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THE SOIL,
AN INTRODUCTION TO THE SCIENTIFIC
STUDY OF THE GROWTH OF CROPS
BY A. D. HALL, M.A. (Oxon.)
DIRECTOR OF THE ROTHAMSTED STATION
(LAWES AGRICULTURAL TRUST)
FOREIGN MEMBER OF THE ROYAL ACADEMY OF AGRICULTURE
OF SWEDEN
SECOND EDITION, REVISED AND ENLARGED
LONDON
JOHN MURRAY, ALBEMARLE STREET, W.
1912
DEDICATED TO
THE WORSHIPFUL COMPANY OF GOLDSMITHS
THE FIRST PUBLIC BODY IN THIS COUNTRY TO
CREATE AN ENDOWMENT FOR THE
INVESTIGATION OF THE SOIL
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First Edition .
March 1903
Reprinted . . ,
September 1 904
Second Edition .
, September 1908
Reprinted .
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Reprinted . . ,
, Afay 1912
Photograph showing transition from Rock into Subsoil and Soil by Weathering
(Hythe Beds, Great Chart, Kent).
[Frontispie
INTRODUCTION
The study of the soil, which is fundamental in any
application of science to that part of agriculture which
deals with the growth of crops, has received greatly
increased attention during the past few years. The
crude chemical point of view, which in the main
regarded the soil as a nutritive medium for the
plant, has been altogether extended, by a considera-
tion of the soil as the seat of a number of physical
processes affecting the supply of heat and of air
and water to the plant, and again as a com-
plex laboratory, peopled by many types of lower
orgariisms, whose function is in some cases indis-
pensable, in others noxious, to the higher plants with
which the farmer is concerned. These three kinds of
reaction — chemical, physical, and biological — interact
upon one another and upon the crop in many ways;
they are affected by, and serve to explain, the various
tillage operations which have been learnt by the
accumulated experience of the farming community,
and the hope for future progress lies in the further
adaptation for practical ends of these processes at
work in the soil. But it must not be supposed that
science is yet in a position to reform the procedure of
farming, or even to effect an immediate increase in the
280282
vi INTRODUCTION
productivity of the land : agriculture is the oldest and
most widespread art the world has known, the applica-
tion of scientific method to it is very much an affair
of the day before yesterday. Nor can we see our way
to any radical acceleration of the turnover of agricultural
operations that shall be economical ; the seasons and the
vital processes of the living organism are stubborn facts,
unshapable as yet by man with all his novel powers.
But even if the best farming practice is still a step
beyond its complete explanation by science, yet the
most practical man will find his perception stimulated
and his power of dealing with an emergency quickened
by an appreciation of the reasons underlying the
tradition in which he has been trained ; and such an
introduction to the knowledge of the soil it is the aim
of this little book to supply. The book is primarily
intended for the students of our agricultural colleges
and schools, and for the farmer who wishes to know
something about the materials he is handling day by
day. While a certain knowledge of chemistry is
assumed, it is hoped that the subject is so treated as
to be intelligible to the non-technical reader who is
without this preliminary grounding. Though the book
is in no sense an exhaustive treatise, it has been my
desire to give the reader an outline of all the recent
investigations which have opened up so many soil
problems and thrown new light on difficulties that
are experienced in practice. The scope of the book
precludes the giving of references and authorities for
all the statements which are made; but, for the sake
of the more advanced student, a bibliography has
been appended, which will take him to the original
sources and give him the means of learning both
sides of the more controversial questions.
The same reason — want of space — has prevented me
INTRODUCTION vii
from giving an adequate justification of some of the
points of view indicated Any worker in so novel and
unsurveyed a field as the study of the soil still presents,
must arrive at certain personal conclusions, and I have
tried to steer a middle course between an over insistence
on these points on the one hand, and the colourlessness
that would come from their entire exclusion on the
other. No great part of a text-book can pretend to
be original, but in the sections dealing with the
chemical analysis and the physics of the soil, I have
incorporated a good many unpublished measurements
and observations ; for the mass of the results on which
the book is based, I am chiefly indebted to the work
of Lawes and Gilbert, as set out in the Rothatnsted
Memoirs, and to the writings of Warington in this
country, of King, Hilgard, and Whitney in America,
of Wollny in Munich.
I have to thank Professor J. Percival, of the South-
Eastern Agricultural College, for notes respecting the
association of plants with specific soils, and many
suggestions on biological questions; Major Hanbury
Brown, C.M.G., head of the Egyptian Irrigation
Department, for information concerning "salted" lands
in Egypt ; Mr F. J. Plymen, who has been associated
with me in carrying out a soil survey of the counties
of Kent and Surrey, and has executed many of the
observations recorded here ; Mr W. H. Aston, one of
my pupils, to whom I owe the observations on p. 135 ;
and finally, Dr J. A. Voelcker, to whom I am greatly
indebted for reading the proof-sheets, and making
many valuable suggestions thereon.
A. D. HALL.
Harpenden, December 1902.
PREFACE TO THE SECOND EDITION
A CONSIDERABLE number of additions and alterations
have been incorporated in the present edition. These
include a revision of the method recommended for the
mechanical analysis of soils, the method now given
being that adopted by the members of the Agricultural
Education Association in this country. ' Owing to re-
searches which have appeared since the publication of
the first edition, I have greatly modified the views I
then expressed on the nature of clay, and on the part
played by zeolitic silicates in the retention of ammonium
and other salts by the soil During the last six years,
however, the greatest additions to our knowledge of the
soil are those dealing with bacteria; in consequence,
the chapter on the living organisms of the soil has been
largely rewritten and added to. A number of minor
corrections have been made in the text, some of which
represent the removal of errors, and others modifications
due to more recent research. For the mistakes which
must still remain, and which will become evident in the
course of time, I must ask my readers' pardon before-
hand ; in dealing with so complex a subject as the soil
we are still far from final conclusions, many of our most
trusted conclusions are only rough approximations to
the truth, and by the progress of research they may at
any time require remodelling until they are hardly
recognisable.
A. D. HALL.
The Rothamsted Experimental Station,
May 1908.
CONTENTS
INTRODUCTORY
PAGE
In the Scientific Study of Soils, Chemical, Physical, and Bio-
logical Considerations are involved . . • I
CHAPTER I
THE ORIGIN OF SOILS
Sedentary Soils, and Soils of Transport — Weathering —
The Composition of Rock-forming Minerals and their
Weathered Products — Distinction between Soil and Sub-
Soil — General Classification of Soils • . .6
CHAPTER n
THE MECHANICAL ANALYSIS OF SOILS
Nature of Soil Constituents : Sand, Clay, Chalk, and
Humus — Methods of Sampling Soils — Methods for
the Mechanical Analysis of a Soil — Interpretation of
Results ....... 32
CHAPTER in
THE TEXTURE OF THE SOIL
Meaning of Texture and Conditions by which it is affected
— Pore Space and Density of Soils — Capacity of the Soil
for Water — Surface Tension and Capillarity — Percola-
tion and Drainage — Hygroscopic Moisture . . 60
xi
xii CONTENTS
CHAPTER IV
TILLAGE AND THE MOVEMENTS OF SOIL WATER
PAOB
Water required for the Growth of Crops — The Effect of
Drainage — Effects of Autumn and Spring Cultivation,
Hoeing and Mulching, Rolling, upon the Water Con-
tent of the Soil— The Drying Effect of Crops— Bare
Fallows — Effect of Dung on the Retention of Water by
the Soil • • . • , . 89
CHAPTER V
THE TEMPERATURE OF THE SOIL
Causes affecting the Temperature of the Soil — Variation
of Temperature with Depth, Season, etc. — Tempera-
tures required for Growth — Radiation — Effect of Colour
— Specific Heat of Soils — Heat required for Evapora-
tion — Effect of Situation and Exposure — Early and Late
Soils. ••••••• 120
CHAPTER VI
THE CHEMICAL ANALYSIS OF SOILS
Necessary Conventions as to the Material to be Analysed
— Methods Adopted — Interpretation of Results — Dis-
• tinction between Dormant and Available Plant Food
— Analysis of the Soil by the Plant — Determination
of "Available" Phosphoric Acid and Potash by the
Use of Weak Acid Solvents .... 139
CHAPTER VII
THE LIVING ORGANISMS OF THE SOIL
Decay and Humification of Organic Matter in the Soil —
Alinit — The Fixation of Free Nitrogen by Bacteria
living in Symbiosis with Leguminous Plants — Soil
Inoculation with Nodule Organisms— Fixation of Nitrogen
by Bacteria living free in the Soil — Nitrification —
Denitrification — Iron Bacteria — Fungi of Importance
in the Soil : Mycorhiza, and the Slime Fungus of
" Finger-and-Toe " . . • . .168
CONTENTS xiii
CHAPTER VIII
THE POWER OF THE SOIL TO ABSORB SALTS
PAGE
Retention of Manures by the Soil — The Absorption of
Ammonia and its Salts ; of Potash ; of Phosphoric
Acid — Chemical and Physical Agencies at Work —
The Non-Retention of Nitrates — The Composition of
Drainage Waters — Loss of Nitrates by the Land —
Time of Application of Manures . • • .211
CHAPTER IX
CAUSES OF FERTILITY AND STERILITY OF SOILS
Meaning of Fertility and Condition — Causes of Sterility:
Drought, Waterlogging, Presence of Injurious Salts —
Alkali Soils and Irrigation Water — Effect of Fertilisers
upon the Textuie of the Soil — The Amelioration of
Soils by Liming, Marling, Claying, Paring and Burn-
ing — The Reclamation of Peat Bogs . • . 233
CHAPTER X
SOIL TYPES
Classification of Soils according to their Physical or
Chemical Nature — Geological Origin the Basis of
Classification — Vegetation Characteristic of Various
Soil Types : Physical Structure, Chemical Composition,
Natural Flora and Weeds characteristic of Sands,
Loams, Calcareous Soils, Clays, Peat, Marsh, and Salt
Soils — Soil Surveys, their Execution and Application . 271
APPENDICES
Appendix I.— Chemical Analyses of Typical Soils . 300, 301
„ II. — Bibliography. .... 302
Index ........ 3°5
LIST OF ILLUSTRATIONS
FIGURES PAGE
Photograph showing transition from Rock into Subsoil
and Soil by Weathering (Hythe Beds, Great Chart,
Kent) • • . . . • • Frontispiece
1. Photograph of Soil-sampling Tools • • .49
2. Diagram illustrating Pore Space . • . 62
3. Diagram illustrating Capillary Rise and Depression of
Liquids ....... 72
4. Photograph illustrating Liquid Film round Soil Particles 73
5. Diagram illustrating Liquid Film round Soil Particles . 75
6. Water Content of Columns of wetted but thoroughly
drained Sand and Soil ..... 76
7. Diagram showing Rainfall and Percolation at Rotham-
sted, 1870-1905 ...... 78
8. Soil Temperatures at 9 a.m., Monthly Means • . 122
9. Soil Temperatures at 9 A.M., Daily Readings . . 123
10. Effect of Nature of Surface upon Soil Temperatures . 128
11. Temperatures of Drained and Undrained Bog . . 132
12. Distribution of the Sun's Rays on Southerly and
Northerly Slopes . . . . 133
13. Temperatures (Max. and Min.) at Various Altitudes . 135
14. Nitrates in Cultivated and Uncultivated Soil . . 195
15. Losses of Nitrogen in Drainage fiom Rothamsted
Wheat Plots ...... 226
16. Nature and Distribution of Alkali Salts . • • 246
XT
> > >
THE SOIL
INTRODUCTORY
In the Scientific Study of Soils, Chemical, Physical, and
Biological Considerations are involved.
The whole business of agriculture is founded upon the
soil; for the soil the farmer pays rent, and upon his
skill in making use of its inherent capacities depends
the return he gets for his crops. Taking rent as a
rough measure of the productive value of land, it is
clear that enormous differences must exist in the nature
of the soil, for in the same district some land may be
rented at £2, and other land at as little as 5s. per acre.
Of course rent is not wholly determined by the nature
of the soil, but depends also on the proximity of a
market, and the adaptability of the land to special
purposes ; a light sandy or gravelly soil, almost worth-
less for general agricultural purposes, may be valuable
in the neighbourhood of a large town, because its earli-
ness and responsiveness to manure make it specially
suitable for market gardening.
In some cases the difference between soils is seen
in the quality of the crop produced rather than in the
productiveness ; for example, the " red lands "of Dunbar
are famous for the high quality of the potatoes
grown upon them : such potatoes will sell at 80s. to 90s.
1 A
t- n i
2 INTRODUCTORY
per ton, when potatoes grown upon the Lincoln warp
soils are at 60s., and those from the black soils of
the fen country are only fetching 45s. to 50s. This
extra price for the red land potatoes is due to
the fact that they can be cooked a second time,
after cooling, without changing colour, whereas the
ordinary potato is apt to blacken a little when once
cooked and allowed to grow cold.
The scientific study of soils is concerned with the
differences indicated above ; its endeavour is to obtain
such a knowledge of the constitution of the soil and the
part it plays in the nutrition of the plant, as will make
clear the cause of the inferiority of any given piece of
land, and ultimately enable the farmer to correct it.
The problems involved are far more complex than
they appear; at first sight nothing would seem easier
than to make a chemical analysis of the soil and find
out in what respects it differs from another • soil of
known value ; then the deficiencies or the excesses, as
compared with the good soil, could be corrected by suit-
able manuring. The matter is not, however, quite so
simple, for if on the one hand the soil can be considered
as a great reservoir of plant food which can be recovered
in crops, on the other hand it is equally correct to
regard the soil as a manufactory, a medium for trans-
forming raw material in the shape of manure into
the finished article — the crop. In new countries where
virgin soil is being exploited, and in districts where the
systems of agriculture are primitive, the former point of
view is the correct one ; nothing is given to the soil
beyond that amount of labour which will enable some
of its inherent value to be realised in a crop. Little by
little the capital, which may be practically boundless,
as in the great wheat lands of Manitoba, or initially
little enough, as on a Connemara heath, is being drawn
INTRODUCTORY 3
upon and not replaced. But in a Kentish hop-garden
or other land where an intensive system of cultivation
is practised, the crop does not remove as much as it
receives ; often the land is intrinsically poor, and owes
its value to the manner in which it will elaborate
the raw material supplied as manure. And not only
are these very special soils gaining, rather than losing
fertility with each crop, but, from a general point
of view, all countries that are being highly farmed,
like parts of Great Britain, are steadily increasing in
fertility at the expense of other countries which are
growing crops on virgin soil ; in the linseed, the maize,
the cotton seed, that are fed to our stock, there travels
to our soil some of the wealth of the lands upon which
these crops were grown. Hence the study of the
inherent resources of the soil is perhaps less important
than an examination of the manner in which the soil
deals with such materials supplied under cultivation.
The complete knowledge of the soil and the part it
plays in the nutrition of the plant requires investigation
along three lines, which may be roughly classed as
— chemical, physical or mechanical, and biological ;
naturally these points of view are not independent of
one another, but are only so separated for convenience
of study.
In the first place, we know that the plant derives
certain substances necessary to its development from
the soil: nitrogen and all the ash constituents reach
the plant in this manner. We have, therefore, to
investigate the proportions in which these constituents
are present in the soil, the state of combination in
which they may respectively exist, and the variations
in these factors normally exhibited by typical soils,
all of which questions may be described under the
head of chemical analysis. Further investigations of a
4 INTRODUCTORY
chemical nature deal with the power of various soils to
retain manure, the causes of sterility or fertility, and
the measures that can be adopted for the amelioration
of soils.
The soil is, however, not merely a storehouse of food
for the plant, since water is equally indispensable to
its existence, and is immediately derived from the soil ;
hence it is of prime importance to study the causes
which underlie the movement of water in the land,
and its supply to the growing crop. In the relation
between soil and water the cultivation to which the
land is subjected plays a prime part, hence it will be
necessary to trace the effect of each of the main opera-
tions of tillage upon the structure of the soil. Again,
the texture of the soil and the proportions of water
and air it retains, affect its temperature and that
responsiveness to change of season which we roughly
indicate by the terms "early" and "late" soils. The
general consideration of these questions may be termed
soil physics.
Finally, the soil is not a dead mass, receiving on
the one hand manure, which it yields again to the
crop by purely mechanical or chemical processes; it
is rather a busy and complex laboratory where a
multitude of minute organisms are always at work.
By the action of some of these organisms, vegetable
residues and manures are reduced, we might almost say
digested, to a condition in which they will serve as food
for plants ; others are capable of bringing into combina-
tion, or " fixing," the free nitrogen gas of the atmosphere,
and therefore add directly to the capital of the soil;
others again are noxious or destructive to the food
stores in the soil.
The work of these organisms is much affected by
cultivation ; in fact, it would not be too much to say
INTRODUCTORY 5
that most of the operations upon the farm have received
a new light from the knowledge that has been acquired
in the last few years of the living processes taking place
in the soil. In this direction also new developments
of agriculture seem to be possible, and though the
progress is only small as yet, we see indications that
the productive capacity of the land may be per-
manently increased by the introduction of certain
organisms capable of assisting the work of the higher
plants.
On the biological side we have also to study the
association of certain plants with particular soils ; an
examination of the natural flora of any district will
show that some species are almost confined to sandy
soils, others to soils containing chalk, rarely wandering
on to different types of soil ; again, particular weeds
are characteristic of clay land, others of sand ; and
some even of our cultivated crops show a marked
intolerance for particular soils.
The full story of the soil cannot yet be told ; small
wonder that in the course of the many centuries man
has been cultivating the face of the earth, he has found
out much which science can barely explain, still less
improve upon. Nor are the problems simple — the
food, the water, the temperature, the living organisms
in the soil are all variables, affected by cultivation and
climate, themselves also variable ; they all act and
react upon one another and upon the crops ; hence we
can easily understand that the smallest farm may
present problems beyond the furthest stretch of our
knowledge.
CHAPTER I
THE ORIGIN OF SOILS
Sedentary Soils, and Soils of Transport — Weathering — The Com-
position of Rock-forming Minerals and their Weathered
Products — Distinction between Soil and Subsoil — General
Classification of Soils.
The study of soils must begin with some knowledge
of their origin and their relationship to the rocks that
underlie them, out of which, in most cases, they have
been formed.
Perhaps the best way of arriving at an idea of the
natural processes which result in soil, is to visit a
river valley and examine, first a quarry on the flanks
of the hills, and then one of the cuttings for gravel or
brick earth, which often lie a little above the river level.
The face of the quarry shows at a depth of 10 feet
or so from the surface the massive rock, unaltered as
yet by any action of the weather. Closer examination,
however, shows that even at this depth the rock is
not quite solid ; if it be a stratified rock the planes of
bedding are apparent, along which the rock can be
split Joints again traverse the rock at right angles
to the bedding planes, and along both joints and bed-
ding planes it is evident that water makes its way, for
the edges of the cracks are slightly altered and dis-
coloured. Nearer the surface, the cracks and lines of
chap, i.] SEDENTAR Y SOILS 7
weakness in the rock become more palpable ; in some
cases the joints have been forced open by the intrusion
of the roots of trees; minor cracks have started from
the main ones, and the disintegration of the rock at
the edges of the cracks has proceeded further, till at a
distance of 3 or 4 feet from the surface the whole
material is loose and shattery, though still preserving
the appearance of solid rock. Still nearer the surface,
the rock structure seems to have disappeared ; rock
may be there in lumps and fragments, but it is em-
bedded in small material that may fairly be termed soil
or earth. Still nearer the surface the rock fragments
become smaller, and the proportion of fine earth larger,
till in the top 9 inches or so a new change begins.
Here the stones are generally small, and the material
is dark from the admixture of decaying vegetable
matter, residues of the crops that have covered the
surface for long ages. This is the soil proper, generally
shading gradually into the subsoil below, which in its
turn passes insensibly into the underlying rock. It is
obvious that a soil such as we have been describing has
been directly formed from the rock — it is, in fact, the
rock disintegrated and reduced by frost and snow, air
and rain; all those agencies we group together under
the name of "weathering." We are dealing with a
soil formed in situ, or, as it is sometimes termed, a
sedentary soil.
The frontispiece shows a photograph of such a case
of weathering of rock into subsoil and soil, as seen in a
section of the Hythe Beds, near Great Chart, Kent.
But when we examine the section of the gravel pit
or the brick earth workings lower down in the valley,
the sequence is not the same ; we still have the soil
proper passing into the subsoil, but this is fairly uniform
throughout instead of showing a progressive change
8 THE ORIGIN OF SOILS [chap.
as we descend ; if it be gravel, the stones continue of
the same size ; if brick earth, neither stones nor hard
stratified clay make their appearance. Should the
exposed section be deep enough, we find at last the
subsoil suddenly giving place to entirely different
material — solid chalk, or massive clay, or sandstone, as
the case may be — perhaps incapable, when disintegrated,
of furnishing the stuff of which the upper stratum of
gravel or brick earth is composed. In this upper
stratum we see the clearest evidence of the action of
water ; the brick earth is free from stones and is of even
texture, the gravel contains hardly any fine material,
and its constituent stones are worn and partly rounded ;
only running water can thus sift the heterogeneous
results of the weathering of rocks, and grade them into
different deposits. From what can be seen of the
present work of the river, it is clear that the brick earth
was deposited where the water was moving very slowly,
in quiet bays and in cut-offs, which only from time to
time get filled up with muddy flood water ; the gravel
must have been laid down in the strongest wash of the
currents.
Soils and subsoils of this type, which bear no
particular relation to the underlying rocks, but have
travelled from a distance by means of running water
or some kindred agency, are known as soils of transport,
or, to use the terminology of the Geological Survey,
as drift soils.
Weathering.
The study of geology teaches us that nearly all the
rocks termed stratified or sedimentary, which cover
the greater part of the surface of the British Islands,
have been formed from the waste of previous rocks by
weathering, and by the subsequent redeposit and con-
I.] WEATHERING 9
solidation of the weathered material. A grain of sand,
for example, is practically indestructible ; it may have
become cemented to the other grains on the sea beach
where it was lying, and give rise to the rock we term
sandstone ; the rock thus formed may have been elevated
into dry land, broken up into loose grains, and washed
down to the sea to form a new beach, over and over again
in the world's history; so long a time has elapsed since
water first began to work on the earliest rocks. For this
reason, if we want to trace out the origin of a soil in
detail, we must in most cases go beyond the sedimen-
tary rock from which it immediately derives, back to the
so-called primitive or crystalline rocks, which represent
in a sense the original materials of the earth's crust
Here we shall find certain fundamental minerals,
which in a weathered state, altered both mechanically
and chemically, go to form both the sedimentary rocks
and the soil which is our immediate study. Though
the number of distinct minerals is immense, practically
the mass of the earth's crust is made up of a few only ;
silica, various complex silicates of alumina, iron, lime,
magnesia, potash, and soda, together with carbonate of
lime, which is generally of organic origin, are all that
need be considered in relation to soils.
The various agencies which reduce rocks to soil,
grouped under the general term of weathering, may be
distinguished as mechanical — including the work of alter-
nations of temperature, frost, wind, rain, and glacial ice —
and chemical, the complex effects of solution and oxida-
tion that are brought about by water, especially when
charged with carbonic acid.
In dry climates the alternations of temperature
between day and night set up sufficient strain to fracture
even large rocks, and eventually reduce them to dust
The dust and sand of the deserts of Central Asia, the
ro THE ORIGIN OF SOILS [chap,
barren lands of the United States, and many parts of
both North and South Africa, are formed in this way ;
because of the dryness of the atmosphere, radiation is
extreme, and the temperature of the rock surface will
rise to 6o° C. in the day and fall below zero at night.
Crystalline rocks soon disintegrate under such alterna-
tions of temperature, and the fine angular dust thus
formed is transported by wind into the plains and valleys,
giving rise to soils largely wind-borne. Richthoven has
supposed that the immense loess deposits of China are in
the main dust that has been blown from the Central
Asian deserts. Even in a humid country like our own the
wind plays a considerable part in forming soil, material
being constantly removed from any bare surface and
deposited elsewhere as dust When all the country
was in its natural state and clothed with vegetation,
the amount of transport as dust must have been con-
siderably smaller than at present, but even then worm
casts brought up in the spring would crumble in dry
weather, and be moved to lower levels by the wind.
The thickness of the dust deposit may be gauged
by the rapidity with which shingle beds newly won
from the sea become covered with vegetation ; in
the neighbourhood of Dungeness shingle beds known
to be less than fifty years old are already clothed
with a scanty flora. On scraping away a few inches
of the shingle the interstices between the stones are
found to be filled with a fine black sand, which
can only have been wind-borne ; this rapidly increases
as the first vegetation checks the velocity of the
wind above the stones and arrests the dust, till at last
it reaches the surface and the grass begins to spread
over the stones. Exact dates are difficult to obtain,
but probably considerably less than a century is suffi-
cient to form a thin turf over a bare shingle bed.
I.] WEATHERING II
But the great weathering agency in temperate climates
is undoubtedly frost acting upon water contained within
the rocks and stones ; the water expands as it changes
into ice, and exerts an enormous pressure — indeed about
ioo atmospheres would be required to keep water in a
liquid condition at — i° C. All rocks when freshly exposed,
hold, by capillary attraction, a certain amount of water
known as the "quarry water," which amounts in the
white chalk to as much as 19 per cent A piece of such
chalk will be shattered into fragments by a single
night's frost Even after the quarry water has been
dried out the most close-grained rocks will absorb a
small quantity of water. The face of polished granite
rapidly deteriorates in severe climates, owing to the
freezing of the water that finds its way into the minute
divisions between the crystals : Cleopatra's Needle,
which had retained its smooth face for centuries in
Egypt, soon became affected after its removal to
London, and has to be protected by a waterproof
varnish, as have all the granite monuments in Canada.
In nature also, all rocks are traversed by joints and
bedding planes ; these cracks are filled with water and
opened and extended by its conversion into ice in the
winter, till finally a block is wedged off and a fresh
surface exposed to the action. Where flagstones are
quarried, the workmen are in the habit of saturating the
surface of the rock with water before the winter sets in :
thus the rock is split along its bedding planes more
effectively than by any artificial means. The fragments
that have been broken off the main rock will be con-
tinually reduced in size by successive frosts, until they
reach the ultimate fragments which are no longer
penetrated by water ; even in a soil the disintegration is
still proceeding.
The weathering agencies just described would gradu-
12 THE ORIGIN OF SOILS [chap.
ally cover any exposed rock with a layer of debris,
which would protect the lower layers from further action
were it not that the rain is always washing the finer
particles into the valleys and so leaving the rock open to
fresh attack. Even on grass land the fine mould brought
to the surface by worms, moles, ants, etc., is constantly
travelling downhill by the agency of rain. On arable
land containing stones it is a common expression to say
that the stones " grow " : however thoroughly the surface
may be picked clean of stones, in a year or two they will
seem as numerous as ever; the fine soil gets washed
away to lower levels, leaving the stones standing upon
the surface. Even the stones themselves gradually creep
downhill, the rain undermines them till they fall over,
they must fall a little lower down the slope, until they
eventually reach the valley and are subject to further
transport by running water. At the bottom of many of
the smaller dry valleys on the chalk rests an enormous
accumulation of flints of all sizes ; in one case in a small
upland valley the deposit was 6 or 7 feet thick, and the
unworn flints were so close as to be practically in
contact, only the interstices being occupied by soil ; yet
the surface carried good crops.
The material which thus creeps down the sides of
the valleys is further sorted out by the streams and
rivers and deposited as beds of gravel, sand, or clay,
the " alluvium " which underlies the level river meadows.
The coarser the material the more readily will) it
settle, the finer particles are only deposited when the
velocity of the stream has been almost entirely checked.
The gravel and sand are deposited in and about the
stream course itself, the finer material falls on the meadows
in flood time, so that their level is gradually raised from
year to year. Wherever the meadows get water-logged
the surface vegetation will begin to accumulate as
I.] WEATHERING 13
peat; the 'stream also wanders about from side to
side of the valley, hence borings through any exten-
sive deposit of alluvium will disclose alternating beds
of gravel, sand, brick earth, and peat, of variable
extent and thickness. The great alluvial flats or
marshes at the mouths of many of our rivers are
formed in this manner ; the deposit takes place in the
sea or in the estuary, until the tides and currents work
the material up to high-water mark, after which only
fresh-water beds are laid down.
Although most of the materials of which rocks are
composed are in the ordinary sense insoluble in water,
few of them, except the pure sand grains, can resist the
attack of water charged with carbonic acid. The rain
water when it reaches the ground has little carbonic acid
in solution, but the gases in the soil contain a consider-
able quantity derived from the decay of vegetable matter
in the surface layer, and the water in contact with these
gases will dissolve a proportionate amount. The pro-
portion of carbonic acid in the soil gases varies very
much both with the permeability of the soil and the
proportion of humus, but at a depth of 1-5 metres
Wollny found it vary from 3-84 per cent to 14-6 per
cent at various periods of the year. At greater depths
the amount is still higher, so that the percolating water
becomes a weak solution of carbonic acid, and attains a
considerable solvent power. Not only are the alkaline
silicates attacked by the weak acid thus formed, but as
lime, magnesia, and iron protoxide also form soluble
bicarbonates, all minerals containing these bases are
liable to attack. Probably some of the organic acids
produced by the decay of vegetable matter in the sur-
face soil aid in the solvent power of soil water; yet,
undoubtedly, water containing carbonic acid is the great
natural solvent, and spme of the more striking cases of
i 4 THE ORIGIN OF SOILS [chap.
its action in breaking down rocks will be discussed later
under the heads of felspar, augite, and calcium carbonate.
The attack of frost and water upon rocks is much
assisted by the roots of plants and trees ; if we examine
a fresh section of the soil over a quarry or brick pit, the
roots of ordinary field plants can be traced downwards
for 4 feet or more, while the roots of a tree may be seen
working far into tiny fissures of the almost unaltered
rock. The roots follow the water in the fissures : at first
they can enter very minute cracks ; as they grow, the
pressure they exert widens the cracks ; finally, the roots
decay and leave a channel down which water can per-
colate freely. The fine roots themselves have a certain
solvent action; after plants had been grown in a pot
filled with powdered granite rock, which had been freed
from all fine particles by washing, an appreciable quantity
of mud and clay was found to have been formed.
The opening up of the subsoil to weathering by the
action of roots is also carried out by worms, which have
been observed making their burrows to the depth of 5
feet, thus introducing both air and water into the lower
strata. But the great work of worms in regard to soil
lies rather in the production of the fine surface layer of
mould rich in vegetable matter : Darwin calculated that
on an ordinary chalky pasture the whole of the fine
surface soil to a depth of 10 inches was passed through
worms and cast up on the surface in the course of fifty
years. During their passage through the gizzard of the
worms the stony particles will receive a certain amount
of rubbing and be reduced in size, so that some of the
finer particles in the soil owe their origin to worms.
The deposit of the fine soil on the surface in the shape
of worm casts, which are afterwards spread by the action
of rain and wind, explains why chalk, ashes, or even
stones placed on pasture land gradually sink below the
l] transport of WEATHERED MATERIAL 15
surface. Darwin found in one case that a layer of burnt
marl spread on the surface had sunk 3 inches in fifteen
years, in another case a layer of chalk was buried 7
inches after an interval of twenty-nine years ; in neither
case, however, can we estimate the part played by the
accretion of dust in forming this deposit. When we
consider for how long a period worms must have been
working in our cultivated soils, it is clear that the whole
must have been through them over and over again, and
that much of the fineness of the surface soil must be due to
their action, both in actually grinding the fragments and
in constantly bringing the finest portions back to the top.
In addition to the alluvial deposits proper, which are
still in process of formation, beds of gravel, sand, and
brick earth occur in many river valleys, as terraces on
the flanks of the hills, often much cut and denuded by
the modern river. These high level formations prob-
ably represent alluvial deposits of a former epoch
where the general slope of the land was greater and the
rivers, fed by a higher rainfall in the hills, ran in greater
volume. That the material of which these deposits con-
sist has been sorted by running water is evident from
the uniformity of size it possesses in each bed : while
the coarseness of the gravel, and the fact that in some
cases the stones are not made from the immediately
underlying rock, all point to a great lapse of time and a
river of higher transporting power than the present one.
The wide deposits of brick earth in the neighbourhood
of London and in East Kent were probably laid down
either by floods on the river meadows or in quiet bays
and lagoons of an estuary.
Over a great part of Britain north of the Thames,
especially in the midlands and the eastern counties,
the surface of the land is covered with beds of clay
and sand which owe their origin to glacial ice. In
i6
THE ORIGIN OF SOILS
[chap.
Scotland, the north of England, and Wales, these beds
are full of ice-scratched stones, and clearly represent
material that has been ground down by a moving
glacier : but the origin of the glacial drift of the eastern
counties is more obscure, for water seems to have played
some part in its formation. The beds are mostly stiff
and clayey in character, and by their included fragments
show from what formation, as a rule not very remote,
they have been derived.
Rock-forming Minerals.
In the solid crust of the earth D'Orbigny has
estimated that the chief minerals are present in the
following proportions — felspars, 48 per cent. ; quartz,
35 per cent. ; micas, 8 per cent. ; talc, 5 per cent. ;
carbonates of lime and magnesia, 1 per cent. ;
hornblende, augite, etc., 1 per cent; other minerals
and weathered products, 2 per cent.
The following table shows the composition of these
chief minerals, with a few others that play some part in
the formation of soil : —
.5
e«
ta
eS
a
ES
m
1
-♦-a
O
P4
e8
"55
8
a
fcO
9
4)
a
3
9
1
3
Ferrou
Oxide.
Ferric
Oxide.
fa
s
Quartz . . .
100
• ••
fOrthoclase
64-2
17
...
...
i's-4
...
• • •
Felspar-! Albite
68-6
iz*S
19-6
• ••
• ••
^Anorthite .
43-1
...
...
• • •
20
36-9
• ••
• ••
f
45
6
26
1
Mica . . \
to
to
to
• * •
. • .
to
• ••
t ••
to
[
*
5o
10
i-5
36
47
/Hornblende .
\ Augite. . j
39
10
10
3
3
to
• •«
...
to
to
to
to
...
...
. v.
49
27
15
15
20
Olivine . .
4i
• • •
...
49.2
9.8
...
Talc
63-5
• • t
...
31-7
...
...
...
4 '.8
I.] ROCK-FORMING MINERALS 17
~ Quartz, the crystalline form of silica, is found
massive and in veins in the primitive rocks, and in
fragments of all sizes in the granites, gneisses, and
similar rocks. From the waste of these crystalline
rocks are derived the sandstones of all geological ages
and directly or indirectly the sands now existing.
In a sandstone rock the grains of quartz are bound
together by a cement, which may be oxide or car-
bonate of iron, as in the Lower Greensand of Surrey
and Beds, and in some of the Wealden sandstones, or
carbonate of lime, as in the Kentish Rag, or even silica
itself, as in the hard blocks of tertiary sandstone, which
are left as " grey wethers " on the surface of the chalk.
In some of the older sandstones the rock is practically
homogeneous ; heat, pressure, and solution having
thoroughly felted the grains together. Many sand-
stones weather rapidly, through the solution of the
cement binding the grains together ; the resulting sand
has the same texture as it possessed before it was
cemented into a rock.
The grains of sand that are first weathered from a
crystalline rock possess an angular shape, but are
soon rubbed down in running water into rounded
grains with a surface like fine ground glass. Hence
the degree of angularity which the sand grains show gives
some indication of the amount of wear and tear they
have suffered since their origin as sand. Below a certain
size, however, quartz grains seem no longer capable of
rubbing against one another, but remain angular even
after long travel in running water. Daubree has shown
that angular fragments of sand of less than 01 mm.
in diameter will travel in water without becoming
rounded, hence any rounding 01 smaller grains of sand
must have been due to solution.
Silica in the crystalline state is very slightly soluble
B
18 THE ORIGIN OF SOILS [chap.
in water, a certain amount of solution taking place
even at ordinary temperatures : most natural waters
show a little silica in solution, though this more
probably arises from the decomposition of natural
silicates by water containing carbonic acid, rather than
from the direct solution of quartz.
Amorphous silica in the form of "flint" plays a
conspicuous part in the constitution of many soils in
the south and east of England ; owing to their
durability and the former greater extension of the
chalk, they are found in many districts remote from
the chalk, even in the drift beds of the Channel Islands.
When first won from the chalk, flints possess a clear
black translucent structure, and are easily fractured and
crushed ; when weathered, either in flint gravels or
on the surface of the soil, they become yellow or brown
in colour, more opaque, and much harder, so that
weathered flints are always preferred for road-making.
The surface also becomes covered with a white incrusta-
tion, extending to a depth of ^ of an inch or more ;
this is, however, only incipient weathering, probably
due to the freezing of the small amount of water that
soaks in at the surface.
The Felspars constitute the most important group of
minerals found in the crystalline rocks : they are
double silicates of alumina and some other base, potash,
soda, or lime, of the general formula R 2 0, A1 2 3 , 6Si0 2 ,
where R 2 may be either K 2 0, Na 2 0, or CaO. In
granites and gneisses the common felspar is orthoclase
or potash felspar; in the volcanic rocks plagioclase
felspars predominate, in which the base is lime, gener-
ally with some admixture of soda and potash.
The felspars are all distinguished by the ease with
which they are attacked by water containing carbonic
acid, those containing lime more readily so than the
I.] ROCK-FORMING MINERALS 19
potash felspar. The lime or the alkali is removed in
solution, some of the silica is also removed ; the alumina
remains as a hydrated silicate, A1 2 3 , 2Si0 2 , 2H 2 0,
called kaolinite. Owing to this disintegration of
felspar, the crystalline rocks in which felspar is present
weather rapidly, the other materials, quartz, mica,
hornblende, become loosened from the matrix, and
the whole rock becomes rotten. The granite of Corn-
wall and Devon is generally covered to a considerable
depth, as much as 100 feet in some cases, with a
layer of kaolinite, in which the unchanged quartz and
mica are embedded; the kaolinite, freed by washing
from the quartz, mica, etc., forms the " china clay " or
kaolin of commerce. In the same way the basalts and
other kindred rocks give rise to a red clay, consisting
of kaolinite and the red iron oxides resulting from
the oxidation of the magnetite and the hornblende,
augite, etc., which contain ferrous silicates. From the
decomposition of the felspars, augite, hornblende, etc.,
all our clays arise; as these minerals also generally
contain potash, they are the source of the potash
required by crops, which is always more abundant as
clay predominates in the soil.
Daubree caused 3 kilos of fragments of felspar
to revolve in an iron cylinder with 3 litres of water,
so that they practically performed a journey of 460
kilometres, with the result that 2-72 kilos of mud were
formed, of which 36 grams were clay, and in the
water there were 126 grams of potash in solution as
silicate.
Senft examined the action of water charged with
carbonic acid upon two granites, one (A) composed of
orthoclase, quartz, and potash mica, the other of (B)
plagioclase, quartz, and magnesia f mica, and obtained
in solution —
10
THE ORIGIN OF SOILS
[chap.
A.
B.
Potash as Bicarbonate
Soda as Bicarbonate .
Lime as Bicarbonate .
Magnesia as Bicarbonate
Silica
Iron as Bicarbonate .
15 to 25 per cent.
2 „ 6 „
1 », 2
a trace
a little
a trace
5 to 8 per cent.
8 „ 10 „
4 » 5 u
10 „ 15 H
a little
a trace
The undissolved residue of A was a white, of B a
yellow, clay containing fragments ot quartz and flakes
of mica.
The following analyses show the change that takes
place in passing from orthoclase felspar to kaolin; in
the third column the analysis of kaolin is recalculated
to show what arises from 100 parts of felspar, on the
assumption that none of the alumina is removed by
solution : —
Orthoclase
Felspar.
Kaolin.
Kaolin
from 100 Felspar.
Silica • •
Alumina • •
Potash • •
Water • •
64.2
1S.4
17
46.8
37-3
2-5
13
23»i
18.4
I.I
6.4
99.6
99.6
49
% Mica is essentially a double silicate of alumina and
potash, with some oxide ol iron : the potash being
replaced by magnesia in black mica or biotite. Mica
splits up into minute flakes as the rock weathers, but
these flakes are fairly resistent to chemical change, and
may be detected in most sands and sandstones. Ulti-
mately, however, they pass into hydrated silicates of
I]
ROCK-FORMING MINERALS
21
alumina, and are rarely to be detected in the soils
resting upon sedimentary rocks.
N Hornblende and Augite, though differing in crystalline
shape, are chemically identical, and consist of silicates of
varying proportions of lime, magnesia, alumina, ferrous
and ferric oxides ; manganese and the alkali metals are
generally also present They constitute, with plagioclase
felspar and magnetic oxide of iron, the chief part of the
rocks that are sometimes roughly termed " greenstone " —
basalts, diorites, etc., of both volcanic and plutonic
origin. They decompose under the action of carbonic
acid charged water, especially those containing much
lime, while those with much magnesia are the most
resistent; the products of the action are kaolinite,
oxides of iron, and carbonates of lime and magnesia.
The following analysis (Ebelmar) show the chemical
change in the weathered layers of a basalt from
Bohemia and a greenstone or dolerite from Cornwall : —
Basalt.
Greenstone.
Unaltered.
Weathered.
Unaltered.
Weathered.
Silica • • •
Potash • • •
Soda • • •
Lime • • .
Magnesia • •
Alumina . . •
Iron Protoxide •
Iron Peroxide .
Titanium Oxide
Water .
44.4
4.8
2.7
H-3
9.1
I2»2
12. 1
3-5
trace
4.4
42.5
} - (
2.5
3-3
17.9
n-5
1.2
20.4
51.4
1.6
3-9
5-7
2-8
15-8
12-9
3-o
0.7
i-7
44-5
I«2
i-7
1.4
2.7
22-1
• •«
17.6
8-6
The loss amounts to about 44 per cent in the
case of the basalt, and 34 per cent in that of the
greenstone.
Another example may be given of the analysis
22
THE ORIGIN OF SOILS
[chap.
(Hanamann) of a basalt from Bohemia, with that of
the weathered crust and of the resulting soil : —
Bock.
Weathered Crust.
Soil.
Silica « • •
41-84
39*7
39-17
Potash • • •
0-82
0-83
0.94
Soda • • •
3-45
2.51
1-03
Lime . . • •
11.16
8- 02
4.72
Magnesia . •
3-63
3.20
2-92
Alumina . . .
17-51
16.94
16.58
Iron Protoxide •
3-71
...
Iron Peroxide . •
12.77
15-05
14*22
Phosphoric Acid
o-5
0.48
0.48
Carbonic Acid . •
o-88
2.67
o-6i
Water . • •
3-56
10.5
19.28
Olivine is essentially a silicate of magnesia and
protoxide of iron, not uncommon in some basalts,
which easily weathers and becomes a soft hydrated
silicate, called serpentine, to which talc is very similar
in composition. These magnesian silicates are not
of great importance in the British Islands ; only in the
Lizard district of Cornwall are they extensively
developed and give rise to poor, barren soils.
Calcium Carbonate ', though present in many of the
older rocks in its crystalline form of Calcite or Iceland
Spar, is there to be regarded rather as a secondary product
brought by infiltering water than an original mineral.
It is soluble in water charged with carbonic acid ;
hence when the complex silicates containing lime
are weathered, the lime is removed in this form. The
calcium carbonate is redeposited when the water loses
the carbonic acid either by evaporation or by diffusion
on contact with air. In a massive form calcium carbon-
ate forms many of the sedimentary formations — the
older ones hardened to limestones, and the more recent
ones soft like the chalk ; in these cases it has been secreted
I.] CARBONATE OF LIME 23
from natural waters by living organisms, foraminifera
corals, etc., and only gets a crystalline structure by
later change. Calcium carbonate from organic sources
is present to some extent in nearly all sedimentary
rocks ; the vast majority of the fossils there found are
constituted of calcite.
In the limestone and chalk rocks the calcium car-
bonate is never quite pure; in the white chalk, which
is the purest, the proportion of calcium carbonate, after
excluding the flints, is only about 98 per cent. ; in
others the proportion of clay and mud which were
simultaneously deposited gradually increases, so that
we can find rocks of every gradation between chalk and
clay or sandstone.
