C, R., 290.

Fischer, Hugo, [B 98, 170], 204, 210,

243.
Fisher, E. A. [B 99], 113, 119, 351.
Fliigel, M., 34.

Forbes, A. C., 135.

Foreman, F. W., 345.

Fowler, G. J., 212, 214.

— L. W., 205.

Frank, B. [B roo], 254, 255, 281.
Franke, E. [B 224], 282.
Frankland, E., 192.

Franklin, T. B., 232.

Fraps, G. S. [B 313], 262.

Fred, E. B., 204, 274.

Free, E. E. [B t00a, 178c], 51, 52,

53:

Fresenius, L., 234.
Freundlich, H., 162.
Friske, K. [B 224e], 45.
Fritsch, F. E., 255.
Frohlich, Otto, 34.

389

Fry, W. H. [B 263], rar.
Fuchs, F., 138.

Fulmer, H. L., 273.
Funchess, M. J., 212.

GaceEr, C. S., go.

Gainey, P. L.,
263.

Gans, R. [B ror], 110, 344.

Gaudechon, H. [B 208d], 279.

Gaunersdorfer, J., 73.

Gautier, A. [B 102], 76, 256.

Gayon, U. [B 103], 209.

Gedroiz, K., 104, 169.

Georgs, R. [B 257c], 38.

Gericke, W. F. [B 175], 77.

Gerlach, M. [B 104], 88, 180.

Giglioli, I., 213.

Gilbert, J. H. [B 161-166], 17-20, 25,
54, I5I, 180, 192.

Gilchrist, D. A., 345.

Gile, P. L. [B 105], 304.

Gillespie, L. J. [B rosa], 114, 116, 176,

185, 188, 204, 262,

243.

Giltner, W., 169.

Gimingham, C. T. [B 106, 120e], 132,
156, 242, 317.

Girard, Aimé, 281.

Giustiniani, E. [B 44], 256.

Given, G. P., 262.

Glauber, Johann Rudolph, [B 107], 3, 4.

Glinka, K. [B 107a].

Godleweski, 189.

Gola, G. [B 108], 123, 237, 239.

Goheen, J. M., 205.

Golding, John, [B 109, 241 d], 204, 206,
287.

Goldthorpe, H. C. [B 113c], 275.

Goodey, T. [B r10], 289, 293, 294.

Goodwin, W., 345.

Gossel, F., 75.

Gortner, R. A., 142.

Gottheil, O. [B 111], 217.

Gotze, C. [B 224a], 193.

Grafe, V., 79.

Graham, Thomas, 148.

Grandeau, M. L. [B 112], 15, 88, 143.

Greaves, J. E. [B 113], 50, 84, 86, 146,
272, 273, 274, 275.

Green, H. H. [B 181g], 84, 318.

Greig-Smith, R. [B 114], 249, 282.

Grégoire, A., 336.

Griffith, J. J., 83, 345.

Groves, J. F., 54.

Gully, E. [B 10, 116], 111, 130, 136,
139, 307.

Gupta, N. Sen, 212.

Gustafson, A. F., 107.

Guthrie, F. B. [B 117], 86.

39°

Haas, A. R. C., 81.

Hagem, O. [B 118], 256.

Hales, Stephen, [B 119], 5

Halket, A. C., 72

Hall, A. D. [B 120, 121], 45, 51, 52, 65,
77, 102, 116, 156, 157, 161, 164,
173, 183, 234, 238, 242, 296, 317,
327) 343) 345-

Hanley, J. A., 155, 170, 351.

Hannig, E., 222.

Hansteen, B., 78.

Hardy, W. B. [B 122], 161.

Harris, F. S. [B 123], 49.

— Jj. E. [B 124], rrr.

Harrison, F. C. [B 125], 206, 208,

— 1 ge » 202.

H. [B 126], 213, 256, 300.

Hart, E. B. {[B 127], 76, 274.

Harter, L. L. [B 1450].

Hartwell, B. L..[B 128], 112, 212,
240.

Hasenbaumer, J., 170.

Hassler, C., 170.

Hatschek, Emil, 148.

Hazen, W. [B 263], r2z.

Heiden, E., 152.

Heinrich, R. [B 129], 221.

Heinze, B., 269.

Hellriegel, H. [B 130], 24, 31, 43, 48,
59, 66, 70, 73, 196, 204, 356.

Helmont, Johann Baptista Van, [B 131],

2, 4; 9-

Helms, R. [B 117], 86.

Hendrick, J. [B 132], 100, 102, 118, 334,
336, 346.

Henneberg, W., 155.

Henslow, J. S., 321.

Herdman, W. A., 279.

Hildebrandt, F. M., 157.

Hilditch, T. P., 215.

Hilgard, E. W. [B 133], 139, 143, 152,
223, 303.

Hill, C. F., 345.

Hills, T. L. [B 134], 199.

Hiltner, L. [B 135, 2150], 205, 208,
276, 281, 305.

Hirst, C. T. [B 1135], 44, 50, 86, 272.

Hissink, D. J. [B 136], 344, 351-

Hoagland, D. R. [B 136a, 260], 80,
81, 116, 118, 125, 127, 238, 356.

Hoffman, C., 212.

Home, Francis, [B 137], 6, 9, 22.

Hopkins, C. G. [B 138], 191, 321.

Hoppe-Seyler, F., 140.

Houston, D., 281.

Howard, A. [B 139], 50, 52, 229.

— G,. L. C. [B 139], 50, 52.

Hudig, J., 294.

Hughes, F. [B 92], 77.

SOIL CONDITIONS AND PLANT GROWTH

Huntemiiller, Otto, 293.

Hurst, L. A, [B 1056], 243.

Hutchinson, H. B. [B 140, 241¢], 27,
58, I1Q, 177, 191, 202, 204, 210, 213,
217, 241, 243, 249, 263, 266, 272,
273, 282 et seq., 295, 350.

IkepA, K., 215.

Iljin, V. [B 141], 46.

Imaseki, T. [B 74], 210.
Immendorff, H. [B 141], 193, 195.
Ingen- pice Jan, [B 142], 9.
Ingle, H., 318.

Irvine, J. C., 138, 139.
Itscherekov, B., 123.

JAVILLIER, M. [B 143], 83.
Jensen, C. A.[B 144, 179], 744, 248,

279.
Jewson, S. T., 257.
Jodidi, S. L., 146.
Joffe, J. S., 81.
Johnson, H. W. [B 60c], 211.
Johnston, E, S., 157.
Joly, J., 89.
Jones, H. M., 186.
Jongh, A. C. de, 339.
Jorgensen, I. [B 231, 272], 51, 88.
Joshi, N. V., 300.
Jupe, C. W. C. [B 92], 77.

KaLMann, W., 152.

Kappen, H. [B 170], 81, 113, 210, 211,
233. :

Kaserer, H., 144, 213.

Kearney, Thomas H. [B 145], 43, 78.

Keen, B. A. [B 146], 167, 173, 221, 224,
226, 230, 232.

Kellerman, K. F., 262.

Kellermann, C. [B 234].

Kelley, W. P., 191, 210, 263, 274.

Kellner, O., 155.

Kellogg, E. H. [B 606], 2rr.

Kreutzhage, C. [B 315], 77.

Kidd, F. [B 147], 85.

Kilroe, J. R., 346.

King, F. H. [B 147a], 27, 173.

— Walter E. [B 148], 276.

Kirwan, Richard [B 149], 7, 8

Kliger, I. J., 186.

Klimmer, M., 205.

Knop, W. [B 150], 19, 76, 154, 157.

Knudson, L., 57.

Koch, Alfred, [B 151], 200, 201, 202,

203, 211, 281.

— G.P. ae 290.

— Kate, 14

Konig, Alfed, [B 153], 153-

Si J: +» 170.

I ee ae

AUTHOR INDEX

Koning, C. J. [B 221], 257.
Kopeloff, N. [B 152], 243, 250, 290.
Kossowitsch, P. [B 154], 200, 255,
310.

Kraus, Pace [B 155], 305.
owt te +) 205.

[B 156], 58, 73, 282.
Krull, ¥. [B 1516}, 201.
Krzemieniewski, S. [B 157], 199, 261.
Killenberg, O., 155.
Kiilbel, 3.

LacuMann, J. [B 158], 21, 25.
Lafar, ae 18g.

Lagatu, H. [B 82], roo.

Lainé, E. [B 208], 190, 194.
Lampadius, 8.

Langworthy, H. V., 169.

Larsen, O. H. [B 67a].

Lathrop, Elbert C. [B 251], 248, 282.
Lau, Erich [B 159], 227.

Laurent, Emil, [B 160, 247], 26, 255.

— j., 15. .

Lawes, J. B. [B 161-166], 17-20, 25, 54,
64, 151, 180, 192.

Leather, J. W. UB 167], 221, 228, 229,
279, 299, 348.

Lebedeff, A. J. [B 168], 208.

Lechatelier, H. L., 106.

Le Clerc, J. A., 79, 297, 300.

Leeden, R. van der, 170.

Lefévre, Jules, [B 64, 169], 15.

Lehenbauer, P. A., 54.

Leitch, I., 54.

Lemmermann, O. [B 170], 75, 210, 234.

Lemstrém, S. [B 171], 83.

Lendner, A., 256.

Leoncini, G. [B 172], 105, 161.

Lepeschkin, W. W., 54.

Lesage, Pierre, [B 173], 88.

Letts, E. A, 193.

Lewin, K. R. [B 191], 289, 295.

Lewis, F. J., 135.

— W.C. McC., 161.

Léwy, [B 48], 227.

Liebig, Justus von, [B 174], 14-20, 22,
28, 154.

Liechti, P., 212.

Linder, S. E., 162.

Linhart, G, A., 199.

Lint, H. C. [B 152], 250, 290.

Lipman, C. B. [B 175], 77, 125, 139,

200, 205, 262, 273, 274, 348, 349.
— J. G. [B 176, 177], 186, 187, 212,
_ 244, 263, 273, 275.

Lippmann, E. O. von, 152.

Litzendorff, J. [B 151], 201.

Livingstone, Burton Edward [B 178-9],
52, 54, 248.

391

Livingstone, Grace J., 54.

Lloyd, W. E., 347.

Lockett, We ee, 212;

Lodge, Oliver, 88.

Loew, Oscar, [B 180], 67, 75, 76, 86,
216, 294.

Léhnis, F. [B 181], 21, 190, 197, 198,
203, 215, 261, 263, 279.

Lucanus, Benno, [B 39, 182].

Luxmore, C. M., 345.

Lynde, C. J., 105.

Lyon, T. Lyttleton, [B 183], 118, 243,
298, 299.

Maasssn, Albert, [B 184], 209.
McBeth, I. G. [B 185], 177, 258, 300.
McCall, as G., 157, 356.

— J. R,, 125.

McCaughey, W. G. [B 186], roo.
McClendon, J. F., r1rq.

McCool, M. M. [B 49¢].

McDole, G. R. [B 3c], 221.
McGowan, W., 281.

McHague, J. S. [B 187], 74.
Mcllvaine, T. C. [B 243], 81.
McLane, J. W., 43, 144.

McLean, H. C. [B 188], 181, 212, 243,

259.

McLennan, K. [B r40c], 119, 241, 295,
350.

Maercker, M., 209.

Maillard, L. C. [B 189], 142, 146.

Manns, T. F., 205.

Maquenne, L. [B 81a], 74, 79, 209.

Marchal, Emile [B 190], 185, 187.

Markham, Gervase, 311.

Martin, C. H. [B rgr], 289, 295.

— D. E., 348.

— J.C., 125.

Marz, S. [B 233c], 124.

Maschaupt, J. G. [B 192], 296.

Mason, J., 184.

Masoni, G, [B 172], 105, 213.

Massart, J., 306.

Massey, A. B., 212.

Matisse, G., 54.

Matthaei, Gabrielle L. C. [B 193], 35
(now Howard, G. L. C., which see).

Matthews, D. J., 350.

Mayer, Ad. [B 194], 139, 140, 164.

Mayow, John, [B 195], 3, 4

Maximow, N. A., 54.

Mawley, A. E., 232.

Mazé, P. [B 196, 7], 56, 58, 76, 80, 85,
236, 304.

Meacham, M. R.,, 81.

Mellish, H., 232.

Merkle, F. G., 176.

Meusel, E. [B 198], 209.

392

Michelet, E. [B 199], 136.

Miklauz, Rudolf, 140.

Miller, H. G., 77.

— N.H. J. [B 120), 140, 200], 58, 129,

179, 242, 296.

Minkman, D. C, J. [B 18].

Mirasol, J. J., 113.

Mitscherlich, E. A. [B 201], 28, 31, 64.
170, 234, 344.

Miyake, K. [B 202], 28, 157, 185, 188.

Mockeridge, F. A., 198, 236.

Molisch, H., 88, go.

Molz, Emil, 304.

Moore, B., 200, 255.

— G. T., 290.

Mooser, W., 212.

Morgan, J. F. [B 203], 123.

Morison, C. G. T. [B 120¢, 204], 77,
161, 164, 165, 351.

Morrow, C. A., 147.

Morton, Chalmers, 192.

Mosier, J. G., 107.

Moss, C. E., 134.

Mosséri, V., Too.

Mulder, C. re [B 205], 129, 138.

Miiller, P. E. [B 206], 136.

— P. Th., 293.

Miinst, M., 164, 165.

Miinter, F., 235.

Muntz, A. [B 207, 208, 245], 23, 185,
190, 194, 195, 279.

Murray, T. J. [B 209].

Nacaoka, M. [B aro, 211], 76, 210.
Nakano, H., 255.

Nazarova, P. [B 141], 46.
Neller, J. R. [B 3d], 137, 185.
Neubauer, Hugo, [B 212], 352.
Neumann, P. [B 213], 206.
Newman, J. E., 88

— L.F., 345. ,

Nilson, Lars Fredrik, 234.
Nobbe, F. [B 215], 69, 205, 208.
Nolte, O. [B 216], 122.

OBERLIN, [B 217], 280.

Odén, Sven, [B 218], 95, 113, I19, 130,
140, 141, 143, 162, 355.

Oehler, R., 289.

Ogg, W. G. [B 132], 100, 102, 118, 334,
336, 346.

Olaru, D. Ea 8

Oliver, F. W.,

Omelianski, W. wi} 219, 311d], 177, 18%,
18g, 190, 191, 261.

Osterhout, W. J. V. [B 220], 78.

Ostrovskaja, M. [B 141], 46.

Ostwald, Wo., 148, 161.

SOIL CONDITIONS AND PLANT GROWTH

Oudemanns, A. C. [B 221], 257.
Owen, Irving L. [B 177], 273.

Pauissy, Bernard, [B 222], 1.

Paine, ’'S.. G,, 72,

Parker, F. W., 74.

Parkinson, S. T., 89.

Parr, A. E. [B 1816], 263.

Pasteur, Louis, 23.

Patten, A. J., 211.

— H. E. [B 65c], 156, 157.

Paturel, G., 67.

Peck,:S.: 3); 202, 2733

Pember, F, R. [B 128a, 179], 112, 212,
248.

Perciabosco, F., 59.

Petersen, J. B., 254.

Peterson, W. H. iB 127, 223], 76.

Petherbridge, F. R. [B sha, 146, 236,
248, 287, 295.

Pettenkofer, M. v., 228.

Pfeiffer, Th. [B 224], 34, 45, 193, 201,
233, 234, 282, 305.

Pickering, S. U. [B 225], 162, 169, 246,
282.

Picton, H., 162.

Piemeisel, R. L., 43.

Pillai, N. K. [B 181d], 198.

Pitra,‘J,, ‘255;

Pitz, W..5:77-

Plymen, F. J. [B t20a], 234.

Portheim, L. R. v., 79.

Potter, R. S., 120, 147.

Pratt, O. A., 257.