Owing to its solubility, the weathering of limestone
takes the form of the removal of calcium carbonate
more or less completely, leaving a fine-grained residue
of the insoluble clay or sand. In the case of chalk
and of the purer limestones, the insoluble residue con-
sists mainly of a fine red or yellow clay; the chalk
downs, when not obscured by drift formations, are
covered with a sticky, reddish soil, only as a rule a
few inches in thickness, and though the actual chalk
is so close, in many cases this soil is almost deprived
of all its calcium carbonate. Almost exactly similar
material may be obtained in the laboratory by dis-
solving a few pounds of chalk or limestone in dilute
hydrochloric acid. Whenever a section is exposed in
chalk or limestone rocks, it will be noticed that the
dividing line between soil and rock is very irregular;
thin as the soil may be as a whole, in places it descends
into cavities and " pipes " in the rock, sometimes 20 or
30 feet deep. In these depressions the soil is the same
reddish clay as occurs on the surface, mixed with flints
in the case of the upper chalk ; they are essentially the
24 THE ORIGIN OF SOILS [chap.
results of solution, and represent the lines along which
the drainage of the rain water has been more active,
owing to a joint or fissure in the rock below.
Other minerals which do not constitute any large
proportion of the earth's crust, but still play some
part in the soil, are apatite, glauconite, selenite, limon-
ite, and iron pyrites.
Apatite, or crystallised phosphate of lime, —
Ca 5 (P0 4 ) 3 F, — is present in small quantities in many
of the fundamental rocks, and is probably the ultimate
source of the phosphoric acid of soils. Apatite also
occurs massive in some of the older strata, and has been
worked as a raw material, for the manufacture of phos-
phatic manures, in Norway and Canada.
Selenite, hydrated sulphate of lime, CaS0 4 , 2H 2 0,
termed gypsum when massive, is not a fundamental
mineral, but occurs in most clay rocks in well de-
veloped crystals. Diffused through the soil and dis-
solved in soil water, selenite doubtless provides most of
the sulphur required by plants.
Limonite ) hydrated oxide of iron, occurs in lumps
and bands in many of the sedimentary rocks; in a
diffused state it is the main colouring matter of soils ;
in heavy, undrained soils it often forms a layer or " pan "
some inches below the surface. It is deposited from
water containing bicarbonate of iron on exposure to the
air ; the rusty deposits and stains from chalybeate springs
and wells consist of limonite. The action appears to be
as follows — the hydrated peroxides of iron in the soil
when in contact with humus (decayed vegetable matter)
and water charged with carbonic acid become first
reduced to the ferrous state by the organic matter, and
then dissolved as bicarbonate. On exposure to the
air, the excess of carbonic 1 *' acid escapes by diffusion,
the ferrous carbonate, as it is precipitated, is also oxi-
I.] ROCK-FORMING MINERALS 25
•
dised by the oxygen of the air, and deposited as Hmonite.
It will be noticed that stones taken from peaty land
are always bleached white, through the removal of iron,
and the surface sand of heathy land is always simi-
larly bleached. On examining a section of any purely
sandy formation, the surface soil will be found to be
bleached below the layer of vegetable matter to the
depth of a foot or more. Then comes a layer an inch or
two thick nearly black in colour, where the sand is more
or less cemented together by Hmonite, and below this
the normal brown or yellow sand begins. The black
band is formed at the depth to which the air usually
penetrates the soil ; it consists of Hmonite deposited
at the evaporating surface of the soil water, which
contains the iron dissolved from the bleached surface
sand. In a similar manner arises the hard layer of
Hmonite, the " iron pan " or " moor-band pan," found just
below the cultivated soil on many undrained lands, and
again the deposit of " bog iron ore " which is generally
to be seen beneath the black peaty accumulation in any
swampy place. The solution of iron as bicarbonate,
and its precipitation as Hmonite, do not occur in soils
containing any calcium carbonate, being essentially a
sign of an acid condition of the soil and its need for
lime or chalk.
Glauconite is a hydrated silicate of iron, alumina,
and potash with a little lime and magnesia, which
occurs as dark green grains in many sedimentary rocks,
especially of the Cretaceous age. It is to the presence of
this material that the Greensand formations owe their
name; it is sometimes also to be seen in chalk and
in the tertiary sandstones. It readily weathers to
brown oxides of iron.
Zeolites, Akin to glauconite are certain hydrated
double silicates of aluminium and the alkalis or alkaline
26 THE ORIGIN OF SOILS [chap.
earths, called generically zeolites, which play a very
important part in the soil, though they may not be
present in large amounts. These bodies, which result
from the weathering of the felspars, contain a consider-
able proportion of water, loosely combined and readily
displaced, but their distinguishing feature is the ease
with which the secondary bases they contain, the calcium,
magnesium, sodium or potassium, are replaced by other
metals, whenever their salts are brought into contact
with the zeolites. Little is known of the actual nature of
the zeolitic bodies in the soil, but certain zeolites occur
from time to time in a pure state. The best known of
them is natrolite, which crystallises in fine needles
possessing the composition — Na 2 0, A1 2 3 , 3Si0 2 , 2H 2 0,
a little calcium being generally present also.
Iron Pyrites, FeS 2 , occurs in small brass yellow
cubic crystals in many of the older rocks, especially
those of a clay character; another form, in fibrous
masses of a lighter colour, is called marcasite, and is
common in the more modern clays, especially the
London clay, and again in round balls in the chalk.
Marcasite readily oxidises in moist air to ferrous sul-
phate and sulphuric acid : and many clay soils contain
basic sulphates, soluble in dilute acids but not in water,
that have arisen in this way. Selenite and the soluble
sulphates present in well waters, especially in clay soils,
are probably secondary products arising from the oxida-
tion of marcasite. In a finely divided condition iron
pyrites forms the colouring matter of many dark green
or olive rocks and clays.
Soil and Subsoil,
Although the transition from soil to subsoil is
gradual, the distinction between the two is, as a rule,
easy to be made; the change begins an inch or so
I.] SOIL AND SUBSOIL 27
below the usual limit of cultivation on arable soils, on
pastures at the depth to which the mass of the roots
penetrate. The most obvious difference between the
two lies in the comparative richness of the staple in
decaying vegetable matter or humus, which indeed
would be entirely confined to the surface layers were
it not for the decay of the deeper roots and the work of
worms. To the humus is also due the difference in
colour ; not only does the colour deepen towards black
as the proportion of humus increases, but by it the
sands and clay are to a greater or less extent bleached
through the removal of the iron oxides which colour
them, hence the inorganic material is lighter and duller
in colour in the soil than in the subsoil. In stiff clays
the subsoil often shows signs of imperfect oxidation at
comparatively slight depths. On an old pasture on the
Gault Clay a trench was dug, the top 3 inches were black
or nearly so and gradually changed to a stiff brown
loam which extended to a depth of 9 or 10 inches,
becoming lighter and more distinctively yellow as
the admixture of humus diminished ; below this depth
the clay became mottled, grey, and yellow mixed, till
at a depth of 4 feet practically the whole was a dark
blue unweathered clay, owing its colour to iron pyrites
and glauconite or kindred silicates of iron protoxide.
One of the greatest distinctions between soil and subsoil
lies in their respective texture ; in humid climates like
our own the soil is almost invariably composed of coarser
grains than the subsoil, though in arid climates soil
and subsoil appear to be almost uniform. This is due
to the rain constantly percolating through even the
stiffest soils and washing down the finest particles ; in
heavy rains also, water runs off the surface into the
ditches, carrying with it the finest particles of the soil
and leaving behind the coarser grains on the surlace.
28 THE ORIGIN OF SOILS [chap.
Naturally, this loss of the finer particles is greater as
the soil is more worked and made open to percolation
and washing ; to some extent it is counterbalanced by
the work of worms bringing the fine mould to the
surface from below, so that the difference is least in
an old pasture. Per contra, it is greatest in an old
garden soil, where the constant working and further
opening of the soil by the introduction of bulky manure
often results in so complete a washing down of all the
finer particles that the soil proper loses its power of
cohering, falls into dust when dry, and is popularly said
to be " worn out"
In addition to its humus the soil is nearly always
richer than the subsoil in all the essential elements of
plant food, despite the fact that crops have been raised
on it for generations ; the crops, in fact, have been the
cause of the difference, for the deeper roots draw food
from the subsoil and leave it behind on the surface as
the plants decay. Potash is perhaps an exception in
this connection; being essentially a product of the
weathering of felspar, and removable from the soil by
water containing carbonic acid, it is often more abundant
in the comparatively unweathered subsoil. The richness
of the humus, its greater warmth and the freer access
of air also cause it to be more abundantly supplied with
those organisms which play such an important part in
preparing the food of the higher plants: as will be
seen later, subsoils become almost without living
organisms at a very slight depth.
For all these reasons, — the absence of humus, and of
the organisms associated with it, the comparative poverty
in inorganic plant food, the presence sometimes of unoxi-
dised material, and on stiff soils the great change of
texture, — the subsoil is often comparatively unfertile
and may be almost barren. Desirable as it is to work
I J CLASSIFICATION OF SOILS 29
the subsoil and open it to the access of air and the free
penetration of roots, all methods of cultivation should
be avoided that would bury the surface soil and bring
the subsoil to the top. A plough which inverts the
soil should not go below the former limit of cultivation,
and if it is desired to deepen this limit, it should be
done by degrees, half an inch or so each year. Immense
damage has been done to the fertility of many of the
heavier soils by rash ploughing with steam, especially
where the old " lands " were thrown down, burying the
fertile soil in the furrows and baring the raw clay on
the tops of the ridges.
General Classification of Soils.
Although a distinction has been drawn between
sedentary soils and soils of transport, there are few
sedentary soils that do not contain material which has
been carried from some other formation at a distance ;
only on great stretches of flat country belonging to a
single geological formation may be expected a soil
purely derived from the rock below. Especially in
Britain, where the outcrops of the different formations
are generally narrow, and where the surface is always
undulating, we find that the continual creeping of soil
particles to lower levels has resulted 111 an admixture
of foreign material in most soils. " La couche tres-
mince de la terre vegetale est un monument d'une
haute antiquite" (Elie de Beaumont), so that in many
places the soil contains the debris of formations now
removed by denudation. In the south-east of England
the soils that rest on the chalk, which may be only
from a few inches to a few feet below, contain
abundance of quartz sand, even up to 75 per cent.
No such sand exists in the chalk itself, so that it has
come from the lower tertiary beds which once over-
35 THE ORIGIN OP SOILS [chaP.
spread the chalk. On the wide flats of Weald Clay
in the same district, the soil contains sand that has crept
from the central hills of the Weald or from the Lower
Greensand escarpment, often several miles away. The
following analysis of a soil resting on a brick earth
bed in the valley of the Kentish Stour, shows that the
brick earth, which itself contains little or no chalk, has
become covered with chalky rain-wash from the hills
flanking the valley : —
Depth — Inched .
o to 6
6 to 12
12 to 18
18 to 24
Calcium Carbonate %
9-20
7.16
2-6
0-96
In the main, however, the bed below gives its char-
acter, both chemical and physical, to the soil ; and the
ordinary rough classification of soils into sands, clays,
marls, and loams, follows closely the nature of the
underlying geological stratum. A coarse - grained
sandstone gives rise to a typically sandy soil, such as
the soils derived from the Bagshot beds, which form
the New Forest and the heathy land in the Aldershot
district ; on the Lower Greensand lie the sandy heaths
in west Surrey, Hampshire, and in Beds ; again, on
the Bunter beds of the New Red Sandstone lie many
of the uncultivated commons and parks of the Midlands,
such as Sutton Park, Cannock Chase, and Delamere
Forest. These coarse sandy soils, which have so often
remained unenclosed as forests and commons, are gener-
ally deficient in chalk, and accumulate peat wherever a
parting of clay gives rise to stagnant water.
Clay soils are common in nearly every part of
Britain; they arise from the great clay strata of all
ages, like the London Clay, the Weald Clay, and the
Oxford Clay, or from metamorphic rocks like slate, or
from the crystalline rocks like granite and basalt, or
even from the limestones by solution.
I.] CLASSIFICATION OF SOILS %\
Between the sands and the clays come mixtures of
all grades, better working than the clays and more
fertile than the pure sands ; sometimes the clay forma-
tion itself contains sand, as in the upper beds of the
London Clay, or we may have a fine-grained sandstone
mixed with clay, as in some of the carboniferous rocks.
In all these cases, when chalk is absent, and drainage
incomplete, there will be an accumulation of humus,
resulting in a peaty formation.
Some argillaceous limestones give rise to typical
"marls," mixtures of chalk and clay; e.g., some of
the beds of the Lias and of the Keuper.
Other limestones with a sandy basis, and fine-
grained sandstones cemented by carbonate of lime,
give rise to * loams," which are free-working soils, mainly
composed of fine sand with some clay and a little
calcium carbonate. The alluvial soils in the valleys
are loams, passing in places into gravels; these are
generally the richest soils ; as a rule they are mixtures
derived from many formations, and so are well supplied
with humus and the mineral elements of plant food ;
they are deep, and not over consolidated, thus admitting
of the percolation of water and the descent of roots ; yet
they are fine-grained enough to prevent them drying
out too rapidly. But though these terms, sands, clays,
marls, loams, and peaty soils, serve for rough descriptive
purposes, a more exact determination of the constituent
particles is necessary to properly characterise a soil,
and for this we must resort to what is termed the
" mechanical analysis " of a soil
CHAPTER II
THE MECHANICAL ANALYSIS OF SOILS
Nature of Soil Constituents : Sand, Clay, Chalk, and Humus—
Methods of Sampling Soils — Methods for the Mechanical
Analysis of a Soil — Interpretation of Results.
It has already been indicated that as soils are derived
from the waste of rocks, they consist of a mass of
particles of various minerals and of all sizes, together
with a certain amount of humus of vegetable origin,
and that they may be roughly classified according to
the predominance of the coarse-grained particles called
"sand" or the very fine material known as "clay"
The mechanical analysis of a soil consists in pushing
this rough " eye and hand " classification a stage further
into the region of exact measurement, and in deter-
mining the minute physical structure of the soil by
estimating the proportions in which particles of various
sizes are mixed together in the soil. Upon the physical
structure of the soil so determined, or as we should
practically term it, the texture, depend some of its
most important features, particularly its behaviour with
regard to the supply of water to crops and its amena-
bility to cultivation.
In the first place, it will be necessary to discuss a
*ittle more thoroughly the nature of the four substances
CHAP. II.]
THE NATURE OF SAND
33
to which the texture of the soil has been referred — the
sand, clay, chalk, and humus — of which the first two
are of most importance, since soils which are mainly
characterised by chalk or humus are less commonly
in cultivation.
Sand. — On the seashore, in beds of an alluvial
nature, and in formations of all geological ages, we are
familiar with sand ; in the main it consists of grains of
quartz, rounded by continual rubbing, and more or less
coloured by oxide of iron. It represents the quartz
contained in the fundamental rocks, weathered and
worn by water : in some cases of comparatively
recent origin, in others it is material that has re-
peatedly been formed into a sedimentary rock,
disintegrated afresh and sorted by the action of running
water. The coarser the grains of which a sand is made
up, the more rapid must have been the current from
which it was deposited. The following table shows
the rate of flow which is necessary to carry sand
grains of various sizes : —
Diameter of Grain3,
mm.
Velocity of Current,
mm. per sec.
o-S
64
o-3
o-i6
32
16
0*12
8
0*072
4
0.047
0.036
2
I
0.025
0-5
A closer examination of most sands will show that
they do not consist wholly of quartz grains, but also
contain rounded fragments of many of the minerals
present in the fundamental rocks which have any
resistance to weathering. Flakes of mica are common,
C
34 THE MECHANICAL ANAL YS1S OF SOILS [chap.
fragments of more or less altered felspar, of oxide of
iron, and even of tinstone, rutile, and zircon, may be
identified. In fine-grained sands the fragments of
minerals other than quartz become as a rule more
abundant, till they begin to predominate over the
quartz grains in the finest silts and muds that are
deposited from very gently moving water. Under the
microscope the quartz grains show a crystalline struc-
ture, and a surface more or less dulled and rounded
according to the travel the grains have suffered. In
mass the chief characteristic of sand is its want of
coherence when dry.
Clay. — The material we call clay is characterised by
certain properties that are shown when the clay has
been " puddled," i.e. y kneaded when in a moist condition.
The clay is plastic^ it can be moulded and worked into
various shapes, even into quite thin leaves, and it will
retain these shapes on drying. During the drying
process a shrinkage takes place: the dry material is
hard and tenacious, and can only be broken or crumbled
with difficulty. The shrinkage is considerable : a little
brick was made of good modelling clay 7 inches long,
and about 1 square inch in section; two marks were
then made on this 6 inches apart; after a fortnight's
drying in a room the marks were only 5-7 inches apart,
showing a shrinkage of 5 per cent. Clay is further
impermeable to water when in the moist puddled
condition, for which reason it is used to line the bottoms
of ponds in pervious soil, and is built up inside the
retaining dams of reservoirs ; quite a thin layer of clay
will hold water indefinitely as long as it is not allowed
to dry and crack, nor to be washed away by the action
of running water.
From a chemical point of view, all clays are found
to consist largely of kaolinite, the hydrated silicate
II.] THE NATURE OF CLAY 35
of alumina which is formed by the weathering of fel-
spar ; the other materials present consist of extremely
fine grains of quartz and other weathered minerals,
together with more or less oxide of iron. "China
clay" and the best "pipe clays" contain little or no
iron ; the deep-seated clay formations are generally
coloured dark green or blue or black by the presence '
of ferrous silicates like glauconite ; on weathering and
exposure at the surface the clays become yellow or
brown, owing to the oxidation of these ferrous to ferric
salts.
Water in which a little clay has been rubbed up
remains turbid for a very long time ; days and even
weeks elapse before the particles settle down to the
bottom — indeed, however long the liquid may be at
rest, a slight haze or cloudiness may be observed
within it Schloesing has drawn a distinction between
the part of the clay, amounting to 1 or 2 per cent
only of the whole, which persists in remaining sus-
pended and the portion which settles down ; he has
called it "colloid clay," and attributes many of the
typical clay properties to the jelly-like medium of
colloidal matter by which the other defined particles
of the clay are surrounded. Schloesing associates this
colloid clay with such typical colloids as the highly
hydrated forms of silica and organic bodies like starch
and gum which, though they appear to be truly dissolved,
yet cannot diffuse through a membrane, and form, on
drying, hard non-crystalline masses, with much shrinkage
and a characteristic fracture. But later researches on
colloids show that they are not essentially different
from suspended matter; they consist of particles too ,
fine to settle down in water, or to be arrested by a
filter even of porous porcelain, but which are still
sufficiently coarse to show their presence when a strong
36 THE MECHANICAL ANALYSIS OF SOILS [chap.
beam of light is passed through the liquid, as is not the
case with bodies truly dissolved. From this point of view
the "colloid clay" would only represent the limiting
state of fineness, differing in degree, but not in kind,
from the other clay particles.
The question still remains whether we shall give
to clay a physical or a chemical definition ; in the first
place, does the fineness of the material alone confer the
characteristic clay properties of plasticity, impermeability
to water, and shrinkage and tenacity on drying, or do
these properties depend on the chemical composition of
the substance making up the clay. It is easy to show
that fineness of division is a necessary factor in the
existence of clay, because we can obtain material
possessing the chemical composition of typical clays
which yet behave physically as if they were sand. A
sample of crude kaolinite rock as dug in Cornwall
from the surface of granite, was powdered and passed
through a sieve retaining all particles above o-2 mm.
in diameter; the remainder, which consisted mainly
of kaolinite with a little mica, was further separated
by sedimentation from water into four fractions : —
Fraction.
Approximate Size of
Particles in min.
Per cent, of
Original Material.
I
2
3
4
0-2 to 0-05
0-05 „ o-oi
o«oi „ 0-005
below 0-005
1
22
39
21
20
Of these fractions the first contained all the mica,
the others were practically pure kaolinite, yet the second
fraction showed none, and the third very little of the
characteristic properties of clay ; when dried they fell
II.]
THE NATURE OF CLA Y
37
or could easily be rubbed into a fine powder, only
the fourth and finest fraction dried into a hard co-
herent mass. Thus we can have material which con-
sists entirely of kaolinite, and yet is not clay ; such as
we see in natural deposits of fuller's earth, which
consists of kaolinite but possesses no plasticity, and
falls on drying into a fine powder.
In the same way a natural soil contains particles
of silicates of alumina of all sizes, though they only
begin to predominate in the fractions of finest grain.
On separating one of the Rothamsted soils into
fractions, according to their size by the method to be
described later, and analysing them, the following results
were obtained : —
Fraction.
Approximate Size
of
Particles in mm.
Per cent.
of
Original
Soil.
Percentages in Material.
Silica.
Ferric
Oxide.
Alumina.
I
2
3
4
5
o-2 to 0-04
0-04 „ o-oi
o-oi „ 0-004
0-004 „ O-0O2
below o-oo2
24
35
II
6
24
94.6
920
88-3
61.7
45-9
I-I
1-2
1-8
7.0
12*2
3-4
6-2
8-5
23-4
30-9
Taking the mean of several analyses, the fifth fraction,
which is to be regarded as clay proper, possessed the
following approximate composition, if all the alumina is
combined as A1 2 3 , 2Si0 2 , 2H 2 — kaolinite 72 to 75
per cent; ferric oxide, 11 to 12 per cent; quartz, 9
to 10 per cent; alkalis and alkaline earths, 4 to 6 per
cent.
From these results we must conclude that kaolinite
is not necessarily clay, but that fineness of grain is also
an essential factor, the characteristic clay properties not
being developed except in material the particles of
38 THE MECHANICAL ANAL YSIS OF SOILS [chap.
which are less than one-fivehundredth of a millimetre in
diameter.
But though fineness of grain is a factor, it is probably
not the only factor, as may be seen from a consideration
of another important property of clay — its power of
flocculating or coagulating under the action of minute
quantities of various salts. To illustrate this point, a few
grams of good clay should be rubbed up with several
litres of distilled water, and the supernatant turbid liquid
poured off into a series of tall jars each holding from
300 to 500 c.c. of the liquid. To one of these jars
nothing is added, to two others -018 and 0-009 gram of
hydrochloric acid respectively, to a fourth 0-028 gram of
calcium chloride, and to the fifth 0-58 gram of sodium
chloride. The contents of the jars are shaken up until
solution is effected, and they are then put aside to stand.
After some time the liquids to which the salts have been
added will begin to clear, and the clay particles will clot
together and fall to the bottom ; the jar containing the
larger quantity of hydrochloric acid will clear the first,
the others will clear approximately together, but the pure
clay water will remain turbid for many days. If a little
of the turbid clay water be examined by a T V-inch
oil immersion lens under the microscope, it is just
possible to see the clay particles in rapid " Brownian "
1 motion, and if a little acid or salt be then introduced
under the cover glass, they will be seen to move together
and form into little clots or aggregates as soon as they
experience the effect of the added acid or salt. By
comparative experiments it can be shown that the
flocculating power of any salt is proportional to its amount
up to a certain limit, when the material is so completely
flocculated that no further addition of salt has any
effect; conversely, the flocculating power of a given
amount of salt is inversely proportional to the quantity
ii.] FL0CCULAT10N 39
of clay suspended in the liquid. The flocculating power
of a salt also varies with both the acid and the metal;
the following table shows approximately their com'
parative effect : —
HC1 .
• 30
HNO3 . . 28
H 2 S0 4 .
• 20
LyELylQ •
. 15
Ca(N0 3 ) 2 . 10
CaS0 4 .
• >5
KC1 .
3
KN0 3 . . >2
K 2 S0 4 .
• <i
NaCl .
• >i
NaN0 3 . <i
NaaSC^
• 0.5
The alkalis and salts like phosphate of sodium, which
give rise to free alkalis on hydrolysis, instead of
flocculating have the opposite effect, and keep the
particles in their finest state of division without any
tendency to settle.
It is y furthermore, possible to show that many
substances, however finely divided, will not assume the
condition of indefinite suspension in water so as to be
flocculated by salts ; in particular, suspensions of finely
divided quartz, ferric hydrate, and hydrated alumina
flocculate spontaneously and will not remain turbid for
many minutes, though in their turn they can be
deflocculated and made to remain in suspension by
adding a trace of free alkali to the liquid.
Without going further into the details of a subject
which is still very obscure, the condition of free suspen-
sion in water and the Brownian motion of the
particles of clay seem to be associated with the presence
of the zeolitic double silicates which contain atoms of
potassium or sodium in their molecule, and which doubt-
less give rise to a little free alkali by their partial
hydrolysis when in contact with a large bulk of water.
We may thus conclude that fineness of grain is
not the only factor in the constitution of clay, but that
the characteristic clay properties which are always
associated with the power of flocculation depend also
40 THE MECHANICAL ANAL YS1S OF SOILS [chap.
upon the nature of the material ; in the soil they depend
upon the presence of the zeolitic double silicates derived
from the weathering of the felspars in the fundamental
rocks.
The power of flocculation plays a very important
part in the cultivation of clay soils. When such a soil
possesses a good texture its finest particles are in a
state of temporary aggregation or flocculation, so that
they behave as if the soil, as a whole, were built up of
much coarser particles. Just as a potter or a brick-
maker brings his material into its highest condition of
plasticity by repeatedly kneading and working it, by
which process the naturally formed aggregates are
resolved into their ultimate particles and the material is
made as fine-grained as possible, so if a clay soil be in
any way worked or disturbed when in a wet condition, it
becomes apparently more clayey than before. It remains
persistently wet and impervious to the percolation of
water, and shrinks when dry into hard tenacious clods.
But if the clay be exposed to the weather for some time,
so that it undergoes alternations of temperature, freez-
ing and thawing, wetting and drying, it will experience
a certain amount of spontaneous flocculation and behave
as though it were coarser grained, so that if caught
in the right state of partial dryness it may easily be
crumbled.
Flocculation may also be aided or otherwise by the
use of certain artificial manures, as will be explained
later ; the incorporation again of humus much improves
the texture, while the action of lime is particularly
effective and is much employed in practice to ameliorate
the working. of clay soils.
Lime itself can be shown in the laboratory to possess
little flocculating power, for though its base is calcium, a
highly effective metal, it is combined as a hydrate, which
il] CALCIUM CARBONATE 41
has a deflocculating effect. However, as soon as lime
is applied to the soil it becomes converted into carbonate,
and some of it will be always going into solution as
bicarbonate, a salt which possesses great flocculating
power.
In practice, the application of such small quantities
of lime as a ton or even half a ton to the acre have the
greatest value in ameliorating the working of clay land ;
not only does it move more readily and fall more easily
into a good tilth, but by becoming coarser grained it
allows the rain to percolate more freely and thus dries
earlier in the season, so that the limed land can often be
worked several days before the unlimed land can be
touched. Though the Rothamsted soil is by no means
of the heaviest, it is only because of the repeated
additions of carbonate of lime in former years that
it can be retained under arable cultivation ; portions of
the same land without carbonate of lime lie so wet
in the spring that they were laid down to grass in
consequence of the repeated failures to secure a good
seed bed.
Chalk, or carbonate of lime, is present in all soils,
with the exception of a few extremely open sands and
peaty soils that are practically of vegetable origin.
The proportion varies enormously, according to the
origin of the soil ; on some of the thin loams derived
directly from the great calcareous formations like the
chalk or the oolite, the calcium carbonate in the soil
may rise to as high a proportion as 60 per cent, but in
the majority of the loams under cultivation the pro-
portion is nearer 1 per cent, and it often falls much
below this in clays and sands. Chalk in the soil is
essentially a transitory substance, as it is constantly
removed by the action of percolating water charged
with carbonic acid, arising from the decay of vegetable
42 THE MECHANICAL ANALYSIS OF SOILS [chap.
matter in the surface soil. Many of the fermentation
changes that also take place in this vegetable matter
give rise to acids, which in their turn combine with
the calcium carbonate. So rapid are these removals of
calcium carbonate that it is difficult to understand how
any of it persists in the surface layers of many soils, the
subsoil of which shows that they must have been initially
poor in chalk, were there not some compensating
agencies at work. Amongst these agencies must be
reckoned the calcium salts in plants, which in many
cases are drawn up by deep-seated roots from
the subsoil and become calcium carbonate on the
ultimate decay of the plant tissues.
In a normal soil the particles of calcium carbonate
are of all sizes, many of the finer particles of silt and
clay are loosely cemented together by calcium carbonate,
as may be seen by the increase in the finer fractions
if a soil be washed with dilute acid before it is separ-
ated by sedimentation.
Humus. — On examining many rocks taken from
such depths that they have undergone none of the
weathering processes which convert them into soil, they
are found to contain both carbon and nitrogen, occasion-
ally in quantities comparable with those found in the
soil itself. This is only the case with the sedimentary
rocks and particularly the indurated clays, the carbon
and nitrogen in fact only represent the organic
matter in the original deposit in a more or less
mineralised condition. But since these carbon and
nitrogen compounds are only slightly affected by any of
the weathering processes by which soil is made, they
must pass into the soil and |here become merged with
the organic matter of more recent origin. Such material,
however, plays a very unimportant part in the soil, and
we may pass on at once to the debris of vegetation of
II.] THE NATURE OF HUMUS 43
recent origin or the humus which is characteristic of all
soils proper.
The term humus is applied to the black or dark
brown material of vegetable origin which gives to
surface soil its characteristic darker colour as compared
with the subsoil. It is essentially a product of bacterial
action ; there are a number of bacteria working in the
absence of air and universally distributed, which attack
the carbon compounds of plant tissues, especially the
carbohydrates, with the production of marsh gas or
hydrogen, carbonic acid, and humus. In the presence
of air the characteristic humus-forming fermentation
is replaced by one which results in the complete com-
bustion of the organic matter to carbonic acid. For
this reason more humus is found in a pasture than in a
continually aerated arable soil, more again in clays than
in the lighter soils through which air is always being
drawn as the rain percolates, and the accumulation
of humus reaches its maximum where considerable
rainfall and an impermeable stratum combine to make
the soil so water-logged that all access of air is cut off,
as in swamps and bogs. The presence of chalk in the
soil also assists in the destruction of humus, since it
neutralises the acids which largely compose the humus,
and which tend to inhibit the further action of bacteria.
The chemical composition of humus is indefinite;
it is a variable mixture of several substances, themselves
of very complex constitution ; it always contains more
carbon and less hydrogen and oxygen than the vege-
table tissues from which it was formed. The following
figures show the composition of grass and of the top
brown layer of turf in a peat bog, also of the same peat
of greater age at depths of 7 and 14 feet, the mineral
matter and moisture being excluded in calculation in
each case ; —
44 THE MECHANICAL ANALYSIS OF SOILS [chap.
Grass.
« —
Top Turf.
Teat at 7'.
Peat at 14'.
Carbon . •
Hydrogen •
Oxygen . •
Nitrogen
5°'3
5-5
42-3
1.8
57-8
5-4
36
0.8
62
5-2
30.7
2.1
64
5
26.8
4.1
Substances akin to humus can be formed from the
carbohydrates (such as sugar, starch, and cellulose), by
heating them for some time with water under pressure,
the action being more rapid if a trace of mineral acid be
present ; the resulting substances are weak acids and
form salts, so are generally termed humic acid : —
Humic Acid.
From Sugar.
Natural.
Carbon . •
Hydrogen •
Oxygen . .
Nitrogen .
63-9
4-6
3i-5
56-3 to 59
4-4 11 4'9
32-7 1, 36
2-8 „ 3-6
As a rule, the active humus of the soil is
there present in the form of salts of calcium, which
on treatment of the soil with dilute hydrochloric acid
are decomposed, a little of the humic acids going into
solution but the greater part remaining undissolved.
By filtering off the acid and then treating the soil with
a weak (4 per cent, by volume) solution of ammonia or
other alkali, the liberated humic acids are dissolved and
may be reprecipitated either as free acids by the addition
of hydrochloric acid, or as calcium salts by the addition
of a solution of calcium chloride. The humic acids thus
going into solution are sometimes estimated as " soluble
humus," they do not include the whole of either the
organic matter or the nitrogen in the soil. The brown
ii.] THE NATURE OF HUMUS 45
solution that is formed is akin to the dark liquid draining
from a dung heap, which contains humus dissolved
by the alkaline carbonates of the fermented urine.
Occasionally soils are found which naturally pos-
sess an acid reaction, and in which the whole or part
of the soluble humus is uncombined with calcium,
so that it goes into solution in ammonia without the
preliminary treatment with acid. The portion of the
natural humus of soils that is soluble in acids contains
nitrogen, and seems to be of the nature of an amide.
Although dark brown humic substances can be
prepared from carbohydrates, and therefore contain
only carbon, hydrogen, and oxygen, yet the soluble
humus of the soil, even when dissolved and reprecipi-
tated, always contains some nitrogen, nor can it be
obtained entirely free from phosphorus and mineral
matter. The original vegetable matter is made up not
only of carbohydrates, but of other carbon compounds
containing nitrogen, and in some cases both nitrogen
and phosphorus ; these all break down under bacterial
action into dark-coloured substances richer in carbon,
and roughly classed as humus. The splitting-up process
continues in the soil, so that humus becomes one of the
great sources of nitrogen for the food of plants, and a
soil well supplied with humus is generally regarded as
fertile. v
During the formation and continued decomposition
of humus the carbohydrates appear to be first attacked,
and the nitrogen-containing bodies, tg. % the nucleins in
particular, resist the action of bacteria. For this reason,
where we find the proportion of humus in a soil is low,
the proportion of nitrogen in the humus itself will be
high, the decay of the humus falls more heavily on the
purely carbonaceous part of the material.
This is seen in the figures obtained by Lawes and
46 THE MECHANICAL ANALYSIS OF SOILS [chap.
Gilbert for the ratio that exists between the propor-
tions of carbon and nitrogen in various soils : —
Ratio -^
N
Cereal Roots and Stubble • <
43
Leguminous Stubble . • <
23
Dung ... •
18
Very old Grass Land • •
137
Manitoba Prairie Soils . • «
13
Pasture recently laid down • <
117
Arable Soil . . . .
, IO-I
Clay Subsoil • . • <
6
Hilgard and Jaffa also found that the humus of soils
in an arid climate, where the deficiency of rainfall causes
the soil to be very open, contains a higher proportion of
nitrogen than is found in the humus of damper soils : —
Number
of Samples
Examined.
Average per cent.
of
Humus in Soil.
Average per cent.
of
Nitrogen in Humus.
Arid Soils . •
Semi-arid Soils •
Moist Soils . •
18
8
8
O.75
0.99
3-04
15-87
10-03
5-24
The following table (p. 47) gives the results of the
determination of carbon, nitrogen, humus, and the
percentage of nitrogen in the humus, in a selection of
extremely rich virgin soils obtained from different
parts of the world ; the Canadian, Russian, and Monte
Video soils were very similar uniform fine-grained grey
or black soils found on the great plains.
These results would seem to indicate that the most
valuable humus, i.e. that which will decay rapidly
and yield nitrogen compounds available as food for
plants, is that possessing a high ratio of carbon to
nitrogen.
II.]
THE NATURE OF HUMUS
47
Locality.
1. Canada
2. Canada
3. Russia .
4. Rhodesia
5. Monte Video
6. New Zealand
Indian Head
Wide Awake
Ploty
Salisbury
{
{
Tararua
Mountains
Description of
Soil.
Black Prairie
Black Steppe
Black Vlei
Black
44 Camp " Soil
Black Sandy
Pasture
)
— o5
2 s
d
a
fc
a
1
w
CM
u
a
©
•— <
.-5
3
eS
^3
tH,Q
n
S
CO
2-59
0-317
8-2
3-94
4-07
2-58
0-330
7-8
5-09
2-71
2-19
0-268
8-2
5-66
2.42
20-15
1-89
10-7
21.3
2-40
1-89
0-261
7*3
4.48
3-5i
12-66
0-949
13-2
10-35
4.67
Against this, Berthelot and Andr6 have investigated
the ratio of carbon to nitrogen in the different portions
of the humus which can be dissolved by alkalis or acids,
and they find that the most soluble portions contain the
highest proportion of nitrogen. It does not, however,
follow that the substances most soluble in acids or
alkalis are necessarily those which will most readily be
converted by bacteria into a form available for plants,
and, on the whole, the evidence seems to show that a
humus rich in nitrogen will yield it very slowly to crops.
Humus acts as a weak cement and holds together
the particles of soil, thus it serves both to bind a coarse-
grained sandy soil, and, by forming aggregates of the
finest particles, to render the texture of a clay soil more
open. In determining the sizes of the constituent
particles of a soil, the " mechanical analysis," it is desir-
able to remove the humus as far as possible, and so
break up these temporary aggregates.
. Sampling of Soils.
The first step in the analysis of any soil, mechanical
or chemical, consists in obtaining a sample that shall
adequately represent the land in question.
48 THE MECHANICAL ANALYSIS OF SOILS [chap.
In this country it is customary to take a sample down
to a depth of 9 inches as representing the soil proper ;
it is, however, doubtful if this is not too deep, being
below the depth to which cultivation is generally carried ;
probably a 6-inch sample would more truly represent the
cultivated soil. In many cases it will be found that the
true soil does not extend to a depth of anything like 9
inches, but that there is a sharp change into subsoil or
even rock before this point : e.g., on the chalk downs the
soil is often not more than 4 inches deep, below which
white broken chalk rock begins. In such cases the
sample must only be taken to the depth at which the
visible change begins.
To obtain the sample two methods are generally
adopted. At Rothamsted a steel box, without top or
bottom, 9 inches deep, and 6 inches square in section, is
used ; the sides are wedge-shaped, about f inch thick
at the top and tapering off to cutting edges below.
The surface, if uneven arable land, is first raked over
and gently beaten level, then the box is placed in
position and driven down with a heavy wooden rammer
till the top of the box is flush with the surrounding
soil. The soil enclosed by the box is then carefully dug
and scraped out into a bag for conveyance to the labora-
tory; two or three samples to the same depth being
taken from the same field and afterwards mixed.
Should samples of the subsoil be required, the box
is left in position after its contents have been scraped
out, and the surrounding soil is dug away to the 9-inch
level, the box is then rammed down for the second
9 inches, and its contents removed : the process being
repeated till the required depth has been reached.
A modification of the Rothamsted method consists in
marking out on the surface a square 9 inches on the
side, and digging away the surrounding soil until a
#
FlG. i. — Photograph of Soil-sampling Tools.
II.] METHODS OF SAMPLING 49
9-inch cube of earth remains standing; over this a
wooden box is slipped, and the cube is cut off by pushing
a spade beneath at the 9-inch level
On soils which do not contain many large stones,
samples may be taken with an auger, both more rapidly
and with greater security of obtaining an average
sample. A convenient tool for the purpose consists of a
cylindrical auger made of steel, about T V inch thick, of 2
inches internal diameter and 12 inches deep, with a slot
f inch wide running from top to bottom ; the lower
edge of the cylinder and the edges of the slot are
sharpened ; to the upper end of the cylinder a handle
carrying a wooden crossbar is riveted. The auger is
forced gently into the soil with a twisting motion until
the required depth is reached, when the tool is with-
drawn and the core scraped out into a bag. Six to ten
cores at least are taken at regular intervals in the same
field and mixed to secure an average sample. Each
boring can be continued to obtain subsoil samples as
deep as the length of the handle permits. It is impos-
sible to obtain samples with the auger when the soil is
dry. Fig. 1 shows a photograph of both types of soil-
sampling tools.
When the samples reach the laboratory they are
spread out on shallow trays to dry, which process may
be accelerated by a gentle warmth, not exceeding 40 C.
In dealing with stiff soils it is advisable to crumble all
the lumps by hand while the earth is still somewhat
moist When the whole is sensibly dry the stones are
separated by a sieve having round holes 3 mm. in
diameter; the material that does not pass the sieve
is gently worked up in a mortar with a wooden pestle,
care being taken not to break the stones, chalk, etc.,
but only to crush the lumps of earth. Finally, the
material upon the sieve is roughly weighed and well
D
50 THE MECHANICAL ANALYSIS OF SOILS [chap.
washed in a stream of water till all the fine earth is
gone, dried, picked over to free it from roots and
stubble, and weighed as "stones." To get the pro-
portion borne by the stones to the soil, the fine earth
is also weighed, an addition being made of the weight
lost by the stones in washing.
Of course the figure obtained for the proportion of
stones is only approximate, for if the stones are of any
size they will be very irregularly caught by the auger
or even by the 6-inch square tool. The material passing
the sieve is again spread out in a thin layer in an
ordinary room, until the surface maintains the same
colour as the lower layers; it is then bottled up as
" air-dry fine earth " for analysis.
The Mechanical Analysis of a Soil
The mechanical analysis that follows consists in
dividing the fine earth into a series of fractions con-
sisting of particles of known size ; we can use sieves to
sort out the coarser grades, but the finer ones must be
separated by their relative powers of remaining sus-
pended in water.
The methods in use depend on two principles:
in one, the hydraulic method (Hilgard, Schoene, Nobel),
soil is washed by successive currents of water of veloci-
ties calculated to carry particles of the required size
according to the table on p. 33 : in the other, the sedi-
mentation method of Osborne, Knop, and Schloesing,
the soil is suspended in water and allowed to stand, the
separation being effected either by the times required
for the particles to settle down through a fixed distance,
or by the distances fallen in a given time. The method
to be described is based upon the latter principle. The
hydraulic method requires special apparatus, and is only
suited to laboratories entirely devoted to soil analysis.
II.] ANALYTICAL METHODS 51
Method of Analysis.
1. Ten grams of the air-dry fine earth are weighed
out into a beaker or basin and treated with 100 c.c. of
Nj$ hydrochloric acid ; the soil is well worked up with a
rubber pestle (made by fixing a glass rod into a small
solid rubber bung) until all the lumps of clay, etc., are
broken up. If the soil contains much calcium carbonate,
a further addition of acid may be required.
The object of the acid is to dissolve the carbonates and
humates, and thus loosen the particles in any aggre-
gates where chalk or humus form the cement. With-
out this preliminary treatment the amount of clay
found will be largely determined by the proportion
of humus present ; the soil of an arable field, for
example, will show more clay than the soil of an
adjoining pasture, when the sedimentation is made with
water alone. But after the preliminary treatment with
acid to remove the humus, both fields will show the
same proportion of ciay (as they should do, since they
are of the same origin), and only differ in the amount
of humus they have accumulated — a temporary factor.
After standing with the acid for an hour, the whole
is thrown on a tared filter and well washed until all acid
is removed. The filter and its contents are dried and
weighed ; the loss the soil has suffered represents the
material dissolved and the hygroscopic moisture.
2. The soil is now washed off the filter with
ammoniacal water (about 1 c.c. of strong ammonia
solution in half a litre of water) on to a small sieve of
100 meshes to the linear inch, the portion passing through
being collected in a beaker which is marked on the side
at a distance of 8-5 cm. from the bottom.
The ammonia completes the dissolution of the humates,
and also masks the effect of any traces of soluble salts
which may be left and would cause aggregation in the
manner indicated earlier, p. 38.
52 THE MECHANICAL ANALYSIS OF SOILS [chap.
The portion which remains on the sieve is dried and
weighed. It is then divided into "fine gravel" and
" coarse sand " by means of a sieve with round holes of
I mm. in diameter, the portion retained by the sieve
being designated " fine gravel."
3. The portion in the beaker is well worked up with
the rubber pestle, ammoniacal water is added up to the
8- 5 cm. mark, and the whole is put aside to stand for
twenty-four hours. The turbid, supernatant liquid is
then rapidly poured off into a large jar, and the residue
is rubbed up again with the rubber pestle and more
ammoniacal water, as before. The whole operation of
filling to the mark, standing for twenty-four hours, and
pouring off the turbid liquid is carried through as
before, and repeated as long as any matter remains
in suspension for twenty-four hours. Generally seven
to ten decantations will be sufficient, after which the
united turbid liquid is evaporated to dryness in a tared
basin, and weighed. This fraction consists of the "clay"
particles less than 0002 mm. in diameter, together with
all the soluble and some of the insoluble humus. The
contents of the dish are ignited over an Argand burner
for some time and reweighed, to obtain the weight of the
" clay " after ignition.
4. The sediment from which the clay has been
removed is worked up as before in the beaker, which,
however, is now only filled to the depth of 75 cm.