Prazmowski, Adam, [B 227]. :

Prescott, J. A. [B 223, 241h], 150, 157,
275, 294, 299, 352.

Prianischnikow, D. [B 229], 58, 155.

Priestley, Joseph [B 230], 8, 20

J. H. [B 231], 38.

Pringsheim, E., 256.

— Hans, [B 232], 202.

Pugh, E. [B 164], 20, 192.

Purvis, O. N., 69

Pusey; Poy:27

QUENSELL, E., 211.

Raun, Otto, 215.

Ramann, E. [B 233], 98, 113, 124, 136,
155, 164, 165, 234, 258, 259.

Rambaut, A. A., 232.

Rathmann, W. [B 224/], 234.

Raumer, E. von [B 234], 74.

Rayner, M. C., 254, 306.

Reed, H. S. [B 236, 249], 67, 72, 199.
247.

Reiset, M., 192.

Remy, Th. [B 237], 199, 204, 261, 263.

AUTHOR INDEX

Rettger, L. F., 186.

Rice, Frank E. [B 238], rr2.

Richards, E. H. [B 238a, 241i and &),
75, 179, 190, 195, 203, 228, 256.

Richmond, T. E., 2rz.

Rigg, Th., 345.

Rindall, A. [B 239], 113.

Risler, E., 346.

Robbins, W. J., 212.

— W. W., 254.

Robertson, G. Scott, 312.

— R.A., 138, 139.

Robinson, G. W.[B 240], 100, ror, 118,

330) 334 345» 347) 353-

— R.H., 18

Rohland, P., 104, 106.

Romer, H. [B 120d, 3096], 297.

Résing, G. [B 2:78], 199.

Rosso, V., 59.

Ross, Ronald, 289.

Russell, E. J. [B 121, 146d, 241], 27,
and elsewhere.

Ruston, Arthur G. [B 71], 82, 268.

Sapascunikorr, A, [B 181¢], 279.

Sackett, W. G., 211, 263.

Salisbury, E. J. [B 242], 241, 305, 314.

Salter, R. M. [B 243], 81, 352.

Sanderson, J. C., 89.

Sandon, H., 289.

Sangiorgi, C., 289.

Satterly, J., 89.

Saussure, Théodore de [B 244], 9, 18,
20, 175.

Saw, M. P., 35r.

Scales, F. M. [B 185c], 258.

Scheele, Carl Wilhelm, g.

Schloesing, Th. [B 244, 5], 23, 122, 148,

161, 175, 210.

— — fils [B 246, 7], 25, 227, 255.

Schneider, Felix, 170.

— Ph. [B 248], 2or.

Schneidewind, W. [B 156a],-282.

Schollenberger, C. J. [B 4], 118, r19,
120.

Epa nag C. F., 208.

Schreiner, O . (B 249-52], 141, 157, 247,

Schréder, J. (B 215a], 69.
Schiibler, Gustav, [B 254], 12, 14.
Schulze, B., 73.

Schultz-Lupitz [B 255], 314.
Schumacher, W., 173.

Searle, A. B., 106.

Seaver, F. J., 282.

Sebelien, J. [B 234], 136.

Seelhorst, C. von [B 256-8], 38, 44, 45,

48, 128, 180.
Senebier, Jean [B 259], 9

393

Sestini, F., 216.

Shantz, H. L. [B 550], 43, 47, 224, 226.

Sharp, L.-T. [B 1360, c, 260], 80, 116,
125, 127, 238, 349.

Shedd, O. M., 212.

Sherman, J. M. [B 261], 283, 285, 290,
293, 295.

Shive, J. W. [B 262], 79, 80.

Shorey, E. C. [B 249, 263], 121, 141.

Shull, C. A. [B 264], 53, 167, 222, 224,
226.

Shutt, F. T. [B 265], 44, 182, 310.

Sibertzev [B 266], 303.

*Sigmond, Alexius A. J. von, [B 266a],
157, 344, 352 (also called A, de
Sigmond).

Sigmund, Wilhelm, [B 267], 86.

Simmermacher, W. [B 224f], 234.

Sjollema, B., 170.

Skene, M., 306.

Skinner, J. J. [B 179, 252], 248.

Smalley, H. R., rr2.

Smith, Angus, 192.

— Geoffrey, 293.

— J. G. [B 97], roo.

— Norman, 216.

— N. R. [B 185], 197, 300.

— Otto M., 162.

— R. E. [B 60d], 279.

Snyder, Harry [B 268], 120, 138, 147,
181.

Séderbaum, H. G., 58.

Sohngen, N. L. [B 269], 213, 239.

Sérensen, S. P. L., 115.

Sostegni, L., r4o.

Sothers, D. B. [B 204], 164, 165.

Sprengel, Carl [B 270], 14, 129.

Stevens, F. L. [B 271], 190, 263.

Stewart, R. [B 1130], 44, 50, 86, 146,
173, 272-

Stiles, W. [B 272], 51, 238.

Stoddart, C. W. [B 307].

Stohmann, F., 155.

Stokes, G. C., 353.

Stoklasa, Julius [B 273-5], 72, 75, 91,
177, 197, 198, 199, 206, 209, 211, 269.

Stormer, K. [B 1350, c}, 276, 281.

Stranadk, Franz [B 275)], 197, 198.

Stremme, H., 165.

Strutt, R. J., 89.

Stutzer, A. [B 276], 206.

Suchtelen, F. H. H. van, 123, 176.

Siichting, H. [B 2780].

Sutton, M. H. F., gr.

Suzuki, Shigehiro [B 277], 76, 208.

Swanson, C, O., 181.

TackE, Br. [B 278], 113, 136, 143.
Tadokoro, T., 170.

394

Tansley, A. G, iB 279], 134.
Tartar, H. V., 185.

Tatlock, R. , 352.

Temple, a ait 263, 273.

Tenant, Smithson, [B 280], 86.
Ternetz, Charlotte, 259.

Thaer, A. von, 11.

Thaysen, A. C. [B 140g], 249.
Theiler, A., 318.

Thiele, R. [B 281], 203, 232.
Thompson, H. S. [B 282], 150.
Thomson, D., 289.

Thornton, H. G., 293.
Thurmann, H. [B 224a].
Tottingham, W. E. [B 283], 79, 212.
Traaen, A. E., 272.

Treub, 255.

Truog, E. [B 285], 74, 81, 113.
Tucker, M. [B 256].

Tulaikoff, N. M. [B 266, 284], 48.
Tull, Jethro [B 286], 5

UnpDERWoop, Lilian M. fs t2td], 51,
52, 173, 238.

VeirTcu, F. P. [B 287], rr9.
Vermuyden, C., 308.

Ville, Georges, [B 288], 19, 192.
Vincent, C., 120.

Virgil, 184.

Vitek, Eugen, [B 2750], 197, 198.
Voelcker, A. [B 289], 128, 150.
— J. A. [B 290], 73-6, 78, 86.
Vogel, I. [B 104], 198.

Wape, H. R. [B 97], roo.

Waggamann, W. H., 157.

Wagner, P. [B 291], 193, 195, 209.

— R,, 212.

Waksman, Selman A. [B 292], 186, 187, | —
257, 258, 259, 260, 276, 290, 204.

Wallerius, Johan Gotschalk, [B 293], 7.

Warburg, O., 57.

Ward, Marshall, [B 294], 205.

Warington, R. [B 1666, 295-97], 23,
180, 189, 190, 209, 298, 357.

Warth, F. J., 351.

Way, J. T. [B 298], 17, 22, 150, 153,

155.
Waynick, D. D., 262, 349.

SOIL CONDITIONS AND PLANT GROWTH

Weber, C. A. [B 299], 134.

Webster, T. A., 200, 255.

Weevers, T., 72.

Weinhold, A. [B 300].

Weir, W. [B 3004], 144.

Weis, Fr. [B 301], 146.

Westermann, T. [B 302], 139.

Wheeler, H. J. [B 303], 116.

Whiting, A. L., 191, 206, 207.