The contents are now allowed to stand for twelve
and a half minutes only, when the liquid is poured
off into a large jar as before. The operations
are then repeated until all the sediment settles
in twelve and a half minutes and the liquid above
is left quite clear. The contents of the jar are now
evaporated to dryness and weighed, as in operation
3, before and after ignition ; this fraction is desig-
II.]
ANAL YTICAL METHODS
53
nated " fine silt," and lies between ooio and 0002 mm.
in diameter.
5. The sediment remaining in the beaker is worked
up afresh just as in the previous operations, the mark
being now placed 10 cm. from the bottom of the beaker,
and the time of settlement fixed at one hundred seconds.
The sediment is dried and weighed as " fine sand," while
the portion that is poured off is obtained by evaporation
as in the previous operations, and is designated as " silt."
The soil has thus been divided into the following series
of fractions : —
Diameter in Millimetres.
Maximum.
Minimum.
I
2
3
4
5
6
7
Stones and Gravel .
Fine Gravel
Coarse Sand •
Fine Sand •
Silt • •
Fine Silt . • •
Clay . •
3
I
0«2
0*04
O-OI
O«002
3
1
0*2
0*04
O-OI
0-002
\ Separated
f Jl
] sitting.
->
Separated
by
subsidence.
j
If there be much " fine gravel " in the soil, it is best to
make a separate determination of its amount on a
sample weighing 50 grams, treating with acid as before,
and then washing the whole on to the 1 mm. sieve.
The result obtained should be taken as the true
percentage, and the other percentages found in the
analysis of 10 grams only should be recalculated to agree
with it.
The sizes of the particles, the depth of the liquid, and the
times adopted above, are purely conventional. The time
of settlement required to obtain a fraction of any
given range of size can be determined by a series of
trials, the material remaining suspended in each case
54 THE MECHANICAL ANALYSIS OF SOILS [chap.
is measured under the microscope until the right time
is hit off to secure the desired range of size in the
sediment. The relationship between the time of settle-
ment, the height of the liquid column, and the diameter
of the particles, is governed by the formula : —
v = 2ga* O-/))
9 V
where <r is the density of the particle, a its radius, p the
density, and tj the coefficient of viscosity of the liquid.
The application of the formula, however, requires to be
checked by observation with the microscope, because
the particles are not spheres.
The hygroscopic moisture and the loss on ignition
also require determination, which is described under
the chemical analysis of a soil.
Interpretation of Results.
It is as yet impossible to predict the behaviour of
a soil under cultivation from a consideration of its
mechanical analysis; in a general way we can see
whether a soil is heavy, whether it is likely to dry
"steely," or whether it will crumble readily under
proper cultivation, and whether it is more suitable for
market gardening or wheat growing, but the more
refined points of difference connected with the manage-
ment of given soils, which become known by experience
to a good practical farmer, cannot as yet be deduced
from the analysis. It is necessary to accumulate more
data, until we possess the mechanical analysis of a
large number of soils whose texture and amenability
to cultivation have been ascertained by long practice;
then we shall be able to assign any soil by its mechanical
analysis to a known type.
The power of a soil to retain moisture and resist
moderate drought depends on a predominance of the
II.] TYPICAL SOILS 55
finer particles and of humus ; good wheat land or
land that will form sound permanent pasture will
contain at least 30 per cent, of silt and clay. The
ease with which a soil suffers the rain to percolate
depends upon the relatively low proportion of silt and
clay rather than on the amount of coarse-grained
material; the fine particles pack in among the larger,
and the soil is equally resistent to the passage of water,
whether the finest material is diffused among coarse
sand and gravel, or among the finer grades of sand.
The shrinkage of a soil on drying, and its tenacity when
dry, are even more dependent on low proportions of
coarse sand, humus, and chalk, than on the actual
amount of clay and silt which cause the shrinkage.
The really difficult soils to work are those containing
less than 20 per cent, of sand above 01 mm. in
diameter.
The table on page 56 will serve to illustrate these
points.
Soil No. I represents one of the lightest of sands,
about the extreme limit of cultivation — a soil, indeed,
which had been found unfit for ordinary farming,
and had been planted with conifers.
It will be seen that more than 83 per cent consists
of " sand," nearly all of the coarser kinds, while the clay
only amounted to 4-7 per cent, most of which was really
ferric oxide. Calcium carbonate is also entirely absent,
owing to which the soil accumulates more humus than
would be expected from its great aeration, and in the
hollows where water lies it often becomes peaty. Such
soils are rarely in cultivation, but are left as wastes,
carrying a natural vegetation of heather and pine.
Because, however, of their lightness and warmth,
they are sometimes valuable for market gardening on a
small scale, if they are so situated that large supplies of
56 THE MECHANICAL ANALYSIS OF SOILS [chap.
farmyard manure or town dung are available, straw-
berries being a favourite crop.
Soil No. 2 was taken from the Stackyard field of the
farm of the Royal Agricultural Society at Woburn, and
represents a light sandy loam, early, and extremely easy
1
2
3
4
5
6
7
8
T3
a
i
a
a
a
%
CD c3
o a>
WT3
*
O
0}
4*
,3
03
O
>>
>
«3
fc-a
f> SB
1
m
a
CD
3
bO
3
1=1
03
03
H
£
Fine Gravel • .
4.1
I»0
3-0
1*2
I *9
1.9
1-3
0-4
Coarse Sand •
70-3
49.9
33-8
5-3
3-3
6.2
21-2
o-8
Fine Sand •
7.0
l6»I
28-0
32'I
36.8
214
12-5
6.4
Silt
i-5
II'I
5-6
33-3
2I«0
32-5
15.0
i8-6
Fine Silt .
5-8
5.6
io-8
5-3
14-3
13-8
II-9
1 3-6
Clay • »
47
97
6.6
1 1.8
13-5
17.6
28-3
42.2
Moisture
2-6
I»2
4-3
1.9
1.4
2»2
1.6
9-5
Loss on ignition
3«o
3-8
6.9
4-5
4-5
5-8
7-8
9.1
Calcium Carbonate .
...
• ...
0.2
01
o-3
2-5
...
0.4
SUBSO
ILS.
Fine Gravel . •
6-5
1*0
4.1
o-3
2 -6
17
07
0«2
Coarse Sand . •
75-5
50I
36.8
2-1
2-8
4*3
n»6
o-5
Fine Sand «
4.9
15*9
26-1
27.O
35-2
15-8
7-3
6.2
Silt
1-7
12.5
5-4
40-8
19.9
24.0
9-8
15-9
Fine Silt
4.2
5-9
8.4
57
16.1
16.7
15-2
IO-2
Clay
2-2
8-6
9-5
16.4
i6«2
28.7
427
48.9
Moisture
1-6
0.9
3-3
3-6
1-2
3-8
2.6
6-3
Loss on ignition
Calcium Carbonate .
27
27
57
2-8
4.1
4.6
8-1
7-3
• ••
O'l
0.1
o-3
0«I
...
0«I
to work in any weather. Owing to the preponderance
of coarse sand, it suffers somewhat from drought and
rarely carries heavy crops ; and though responding well
to manuring, the soil is hungry and does not long retain
organic manures. The soil contains enough silt to
possess a distinct power of lifting the subsoil water by
II.] TYPICAL SOILS 57
capillarity, and similar soils containing less coarse sand
and rather more fine sand and silt are often among the
most valuable, because they combine free working
with a capacity to resist drought through capillary action.
This soil is more suited to market gardening than to
mixed farming, makes poor pastures, grows good barley
and turnips, but is too light for wheat and mangolds.
Soil No. 3 is a light sandy loam from one of the
most valued of the " red land " potato soils, near Dunbar.
In the cool climate, with a fair rainfall here prevailing,
this forms an excellent arable soil for all crops, specially
prized as yielding potatoes which retain their colour
and are mealy after boiling.
Soil No. 4 is a typical free working loam from the
Thanet sand formation, but rather lighter than usuaL
It is easy to work, warm and early, stands drought well,
and is grateful and fairly retentive of manure. This is
a highly valued soil for all ordinary arable cultivation,
but is rather too light for wheat and pasture in the
south or east of England. No particular fraction of
the soil is predominant, but the soil is a fairly uniform
mixture of particles of all grades.
It should be noticed that in these first four soils of
a sandy type soil and subsoil are of very similar
structure, whereas as soon as the smaller particles
predominate on the heavy lands, then the soil is coarser
grained than the subsoil.
Soil No. 5 comes from the Hastings Sand in Sussex,
and represents a light example of a type of soil which,
with a certain amount of variation in the relative
proportions of fine sand and silt, covers a considerable
area in the high Weald country.
Generally it forms a sticky, heavy working soil,
commonly described as a clay, though the sand and silt
fractions predominate and no excessive proportion of
J8 THE MECHANICAL ANALYSIS OF SOILS [chap.
clay is present. The soil, however, is kept very close by
the lack of coarse sand and of any of the still coarser
gravel and stones, the absence of carbonate of lime also
makes it stickier and more difficult to work. If a good
tilth is obtained, as for instance a seed bed for roots, and
heavy rain follows, these soils are particularly liable to
run together and set on drying to a glazed caked surface,
very inimical to germination. When well supplied with
lime and organic matter, these soils are fertile and carry
magnificent crops ; but they are rather late and expensive
to work, so that they have in great measure been laid
down to grass. They carry good grass when well
treated, and particularly when dressed with lime and
basic slag.
Soil No. 6 is taken from the Broadbalk Wheat Field
at Rothamsted : it is a heavy loam, stubborn and
intractable to work, which would lie very wet were not
the land naturally under-drained by the chalk rock at a
depth of ten or twelve feet below. The surface soil also
contains a large number of flint stones, not shown in
the analysis, and these help to keep the soil more open
and assist the drainage. Heavy as it is, the soil is not
a true clay ; it is the silt and fine sand fractions which
predominate, and to these must be attributed the
tendency of the soil to run and dry with a caked surface,
if much rain falls after a fine tilth has been attained.
In the soil but not the subsoil there is a fair proportion
of calcium carbonate, of artificial origin, and this con-
tributes greatly to the workability of the soil, for it has
been found unprofitable to retain some of the fields,
in which the calcium carbonate is absent, under
arable cultivation. Land of this class is still largely
under the plough, and is good wheat, mangold,
and bean land, but is too heavy for barley or turnips.
An occasional bare fallow is desirable to clean the land
II.] TYPICAL SOILS 59
and bring it into tilth again ; it also yields very fair
permanent pasture.
Soil No. 7 is situated on the Kimeridge Clay forma-
tion in Cambridgeshire ; it is heavy land, difficult to
cultivate, and when under the plough requires a bare
fallow from time to time to restore the tilth. This
represents one of the heaviest soils which respond to
arable cultivation, which indeed is only practicable
because the soil, though containing so high a proportion
of clay, also contains a good deal of coarse sand, which
keeps it open and helps to render it friable.
Soil No. 8 is a heavy, undrained London Clay, which
will carry nothing but poor pasture. At one time it
would carry in favourable seasons heavy crops of
wheat and beans, but the expense of cultivation and
the danger of missing a season have rendered it quite
unprofitable to farm under the plough. It will be
noticed that the soil consists almost wholly of the
finer fractions, nearly one-half being " clay " ; nor
is there any difference between soil and subsoil,
except in the humus, which improves the texture
of the surface.
CHAPTER III
THE TEXTURE OF THE SOIL
Meaning of Texture and Conditions by which it is affected — Pore
Space and Density of Soils— Capacity of the Soil for Water —
Surface Tension and Capillarity — Percolation and Drainage —
Hygroscopic Moisture.
In the preceding chapter, the nature of the particles
composing the soil has been discussed ; it now remains
to consider the manner in which they may be arranged,
and the structure that results from the interaction
of the soil particles, the water, and such salts as may
be dissolved in the water. On these factors depend
what the farmer knows as the " texture " of the soil,
the degree of resistance it affords to the passage of a
plough, etc., the ease or otherwise with which that prime
object of cultivation, the preparation of a seed bed, can
be attained.
It is clear that as a soil consists of particles there
must be between them a certain amount of space
which is occupied by air or water ; this is known as the
"pore space," and on its amount will largely depend the
density of the soil. Taking the simplest theoretical
case, a soil made up of equal spheres in contact with one
another, it will be found that the pore space is de-
pendent upon the method of packing, but not upon the
60
chap, in.] PACKING OF SOIL PARTICLES 6 1
size of the spheres. If the system of packing shown in
A and B, Fig. 2, is adopted, the pore space reaches its
maximum and amounts to 47-64 per cent of the whole
volume occupied by the soil ; this proportion is the same
when the soil particles have a smaller diameter, as in B ;
as long as the spheres are uniform in size, whatever that
may be, and are packed as shown in the diagram, the
pore space will be at its maximum. The minimum pore
space is attained by the packing shown in C and D ; it
amounts to 25-95 per cent, and is again independent of
the size of the particles, provided they are uniform. If
the spheres are, however, of very different sizes, so that
smaller spheres lie wholly within the spaces between
the larger spheres, as in the arrangement shown in E,
the pore space may be indefinitely reduced. Per contra^
if aggregates of particles exist in the soil, containing
both pore space between the ultimate particles and
between the aggregates which behave as single particles,
as in F, the pore space may rise much above the
maximum of 49 per cent A soil in situ generally
possesses a pore space larger than the proportions
indicated above; various causes, such as the stirring
due to cultivation, the decay of vegetation, etc., leave
definite cavities in the soil : for example, if a hole be
dug for any purpose in ordinary cultivated ground and
afterwards filled up with its own soil, it is rarely possible
to fill the hole completely, especially if a little pressure
4ias been used to trample down each layer.
In ordinary soils the pore space varies from a little
over 50 per cent among the stiff clays, down to 25 or 30
per cent, in the case of coarse sands of uniform texture.
The reason for the greater pore space with the finer
grained soils lies in the fact that the weight of the
small particles of clay is not sufficient to overcome the
friction and move the particles into the arrangement
62
THE TEXTURE OF THE SOIL
[chap.
CO
<— »
+3
t-.
ri
Oh
ft
u
*n
a,
CO
c
<u
<L>
o
rt
O.
<U
o
a.
a
fa
III.]
TRUE AND APPARENT DENSITY
63
giving the minimum pore space. If some small shot are
shaken into a graduated measure and the pore space
determined by pouring in a measured volume of water,
the indicated minimum will be found ; but if the experi-
ment be repeated with sand which has been sifted to get
approximately a uniform size, a higher figure will result.
In the one case the particles are too light to exert
much force towards the rearrangement of the mass ; in
the former case the heavy smooth shot slip straightway
into the most compact arrangement, because by it the
shot attain their lowest position. In consequence of the
pore space, the density of a soil in situ will differ very
much from that of the materials of which it is composed,
nor will all soils possess the same apparent density when
dry. Perhaps the best way of ascertaining the apparent
density of a soil or soil materials is to get a smooth
metal pint pot or like measure, fill it with the material
in question with gentle tapping, and then strike off the
upper surface smooth with a rule. The weight of the
contents divided by the volume gives the apparent
density, from which the true volume and the pore space
can be calculated, if the true density of the material be
known. The following table shows the true and ap-
parent density of the chief soil materials ; as a mean
figure for purposes of calculation, 2-65 can be taken as
the true density of ordinary soils : —
True Density.
Apparent Density
when dry.
Humus • • • .
Clay . • • • .
Sand .....
Calcium Carbonate.
Hydrated Oxide of Iron .
1-2
2-5
2-6
2-75
3-4 to 4
•34
1
1.45
...
The following table shows a few determinations
64
THE TEXTURE OF THE SOIL
[chap.
made in the laboratory, of the apparent density of
various soils in a roughly powdered state and without
the stones, which, being solid, would add to the apparent
density of the soil. The results are also recalculated
to show the weight of a cubic foot of the soil, and the
weight per acre of a layer 9 inches deep : —
Apparent
Density.
Weight per
cubic foot.
Lbs. per acre
to 9".
Heavy Clay . • .
Sandy Clay .
Sandy Clay Subsoil .
Light Loam .
Light Loam Subsoil
Sandy Loam • . •
Sandy Peat • •
Light Sand • • •
I«o62
1*279
Ll8
1*222
LI44
1.225-^
O.782
I«266
664
80
737
764
71-5
76.7
49
79.2
2,150,000
2,6oo,000
2,380,000
2,480,000
2,320,000
2,490,000
1,580,000
2,560,000
The figures given above are not exactly comparable
with soils under natural conditions, because of the
powdering, the exclusion of stones, etc., but they
serve to show that the clay soils usually described as
"heavy" are really less dense, and weigh less per cubic
foot than some of the lighter soils, whereas pure sands
are the densest of all. The farmer's terms of " light "
and " heavy " land refer to the draught of the plough,
the resistance the soil opposes to being torn asunder,
and not to the actual weight of the portion moved ;
sands which he calls " light," being, as the table shows,
heavier per cubic foot than the clays which the farmer
calls heavy soils.
This point will be further elucidated by the following
table, which shows the weight per cubic foot of the
arable soils at Rothamsted and Woburn down to a
depth of 3 feet These results represent the real weights
of the soil as obtained by cutting out a block 6 inches
square by 9 inches deep, weighing it, and afterwards
III.]
WEIGHT OF SOIL
6$
ascertaining the deduction to be made for water. The
Rothamsted soil is a stiff clay with many flints, the
Woburn soil is a loose, coarse-grained sand, containing
only a little stone derived from the rock below. It will
be seen that, if the stones are excluded, the density
increases with the depth, because of the greater consoli-
dation caused by the weight above and to some extent
by the washing down of the finest particles, but the
increase does not continue much below the depth of 3
feet, the limit of these measurements : —
Weight per
Per cent.
Weight per
cub. foot.
of Stones.
acre.
Lbs.
Lbs.
r °"
to
9"
95-4
16.8
3,116,000
Rothamsted Arable
9"
»»
18"
93*o
I20
3,037,000
Broad balk
1 8"
»i
27"
92*0
7-1
3,004,000
I 27"
)»
36"
92^
7-5
3,012,000
f °"
»!
9"
96.6
2.96
3,157,000
Woburn Arable
9"
1 8"
18"
27"
103.8
106.2
5-95
4.92
3,382,000
3,462,000
1
[ 27"
n
36"
106.9
7-83
3,501,000
From the data thus obtained as to density and
pore space, together with a mechanical analysis to
show the proportion of particles of various sizes, it is
possible to calculate for any given soil both the number
of soil particles and the area of the surface they expose,
on the assumption that the particles are spherical.
Approximately with grains I mm. in diameter, there
would be 700 grains in I gram of the soil, and the
number of grains to the gram will vary inversely as
the third power of the diameter, i.e., if the diameter be
divided by 10 and become 01 mm., there will then be
700,000 grains to the gram. The surface possessed by
all the soil grains can be similarly deduced by calcula-
tion, and will be found to vary inversely as the diameter
E
66
THE TEXTURE OF THE SOIL
[CHAP.
of the individual grains ; a sphere I inch in diameter
will have only half the surface of the eight spheres of
half an inch in diameter which possess the same volume
Hence it follows that the surface of an ordinary soil
must be extremely extensive, and since many of
the properties of the soil are dependent upon the surface
it becomes important to arrive at some measure of
this quantity. By calculation only a very rough idea
of its extent can be formed, both because every
departure of the soil grains from the spherical form
will increase the surface without affecting the weight,
and also because the mechanical analysis of a soil
gives only a generalised statement of the distribu-
tion of soil particles of various sizes in the soil. But
the surface of the soil grains in the case of a sandy
soil where the grains are all free, may be calculated
from the observed rates of flow of fluids like air or
water through a measured portion of the sand ; and by
using this method King has computed the surface of
the constituent particles of various types of soil with
the results set out below : —
Pore Space, per
cent.
Area of Surface in
square feet,
per cubic foot of Soil.
Finest Clay . .
Fine Clay Soil .
Loamy Clay Soil •
Loam •
Sandy Loam . • •
Sandy Soil
52-9
48
49.2
44.1
38-8
32-5
173,700
110,500
70,500
46,500
36,900
II,000
As a rough figure to remember, the surface of the
particles in one cubic foot of an ordinary light loam
may be taken as about an acre; this will increase
as the soil approaches more and more to clay, and
diminish as the soil becomes increasingly sandy. The
in.] WATER IN SATURATED SOIL 67
extent of surface exposed by the soil particles is im-
portant because it is their active part ; other conditions
being equal, the amount dissolved from a solid body in
a given time by any solvent will be proportional to
the surface exposed.
Again, the water in a soil usually exists as a film,
coating the surface of the soil particles, and the amount
of water that can be held under particular conditions
becomes a function of the extent of surface ; even the
power of a soil to remove certain substances from solution
is likewise dependent on the surface.
Capacity of the Soil for Water.
So far, the structure of the soil in a dry state has
only been considered, it is now necessary to consider
its behaviour when fully saturated with water, before
passing on to the more usual state when the soil con-
tains both air and water.
The amount of water which a soil will hold when com-
pletely saturated will depend upon the pore space, will,
in fact, be the pore space together with whatever water
the material of the particles can imbibe without causing
any swelling. Perhaps the best method for determining
the water capacity of a soil is one devised by Hilgard.
A small cylindrical brass box. is constructed, 1 cm.
deep and 6 cm. in diameter. The bottom is a sheet of
perforated brass, and the whole is supported on three
legs ; the capacity of the box is about 30 c.c The
exact capacity is determined by waxing up the holes,
weighing, filling with water, and reweighing. A circle
of thin filter paper cut to fit the box is laid inside
and wetted, any superfluous water that comes through
being wiped away. The box is then weighed, care-
68 THE TEXTURE OF THE SOIL [chap
fully filled with fine earth, and gently tapped to settle
the soil down ; finally, the surface is struck off level
with a straight-edge. The box is now weighed again
to find the quantity of dry soil taken, and placed in a
dish of distilled water, so that the water stands about
I mm. above the lower surface of the soil inside the
box ; the dish is then covered over to prevent evapora-
tion. The water rises in the soil, displacing the air,
and in about an hour's time the soil will have absorbed
all the water possible. The box is lifted above the water
a little, allowed a few minutes to drain, the excess of
water clinging to the under-surface is wiped away with
a clean cloth or filter paper, and the whole is then
weighed. A previous "determination of the moisture
present in the " air-dry fine earth " must also be made,
to provide all the data necessary for the calculation of
the water contained in the saturated soil. This calcu-
lation may be made in three ways: either the pro-
portion the water in the saturated soil bears to the
dry soil, or the proportion of water in the wet soil •
may be estimated, or again, the proportion by volume
that is occupied by water and soil respectively may
be calculated. The figures thus obtained will vary very m
considerably, because the less dense the soils, because of
the clay and humus they contain, the more water they
will absorb; thus the proportion which the water
absorbed bears to the weight of the dry soil becomes
exaggerated in their case. Perhaps the soundest picture
of the state of affairs is attained by considering the
volume that is occupied by the water in the soil, and
expressing it either as a percentage by volume, or as
lbs. or inches of water per cubic foot of wet soil. The *
following figures show the results obtained for four
distinctive soils, calculated out in the different ways
described above.
III.]
WATER RETAINED BY SOIL
6 9
Maximum.
Minimum.
u
<D
^^
S3 CI
<D 33
©JS
a >*
O >H
<D O
08 i-l
Pi
Per cent, of Water
in Saturated Soil
by Weight.
Per cent, of Water
in Saturated Soil
by Volume.
1
b
Q
1— 1
u
B
Ph
JS
s
Coarse Sandy Soil
Light Loam.
Stiff Clay .
Sandy Peat . •
45
50-5
98-6
155
31
33-5
49.6
60.8
50.5
55-8
67.6
63.2
18
29-2
56.4
Il6
15*3
22-6
36-1
53-7
22*2
35*4
45.6
52-8
0-8
2.9
6.9
8.3
Under natural conditions a soil is rarely saturated
to the extent indicated in the previous table ; as the
rain water enters from above, the surface of the soil
is wetted first and the air within the soil finds a diffi-
culty in escaping, so that even after long-continued
rain the pore space does not become entirely filled
with water.
The following table shows the water contained in
a few field soils sampled a day or two after* the cessa-
tion of long-continued rain, and calculated as per-
centages of the wet soil by weight : —
Per cent, of
Water
in Wet Soil.
Sand at Water Level
18.4
Rothamsted Wheat Land, unmanured ....
23-0
„ „ manured with artificials
24.7
„ „ manured with dung for 26 years
37-6
Light Loam above Chalk .
20.3
Hellriegel has shown that the optimum proportion
of water in the soil for the growth of the plant is
40 to 50 per cent, of the maximum required for satura-
tion.
7o
THE TEXTURE OF THE SOIL
[CHAP.
Flow of Water through Soils.
The freedom with which water will move through
soils under the action of gravity or other force will
depend not only on the pore space, but upon the
mean size of the channels formed between the soil
grains. King made some experiments with sands
graded by sieves and formed into columns 14 inches
long and 1 square foot in section, above which the water
was maintained at a head of 2 inches. He obtained
the following results expressed in inches of water
passing in twenty-four hours ; the second column gives
the number of meshes to the inch of the sieves which
respectively passed and retained the sand : —
Medium.
Sieves.
Inches.
,, . • • • .
>i • • • • •
Clay Loam . •
Black Marsh Soil .
_
40 to 60
60 „ 80
80 „ IOO
IOO
Ml*
•*•
301
160
73-2
39*7
1.6
•7
It will be noticed that there is a great diminution
in the rate of flow as soon as a soil containing small
clay particles is introduced ; of course, one of the
characteristic properties of clay is that it will not
allow any flow of water through it when it has been
puddled. In the puddled condition, the particles
constituting the clay are no longer aggregated, the
material is in its finest-grained condition, so that the
pore spaces between them must have become extremely
small. Not only is the flow diminished by the increase
of friction in the narrow channels, but in the case of
clay their dimensions have become so small that prob-
ably the contained water wholly within the range
hi.] SURFACE TENSION 71
of the molecular forces to be described later ; it is thus
prevented from flowing at all, and only moves by
diffusion. If we assume for clay particles a mean
diameter of 00002 mm., and a structure similar to A in
Fig. 2, p. 62, it is easy to show that no molecule in the
space between the spheres can be further than about J
of the diameter of a sphere, or 000004 mm - from one or
other surface, while the range of molecular forces as
calculated by Quincke extends to about 000005 mm.
from the surface. Spring has indeed shown that infiltra-
tion of water is impossible through clays or loams unless
they are first allowed to expand by taking up water.
Surface Tension and Capillarity.
The existence of attraction between the molecules
causes the free surface of any liquid to become a sort
of stretched elastic film, in tension itself, and exerting
a certain pressure inwards when free. The molecules
within the liquid are equally attracted in all directions
by the surrounding molecules, and are therefore in equili-
brium ; the molecules on the surface, having nothing on
one side, are only attracted inwards, and so, as a whole,
exert a pressure on the liquid similar to that which
would be caused by a stretched elastic skin over the
liquid.
The existence of this force of "surface tension," as
it is called, may be demonstrated by many simple ex-
periments, e.g. y by the familiar fact that a clean needle
will float when placed carefully on the surface of water ;
or, by the fact that any portion of a liquid which is so
small that the force of gravity on it is not large
compared to the molecular forces, immediately assumes
the spherical shape. Of all figures, a sphere has the
smallest surface in proportion to its contents, *>., the
72
THE TEXTURE 01 THE SOIL
[chap.
stretched film on the surface of a drop of liquid shrinks
as far as it can until the liquid is packed into the
smallest possible compass, into the form of a sphere.
When a liquid and a solid are in contact, the form of
the surface and the resulting pressure or tension depend
on whether the liquid "wets" the solid or not For
Water
FlG. 3. — Capillary Rise and Depression
of Liquids in Glass Tubes.
example, if a series of very fine or "capillary" glass
tubes are dipped into water and mercury respectively,
the water will rise up the tubes in inverse proportion to
their diameters, the mercury, which does not wet the
glass, will be correspondingly depressed.
The water surfaces a, b y c (Fig. 3), are convex to
the water, and become more convex the narrower the
tube is; the pressure below the convex surface must
hi.] SURFACE TENSION 73
be less than atmospheric, or the water would not
stand higher within than without the tube ; further,
the pressure beneath a y the most convex and therefore
most stretched surface film, is lower than the pressure
beneath b> and still lower than that beneath c. Per
contra^ the mercury surfaces are convex outwards, and
exert pressure on the liquid beneath, depressing it
below the general surface of the liquid in proportion
to the degree of convexity. These instances will help
us to realise that the surface of a liquid may exert
either a pull or a pressure on the liquid within,
according to the curvature of the surface, and the
greater the curvature the greater will be the force
exerted. It is this tension of the surface film which
causes movements of water in soil, other than those due
to gravity; for example, if a flowerpot stands in a
shallow dish of water, the whole of the soil within the
pot is kept moist ; or if water is poured on to dry soil, it
is seen to work outwards through the soil, the water
advancing from particle to particle as it wets them, just
in the same manner as it rises up the capillary tubes.
When a soil is saturated, the whole pore space is filled
with water ; if this soil be allowed to drain, some of the
water is pulled away by gravity, but much remains
clinging round the particles in the stretched film con-
dition, the tension in the film balancing the pull due to
gravity. Perhaps the best illustration of the state of
affairs in a wet but drained soil may be obtained by
linking a series of toy balls together, as shown in the
photograph (Fig. 4), and then dipping the whole into oil.
When the oil has ceased to drip it will be seen that
every ball is covered by a thin film of oil, and that
between the balls there is a layer of oil much thicker in
the lower than in the upper layers. The whole surface
film is equally stretched, but the stretching in the upper
74 THE TEXTURE OF THE SOIL [chap.
layers is largely due to the pull from the oil below,
while in the lowest layer of all the whole tension
exerted by the stretched film is devoted to holding up
its own thick film of oil. If oil be taken away at any
point, the curvature of the film, and therefore the tension
of the surface in that region, is increased : a readjust-
ment then takes place till the stretched film regains the
same tension everywhere, which is effected by a motion
of the oil to the place where the tension has been
increased. If the withdrawal of the oil be continued, the
film round the balls becomes thinner and thinner ; the
more it is stretched, the more closely it clings to the
surface, so that the removal becomes progressively more
difficult ; at last the film becomes so much stretched
that it ruptures and reunites again over a smaller
surface, hence with a diminished tension. The rupture
naturally takes place where the film is thinnest, on the
top layer of balls, which becomes more or less "dry"
while the lower balls are still surrounded by their
film.
Just in a similar way water will always move in a
soil from a wet to a dryer place, till the film surround-
ing the particles is equally stretched throughout.
For example, if A, B, C (Fig. 5) represent three soil
particles, of which A and B are surrounded by a thin,
and C by a thicker, film of water : when the spheres
are in contact the water will fill up part of the angle
between the spheres, as shown in the diagram. But
the water surface at a is more curved than at 6, i.e.,
it corresponds to the surface at a in the fine capillary
tube (Fig. 3) as compared with the surface at b in the
wider tube. But the diminution of pressure caused by
a is greater than that caused by fr, as shown by the
greater height to which water is raised in the tube ;
hence in the same way the pressure inside the liquid
in.] SURFACE TENSION 75
at a (Fig. 5) will be lower than that at b y and there
will be a flow of water from b to a, until the curva-
tures and corresponding surface tensions are equalised.
In a wet soil, then, surface tension is a force tending
on the one hand to retain a certain amount of water
round the particles, and on the other to equalise the
distribution of water, by causing movement towards any
point where the surface tension has been increased.
For example, if the water in a soil is in equilibrium and
evaporation begins at the surface, the film there is made
FlG. 5. — Diagram illustrating Liquid Film round Soil Particles.
thinner, and the curvature increased in the angles
between the soil particles : hence the pull exerted by
the film is increased, and water is lifted from below
against gravity. Per contra, if rain fall on such a soil
the films round the upper particles are thickened, their
tension is lowered, and the pull of the film below now
acts with gravity in drawing the water down into the
soil.
Percolation,
The state of affairs illustrated by the model of balls
dipped in oil is seen in the case of a soil which has been
thoroughly saturated so that all the pore space is occu-
pied by water, and then allowed to drain until the remain-
ing water is held in the soil by surface tension only.
76
THE TEXTURE OF THE SOIL
[chap.
In the upper layers the film will be stretched to the
utmost, or even broken by the pull of the water below ;
in the lower layers the film will be wholly engaged in
holding the water immediately in contact with the
particles of the layer: these layers may be saturated,
while the upper layers hold an amount dependent on
their distance from the saturated zone, and on the
extent of surface exposed by the particles.
The accompanying diagram (Fig. 6) expresses the
results of an experiment of King's, where columns of
sand and soil, 8 feet and 7 feet long respectively, were
saturated and then allowed to drain till they parted
with no further water, which required a period of sixty
days in the case of the soil columns, and of more
than two years for the sand. The tubes were then
cut up, and the proportion of water in the sand or
soil in successive 3-inch lengths of the tubes was
determined.
It will be seen that the sands retain very little water
by surface tension in the upper layers, whereas the clay
loam, with the enormous area its particles expose, holds
practically the same proportion throughout
v If we also consider the following table, showing the
time taken by the same sands and soils to part with
their water, the difference of the texture of the soils will
be even more evident : —
Inches of Water Lost in
SO min.
31 to 60
min.
24 hrs. (?)
2 to 11
days.
12 to 21
days.
No. 20 Sand .
11 60 „
„ 100 „
Sandy Loam .
Clay Loam
IO«25
5-67
1*21
4.68
4-52
.84
• • •
• ••
2-64
1-96
• » •
5-07
2-II
• ••
•9
•49
. . '
• ; —
^aMjul
bo
30
t \A~d/M ^.O^-Wt
! I ) ! < ' ' \ I ' ! I 'i I I — • • <* ' 'i » i i
FlG. 6. — Water Content of Columns of wetted but thoroughly drained
Sand and Soil.
Hi.] PERCOLATION OF WATER 77
The downward movement of rain water through
soils is known as "percolation," and is distinguished from
" flow " by the fact that the water is supposed to have
free surfaces, so that surface tension comes into play.
It takes place under the action of gravity through the
pore space proper, and also through the cracks, the
worm tracks, the passages left by decayed roots, and
other adventitious openings in the soil. The percola-
tion proceeds until the zone is reached where the pore
space is completely filled ; this is known as the " water
table," and is the level at which water stands in the
wells. Above the water table the soil will be more or
less in the state represented in the diagram showing
sands and soils in which percolation has ceased ; though
there will be most probably a more irregular distri-
bution, with zones which contain an excess of water
travelling downwards with greater or less rapidity,
according to the texture of the soil. It is these
temporarily saturated zones which cause the ordinary
tile drains to run, although situated many feet above
the permanent water table. A soil in which percola-
tion has ceased, though it may still contain much
water, will not part with it to a drain ; the water
cannot break away from the elastic film and run
off down the drain, unless it be present in such an
excess that the surface tension is insufficient to hold
it against gravity. But in a clay soil, percolation is
so slow that the upper few feet of soil may become
saturated by the winter rains and remain so for
months, if percolation has to proceed all the way down
to the water table ; by the introduction of drains, the
percolating column is shortened to the distance between
the surface and the drain. In a coarse-grained sandy
soil percolation is very rapid, the land dries quickly
after rain, and retains a minimum of water by surface
78 THE TEXTURE OF THE SOIL [chap.
tension; in fine-grained soils, which are, however, not
too fine for percolation, the excess of rain will be
removed rapidly enough to keep the soil below the
saturated condition, yet enough water may be retained
to supply the needs of the crop between the intervals of
rain.
While the flow of water from a field-drain may be
taken as a rough measure of the amount of percolation
going on at any given time for that soil, the movement
may be followed more closely by means of a lysimeter
or drain-gauge. As a rule, the records of these instru-
ments are vitiated by the disturbance undergone by the
soil in filling them ; but the drain-gauges at Rothamsted
were constructed by building cemented walls round
blocks of earth in situ, and then gradually introducing a
perforated iron plate below to carry the soil. The
following diagram (Fig. 7) shows the mean monthly
records of rainfall and percolation through a depth of
20 inches and 60 inches respectively, over a period of
thirty-five years.
It will be seen that of the total rainfall a little less
than one-half percolates through 60 inches of the
Rothamsted soil; it should be remembered, however,
that the surface of these gauges is kept free from
weed or any growth. The total drainage through
20 inches of soil is practically the same as that through
60 inches, but rather a greater proportion of the rainfall
comes through the 60-inch gauge in the winter and
through the 20-inch gauge in the summer. In the
winter months the percolation reaches as much as
80 per cent, of the rainfall, in August little more than
20 per cent, of the rainfall finds its way through the
layer of soil. When the ground has become dried
to any depth in the summer, percolation may be much
hindered by the air within the soil and the want of a
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in.] PERCOLATION OF WATER 79
continuous film of wetted surfaces to lead the water
down by surface tension. The top layer of soil becomes
thoroughly wetted and will not allow the air below to
escape ; only after some time are local displacements of
the air set up, which enable the water above to make
connection with the wetted subsoil below, so that
percolation can begin. For this reason summer rains
falling in a season of drought are often noticed to be
of little benefit to the crop, because they are retained
near the surface until wholly evaporated, instead of
increasing the stock of moisture at the lower levels
where the roots of the plant are then operative in
obtaining water.
Minimum Capacity of Soil for Water.
The amount of water retained in a soil by surface
tension alone, when percolation has removed as much
as possible, is rather an important factor to determine,
as upon it depends to some extent the power of the
soil to resist drought by retaining water for the crop
between intervals of rain. If short columns of sand and
soil which have been saturated and then allowed to
drain away into a state of equilibrium are considered,
it is clear that very different proportions of water
are retained by the various soils. Suppose, however,
the column be of such a length that at some level the
upper film of water cannot be further stretched, but
the particles cease to be wet; the layer immediately
below this dry soil contains the minimum amount of
water consistent with a continuous film at all. Soil
in this condition may be regarded as at the minimum
of saturation; it will part with no more water by
drainage, and will become drier only by evaporation.
In order to obtain such a sample of soil for determina-
80 THE TEXTURE OF THE SOIL [chap.
tion of its water content, much trouble would be
necessary, and a long time must elapse before equili-
brium could be obtained in the long tube filled with
wet soil. Practically, however, the same result can
be reached with the apparatus previously described
for estimating the maximum capacity of the soil for
water, by constantly bringing the thin layer of soil
there used into contact with dry soil, until the
previously saturated soil no longer parts with water
to the new soil.
After the determination of maximum water capacity,
as previously described (p. 6y\ a little more fine earth,
which has been standing for some time over water in a
closed space so that it has acquired all the hygroscopic
moisture it can, is shaken lightly over the surface of the
wet soil in the box to the depth of \ inch or so. It
rapidly becomes wet, as will be evident by a change in
colour, whereupon it is struck off by drawing a fine
tightly stretched wire across the top of the box and
shaking the loosened layer off. More fine earth is then
shaken on and struck off as before when wetted, the
operation being repeated again and again, until a thin
dry layer remains on the surface for half an hour or so
without showing by change of colour any absorption of
water. During this wait the box should be in a closed
chamber over water. With fine-grained clays and loams
the process does not take long, with a coarse sand the
water moves slowly into the dry layer, and it is difficult
to hit off the exact end-point, when the soil particles
are still surrounded by water but the surface tension
is too great to allow this water to pass into dry soil.
Finally, the box and its contents are weighed, dried
in the oven, and reweighed. The second weighing
when dry is necessary, because the box will be found
to hold more dry soil than was originally filled into
in.] WATER LIFTED BY SURFACE TENSION 81
it ; a certain amount of consolidation has taken place
through the addition of the dry earth and the subsequent
striking off when wet The numbers in the columns
headed Minimum in the table on p. 6g were obtained
in this way: they show that though the maximum
capacity for water, or pore space, may not vary very
greatly for different soils, there is a much wider and
more important divergence between the amounts of
water they will retain by surface tension alone, this
latter being the important factor in judging of the power
of the soil to retain a reserve of moisture for crops.
Variations in Surface Tension.
The surface tension of water is very high, but it is
easily raised or lowered by the presence of small
amounts of material in solution. The effect of altering
the surface tension of a film at any point is to cause
motion, as is seen in the well-known experiment of
covering a plate with a thin film of coloured water and
dropping a little alcohol into the middle of the film.
The alcohol immediately weakens the surface tension of
the film in the middle to such an extent that all the
liquid runs to the outer edge and leaves the plate bare
in the middle. Most of the salts which are used as
artificial manures and are soluble in water increase the
surface tension of the soil water, hence an application of
salt or nitrate of soda may, by increasing the tension of
the surface film, lift more water from the subsoil and
maintain the top layer of soil in a moister condition.
Per contra^ solutions of organic matter, particularly of the
many organic substances used as manure which have a
little oil in them, extracts of dung, etc., have a surface
tension below that of water. To this fact may be
attributed the "burning" of soils which is sometimes
F
82
THE TEXTURE OF THE SOIL
[chap.
seen when organic manures are applied late in the
season and dry hot weather succeeds j the soil water at
the top in contact with the manure has a lower surface
tension and consequently less lifting power for the
subsoil water ; hence any shallow rooted crop is deprived
of some of the subsoil water which would have otherwise
been lifted to it. A rise of temperature diminishes the
surface tension of water, and therefore lessens the
sustaining power of the film ; as it also lessens the
viscosity of water, it will often cause percolation to
begin afresh from soil that had apparently ceased to
yield any more drainage. This effect may sometimes
be seen in variations in the flow of land drains or in the
level of water in shallow wells. The following table
shows the comparative surface tension of water and
various solutions: —
Nature of Solution.
Density.
Surface Tension.
Water . . •
Common Salt • ,
Kainit • ,
Nitrate of Soda • ,
Dung . . . ,
Superphosphate • «
Soil Extract . . ,
Garden Soil Extract
l«l
I'l
I«I
1-0013
1-0104
I-O
I-O
Dynes per sq. cm.
7*532
7.9H
7.9
7*73
7.464
7.414
7.244
7-089
Cohesion caused by Surface Tension.
In certain cases the stretched film surrounding soil
particles will give them an apparent cohesion by
enclosing them and drawing them together. A handful
of wet sand can be moulded into shape, but falls in
pieces as soon as it is dry : just as in a camel-hair
pencil the bristles, which stand apart when dry or
wholly immersed in water, are drawn together to a
III.] COHESION DUE TO SURFACE TENSION S3
point if the brush be dipped in water and withdrawn.
Or again, a flat sandy beach from which a smooth sea
is receding will often show above tide-mark one stretch
of sand quite dry and loose, in which the feet sink
deeply, and another very soft stretch immediately left by
the tide where the sand grains are completely sur-
rounded by water. Between the two is a stretch of
sand of the same character, but firm to walk upon;
this is partly wet, and there is enough water to form a
film round the grains and hold them in position with a
certain amount of force. That this sand is really just
as loosely arranged as the softer tracts that are either
wetter or drier, may be seen by the fact that it will
easily pack more closely together under repeated gentle
pressure with the foot The shrinkage of soils, especially
of clays, as they dry, may be attributed to the surface
tension of the films surrounding the groups of soil
particles; as the water content is lessened the films
exert more force in their effort to contract, and drag
some of the particles closer together, especially the very
small particles whose weight is trivial compared to the
forces exerted by the film. Clay shrinks more than
other soils because of the greater number of particles,
their small size, and the higher proportion of pore space
into which motion can take place. The tenacity of wet
clay is due to the number of water films that have to be
ruptured, the vastly greater cohesion of dry clay
probably to the fact that many of the particles have
been dragged within the range of one another's
molecular forces. There is a stage in the drying of
clay when it will fall to pieces when worked ; probably
this represents the stage analogous to the partly wet
sand, when cohesion is due to the surface films. The
clay is neither so wet that the particles just slip over
one another when pressure is applied — the pasty
84 THE TEXTURE OF THE SOIL [chap.
condition; nor have they been drawn so closely
together as to cohere without the aid of any water.
It is not quite intelligible why a piece of dried clay
becomes soft and swells again when wetted, nor
why the particles should once more move apart.
Hygroscopic Moisture,
If the withdrawal of water from a soil by evapora-
tion be continued, a point is at last reached when the
soil becomes air-dry : it still retains some water, which
will vary in amount with the degree of humidity of the
atmosphere and the temperature. This last film of
water is held very closely and in a somewhat different
manner from the ordinary film held by surface tension,
though the two shade off into one another. For example,
the film of hygroscopic moisture can be produced by
condensation alone, when perfectly dry soil is placed in
an atmosphere containing water vapour : the surface
of materials like glass, sand, etc., has sufficient attrac-
tion for water to condense it from a state of vapour.
The amount of hygroscopic water retained by different
types of soil when air-dried and then allowed to
stand in a saturated atmosphere at ordinary tempera-
ture, is given in the table below ; it will be seen to
be more or less proportional to the surface possessed
by the soil particles, clay and humus retaining the
most.
This hygroscopic moisture cannot be of any service
to plants : Sachs has shown by experiments in pots that
tobacco plants will begin to wilt before the soil has
parted with all its moisture. When wilting began with
a sandy soil the sand in the pot still contained 1-5 per
cent, of water, with a clay soil 8 per cent, of water ; with
a mixture of sand and humus there was as much as
III.]
HYGROSCOPIC MOISTURE
85
i2«3 per cent of water retained by the soiL Heinrich
has further shown that wilting begins before the water
content of the soil has been reduced to the hygroscopic
water limit, as the following figures demonstrate —
Water per 100 of Dry Soil.
When Plants Wilt.
Hygroscopic Water.
Coarse Sand . • •
Sandy Garden Soil .
Fine Sand, with Humus .
Sandy Loam
Chalky Loam . •
i-5
4*6
6-2
7-8
9-8
49-7
115
3
3-98
5-74
5-2
42-3
This is easily intelligible in view of the fact that the
root hairs cannot be in contact with all the soil particles,
nor can the water move from particle to particle when
/ it has been reduced to so low a proportion. King has
made some observations of the amount of water still
present in soils in the field which had become so dry
that growth was at a standstill and the plants were
wilting.
Depth.
Nature of Soil.
Per cent, of Water
under Clover.
Under Maize.
o" to 6"
6" „ 12"
12" „ 1 8"
1 8" „ 24"
24" „ 3o"
40" „ 43"
Clay Loam
>>
Reddish Clay
n
Sandy Clay
Sand
8.4
8-5
I2»4
13-3
13-5
9-S
7
7-8
11.6
12
io-8
4.2
Under these conditions all the soils were about
equally dry, so far as any power to part with their
water went; if the estimates made previously of the
area of surface of the particles constituting various kinds
86 THE TEXTURE OF THE SOIL [chap.
of soil be combined with these percentages to calculate
the thickness of film of the water on that surface, it will
be found that on all soils the film possesses approxi-
mately the same thickness, about 0-00003 inch.
Because of the quantity of water which some soils
will retain rather than give up to the plant, it is possible
that such soils may have less available water for the
plant than a much coarser grained soil which starts
with a lower initial amount of water. For example, the
clay and sand in the table above contain when satu-
rated about 26 per cent, and 18 per cent of water
respectively; as the crop can reduce this to 12 per cent,
in one case, and 4-2 per cent in the other, both sand
and clay yield about the same amount of water to the
crop.
A good example of the fact that only the water in
the soil which is in excess of the hygroscopic moisture
is available for the crop, is seen in F. J. Alway's studies
of soil moisture conditions in the "Great Plains" region
of north-western America. There the rainfall is only
from 12 to 15 inches annually, and falls chiefly during
the summer months ; because of its insufficiency for the
production of continuous crops, it is customary to take
a bare fallow one season in three in order to accumulate
the rainfall for the benefit of the two succeeding grain
crops.
The table shows the water content of the soil down
to the depth of 6 feet on two fields near Indian Head,
Saskatchewan, taken at the end of July 1904, field B.
having been fallowed, and C. having carried a crop of
oats which had shown the effects of drought Figures
are given for the total water in the soil as sampled, the
hygroscopic moisture as determined in the laboratory,
and the difference, which may be termed the free
water : —
III.]
HYGROSCOPIC MOISTURE
87
Foot Section.
B, after Fallow.
Water.
C, after Oats.
Water.
Total.
Hygro-
scopic.
Free.
Total.
Hygro-
scopic.
Free.
First
Second .
Third .
Fourth .
Fifth .
Sixth .
Mean
29-4
I4.9
1 6-4
17.4
21.8
19*6
I2'0
3-9
4-7
5-S
7-6
8-0
17.4
II-O
11*7
11.9
I4'2
1 1-6
20-0
22 '4
21-6
l6-I
15.I
15.9
12.7
13-2
13-5
4-6
4.2
5-9
7*3
9.2
8-1
II.5
10.9
IO'O
19.9
...
I2»9
18.5
...
9-5
In each field the upper layer of soil possesses a
higher capacity for retaining hygroscopic moisture than
does the lower layer, but in field C. this upper layer is
thicker than in field B. It will be seen from the table
that as regards total water there is no great difference
between the two fields, but when the hygroscopic
moisture is deducted, B. contains 3-4 per cent more
water available for the plant This difference was
manifested in the following season in the yield, which
was only 2160 lbs. of grain and straw on C. and 9200
lbs. on B.
Though it is doubtful if the hygroscopic moisture
gathered by the surface soil in the cooler and damper
periods of the night, can be passed on to the subsoil and
given up to the roots, yet by its evaporation the next
day it may help to keep the temperature of the soil
down, and so indirectly diminish the loss of water to the
soil. Of course in certain conditions of air and soil
temperature there is condensation upon the soil of visible
water which can be available to the crops ; for example,
in some months drain-gauges yield more water than the
rainfall, though a certain amount of loss by evaporation
must also have taken place. This usually happens in
88 THE TEXTURE OF THE SOIL [chap. hi.
the early spring, and can be set down to the conden-
sation of dews by the thoroughly chilled ground from
a warm and moist atmosphere. Warington has sug-
gested that the persistent wetness of the soil in February
must be attributed to this cause. In a coarse-grained
soil mostly filled with air, the cooling of the surface that
comes by radiation at night may result in an upward
distillation of water from the wetter and warmer subsoil.
Hilgard has suggested this explanation to account for
the capacity of some Californian soils to maintain a
crop during a rainless winter, when the soil itself shows
only 3 per cent or so of water.
A. Mitscherlich has made a number of determina-
tions of the heat that is evolved on moistening dry soil
(benetzungs-warme), due to the condensation of the
hygroscopic water on the surface of the soil particles,
and regards the figure thus obtained as of great
significance in judging of the physical properties of a
soil, since it provides a measure of the total surface of
all the particles composing the soil. He obtained results
of the following order, in calories evolved per gram of
soil — sand ooi, calcium carbonate 0-38, sandy soil 0-79,
sandy loam 2-37, strong clay 14-98, peat 22-66. Un-
fortunately the determination is by no means an easy
one to make, and no sufficient number of results have
been obtained for soils of known behaviour in the field
to enable one to form a judgment of the value of the
method.
\
CHAPTER IV
TILLAGE AND THE MOVEMENTS OF SOIL WATER s
Water required for the Growth of Crops — The Effect of Drainage
— Effects of Autumn and Spring Cultivation, Hoeing and
Mulching, Rolling, upon the Water Content of the Soil —
The Drying Effect of Crops — Bare Fallows — Effect of Dung
on the Retention of Water by the Soil.
The amount of water transpired by various plants
during their growth has been investigated by Lawes
and Gilbert at Rothamsted, by Hellriegel in Germany,
and Wollny in Munich, and by King in America. The
general principle upon which these observers have
worked, has been to grow the plants in pots and
measure the amount of water consumed during growth,
care being taken to eliminate or allow for losses by
evaporation from the bare ground, and also to render
the conditions of the plant's life as similar to those of
the open field as possible. Finally, the plant is washed
free of soil, dried, and weighed, so that a ratio is
obtained between the dry matter produced and the
water consumed during growth.
Some of the numbers obtained are given in the table
below : it will be seen that the same plant gives very
different results with the different observers.
89
90 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
Lawes and
Gilbert.
Hellriegel.
Wollny.
King.
Wheat .
22;
359
Barley . • •
262
310
393
774
Oats
...
402
557
665
Red Clover . •
249
330
453
...
Peas • • •
235
292
477
447
The divergencies in these results are intelligible, if
we consider that the " transpiration " process by which
the water is lost, and the "assimilation" process by
which the plant gets heavier, have no necessary con-
nection, though both become active under the same
stimuli of light and warmth. Some leaves transpire
rapidly as a means of maintaining a low temperature
whilst absorbing large amounts of radiant energy from
the sun; other plants which have to resist drought
reduce the transpiration by a thickened cuticle, or by
a more concentrated cell sap. Dr H. Brown has shown
that of the radiant energy falling upon a sunflower leaf
on a bright August noonday, about 95 per cent, was
consumed in evaporating the transpiration water; of
the energy falling upon the same leaf in bright diffuse
daylight, only 28 per cent was used up in evaporation.
Comparing in these two cases the water transpired with
the carbohydrate produced (and this will be about -£$ of
the total dry matter) we find in the sunlight the ratio
was 347 to 1, in the diffuse daylight 234 to 1. Further
investigations are desirable ; but, taking the whole
group of observations, we shall be justified in assuming
that our ordinary field crops transpire about 300 lbs.
of water for each lb. of dry matter produced. It now
remains to translate this approximate figure into tons
of water per acre required to grow the ordinary crops.
The following table shows the weight at harvest of a
IV.]
WATER REQUIRED FOR GROWTH
91
fair yield of the crop in question, the percentage of
water contained in the crop, the weight of dry matter
produced per acre, then the water transpired as deduced
from the dry matter produced, and in the last column
this same amount of transpired water recalculated as
inches of rain.
Weight
Per cent.
Weight of
Calculated Water
Crop.
at
of
Dry Matter
transpired
Harvest.
Water.
at Harvest.
during Growth.
Tons
Tons
Tons
Inches of
per acre.
per acre.
per acre.
Rain.
Wheat
2'5
18
2*05
615
6 '09
Barley . •
2
17
1-66
498
4-93
Oats
2-5
16
2»IO
630
6*24
Meadow Hay .
i : 5
16
I «26
378
3«74
Clover Hay
20
16
1.68
504
(?)
Swedes
17
88
204
612
606
Mangolds
30
88
3-6o
1080
10*69
Potatoes • .
7-5
75
1.87
561
5-55
Beans • .
2
17
1.66
498
4.94
It will be seen that in all cases the amount of water
transpired by the crop is a notable fraction of the total
annual rainfall, particularly so in the case of a root crop
like mangolds, which in the south and east of England
will often require a full half of the total rain falling
within the year. As much of the rainfall runs straight
off the surface into the ditches, and another portion
is lost to the land by percolation into the springs, as
again a considerable fraction is evaporated at certain
seasons from the bare surface of the soil, it is evident
that the water supply, even in our humid climate, is
far from sufficient for the maximum of production, and
may easily fall below that which is required for an
average crop. Indeed, we may take it as a truism that
the yield is more often determined by the water avail-
able than by lack of the other essentials of growth —
92 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
light and heat, manure, etc. Of this we can have no
better proof than the enormous crops grown by irriga-
tion on sewage farms. Where the conditions are
favourable, and the farm is situated on a free draining
sandy or gravelly soil, so that the water can be often
renewed and drained away to keep the soil supplied
with air as well as water, the production of grass,
cabbages, and other green crops is multiplied five or
even tenfold by the unlimited supply of water. Speak-
ing generally, over a great part of England, where the
annual rainfall is from 35 to 25 inches, a large proportion
of which falls in the non-growing season, it is necessary
to husband the water supply, and it will be found that
one at least of the objects of many of our usual tillage
operations is the conservation of the moisture in the
ground for the service of the crop. From this point
of view, the various operations dealing with the land
can now be considered, such as drainage, ploughing,
hoeing, rolling, and other cultivations.
The Effect of Drainage.
Drainage is usually regarded as a means of freeing
the land from an excess of water, but it also has an
important effect in rendering a higher proportion of the
annual rainfall available for the crop, so that drained
land will suffer less from drought than the same land
in an undrained condition.
Land may require drainage for various reasons : it
may possess a naturally pervious subsoil, and yet be
water-logged owing to its situation, or the subsoil may
be so close in texture that percolation is reduced to a
minimum and the surface soil remains for long periods
almost saturated with water, especially if the slope is
gentle and water lies after rain until very large amounts
IV.] DRAINAGE 93
soak in. The flat meadows adjoining a river are
often water-logged because their surface is little higher
than the water in the river and the general water table
in the adjoining soil. In these cases tile drains are of
no value because of the want of fall ; open cuts and
ditches draw off the water best, and by exposing some
of the subsoil water both to aeration and evaporation,
lead to the improvement of the land. Another cause of
swampy water-logged land is the rising to the surface
of a spring or a line of soakage, such as is always
formed at the junction of a clay or other stiff soil
with an overlying pervious formation, " when the sand
feeds the clay," as the old rhyme runs. Such wet spots
can be drained by tiles or by an open ditch cutting the
springs or the line of soakage. Land lying on an
impervious subsoil at the foot of a slope is often very
wet because the water which has accumulated in the hill
and soaked downwards is forced to the surface by the
hydraulic pressure of the water above ; such seepage
water rising to the surface from the subsoil is character-
istic of many valley soils, and can best be dealt with by
a system of tile drains. But tile drains are most
generally employed and are of greatest value in dealing
with stiff impervious subsoils, which cannot get rid of
the rain falling upon them ; indeed, one of the prime
improvements effected in English agriculture was the
drainage of something like 3,000,000 acres of heavy
land between the years 1840-70. A great portion of
the work was unfortunately of little avail, because at
first there was a tendency to set the drains too deep,
at 4 feet instead of the 2 to 3 feet which have been
found to answer best. The benefits conferred by
drainage depend upon the lowering of the permanent
water table to the depth at which the drains are laid,
so that instead of constantly stagnant water a movement
94 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
of both water and air is established in the soil above
the drain. In the first place, the introduction of the
air which follows the water drawn off by the drains
brings the whole depth of soil into activity, whereas
previously only the portion not water-logged was avail-
able. Plant roots cannot grow without oxygen from
the air, hence in a water-logged soil the roots
are confined to the surface layer only ; after drainage
the roots can penetrate as far as the air extends.
At the same time, all the fundamental chemical and
biological processes of the soil, such as nitrification and
weathering, are brought into action by the introduction
of the oxygen upon which they depend. Later it will
be seen that a water-logged soil results in the loss of
nitrogen to the land when such manures as nitrate of
soda are applied to it It is the extended root range of
the crop resulting from the introduction of air by
drainage which enables the drained land to resist a
drought better than before. In an undrained soil
the roots are confined to a shallow layer, which
they soon deprive of all moisture; further supplies
of water from the saturated soil move upwards very
slowly in a clay soil, so that the plant may suffer
greatly. In a drained soil, on the contrary, the roots
traverse the whole 3 feet or so into which air has been
admitted ; this mass of soil, even after it has given up by
percolation all the water it can, will still hold much more
than is contained in the shallow layer alone traversed by
roots before drainage. Following upon drainage, a slow
improvement in the texture of clay soil is always
manifest: by the drawing of air into the soil, by the
consequent evaporation and drying, a certain amount of
shrinkage and a clotting of the fine clay particles result,
which is never entirely undone when they are wetted
again. Roots, which afterwards decay and leave holes,
iv.] DRAINAGE 95
and deep worm tracks, are all brought into the soil by its
aeration, and result in more rapid percolation. Again,
the washing through the soil of soluble salts derived
from the surface, especially the bicarbonate of lime
which is so characteristic a constituent of drainage
water, also induces flocculation of the fine clay particles.
Lastly, there is a steady removal by the drains of the
finest clay stuff, for whenever tile drains are running
freely the water will be found slightly turbid with clay
matter. All these causes contribute to establish a better
texture in the drained soil, beginning at the tiles and
spreading slowly outwards. The other result of drainage
which may be noted here is the greatly increased warmth
and earliness of a drained soil ; the high specific heat of
water, and the cooling produced by evaporation when
the water table is near the surface, combine to hinder
a water-logged soil from warming up under the sun's
heat in the spring, so that undrained land is notoriously
cold and late.
Effect of Autumn Cultivation upon the Water Content
of the Soil.
In regions where the annual rainfall is not very high
and occurs chiefly during the early winter months, it is
important to get as much of it as possible into the soil
for the use of the subsequent crop. Breaking up the
stubbles after harvest is an important factor in catching
the winter rain ; all land which is to lie idle through the
winter, previous to the sowing of roots or spring corn
should be early turned over with the plough and left
rough through the rainy season. On the old stubble
which has been made solid by the weather and the
trampling during harvest, the rain lies for some time and
evaporates, and if the land be at all on a slope the water
shoots off into the ditches. But the broken surface of a
96 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
ploughed field both hinders the flow of the water and
affords it many openings by which to sink in ; at the
same time the increase of pore space in the loose
ploughed layer enables this portion to absorb more
water before percolation begins. King has observed in
May a difference of 2-3 per cent, of water in the top 3
feet of soil between land ploughed in the autumn and
the adjoining land not ploughed ; the gain in this case
due to the ploughing was no tons of water per acre or
rather more than 1 inch of rain.
The following table shows the effect of ploughing up
a stubble in autumn on a thin chalky loam at Wye,
Kent, where the soil is only about 2 feet deep. The
samples were taken on 3rd March 1902 ; there had
been but little rainfall except in the previous December.
The figures show mean percentages of water in the wet
soil exclusive of stones.
Land Ploughed
in Autumn.
Adjoining Land
not Ploughed.
1st foot
2nd foot
16.45
15-8
16
I4'6
Of course the autumn ploughing has many other
beneficial effects in addition to the above-mentioned
gain of water ; the ploughed soil gets alternately frozen
and thawed, wetted and dried, with the result that on the
stiff lands the puddling effects of trampling, etc., are
obliterated, and the soil acquires a loose, open texture,
out of which a seed bed can be made. Again, the
additional surface which is exposed to the action of frost
and rain causes increased weathering, and some of the
dormant mineral plant food is brought into a more
available condition.
IV.] CULTIVATION AND SOIL WATER 97
Spring Cultivation,
In such climates as prevail in parts of England,
where it is necessary to retain as much of the winter's
rainfall in the land as possible, and where spells of
drying weather are apt to set in with the spring, it is
desirable to cross plough or otherwise move any land
that is destined for a summer crop at as early a date
as it will bear cultivation. This spring working is
necessary for two reasons : to obtain a mulch, or layer
of loose soil, which will conserve moisture in the sub-
soil during the dry periods that follow, and to give the
surface soil an opportunity of drying gradually into a
condition that will yield a good tilth. The land, even
though ploughed in the autumn, will become consoli-
dated again to a considerable degree by the beating
rains of winter. In this closely packed material capil-
lary water can move freely, and as the surface layer
dries under the action of the sun and wind, fresh supplies
of water are lifted from the subsoil by surface tension,
with the result that there is a steady and continuous
drain of subsoil water through its connection with the
exposed and rapidly evaporating surface. But if the
top layer of soil is broken up and left loose upon the
land by the cultivator, there is no longer a continuous
film joining the exposed surface and the subsoil water ;
surface tension can only lift water as far as the film is
unbroken, i.e., as far as the unstirred soil extends, and
this layer is protected from evaporation by the loose
soil above. Regarding it from another point of view —
in the undisturbed land there exist fine passages and
capillary spaces extending from the surface down to the
subsoil ; up these passages water will rise as long as it is
withdrawn by evaporation at the top ; in consequence,
the surface soil is not allowed to dry, being fed with
G
98 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
subsoil water which is constantly withdrawn from below.
But when the land is cultivated the capillary channels
are broken, water cannot rise into the loose layer of
surface soil, which in the main is separated from the
firm soil below by large spaces across which water
cannot rise; hence the surface soil can become dry,
because it is cut off from the subsoil water, which in its
turn is preserved for use later. The drying of the
surface soil which ensues, through its severance from
the water-yielding subsoil, is of the greatest possible
importance in obtaining a tilth. At a certain stage
the soil can be dragged and will fall in pieces, but if
it be not detached from the subsoil it will either remain
persistently wet, so that it cannot be harrowed down,
or if it be forced to dry under the action of wind and
sun, it will set very hard and " steely," should it contain
any admixture of clay. The sudden forced drying of
strong land always produces hard and intractable clods,
which may defy all the efforts of the cultivator during
the rest of the season, unless a fortunate succession of
weather enable him to begin to make his tilth over
again.
It may be thought that the amount of water lifted
by surface tension cannot be so large as to result in
any serious losses to the subsoil store, but in soils of
suitable texture enough can certainly be raised to keep
the crop alive during periods of drought. In some of
King's experiments with a cylinder full of very fine
sand, he found that the evaporating surface lost daily
an amount of water equal to 0-46 inch if the per-
manent water level were 1 foot below, 0-405 if the
water had to be lifted 2 feet, and 0-18 inch if the
water had to rise 4 feet to the evaporating surface.
When the sand was replaced by a clay loam, the lift
of water to the surface was somewhat less, but in all
IV.] WATER LIFTED BY SURFACE TENSION 99
cases the amounts were probably less than would be
realised under field conditions, because the evapora-
tion was not enough to dry the surface, and was
further checked by the formation of a saline crust on
the surface. Working in the field, King obtained a
daily loss at the evaporating surface of 1-3 lb. per
square foot, or 0-19 inch of water, the water table
being from 4 to 5 feet below the surface.
The relative powers of different soils to lift water by
capillarity alone is well seen during any long summer
drought, such as prevailed in the south of England
during 1899 and 1900. In the Thames valley, fields of
swedes grew till the roots were one or two inches in
diameter, and then died outright, although the water
table was not more than 16 or 20 feet below; yet the
coarse-grained gravel of which the subsoil was composed
could not lift the water in any appreciable quantity to
the surface. In the same seasons the crops upon the
chalk hills were quietly growing ; though the water table
was as much as 200 feet below the surface, there was
still a steady capillary rise of water through the fine-
grained chalk. In a drought it is always the gravels
and coarse sands which suffer first, and this not because
they start with less water, for we have already seen
that what they absorb they can give up almost wholly
to the plant, whereas a clay, which absorbs much more,
can only hand over about the same proportion to the
plant as the sand did, so much being held as hygroscopic
moisture. The plant suffers because the small surface
of the soil particles gives the coarse-grained sand or
gravel a very limited power of lifting the subsoil water
to the roots of the plant. Should a drought continue,
the clay soils begin to suffer next, for though they start
with large supplies of water and have an extensive sur-
face of soil particles, yet water can be moved so slowly
loo TILLAGE— MOVEMENTS OF SOIL WATER [chap.
through the very fine pore spaces that the upward lift
cannot keep pace with the loss by transpiration and
evaporation. The soils which are least affected by
drought are the deep loamy sands of very uniform tex-
ture, fine-grained enough to possess a considerable lift-
ing surface, and yet not too fine to interfere with the
free movement of soil water. The western soils which
the American writers describe as capable of growing
wheat with a winter rainfall of 10 to 12 inches and an un-
broken summer drought of three months' duration, are
deep, fine-grained, and uniform, with practically no
particles of the clay order of magnitude to check the
upward lift by capillarity.
The following table illustrates how the subsoil acts
as a regulator to the amount of water contained in the
surface layer, absorbing the water which descends by
percolation during rainy periods, and giving it up again
by capillarity to the surface soil during periods of
drought The first line shows the rainfall during the
periods indicated, the second line the amount of
evaporation during the same period, while the third line
shows the changes in the water content of the top foot
of soil. As this change is not represented by the
difference between the rainfall and the evaporation, it
is clear that water must have been in some cases passed
down to the subsoil, in others lifted from it, in quantities
shown by the last set of figures.
30/iv.
80/v.
9/vii.
7/ix.
Water in inches.
to
to
to
to
30/v.
9/vii.
7/ix.
27/x.
Rainfall • • • • .
0-18
4-53
3-17
5-63
Evaporation .....
3-45
2^96
5-71
1-83
Gain or Loss of Water in top foot
-10
+ 1-4
-0*24
+ 0'6i
Water furnished by ( - ), or passed
on to ( + ) Subsoil . .
-2.27
+ 0.17
-20
+ 3-21
IV.]
CULTIVATION AND S'6/L WATER
101
During the first period, the month of May, a dry
spell prevailed, only 018 inch of rain fell, while the
evaporation amounted to 3-45 inches; despite this
loss the top foot of soil only contains 1 inch of water
less than at the beginning, so that the rest of the
excess of evaporation over rainfall must have come
from the subsoil, which had in fact to furnish 2-27
inches. In the second period more water fell as rain
than was evaporated ; the surface soil gained 1-4 inch,
which did not account for all the excess of rain over
evaporation, a further -17 inch must have descended
into the subsoil.
The following figures, obtained by King, illustrate
how a spring ploughing preserves the soil moisture
during a period of dry weather, by establishing a
loose protecting layer over the water bearing subsoil.
The upper line shows the water content of the top
4 feet of a certain piece of land on 29th April, on
which date part of it was ploughed and part left
untouched. On 6th May, no rain having fallen,
the soil was sampled again, both on the ploughed
and the unploughed piece, with the results set out
in the lower figures : —
Lbs. of Water in each successive cubic foot.
1st.
2nd.
3rd.
4th.
Land on 29th April ....
Land on 6th May, ploughed 29th April .
Land on 6th May, not ploughed
14-1
13-9
io-6
20-1
207
18
18
18-3
17-3
16.6
16
13-9
It is seen that the ploughed land practically lost no
water during the week ending 6th May, whereas
during the same period the land not ploughed lost
9* i lbs per square foot of surface, a quantity equivalent
to if inch of rain.
102 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
A similar trial made on a light loam at Wye during
a dry period in the spring of 1902, gave the following
percentages of water in the wet soil.
Land Ploughed
Autumn and Spring.
Ploughed Autumn
only.,
1st foot
2nd foot
16.7
154
15-9
13*9
There can be little doubt that the earlier land
which is intended for spring corn, or particularly for
roots, can be moved in the spring, the more water
will be saved for the use of the subsequent crop, and
the easier will a good tilth be established. The chief
danger lies on the very fine sandy soils which, when
in a loose condition, are apt to run together under
heavy rains and afterwards cake on drying.
Hoeing and Mulches.
The principles which have already been developed
to explain the effect of an early spring ploughing in
saving subsoil water, apply even more markedly to
all the later spring and summer cultivations, hoeing
and the like, which have for their object the mainten-
ance of a loose tilth upon the surface. The loose soil
becomes itself dry, but by reason of its discontinuity
and coarse-grained condition, does not conduct the
moisture from the firm subsoil to the surface exposed
to sun and wind. Under these conditions the only
loss will be of that water which evaporates from the
moist soil into the air spaces of the loose upper layer
and then diffuses into the atmosphere ; the deeper the
loose layer thus formed, the more effective will it be,
and if it is destroyed by a fall of rain, which consolidates
V
IV.] CULTIVATION AND SOIL WATER 103
the ground and establishes a continuous liquid film
from the subsoil water right up to the surface, it should
be renewed by a fresh cultivation as soon as the land
will admit of working. It is often noticed that a casual
shower during a dry period, or watering a garden unless
the operation is done very thoroughly, may result in a
greater drying up of the soil than ever, just because
a film of water is created able to lift water from the
subsoil up to the evaporating surface. The loose hoed
ground practically forms a mulch, though the protect-
ing material is the soil itself instead of straw or kindred
substances.
Of course, the conservation of soil moisture is
not the only good effect brought about by the surface
cultivation during the summer: the aeration of the
soil, the mechanical distribution of the nitrifying
bacteria that is effected, the warmth of the surface
layers due to their dryness, all combine to render
nitrification active, and to bring into a form available
for the plant the reserves of nitrogen in the humus of
the soil. This point will be dealt with more at length
later : for the time, it will be sufficient to remind the
reader how a turnip crop with its frequent spring and
summer cultivations is almost independent of any
nitrogenous manure, though it removes something like
100 lbs. of nitrogen per acre : whereas a wheat crop,
removing less than half that quantity of nitrogen per
acre, often requires the application of a nitrogenous
manure, because it is grown on undisturbed soil in
the cooler season of the year.
The saving of soil moisture which can be effected by
hoeing is illustrated by one of King's experiments,
when, during a dry period, the soil on a piece of land
kept cultivated to a depth of 3 inches was sampled from
time to time down to a depth of 6 feet, samples being
104 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
taken simultaneously from an adjacent piece of land
where the surface was kept smooth and firm. On the
cultivated land there was a daily loss equivalent to
14 \ tons of water per acre, which was increased on the
uncultivated land to 17-6 tons per acre; the difference
during the 49 days over which the trial was spread,
amounting to 1-7 inch of rain saved by the cultivation.
The value of surface cultivation is well seen in other
trials of King's, where the water content down to a
depth of 4 feet was compared on two adjacent pieces of
land, one stirred to the depth of 3 and the other to 1 \
inches only. The 3-inch soil mulch, taking the whole
season through, preserved more soil moisture than the
shallower cultivation, but by keeping the soil immedi-
ately below the mulch more moist and therefore with a
better developed water film, it also enabled this layer to
lift more moisture from the 3 or 4 foot depth into the
top or second foot, a position more available for the
crop. Thus the average of three determinations of
water content on 16th July gave the following results —
Per cent, of Water.
1st foot.
2nd foot.
3rd foot.
4th foot.
Soil cultivated, 3" deep
„ 1 J" deep .
12.3
II-2
l8-6
17.6
1 6-8
17-8
14*6
16.2
On this occasion it is seen that the upper 2 feet of
soil are being kept moister by their greater power of
lifting water from the lower layers, which actually con-
tain more water under the ij-inch mulch than under
the 3-inch mulch.
Although the gardener uses the hoe freely to estab-
lish soil mulches, he also employs dung, grass-clippings,
and even straw to the same end, anything to break the
IV.] MULCHES 105
connection between the water-bearing subsoil and the
exposed evaporating surface. Such mulches of loose
organic material are even more effective in conserving
soil moisture than a fine tilth, there is less tendency to
form any continuity of water film between subsoil and
mulch; moreover, the evaporation of the water they
themselves contain helps to keep the temperature down.
The great drawback to their employment is that they
prevent the continual stirring of the ground which
promotes aeration and nitrification.
Stones serve almost the same purpose as a mulch,
especially when they are impermeable, like flints, and
cover the surface at all thickly. They shield the land
below from evaporation ; indeed, on picking a flint off an
arable field the ground below will generally be found
cool and damp. The vineyards of the Rhine, etc., are
generally set on steep slopes very thoroughly drained
and exposed to the sun; it will be noticed that the
utmost care is taken to keep the surface of the soil
covered with the broken slaty rock.
Effect of Rolling.
Though it has been pointed out that maintaining a
loose tilth on the surface is the most effective means
possessed by the farmer of saving the soil water and
minimising losses by evaporation, yet one of the funda-
mental acts of husbandry in the spring consists in roll-
ing and otherwise consolidating the land. Particularly
is this the case on the chalk and similar light soils ; when-
ever a spell of dry weather prevails in the early part of
the year the farmer will be observed rolling his seeds,
or his spring corn, or his newly sown turnip land, as
the case may be ; he will even take a heavy cart
wheel down between the drills when the roller will
not give him pressure enough. The result of the
106 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
consolidation of the surface soil thus effected is to
improve its power of lifting the soil water from below
by capillarity, because the pore space is diminished
and the wide intervals across which the water film
cannot exist are largely closed up ; just as the motion
of water through surface tension almost ceases in a
thoroughly loose soil, it is, per contra, increased when
the particles are brought more closely together.
Hence, on the rolled land there will be a greater lift
to the evaporating surface and subsequent loss of
water, but the farmer faces this loss in order to keep
the upper few inches of soil supplied with moisture.
Rolling is only done on land occupied by germinating
seeds, young spring corn, or a young ley, where the
roots, if any, are so close to the surface that the whole
crop will perish if the top layer is allowed to dry. The
effect of rolling is to increase the capillary lifting power
of the top soil, so maintaining it in a moister condition,
although the land as a whole is made dryer by the extra
evaporation which must accompany the rise of subsoil
water to the surface. It is a maxim in farming on the
chalk, where there is always a store of subsoil water at
some depth or other, and where also the surface soil is
peculiarly liable to become open in texture through the
action of worms and the rapid decay of dung, that the
land will become moist if it can only be got "tight"
enough. On any light cultivated land it is easy to
notice how much moister the soil remains when it has
been consolidated by a foot mark ; a gardener again,
whose rich and deeply-worked soil is apt to get very
open, always treads the ground as solid as possible in
preparing a seed bed for onions and other small seeds.
The following figures given by King as mean values
from a number of measurements show how rolling dries
the soil as a whole when samples are taken down to 2
iv.] ROLLING 107
feet or more, but maintains the surface soil, sampled
only down to 1 8 inches, in a moister condition.
Depth of Sample.
Down to 18" .
Down to 24" .
Down to 36"- 54"
Percentage of Water.
Boiled Ground.
15-85
19.49
1872
Unrolled Ground.
15.64
19-85
19-43
Since rolling dries the soil as a whole, it is only desirable
when shallow-rooted crops must be kept supplied with
water at any cost ; as soon as they get their roots down
hoeing should begin to diminish the inevitable evapora-
tion from the firm surface. Thus a tool like the old
broadsharing plough, still used on the chalk, is particu-
larly valuable in preparing a tilth for roots, for, while
creating a loose surface tilth, it is consolidating the soil
below and increasing its power of lifting water from the
subsoil.
Similarly, in the semi-arid regions of Western
America and in Australia, where the rainfall is barely
sufficient for the needs of the crop, in the preparation of
the land great importance is laid on the two operations
of "subsoil packing," and "the establishment of a soil
mulch." This is the equivalent of the English practice
of preparing a seed bed for roots ; frequent cultivation
without inverting the soil to work it down to a fine
tilth, constant use of the ring roller or subsoil packer to
consolidate this crumb until it will lift water by capillarity,
and finally the production and continual renewal by
means of light cultivators or horse hoes of a very thin
skin of loose soil on the surface.
Valuable as the operation of rolling is on grass
land in the early spring, in order to consolidate the soil
108 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
round the roots of the grass after the surface has been
lifted by the winter frosts and by the action of worms,
it should be borne in mind that it is easy to do harm
by injudicious rolling in wet weather on soils that are
at all heavy. Even on grass land the clay may become
so puddled or tempered that it dries round the roots
with a very harsh caked surface, little permeable to
air and water. This sort of damage is perhaps most
often seen on lawns and cricket grounds which are
often rolled repeatedly with heavy rollers when the
ground is thoroughly wet; a smooth, pasty surface is
produced to the ultimate great detriment of the growth
of the grass. Of course upon arable land the greatest
care must be taken never to roll when the top is at all
wet or even damp, lest a pasty surface be developed,
which will dry to a glazed baked crust. It is necessary
even to wait until the dew has been dissipated before
rolling strong land that has been well worked and
drilled for roots.
The Drying Effect of Crops.
Since a crop transpires about 300 lbs. of water for
each pound of dry matter produced, any land which
is carrying a heavy crop must contain much less water
than the adjoining uncropped land, unless there has
been such an excess of rainfall as to saturate the soil
in either case. Any summer growing crop, however,
especially one of roots, transpires so large a proportion
of the customary rainfall during the period of growth,
that it must leave the soil much drier for its growth.
As an example of this removal of water by the growing
crop, the following figures obtained at Rothamsted
during the very dry summer of 1870 may be quoted,
showing as they do the water present in successive 9
IV.]
WATER EVAPORATED BY CROPS
109
inches of fallow and of adjoining land carrying a barley
crop —
Percentages of Water in fine Soil,
June 27-28, 1870.
First 9"
Second 9" .
Third 9" .
Fourth 9" .
Fifth 9"
Sixth 9" .
FalloNjy.
20-36
29-53
34-84
34-32
3I-3I
33-55
Barley.
II* 91
19.32
22.83
2509
26.98
26.38
The total difference between the cropped and un-
cropped land down to the depth of 54 inches, amounted
to more than 900 tons of water per acre, or 9 inches
of rain, which is quite half as much again as would
be accounted for by the crop on the assumption that
only two tons or so of dry matter had been grown at
the date of sampling.
Another example of the withdrawal of water from
the soil by the crop is seen in the proportions of water
in the soil of certain of the permanent grass plots at
Rothamsted, taken in July of the same year, 1870 —
Plot 8.
Plot 9.
Plot 14.
No Manure.
Mineral Manure
+
Ammonium Salts.
Mineral Manure
+
Nitrate of Soda.
Crop, 1870 .
5$ cwt. of Hay.
20J
M|
Per cent, of Water.
First 9"
Second 9" • . .
Third 9" .
Fourth 9" . .
Fifth 9" .
Sixth 9" .
10-83
13-34
19.23
22.71
24-28
25.07
13-00
10.18
16-46
18-96
20-54
21.34
12-16
ii-8o
15-65
16.30
17.18
18.06
Means . • •
19.24
16-75
15.19
no TILLAGE— MOVEMENTS OF SOIL WATER [chap.
Down to the depth of 54 inches the plot receiving
minerals and ammonium salts contained 200 tons, and the
plot receiving minerals and nitrate 325 tons, less water
than the unmanured plot, quantities in this case some-
what less than would be indicated by the amount of
dry matter produced.
There are two important cases in which the drying
effect of vegetation needs to be taken into account, in
the use of catch crops and in the planting of fruit trees.
On the lighter lands of the south of England catch
crops are not uncommonly taken on the land before
roots. The stubbles are quickly broken up, and
vetches, trifolium, or rye, are sown in time to make a
start while the land is warm, and to be either cut green
or fed off before the land is wanted for turnips in the
following spring. The advantages of the practice are
that the summer-formed nitrates in the stubble-ground
are saved from washing out, and that a valuable bite of
early fodder is obtained : with the leguminous crops also,
the farm is enriched by the nitrogen gathered from the
atmosphere. The difficulty of getting catch crops lies
in the fact that the stubble ground is left very dry by
the preceding crop, so that a timely rainfall is needed to
obtain a plant. The danger of their use is that they
may so deplete the available soil water as to give the
succeeding crop of roots a very poor chance of germin-
ating or growing well. In America the practice has been
suggested of sowing some leguminous crop like clover
in the tillage orchards about the end of July, so that the
new surface crop should so dry the ground as to forward
the ripening of the apples on the trees; again, any
second growth of the trees due to a late summer rainfall
would be prevented, this moisture being dealt with by
the catch crop.
The second illustration worthy of notice is that fruit
iv.] PLANTING FRUIT TREES IN GRASS LAND ill
trees when newly planted in grass land often make a
very poor growth for a year or two. This is because a
fruit tree when planted is but indifferently supplied with
water-collecting roots ; inevitably they are few in number
and have a very restricted range. Hence they must be
in a soil well supplied with moisture if they are to provide
the tree with the necessary water, and they are very ill
fitted to compete with a crowd of fibrous grass roots
surrounding them, should the season turn out dry. In
one experiment the moisture in the top foot of a
pasture was found to be only half that present in the top
foot of neighbouring uncropped land.
The following table shows the percentages of water
in the fine earth of an orchard on heavy soil, part of
which was under grass and part kept tilled ; it will be
seen that in the winter the grass land carries as much
or even more water than the bare soil, but towards the
end of the summer the drying effect of the grass becomes
very pronounced, even down to the third foot
1st 9 Inches.
2nd 9 Inches.
3rd 9 Inches.
Grass.
Bare.
Grass.
Bare.
Grass.
Bare.
December 19/05
March 3/06
May 24/06
July 25/06
September 27/06
April 6/07.
October 9/07 .
250
26.7
17-7
13-8
I2«6
2I-I
22<6
24.7
23-3
24.0
15-6
15-7
21.6
27.1
23-5
21-2
I87
13-8
21.
237
23-5
21.9
24-5
18-2
24.0
••*
25-0
20'6
22.3
148
15-6
19*6
23.6
27.O
19-6
25-0
24.4
l8-8
21.3
25-0
Few crops so effectually dry the surface soil as grass
does, because of the intimate way in which its roots
traverse the soil ; hence a fruit tree cannot compete
with grass for water as long as the two sets of roots are
confined to the same layer. The experiments at the
H2 TILLAGE— MOVEMENTS OP SOIL WATER [chap.
Woburn Fruit Farm of planting fruit trees and sowing
the seed of coarse meadow grasses at the same time,
show this competition at its highest degree, but even
when trees are planted in old pasture care should be
taken to keep a ring round the tree free from grass
and well cultivated or mulched for at least two years.
For similar reasons, when trees are planted in arable
land weeds should be kept down, nor should crops like
cabbages or mangolds be grown between the rows
of trees ; such crops are usually considered to " draw
the land " and deplete it of plant food, but the harm
they do lies in the water they withdraw just at the most
critical season, when the tree is making its first start in
its new quarters.
Bare Falloivs.
The custom of fallowing land, of leaving it entirely
bare for a season, during which the land is worked as
often as possible, is one of the oldest in agriculture ; a
rotation of wheat, wheat, fallow, .or of beans, wheat,
fallow, being almost universal, until the introduction! of
turnips gave the farmer a chance of cleaning his land
and yet growing a crop at the same time. The objects
of a fallow were various : in the first place, the summer
cultivations resulted in a thorough cleaning of the land
and in a free development of nitrates for the succeeding
crop ; also on the heavy soils, which are the most suited
to fallowing, a good tilth was obtained that was often
impossible otherwise. Indeed, at the present day it is
found desirable and even necessary to introduce an
occasional bare fallow when farming on the heavy clays
of the south and east of England, in order to obtain a
satisfactory tilth in that dry climate.
One of the most notable effects of fallowing lies in
the production of a stock of nitrates from the stores of
IV.] STORAGE OF WATER BY BARE FALLOWS 113
combined nitrogen in the soil ; these nitrates are at once
available for the ensuing wheat crop if the autumnal
rains are not too great to wash them out of the soil
(see p. 116).
But in addition to the gain in available nitrogen due
to fallowing, the land which does not carry a crop during
a season will accumulate a store of water which may be
of the utmost service to the succeeding crop. In the
preceding section some figures have been given showing
how much more water is present at the end of the
summer in the fallow land than in the land which had
carried a crop, so that in districts where the winter rain-
fall is small the fallowed land will start the next season
with a great advantage. Indeed, in a semi-arid climate
where the annual rainfall is insufficient, satisfactory crops
may yet be grown in alternate years by using an inter-
mediate fallow period in which to accumulate a reserve
of subsoil water.
The following series of measurements will illustrate
this point ; it shows the percentages of water in spring
and autumn on fallow and cropped land respectively,
also the water present in the same land in the following
spring and autumn, when both plots were in oats.
Spring.
Autumn.
Following
Spring and Autumn.
Fallow.
1.
Corn.
2.
Fallow.
1.
Corn.
2.
Oats.
1.
Oats.
2.
Oats.
1.
Oats.
2.
1st foot •
2nd foot •
3rd foot •
24%
20 „
18 „
22%
19 *
18 „
17%
20 „
16 „
7%
12 „
4 H
19%
21 „
18 „
16%
18 „
IS „
6%
10 „
9 N
4%
5 N
8 „
The effect of the fallowing in retaining more moisture
in the soil is seen throughout the whole of the second
season.
H4 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
At Rothamsted portions of the wheat field were
fallowed during the summer of 1904, and the following
table shows the percentage of water in the fine earth on
13th September, 2-849 inches of rain having fallen since
the crops had been cut.
TJnmanured.
Dunged.
Mean of 8 Plots.
Cropped.
Fallow.
Cropped.
Fallow.
Cropped.
Fallow.
1st 9" .
2nd 9'' . t
3rd q" •
4th 9" .
15-8
18.9
20-8
23.1
i6«o
19.8
23-3
25-2
20-2
I4.5
13-7
15-5
19-3
17-0
184
19.7
17.4
l8-8
20*1
20-9
17-2
20-0
22-3
23.I
Mean •
19-6
20-8
16-0
18-6
19-3
20-6
In the surface layer there is practically no difference,
both having become equally wet by the rains after
harvest, but in the lower depths the fallow soils are the
wetter, and the differences are more pronounced for the
unmanured plot where a small crop had been grown
than for the dunged plot with its larger crop.