Whitney, M. [B 304-6], 27, 124, 172,
173, 233, 238, 247, 343-

Whitson, A. R. [B 307], 321.

Wieler, A., 82.

Wiegner, Georg, [B 308], 97, 156.

Wigham, J. T., 89.

Wild, L. J., rz9.

Wiley, Hy W., 355-

Wilfarth, H. [B 130¢c, d: 309], 24, 49,
59, 196, Hess

Willcox, W. H.,

William, H. F, iB. 136), roo, 199."

Williams, Bruce, 199.

Willstatter, Richard, [B 310], 75.

Wilms, J. [B 258], 128.

Wilson, G, W. [B 188], 259.

— Jj. B., 78.

— J. K., 207.

Wimmer, G. [B 130d, 3096], 297.

Winogradsky, S. [B 311-12], 23, 189,
Igo, 196, 260.

Wise, L. E. [B 1o5¢e], 114.

Withers, W. A. [B 271, 313], 190, 262,
263.

Wojtkiewicz, A. [B 314], 279.

Wolff, E. [B 315], 77.

Wolkoff, M. I. [B 316], 79, 80, 161.

Wollny, E. [B 317-18], 26, 139, 175,
i 232.

Wood, T a [B 319-20], 234.
Te T, 19

Woodwark i [B 321], 4

Woronin, M. [B 322], 25.

Youna, Arthur, 6

ZALESKI, W., 35.

Zsigmondy, R., 99, 148, 160.
Zwaardemaker, H., 72, 91,

Zyl, J. P. van, [B 323], 125, 127.

SUBJECT INDEX.

ABSORPTION compounds in soil, 104.
— of nutrients by plants, 296, 357.
— — substances by soil, 17, 149 et seq.
chemical explanation
(Way), 153.
—'— — — — colloidal explanation,
155.
combined chemical
and physical ex-
planation (Knop),
154.
— — — — — physical explanation
(Liebig), r54.
Equations expressing:
amount, 156.
rate, 157.
Acidity, effect on plants, 81.
— insoil, 109. See also Soil Acidity.
— — atmosphere, effect on plants, 82.
— methods of studying ; hydrogen ion
concentration, 113 ; titration, 113.
Acids, dilute, action on soil, 157 et seq.,
234.
— use in soil analysis, 234, 334.
effect on activity of micro-organisms
in soil, 268.
— — plant growth, 81, 268.
supposed excretion by plant roots,
16, 233.
Actinomyces in soil, 259.
— producers of humus, 177.
Adsorption, 156.
— equation expressing, 149, 156.
Aeration of soils, effect on plant growth,
51, 84.
— swamp soils assisted by alge, 256.
Agricere, 282,
Agriculture, early literature of, 1.
Air, effect on plant growth, 9.
— of vegetation on, 8, 10, 20.
in soil, composition of, 227.
— — volume of, 220,
supply to plant roots, 51.
in swamp soils, 256,
Algz in soils, 254.
Alkali soils, 74, 86, 94.

—

eee ee

Alkali soils, bacterial activity, 274.
Alkalis necessary for plant growth, 15,

18,

— supposed production by plants, 8, ro,

Alumina in soil, 338.

Aluminium, beneficial in small quan-

tities, 56, 75.

— harmful in larger amounts, 84.

Amino-acids produced in soils, 185.

Amino-nitrogen in soils, 147.

Ammonia, assimilation by alge, 256.

— — — fungi, 259.

— — micro-organisms, 210 et seq.

— — plants, 57.

in soil, determination of, 349.

— — amount of, 146.

nutritive value to plants, 58.

production in soil, 185 et seq.

— — — organisms effecting, 187.

— — — by fungi, 259.

toxic effects in larger quantities, 58,

80.
Ammonifying power, 263.
Ammonium salts, absorption by soil,
155 et seq.

— sulphate acidifies soil, 116.

Amoebae in soil, 289 et seq.

Analysis of plant ash and study of plant
nutrients, 7.

— — soil and manurial requirements,

326, 334.
— — -— mechanical and _ chemical
contrasted, 342.

— — — methods of, 347.

Antagonism of ions (bacteria), 274.

— — — (plants), 79.

Arid conditions, value of phosphates in,

65.
-— soils, comparison with humid:
mineral composi-
tion, roo.

—-—-— organic matter, 139.

Arsenic compounds, effect on micro-
organisms, 84.

— — — — plant growth, 84.

— — oxidation by bacteria, 84.

395

396

Artificial manures, 18, 19, 20.

Ash constituents of plants; composi-
tion of, ro.

— — — — essential to growth, Io,
I4.

— — — — functions of, ro.

— — — — importance as manure (see
also Mineral constitu-
ents), 2, 15, 16.

Asparagine as plant nutrient, 57.

Aspartic acid as plant nutrient, 57.

Assimilation, effects of temperature on,

35.
Auximones, 236.
Availability of mineral nutrients, 234,
245, 353-

Azotobacter. See Nitrogen fixation.

— activity in natural conditions, 200.

— effect of humus on, 199.

— test for soil acidity, 242.

— and clostridium, relative distribution

of, 204.

Bacreria as makers of plant food, 26,
175, 216.

Bacterial activity and nitrate produc-

tion, 270 et seq.
—- — soil productiveness, 176, 216,
260 et seq.
numbers in soil, effect of external
conditions on, 272, 275 et seq.,
286,
in field soils, 275 et seq., 292.
in soil, 217, 262, 264 et seq., 276,
292.
— — not clearly related
plant growth, 266.
rapid fluctuations in, 270, 292.
under different crops, 297.
Bacteriotoxins in soils, supposed, 249.
‘* Bake ” land, 241.
Barium salts, effect on plant growth,

to

74:
Basicity in soils, harmful effect of lack
of, 117.
Black soils, properties of, 307, 310.
Bleisand, 163.
Bones, fermentation of, 211.
Boron, harmful in large amounts, 78.
— helpful in small quantities, 56, 78.
Brownian movement of clay particles,
105.
Buffer action, 115.

CaEsIvM salts and plant growth, 74.

Calcareous soils, 94, 303.

— — characteristics of plant growth
on, 304.

Calcicolous plants, 74, 304 et seq.

SOIL CONDITIONS AND PLANT GROWTH

Calcifuges, 74, 304 et seq.

Calcium in plants: association with
nitrogen, 74.

— — more in leaf than in seed, 74.

carbonate, amount desirable in soil,

11g.

effect in soil, flocculation, 117.

neutralisation of acid,
Eke

other effects, 117, I19,
329.

— on soil organisms, 273.

— — plant, small quantities,
239.

excess, 86, 304.

not always essential to fertility,
118.

production in soil, 296.

rate of removal from soil, 119.

compounds in soil, 121.

oxide. See Lime.

salts, effects on plant growth, 74,

78.

— — — soil organisms, 274.

sulphate. See Gypsum.

Carbohydrates, absorbed and utilised
by plants, 57.

effect on ammonia production, 186.

— — denitrification, 202, 209 et seq.

— — nitrogen fixation, 198.

— — soils, 201.

elaboration in plants affected by

potassium salts, 70.

phosphates, 67.

Carbonates, amount in soil, 337.

— determination in soil, 335, 350.

Carbon cycle in soil, 175.

— nitrogen ratio in plant residues,

175-
— — — soil, 175, 273.
dioxide, effects of variation on
plant growth, 57.

— evolution in soil, 175, 177; rela-

tion to ammonia production,

185.

— — inhibiting effects on germina-
tion, 85.

— — in soil air, variations in, 227,
277:

necessary for plant growth, ro.

percentage in soil atmosphere,
227.

toxic effect on root, 51.

variations in atmosphere, 57.

source of, for plants, 10, 12, 56.