The way in which fallowed land is of benefit to
the crop, both by making nitrates and particularly by
saving water in a dry season, is easily seen in the
superior plant always found on the outside rows or
edges of an experimental plot divided from the others
by a bare path ; on one side the plant has the benefit
of fallow ground as well as of extra space, light, and
air, and flourishes accordingly. The Lois-Weedon
system of husbandry, where the land was divided
into alternate 5 -foot strips of corn and cultivated
fallow land, was nothing but an application of this
principle on a large scale, as indeed is any system of
growing a crop in wide rows to admit of some form
of hoe or cultivator working regularly at the ground
IV.]
VALUE OF BARE FALLOWS
US
between. In a humid climate or on a porous soil there
is great danger of losing the nitrates formed in the
summer by washing out during the autumnal and winter
rains, nor is there any advantage gained by storing water
where the usual winter rainfall is sufficient to saturate
the soil. For this reason, in the Rothamsted experi-
ments, the plot growing wheat continuously has given a
greater crop per acre per annum than the plot fallowed
and sown with wheat in alternate years, though the
wheat crop following fallow has always been larger than
the crop grown the same year on the unmanured plot
Average Crop.
Wheat every Year.
Fallow and Wheat.
Grain.
Straw.
Grain.
Straw.
1856-189$ • •
Bushels.
I2|
Lbs.
II27
Bushels.
H
Lbs.
798
Of course the average crop on the fallowed ground
was twice the above figures, i.e., I7§ bushels of grain and
1595 lbs. of straw, but it was only grown every alternate
year.
That the autumnal rainfall is the great factor in
determining whether a bare fallow shall be profitable
or not to the following crop, may be well seen by a
further examination of the results obtained at Rotham-
sted on these plots, by comparing the crops with the
percolation which took place in the autumn previous.
The percolation through 60 inches of bare soil for
the four months, September to December inclusive, as
measured by the drain gauge previously described on
p. 78, amounted on the average to 6-45 inches for the
31 seasons 1870-1901. If, then, we divide the harvest
years into two groups according as the autumnal
percolation is above or below the average, and allot
n6 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
to each year the crops on the continuous wheat and
wheat after fallow plots for the harvest following the
given percolation, we shall obtain the following
average results, which show in group I the mean
crops following autumns of less than average percola-
tion, and in group 2 those following autumns of
comparatively high percolation. The percolation is
given in inches, the crops in lbs. per acre of total
produce, both grain and straw ; and as further evidence
of the extent of percolation, the average number of
days are given during the four months on which
water ran from the tile drain underlying the con-
tinuous wheat plot
I.
2.
si
1
9 -;
£ to
3 &
•+= eS
<D
a
OS
O
Crop, lbs. per acre.
Wheat after
Wheat
each Year.
Gain due to
Fallow.
15 Years of Percolation
below average .
16 Years of Percolation
above average .
8.94
13-78
3-99
8.92
4
13
1807
1627
2677
1757
870
130
Thus the bare fallow which increased the succeeding
crop above that given by the continuous wheat plot
by nearly 48 per cent, in the seasons when a com-
paratively dry autumn succeeded the fallow, increased
it by less than 8 per cent, when there was much
percolation after the fallow.
It follows, therefore, that the practice of fallowing
land is only an economical one where the annual rain-
fall is low and where the land is too strong to admit
of free percolation ; it is, however, admirably adapted to
the successful cultivation of clay land in dry, hot climates.
IV.]
HUMUS AND SOIL WATER
117
Effect of Dung on the retention of Water by the Soil.
A soil which has been enriched in humus through
repeated applications of farmyard manure will resist
drought better than one in which the humus is low ; the
difference is seen not so much in the greater amount
of moisture present in the soil containing humus, as
in the way it will absorb a large amount of water
temporarily during heavy rainfall, and then let it
work more slowly down into the soil, thus keeping
it longer within reach of the crop. Good examples
are afforded by the Rothamsted plots; samples of
soil from the wheat land were taken on 13th September
1904, on the previous day 0262 inch of rain had
fallen, but for nine days before there had been little
or np rain. The portions of the plots from which
the samples were drawn had been fallowed through
the summer, so that the drying effect of the crop
is eliminated. Samples were also taken from the
barley plots on 3rd October of the same year; 0-456
inch of rain had fallen on the 30th September, before
which there had been fifteen days of fine weather.
The following table shows the water in the soil of the
unmanured and the continuously dunged plots respec-
tively, as percentages of the fine earth from which the
stones had been sifted
Percentages of Water in Rothamsted Soils.
Depth.
Broadbalk Wheat.
Hoos Barlky.
Unmanured.
Dunged.
Unmanured.
Dunged.
0" to 9"
9" „ 18"
18" „ 27"
160
19.8
23-3
19-3
170
184
17-0
22-5
22-1
20-7
177
18.3
n8 TILLAGE— MOVEMENTS OF SOIL WATER [chap.
It is thus seen that in both cases the dunged soil,
rich in humus, had retained more of the comparatively
recent rainfall near the surface, so that the top soil was
moister while the subsoil was drier. The difference
in favour of the surface soil was about 3-5 per cent, which
on that soil would amount to about 30 tons per acre,
or approximately 0-3 inch of rain. It is thus seen
that the surface soil of the dunged plot had retained
practically the whole of the preceding rainfall ; and
the greater dryness of the subsoil was due to the way the
soil had kept back the small rainfalls, which have
been evaporated instead of passed on to the subsoil
as they were on the unmanured plots. The same fact
is illustrated by the behaviour of the drains which run
below the centre of each of the wheat plots at a depth
of 30 inches ; below the dunged plot the drain very
rarely runs, only after an exceptionally heavy and
long - continued fall, whereas the drain below the
unmanured plot runs two or three times every winter.
Putting aside the greater drying effect of the much
larger crop on the dunged plot, the difference is mainly
due to the way the surface soil rich in humus first
of all absorbs more of the water, and then lets the
excess percolate so much more slowly that the descend-
ing layer of over-saturation, which causes the drain to
run, rarely or never forms.
The water-retaining power of the dung may also
be seen in the superior yield of the dunged plots
in markedly dry seasons. The following table shows a
comparison of the yield on plot 2, receiving 14 tons
of dung, and plot 7, receiving a complete artificial
manure, for the years 1879, which was exceptionally
wet and cold, and 1893, which was hot and dry
throughout the growing period of the plant The
rainfall for this period, /.&, for the four months March
IV.]
HUMUS IN WET AND DRY SEASONS
119
to June, was 13 inches in 1879 an ^ only 2-9 inches
in 1893.
Wheat. Yield in Bushels of Grain.
Plot.
1879.
1893.
Average
51 Tears.
2
7
16-0
16.25
34-25
20.25
357
329
The average yield on the dunged plot is about 3
bushels more than on plot 7, but in the dry year its
superiority amounted to 14 bushels, whereas in the
very wet year the two plots sank to the same low level.
In a bad season the bacterial changes which render the
plant food in dung available for the crop go on very
slowly.
CHAPTER V
THE TEMPERATURE OF THE SOIL
Causes affecting the Temperature of the Soil — Variation of
Temperature with Depth, Season, etc. — Temperatures
required for Growth — Radiation — Effect of Colour — Specific
Heat of Soils — Heat required for Evaporation — Effect of
Situation and Exposure — Early and Late Soils.
The life of a plant is practically suspended below
a certain temperature, which is about 41 ° F. for the
majority of cultivated plants ; all the various changes
which are essential to the development of the plant
such as germination, vegetative activity, and the bac-
terial processes in the soil, show a similar dependence
upon temperature.
These vital actions cease below a certain minimum,
above which they usually increase with the tempera-
ture until an optimum is reached, when the action is
at its greatest ; beyond this point the action decreases
until a superior limit is reached, which again suspends
all change. It therefore becomes important to study
the manner in which heat enters and leaves the soil,
because upon the temperature acquired depend such
practical questions as the suitability or otherwise of
the land for particular crops, the season at which to
sow, and the earliness or lateness of the harvest
The surface soil receives heat in four ways : —
(1) By direct radiation from the sun, whose rays
both of light and invisible heat are absorbed
120
chap, v.] AGENCIES AFFECTING TEMPERATURE 121
and raise the temperature of the absorbing
soiL
(2) By precipitation, as in the spring when warm
rain enters the ground and brings with it a
considerable quantity of heat, or when aqueous
vapour in the air is condensed on the colder
soil
(3) By conduction from the heated interior of the
earth a small amount of heat reaches the
surface.
(4) By the changes which result in the decay of the
organic material of the soil, when as much heat
is developed as if the same material had been
burnt in a fire.
The surface soil loses heat : —
(1) By radiation; like any other body possessing
heat, the surface of the soil is always emitting
invisible radiant heat, which may, or may
not, be counterbalanced by the corresponding
radiations it is absorbing.
(2) By conduction either to cooler layers of earth
below or to cooler air above.
(3) By the evaporation of # the water contained in
the soil ; at ordinary temperatures the evapora-
tion of 1 lb. of water would absorb enough
heat to lower the temperature of about 7500 lb.
of soil by 1° F.
The actual temperature attained by a given soil at
any time depends upon the relative effect of the heat-
ing and cooling actions set out above.
122 THE TEMPERATURE OF THE SOIL [chap.
Soil Temperatures.
The accompanying curves (Fig. 8), show the
monthly mean temperatures of the soil at 6 inches, 3
feet, and 6 feet respectively, as compiled from readings
taken at 9 A.M. at Wye during 1896, the soil being a
light well-drained loam under grass. It will be seen
that the variations in temperature diminish with the
depth: in fact a point is soon reached, about 50 feet
down, below which the effect of the gain or loss of
heat at the surface is inappreciable, and the tempera-
ture is constant from day to day, only increasing with
the depth, according to the well-known law. Each
curve cuts each other curve at least twice ; for a
certain period the upper layer is giving, and during
the rest of the year, receiving heat from the layer
above or below. The maximum temperature attained
at a depth of 3 feet comes a little later in the year
than the maximum for 3 inches, and the maximum
at 6 feet lags still further behind, owing to the slow-
ness with which the heat is conducted. It will be
seen that the curve indicating the temperature at 6
inches (and the mean figures for 3 and 9 inches are
almost identical) does not reach the 41 F. required for
the beginning of vegetative growth until April ; it
is, however, constructed from monthly averages only,
and from observations taken at 9 A.M., when the
surface soil has been considerably cooled during the
night. Much higher temperatures are obtained during
certain parts of the day even in the early spring
months, otherwise no germination could take place;
these diurnal and hourly fluctuations are, however,
chiefly confined to the surface soil. The following
vo
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v.]
VARIATIONS OF TEMPERATURE
123
curves show firstly (Fig. 9), the daily results during
a fortnight of April 1902, also (Fig. 10) certain hourly
readings obtained in the same month, in this case
beneath smooth, well-worked arable land. The diurnal
variations die away before the depth of 3 feet is reached,
nor are hourly variations perceptible at the depth of
one foot, except in the case of heavy precipitation and
a pervious soil. It will also be noticed from the
last curves that during part of the day the temperature
at the depth of 6 inches ran up to a point well above
the minimum required for germination, although the
mean soil temperature at 9 A.M. was near that limit
Temperatures required for Growth.
Reference has already been made to the fact that a
certain temperature is necessary before the vital
processes involved in growth become active; this
temperature is not always the same, but may be con-
sidered to lie between 40 and 45 F. for most of the
plants grown as crops in this country.
The following table shows minimum, optimum, and
maximum temperatures of growth for a few plants.
Minium in.
Optimum.
Maximum.
Mustard • •
32° F.
8i°F.
99° F.
Barley . • •
4i
83-6
99.8
Wheat . •
41
83-6
108.5
Maize . • .
49
92-6
115
Kidney Bean.
49
92-6
115
Melon .
65
91.4
in
The next table shows the effect of soil temperature
uoon the growth of the root of maize.
124
THE TEMPERATURE OF THE SOIL [chap.
Root Growth of Maize in 24 Hours.
Temperature.
Millimetres.
6 3 °F.
1-3
79
24-5
92
39
93
55
IOI
25-2
108.5
5-9
The osmotic absorption of water by the roots of
a plant is much affected by the temperature of the soil ;
although some plants, like cabbage, will continue to take
in a little water even near freezing point, others require
a higher temperature; for example, Sachs has shown
that tobacco and vegetable marrow plants will wilt
even at night, when transpiration is very small, if the
temperature of the soil falls below about 40 F.
The killing of plants like rose trees during frost is
generally due to drying out from this cause rather than
to the actual cold. As the air is often very dry during
a frost, evaporation continues, especially if a wind be
blowing at the same time ; thus the exposed shoots of
the plant are losing water which is not being replaced
by the roots, whose action is suspended by the low
temperature. A covering of snow or dead leaves, or
even the protection afforded by a little straw, bracken,
or spruce boughs, prevent the destruction of the plant,
not by keeping it so much warmer, but by pro-
tecting it from evaporation.
The connection between soil temperature and vital
processes is perhaps most apparent in the case of
germination, for which not only is a certain minimum
temperature requisite, but for several degrees above this
minimum the germination is so slow and irregular that
the young plant is very liable to perish while remaining
v.]
GERMINATION
125
in such a critical condition. The following table shows
the range of temperatures for the germination of various
cultivated plants.
Temperatures of Germination.
Fahrenheit.
Minimum.
Optimum.
Maximum.
Wheat ....
Barley • •
Oats • • •
Pea
Scarlet Runner • •
Maize ....
Cucumber, Melon, etc
32° to 41°
40°
32° to 41°
38° to 41°
49°
49°
6o° to 65°
77° to 88°
77° to 88°
9*i°
9i°
88° to 99°
88° to 110°
ioo° to IIO°
88° to ioo°
115
ii5°
110° tO I20°
The practical bearing of these figures is obvious;
it is necessary to sow some seeds, like the melon, in
heat, and to defer the seeding of other crops, like
mangolds or maize, until the ground has acquired not
only the temperature necessary for germination but
one that will ensure a subsequent rapid growth of
the seedling plants.
For example, turnips will germinate at almost as
low a temperature as barley, but the optimum tempera-
ture is higher for turnips ; they are therefore sown much
later in the spring, when the ground has more nearly
reached this temperature, because the seed is small and
the young plant very susceptible to insect attacks, so
that the turnip seed must germinate and grow away
rapidly if it is to succeed.
Under ordinary field conditions much of the
nutrition of the crop depends upon the activity of
certain bacteria in the soil which break down organic
compounds containing nitrogen, and ultimately resolve
them into the nitrates taken up by the plant Most
126 THE TEMPERATURE OF THE SOIL [chap.
bacteria are active within about the same limits of
temperature as have been indicated above for the
higher plants ; the nitrification bacteria, for example,
cease their work below 41 ° F. and above 130 F., their
optimum temperature being about 99 F.
The way a low temperature will check the production
of nitrates until they are inadequate for the needs of
the crop is often seen in spring, and may be connected
with the yellow colour of the young corn during a spell
of cold and drying east wind.
Radiation.
The main source of the soil warmth consists in the
heat received from the sun by radiation ; this, according
to Langley, amounts to about 1,000,000 calories per
hour per square metre of surface from a vertical sun in
a clear sky. Supposing this energy were wholly
absorbed by a layer of dry soil 10 cm. thick, its
temperature would rise by as much as 90 F. in an hour.
Of course in nature many other factors are at work to
reduce this temperature; the sun is rarely vertical,
the soil material does not completely absorb but reflects
some of the sun's rays unchanged ; at the same time it
is always radiating in its turn rays of lower pitch than
the majority of those received. The latter rays are
easily caught by many substances, glass and water
vapour in particular, which are transparent to the rays
of higher refrangibility proceeding from the sun. A
greenhouse, for example, is practically a radiant heat
trap ; the temperature inside runs up because the sun's
rays of light and heat can penetrate the glass, whereas
the obscure heat rays radiated back again from the
warmed-up surfaces inside the house are not able to
pass through the glass again. Just in the same way the
temperature rises and the sun's heat becomes oppressive
v.] COLOUR AND TEMPERATURE 127
when the air is laden with water vapour, because it
retains the radiations emitted by the surfaces heated by
the sun. Per contra, the temperature of the ground
falls more rapidly at night when the sky is clear and
the air dry, for then there is no blanket of cloud or
water vapour to arrest or reflect the radiations from the
surface.
The power of soils to absorb the sun's rays depends
very much upon colour : with black soils the absorption
is almost complete ; it is greater for red than for yellow
soils, least of all for those which look distinctly white or
light coloured. It has already been shown that the
colour of soils depends mainly upon humus and
hydrated ferric oxide, the latter accounts for all the
red, yellow, and brown shades, the former for the black
coloration of the soil Deep-seated clays are often
blue or green, due to various ferrous silicates or to
finely divided iron pyrites, which afterwards oxidise
to brown ferric oxide. The more finely grained a
soil is the more surface it possesses, and the greater
amount of colouring matter that is required to pro-
duce a given colour; a coarse sand is often quite
black though it contains but a small percentage of
humus.
Though the colour of a soil affects the rate at which
it absorbs heat, it does not follow that the dark soils
will lose with a corresponding rapidity when radiation
is taking place at night; the emissive power of the
substance for rays of low refrangibility, such as are
emitted at ordinary temperatures, is not affected by
colour. Hence, the extra heat gained by a dark soil
is retained and not lost by a corresponding increase in
its radiating power.
The curves in the accompanying diagram (Fig. 10),
show the temperatures of the soil at a depth of 6
128 THE TEMPERATURE OF THE SOIL [chap.
inches during an April day with a bright sun and a
strong drying wind. The land was a light loam of
a grey-brown colour when dry; it had been culti-
vated, rolled, and the surface hoed over before the
thermometers were inserted ; on plot I the bare
ground was left untouched, plot 2 received a dressing
of soot until the surface was black, plot 3 was
similarly whitened over with lime. It will be seen
that the covering of soot warmed the soil until at 3
P.M., when the maximum temperature was attained,
the difference was 2-4°; this superiority is also re-
tained during the later cooling stages; even at 9 P.M.
the blackened soil was still 2-5° warmer than the bare
ground. The whitening with lime had caused so con-
siderable a reflection of the radiant heat that the soil
beneath was always 2 to 3 cooler than the bare ground.
In carrying out this experiment it is necessary to use no
more lime or soot than will distinctly colour the soil ;
the results will be disturbed if an excess of either loose
powder acts as a mulch.
Specific Heat.
The specific heat of the substances of which soil
is composed is comparatively low, ranging from 01
to 02, z>., only from one-tenth to one-fifth as much
heat will be necessary to raise the temperature of 1
lb. of dry soil by i°, as would be required to produce
the same rise of temperature in an equal weight of
water. The humus possesses the greatest specific heat
and the sand the least ; against this must be set off the
fact that the densities of these soil constituents vary
in the opposite sense, so that the amounts of heat
required to bring about a given rise of temperature to
a certain depth in different soils are more nearly equal.
The specific heats are, however, small in every case
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V
s
•.
v
-
OS
-
'-
-
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S
v.]
HEAT REQUIRED TO WARM SOIL
129
when compared with that of water ; hence soils which
retain much water will require far more heat to raise
their temperature than dry soils would. In consequence,
clay and humus soils are cold because the water they
retain gives them a high specific heat, they require more
of the sun's rays in spring to bring them up to the
proper temperature for growth, while sandy and other
open-textured soils are warm because of their dryness.
If the figures given by Oemler for the specific heats
of various soils be combined with their approximate
densities and with their minimum capacity for water, the
following results are obtained for the specific heats of
certain typical soils in a saturated but completely drained
condition —
Specific Heat.
Equal Weights.
Equal Volumes.
Water . .
Humus . • •
Sandy Peat • •
Loam . • •
Clay • • •
Sand . • • •
Dry.
IO
0-21
O.I4
OI5
OI4
O-I
Dry.
I-O
O-I I
0-18
0.15
0-125
Wet.
I-O
0-72
o-53
o-6i
o-34
The sandy soil only requires about half as much
heat to raise its temperature by a given amount as
would be needed by the peaty or clay soil, when all the
soils are in a wet but thoroughly drained condition ; of
course if the clay or peat were inadequately drained, so
that a higher proportion of water was retained, their
specific heats would approximate still nearer to that
of water.
Just as a clay soil is slow to warm in the spring, its
high specific heat causes it to cool correspondingly
*3o THE TEMPERATURE OF THE SOIL [chap.
slowly after the heat of the summer. On clay soils
growth will be noticed to continue later into the autumn
than on the lighter lands.
Heat required for Evaporation,
The coldness of a wet and undrained soil is due, not
only to its high specific heat, but to the fact that so much
of the heat it receives is spent in evaporating some of its
retained water, without causing any rise in temperature.
The evaporation of I lb. of water at 62 F., z>., its con-
version into water vapour at the same temperature,
requires as much heat as would raise the temperature of
1050 lbs. of water by i°F., and, if there be no source of
external heat bringing about the evaporation, the sub-
stance from which the water is evaporated must become
cooled to a corresponding extent The cooling effect of
evaporation is well known, but its application to the
soil is not always realised ; clays and even more so un-
drained soils are cold and late, not only because of their
high specific heat, but because they retain so much
water which can be evaporated. The drying winds of
early spring exercise a great cooling effect whenever
the soil moisture is allowed to evaporate freely, hence
the importance of establishing a loose tilth, if the
seed bed is to warm up the temperatures requisite for
germination.
Anything providing a little shelter to check evapora-
tion and break the force of the wind in the spring will
have a considerable effect in raising the soil temperature.
The dotted curve in Fig. 10 shows the effect of enclosing
a plot of the same land with a slight hedge made of
spruce fir boughs about 2 feet high. In the morning
the temperature of the sheltered plot was below that of
the open ground because of the shading from the direct
rays of the sun, but as soon as this effect was over
v.] EVAPORATION COOLS THE LAND 131
it will be seen that the wind break, by checking evapora-
tion, maintained the soil temperature more than 2°
above that of the open ground. Sufficient attention is
not given in practice to the value of even slight wind
breaks for checking evaporation and so raising the
temperature of the soil in early spring. The raisers of
specially early vegetables, radishes in particular, on a
strip of light land close to the sea in Kent are, however,
in the habit of breaking the sweep of wind across their
fields by erecting temporary fences of lightly thatched
hurdles.
Even the stones upon the surface of the land help.
In the Journal of the Royal Agricultural Society for
1856, an experiment is described in which the flints
were picked off the surface of one plot of ground and
scattered over an adjoining plot, with the result that the
plot with double its usual allowance of stones was three
or four days earlier to harvest than the rest of the field,
while the plot without stones was a week later still. It
will always be noticed how the grass upon a field coated
with dung starts earlier into growth, because the loose
manure acts as a mulch and protects the soil from the
cooling due to evaporation.
Land which is protected from evaporation, and to
some extent from radiation, by a layer of vegetation, is
always both warmer and less subject to fluctuations of
temperature than bare soil.
The warming up of a well-tilled surface soil is
increased by the fact that the conduction of heat into
the soil below is much checked by a loose condition. A
solid body will always conduct heat far better than
the same substance in the state of powder, and the
more compressed the powder is the better it will conduct,
simply because there are more points of contact Hence
a rolled and tightened soil will conduct the heat it
132
THE TEMPERATURE OF THE SOIL [chap.
receives more rapidly to the lower layers than one
which is loose and pulverulent. King has shown that,
despite the increased evaporation, there is always a
higher temperature below a rolled than an unrolled
surface.
A few observations may be given showing the effect
of drainage in enabling the sun's heat to raise the
temperature of soil. The curves (Fig. n) show the
hourly temperatures of the drained and undrained por-
tions of a peat bog during two last days in June (Parkes,
/. R. Ag. Soc, 1844, I 4 2 )> at depths of 7 inches and 13
inches respectively; the sudden rise of temperature
between 3 and 4 P.M. on the second day was due to a
thunderstorm, during which heavy rain at a temperature
of j2>° F. was falling.
The figures in the table below are derived from
observations made by Bailey-Denton in 1857 (/. R.
Ag. Soc, 1859, 273), on a stiff clay soil situated on the
Gault at Hinxworth, the drains being 4 feet deep and
25 feet apart in the drained part It is noteworthy
that the temperature of the air 9 inches above the
surface is higher for the drained than for the undrained
land, thus supplying further evidence of the cooling
effect ot evaporation.
Mean Temperature °F. at 9 a.m.
A.— Land Drained.
B.— Land Undrained.
March • • • •
April # #
May . . .
Mean excess over B. .
Air.
39-4
43
52-9
At 18".
40-6
46
51
At 42".
4I.7
44.8
48.4
Air.
39
42.4
52-7
At 18".
38-2
44
48-8
At 42".
40-3
43-8
47.1
0.4
2»2
1*2
.»•
»•«
.»»
■a
03
S3
T3
03
d
J3
H CO
OS
i— 1
BQ
03
03 00
E
<T>
go
u
S^
'a
r
fh
a
9
a
a
•-3
—.
>•
K>»
r-> -
F
i
I
'
II l 'jl I » I i n il-} - . . / , I I , I | i > ,i , ., «
s" J I
!
I
!
01
-r—
— - w
I
i
— r
01
to
J
v.] SPRING FROSTS 133
Effect of Situation and Exposure,
Other conditions being equal, in the northern hemi-
sphere the soil temperatures will always be higher on
land sloping toward the southern quadrant than with
any other aspect. King found a difference of about
3 F. down to the third foot between a stiff red-clay soil
with a southern slope of 18 and the same soil on the
flat ; Wollny obtained a mean difference of i°-5 between
the north and south sides of a hill of sandy soil inclined
at 1 5 . The chief cause of these differences is the fact
that in this country the sun is never vertical, hence a
beam of sunlight represented by xy, Fig. 12, is spread
FlG. 12. — Distribution of the Sun's Rays on Southerly and
Northerly Slopes.
over an area represented by AB when the ground is
flat ; if the ground slopes to the south, the same beam is
spread over the smaller area represented by AC ; if the
ground slopes to the north, it is spread over the larger
area represented by AD. During the winter half-year,
also, the southern slope will have a longer duration of
sunlight than the northern slope.
Though in a general way the temperature both of
the air and the soil decreases with elevation above
134 THE TEMPERATURE OF THE SOIL [chap.
sea- level, yet it is well known that the severest frosts
occur locally at the bottom of valleys and hollow places.
This is particularly noticeable in the sudden night frosts,
characteristic of early autumn and late spring, which
are so dangerous to vegetation ; it is usual to find the
tenderer plants of our gardens, such as dahlias, cut
down by frost on the lower levels long before the
gardens on the hill are affected. Spring frosts, again,
will often nip the early potatoes in the valleys when
the higher lands are untouched. Fruit plantations
should not be set in the valleys, for no crop suffers
more from these unseasonable snaps of cold ; so clearly
is this fact recognised, that in some fruit-growing
districts only land above a certain elevation is regarded
as suitable for fruit, and commands a higher rent in
consequence. Two causes co-operate in producing the
excess of cold at the lower levels. The night frosts
in question are always the result of excessive radiation
when the sky is clear and the air still. The ground
surface loses heat rapidly and cools the layer of air
above; the cold air thus produced is denser, and pro-
ceeds to flow downhill and accumulate at the lower
levels. There is thus a renewal of the air above the
higher slopes, and the effect of radiation is mitigated
by the inflow of warmer air ; at the bottom no change
of air is produced and the radiation proceeds to its
full effect.
At the same time the vegetation in the valley
is generally more susceptible to a frost; the greater
warmth by day, together with the extra moisture and
shelter, induce an earlier and a softer growth.
The other cause that operates to produce severer
frosts in the valleys arises from the fact that frosts,
and radiation weather generally, accompany the dis-
tribution of pressure known as an " anti-cyclone," during
VMax,
Min.
May 11th 12th 13th 14th 15th May 16th, 1902
FlG. 13. — Temperatures (Maximum and Minimum) at Various Altitudes.
A = 100 feet. B = 150 feet. C = 500 feet.
v.] DRY SOILS ARE EARLY 135
which there is always a gentle downdraught of cold
air from the clear sky above. But the circulation of
air in an anti-cyclone is always reversed at a certain
elevation, so that as one ascends, the downdraught of
cold air becomes less and eventually ceases ; the mean
temperature at the same time rises instead of falling
with the elevation.
Observations of the minimum temperature on the
grass made at two stations on a gentle slope of the downs
at Wye, about a mile apart and differing in level by
100 feet, showed during a period of thirty-six days
in February and March 1902 (which was exceptionally
calm and mild), a mean difference of i° in favour of
the upper station, but on two occasions the lower
thermometer fell to 24°- 5 and 2i°-5, when the upper
thermometer was 30°* 5 and 29°- 5 respectively.
The accompanying curves (Fig. 13) show the daily
maxima and minima of the air temperatures of 4 feet
from the ground for a few days before and after the
occurrence of a disastrous late spring frost in May 1902.
One station, A, was in a river meadow about 100 feet
above sea-level, the second, B, was on a terrace about
50 feet higher and half a mile away ; the third, C, was on
the flat summit of the down, 500 feet above sea-level
and about ij mile from the first station. It will be
seen that the river-side station gave always the highest
maxima during the period and the lowest minima,
showing on the one occasion n° of frost.
Early and Late Soils.
From all the considerations developed above it will
be seen that an early soil is essentially a coarse-textured
and well-drained one. Such a soil retains little water,
thus possessing a low specific heat, and is easily warmed ;
136 THE TEMPERATURE OF THE SOIL [chap.
at the same time the low water content gives rise to
less evaporation at the surface, and this great cause of
cooling is minimised. The dryness of the soil permits
of early cultivation, which, by cutting off the access of
subsoil water and diminishing the conduction of heat
from the surface, quickens the warming up of the
seed bed. The early aeration and warming of the soil
promotes the nitrification which is also necessary to
growth. If, further, the soil be stony, the conduction of
heat from the surface layer into the soil is more rapid,
solids being better conductors than powders. Such
soils, again, are generally dark coloured, because on the
comparatively small surface exposed by the coarse
grains the same proportion of humus has a greater
colouring effect.
These conditions are generally fulfilled by the
alluvial soils bordering the larger rivers; in the
neighbourhood of large towns, which are generally
situated on a river, they form the typical market-
gardening soils, especially as their natural poverty can
be alleviated by the large supply of dung which is easily
obtainable from the town.
At the same time these soils have their dis-
advantages ; from both their nature and their situation
they are subject to rapid changes of temperature ; they
suffer much from night frosts both in spring and
autumn, and dry out easily in the summer, so that
some crops do not come to their full growth. Autumn
planted vegetables grow away rapidly, and are apt to
become " winter proud " and killed by severe weather.
Of course, to ensure the maximum of earliness and
freedom from spring frosts, the geographical situation
and the climate must be considered as well as the nature
of the soil. The neighbourhood of the sea or any large
body of water has a great effect in equalising the
v.] EARLY AND LATE SOILS 137
temperatures and preventing severe frosts ; in the
British Islands, for example, the earliest potatoes are
grown in Jersey, near Penzance, and on other light land
along the southern coast of Cornwall, and again a little
later near the sea in Ayrshire. Light land round the
coasts of Kent and Essex, which borders, and in some
cases is almost surrounded by the sea, is also specially
valued for the growth of early vegetables.
The soils naturally retentive of water are late,
both because they dry slowly and are rarely fit to work
early in the year, and because the high water content
keeps their temperature down. Except in long-con-
tinued droughts they maintain a supply of water to the
plant, their high specific heat keeps them at a com-
paratively equable temperature and prevents them from
cooling down so soon when the summer heats are
past. In consequence, the crop is neither forced early
to maturity nor is growth checked so soon in the
autumn, the period of development is prolonged until
in some cases the season becomes unsuitable for
ripening. Many subtle differences may be noticed
between the quality of produce grown upon early and
late soils ; for example, a comparison made by the
author of the same variety of apple grown upon
adjoining clay and sandy soils showed that the apples
from the clay land were smaller and greener, but
contained a greater proportion of sugar and acid,
and possessed a higher aroma than the apples grown
upon the lighter and earlier soil. Wheat grown on
the clays is generally of better quality and " stronger "
than that yielded by the lighter soils ; whereas the
lighter soils yield the finer barley, in which carbo-
hydrates and not nitrogenous materials are characteristic
of high quality. On a light soil, becoming both warm
and dry early in the season, the plant ceases the sooner
138 THE TEMPERATURE OF THE SOIL [chap. v.
to draw nutrient material from the soil; assimilation,
however, continues after the plant has ceased to feed ;
finally, maturation — the removal of the previously-
elaborated material into the seed — begins earlier and
can be more thoroughly accomplished. Hence grain
from an early soil is, on the whole, characterised by
a higher proportion of carbohydrate to albuminoid,
always provided that no excessive or premature dry-
ing-out has taken place, for that ripens off the grain
before the transference of starch has been completed
CHAPTER VI
THE CHEMICAL ANALYSIS OF SOILS
Necessary Conventions as to the Material to be Analysed —
Methods Adopted — Interpretation of Results — Distinction
between Dormant and Available Plant Food — Analysis of
the Soil by the Plant— Determination of " Available " Phos-
phoric Acid and Potash by the Use of Weak Acid Solvents.
The chemical analysis of a soil aims at ascertaining the
amount which the soil contains of the various elements
necessary to the nutrition of the plant, with a view of
either making good the general deficiencies of the soil or
of adjusting the supply of plant food to such special
requirements of a particular crop as may have been
indicated by previous experiment
The analysis of plants grown under ordinary con-
ditions shows that a comparatively limited number of
elements enters into their composition ; in the main they
are composed of water and certain combustible com-
pounds of carbon, hydrogen, nitrogen, and sulphur. In
the mineral residue that is left after the combustible
material has been burnt off, will be found potassium,
sodium, calcium, magnesium, and a little iron among
bases ; and phosphorus, chlorine, sulphur, and silicon
among non-metallic elements. Manganese in very small
quantities occurs in nearly all plants : other elements
like lithium, zinc, copper, are found in traces under
140 THE CHEMICAL ANALYSIS OF SOILS [chap.
special conditions of soil. By pot cultures in the
laboratory it can be shown that of the above elements,
the carbon, hydrogen, and oxygen are drawn from the
atmosphere or the water, and that nitrogen, chlorine,
sulphur, phosphorus, among non-metals, and potassium,
calcium, magnesium and iron, among metals, are ele-
ments indispensable to the plant, and are derived by
way of the root from the soil. In view of the above
facts it is clearly unnecessary to make an ultimate
determination of all the elements present in the
soil, which has already been shown to consist largely
of sand and various silicates of alumina, etc. These
materials constitute the medium in which the plant
grows, but do not themselves supply it with any
food ; they need not, therefore, be estimated chemi-
cally.
The chemical analysis of a soil, then, resolves itself
into determinations of the nitrogen, phosphorus, potas-
sium, calcium, and (of less importance) of sodium, mag-
nesium, iron, aluminium, chlorine, and sulphur. To
these must be added the determination of the carbon
compounds of the soil, which have already been touched
on under the head of humus, and of the carbonates of
calcium and magnesium, which in most soils constitute
the bases available for neutralising any acids that may
be produced. Having decided upon the elements to be
determined it would then be possible to proceed as in
an ordinary mineral analysis : the sample of soil would
be reduced to such a state of division as would admit
of drawing an accurate small sample, and then entirely
disintegrated by some such reaction as fusion with
ammonium fluoride. But results obtained in this way
would give very imperfect information about the soil, for
the procedure draws no distinction between material
present in the imweathered interior of the stones and
vi.] NECESSARY CONVENTIONS 141
coarser particles, which could not reach the plant for
generations, and that which exists as very small particles
or as a coating on the larger ones, and is therefore open
to attack by the water in the soil. The nutrient material
of the soil can only reach the plant in the dissolved
state, and in dealing with slightly soluble substances
such as constitute the soil, the amount which goes into
solution is practically proportional to the surface
exposed. But the surface exposed increases as the
material is subdivided, one gram of soil in pieces
1 mm. in diameter would only expose one-thousandth
of the surface exposed by the same amount of soil in
particles 0001 mm. in diameter, so that to all intents
and purposes the stones and coarser particles con-
tribute such a small proportion of the surface of the
soil that the material dissolved from them can be
neglected. ■"*
For these reasons — the small surface exposed by
the larger particles and the unweathered nature of
the compounds within them — the stones above a
certain size are not included in the analysis, nor is
any attempt made to bring into complete solution even
the selected material. Hence it becomes necessary
in soil analysis to accept certain "conventions" as to
the preparation of the soil for analysis, the nature of the
acid used for solution, and the duration and temperature
of the attack ; all of which factors so affect the mineral
matter going into solution that results are only com-
parable when obtained in the same way. It must
always be remembered that soil analysis is only a
relative process, by which soils that are unknown can
be compared with others whose fertility has been
tested by experience; no means exist of directly
translating the results into terms of the crop the soil
will carry. The methods of analysis that are indicated
142 THE CHEMICAL ANALYSIS OF SOILS [chap.
below are those adopted by the members of the
Agricultural Education Association in this country :
unfortunately, there is no uniformity in the methods
pursued even among chemists in the same country,
wide as are the variations introduced by the different
processes in vogue. For example, an acid such as
hydrochloric will dissolve very different amounts of
potash from a given soil, according as the soil is
treated directly with the acid or first ignited, nor is there
any constant relation between the amount dissolved
from ignited and from raw soil.
Method of Analysis.
The soil sample is taken, passed through the 3 mm.
sieve, and air-dried, exactly as previously described
for the mechanical analysis. From the large air-dried
sample of "fine earth" a portion of about 100 grams
is drawn, and either ground in a steel mill or broken
in a steel mortar till it will all pass through a sieve
with round holes 1 mm. in diameter. This is done
to enable the analyst to draw a fair sample weighing
only a few grams : if the " fine earth " which passes
the 3 mm. sieve were itself used, it would be impossible
to adjust the relative proportions of coarse and fine to
correspond with the bulk. It is not uncommon to find
coarse particles of carbonate of lime sparsely scattered
through the soil when the land has been limed ; only by
grinding and mixing can this matter become evenly
distributed through the soil. On the ground material
the following determinations are made:—
(1) Moisture lost at ioo°C.
(2) Loss on ignition.
(3) Nitrogen.
(4) Earthy carbonates.
VI.] ANALYTICAL METHODS 143
(5) Phosphoric acid and potash soluble in strong
hydrochloric acid. If necessary, soda, lime,
magnesia, oxides of iron, alumina, and
sulphuric acid can be determined in the same
solution.
(1) About 5 grams are weighed out into a plati-
num dish or porcelain basin and dried in the ordinary
steam oven, the temperature of which is never quite
ioo°C. If the soil contains much organic matter, it
will be difficult to bring it to a constant weight, the
material will slowly lose water for weeks. An arbitrary
limit of twenty-four hours drying should be taken.
(2) The loss on ignition should represent the organic
matter which is burnt to carbon dioxide and water when
the soil is heated in the air, but it is impossible to
avoid at the same time driving off some of the water
of constitution in the zeolites, kaolinite, and similar
hydrated silicates in the soil. It is difficult even to
obtain consistent results, because of variations in the
temperature and time of the operation. The best plan
is to heat the soil, as dried in the previous operation,
at as low a temperature as possible, to a barely visible
redness, preferably in a platinum dish, for some hours
with occasional stirring.
The loss on ignition is wanted as a measure of the
organic matter of the soil, but we have no means of
estimating the varying part, great with clay soils,
that is played by the water of constitution. It is possible
to get a better measure of the organic matter by estimat-
ing the total carbon in the soil and assuming that the
organic matter of the original soil contained about
55 per cent, of carbon. The combustion of a soil by the
ordinary method for determining carbon is rather a
tedious process even in skilled hands ; in dealing with
soils it is convenient to effect the oxidation by means of
144 THE CHEMICAL ANALYSIS OF SOILS [chap.
a mixture of sulphuric and chromic acids, taking care to
interpose a tube of heated copper oxide between the
flask containing the soil and acids and the apparatus
used for absorbing the carbon dioxide, in order to
complete the oxidation of some of the products formed.
This process can be made to follow the determination of
the carbon dioxide evolved by the action of acid alone,
the same apparatus and the same portion of soil serving
for both.
(3) The nitrogen in 10 to 20 grams of the ground
" fine earth " is estimated by Kjeldahl's process. Though
there is some nitrate present in the soil, no special
precaution need be taken on its account, the proportion
it bears to the total nitrogen is so small as to be
negligible.
(4) The earthy carbonates of the soil are estimated
from the quantity of carbon dioxide, which is evolved
on treating the ground "fine earth" with an acid.
When the proportion of calcium carbonate is high the
determination can be made by the usual gravimetric
methods. Scheibler's apparatus for measuring the
volume of carbon dioxide evolved is suitable when the
proportion of calcium carbonate does not fall below
1 per cent. ; below that point some other method must
be employed, because all the carbon dioxide evolved goes
into solution in the reacting acid. The most exact and
convenient method for determining calcium carbonate,
especially when the quantity involved is very small,
consists in absorbing the carbon dioxide evolved by
dilute caustic soda in a Reiset tower, and estimating the
carbon dioxide by titrating the alkali first with phenol-
phthalein and then with methyl orange as an indicator
(see Amos. Jour. Agric. Sa\, 1, 1905, 322). Certain acid
soils rich in humus contain other organic substances
which yield carbon dioxide on boiling with dilute acid,
VI.] METHODS OF ANALYSIS 145
in which case the soil should be attacked with a boiling
dilute solution of ammonium chloride. It is not
sufficient in such cases to estimate the calcium dissolved
by dilute acids from the soil, because there are always
present other compounds of calcium, e.g., silicates and
sulphates, which are soluble in the acid and would be
reckoned as calcium carbonate. The factor that is
required is not the calcium, but the amount of
carbonate which will serve as a base in the soil and
combine with the acids liberated by decay, nitrifica-
tion, or from some of the artificial manures. To this
end it is not necessary to discriminate between the
carbonates of calcium and magnesium, accordingly
the carbon dioxide evolved is calculated back to
calcium carbonate. In a few soils ferrous carbonate
may be present; this is oxidised to ferric hydrate
when the powdered soil is boiled with water, and
may be so removed before determining the carbon
dioxide. In temperate climates, however, it is only
a few bog soils that need be examined for ferrous
carbonate.
(5) For the determinations of soluble constituents
20 grams of the powdered soil are placed in a flask
of Jena glass, covered with about 70 c.c. of strong hydro-
chloric 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 cc and aliquot
portions are taken for the various determinations.
The analytical operations are carried out in the usual
K
146 THE CHEMICAL ANALYSIS OF SOILS [chap.
manner, but special care must be taken to free the
solution from silica and organic matter. For phos-
phoric acid a portion of the solution is evaporated to
dryness and ignited, the residue is taken up with
hydrochloric acid, filtered, again evaporated to dry-
ness, and heated in an air - bath for half an hour at
1 05 . This residue is then taken up with dilute
nitric acid, filtered, and made up to about 50 c.c.
Five grams of ammonium nitrate are added, and 50
c.c. of a solution of ammonium molybdate containing
60 grams molybdic acid per litre. The mixture is
put aside in a warm place for twenty-four hours, the
precipitate is filtered off, and, after washing with
ammonium nitrate solution, is dissolved by ammonia
into a tared porcelain basin, evaporated to dryness,
and gently ignited over an Argand burner. The
resulting material contains 3-794 per cent of phos-
phoric acid. For the determination of potash the
same procedure is followed, but the residue after the
second evaporation is taken up with dilute hydro-
chloric instead of nitric acid. To the solution 25 c.c.
of a solution of chloroplatinic acid containing 0-005
gram platinum per c.c. is added, and the whole gently
evaporated over a water-bath till almost dry. It is
then thrown on to a filter and washed with alcohol,
then washed again with a solution of ammonium
chloride which has been saturated with the double
chloride of platinum and ammonium, and finally dis-
solved off the filter paper with a little hot water in a
tared basin, evaporated, and weighed. A Gooch crucible
is most convenient for handling both the phosphoric
acid and potash precipitates.
The other determinations which may be made in
this solution consist of soda, lime, magnesia, iron,
alumina, manganese, and sulphuric acid, but in most
VI. ] METHODS OF ANALYSIS 147
cases these may be omitted. It is occasionally desirable
to examine the soluble salts in the soil ; about 200
grams of the fine earth should be successively washed
with small portions of hot water by the aid of a
filter -pump. In the solution the total solids are
determined ; they consist, in the main, of the nitrates,
sulphates, and chlorides of sodium, potassium, mag-
nesium, and calcium, which can be determined by
the usual methods. Of course, the amount of soluble
salts to be found in the surface soil at any time is
largely regulated by the previous weather ; after con-
siderable rainfall the soluble salts are washed down
into the subsoil, after long evaporation they are con-
centrated in the surface layers. The amount of nitrates
that is present is further affected by the previous
cropping, temperature, and working of the soil, and
by the manipulation the soil receives after it reaches
the laboratory. Thus the determination of the soil
constituents that are soluble in water does not enter
into the ordinary routine of analysis, their presence
is affected by so many temporary factors which pre-
vent the comparison of one soil with another.
As, however, the determination of the amount of
nitrate present in a soil is often required for other
purposes, it will be convenient here to indicate the
method to be followed. In the first place, the soil must
be analysed either immediately after it has been sampled
and after rapid drying with the aid of heat, for the
manipulation a soil sample usually receives in the
drying, sifting, and other preliminary operations,
will cause the production of large quantities of
nitrates in ordinary soils.
A funnel with a large filtering surface, at least 2
inches in diameter, must be taken ; Warington originally
made use of the inverted upper portion of a Winchester
148 THE CHEMICAL ANALYSIS OP SOILS [chap.
quart bottle with a disc of copper gauze, 2 inches in
diameter resting in the neck, but this may be replaced
advantageously by a Buchner funnel 6 inches in
diameter. In either case the funnel is connected with
an exhaust - pump, the disc is covered with a good
filter paper wetted, then at least 500 grams of the soil
are packed carefully into the funnel and pressed down a
little, care being taken to avoid plastering if the soil is
clayey. The soil sample as it comes from the field is
spread out, roughly crumbled, and mixed ; from this the
500 grams or so are taken and weighed before putting
on the funnel. Another portion is weighed out and
dried in the steam oven, to ascertain the proportion of
water in the sample. 50 cc of hot distilled water are
now poured on the soil, allowed to stand a few minutes,
and the pump started. When the liquid has been drawn
through, successive small portions of hot water are put
on, and the pump started afresh; it will be found
possible to wash out practically the whole of the nitrate
with 100 cc. of water.
If the liquid shows any tendency to come through
the filter turbid, this can be obviated by adding a few
drops of sulphuric acid to the water. In the filtered
liquid the nitrates may be determined by reducing with
the zinc copper couple, distilling off the ammonia and
determining it either by Nesslerising or by titration,
according to its amount. The couple is prepared by
dipping half a dozen strips of thin sheet zinc, 6 inches
long by I J broad, successively into dilute caustic soda,
very dilute sulphuric acid, and then into a 3 per cent,
solution of copper sulphate, in which it is allowed to
remain until it has acquired a good black deposit of copper.
They are washed by immersion in water, and finally in
ammonia -free distilled water, and placed in a bottle
with 200 cc. of the soil extract and a crystal of oxalic
VI.] ANALYSES OF TYPICAL SOILS 149
acid. The bottle is kept in a warm place or an incubator
at 25° for twenty-four hours before distilling off the
ammonia.
The table (Appendix I.) shows the analyses by the
method above described of a few typical soils.
It will be seen, as a rule, that the water retained by
the soil when air dry, the loss on ignition, and the
nitrogen, rise and fall together, because the humus
which contains the nitrogen is the most hygroscopic
constituent of soils. Clay soils which tend to conserve
humus also contain the most constitutional water ; this
further tends to increase the loss on ignition in their case.
The proportion of nitrogen found ranges from 0-5
per cent, in very rich pasture soils down to below 01
per cent, on light arable soils, it is rarely up to 02 per
cent, in arable soils, and the warmer, the more open,
and more worked the soil is, the less will be the pro-
portion of nitrogen. In the fertile hop-gardens of
East Kent the percentage of nitrogen is rarely as much
as 02 per cent., despite the great dressings of nitro-
genous manure that are annually applied.
The proportion of phosphoric acid in soils is not so
variable as the proportion of nitrogen ; it ranges from
about 006 per cent, to 02 per cent. ; the lower amounts
occur generally on the sands and clays, the higher on
loams and soils well provided with calcium carbonate.
The proportion of potash shows extreme variations,
a clay soil may yield one per cent, of potash to strong
hydrochloric acid, a sand only one-tenth as much. It
has already been pointed out that "clay" is chiefly
the result of the weathering of felspars and kindred
minerals containing potash; this weathering is never
chemically complete, so that all soils containing any
considerable admixture of clay are necessarily rich in
potash. The amount dissolved out by hydrochloric
150 THE CHEMICAL ANALYSIS OF SOILS [chap.
acid is also somewhat of an accidental figure, as it
depends very much on how far the previous treatment
of the soil has forwarded the weathering process, for
there remains in all soils rich in potash much material
that will not tyield potash to strong hydrochloric acid
even after forty-eight hours' digestion. For example,
the soil from one of the plots in the Broadbalk wheat-
field at Rothamsted only yielded 0-5 per cent, of potash
to hydrochloric acid, but when completely broken up
by ammonium fluoride it was found to contain 2-26 per
cent, of potash.
Of all the soil constituents calcium carbonate shows
the widest fluctuations ; it may constitute 40 or 50 per
cent, of some of the thin soils resting on the chalk,
or it may sink on some of the sands and clays to such
small proportions as only to be detected by the most
refined analysis.
The importance of the calcium carbonate lies not
in the calcium that it supplies for the nutrition of
plants, but in that it acts as the chief base, maintain-
ing the neutrality of the soil. Many plant diseases,
like the slime fungus which causes " finger-and-toe "
in turnips, etc., are only prevalent when the soil is
losing its neutral condition, and are not found when a
sufficiency of calcium carbonate is present. The normal
changes in a soil are brought about by bacteria, which
only flourish when the medium is neutral or very faintly
alkaline ; as soon as the soil becomes acid the bacterial
actions are largely suspended, and in their place moulds
and other micro-fungi become predominant. It is for
this reason always desirable to test the reaction of a
soil by putting a little on litmus paper, moistening
it, and after a few minutes washing away the soil.
What proportion of calcium carbonate is required for
fertility and health is difficult to say, probably an
VI.] INTERPRETATION OF RESULTS 151
inferior limit of 0-5 per cent, is the lowest that is safe.
In the case of soils containing about this proportion
much will depend on how finely it is disseminated,
05 per cent, in visible pieces will not be so effective
as o- 1 per cent, of the amount in particles of the same
order of size as the clay or silt particles. For this
reason it is advisable when analysing a doubtful soil of
this kind, to make a rough separation of the finer
particles, by pestling up 10 grams of the soil with water,
and pouring off the supernatant liquid after one minute's
standing, as in a mechanical analysis. Having washed
away the finer portion of the soil two or three times in
this way, the residue is dried and the carbonates which
remain are estimated as before, thus a rough idea is
obtained of their distribution among the finer or coarser
sets of soil particles.
Interpretation of the Results of a Soil Analysis.
Though much may doubtless be learnt by a com-
parison of the analysis of a given soil with the analysis
of others whose fertility has been proved by experience
or by actual manurial experiments, there are yet many
considerations which prevent much weight being attached
to the results thus obtained.
A comparison of the total amount of any of the
elements of plant food in the soil with the amount that
is withdrawn by an ordinary crop shows at once that
even in the poorest soils there is sufficient material for
something like a hundred average crops.
The density of the surface soil has already been dis-
cussed ; it will be sufficiently accurate for our purpose
if we consider that the top 9 inches of one acre of
an ordinary arable field weighs 2,500,000 lbs. On this
basis, and without taking into account the fact that
152 THE CHEMICAL ANAL YSIS OF SOILS [chap.
the roots of most cultivated plants range far deeper than
9 inches, there is yet present about 2500 lbs. per acre
of nitrogen, potash, and phosphoric acid in a soil con-
taining only 01 per cent, of these constituents*, which
is about the lower limit usually found. The following
table shows the amounts of these food materials —
nitrogen, phosphoric acid, and potash — which are taken
from the soil by an average crop grown in rotation.
Lbs. per acre.
Wheat.
Swedes.
Barley.
Clover.
Nitrogen
Phosphoric Acid .
Potash .
41.7
20-5
3-56
94.O
24.I
93-5
49.O
207
35-7
159*3*
28.2
102 4
* Partly derived from the atmosphere.
It is clear from a comparison of this table with the
quantities previously specified, that even the poorest soil
contains the nutrient material required by any ordinary
crop many times over, yet we know that crops respond
vigorously to dressings of manure which only add a
fraction to the plant food already stored in the soil.
For example, a wheat crop on poor soil would often be
doubled by the use of 2 cwt. of nitrate of soda per acre,
i.e., by the addition of 35 lbs. of nitrogen in nitrate of
soda to a soil that already contained in the top 9 inches
more than 2000 lbs. per acre. Again, 4 cwt. per acre
of superphosphate, containing about 60 lbs. of phosphoric
acid, will be necessary in the usual rotation to secure
a good swede crop, though there may be already 2000
to 3000 lbs. of phosphoric acid in the soil. We are
then driven to conclude that the nitrogen, potash, and
phosphoric acid are present in the soil in some other
mode of combination than the form in which they exist
in manures ; so that although they may be in the soil
VI.] RESERVES OF PLANT FOOD IN THE SOIL 153
they are in such a state as to be very partially of service
to the growing plant. Further evidence of the enormous
stores of plant food in the soil and the comparative slow-
ness with which they can be utilised may be obtained
by considering the results obtained at Rothamsted,
where on one plot wheat has been grown continuously
without manure for sixty-four years (to 1907). The
average yield from this plot was for the first twenty years,
1844-63, 16-3 bushels of grain and 15-1 cwt. of straw;
1 1-6 bushels of grain and 9-3 cwt. of straw for the next
twenty years, 1864-83 ; and 12-3 bushels of grain and
8-7 cwt. of straw for the third period of twenty years,
1 884- 1 903. It is calculated that during the last fifty years
there have been removed from this plot about 900 lbs.
per acre of nitrogen, 470 of phosphoric acid, and 760 of
potash, i.e., about 18, 9, and 15 lbs. per acre per annum
respectively ; yet from analyses of a sample taken in 1893
the surface soil to a depth of 9 inches still contained
oil per cent, of nitrogen, 0-114 per cent, of phosphoric
acid, and 0-38 per cent, of potash soluble in strong
hydrochloric acid, or 2750, 2850, and 9500 lbs. per
acre respectively. The soil must therefore be re-
garded as possessing most of its plant food in states
of combination that cannot be utilised by the plant,
and these forms slowly pass, by weathering and other
changes, into material which is available for the crop.
The plant food of the soil represents so much capital,
and, as in many another business, but a small proportion
of the capital is liquid at any given time : it is largely
the object of cultivation to effect such a turnover of the
capital as will liquidate some of it in a form available
for the nutrition of the crop.
In the old systems of agriculture, before the land was
enclosed, the whole crop was grown out of capital,
nothing but labour was put into the soil : in which con-
154 THE CHEMICAL ANALYSIS OF SOILS [chap.
nection it is interesting to note that the original mean-
ing of manure was to work by hand.*
It becomes important, then, to attempt to discriminate
between the various forms in which the nitrogen,
potash, and phosphoric acid may be present in the soil,
according as they are soluble, or likely in a short time
to become sufficiently soluble to reach the crop. In the
case of nitrogen we know that of the various compounds
such as proteins and protein residues, amides, ammonia
salts, and nitrates which can be detected in the soil, only
the latter can enter the plant, but that, by processes of
fermentation, all of the other compounds will eventually
pass into the state of nitrate. Of the immediately soluble
nitrogen compounds — nitrates, nitrites, and ammonia —
a very small amount, varying from 5 to 200 lbs. per
acre, is ever present in the soil at any given time,
though it is constantly being renewed by fermentation
processes.
Phosphoric acid also exists in the soil in many
distinct compounds: in combination with carbon, etc.,
it is found in nuclein and lecithin, which in a more
or less humified condition are found among the plant
and animal residues : it also occurs as phosphate of
the sesquioxides of iron and alumina ; as tribasic, and
probably also as dibasic phosphate of lime. Of these
compounds the latter are undoubtedly the most soluble
in either pure water or the carbonic-acid-charged water of
the soil, but much must depend on the physical condition,
as well as on the chemical combination, in which the
material exists. For example, when using tribasic phos-
phate of lime as a manure, the softer phosphates, such as
steamed bone flour, are more effective than the chemically
similar but harder material in ground rock phosphate.
* Cf. Defoe, Robinson Crusoe (17 19)— "The ground that I had
manured or dug up for them was not great."
vi.] DORMANT AND ACTIVE PLANT FOOD 155
It is not so easy to classify the various compounds
of potash existing in the soil : we know that as felspar
passes into kaolinite there are intermediate stages of
weathering in which the potash is gradually becoming
more soluble in soil water, but it is impossible to isolate
or classify the various hydrated silicates containing
potash that must exist. Potash, again, which has once
been dissolved, is caught and retained by the soil in
various ill-defined compounds, some of which must
reach the crop more rapidly than others.
The work, then, of soil analysis must be extended
to include some investigation of the condition of the
plant food in the soil, as well as its absolute quantity :
it is not enough to determine what constituents are
present with the view of making good the deficiencies,
because there is always more than enough for many
crops ; inquiry must be rather directed towards finding
how much is likely to reach the crop. The attempt
to discriminate between the total and what may
be termed the available plant food in the soil, i.e.,
that which is in a form the crop can immediately
utilise, has been made in two ways — by using the
growing plant as an analytical agent, or by attacking
the soil with very dilute acids, whose action is akin
to the natural solvent agencies at work when the
plant is growing. The former process proceeds upon
the assumption that any given plant has a certain
average composition which it will acquire when freely
supplied with all the elements of nutrition; if this
plant be grown upon a soil deficient in one particular,
that deficiency will be reflected in the analysis of the
plant when fully grown. It is thus necessary to select
a standard plant and grow it under normal conditions
of manuring to ascertain the proportion that nitrogen,
phosphoric acid, and potash usually bear to the ash.
156 THE CHEMICAL ANALYSIS OF SOILS [chap.
The selected plant is then grown upon the soil in
question, gathered at the appropriate stage and ana-
lysed, when the composition of the ash, as compared
with its composition under normal conditions, should
give indications of the state of the soil. Various dis-
turbing factors come into play ; for example, the presence
in the soil of large quantities of a non-essential material
like calcium sulphate or sodium chloride lowers the
proportion that potash bears to the total ash without
necessarily indicating any want of potash; again, a
deficiency of nitrogen is more seen in a general stunting
of the whole development of the plant than in a com-
parative poverty of nitrogen in the final growth. But
by selecting suitable test plants, valuable indications can
be obtained as to the need or otherwise for specific
manuring. As a rule, cereals are unsuitable test
plants, since they are well able to satisfy their require-
ments for mineral nutrients from comparatively im-
poverished soils ; the straw of barley, however, shows
considerable variations from which the condition of the
soil as regards its supply of phosphoric acid and potash
can be interpreted. The phosphoric acid in the ash of
barley straw will vary between 2 and 4 per cent, and
the potash between 6 and 24 per cent, and as the
straw of barley grown without special manuring can
readily be obtained, it forms a convenient test plant.
The most sensitive test plants are provided by roots —
swedes for estimating the phosphoric acid, and mangolds
for estimating the potash in the soils on which they
have been grown. The phosphoric acid in the ash of
swedes has been found as low as 9 per cent, when the soil
was one that responded readily to phosphatic manures,
rising to 16 per cent, when the soil was one that required
no phosphatic manure. Similarly, the potash in the ash
of mangolds will vary between 12 and 40 per cent
VI.] ENTRY OF PLANT FOODS BY OSMOSIS 157
The method which is now very largely employed
to determine the mineral plant food in the soil that
may be regarded as immediately "available" for the
crop, consists in attacking the soil with a very dilute
acid, whose action shall be comparable with the natural
solution processes bringing nutriment to the plant. The
mineral matter finds it way by osmosis into the plant
in two ways : either from the natural soil water, or from
the more concentrated solution formed in immediate
proximity to the root-hairs by the attack of the excreted
carbon dioxide upon the soil particles.
The natural soil water is constantly dissolving
small quantities of phosphoric acid, potash, and other
materials, in which it is aided by the carbonic acid
it also contains ; as this water passes by osmosis into
the root-hairs it will carry with it the dissolved
material, with the exception of any particular ion or
radicle which has already attained in the cell sap a
higher concentration than it possesses in the external
soil solution. But if the soil water alone brought the
mineral matter with it, not enough enters the plant
to account for the observed facts. For example, the
growth of a crop of a ton and a half of clover hay
requires the transpiration through the leaves, and
therefore the absorption at the root, of about 400
tons of water (see p. 91) ; the same crop would also contain
about 50 lbs. of potash. If, then, the crop derived all
its mineral matter by the simple inflow of the soil
water into the root, the 50 lbs. of potash must have
been originally dissolved in the 400 tons of water
that passed through the crop, which means that the
soil water contained as much as 0006 per cent, of
potash, a greater concentration than is observed in
humid climates. In fact, the particular ions or radicles
concerned in nutrition enter the root faster than the
ISS THE CHEMICAL ANALYSIS OF SOILS [chap.
water does ; they diffuse through the cell wall because
the sap within is maintained in a less concentrated
state as far as they are concerned than the external
soil water, because they are constantly being withdrawn
from solution by the living protoplasm of the cells.
It has been supposed that solvent action of the soil
water is also assisted by the cell sap of the root-hairs, which
is always distinctly acid in its reaction ; these root-hairs
are always very closely in contact with soil particles,
and some of the acid has been supposed to diffuse out-
wards through the cell wall. Sachs has shown that a
polished slab of marble is etched wherever the fine
roots of a plant came in contact with it, and on the
strength of this and similar experiments, the cell sap
has been regarded as a factor in bringing the minerals
of the soil into solution for the plant. All the solvent
actions, however attributed to the cell sap, can be
brought about by the carbon dioxide which is always
being excreted by the root, and more critical experiments
seem to negative the opinion that any fixed acids pass
outwards through the cell wall of a living plant, at any-
rate after it has passed the seedling stage.
Whatever the theories which have been formed as
to the manner in which the mineral constituents of the
soil pass into solution for the plant, it is improbable that
the conditions can be reproduced in the laboratory, and
for the practical purposes of analysis the desideratum is
a solvent that will dissolve the class of material which
is found by experience to reach the immediate crop, but
which will not touch the same material should its state
of combination or physical condition be such as to render
it unavailable for the plant. Various solvents have been
proposed: for example, Deherain showed that dilute
acetic acid, while dissolving some phosphoric acid from
ordinary soils, was incapable of extracting any from a
vi.] SOLVENTS FOR AVAILABLE PLANT FOOD 159
particular soil which yielded very poor crops unless man-
ured with superphosphate, though it contained o-i per
cent, of phosphoric acid soluble in strong hydrochloric
acid. Hence he concluded that dilute acetic acid
forms a solvent only capable of attacking the avail-
able phosphoric acid. A solution of carbonic acid has
been suggested as akin to the natural soil water;
other solutions have been employed because they will
dissolve certain of the compounds of phosphoric acid
in the soil, but not all — the calcium phosphates, for
example, but not the phosphates of iron and alumi-
nium ; other solvents, again, are recommended as
akin to the acid cell sap. However, experience seems
to show that the 1 per cent, solution of citric acid pro-
posed by Dyer in 1894 gives results that are most in
accord with what is known of the soil, either from its
past history or by cropping experiments.
The method of conducting the analysis is as follows :
— 200 grams of the " fine earth " that has passed the 3
mm. sieve, in its air-dried state, is placed without any
further grinding in a dry Winchester quart bottle with
20 grams of pure crystallised citric acid and 2 litres of
water. The bottle should either be one previously used
for the storage of strong acids or should have a pre-
liminary soaking in dilute hydrochloric acid. The
mixture of soil and dilute acid is thoroughly shaken from
time to time, as often as may be convenient, during the
seven days the solvent action is allowed to proceed.
After seven days the solution is filtered, and two aliquot
portions of 500 c.c. each are evaporated to dryness and
ignited to get rid of the citric acid and other organic
matter. The residues are dissolved in hydrochloric acid,
again evaporated, and heated for a time to I05°C. to
render all the silica insoluble. In one portion the
phosphoric acid, and in the other the potash, are deter-
160 THE CHEMICAL ANALYSIS OF SOILS [chap.
mined by the processes previously described. The time
of extraction may be shortened to twenty-four hours if
the bottle be put in a good end-over-end shaking
machine which will keep the soil and the solvent
thoroughly agitated.
An examination of the citric acid solution shows that
all the compounds of phosphoric acid that have been
indicated as existing in the soil are more or less
attacked; at any rate, the resulting solution contains
organic matter and salts of aluminium and iron, in
addition to calcium. It has been suggested that the
varying amounts of calcium carbonate contained by soils
will much affect the material dissolved by the citric acid,
some of which becomes neutralised by the calcium
carbonate. But though the amount of phosphoric acid
dissolved from a given soil by the citric acid solution will
be diminished if the calcium carbonate in the soil is
increased, a very similar reduction will be effected in the
natural processes of solution of the soil phosphates under
field conditions. No attempt should be made to add an
extra amount of citric acid to combine with the calcium
carbonate ; secondary solvent actions are set up both by
the carbon dioxide evolved and by the calcium citrate
formed, moreover, the real comparative basis of the
method of analysis is destroyed.
It must not be supposed that the citric acid solution,
nor indeed any of dilute acid solvents that have been
proposed for this purpose, are real differential solvents,
which extract the material in the soil which is available
for the plant and leave untouched whatever is combined
in some other form. In reality, as soon as the acid has
been for a sufficient time in contact with the soil a state
of equilibrium is attained between the phosphoric acid,
for example, that has gone into solution and that which
remains in the solid state. The precise equilibrium
VI.] t/SE OF WEAK ACID SOLVENTS 161
attained depends not only upon the strength of the acid
solution and the nature and amount of the phosphoric
acid compounds in the soil, but also on the nature and
amount of the bases that are there present. If, for
example, the citric acid solution is filtered off after it has
extracted all the phosphoric acid it can, and a second
portion of solution is added and the soil extracted afresh,
then more phosphoric acid will go into solution, the
amount being smaller than before but still considerable.
A third, a fourth, and even a fifth extraction does not
remove from the soil all the phosphoric acid that will
go into solution in the dilute citric acid solution. Thus
it is impossible to say that the dilute citric or any other
acid dissolves out and measures the "available" phos-
phoric acid or potash ; it does, however, provide a figure
indicating the comparative rate at which the soil is likely
to yield up its nutrient constituents to the normal solvent
actions going on in the soil. The results, then, of this
method of analysis are not to be regarded as absolute
amounts, but as empirically obtained figures which must
be interpreted in the light of experience. The type of
the soil plays a part ; for example, a quantity of citric acid
soluble phosphoric acid that would indicate poverty in a
strong loam or in a soil rich in organic matter like an
old pasture, would be ample for ordinary crop purposes
if the soil were light and sandy. Again, the crop must
be taken into account ; a percentage indicating enough
available phosphoric acid in the soil for wheat or man-
golds would indicate deficiency when the swede crop
came round.
In certain cases, by continuing the extraction with
citric acid until the amount going into solution at each ex-
traction becomes approximately constant at some low
figure, it is possible to differentiate between the phos-
phates in the soil that are easily soluble and may
L
162 THE CHEMICAL ANALYSIS 0£ SOILS [chap.
therefore be termed " available," because they possess a
comparatively high solubility factor, and other phosphates
which would yield, under natural conditions, solutions
too dilute to nourish the crop efficiently.
For example, the following table shows the amounts
of phosphoric acid dissolved by successive extractions of
certain Rothamsted soils with I per cent, citric acid
solution, from which it will be seen that at about the fifth
extraction the quantity dissolved begins to approach a
constant.
Phosphoric Acid, Mgms. per ioo Grams Soil.
Field.
Plot.
Treatment each Year.
Extraction.
1st.
2nd.
8rd.
4th.
5th.
6th.
Broadbalk
»>
»»
11
»»
11
3
5
7
8
10
2
Unmanured .
64 lbs. P 2 5 , no Nitrogen
64 lbs. P 2 5 , 86 lbs. N. .
64 lbs.P 2 5 , 129 lbs. N. .
86 lbs. N. only
Dunged ....
6«4
69-0
56.1
46-3
7-7
49-3
6-8
28-0
22-8
18.9
5.2
15-3
3-9
H-3
8-9
7.8
3-3
7-5
3-0
7-3
6-5
5-3
2.7
6.0
2-5
4-5
4.4
4.0
27
4.4
2-3
4.4
3-o
27
Hoos
»»
n
»
iAA
2AA
3AA
4AA
43 lbs. N. only
64 lbs. P 2 5 , 43 lbs. N. .
43 lbs. N. and Potash
64 lbs. P 2 5 , 43 lbs. N.,
Potash
6-3
52-2
6-3
53-5
3-5
2I«2
2-7
io-6
2*2
8.9
2-3
6.4
1.9
6-5
21
4.9
2«0
3-8
1.9
4-5
I «2
2-9
i-5
3-8
The successive amounts going into solution in the
first four or five extractions of the soil from the plots
which had received soluble phosphoric acid every year
are found to be decreasing in a logarithmic series, and
this may be supposed to indicate that the solvent is
dealing with only one class of material, which is entirely
removed at about the fifth extraction. After the
fifth extraction there only remains the more insoluble
classes of phosphates which form the main stock in
VI.]
AVAILABLE PLANT FOOD
163
the soil. The unmanured plot and that which has
received dung do not show the same regular decrement,
indicating that the solvent is each time dealing with a
more complex mixture of phosphates successively going
into solution. This conclusion is strengthened when the
total amount of phosphoric acid dissolved in five extrac-
tions is compared with the amount known to have been
applied to the land during the period the plots have
been under experiment, after deduction has been made
of that which is also known to have been removed in the
crop.
Phosphoric Acid in Rothamsted Soils.
Field.
Plot.
Dissolved in Five
Extractions.
Supplied
in
Man are.
Removed
in
Crop.
Surplus.
Per cent.
Lbs.
per acre.
Broadbalk
>>
11
>•
3
5
7
8
2
0-0226
0-I20I
0-0987
0-0823
0-0825
565
3000
2470
2055
2060
O
39 6
3810
38IO
4780
550
790
I370
1520*
1650
-550
3170
2440
2290
3I30
Hoos
iAA
2AA
4AA
0-0159
0*0926
00799
400
2315
2000
3390
3390
555
1200
1240
-555
2190
2150
* Approximate estimate, since the crop has rarely been analysed.
It will be seen from the above table that the amount
of phosphoric acid dissolved by the five extractions
agrees closely with the surplus left by the manuring in
all the cases where the phosphoric acid has been put on
as soluble mineral superphosphate. This is not the case,
however, for the plot manured with dung, which contains
a considerable proportion of difficultly soluble phosphate.
It should not be supposed that the whole of the
so-called " available " phosphoric acid or potash will be
164 THE CHEMICAL ANALYSIS OF SOILS [chap.
removed by the crop; even the minimum of o-oi per
cent., soluble in citric acid, which has been suggested,
as marking the limit of fertility, means about 250 lbs.
per acre in the surface layer 9 inches deep : and few
crops will take away as much as 50 lbs. per acre of phos-
phoric acid or 150 lbs. per acre of potash. No crop
searches the soil so thoroughly for food as does the
solvent acid : if we assume that the roots themselves by
their excretion of carbon dioxide effect some of the
solution, it is obvious that they come in contact with but
a small proportion of the soil particles ; nor can the soil
water, limited in amount and moving slowly, attack the
soil with the vigour displayed by an acid which is
continuously shaken with a comparatively small propor-
tion of soil. Even in the case of material so essentially
* available " as a manure soluble in water, the whole of
the manure applied is never recovered in the crop ; e.g.,
in the experiments with wheat at Rothamsted, only 73
per cent, and with mangolds 78 per cent, of the nitrogen
supplied as nitrate of soda has been recovered in the
crop, though there was an abundant supply of the other
manurial constituents. In the same way, on the plot
with an excess of nitrogen there was recovered only
36 per cent, of the phosphoric acid supplied as super-
phosphate, and 50 per cent, of potash supplied as sulphate
of potash.
In other words, the "available" plant food in the
soil represents not that which the succeeding crop will
remove, but that which it can draw upon : how much
it will acquire will depend on a variety of factors, such
as the nature of the plant, the texture of the soil,
the supply of water, and other necessaries of nutrition.
No method akin to solution in dilute citric acid
has yet been devised for determining what proportion
of the nitrogen reserves in the soil is likely to be avail-
vi.] DETERMINATION OF SOLUBLE HUMUS 165
able. The conversion of the nitrogenous matter of
the soil into soluble nitrates, in which form nitrogen
enters the plant, is a biological process which is influ-
enced by a number of conditions, such as temperature,
degree of moisture and aeration of the soil, the mechani-
cal treatment it receives, all impossible to predict.
Some idea of the condition of the organic matter
and the readiness with which it is likely to change, may
be obtained by a determination of the humus soluble in
dilute ammonia and the percentage of nitrogen in this
humus, or again by a study of the ratio of carbon to
nitrogen in the organic matter as previously indicated
(p. 46). In order to determine the soluble humus, 20
grams of the soil are digested with enough 1 per cent,
hydrochloric acid to dissolve all the calcium carbonate,
thrown upon a filter, washed with a little more of the
hydrochloric acid and then with water until neutral.
The soil is then washed off the filter into a bottle with a
4 per cent, solution of ammonia and shaken for twenty-
four hours, after which the bottle is left to stand until
the bulk of the inorganic matter of the soil has settled.
150 c.c. are pipetted off and evaporated to dryness over
the water-bath, weighed and ignited, a deduction being
made of the inorganic matter remaining after ignition.
To determine the nitrogen content of the humus a
second 1 50 c.c. are placed in a Kjeldahl flask. Two or
three grams of magnesia are added and the whole
evaporated to dryness to get rid of the ammonia in the
solution, after which the contents of the flask are
digested with sulphuric acid in the usual manner.
Valuable as these determinations may become in
judging a soil, a sufficient body of data do not as yet
exist to enable them to be interpreted with precision.
In the analysis of a soil, without doubt the most
important figure is the proportion of calcium carbonate,
166 THE CHEMICAL ANALYSIS OF SOILS [chap.
for on that must be based the decision not only of
whether liming is necessary, but what class of artificial
manures should be employed. Where the calcium
carbonate is scanty, manures like superphosphate and
sulphate of ammonia should never be employed, but basic
slag or some neutral phosphate on the one hand, and
nitrate of soda as a source of rapidly acting nitrogen on
the other. The texture of the soil, the rapidity with
which decay and nitrification of organic matter take
place, freedom from fungoid diseases, all depend on an
adequate proportion of calcium carbonate in the soil,
say from half to one per cent. ; so that of all the
determinations this is the most important.
The determinations of the loss on ignition, the
nitrogen, and possibly the humus, give the analyst an
idea of the reserves of organic matter in the soil;
judged in conjunction with the mechanical analysis and
the proportion of calcium carbonate, an opinion can be
formed as to the condition of the soil and how far these
reserves are likely to be brought into play by cultiva-
tion. An opinion may, again, be formed as to the need
for organic manures to increase the humus content of
the soil, or whether fertility is likely to be maintained
with purely mineral manures.
A consideration of the available phosphoric acid and
potash will give the analyst an idea of the immediate
need or otherwise of mineral manuring ; the proportions
these bear to the "total" phosphoric acid and potash
give him grounds for deciding whether the lack is only
temporary or real. In the former case measures may
be taken to liberate some of the reserves, as by the
judicious use of lime or of organic manures which will
generate carbonic and other acids within the soil.
Such further questions as the presence of harmful
substances, or even of an excess of more normal con-
VI.] INTERPRETA TION OF ANAL YSES 1 67
stituents of the soil, must be considered by the analyst,
but will be dealt with in a later section.
In some cases it will be possible by a chemical
analysis to pronounce a given soil to be unsuited to a
particular crop : as a rule, however, it is not its chemical
composition which fits the land for a particular crop,
but its mechanical texture, water-bearing power, drain-
age, etc. In most cases the soil can be adjusted to the
crop by manure, though the process may be unsound
from an economic standpoint, but no expenditure can
ever rectify unsatisfactory texture, e.g., convert a light
sand into good wheat land.
Even in considering the chemical analysis of a
soil, no hard-and-fast rules can be laid down, the
judgment and experience of the analyst must come into
play in deciding how far the deficiency or excess of one
constituent is likely to affect the action of some of the
others : and again, how far the texture, the aspect, and
other factors that can only be ascertained in situ, will
exercise an influence upon the enormous reserves of
plant food contained in every soil.
CHAPTER VII
THE LIVING ORGANISMS OF THE SOIL
Decay and Humification of Organic Matter in the Soil — Alinit—
The Fixation or Free Nitrogen by Bacteria living in Sym-
biosis with Leguminous Plants — Soil Inoculation with Nodule
Organisms — Fixation of Nitrogen by Bacteria living free in the
Soil — Nitrification — Denitrification — Iron Bacteria — Fungi
of Importance in the Soil : Mycorhiza, and the Slime Fungus
of " Finger-and-Toe."
The soil is the seat of a number of slow chemical
changes affecting the organic material it receives:
residues of an animal or vegetable nature, when applied
to the soil, are converted into the dark-coloured complex
known as "humus," which becomes eventually oxidised
to carbonic acid, water, nitric acid, and other simple
substances serving as food for plants. These changes,
at one time regarded as purely chemical, are now
recognised as dependent upon the vital processes
of certain minute organisms, universally distributed
throughout cultivated soil, and subject to the same
laws of nutrition, multiplication, life and death, &s
hold for the higher organisms with which we are more
generally familiar.
The microscopic flora of the soil, roughly classed
as fungi and bacteria, is vast, and has been very in-
adequately explored as yet: certain types of change
m
CHAP, vil] TYPES OF BACTERIAL ACTION 169
in the soil materials have, however, been associated
with particular organisms or groups of organisms, and
many of these changes are of fundamental importance
in the ordinary nutrition of plants. The organisms in
the soil which so far have received the chief attention
are those concerned with the supply of nitrogen to the
plant. Certain organic compounds of nitrogen, chiefly
of a protein nature, become gradually broken down by
the action of soil bacteria into simpler compounds, e.g.,
into amino-acids, and then into ammonia, which latter
substance is seized upon by other organisms and oxidised
successively to nitrous and nitric acid. As nitric acid
is almost the only form in which the higher plants
obtain the nitrogen they require, the fertility of the
soil is wholly bound up in the maintenance of this
cycle of change. Under certain conditions the work
of other organisms intervenes, and the nitrogen com-
pounds, instead of becoming nitric acid, are converted
into free nitrogen gas, and are lost to the soil. Per
contra, another group of organisms possesses the power
of "fixing" free nitrogen, i.e., of taking the gaseous
element nitrogen and combining it with carbon,
hydrogen, oxygen, etc., into forms available for the
higher plants. Such organisms sometimes act when
living in "symbiosis" with plants possessing green
carbon-assimilating tissue: the two form a kind of
association for mutual support, the bacteria deriving
the carbohydrate which they must consume from the
higher plant supplied by them with combined nitrogen.
Other symbiotic processes have been traced in the
soil, and may yet be made to play an important part
in the nutrition of field crops. Indeed, a number of
tentative trials have already been made with the view
of increasing the productiveness of the soil by introduc-
ing either useful organisms that were wanting, or
170 THE LIVING ORGANISMS OF THE SOIL [chap.
improved types to replace already existing kinds of
less effective character.
The Changes of Organic Matter in the Soil.
The surface layer of soil is constantly receiving
additions of organic matter, either leaves and other
debris of vegetation covering the ground, together with
the droppings of animals consuming that vegetation,
or dung and other animal and vegetable residues which
are supplied as manures to cultivated land. These
materials rapidly change in ordinary soil, losing almost
immediately any structure they possess, becoming
dark-coloured humic bodies, or even burning away as
thoroughly as if placed in a furnace. That these
changes are due to micro-organisms is seen by their
immediate cessation if the soil be treated with anti-
septics like chloroform or mercuric chloride : or if the
mixture of soil and organic matter be sterilised by heat-
ing. Attempts have been made to estimate the number
of bacteria contained in the soil : the prodigious numbers
obtained, 2 up to 50 millions or more per cubic centi-
metre of the upper soil, show little beyond the fact that
the soil is tenanted much as any other decaying organic
material would be. The soil bacteria are always associ-
ated with a certain number of fungi and yeasts,
especially when the reaction of the medium is at all
acid: the organisms are most numerous in the surface
layer, though they are still to be found in the deepest
subsoils. Below a certain depth they must disappear,
because deep well water often comes to the surface in
an absolutely sterile condition. The changes which
organic materials undergo in the soil may be roughly
grouped into two classes ; according as there is free
access of oxygen or not, either decay (eremacausis)
vil] FORMATION OF HUMUS 171
with eventual resolution into the simplest inorganic
oxidised compounds, or " humification " will set in.
These changes can be best indicated by the fate of a
dead branch when it falls either upon the ground, or
into a pond or swamp where it becomes buried in the
mud at the bottom. In the latter case the fermentation
changes cause the wood to darken even to blackness ;
gases like carbonic acid and marsh gas are split off, so
that the material becomes proportionally richer in
carbon and poorer in oxygen. Eventually, however, the
process slackens, the losses practically cease, and a
large proportion of the original material persists. On
the other hand, the branch exposed to the air, without
darkening very much, becomes slowly resolved by the
action of fungi and bacteria into carbonic acid and
water, ammonia, nitrogen gas, and mineral salts, with
much the same final result as though it had been placed
in a furnace. In soil, both these types of change may
go on, and the conditions of the soil as regards aera-
tion, drainage, temperature, and cultivation, determine
which will predominate.
Practically, the whole group of aerobic bacteria, i.e.,
those which require free oxygen for their development,
and fungi are capable of bringing about the oxidation
changes which result in the production of carbonic
acid, the combustion of some carbohydrate being
essentially the means by which they derive their
energy. As an intermediate step between the carbo-
hydrate and the carbonic acid, a certain amount of
humus is produced — " mould," or the " mild humus "
of the German writers. Examples of this material
can be seen in the leaf-mould collected by gardeners
from woods, or the fine, brown powder which can be
scraped out of the inside of a hollow tree, particularly
of a willow ; this mould differs from the peaty humus,
172 THE LIVING ORGANISMS OF THE SOIL [chap.
to be described later, in its neutral reaction and in
the readiness with which it can be further oxidised.
Neutral in its reaction, it yields but little soluble
"humic acid" to the attack of an alkali.
Besides carbohydrates, most aerobic bacteria require
some carbon compound of nitrogen, and will begin to
break down protein and other nitrogen-containing
materials. The products of their attack are succes-
sively peptones, bodies like leucin and tyrosin, even-
tually ammonia, and probably free nitrogen, but the
ultimate production of ammonia is perhaps the most
characteristic feature of the aerobic fermentation of
protein bodies. Other amides are also resolved into
ammonia, of which a characteristic example is afforded
by the change of urea into ammonium carbonate.
This process [which is one of hydrolysis, not of oxi-
dation, being represented in the gross by the equation
CO(NH 2 ) 2 +2H 2 = (NH 4 ) 2 C0 3 ] is brought about by
more than one organism, universally distributed and
abundant in such places as stables and cattle stalls.
In warm weather the conversion of the urea of the
urine into ammonium carbonate is very rapid, and as
the re3ulting product dissociates into gaseous ammonia
and carbonic acid, to this cause is due the smell of
ammonia which is always to be noticed in such places.
These changes to ammonia are the necessary prelim-
inaries to the final oxidation process or nitrification,
which, as the means by which the higher plants receive
their supplies of nitrogen, will be discussed separately.
The various oxidation processes in the soil are, like
all other bacterial actions, promoted by a certain
warmth, the optimum temperature being about 25°-30°,
by a sufficiency of moisture, and by the presence of
mineral food, like phosphates and potash salts. In
any great quantity, however, salts are harmful,
vil] DEC A V OF ORGANIC MATTER 1 73
particularly sodium chloride; an acid reaction also
diminishes considerably the rate of decay. Speaking
generally, bacteria do not thrive as soon as the
medium passes the neutral point, and all the decay
processes must be carried out by the development of
fungi when the medium is acid. The presence of
chalk, or any form of carbonate of lime, by neutral-
ising any acids as fast as they are formed, promotes
the destruction of organic matter. Wollny has also
shown that calcium humate will oxidise much more
rapidly than uncombined humic acid placed under
similar conditions. To the absence of carbonate of
lime and mineral salts generally, may be ascribed the
tendency of -humus to accumulate and persist on the
very light, sandy heaths, where the soil is dry and
hot in summer, and also well aerated. It has already
been indicated, in treating of humus, that the various
organic compounds of nitrogen show very different
susceptibility to the breaking-down process which even-
tually renders the nitrogen available for the crop —
amongst the most resistent substances being the nucleo-
proteins in the undigested portions of food which form
dung, and the humus residues from poor, cropped-out
land. As in all cases much of the nitrogen of both soil
and manure seems to pass into obstinately persistent
compounds yielding slowly, if at all, to oxidation, and
hence wasted to the farmer, an attempt has been made
to increase the preliminary breaking down of nitrogen
compounds in the soil by the introduction of certain
very active bacteria. Stoklasa has shown that various
organisms — B. megatherium> B. fluorescens, etc. — when
seeded into soil manured with bone-meal or similar
materials, increase both the nitrogen and the phosphoric
acid obtained by the plant. A pure cultivation of some
such organism, B. Ellenbachensis> was for a time sold
174 THE LIVING ORGANISMS OF THE SOIL [chap.
commercially under the name of alinit, and though
the power of fixing nitrogen was claimed for it, its
chief action was probably such as was described above. It
has been found to cause increased crop returns on peaty
or other soils rich in humus, or where slow-acting
nitrogenous manures have been applied.
The fermentation which goes on in absence of
oxygen, is brought about by a large number of bacteria,
some of which are only active in the absence of oxygen,
others are aerobic, but will continue their work when
deprived of free oxygen. Carbohydrates are decomposed
with formation of carbonic acid and other gases like
hydrogen and marsh gas, butyric and other fatty acids,
a residue of humus being always produced at the same
time. The protein bodies readily undergo putrefactive
change, with the production of tyrosin and various
amino-acids, fatty acids, ammonia, phenol, and other
bodies containing an aromatic nucleus, gaseous com-
pounds of sulphur, etc. In the main, however, the
changes of organic material in the soil fall upon the
cellulose; it loses carbonic acid, marsh gas, hydrogen,
etc., and becomes humus with a gradually increasing
proportion of carbon ; the nitrogenous materials resist
attack more than the carbohydrates, and hence tend to
accumulate, so that an old sample of deep-seated peat
is richer in nitrogen than a more recent sample taken
from nearer the surface. Finally the humus thus pro-
duced, which may be called peat, is essentially an acid
product, and even when aerated and supplied with
mineral materials will oxidise with extreme slowness.
The Fixation of Free Nitrogen.
In the earliest theories regarding the nutrition of the
plant which were accepted after chemistry had become
an exact science, it was considered that the plant
Vii.J EARLY THEORIES 175
derived its nitrogen from the humus of the soil, as,
for example, in de Saussure's statement that " Plants
receive their nitrogen almost entirely by the absorption
of the soluble organic substances." This view was
displaced by the so-called " mineral theory " of Liebig,
who, in laying down the broad principle that the plant
only derived certain necessary mineral constituents, its
"ash," from the soil, and the whole of its carbon
compounds from the atmosphere, was led to regard
the nitrogen as well as the other combustible matters
of the plant as due to the atmosphere, largely because
of the exaggerated estimate which then prevailed as to
the amount of ammonia from the air that was brought
down in the rain. Boussingault had already shown, by
weighing and analysing the crops on his own farm for
six separate courses of rotation, that from one-third to
one-half more nitrogen was removed in the produce
than was supplied in the manure. The gain of nitrogen
was little or nothing when cereal crops only were
grown, but became large when leguminous crops were
introduced into the rotation. Liebig, however, con-
sidered that cereals, as well as the other plants, were
able to draw their ammonia from the atmosphere,
and that, provided sufficient mineral plant food were
forthcoming, there was no need of ammonia compounds
in the manure.