Cardiganshire, damage done by lead

mines, 85.
Carr, 134.

transformation in plants affected by |

e

SUBJECT INDEX

Catalytic actions of soils, 215.
Cellulose, decomposition in soil, 177.
— — — — by fungi, 258.
———— by _ Shirochaeta
phaga, 177.
Chalk soils, 120, 306 et seq.
Charlock, spraying for, 83.
Chemical analysis of soil, 233.
Cheshunt Experimental Station, 71.
Chlorides and plant growth, harmful in
larger amounts, 73, 75,
76.
-_--—--— necessary in small quan-
tities, 56.
Chiocopiryit a magnesium compound,

cyto-

75+
Chlorosis, 86, 304.
Clay as electro-negative colloid,
161,
— semi-permeable.membrane, 105.
colloidal (Schloesing’s) preparation
of, 148.
composition of, ror et seq.
definition of, 104.
effects in soil, 106.
justification of British definition, go.
particles, constitution of, 162.
properties of, 104 e¢ seq.
reation to K,O content, 340.
soils, 310.
— characteristics of plant growth
on, 312,
— need some excess of calcium
carbonate, I1g, 336.
— water supply in, 219.
two groups of, ror.
Climate and crops, 45.
— a factor in determining soil type, go,
303.
fertility, 331.
Clostridium and azotobacter, relative
_ distribution of, 204.
Coarse sand, composition of, 102.
— — effect in soil, 108.
roperties of, 108.
Combetuctts, ror.
Colloidal clay, preparation of, 148.
Colloids in soil, effect in determining
plant habitats, 239.
— — — estimation of, 169.
— — — importance, 149, 172.
— — — influence on micro-organ-
isms, 239.
reactions, 168.
— properties of, 148.
Competition between plants, effect on
flora, 241.
— — — balance in, 241.
Composition of soil particles, ro2.

104,

397

Compound interest law (V. H. Black-
man), 28.
Compound particles in soil, 170, 171,

303.
Condition and fertility, 320.
Constitution of clay, 162.
— — soil, 170.
— — — particles, 172.
Cooper-Hewitt mercury lamp and plant
growth, 88.
Copper salts harmful to plant growth, 83.
— — use as plant poisons, 83.
Crenic acid, 141.
Crops, distribution of; influence of
rainfall, 45.
temperature, 45.
— specially suited to certain soils, 240.
Cultivation, exhaustion due to continu-
ous, 321.
— loss of nitrogen during, 181.

Decomposition of organic matter in
soil, 175 et seq.

Deflocculation. See Flocculation.

Denitrification, 208.

— not extensive in normal soils, 209.

Depth of soil, importance of, 306.

Dicyanodiamide, toxic to plants, 85.

Disease organisms in soil, 253.

— resistance as affected by soil con-

ditions, 53, 61, 70, 71.

Dominant constituents of fertilisers,
19.

Drainage water, composition of, 127,

152.

— — nitrogen lost in, 178, 299.

Dried soil and productiveness, 169.

Drought, effect on soil reactions, 275.

Dry farming, 218.

Eartu the ‘ principle” of vegetation,
4.

Earthworms in soil, action of, 136, 137.

Ecological classification of humus, 133.

Ecology, province of, 43.

Edinburgh Society for Improvement of
Arts, 6

Eelworms in soil, 253.

Efficiency values of plant nutrients
(‘* Wirkungswert”’), 33.

Elective method, 196.

Electric discharge, effect
growth, 87.

Electrolytes, effect on clay, 105.

Electro-osmosis, 161.

Emulsoids, 160.

Energy relationships ; micro-organisms,

129, 252, 272.
— — — ammonifying, 186.

on plant

398

Energy relationships; micro-organisms,
denitrifying, 209.

— — nitrifying, 188.

— — nitrogen-fixing, 197, 199, 201.

— — other groups, 250 e¢ seq.

algze, 252.

bacteria, 128, 176, 252.

fungi, 252.

protozoa, 252.

—- plants, 128.

— sources in soil, 93, 129.

supply and soil population, 176, 2rr.

Engrais complementaires, 76.

Eremacausis, 23.

Excretions from plants, possible toxic,

244 et seq.

Factors affecting the growth of plants,
30, 38.
Feces and nitrogen-fixation, 203.
Fallowing, effect on nitrates in soil,
297 et seq.

— — — soil, 15, 18, 297. -

— harmful effect on fen soils, 310.

— losses during, 180.

Farmyard manure as fertiliser, 19,

236.
effect on clover crop, 207.
— increases moisture content of
soil, 132.

losses during storage, 193.

loss of nitrogen in soil, 181.

retained in surface soil, 152.

and solubility of phosphates,
212.

— bacterial numbers, 269, 273,
276.

— denitrification, 209.

Fatting fields in Romney Marsh, 317.

Fen, 134, 307, 308.

Fertility, transfer from new to old

countries, 318.

Field trials, necessity for, 19, 324.

Fine sand, composition of, 102.

— — effect in soil, 108.

— — properties of, 108.

silt, composition of, 102.

— effect in soil, 107.

— properties of, 107.

Flagellates in soil, 291 et seq.

Flocculation, absorption hypothesis,
161.

— electrical hypothesis, 16r.

— of clay, 105, 160 e¢ seq.

— — silt, 160 e¢ seq.

— reversibility of, 161.

Flooding, effect on soil, 325.

Flora, plant, factors determining, 301.

— — and soil reaction, 240.

SOIL CONDITIONS AND PLANT GROWTH

Fluorides and plant growth, harmful in
larger amounts, 76,

— — — — necessary in small quanti-
ties, 56, 76.

Folding, 306.

Food supply and plant growth, 30,

55:
— — effect of on water requirements
of plants, 48.
Forest soils, humus of, 136.
Fractions obtained during mechanical
analysis, 98, 326.
Frost, late, 303.
— effect on soil bacteria, 279.
Fungi in soils, 137, 256 et seq., 282.
— — — favoured by acid conditions,
243.
— — — producers of humus, 177.
Fulvic acid, 141.
Fungoid pests, liability to, 53, 61, 79.

GERMINATION, capacity of buried weed
seeds, 85.

— — — seed, 54.

— effect of salts on, 86.

— — — CO, on, 85.

Glacial soils, roo.

Glasshouse practice, effects of tempera-
ture, 46.

— — — — watering, 46.

Glutamic acid as plant nutrient, 57.

Grain, composition of. See Seed.

— proportion to straw, factors affect-
ing, 61.

— — — — increased by phosphates
and silicates, 77.

Grass, growing, effect on fruit trees,

246.

Gravel, composition of, 102.

— effect in soil, rog.

— properties of, 109.

Green manuring, 314.

Gypsum, effect on plants, 74, 79, 86.

— — — soils, counteracts alkalis, 74.

— — — supply of potassium salts,

152.

Heatep soil, changes in, 169, 281.
Hochmoor, 134.

Hoeing as manure (Tull), 5.
Hormones, 87.

Hot water treatment for forcing plants,

Humic acid, composition of, 138.

— —- fractionation, 140, 141.

— preparation and properties, 139.

— proof of acid nature, 143.

— protective effect on flocculation,
162.

SUBJECT INDEX

Humin, 141, 146.

— hydrolysis of, 146.

Humus, 133, 137, 303.
ganic matter in soil,

— i dese of, 138.

See also Or-

— artificial production of, from dihyd-
roxyfurfuraldehyde,
143.
—--—--—- quinone, 143.
—--—--—-_— sugar, 142.
— colloidal properties, r4o.
— effect on iron compounds in acit
165.
— formation in soils, from cellulose,
141, 177.
— — — — field observations, 136 ef
seq.

function in soil, 15, 130, 144.

not effective in denitrification, 209.
of field soils, 137. |

— — — similarity to peat, 137.
— forest soils, 136.

relation to Azotobacter, 199.

soils, 94, 307.

— characteristics of plant growth
on, 310.
soluble, supposed

fertility, 143.
— supposed food of plants, 7, 8, 12,

SEES 2 eo

necessity for

15.
“ Hungry ”’ Soils, 108, 312.
Hydrogen ion concentration of plant
juices, 81.
— — — — soil solution, 116.
— — — in relation to plant growth,
81.
Hydroxyl ion more toxic than hydrogen
ion, 81.
Hygroscopic coefficient, 223.
— — relation to other moisture co
efficients, 226.
— water, 166 et seq.
Hymatomelanic acid, 141.