This view of Liebig's, though modified later, when
he admitted that cereals must obtain their nitrogen
from a manurial source in the soil, led to considerable
investigation of the source of the nitrogen in the plant.
Boussingault himself carried out a long series of
laboratory experiments, in which weighed seeds con-
taining a known proportion of nitrogen were grown
in artificial soils containing no nitrogen, but supplied
with the ash constituents of the plant. Care was
176 THE LIVING ORGANISMS OF THE SOIL [chaP.
taken to remove all ammonia from the air in which
the plants were grown, and from the water and
carbonic acid supplied to them ; finally, after growth
had ceased, the amount of nitrogen in the plant and
in the soil was determined. In some cases a known
quantity of nitrogenous compounds was supplied as
manure; but all the results went to show that there
was no gain of combined nitrogen during growth ; the
seed and manure at starting contained as much nitrogen
as was found in the plant and soil at the end.
Similar experiments were carried out with the
utmost precautions by the Rothamsted investigators,
who likewise found no gain of nitrogen by the plant
from the atmosphere. The following results, obtained
by Lawes and Gilbert in 1858, will serve to show
the agreement between the nitrogen supplied and
recovered : —
d
a
d
d
and
nts.
d a £
8>5 1
geni
and
nts.
a 2
o& f*
■3
o'd fl
O ■— < c3
.;-
£0Q <h
Nitr
See
Ma
88 tJ
Wheat
0-0078
0-0081
+ 0*0003
0-0548
OO536
-OOOI2
Barley
0-0057
0-0058
+ O-OOOI
0-0496
OO464
-0-0032
Oats .
0-0063
0-0056
- 0-0007
0O3I2
002l6
- 0-0096
Beans
0-0750
0-0757
+ 0.0007
0-07II
00655
- 0-0056
Peas .
O.OI88
00167
-0'002I
0-0227
0O2II
-OOOl6
Clover
...
• • •
...
0O7I2
OO665
-0-0047
Buckwheat
Q.0200
0-0182
-0.0018
00308
0O292
-OOOl6
It has sometimes been objected that the plants in
these experiments made such a poor growth as compared
with their normal development in the open air that they
never attained their usual power of fixing nitrogen.
However, HellriegeFs experiments on plants which
were supplied with limited amounts of nitrogen showed
that growth is practically proportional to the supply of
VII.]
FIXATION OF NITROGEN
177
nitrogen as long as that is below the maximum required
by the plant. Field experiments at Rothamsted with
leafy crops like mangolds, to which a very small amount
of nitrogen was supplied in order to give them a start,
showed that the increase thus produced was only pro-
portional to the nitrogen supplied, so that there is no
evidence that even a plant which has begun to grow
vigorously can then continue its development by taking
nitrogen from the atmosphere.
From all these experiments the conclusion was
drawn that cultivated plants are unable to "fix"
atmospheric nitrogen, but obtain this indispensable
element in a combined state from the soil together
with the ash constituents; and such was the opinion
that prevailed for something like thirty years.
Notwithstanding the conclusive nature of all the
laboratory experiments, there was still a residuum of
facts obtained under field conditions which were in-
explicable on the theory of the non-fixation of nitrogen,
and these facts were chiefly connected with the growth
of leguminous crops.
Boussingault's crop statistics have already been
referred to ; the following table gives a short summary
of the kind of results he obtained : —
Rotation.
Nitrogen.
Kilos per hectare.
Supplied in
Manure.
Removed in
Crop.
Wheat, Wheat, Fallow
Potatoes, Wheat, Clover, Wheat or Turnips,
Potatoes, Wheat, Clover, Wheat . ,
Lucerne, 5 years
87.2
202 -2
182
• t •
82-8
268.5
339
1035
The amount of nitrogen removed was equal to
If
178 THE LIVING ORGANISMS OF THE SOIL [chap.
that supplied only when wheat was grown, but became
progressively greater the more frequently leguminous
crops occupied the ground.
At Rothamsted the following average quantities of
nitrogen were removed per acre per annum in the
crop, when mineral manures only were applied : —
Wheat (24 years) • • • • 22-1
Barley (24 years) • • • • 224
Roots (30 years) • . . • 16*4
Beans (24 years, only 21 years in Beans) • 45*5
Red Clover (22 years, only 6 years Clover) • 39*8
In this case also the amount of nitrogen in the
produce was much increased when a leguminous crop
was grown.
Another of the Rothamsted experiments showed still
more strikingly the accumulation of nitrogen by a
leguminous crop. A piece of land which had been
cropped for five years by cereals, without any nitro-
genous manure, v/as divided into two portions in 1872,
one being sown with barley alone, and the other with
clover in the barley. In 1873 barley was again grown
on the one portion, but the clover on the other, three
cuttings of clover being obtained. Finally, in 1874,
barley was grown on both portions. The quantities of
nitrogen removed in the crops of 1873 an d 1874 are
shown in the table.
Nitrogen in Crop— Lbs. per
Acre.
1873
1873
Barley . 37-3
Clover . 1 5 1. 3
1874
1874
Barley
Barley
. 39*1
. 694
Thus, the barley which followed clover obtained
30-3 lbs. more nitrogen than the barley following barley
though the previous clover crop had removed H4lba
more nitrogen than the first barley crop. An analysis
Vii.] LEGUM1N0S& AND NITROGEN FIXATION 179
of the soil was made in 1873, after the clover and barley
had been removed ; this showed down to the depth of
9 inches an excess of nitrogen in the clover land, despite
the larger amount which had been removed in the crop.
In Soil after Barley • 01416 per cent Nitrogen.
In Soil after Clover • 0*1566 „ „
In another experiment, land which had previously
grown beans and then been fallow for five years, was
sown with barley and clover in 1883, the clover being
allowed to stand in 1884 and 1885. At starting the
soil was analysed ; the surface 9 inches contained on
an average 2657 lbs. per acre of nitrogen, while of
nitrogen as nitric acid the soil only contained 24-7 lbs.
per acre down to a depth of 6 feet. As a result of the
three years cropping with barley and clover, and then
with clover only, an average amount of 319-5 lbs. of
nitrogen was removed, yet the soil contained, on
analysis at the end of the experiment, 2832 lbs. of
nitrogen per acre in the top 9 inches, or a gain of
175 lbs. per acre in the three years, making a total,
with the crop removed, of nearly 500 lbs. of nitrogen
per acre to be accounted for.
The consideration of field trials of this description
led many observers to think that there still might be
some fixation of free nitrogen, particularly by legumi-
nous plants. Voelcker, in England, when discussing
the power of a clover crop to accumulate nitrogen,
expressed the opinion that the atmosphere furnishes
nitrogenous food to that plant; in France, it was
maintained by Ville; Berthelot also brought evidence
to show that the soil itself, by the aid of its micro-
scopic vegetation, assimilated some free nitrogen. Even
in the laboratory experiments, some of Boussingault's
results, and others of Atwater, in America, showed a
1S0 THE LIVING ORGANISMS OF THE SOIL chap.
gain of nitrogen. But the clearing up of the whole
subject came with the publication, in 1886, of the
researches of Hellreigel and Wilfarth. These investi-
gators found that when plants were grown in sand
and fed with nutrient solutions, the Gramineae, the
Cruciferae, the Chenopodiaceae, the Polygonaceae, grew
almost proportionally to the amount of - combined
nitrogen supplied; and, if this were absent, nitrogen
starvation set in as soon as the nitrogen of the seed
was exhausted. With the Leguminosae, however, a
plant was observed sometimes to recover from the
stage of nitrogen starvation, and begin a luxurious
growth which lasted until maturity, though no com-
bined nitrogen was supplied. In such cases the root
of the plant was always found to be set with the little
nodules characteristic of the roots of leguminous plants
when growing under natural conditions. Further experi-
ments were made in which the plants were grown in
sterile sand, but as soon as the stage of nitrogen hunger
was reached, a small portion of a watery extract of
ordinary cultivated soil was added; whereupon, the
plants receiving the extract recovered from their nitrogen
starvation and grew to maturity, assimilating consider-
able quantities of nitrogen. The renewed growth and
the assimilation of nitrogen were always found to be
attendant upon the production of nodules on the roots.
The nodules were found to be full of bacteria, to which
the name of Pseudomonas radicicola has been given.
They could only be produced by previous infection either
by an extract of the crushed nodules or of a cultivated
soil ; in some cases (lupins, serradella) only by soil which
had previously carried the same crop.
These results, though not at first accepted by Lawes
and Gilbert, led to a repetition of the experiments,
which brought out the fact that in their earlier
VII.] HOW IS NITROGEN FIXATION EFFECTED? 181
trials with leguminous plants the necessary inocula-
tion had always been wanting because of the great
care that had been taken to prevent the entry of any
accidental impurity. Eventually, both at Rothamsted
and by other investigators, the conclusions of Hellreigel
and Wilfarth were confirmed, that when leguminous
plants are grown under sterile conditions, without a
supply of combined nitrogen there is very limited
growth, no formation of nodules, and no gain of nitrogen.
But when the culture is seeded with soil extract there is
luxuriant growth, abundant nodule formation, and coin-
cidently, great gain of nitrogen, many times as much in
the products of growth as in the seed sown. Gilbert also
showed that there is a gradual accumulation and then
withdrawal of nitrogen from the nodules. Lastly,
Schloesing^/.r and Laurent, by growing Leguminosae in
closed vessels, and analysing the air before and after
growth, found an actual disappearance of nitrogen gas,
agreeing with the amount gained by the plant during
growth. Thus, a conclusion was reached that the
leguminous plants can assimilate and fix the free
nitrogen of the atmosphere by the aid of bacteria living
symbiotically in the root nodules, — a conclusion which
served to explain, not only the discrepancies in the
previous experiments, but the long-accumulated experi-
ence of farmers that crops like clover and lucerne enrich
the soil, and form the best preparation for cereals like
wheat, which are particularly dependent on an external
supply of nitrogen. The mechanism of the fixation of
free nitrogen is still incompletely understood. It has
been found possible to grow these bacteria apart from
the leguminous plants, if they are cultivated on a medium
containing only a trace of nitrogen but supplied with
the ash constituents of the plant and also with
some carbohydrate like dextrose or maltose. The
Ib2 THE LIVING ORGANISMS OF THE SOIL [chap.
quantities of nitrogen fixed in this way are always,
however, very much smaller than are fixed by a
leguminous plant on whose root the nodules are well
developed. To fix the nitrogen, some expenditure of
energy is required, and this is derived from the
combustion of carbohydrate supplied to the bacteria by
the higher plant ; indeed it has been observed that the
nitrogen fixation and general growth of the Leguminosse
is stimulated by a supply of sugar or other carbohydrate
to the soil. The organism, Pseudomonas radicicola,
appears to be capable of considerable modifications;
in the nodules it forms rather large rod or Y-shaped
organisms, but if an active subculture be obtained by
inoculation from a nodule into a non-nitrogenous
medium as described above, excessively minute rod-
shaped organisms appear, generally in rapid motion. It
is in this minute unspecialised form that they exist free
in the soil, and it has been shown that they infect the
leguminous host by getting through the thin walls of
the root hairs. The characteristic Y forms have also been
obtained in artificial cultivations by introducing certain
substances into the medium. Much investigation has
also been applied to the question of whether there
is only one kind of bacterium living in symbiosis
with all the Leguminosae, or whether there is not a
definite race appropriate to each species of leguminous
plant, with which it alone can bring about nitrogen
fixation to the full extent. The earliest investigations
had already shown that lupins and serradella did not
develop nodules when infected with an ordinary
garden soil, but only when an extract was added
from a sandy soil on which these plants had been
previously grown ; and Nobbe brought further evidence
to show that, though there is very widely distri-
buted in the soil an organism which will cause
vii.] CULTURES FOR SOIL INOCULATION 183
some nodule formation and fixation of nitrogen,
yet it becomes so modified by growing in symbiosis
with the different leguminous plants, that the best
results are only obtained when each species is directly
infected from nodules taken from the same kind of plant.
Accordingly, he proceeded to the introduction, on a
commercial scale, of pure cultivations on a gelatine
medium of the races of bacteria appropriate to each of
the leguminous plants grown as field crops. The jelly,
which was called " Nitragin," was to be dissolved in a
large bulk of water and sprinkled over the seed before
sowing ; thus ensuring inoculation with the appropriate
organism, which might not happen to be present in the
soil. Nitragin failed to fulfil the expectations which
were formed at its introduction, partly because of the
nitrogenous character of the medium, in consequence of
which the organisms possessed very little vitality or
power of fixing nitrogen. Since that time, however,
several other methods of cultivating the organism for
inoculation purposes have been introduced, either by
growing it on an agar jelly, which contains practically
no nitrogen (Hiltner), by drying up cotton wool which
has been soaked in an active liquid culture (Moore),
or by drying soil which has been treated in the same
way. The culture thus obtained is added to a large
bulk of water, containing a little separated milk to
protect the organisms from substances excreted during
germination, and the seed is dipped into it and
allowed to dry before sowing. The culture may
also be sprayed over the ground or absorbed by a
large quantity of earth which is afterwards sown. The
results of such inoculation are very conflicting ; where the
land has been regularly under cultivation and has carried
the leguminous crop in question many times previously,
nodules are practically always formed whether the seed
184 THE LIVING ORGANISMS OF THE SOIL [chap
be inoculated or not. In such cases inoculation can
only be beneficial if the bacteria introduced either
belong to a more vigorous race of nitrogen fixers
than those normally present in the soil or are more
specifically adapted to that particular crop. It has not
as yet been conclusively demonstrated that such im-
proved races can be cultivated in the laboratory, or that
they can maintain themselves in the soil in competition
with the kindred organisms already present. It should,
moreover, be borne in mind that even if such improved
races of the nodule-forming organism can be introduced
to the plant, the improvement they can produce in the
yield is likely to be something of the order of a ten per
cent, increase, a gain which is only really perceptible
after careful and continued field experiments, and one not
to be detected by the ordinary farmer's eye. Of a very
different order are the results attained by inoculation
when the land contains none of the appropriate organ-
isms; inoculation will then change a stunted, sickly
looking growth into a profitable crop. It is only in
special cases that land devoid of the nodule organisms is
to be met with, most commonly when land is being
brought under cultivation for the first time, as in break-
ing up a virgin soil or in reclaiming heath and bog land.
Such peaty and heathy soils, which are devoid of
carbonate of lime, rarely carry any leguminous plants
the nodules of which could supply the necessary bacteria
to farm crops like clover and lucerne ; when such land
has been reclaimed and limed an inoculation is advisable
before sowing a leguminous crop.
Similarly when the cultivation of such leguminous
crops as lucerne or even sainfoin is being extended into
districts where they have not been grown previously, an
inoculation is often necessary before the roots will nodu-
late freely and the plant make its proper growth. Lucerne
vii.] GREEN MANURING 185
grown for the first time on heavy land in a new district
has been observed to fail completely, the failure being
attended by a complete absence of nodules from the roots.
Inoculation with soil from a field which has pre-
viously grown the crop about to be sown has often
proved a signal success in reclaiming the poor heath
lands of East Prussia, by the system of green manuring
worked out by Dr Schultz at Lupitz. Very large
areas of barren sandy heath land have been re-
claimed and rendered fit for the cultivation of the
ordinary crop by a system of growing lupins and
ploughing in the green crop. Mineral manures alone
are employed, latterly basic slag and the Stassfurt
potash salts; the lupins accumulate nitrogen from
the atmosphere, thus gradually there is built up both
humus to bind together the loose sand and make it
retentive of moisture, and also a store of nitrogen for
the nutrition of succeeding crops. The soil of a field
growing lupins every year from 1865 was found in
1880 to contain 0-087 per cent, of nitrogen in the
surface 8 inches, as compared with 0-027 per cent.
in an adjoining pasture. By 1891 the proportion of
nitrogen had increased to 0-177 per cent, despite the
annual removal of the lupin crop and the fact that
the manuring had been with phosphates and potash
only. It is in reclaiming these heath lands which
have not previously been under cultivation, nor, in
many cases, carried any leguminous vegetation what-
ever, that soil inoculation from land previously cultivated
has given successful results. Dr Salfeld of Hanover has
recorded several cases of the successful cultivation on a
large scale of various leguminous plants, beans, clover,
serradella, lupins, only after previous inoculation with
soil. The experiments were made on both peaty (moor)
and sandy soils, on which, without inoculation, legumin-
1 86 THE LIVING ORGANISMS OF THE SOIL [chap.
ous plants made but little growth and developed no
nodules. Success followed when about 8 cwt. per acre of
soil from a field which had previously carried the crop in
question were sown broadcast over the land in April,
and harrowed in just before seeding. In one case, over
7 tons per acre of green serradella were grown where
the land had been treated with 8 cwt. of soil from an
old serradella field, whereas the crop failed after germi-
nation where no inoculation had been practised.
Fixation of Free Nitrogen by the Soil.
As already indicated, Berthelot attributed to the
soil itself the power of fixing a small quantity of atmo-
spheric nitrogen, a power which was lost when the soil
was sterilised and maintained under conditions prevent-
ing infection. This gain of nitrogen was independent
of the small amount of ammonia absorbed by soil from
ordinary air, which always contains a trace of ammonia ;
and at first it was attributed to the microscopic green
algae which clothe the surface of ordinary moist soil.
The experiments of Kossowitsch, and of Kriiger and
Schneidewind, have, however, shown that the growth of
pure cultures of these algae is dependent on a supply
of combined nitrogen, and that no fixation of free
nitrogen takes place whether the algae growth be small
or large. It is possible, however, that they may live
in symbiosis with nitrogen-fixing bacteria and supply
the carbohydrate, by the combustion of which the
energy needed for the fixation of nitrogen by the bacteria
is obtained. More recently, however, several organisms
have been isolated from the soil which are capable when
growing in a free state of fixing nitrogen drawn from the
atmosphere, and it is to these that the gains of nitrogen
observed by Berthelot must be attributed. Winogradsky
was the first to isolate an organism of this type, which,
vii.] FIXATION WITHOUT LEGUMINOUS PLANTS 187
when grown under anaerobic conditions and supplied
with soluble carbohydrate, breaks the latter down with
the formation of butyric and other acids, and at the same
time draws some of the gaseous nitrogen present into com-
bination. This particular organism Clostridium Pastori-
anum is very widely diffused and can readily be isolated
from pond mud and similar material, where organic matter
is decaying under comparatively anaerobic conditions.
The extent of the nitrogen fixation is, however, small ;
in the laboratory not more than 2 to 3 mg. of nitrogen
are brought into combination for each gram of carbo-
hydrate oxidised. By far the most effective of the
nitrogen-fixing bacteria that are free in the soil is a large
organism, named by its discoverer, Beijerinck, Azotobacter
chroococcum. It may be easily isolated from most soils
by adding a small portion of soil to 50 c.c. of a culture
medium containing per litre 10 grams of mannite or
glucose, 02 gram each of potassium phosphate, mag-
nesium sulphate, and sodium chloride, and 01 gram of
calcium sulphate, half a gram of calcium carbonate being
also added to each flask. The solution and its flask
and plug of cotton wool are previously sterilised by heat.
After inoculation, the flask is placed in an incubator,
and after a week's time a considerable fermentation will
be observed to have taken place, attended by the
evolution of gas and the formation of a brown scum upon
the surface. By making a subculture in a similar
medium, inoculated with a trace of the brown scum, a
fairly pure growth of the Azotobacter can be obtained
for examination, or the amount of nitrogen fixed may
be determined by Kjeldahling the contents of the
flask.
Azotobacter chroococcum is a large oval organism, 4 to
5 jul in length and 3 fj. in width, which differs from most
bacteria in containing glycogen, so that it stains a deep
188 THE LIVING ORGANISMS OF THE SOIL [chap.
brown colour with a solution of iodine, a method which
is convenient for the observation of the organism. It is
aerobic, and is, in fact, a strong oxidising agent, the
dextrose or other carbohydrate which it requires being
converted by it into carbon dioxide and water, together
with small quantities of lactic and acetic acids, alcohol,
and sometimes butyric acid.
A very characteristic bye-product is the dark brown
or black pigment from which the organism derives its
specific name, a pigment which may play its part in the
usual coloration of humus.
As a rule, about 9 or 10 mg. of nitrogen are fixed
for each gram of carbohydrate oxidised, but the ratio
obtained varies considerably under different conditions ;
cultures which have been repeatedly transferred, being,
as a rule, less effective than the impure culture derived
directly from the soil.
Azotobacter chroococcum and its kindred forms are
widely distributed in soils from all parts of the world ;
it has been found in most cultivated soils, and the author
has observed it in virgin soils from East Africa, India,
New Zealand, Egypt, Russia, Monte Video, Ohio, and
Sarawak.
It is, however, not to be discovered in acid soils ; the
presence of calcium carbonate appears to be essential to
its development. Certain minor differences are to be
seen in the Azotobacter organisms present in the soil
from different parts of the world. From tropical and
semi-tropical soils in East Africa, for example, a form
has been isolated which is a very effective fixer of
nitrogen, but which differs from the normal in not giving
rise to the brown pigment ; another form, again, from
Monte Video gives rise to a green fluorescence in the
culture medium.
The amount of nitrogen fixed by Azotobacter may
Vii.) NITROGEN FIXATION IN VIRGIN SOILS 1S9
easily be rendered evident by an increased yie'd of
crop. Koch treated soil in pots with large quantities of
sugar, 2 per cent, 4 per cent., and even more of dextrose,
and then sowed oats, buckwheat, etc. At first the sugar
was injurious, and the first crop suffered in consequence ;
but the proportion of nitrogen in the soil increased, and
the second and third crops were far greater than those
in the check plots of untreated soil. When the soil,
after the application of the sugar, was placed in an
incubator for a month, in order to complete the oxidation
of the sugar, the increased yield due to nitrogen fixation
was also seen in the first crop.
To the Azotobacter and kindred organisms must
certainly be ascribed a large part in preparing and
maintaining the world's stock of combined nitrogen. It
is customary to regard such virgin soils as the black soils
of the Russian Steppes, of Manitoba, and of the Argen-
tina, as rich in nitrogen because of the accumulation of the
vegetable debris of many epochs ; but since plants other
than the Leguminosae do not fix nitrogen themselves,
there could in this way be no addition to the original
stock, which would only circulate from the soil to the
plant and back to the soil again. Under such conditions,
however, there is a continual addition to the soil of the
carbon compounds which the plant derives from the
atmosphere, and this is material which the Azotobacter
can oxidise, and so derive the energy required for the
fixation of nitrogen. It is the constant return to the soil
of oxidisable organic matter which differentiates the
wild from the cultivated land, and renders possible the
long-continued storing up of nitrogen in the virgin soils.
Interesting evidence on this point may be derived
from the Rothamsted experiments; on the Broadbalk
wheatfield the unmanured plot has, during the fifty
years 1844-93 yielded a crop containing on the average
igo THE LIVING ORGANISMS OF THE SOIL [chap
17 lbs. of nitrogen per acre per annum. Analyses of
the soil at the beginning and end of the period showed
a decline in the amount of nitrogen equivalent to a
removal of 12 lbs, per acre per annum, and the rainfall
is known to bring down between 4 and 5 lbs. per acre
per annum. The annual withdrawal in the crop would
thus be closely balanced by the loss experienced by the
soil and the additions, were there not other unknown
withdrawals in the weeds which are removed from the
plot, and in the nitrates which are washed down into the
subsoil and the drains. Doubtless, neither of these two
withdrawals are large, but because of their existence, un-
balanced by any corresponding falling off in the nitrogen
content of the soil, it must be concluded that even on
the arable land some small restorative action is going
on. A portion, however, of the same field has been
covered with a wild vegetation of weeds and grasses for
the last twenty-five years, and this is never cut or
harvested, so that all the debris fall back on the land
just as it would on a virgin soil. Analysis of samples of
this soil taken in 1881, when it ceased to be under cultiva-
tion, and in 1904, showed an annual accumulation of
nitrogen of more than 100 lbs. per acre. The enormous
difference in the fixation on this plot as compared with
the unmanured plot carrying wheat, must be set down
to the difference in the supply of non-nitrogenous carbon
compounds to the two plots ; in the one case the wheat
is all removed except a small portion of root and stubble ;
in the other the whole of the vegetable growth falls back
on the land. The wild vegetation on this plot did include
a considerable proportion of leguminous plants, but a
similar, though smaller accumulation of nitrogen was
observed in another plot of land which had been allowed
to run wild in the same manner, but which carried no
leguminous vegetation in consequence of the small
vil] FORMATION OF NITRATES IN SOIL 191
amount of calcium carbonate in the soil. These two
plots present a very close parallel to the actions which must
have been taking place in all virgin soils where the soil
similarly contains the Azotobacter organism. Doubtless
also some of the value of laying down land to temporary
pasture must be due to the accumulation of nitrogen by
the same agency, because we know that land under grass
accumulates carbon compounds from the roots and
stubble that is not removed during grazing.
Nitrification.
It has long been known that when any organic com-
pound of nitrogen is applied to the soil it becomes event-
ually oxidised to a nitrate, which is practically the only
compound of nitrogen taken up by cultivated plants, the
Leguminosae excepted. The potassium nitrate collected
from Indian soils, the calcium nitrate made artifically in
nitre beds in Europe, owe their origin to this oxidation
of organic compounds of nitrogen. That the process
was a biological one was first indicated by Miiller in
1873, but any widespead recognition of the fact did not
take place before the work of Schloesing and Muntz in
1877. These investigators showed that the formation
of nitrates in the soil ceased at temperatures below 5°
and above 55° C, that it could be stopped by chloroform
vapour and similar antiseptics, and that the soil lost
entirely its power of nitrification if it were heated to the
temperature of boiling water. The investigations of
Warington confirmed these results, and brought to light
the further fact that there were two stages in the oxida-
tion process, one being the formation of a nitrite,
followed by the conversion of this nitrite into the com-
pletely oxidised product. It was found possible to obtain
cultures which would only push the oxidation to the
nitrite stage, thus indicating that there must be at least
192 THE LIVING ORGANISMS OP THE SOIL [chap.
two organisms concerned in the complete nitrification
process. The further study of the organisms was for
a long time hindered by the fact that they could not be
got to grow upon the gelatinous media employed in the
ordinary methods of isolating specific bacteria; and
though P. F. Frankland, by a dilution method, succeeded
in isolating and describing a nitrifying bacterium, it was
not until 1890 that Winogradsky cleared up the problem.
He prepared a solid nutritive medium containing no
organic matter but with silica in its gelatinous form as a
basis, and thus was able to separate nitrifying bacteria
from the large number of other species simultaneously
present in the soil. Winogradsky was able to isolate
two species of bacteria capable of transforming ammonia
compounds into nitrites. One of these, termed Nitro-
somonas europcza, was obtained from all the soils of
the old world he examined ; the other, ascribed to
the genus Nitrococcus, was peculiar to the soils of
America and Australia. The former occurs both as a
single, free-swimming form, and clustered together in a
colony or zooglcea state.
Finally, there appears to be one type of organism
only, included in the genus Nitrobacter, which oxi-
dises the nitrites to nitrates. Winogradsky and other
observers have worked out the conditions of life of
these nitrifying organisms — the limits of temperature
for their growth, 5 and 55° C, have already been given,
the optimum temperature is about 37 C. Their action
is much restrained by the presence of organic matter,
or any quantity of alkaline carbonates or chlorides ; at
the same time, some base * must be present to combine
* Instruction sur la fabrication du nitre : — Par les rdgisseurs
gMraux des poudres et salt pHres> 1777, "Elles doivent F6tre
toujours avec une addition de terre calcaire qui puisse servir de
base a l'acide nitreux."
Vli.] CONDITIONS FAVOURING NITRIFICATION I9J
with the nitrous or nitric acids produced, for nitrification
ceases as soon as the medium becomes at all acid.
While calcium carbonate is the substance which,
as a rule, is effective to this end, many organic salts will
also supply the necessary base. Ammonium salts of
the strong acids will not nitrify directly in the absence
of a base, and the function of the calcium or magnesium
carbonate is usually added to form, by double decomposi-
tion, ammonium carbonate, which the nitrifying organ-
isms can attack. The complex salts formed by the
interaction of the zeolites of clay with ammonium
salts can be nitrified directly, but not, however, the
ammonium humate formed by the corresponding
interaction of ammonium salts and humus. Humus
itself does not inhibit nitrification, and, indeed, the
organisms can be brought to tolerate considerable
quantities of other organic matter, by transferring
them into successively stronger solutions. The organ-
isms are able to obtain the carbon necessary to their
growth from carbonates in the culture medium or
carbonic acid in the air ; the energy necessary to
decompose the carbon dioxide and fix the carbon is
derived from the oxidation of the ammonia, about
35 parts of nitrogen being oxidised for each part of
carbon that is fixed. The nitrifying organisms are
chiefly confined to the cultivated surface layer of
the soil. Warington found that, in the close-textured
Rothamsted soil they were by no means uniformly
distributed below the top 9 inches, and that they were
never present, except accidentally, in the subsoil below
a depth of 2 feet. It has also been shown that they
are entirely absent from many heath and moor soils,
even in the surface layer. They are abundantly found
in the water of shallow wells and rivers.
Summing up the above facts, it is seen that for the
N
194 THE LIVING ORGANISMS OF THE SOIL [chap.
active production of nitrates from the organic com-
pounds of nitrogen present in the soil — and this is
necessary if the crop is to be kept supplied with the
nitrogen required for its growth — the following condi-
tions are requisite : — The presence of the nitrifying
organisms in sufficient quantities, a certain degree
of temperature, darkness, sufficient moisture for the
development of the bacteria, free aeration of the soil
to supply the oxygen necessary, and a base to neutralise
the acids as they are produced.
The scanty number of nitrifying bacteria in any
subsoil below the cultivated layer helps to explain
both its sterile nature when brought to the surface, and
the difficulty and length of time required to develop
a state of fertility, especially when dealing with a
clay soil in which percolation and aeration have been
deficient.
The effect of a low temperature in checking the
formation of nitrates is well seen in the way the growing
corn turns yellow through nitrogen starvation whenever
a cold and drying north-east wind chills the ground
in spring: the bright green colour returns as soon as
warmer and moister soil conditions restore the activity
of the nitrifying bacteria in the surface layer. King
found in the top foot of soil when oats were turning
yellow only 0-026 parts of nitric nitrogen per million
of dry soil, whereas in soil where the oats were green
on the same date there was 0-255 parts of nitric nitrogen
per million, itself a small amount. The greater warmth
of a light soil also causes it to form nitrates quickly in
the spring, and so assists in producing an early growth.
But in obtaining early crops, even when the land is
rich, a dressing of ready-formed nitrate is often of the
greatest assistance, for the development of very early
crops may easily outstrip the rate at which the nitrates
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vil] NITRATES IN SOIL 195
they require can be formed in the still unwarmed soil.
Nitrates are much more freely formed in the summer
than in the winter, and as they are not retained by
the soil, they may easily be washed away when the
crop has been removed, unless weeds or a catch crop
sown to that end are present to take up the nitrates
and store them as organic compounds of nitrogen for
the future enrichment of the land.
The need for aeration in connection with the nitrify-
ing process has already been alluded to when discussing
drainage : all processes of working and cultivating the
soil assist nitrification, both by the thorough aeration
they effect, and by the mere mechanical distribution
of the bacteria into new quarters, where there are fresh
food supplies. In some experiments of DeheVain's he
found that the drainage water from pots of cultivated
soil, which had been sent from a distance, and thus
much knocked about in travelling and filling into the
pot, contained as much as 466 to 664 parts of nitrogen
as nitric acid per million. The drainage water from the
Rothamsted wheat plots contains only from 10 to 20
parts per million; even the cement tanks at Grignon,
2 metres cube, into which the soil had been filled, gave
drainage water containing only 39 parts of nitric
nitrogen per million. In another experiment a quantity
of soil was thrown upon a floor, and worked about
daily for six weeks ; on analysis it contained 0-05 1
per cent, of nitric nitrogen, as against -002 per cent, of
nitric nitrogen in the same soil left in situ. The diagram
(Fig. 14), due to King, shows the dependence of nitrate
production on temperature and the cultivation of the
soil. The lower curve shows the amount of nitrate
in parts per million in dry soil in the top foot of land,
which was not being cultivated because it carried clover
and oats. The upper curve shows the same results
196 THE LIVING ORGANISMS OF THE SOIL [chap.
obtained on well-tilled land carrying maize and potatoes.
On the cultivated land the proportion of nitrates rises
rapidly until the end of June, when the crop begins to
draw freely upon them, and reduces them to a minimum
throughout August and September.
One of the best examples of the manner in which
the thorough working and aeration of a warm soil
promotes nitrification is seen in the management of
the turnip crop as usually grown in this country.
Though shallow-rooted, and taking away large quan-
tities of nitrogen per acre, it is usually grown with
but little nitrogenous manure ; phosphates with a little
dung or with a comparatively small nitrogenous dress-
ing, being sufficient. The rest of the nitrogen is derived
from the rapid production of nitrates, due to the very
thorough working of the soil in the warm season of the
year that is characteristic of the cultivation of the turnip
crop. The production of nitrates by cultivation for
the benefit of a succeeding crop by bare fallowing,
or of an adjoining crop as in the Lois-Weedon system
of alternate husbandry, has been already alluded to.
At Rothamsted, nearly 60 lbs. per acre of nitric nitrogen
were found in October in the top 27 inches of soil
that had been fallowed, as against about half that
amount in land which had been under crop. The un-
manured alternate wheat and fallow plots showed in
September 1878 to a depth of 18 inches 33-7 lbs. of
nitric nitrogen per acre after fallow, and only 2-6 lbs.
after wheat. In land occupied by cereal crops the
drainage waters show that there is practically no nitrate
left in the soil by May, or, at the latest, June; they
reappear again towards the end of July or in August,
and after harvest, if rain falls, and especially if the land
be ploughed, nitrification becomes very active. It
depends upon the rainfall of the autumn and winter
vil] LOSSES OF SOIL NITROGEN 197
whether these nitrates, formed after harvest, are retained
for the succeeding crop or are washed out of the soil.
To sum up, increased nitrification, together with the
conservation of soil moisture and the warming of the
surface soil, are among the chief benefits derived from
all forms of surface cultivation.
As so much of the fertility of a soil must depend on
the number of nitrifying organisms it contains, attempts
have been made to compare soils in this respect, by
seeding small quantities of them into a standard solu-
tion capable of nitrification and determining the amount
of nitric acid formed after a given time. Although con-
siderable differences are seen in the action of different
soils, satisfactory quantitative results have not yet
been obtained, because of difficulties in the way of
drawing strictly comparable samples of the soils,
and the uncertainty still attaching to the amount
which should be used for inoculation or the best period
of incubation.
Denitrification.
The term denitrification is most properly applied
to the reduction of nitrates to nitrites, ammonia, or
particularly to gaseous nitrogen, which is brought
about by bacterial action under certain conditions.
Of late, however, the term has been more loosely
used to denote any bacterial change which results
in the formation of gaseous nitrogen, whether de-
rived from nitrates, ammonia, or organic compounds
of nitrogen.
Angus Smith was the first to observe the evolution
of gas from a decomposing organic solution containing
nitrates, which were destroyed in the process. Other
198 THE LIVING ORGANISMS OF THE SOIL [chap.
observers, particularly Deherain and Maquenne (1882),
with regard to soils, confirmed these results and showed
that they were due to bacterial action. In a paper
published in 1882, Warington described an experiment
in which sodium nitrate was applied to a soil saturated
with water ; after standing for a week, the nitrate was
washed out of the soil, part of it had disappeared, and
part had become nitrite. The total of both nitric and
nitrous nitrogen only amounted to 20-9 per cent, of that
which had been originally applied. That the nitrate
had been reduced to gaseous nitrogen was seen by the
development of transverse cracks filled with gas in the
soil, and it was concluded that some of the nitrogen
applied in manures and unaccounted for in crop and
soil may well be due to the reduction of nitrates to
gas, by the combustion of organic matter with the
oxygen of the nitrate, especially in ill-drained soils in
wet weather. Gayon and Dupetit, in 1886, isolated two
organisms from sewage which would reduce nitrates
to gas in the presence of organic matter, the action
being chiefly carried on when oxygen was absent; it
came to a standstill when plenty of air was supplied, so
that the organism had no need to attack the nitrate
to obtain oxygen. Both in their experiments and in
others, the presence of an abundant supply of soluble
organic matter was one of the necessary conditions
for the destruction of the nitrates. The denitrifying
bacteria are widely distributed. Warington found, out
of thirty-seven species of bacteria examined, only
fifteen failed to reduce nitrate, twenty-two reduced
it to nitrite, and one of them liberated gas. P. F.
Frankland, again, found that fifteen out of thirty
organisms derived from dust or water would reduce
nitrate to nitrite. In fact, a large number of bacteria,
when deprived of oxygen and in contact with abundant
VII.]
DEN1TR1FICA TION
199
organic matter, will obtain the oxygen, which they
normally require for the breaking up of the organic
matter, at the expense of the nitrate.
Many experiments, in which farmyard and other
organic manures have been employed in conjunction
with nitrate of soda and similar active compounds of
nitrogen, have shown a smaller crop for the manures
used together than when either was employed singly.
These results were particularly apparent when large
quantities of material like fresh horse-dung or chopped
straw were used in pot experiments. With well-rotted
dung, the effect of organic material in depressing the
yield which should be given by the nitrate was not
so great.
The nature of the results obtained may be seen
from the following table, which gives the percentage
recovered in the crop of the nitrogen supplied in the
manure, when used alone, or in conjunction with fresh
horse-dung : —
Percentage of Nitrogen Recovered (Wagner).
Per cent.
recovered when
used alone.
When used
with
Horse-dung.
Nitrate of Soda . • • .
Sulphate of Ammonia • • •
Urine . . . • • •
Vjrass • • • • • •
77
69
69
43
52
50
40
20
Numbers of similar experiments in pots have been
recorded. In some cases the use of fresh dung has
even resulted in a smaller crop than was obtained with-
out any manure at all ; but it should be noted that
very large amounts of the organic manures were used,
200 THE LIVING ORGANISMS OF THE SOIL [chap.
equivalent to ioo tons or more per acre. Similar results
have, however, been recorded in field trials, as in some
experiments of Kriiger and Schneidewind's, where fresh
cow-dung was applied at the rate of 23 tons per acre,
horse-dung, 21 tons per acre, and wheat straw at 5-8
tons per acre, on 10th July. These three plots were
in part cross-dressed with urine or with nitrate of soda,
each supplying 43 lbs. of nitrogen per acre. Two suc-
cessive crops of mustard were immediately grown, and
the amount of nitrogen removed by the crop was
ascertained. Compared with the wholly unmanured
plot, the cow-dung alone slightly depressed the crop,
about 1 \ lbs. per acre less nitrogen being recovered ;
the horse-dung produced a depression of nearly double
this amount; the wheat straw produced the greatest
depression, its crop containing about 18 lbs. per acre
less nitrogen than that given by the unmanured plot.
Where straw was used with nitrate of soda the two
gave a crop containing 23 lbs. less nitrogen per acre
than the nitrate alone; where urine was used alone,
the produce contained 25 lbs. more nitrogen per acre
than when it was used in conjunction with cow-dung
and straw.
In fine, all the results pointed to the same con-
clusion — that large amounts of fresh organic manure
not only do not themselves help the crops, but
diminish the effect of other rapidly acting nitrogenous
manures like nitrate of soda, sulphate of ammonia, or
urine.
The action cannot, in the two latter cases at least,
be put down to denitrification proper, unless it is
supposed that nitrification and subsequent denitrifica-
tion can proceed practically simultaneously in the
same soil. It must either be attributed to the fact
that nitrification is very much checked by the
vil] LOSSES OF SOIL NITROGEN 201
presence of large amounts of organic matter; or tc
the conversion of readily available nitrogen into a
comparatively insoluble albuminoid form in the actual
material of the enormous numbers of bacteria that
are developed by the free food supply; or, lastly,
to those fermentation changes of organic nitrogen com-
pounds which result in the liberation of free nitrogen.
Several of these changes may take place together ; the
essential point is, that nitrification does not go forward
in the presence of much organic matter, which instead
favours all the bacterial processes resulting in the
development of free nitrogen.
The conditions indeed which prevail in these experi-
ments are scarcely comparable with the ordinary practices
of agriculture. Enormous quantities of fresh organic
manure are employed immediately before the crop is
sown, the temperature of the pots, or of the ground in
the field experiment quoted, is very high, so that it
is easy to see that an abnormal condition, both as
regards nitrification and the supply of oxygen and water,
must be developed.
There are not lacking both long-continued experi-
ments and ordinary farming experience to show that
nitrates and other artificial manures can be used in
conjunction with dung with the best effects.
For example, the mangold crop at Rothamsted
shows the following average results for the recovery of
nitrogen from various nitrogenous manures used first
with mineral manures alone and then with annual
dressings of 14 tons per acre of farmyard manure, a
quantity that never would be employed so frequently
in practice: —
[Table,
202 THE LIVING ORGANISMS OF THE SOIL [chap.
Nitrogenous Dressing.
Yield.
Tons
per acre.
Nitrogen.
In
Manure.
Recovered
in Crop.
Percentage
recovered
per 100 in
Cross-
dressing.
PLOTS MANURED WITH PHOSPHATES AND ALKALINE SALTS.
Nitrate of Soda .
Ammonium Salts .
Rape Cake . •
Ammonium Salts and
Rape Cake
17*95
15*12
20.95
24.91
86
86
98
184
67.2
49*3
69.4
103-0
78.1
57-3
70.9
56-0
PLOTS MANURED WITH DUNG.
Nothing
Nitrate of Soda .
Ammonium Salts .
Rape Cake .
Ammonium Salts
Rape Cake
and
17.44
24.74
21-73
23-96
24-05
200
63-3
286
H5-8
286
105.6
298
iii»i
384
129-8
3i-6 4
61 -o
49.2
48-8
36-2
* Percentage of Nitrogen in dung recovered.
It will be seen that all the nitrogenous cross dressings
produce an increase of crop when added to the farm-
yard manure. When the cross dressings are used on
plots receiving only non - nitrogenous manures, the
nitrogen recovered varies between 56 and 78 per cent,
of that supplied in the manure; when they are used
in conjunction with dung, the recovery of nitrogen in
the increased yield above that produced by dung alone
varies between 36 and 61 per cent. That the recovery is
smaller in the latter cases is due to the fact that with such
excessive amounts the yield ceases to be proportional
to the supply of nitrogen, being limited by other factors.
Denitrification is only likely to cause rapid loss
of nitrogen when large quantities of nitrate are
applied to undrained or sour land, or when they are
vil] LOSSES OF SOIL NITROGEN 203
used with excessive amounts of fresh dung, which has
not been rotted and so deprived of much of its soluble
organic matter. Of course, a steady loss of nitrogen
due to such causes as have been enumerated above must
also be expected wherever large quantities of organic
nitrogenous manures are accumulating in the land. If,
for example, we compare 2 and 3 of the Broadbalk wheat
plots at Rothamsted, the latter of which is unmanured
and the former receives dung containing 200 lbs. of
nitrogen per acre every year, we find that at the end
of the fifty years, 1844-93, the dunged plot contained
in the top 18 inches about 2680 lbs. more nitrogen than
the unmanured plot, or a mean annual accumulation of
50 lbs. The extra crop grown on the dunged plot
would remove a further 31 lbs., thus leaving 119 lbs.
per annum to be accounted for, either as nitrogen
washed away in the drainage water or lost as gaseous
nitrogen by denitrification processes.
Iron Bacteria.
Another series of bacteria playing an interesting
part in certain soils, consists of those which secrete
hydrated ferric oxide or bog-iron ore in undrained soils,
where the soil water contains ferrous bicarbonate in
solution. Winogradsky investigated four of these
organisms, to whose vital processes he considered the
presence of soluble ferrous salts was essential. Molisch,
however, regards the secretion of ferric hydrate as, in
a sense, an accidental accompaniment of their growth,
much as the separation of large quantities of silica, so
characteristic of the straw of cereals, is unessential to
their development. It has already been noted that
these iron earths do not form in soils containing calcium
carbonate, which seems to prevent the formation of any
soluble ferrous compounds.
204 THE LIVING ORGANISMS OF THE SOIL [chap.
Fungi of Importance in the Soil.