IMPROVEMENT of exhausted soils, 321.
— — soil, methods of, 320.
Injurious factors (to plants), effect of,
42, 80.
— substances (to plants), 80
Inoculation of soil with Azotobacter,
203.
— — — — nodule organisms, 208.
Iodides and plant growth, beneficial in
small quantities, 56, 76.
— — — — harmful in larger
amounts, 76.
Ions, antagonism of, 79.
— complex equilibrium in plant cell,
79-

399

Iron compounds in soil, 338.
— — — — part played in pan forma-
tion, 163.
— — effect on plant growth, 75, 84.
Irrigation, 50, 86.
— and varying requirements of plants,
50.
— effect on soil organisms, 272.
— injurious effects of excessive, 50.
Iso-electric point, 161.
LAMZIEKTE, 318.
Law of minimum, Liebig, 16.
— — — Mitscherlich, 28.
Leaching, effect on soil, gg.
Lead mines, damage done to vegetation
by, 8
Leguminous crops, effect on soil, 184,
208.
— — nitrogen fixation, 24, 207.
Liebig’s patent manure, 16, 17.
Light and plant growth, 54.
— indirect effect on soil, 55.
Lime as a sterilising agent, 273.
— effect on soil, r1. Seealso Calcium
carbonate.
— — — — organisms, 273.
— indications of lack in soil, 336.
— magnesia ratio, 75, 339.
— requirement of soil, 114, r19.
— — — — determination of, 35c.
Limiting factors, 28, 38.
Lithium salts, harmful to plant growth,
73+
Loams, 315.
— characteristics of plant growth on,

315.
Lodging of cereal crops, 317.
Loess soils, 100.
Lysimeter experiments, 178, 180,

Magma unguinosum, the principle of
vegetation, 4.
Magnesian limestone, effect as fertiliser,
86.
Magnesium associated with formation
of oil in plants, 75.
— carbonate, effect on soil organisms,
273.
— compounds in soil, 339.
— in seed, 75.
— salts, effect on plant growth, 75, 86.
— — — — supply of potassium salts,

151.
Manganese salts, beneficial effect on
nodule organisms,
207.

boa cat tea Aa plant growth, 56, 76.

400

Marsh gas, oxidation of, in paddy soils,
213.
Mathematical expressions, for plant
.growth, 31, 34, 42.
— — search for, 27.
— — value of, 28.
Matiére noire (humus), 138.
— — supposed index of fertility, 143.
Maturation, 54.
Mechanical analysis, defects of ordin-
ary, 95-
— difficulty of interpreting quantita-
tively, 170.
fractions obtained during, 98.
interpretation of, 326.
methods, 353.
not applicable to chalk or peat
soils, 344.
Odén’s distribution curve, 95.
Melanoids, 141.
Mercury vapour lamp, effect on plant
growth, 88.
Methods of investigation ;
logical, 260.
correlation, 104, 130.
modern combined, 28.
quantitative, 9.
statistical, 18, 29.
Micro-organic population of soil, effect
on productiveness, 294.
Micro-organisms in soil; active and
resting stages, 216, 290.
action of plant on, 296.
agents causing decomposi-
tion, 175.
— producing plant nutrients,
oy
general relationships, 252.
groups Of, 250 et seq.
methods of investigation, 257
et seq.
relation to growing plants,
129, 251, 252, 260, 294, 296.
— — — sources of energy for, 129.
Mineral constituents of plants, functions
of, Ig.
— — — — list of essential, 19.
— — — — origin of, 9.
— part of soil, 94 e¢ seq., 351.
— soils, 94.
Mineralogical investigations of soil, 100.
Minimum, law of, Liebig, 16, 28.
— — — Mitscherlich, 28.
Mining (metals), injury to vegetation
caused by, 85
Mixed crops, 249.
Moisture in soil, amounts of, 200.
Water in soil.
— equivalent of soil, 224.

bacterio-

See

SOIL CONDITIONS AND PLANT GROWTH

Molkenbéden, 163.

Moorland soils, 307-

— — bacterial activity in, 269, 307.
Muck land, 310.

Mull, 136.

Mycorrhiza, 253.

Myxomycetes, 253.

NEMATODES, free living, 252.
— parasitic, 253.
Niedermoor, 134.
Nitrate immobiliser in soil, 179.
Nitrates, assimilation by micro-organ-
isms, 210 é¢ seq.
— — alge, 256.
as plant food, 22.
contrast between production and
destruction, 209.
effect on bacterial action, 199, 275.
essential plant food, 22.
formation in soil, 21. See also Nit-
rification.
— — — effect of growing plant on,
297. :
of season, etc., 276.
Nitrates and plant growth :
— — — — effect of water supply, 40.
— qualitative relationships,
57, 61.
affected by potas-
sium salts, 63.
— quantitative relationships,
24, 40, 59, et seq.
(See also Saltpetre.)
in soil, alleged catalytic formation
of, 216,
amounts of, 145.
decomposition of, 300.
determination of, 348.
maximum producible, 178.
removal of, 178, 180, 235.
seasonal fluctuations, 146.
supply oxygen to organisms,
209.
Nitre beds, 21.
— — resemblance to fertile soil, rz.
Nitrification, 187 e¢ seq., 216.
— an autocatalytic process, 188.
bacterial origin demonstrated, 23.
culture solutions, 188.
effect on soil phosphates, ror.
influence of organic matter, Igo.
in sewage, more rapid than in soil,
215.
— soil, limited by rate of ammonia
production, 188.
not an oxidase action, 188.
organisms causing, 188, rgt.
relation to soil fertility, 22.

—

SUBJECT INDEX

Nitrification, supposed chemical origin,
23.

— two stages demonstrated, 23, 188.

Nitrifying power, 262, 300,

Nitrites, assimilated by plants in small
quantities, 59.

— bacterial production from nitrates,

— harmful in larger amounts, 85.
Nitrobacter. See Nodule organism.
Nitrogen, determination of, 348.
evolution of gaseous, in decomposi-
tion of organic matter, ror.
fixation of, 182, 195. See also
Nitrogen fixation.
fixing power, 265.
gaseous, supposed assimilation by
plants, 14.
— supposed evolution from soil,
182.
in plant, association with calcium,

74:

nutrition, distinction between legu-
minous and_ non - leguminous
plants, 18, 21, 24.

recuperative agencies in soil, 182.

source of, for vegetation, 10, 14 et
Sequy 18, 20.

content of soils; Broadbalk, 153,

183.

— — relation toloss on ignition,
335-

— upper and lower limits,
184.

— — Wilderness, 183.

grain, effect of length of
ripening period, 54.

— increased by magnesium
oxide and car-
bonate, 75.

sodium hydrate and
carbonate, 73.

— unaffected by many salts,

eae

73+
cycle in soils, 175, 178.
fixation, attempts to induce in soils,
201.

— by electric discharge, 196.
— free-living organisms—

Algz, 200, 255.
Azotobacter, 197.
Clostridium, 196.
— fungi, 259.
— nodule organisms, 25, 204.
— other organisms, 200, 208.
in soil, 182.
— — estimated amount of, 207.
in soils, compounds present, 145.
— — loss of, 178, 181.

401

Nitrogen in soils, percentage present,

335.
— — — relation to fertility, 334.
Nitrogenous nutrients for plants, 18,
22, 57-
Nodule organism, artificial culture of,
204, 206.
entry into plant, 205.
isolation from soil, 205.
life cycle, 204.
— specific nature of, 25.
stimulated by manganese com-
pounds, 207.
Nodules on roots of leguminose con-
tain bacteria, 21, 25.
Normal conditions in heavy soil, 249.
Nutrient solution, effect of concentra-
tion on plant growth, 238.
Nutrients become toxic when given in
larger quantity, 56.
needed in large quantities, 56.
— — small quantities only, 56,
236.
quantity auasebad by plants from
soil, 284, 357.
raw materials out of which plant
food is made, 55.
Nutrition of plants, effect of water

supply, 38 e¢ seq.
Nutritional disturbances in plants, 236,

304.

OrGANIC compounds absorbed and. util-
ised by plants, 57.
— decomposition in soil, 212.
— — effect on plants, 248.
matter in soil, chemical properties
of, 138 et seq.

—

— — — — colloidal properties of,
130.

— — — — decompositions, summary
of, 214.

— determination of, 348.
— effects of, 130, 329.
on fertility, 130,
334-
water supply,
131, 132.
partially | decom-
posed, 130.
undecomposed,
130.
fractionation of, 141.
masks effects of clay and
sand, 132.
nature of, 129.
— original deposit, 12g.
recent formation, 129.

——— — — — on

402

Organic matter in soil, relation to
micro-organic population, 128, 129,
252, 272.

Ortstein, 137, 163.

Osmotic pressure; cell sap, 46, 222,

237.

in relation to soil moisture, 46.

root Sap, 222, 237.

soil solution, 125, 237.

— —and classification

habitats, 237.

Oxidation in soil, 175.

— potential in soil, 177.

— relation to fertility, 176, 283.

Oxygen, dissolved, effect on bacterial

action, 275.
— supply necessary for root, 51.
— — to plant root in swamp soils, 213.

of

Pappy soils, 212, 300.

Pan, composition of, 164.

— formation, 137, 162 e¢ seq.

— — as collodial phenomenon, 165.

— — chemical hypotheses, 163.

Partial sterilisation of soil, 280 et seq.

Pasture soils, humus of, 137.

— — nitrates in, 146.

Pathological conditions, possible nutri-
tion effects, 236.

Peat, 133.

— English types, 134, 135.

— Irish types, 135.

— Scotch types, 135.

Peat ; acidity, cause of, III.

chemical analysis of, 136.

in relation to pan formation, 163.

soils, 94, 307.

— assimilation of ammonia in, 210.

— denitrification and manuring,

210.

— effect of lime on, 119, 210.

— water supply in, 219.

Perchlorates toxic to plants, 85.

Permeability in soil, 105.

Permutit, 156.

Petrographic methods of soil analysis,
I2I.

Py value, definition of, 115.
Hydrogen ion concentration.

Phenol, decomposition of, in soil, 212,
216.

Phosphate exhaustion, 321.

Phosphates, effect of bacteria on, IgI,

2It.
— on soil bacteria, 274.
in plants, economised by presence
of silica, 78.
in soil, 120, 341.
— — determination of, 352.

See

SOIL CONDITIONS AND PLANT GROWTH

Phosphates in soil, reaction with dilute
acids, 157.

— and plant growth, necessity for, 8,
15, 1

— — — qualitative effects, 64.

— — — quantitative relationships, 3.,
64. ‘

Phosphatic fertilisers : increase quality

in crops, 67.

use on cereals, 65,

— — clay soils, 64, 312.

— dry soils, 65.

— fen soils, 309.

— grassland, 67, 183.

— root crops, 64.

Phosphorus compounds in soil, 120,

— — effect on cell division, 67.

— — — — starch transformations, 67.

— — function in cell, 66.

— — more in seed than in leaf, 68.

Physical properties of soil, early studies,

—

12.

Physiological balance of inorganic nutri-
ents in culture
solutions, 78.

—-— eee soil, So,

— grouping of soil organisms, 261.
Physiology, plant, foundation of, 9.
Plant food, amount in soil, 233.
— — available, 233.
growth, effect on soil, 296.
— — soil organisms, 297.
factors affecting, 30 et seq.
in relation to soil types, 315.
optimum ratio of nutrients, 80.
periods in, 296.
relation to concentration of soil
or culture solution, 80.
materials from which it is produced,
6.
nutrients, circulation between plant
and soil, 93.
nutrition, methods of studying, 7.
— realisation of complex nature of,

Ts
— residues, action in soil, 213.
— — decomposition in soil, 214.
Plants, supposed resemblances
animals, 8
Plasticity of clay, effects of, 106,
— — — theories of, 106.
Plating methods, soil bacteria, 261,

to

265.

Ploughsole, 166,

Poisons, inorganic, effect on plants,
80 et seq.

-|— organic, effect on plants, 248.

Pore space, 220.
Pot cultures, introduction of, 7.

ee, is i ee 8

tet

ee

ee ee ee ee ee

}

- Protein content of grain.

Pot cultures, modern methods, 356.
Potash in soil, determination of, 352.
Potassic fertilisers, effect on leguminous
crops, 72.
— — value for thin soils or dry dis-
tricts, 340.
— — — in unfavourable seasons, 71.
Potassium compounds in soil, 120, 339.
— — — — distribution among soil
particles, 121.
— — reported harmful effect on soil,
116,

_ — function in cell, 72,

in plants, economised by presence
of sodium, 73.
salts, absorption by soil, 157.
— effect on bacterial action, 274.
— — influence on nitrogen effects, 63.
— necessary for plant growth, 7.
— — qualitative effects on plant, 69.
— quantitative relationships, 31.
— — suggested replacement in cell by
equi-radioactive elements, 72.
Potatoes, quality in, 318.
Potsherds, effect of adding to soil, 52.
Prairie soils, loss of nitrogen from,
181.
Principles of vegetation, search for, 2.
See Nitro-

en.

— saneabolise : relation to calcium, 74.

Protozoa in soil, 288 e¢ seq.

— — — inverse relation to bacteria,
292.

— — — numbers of, 291.

Putrefactive power (Remy), 263.

RADIO-ACTIVITy an1 potassium supply,
rh

— — of soil, 89.

Radium, effect of, on plant growth, 89.

Rainfall and crops, effect, 45.

— effect on scheme of manuring, 341.

— — — soil, 119, 235, 330.

Rain water ; ammonia and nitrate in, 18.

— — chlorine content, 76.

— — dissolved oxygen in, 228.

— — sulphates in, 76.

Reaction of soil ; methods of controlling,
244.

Reactive inorganic constituents of soil,
109g, 215.

Reclamation of clay soils, 312.

— — sandy soils, 314.

Renewal of plant nutrients, effect of rate
of, 239.

Respiration of plants, general pheno-
mena, 10,

Resting period of plants, 88.

SUBJECT INDEX

403

Ripening processes hastened by phos-
phates, 65.

— — — — drought, 65.

Rohhumus, 138.

Rothamsted. experiments, commence-

ment of, 17.
— — results, 18, and many subsequent
pages.
Root, activity of, makes soil alkaline,
296.
crops, use of phosphatic fertilisers,
64.
development, increased by phos-
phates, 64, 341.

— marked in sandy soils, 316.

— impeded in clay soils, 64.

excretion, nature of, 33, 233.

— of mineral matter, 296.

— possible toxic, 244.

formation of, increased by aeration,

53.
calcium salts, 74.
phosphates, 64.
— — influence of soil type, 64.
water supply, 44.
functions of, 19.
sap, acidity of, 81.
— concentration of, 222, 237.
— — osmotic pressure of, 222, 237.
Rotation of crops, 184, 244.
— — — not essential, 5.
— — — statistics of, 13.

SALTPETRE promotes vegetation, 7.
— the ‘‘ principle” of vegetation, 3.
Salts, alkali, effects on plants, 85.
— — — — soil bacteria, 274.
Sand, composition of, 102.
— cultures, methods of, 356.
— dune, rog.
— effect in soil, 108.
— — — — masked by organic matter, ~
132:
— properties of, 108.
Sandy soils, 302, 312.
— — characteristics of plant growth
in, 312.
— — need no excess of calcium carbon-
ate, 11g, 336.
— — subject to pan, 163.
— — water supply in, 219.
Scouring land in Somerset, 317.
Seasonal effects on bacterial activity in
soils, 276.
— variation in activity of soil organ-
isms, 279.
Seed, composition of, 68, 74, 75.
— effect of length ofripening period, 54.
— — — manuring, 44.

26 *

404

Seed, effect of water supply, 44.
— richer in phosphorus and magnesium
than in calcium and potassium,

75:
— size and weight affected by potassium
susply, 70.
slightly by nitrogen
supply, 61.
phosphate sup-
ply, 66.
Seeds, viability after long period in soil,

eee

Sewage, absorption by soil, 152.

purification ; changes during, 214.

effect of disinfecting filters, 251.

— nitrification during, 23, 1go.

a nitrogen lost during, 192, 194.

reactions during, 214.

Shade crops, 229.

Shrinkage of soils, 106, 133, 219.

Sickness in soil, 248, 287, 294.

Silica, supposed production in plants, 8.

Silicates, beneficial effect on plant
growth, 15, 56,

77:
especially with lack
of phosphate, 77.

— in soil, 120, 121.
— — — Van Bemmelen’s groups, tor,
103.
— Way's double, 153.
Silt, composition of, 102.
— effect in soil, 107.
— properties of, 107.
Silty clays, 312.
Sodium carbonate, toxic effects, 72, 85.
— nitrate, effect on soil tilth, 107, 116.
— salts, effect on plant growth, 72.
— — — — supply of potassium salts,
151.
Soil acidity, effect on flora, 240.
— — — — micro-organisms, 242.
— — hypotheses to account for :—
Chemical: acid silicates, 110,
— aluminium salts, 112.
— exchange of bases, 112.
— hydrogen ion concentration, 113.
— organic acids, 110, 118.

Physical: preferential absorption,
IIo, 118.

Soil acidity, summary of present posi-
tion, II7.

— analysis, 234, 322 et seq., 347 et seq.
— colloidal properties of, 149.
constitution of, 172 et seq.

— — American hypotheses, 172.
— cultures contrasted with sand, 4o, 41.
— exhaustion, 319.

fertility, 319.

SOIL CONDITIONS AND PLANT GROWTH

Soil fertility, theories of, 27.
— formation of, 92, 303.
in relation to plant growth, 3or.
minute particles the ‘* pabulum ” of
plants, 5.
sampling, 324, 347.
solution : concentration of, 125, 126.
— composition of, 93, 125, 127, 173.
— importance in soil fertility, 27.
— investigation in situ by freezing-
point method, 125.
methods of extraction: absorp-
tion, 125,
centrifugal, 124.
displacement, 122,
pressure, 124.
— osmotic pressure of, 125, 126.
— relation to climate, crop and
season, 122, 127.
— — plant growth, 173.
— — soil mineral matter, 122,
125.
supposed constancy of composi-
tion, 173.
surveys, 322, 345.
— based on mechanical analysis,
343.
— petrogeological analysis, 346.
type, characterisation of, 326.
— effect on plant, 45.
water, 122 et seq.
Soils, classification of, 94.
— specially suited to certain crops or
varieties, 316.
Sourness in soil, 118, 239 et seq., 315.
Specific gravity of soils, 220.
— heat of soils, 229.
Spores, bacterial in soil, 216.
Stages of development of plant, effect
of nitrates on, 61.
Statistical method of investigation, 18,
29.
Steppe, 314
Sterilisation of soil.
isation.
— — — not achieved without decom-
position, 250.
Stimulation of plant growth, possibility
of, 87 et seq.
Stones, effect in soil, rog.
Storage of soils, change during, 169.
Straw and stubble, effect on soil, 213.
— effect of nitrates, 61,
— lodging of, 62.
as ne effect OF potassic fertilisers,

See Partial steril-

— relation to grain, 62.
Strontium salts, effect on plant growth,

74-

es

a

Subsoil, distinction from surface, 332 et

seq.
— effect on fertility, 332.
— water, possible supply for plants,
221,
Sugar, effect on soil, 201.
Sulphates and plant growth, 76.
Sulphocyanides, effect on plant growth,

5.

Sulphur cycle in soil, 211.

— dioxide, harmful effect on plant
growth, 82.

— in plants, more than formerly sup-

posed, 76.
Sulphuretted hydrogen, effect on plant
growth, 85.
Sulphuric acid and plant growth, 82, 268,
— — — bacterial action in soil, 268.
Superphosphate as fertiliser, effect on
agriculture, 65.
Surface of soil, caking of, 108.
Suspensoids, 160.
Swamp soils, 212. See also Paddy.
— — air supply to plant root, 256.
— — changes during green manuring,

213.
— — denitrification and manuring, 210.
— — toxic compounds in, 210, 248.
Swelling of soil when wetted, 133.

TCHERNOZEM, 310.

Temperature and assimilation, 34.

— — enzyme action, 37.

— coefficient for cell growth, 35.

— effect on plant growth; qualitative,

455 46, 53-

at, Serar rere eet? quantitative, 34.

— of soil, 229.

— — — effect on bacterial numbers,
272, 276, 286.

— — — lowered by clay, 106.

— — — relation to air temperature,
231.

— — — — — bacterial activity, 272,
276.

— optimum conditions for growth, 53.
— physiological effects, 54.

Texture of soil as affected by calcium
carbonate, 117.
organic matter,

132.

Tilth and compound particles, 106, 171.

— Tull on, 5.

Topographical position and soil pro-

perties, 302.

Torf, 136, 137.

Toxic effects of salts, 78.

— — — — reduced by calcium ion,
78.

SUBJECT INDEX

405
Toxic effects of salts reduced by sugar,

79:
— salts in soil, treatment of, 85, 117.
Toxins in soil, 173, 244 et seq.
Transpiration _and concentration of
nutrient solution, 238.
— coefficient, 46 et seq.
Tull’s method of crop production, 5.

UNDERGROUND water supply and soil
fertility, 331.

Unfavourable seasons, value of potassic
fertilisers in, 71.

‘* Unfree ’’ water in soil, 226.

VALLEYS, fertility of, 332.

Vegetation, effect on soil formation, 93.

— relationships; effect of soil, 301
et seq.

— — other factors, 301 et seq.

— type of, effect on soil, 302.

Vigour of plant affected by calcium, 74.

—-—-- potassium supplies, 70.

WARPING, 308, 315.

Waste land, 327.

Wasted fertility, 321.

Water consumption by plant, 47.

— — — — effect of manures, 48.

meteorological con-
ditions, 47.

soil conditions, 46.

—-— ee water supply, 48.

— cultures, method of: Knop, 19.

— — — — Modern, 356.

— — — — Woodward (1699), 4.

— evaporation from soil, 166 e¢ seq.

— — — — Keen’s equation, 167.

— holding capacity of soil, 223.

— insoil, coefficients, relations between

various, 226,

— — — continuity of state, 224.

— — — determination of, 348.

— — — supposed various states, 166.

— — — volume of, 220.

— retention by soil, 166,

— — — — magnitude of forces de-

termining, 225.
— supply and interpretation of mechani-
cal analysis, 329.
— — effect on leaves, 44.
— — — — plant growth; qualitative,

43-
—_— — — quantitative, 38.
— — roots, 44
— — — — seed formation, 44.
— — influence in determining value of
manures, 64.
in soil, 218, 301.

406

Water supply in soil, available for
* plants, 221 et seq.

— — — effect on bacterial activity,
272, 276.

numbers,
276, 286,

— — — movement of, 218 et seq.

— utilisation of food, 38.

the “ principle” of vegetation, 3.

vapour, absorption by soil, 170.

Wazxlike constituents of soil, 144.

Weathering of soil, 93, 99.

Weeds, checked by heavy crop, 55.

272;

SOIL CONDITIONS AND PLANT GROWTH

Weeds, control by metallic salts, 85.
Wilting coefficient, 221.

— phenomena of, 221.

— point, 168, 221.

Woodland soils, nitrates in, 146.

— — nitrification in, 146.

— — nitrogen content, 55.

ZEOLITES, reactions of, 109,

— supposed presence in soil, 10g.

Zinc salts, beneficial in small quantities,
56, 85.

— — harmful in larger amounts, 85.

PRINTED IN GREAT BRITAIN BY THE UNIVERSITY PRESS, ABERDEEN

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Ss Russell, (Sir) Edward John
591 Soil conditions and
RS. plant growth c4th ed.3
1921

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