Allusion has already been made to the fact that a
large number of fungi inhabit the soil — Penicillium y
Mucor, Trichoderma, Spicaria, etc., Cladosporium, Clado-
thrix, and various wild yeasts, Monilia, etc. — all of
which aid in breaking down the organic matter. Many
of these fungi possess the power of attacking ammonium
salts applied as manure, withdrawing the ammonia and
setting free the acid. To this action is due the acidity
produced by the long-continued use of ammonium salts
as manure, as seen on the experimental plots at Rotham-
sted and Woburn. At Woburn the soil is light and
sandy, containing but little lime, and the application of
ammonium salts containing 50 lbs. ammonia per acre
every season for twenty-four years, has rendered the
land practically incapable of carrying the crops. A
moderate dressing of lime, however, restores the fertility.
The following crops were obtained in 1900 on the barley
plots : —
Ammonium Salts
only.
Minerals
+ Ammonium Salts.
With no Lime ....
With 2 tons Lime, applied Nov.
1897 . . . . .
5-6
28-9
12.3
33-7
The soil had become acid to litmus paper where the
lime had not been used : it is interesting to note that
though barley would not grow, oats flourished freely on
this sour soil. There are, however, two special organisms
which merit further consideration — the fungus which
clothes the finer rootlets of many classes of plants,
forming mycorhiza and the slime fungus, or Plasmodio-
phora which causes the disease known as "finger-and-
toe " or " club " in turnips and other cruciferous plants.
VII.] MYCORHIZA 205
The term " mycorhiza " is applied to the symbiotic
combination of the filaments of certain fungi, whose
complete development is as yet unknown, with the
finest rootlets of certain plants. Sometimes the fungus
forms a sort of cap on the exterior of the short root-
lets, which are generally without root hairs; in other
cases it penetrates the cortical tissue of the root itself,
which may also be furnished with root hairs. Accord-
ing to the researches of Frank, the fungus of the
mycorhiza lives in symbiosis with the higher plant,
attacking the humus and also the mineral resources of
the soil, and passing on the food thus obtained to the
host plant. In some few cases the host plant possesses
no green assimilating leaves, and is wholly dependent
upon mycorhiza to obtain its necessary carbon from
the humus. Such a case is seen in the Neottia Nidus-
avis, or Birds' Nest Orchis, to be found chiefly amongst
beech underwood in this country.
More generally, the host plant is capable of nutri-
tion in the ordinary way when growing in media in
which nutrient salts are abundant, but becomes myco-
trophic in soils and situations unfavourable to the pro-
duction of directly absorbable food — as, for example, in
heaths and moors, where the soil is almost wholly
humus, or beneath the shade of trees, where nitrates are
rarely found and where illumination is insufficient for
much assimilation. Later researches, particularly those
of Stahl, have shown that the symbiosis of mycorhiza,
instead of being a phenomenon restricted to a few
species, is widely diffused among many classes of plants,
and is indeed causally connected with other facts of wide
general importance in plant nutrition. It has already
been indicated that the cultivated plants give off con-
siderable quantities of water by transpiration ; the form
and arrangement of their leaves are adapted to expose
206 THE LIVING ORGANISMS OF THE SOIL [chap.
a large evaporating surface, the root is well developed
and provided with root hairs to keep up the supply of
water to the plant. There are, however, a number of
plants in which transpiration is much less active, and
the leaf area is restricted or otherwise arranged to
diminish the loss of water, so that the proportion
previously stated as existing between the dry matter
produced and the water passing through the plants
is greatly diminished. A diminished supply of water to
the root would, however, necessitate a loss of nutri-
ment to the plant, as both nitrates and other mineral
salts enter the plant with the transpiration water.
Stahl has shown that, in general, these plants with a
small transpiration activity are furnished with mycor-
hiza, by means of which they obtain food of all kinds
from the soil ; whereas, on the contrary, the plants,
like the cereals, the cruciferous and leguminous plants,
Solanaceae, etc., which give off water freely, are never
associated with mycorhiza. Many of the conifers and
heaths which grow on dry soils show this correlation
of a low evaporation and restricted leaf development
with a root-system furnished with mycorhiza.
Another interesting generalisation has also been
brought into line with the above facts by the observa-
tions of Stahl that the mycotrophic plants with a
feeble transpiration do not store starch in their leaves,
but contain instead considerable quantities of soluble
carbohydrates, chiefly glucose. In normal plants, though
sugar is the first tangible result of assimilation, it is
rapidly removed from the sphere of action by being
converted into starch, such withdrawal of the product
of the reaction being necessary if a rapid rate of
assimilation is to be maintained. Should, however,
sugar accumulate in the cells, the concentration of the
cell sap is increased, so that it parts with its water
vil] MYCORHIZA 207
by transpiration less readily. Though many excep-
tions can be observed, there seems to be a very
general association of the development of mycorhiza
with a diminished transpiration and the absence of
starch from the leaf, especially among plants like the
orchids, lilies, iris, etc., which often grow in dry or
shady situations, such plants being further distinguish-
able by a shiny, glossy leaf surface. Stahl has again
shown that the average proportion of ash to dry
matter in the leaf is lower for mycotrophic than for
normal plants; the former grow, as a rule, in situa-
tions containing but little mineral salts, particularly
in humic soils, where, in addition, the plant is put
into competition for whatever nutriment may be present
with the mycelia of fungi, which everywhere traverse
humus in its natural state. By direct experiment, it
has been shown that normal plants grown in humus
develop better when the humus is previously sterilised
by long exposure to chloroform vapour than when it
is in its fresh condition, full of living mycelia com-
peting successfully for the nutriment. The absence
in the leaf of calcium oxalate and of nitrates is
particularly characteristic of mycotrophic plants.
Stahl concludes that symbiosis between the roots of
plants and the mycelia of fungi is a very general
phenomenon, especially characteristic of plants growing
in soils subject to drought, or poor in mineral salts,
or rich in humus. These mycotrophic plants are
generally of slow growth, possess a feeble transpiration,
and limited root development ; their leaves rarely con-
tain starch ; they are also characterised by containing a
comparatively small proportion of mineral salts, among
which calcium oxalate and nitrate are notably absent.
To the mycorhiza associated with plants of the
genus Erica the power of fixing atmospheric nitrogen
2o8 THE LIVING ORGANISMS OF THE SOIL [chap.
has been attributed, but the question still requires further
investigation.
The existence of mycotrophy has certain interesting
applications in practice; there are many plants which
can only be cultivated with difficulty in gardens ; for
example, some of the orchids, ericas, lilies, and others,
generally plants which must be grown in leaf-mould,
peat, or other material rich in humus. Yet humus
alone is not always sufficient for the purpose, the peat
or leaf-mould has often to be obtained from a particular
place; other materials, though equally rich in humus
and possessing similar mechanical properties, prove
quite unsuitable. It is easy to surmise that this effect,
confined in the main to mycotrophic plants and humic
soils, may easily be due to the absence of the proper
fungus from the soils found to be unsuitable.
It has also been shown that the difficulty usually
experienced in raising seedlings of exotic orchids,
which die off in great number just after they have
germinated, may, to a large extent, be obviated by
mixing with the medium in which the seeds are sown
a little of the material in which the parent plants are
growing. The young seedling is found to develop
mycorhiza at a very early stage, and then only will
grow properly.
" Finger-and- Toe"
On many soils, particularly those of a sandy nature,
the turnip crop is often almost wholly destroyed by the
disease known as " finger-and-toe," " club," or " anbury."
Cabbages and other cruciferous crops are equally at-
tacked ; so much so, that in gardens which have become
infected it is practically impossible to raise crops of
this nature. The disease is caused by an organism,
Plasmodiophora brassicce, belonging to the slime fungi,
VII.]
* FINGER-AND-TOE »
209
and forming spores which may remain dormant in the
soil for some time, certainly for two or three years. It
has long been known that the best remedy against finger-
and-toe consists in the application of lime; and as far
back as 1859, Voelcker showed that soils on which this
disease is prevalent are deficient in lime ; and in many
cases in potash also. Later researches have only served
to emphasise the fact that the disease is associated with
soils of an acid reaction, in which calcium carbonate is
wanting, or present in very small proportions. The
fungus, as is generally the case with fungi, refuses to
grow in a neutral or slightly alkaline medium, and the
only way to get rid of the infection in the land is to
restore its neutrality by repeated dressings of lime. At
the same time, the land should be rested as long as
possible from cruciferous crops ; uneaten fragments of
diseased turnips, etc., should not be allowed to go into
the dung, or if they do, the dung should be used on
the grass land. Manures, again, which remove calcium
carbonate from the soil, like sulphate of ammonia, or
acid manures like superphosphate, should not be em-
ployed ; neutral or basic phosphates, with sulphate of
potash on sandy soils, should be employed instead.
The following figures show the amount of lime dis-
solved by hydrochloric acid from soils affected with
" finger-and-toe," as compared with spots in the same
field where the disease was not in evidence : —
Lime per cent.
Voelcker.
Voelcker.
Hall.
Sandy Soil.
Clay Soil.
Soils affected
by disease .
Soils free from
disease . .
.14
.89
.084
•52
•13
•43
•31
•••
•39
o
210 THE LIVING ORGANISMS OF THE SOIL [chap. vii.
It must be remembered that in these cases the total
lime soluble in acids is given, not merely the lime
present in carbonate.
Whenever a turnip crop is seen to be infected with
" finger-and-toe " the land should be well dressed with
3 or 4 tons per acre of quicklime immediately the
crop has been removed ; as long an interval as possible
should be given before again taking a cruciferous crop,
substituting, for example, mangolds for turnips in the
next rotation ; every effort should be made to destroy
cruciferous weeds like charlock ; turnip-fed dung should
not be applied, and another dressing of finely divided
quicklime should be put on for the crop preceding
the sowing of the new turnip crop.
CHAPTER VIII
THE POWER OF THE SOIL TO ABSORB SALTS
Retention of Manures by the Soil — The Absorption of Ammonia
and its Salts ; of Potash ; of Phosphoric Acid — Chemical and
Physical Agencies at Work — The Non-Retention of Nitrates
— The Composition of Drainage Waters — Loss of Nitrates
by the Land — Time of Application of Manures.
Many of the substances employed as manures are
soluble in water, hence it becomes important to ascer-
tain what is likely to be their fate in the soil, when
their application is followed by sufficient rain to cause
percolation into the subsoil. Of substances contain-
ing nitrogen, nitrate of soda, the salts of ammonia,
urea, and kindred bodies, are freely soluble in water;
superphosphate alone of the compounds of phos-
phoric acid commonly used as manure is soluble; but
sulphate, chloride, and carbonate of potash are easily
soluble.
Not long after the principles underlying the nutri-
tion of plants had been established, Thompson and Way
showed that ordinary soil possesses the power of with-
drawing most of the above substances from solution,
and so saving them from washing away into the subsoil
or the drains. Some of the salts, like sulphate of
ammonia, are decomposed, the base alone being re-
211
212 POWER OF THE SOIL TO ABSORB SALTS [chap.
tained and the acid draining through. Way found
that liquid manure from a dung-heap, which contains
both organic and ammoniacal compounds of nitrogen,
potash, and a little phosphoric acid, when filtered through
a short column of soil, parted with almost the whole of
its organic matter and much of its salts to the soil;
compounds of calcium were, however, more abundant in
the filtered liquid than before. Way's observations
were extended by Voelcker, who compared the absorb-
ing powers of different types of soils, and so obtained an
idea of the method by which the absorption of each
substance was effected ; and later researches have only
served to confirm the results then obtained. It was
found that all the organic compounds of nitrogen,
ammonia — either free or in combination — phosphoric
acid, and potash were wholly removed from solution
by ordinary soil, though some soils were more effective
than others ; whereas nitrates, sulphates, chlorides, and,
among bases, sodium and calcium, were only slightly,
if at all, retained. These results are confirmed by the
analysis of the water which flows from land drains
under normal conditions; this will generally be found
to contain nitrates (sometimes in fair quantity), sulphates
and chlorides of calcium and sodium, and considerable
amounts of calcium bicarbonate, but rarely shows more
than a trace of ammonia, phosphoric acid, or potash.
The absorptive action of the soil is partly a chemical
process, due to interactions with the humus, the zeolitic
double silicates, and the calcium carbonate of the soil ;
and partly physical, dependent upon the extent of
surface offered by the soil particles (for the surface of a
solid possesses the power of concentrating molecules of
any dissolved substance in the layer of solution with
which it is immediately in contact). The mechanism of
this physical " adsorption " is but imperfectly understood
Vlii.] ABSORPTION OF NITROGENOUS COMPOUNDS 213
but even pure sand will remove sodium chloride from
solution if the filtering column be sufficiently long; it
may, again, be illustrated by the phenomenon of
"laking," i.e., the power of certain colloid bodies, like
the hydrates of iron and alumina, on precipitation, to
drag down with themselves many organic substances
from solution.
The absorption of the organic compounds of nitrogen
by the soil seems to be a physical process of this kind,
comparable to the action of charcoal in absorbing
ammonia or the strongly smelling products of putre-
faction, etc. The deodorising powers of earth for
faecal and decomposing matter are very familiar; this
means that fixation in a more or less insoluble and
non-volatile state of various organic nitrogen and sulphur
compounds is effected, and other inodorous nitrogen
compounds are retained in the same way. The
absorption is most marked with soils rich in humus
or in clay — the soil materials which present the
largest surface. The absorptive power of soil for
organic compounds of nitrogen is well seen in a
sewage farm, the object of which is to so far purify
sewage by percolation through a few feet of soil, as
to fit it to be turned into a river without danger to
health. For example, on the Manchester Sewage
Works, in 1900, percolation through 5 feet of soil
reduced the organic nitrogen in the liquid from 0-26 to
0-056 parts per million, and the free ammonia from 1-89
to 0-92. It is necessary, also, on a sewage farm to work
with soils possessing but a small absorbing power ; only
sandy and gravelly soils will permit of rapid enough
percolation, both to deal with large volumes of sew-
age and afterwards to aerate themselves and accom-
plish the destruction by bacterial action of the absorbed
material. Stiffer soils would be far more effectual
214 POWER OF THE SOIL TO ABSORB SALTS [chap.
absorbents of the sewage material, but are unsuitable
because they do not admit of percolation.
Absorption of Ammonium Salts.
The absorption of free ammonia follows the lines
indicated above for the absorption of the organic com-
pounds of nitrogen ; but the salts of ammonia are
retained by the soil by purely chemical processes which
result in the formation of insoluble salts of ammonia in
the nature of double silicates and humates. Way and
Voelcker first found, that when either the sulphate,
chloride, or nitrate of ammonia in solution is allowed to
remain in contact with soil, the base is absorbed, but the
acid portion of the salt remains in solution in combina-
tion with lime. Voelcker also showed that when soil
was shaken up with dilute solutions of ammonium salts,
the withdrawal of ammonia from the solution was never
complete, but varied both with the nature of the soil and
the strength of the solution, a greater proportion being
taken from weak than from strong solutions.
The absorption of the ammonium salts by the soil is
now known to be due to the combined effects of at least
three actions — upon the zeolites, upon the humus, and
upon calcium carbonate. With the zeolites a double
decomposition takes place, ammonium becomes insoluble,
and equivalent amounts of calcium, magnesium, potas-
sium (sodium also on occasion) enter into combination
with the acid in the solution, for no acid is absorbed
and the whole solution remains neutral.
The reaction is a reversible one, but the clay con-
taining the zeolites is not capable of absorbing more
than a certain small amount of ammonium from the
strongest solutions of its salts. The following table
shows the ammonia absorbed by ioo grams of very
VIII.]
ABSORPTION OF AMMONIA
215
pure clay when shaken with 300 c.c. of ammonium
chloride solution of varying strengths.
Original
Concentration of
Solution.
Ammonia
Withdrawn.
Ammonia
Withdrawn.
N/IO
N/12
N/15
N/20
N/30
N/50
N/100
Grams.
0-126
0-II5
0-104
0-090
0-068
0-049
0.031
Per cent.
247
28-1
30-5
35-3
40-3
48.0
60-0
Thus from the weaker solutions a smaller total but a
larger proportion of the ammonia was removed by the
clay, and the removal was never complete. Of course,
in the field the amount of soil is so enormously in excess
that the absorption of ammonium salts applied as
manure is practically complete. Thus at Rothamsted
the presence of ammonia in the drainage water is rarely
detected, even when heavy rain immediately follows the
application of the manures.
With humus, ammonium salts interact in a very
similar fashion, calcium coming into solution, and the
ammonium forming some insoluble compound with the
complex " humic " acids.
With calcium carbonate a double decomposition of
the type —
(NH 4 ) 2 S0 4 + CaC0 3 ^Z± (NH 4 ) 2 C0 8 + CaS0 4
— takes place, not only with such substances as
ammonium sulphate, but also with humic and zeolitic
compounds of ammonium, and though the proportion
converted into carbonate is small, it is constantly
renewed as the ammonium carbonate is nitrified, so that
eventually the whole of the ammonium salt applied to
the land undergoes this change.
216 POWER OF THE SOIL TO ABSORB SALTS [chap.
In consequence, the continued use of ammonium
salts as a fertiliser results in the depletion of the stores
of calcium carbonate in the soil, as may be seen from
the following determinations of the rate of disappearance
between 1865 and 1904, of calcium carbonate from
some of the soils of the Rothamsted wheat field where
the calcium carbonate is of artificial origin and is con-
fined to the surface layer of the soil.
Plot.
Manuring.
Bate of Loss
of
Calcium Carbonate.
3
5
6
7
8
9
10
2
Per acre per annum.
Unmanured ...••••
„ „ + 200 lbs. Ammonium Salts
»» it +400 „ „
M »> +600 „ „
„ „ +412 lbs. Sodium Nitrate .
400 lbs. Ammonium Salts only •
Farmyard Manure ......
Lbs. per acre
per annum.
800
880
1170
IOIO
1170
565
1045
590
Thus the use of ammonium salts increases the
normal loss of calcium carbonate experienced by the
soil (due to solution as bicarbonate), and the amount
removed increases with the larger applications of
ammonium salts. Taking the mean of these and other
results obtained at Rothamsted, 200 lbs. of ammonium
salts causes a removal of about 120 lbs. calcium
carbonate, whereas the amount calculated from the
equation given above would be about 160 lbs. That
the loss from the plots receiving sodium nitrate and
dung is less than from the unmanured plot, is due, in
the former case, to the base left in the soil by the growth
of plants which derive their nitrogen from sodium nitrate,
and in the latter, to calcium carbonate formed by
bacterial action from organic calcium salts in the dung.
VIII.J
POTASH
217
The Absorption of Potash.
In all respects the absorption of potash follows the
same laws as that of ammonia : Le. y caustic potash
is absorbed directly, but sulphate, nitrate, and chloride
of potash undergo a double decomposition, by which the
potash is retained and calcium sulphate, nitrate, or
chloride, appear in the water draining through the soil.
Voelcker found in laboratory experiments with small
quantities of soil that potassium carbonate was more
freely absorbed than sulphate, and that clays, marls,
and pasture soils were more effective in retaining potash
than light loams or sands, which latter had but little
absorbing power.
The following table shows some of the results
obtained when potash and soda salts were compared : —
Percentage retained.
Potash.
Soda.
Chalky Loam •
Clay • •
Sandy Loam •
Pasture . • •
Loam • . • •
Ironstone Sand •
3-6
4.0
2-6
3-8
3-4
I- 1
o-8
i-i
06
I-O
I-O
0.6
Both the humus and the zeolitic double silicates take
part in the retention of the potash salts, the reactions
being exactly similar to those taking place with the
ammonium salts. In some of Way's experiments with
pure clays the application of potash salts was followed
by the appearance of the corresponding sodium salts in
the percolating water, though with most soils it is
calcium that is turned out of combination. Potash salts
applied to the soil also react to a certain extent with the
calcium carbonate, giving rise to a little potassium
carbonate, the bad effect of which upon the tilth of the
218 POWER OF THE SOIL TO ABSORB SALTS [chap.
soil will be considered later (p. 253). Dyer has examined
the soils of the Rothamsted wheat plots which had then
been continuously manured in the same way for fifty
years, with the view of tracing the fate of the mineral
manures applied. The following table shows a com-
parison of the amounts of potash soluble in strong hydro-
chloric acid, in lbs. per acre, found in the top 9 inches
of soil from four of the plots; one (No. 11) received
nitrogen and phosphates, but no potash, every year, the
others were variously manured, but all received 200 lbs.
per acre of sulphate of potash. Estimates are also given
of the total amount of potash applied as manure and
removed in the crops over the whole period, so that in
the last two columns a comparison can be made between
the actual surplus of potash in the manured over the
unmanured soils, and the surplus calculated from the
differences between the potash added in the manure
and removed in the crops : —
Plot and Manuring,
per Aero.
Potash— Lbs. per Acre.
Surplus
over Plot 11.
In top
9 inches
of Soil.
3 s
3S
8? &
>
n
rS a
3
c— 1
O
d
11, receiving no Potash .
7, receiving 200 lbs. Sul-
phate of Potash
5, receiving 200 lbs. Sul-
phate of Potash
13, receiving 200 lbs. Sul-
phate of Potash
5107
6793
7233
7078
15
5037
5203
5287
1 190
2550
1136
2410
•••
3662
5242
4052
••*
1686
2126
1971
On the whole, about one-half of the estimated surplus
of potash received by the manured plots still remains in
the top 9 inches of soil.
Dyer further estimated the proportions of potash in
the same soils which was soluble in a I per cent, solu-
VIII.]
PHOSPHORIC ACID
2I<)
tion of citric acid, and found that both the surface and
the subsoil down to a depth of 27 inches contained more
of this readily soluble potash where it had been applied
as manure, than did the companion plot receiving no
potash, as will be seen from the following table : —
Plot.
Lbs. per Acre of Potash, soluble
in 1 per cent. Citric Acid.
Surplus over Plot 11.
First
9 inches.
Second
9 inches.
Third
9 inches.
Calculated.
Found.
II
7
5
13
83
602
799
487
75
374
598
363
IOI
179
257
235
3662
5242
4052
896
1395-
826
These determinations show that soluble potash salts
applied to the land are retained chiefly by the surface
soil, as much as one-half of the estimated additions of
potash during fifty years' manuring being found there.
Some of the potash, however, sinks further and is
retained in the subsoil ; in the top 27 inches a large
proportion — nearly one-quarter of the whole — remains
in such a loose state of combination that it is soluble
in 1 per cent, citric acid, and so may be regarded as
available for the plant
Absorption of Phosphoric Acid.
The retention of soluble phosphoric acid by the soil
is more easily intelligible, for there are present several
substances capable of forming insoluble compounds
with phosphoric acid — e.g., calcium carbonate, hydrated
ferric oxide, and the hydrated silicates of alumina which
make up so much of clay. Sand and powdered silicates
like felspar have been found to possess little or no
power of removing phosphoric acid from solution, nor
220 POWER OF THE SOIL TO ABSORB SALTS [chap.
have either soil, clay, or peat which have been pre-
viously washed with hydrochloric acid.
The following table shows the percentages of the
total phosphoric acid supplied, which were removed
from solution by various soils after remaining in con-
tact for the specified times, the ratio between soil and
phosphoric acid being about iooo to E.
Percentage of Phosphoric Acid Absorbed (Voelcker).
After 1 day.
After 8 days.
After 26 days.
Red Loam • •
Chalky Soil • • •
Stiff Clay • • •
Stiff Subsoil • • •
Light Sandy Soil • .
60
89
51
48
53
78
99
62
69
59
95
100
86
74
73
In an ordinary soil containing a sufficiency of
calcium carbonate, the application of soluble phosphoric
acid like superphosphate will chiefly result in the
precipitation of di-calcium or "reverted" phosphate,
wherever the solution meets with a particle of calcium
carbonate. This di-calcium phosphate is a compound
easily soluble in weak organic acids or in water con-
taining carbonic acid : hence the great value of applica-
tions of superphosphate on soils rich in lime, for thus a
readily available phosphate is very quickly disseminated
throughout the ground in a state of fine division. But
on soils poor in calcium carbonate the precipitation will
be chiefly effected by the hydrated iron and aluminium
compounds, and the resulting phosphates are practically
insoluble in water containing carbonic acid, and but
little in saline solutions or in weak organic acids.
Hence applications of superphosphate to such soils
become much less available to the crop, and should be
preceded by a thorough liming of the land. Even a
Viii.] PHOSPHORIC ACID 221
subsequent liming on soils containing phosphates of
iron or alumina will help to bring them into a more
available form, because a double decomposition result-
ing in calcium phosphate and aluminium or ferric
hydrate, will proceed to an extent dependent on the
mass of lime present in the medium.
Further evidence of the precipitation of phosphoric
acid within the soil is afforded by Dyer's examination
of the Rothamsted wheat soils at various depths, after
fifty years' continuous manuring with and without super-
phosphate. By comparing the amount of phosphoric
acid contained in the soil of the unmanured plot
with that contained in the soils of the plots receiv-
ing superphosphate every year, and knowing also the
amount removed by the successive crops in each case,
it is possible to calculate the surplus that should
remain in the manured over the unmanured plots, on
the assumption that the soil was uniform at starting.
Calculating in this way, Dyer found that no less than
83 per cent, of the phosphoric acid which six of the
plots should possess after fifty years' manuring was
still present in the top 9 inches of soil, whereas the
subsoils from 9 inches to 18 inches, and 18 inches to
27 inches, showed no accumulation of phosphates.
Dyer further determined the phosphoric acid which
was soluble in a I per cent, solution of citric acid,
and found that on the manured plots the top 9 inches
of soil contained about 39 per cent, of the estimated
surplus of phosphoric acid so combined as to be
soluble in this medium, whereas in the subsoils the
"available" phosphoric acid was, if anything, less for
the manured than for the unmanured plots. It has
already been pointed out (p. 163) that if the extraction
with citric acid be repeated, practically the whole of the
phosphoric acid applied as manure and not removed in
Ill POWER OF THE SOIL TO ABSORB SALTS [chap.
the crop can be recovered from the top 9 inches of these
Rothamsted soils. It is clear, then, that soils well
provided with calcium carbonate, as the Rothamsted soil
is, will precipitate very near the surface any soluble
phosphoric acid applied, and retain it for a long time in
a form easily redissolved and obtainable by the plant.
It follows, therefore, that superphosphate, the most
soluble of the phosphatic manures, can be applied to
normal soils in the winter or early spring without any
fear of the phosphoric acid being washed out.
The Composition of Drainage Waters.
Further evidence of the fate of the various substances
applied as manures, their retention or otherwise by the
soil, can be obtained by studying the composition of the
water flowing from land drains.
The drainage from the continuously manured wheat
plots at Rothamsted, each of which possesses a tile drain
running down the centre at a depth of 2 feet to 2 feet
6 inches, has been collected from time to time and com-
pletely analysed by Voelcker and Frankland ; in addition,
systematic determinations of the nitrogen contents have
been made for many years. In a general way, the chief
constituent of the various drainage waters is lime, either
as bicarbonate, sulphate, chloride, or nitrate; soda is
the only other base present in any quantity, very small
amounts of magnesia, potash, and ammonia pass into
the drains. Of the acid radicles, chlorine and
sulphuric acid predominate according to the manuring,
and the proportion of phosphoric acid is minute ; but
the amount of nitric acid varies according to the
manure applied and the season at which the water is
collected.
The following table shows the complete analysis of
the drainage water from twelve of the plots : —
VIII.]
ROTHAMSTED DRAINAGE WATERS
223
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224 POWER OF THE SOIL TO ABSORB SALTS |chap.
An examination of these figures shows that the
amount of organic matter and ammonia reaching the
drains is practically nil; the organic matter supplied
as dung, and the ammonia, which is employed up to
400 lbs. of mixed ammonium chloride and sulphate per
acre, are wholly retained by the soil. The effect, how-
ever, of adding either organic compounds of nitrogen or
ammonium salts is to increase the proportion of nitrates
in the drainage water. Lime is the chief constituent of
the dissolved matter in the drainage waters, the propor-
tion is lowest for the unmanured plot (3), it rises with
the application of minerals (5), and rises again with each
successive application of ammonium salts (6) and (7). The
formation of calcium chloride and sulphate respectively,
when the corresponding ammonium salts are applied to
land containing calcium carbonate, has already been
discussed : it is well seen in the increased richness in
lime, sulphuric acid, and chlorine of the drainage water
from 6 and 7, which receive 200 and 400 lbs. respectively
of ammonium salts, as compared with 5, which receives
the same minerals without any nitrogen compounds. Plot
1 1 receives superphosphate in addition to the ammonium
salts which 10 receives: the effect of the gypsum con-
tained in the superphosphate is seen in the increased
lime and sulphuric acid content of the drainage water of
1 1. The increase is not so great, however, as that caused
by the addition of sulphates of potash and magnesia to
the superphosphate and ammonium salts (plots 13 and 14)
whereas sulphate of soda causes little loss of lime (12).
The use of nitrate of soda on plot 9 causes no increase
in the proportion of lime in the drainage water, but a
large quantity is removed, chiefly as sulphate, from plot
2, receiving dung every year. The quantity of lime
removed annually in this way will be very great:
assuming a mean annual drainage equal to 10 inches
Viii.j LOSSES TO SOIL IN DRAINAGE WATERS 2S5
of water, the unmanured plot will lose about 220 lbs.
per acre per annum of lime : equivalent to about 400 lbs.
of carbonate of lime, whereas the analysis of the soil
shows (p. 216) an annual loss of about 800 lbs. per acre.
The discrepancy between these two figures is due to the
fact that the results are calculated from but a small
number of analyses of the drainage water, the amount
of which is also very uncertain. When 400 lbs. of
ammonium salts are used as manure, either alone or
with minerals, the increased loss of lime calculated on
the same basis amounts to 126 lbs. or 225 lbs. of
carbonate of lime per acre per annum, as against about
240 lbs. found from the analysis of the soil.
The amount of magnesia lost is small, 5 to 20 lbs.
per acre per annum, nor is the amount reaching the
drainage water much increased by its application as
manure to plots 5, 6, 7, 9, and 14.
The amount of potash lost is still smaller, from
3 to 12 lbs. per acre per annum, but it is distinctly
dependent on the amount supplied as manure, being
at a maximum with the dunged plot (2) and the plot
receiving minerals only (5), and greater from all the
other plots receiving potash than from those without
it, i.e., 3, 10, 11, 12, 14. The use of sulphate or
nitrate of soda increases the amount of potash in the
drainage water, not so, however, the use of sulphate
of magnesia. Practically all the soda, chlorine, and
nearly all the sulphuric acid, that are applied in the
manure pass through into the drainage water.
A comparison of the drainage waters in winter
and spring shows that they are more concentrated
in the winter, because the manures (excepting the
nitrate of soda) have then been recently applied : the
chlorides wash out first, then the sulphates, and as
the season advances not only is the total amount of
P
226 POWER OF THE SOIL TO ABSORB SALTS [chap.
lime present much diminished, but it comes away
chiefly as carbonate. With the growth of the crop
in spring the nitrates disappear from the drainage
waters.
The amount of nitrates found in the drainage water
varies not only with the time of year, but also according
to the interaction of temperature, growth of crop, culti-
vation, and percolation. Nitrates are only rapidly pro-
duced when the temperature of the soil has risen : if the
percolation is not excessive the crop may remove the
nitrates as fast as they are formed, but a heavy rainfall
in the spring before the nitrates have been much drawn
upon by the crop, or one just after the land has been
broken up in the autumn and is still warm, will result in
a considerable washing out of nitrates. At the same
time a certain amount of moisture in the soil is necessary
for the formation of nitrates, and the crop itself may so
dry the soil as to reduce nitrification considerably. The
following table (p. 227) shows the estimated loss of
nitrates from the same wheat plots at Rothamsted as have
previously been dealt with, during two years, each of
which has been divided into two periods : firstly, from
the date at which the nitrogenous manures were sown up
to harvest ; and secondly, from harvest round again to
the sowing of manures in spring.
The diagram (Fig. 15) shows the same results in a
graphic form.
The seasons were rather exceptional, the summer
rainfall and drainage in 1879 and the winter rainfall in
the following year being both above the average. It
will be seen that except on the autumn manured plot
15, the loss was greatest from plot 9 receiving 550 lbs.
of nitrate of soda, and this excess of loss was chiefly in
the summer drainage water of 1879; the figures are,
however, exaggerated by the fact that half the nitrate
of Nitrogen per acre. 10 20 30 40
Fig. 15. — Losses of Nitrogen in Drainage from Rothamsted Wheat Plots.
Black = Losses in Summers 1879, 1880.
Shaded = Losses in following Winters.
VIII.]
NITRATES IN DRAINAGE WATER
227
plot received no mineral manures, and therefore grew
but a scanty crop. The losses during the winter months
are more nearly the same for all plots, and represent to
a large degree the nitrification of the organic residues in
the soil. The losses from the plots receiving minerals
and varying amounts of ammonium salts (5, 6, and 7)
increase with each application of nitrogen : the losses
Nitric Nitrogen in Drainage Water.— Lbs. per Acre.
Plot.
3
5
6
7
9
10
11
12
13
14
15
Manuring, per Acre.
Unmanured
Minerals only
Minerals + 200 lbs. Ammonium Salts
„ +400 „ „
>» +55° i) Nitrate of Soda
400 lbs. Ammonium Salts alone
Do. do. + Superphosphate
Do. do. 4-Sulph. Soda
Do. do. 4-Sulph. Potash .
Do. do. 4-Sulph. Mag.
Minerals 4- 400 lbs. Ammonium Salts
in Autumn
Estimated Drainage — inches
1879-80.
1880-81.
bo
bb
M
bo
iTj
~
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co £
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17-1
1.6
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17.7
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19-8
18.3
12-6
4-3
21-4
45-o
15-6
15-0
41*0
42.9
14-3
7-4
35-2
28.3
17.7
3-4
29.6
21-2
17-5
3-3
27^2
I9-0
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3-7
25-3
26-0
16.8
4.2
25.9
9.6
59-9
3-4
74-9
II'I
4-7
1-8
18.8
from the plots receiving ammonia and various mineral
manures diminish as the mineral manure becomes a
more complete plant food, because the greater growth of
crop thus secured more completely removes the nitrates
as they are formed, besides hindering nitrification by
drying the surface soil.
The effect on nitrification of crop and surface
cultivation is well seen in the following table of results
328 POWER OF THE SOIL TO ABSORB SALTS [chap.
obtained by Deherain, who collected the drainage from
cement tanks 2 m. cube and systematically filled with
soil taken from corresponding depths in the field. The
soils had been several years in the tanks, so that they
had settled down into practically normal conditions,
though the effect of the aeration and disturbance of
the soil in filling the tanks is still visible in a rather
high rate of nitrification. Each tank carried the crop
indicated in the first column.
Cropping.
Drainage.
Nitrogen as Nitric
Acid.
Fallow, no cultivation .
Rye Grass ,
Oats
Maize
Wheat, followed by Vetches ,
Wheat
Fallow, hoed ....
Fallow, no cultivation .
Fallow, hoed and rolled . ,
Vine ....
Sugar Beet •
Inches.
II-2
7-8
7-3
6-9
6-6
7-5
II*
II-2
II-2
7-5
7.2
Lbs. per acre in
Drainage Water.
18670
2.28
7-37
2l«6o
12.90
28.70
196-56
15800
183.20
36.20
0-27
The rainfall of the year in question, March 1896 to
March 1897, amounted to 28*8 inches, most of which
fell in the autumn. The most noteworthy results are
the effect of the various crops in diminishing the loss
of nitrates, which is not wholly to be attributed to the
quantity taken up by the crop, because the sum of the
nitrogen removed in the crop and that carried off in
the drainage water is never equal to the nitrogen
removed from the uncropped plots by the drainage
water alone. ' During the comparatively dry spring
months the crops leave so little moisture in the soil that
nitrification is checked, and the total production of
viii.] TIME OF APPLICATION OF MANURES 229
nitrates is less where there is a crop than on the moister
uncropped plots.
When the wheat was followed by a crop of vetches
the loss of nitrates during the comparatively wet autumn
was considerably reduced. Lastly, the hoeing of the
fallow plots resulted in a considerably increased produc-
tion of nitrates.
Time of Application of Manures.
The facts set out above as to the retention of most
of the soluble constituents of manures by the soil, while
the nitrates are liable to wash out, have an impor-
tant bearing on the season at which artificial manures
should be sown. In the first place it is evident that
there is no danger of losing phosphates, or even of
their washing deep into the soil, when employed in
their most soluble form as superphosphate. It is the
general custom to sow superphosphate with the drill
for roots at the same time as the seed : the large
quantity of manure near the seedling in its early critical
stages is probably valuable, and as the roots of swedes
and turnips do not extend very deeply, the phosphoric
acid may be placed in the most likely place to reach
them.
But for more deeply rooting crops, hops and fruit
or even mangolds, it seems probable that superphos-
phate is often applied rather too late in the season,
and that if used as a winter instead of a spring dressing
it would have a better chance of getting well diffused
through the soil. Basic slag and other insoluble phos-
phates should be used in the winter or even the autumn :
there is no risk of loss, and as much rain as possible
is wanted to get them distributed in the soil. As
regards potash salts, Dyer's experiments go to show
that they descend further in the soil, and are a little
230 POWER OF THE SOIL TO ABSORB SALTS [chap.
more subject to washing than the soluble phosphates :
for this reason, where sulphate of potash is employed,
as for potatoes, it will best be sown with the seed.
Where kainit is used, it is best employed as a winter
or autumn dressing ; there will be little loss of potash,
for this will get fixed chiefly in the surface soil. But
the chlorides, which are present in kainit and are
sometimes not wholly beneficial in their action upon the
crop, will be removed and washed out into the drains or
the subsoil water by the winter rains : the magnesia
salts also will be precipitated within the soil, and to a
large extent removed from possible action upon the
crop. Turning to the nitrogen compounds, it is
necessary to keep in mind that all of them will become
transformed into nitrates which are liable to be washed
out All insoluble organic manures should be put
on before or during the winter : the decay processes
will begin, resulting in the formation of amino-acids,
ammonia, etc., which will become fixed in the soil, but
the low winter temperatures will not permit of much
nitrification. Liquid manure and similar materials
containing such readily nitrifiable substances as urea
and ammonium carbonate, should be reserved until
early spring, so that the crop may be growing whenever
nitrification begins. Ammonium salts are very rapidly
nitrified, so that they should only be used in spring.
At Rothamsted nitrates begin to appear in the drainage
water immediately after the application of the ammonium
salts to the wheat plots in March if rain falls, and one
of the plots which has its ammonium salts applied in
autumn shows not only a considerable falling off in
crop but also large quantities of nitrates in the winter
drainage waters.
The following table shows the amounts of nitrates
removed in the drainage water from the two plots which
VIII.]
NITRATES IN DRAINAGE WATERS
231
receive 400 lbs. per acre of ammonium salts with mineral
manures, only differing in the fact that on plot 7 the
ammonium salts are sown in March, and on plot 15 in
October ; also the average crops of grain and straw for
the twenty-three years, 1875-97.
1879-1881.— Lbs. of Nitric Nitrogen per Acre.
Plot.
1879-80.
1880-81.
M
s, >
02
si
W Q
fi'C
H
<Q
'u
m
a
II
OS Q
® CO
m Oi
m
LRur.
Grain.
Straw.
7
15
I8.3I
9*62
12-63
59-92
4.29
21.38
74-94
Bushels.
314
28J
Cwt.
31
26J
Drainage
1 1- 1"
4-7"
1.8"
18.8"
• ••
...
That sulphate of ammonia will to some extent
persist in the soil, and become available for a succeed-
ing crop, after even a whole year has elapsed, is to be
seen from the results of the Woburn experiments upon
wheat. Some of the plots at Woburn receive mineral
manures every year, but ammonium salts or nitrate of
soda only every alternate year : in both cases the crop
falls very much in the years of no nitrogen, but the
decrease is by no means so marked with ammonium salts
as with nitrate of soda, which latter seems to leave no
residue whatever.
The soil at Woburn is an open sandy loam ; but in
the years for which the results are quoted (see Table,
p. 232) the rainfall was low.
The difference between the results set out in the
above-mentioned table and those obtained upon the
corresponding plot at Rothamsted, where the dressing
232 POWER 0* SOIL TO ABSORB SALTS [chap. vin.
of ammonium salts every other year seems to leave no
residue for the following year, may perhaps be set down
to the different texture of the two soils. The ammonium
salts are converted which are washed down into the
subsoil ; at Woburn they can rise again by capillarity,
as the soil, though sandy, is still fine in texture; at
Rothamsted the soil is too close-grained to admit of
any considerable movement of the subsoil water back
to the surface.
Plot.
1899.
Bushels.
1900.
Bushels.
f8B
Minerals only .
20-3
Minerals + Ammonium
^8A
Minerals + Ammonium
Salts .
27-3
1
oilitS • )
33-1
Minerals only .
l6«2
|9B
Minerals only •
9.8
Minerals + Nitrate of
i9A
Minerals + Nitrate of
Soda . •
27.7
1
Soda .
4i
Minerals only .
6-8
4
Minerals only, every
year •
6.9
Minerals only • •
5-9
The experiments recorded above and the results
of the examination of drainage waters go to show
that nitrate of soda should only be employed when
there is a crop in possession of the ground and ready
to seize upon the salt as soon as it becomes diffused
through the soil. Only on dry soils can it be safely
applied as early as the sowing of the seed ; in wet
climates sulphate of ammonia will often be preferable
it the soil is warm enough to induce reasonably quick
nitrification, and when large quantities of nitrate are
wanted they should be put on by successive applications
of not more than 1 cwt. per acre at a time.
CHAPTER IX
CAUSES OF FERTILITY AND STERILITY OF SOILS
Meaning of Fertility and Condition — Causes of Sterility : Drought,
Waterlogging, Presence of Injurious Salts — Alkali Soils and
Irrigation Water— Effect of Fertilisers upon the Texture of
the Soil — The Amelioration of Soils by Liming, Marling,
Claying, Paring, and Burning — The Reclamation of Peat
Bogs.
In discussing the question of fertility, a difficulty at the
outset crops up in the definition of the term " fertility " :
we are dealing with something intangible and dependent
upon so many varying factors that it becomes a matter
of judgment and experience rather than of scientific
measurement We have to distinguish between the
fertility proper, "the inherent capabilities of the soil,"
to use the language of the old Agricultural Holdings
Act, which is the property of the landlord, and for
which the tenant pays rent; and the "condition" or
" cumulative fertility," the more temporary value which
is made or marred by the tenant. Though in the main
it is easy to feel the distinction, it is often difficult, if
not impossible, to draw a line of demarcation between
them. Clearly the farmer in a new country on virgin
soil is dependent wholly on the inherent fertility of
the land, but with much of the land in this country it
is hard to say how far its value is inherent, or due to
long-continued cultivation. When a tenant by many
288
234 CAUSES OF FERTILITY AND STERILITY [chap.
years of skilful management makes a good pasture,
the improvement is rightly credited to him, as fertility
which he has accumulated : the next tenant must
regard the same pasture as part of the inherent capacity
of the soil. Again, a farmer working on the old four
course rotation, selling only corn and meat and purchas-
ing neither feeding stuffs nor manures, is dependent on
the fertility of the soil : another farmer, carrying on an
agricultural business such as market gardening or hop-
growing, and putting more into the land every year as
manure than he takes out as crop, is only using the
land as he would a building, as a tool in a manufactur-
ing process.
Fertility proper is by no means a wholly chemical
question, dependent upon the amount of plant food the
soil contains; in many cases the physical conditions
which regulate the supply of air and water to the plant,
and as a corollary, the bacterial life, are far more potent
in producing a fertile soil than the mere amount of
nutrient material it contains. Especially is this the
case in an old settled country like England, where
manure is cheap and abundant ; here a fertile soil is
often one which is not rich in itself, but one that is
responsive to, and makes the most of, the manure
applied. Clay soils are not uncommon which show on
analysis high proportions of nitrogen compounds and
potash, and again no particular deficiency in phosphoric
acid, but from their closeness of texture they offer such
resistance to the movements of both air and water as to
carry very poor crops. Some light soils again, such as
those on the chalk, would be regarded on analysis as
rich, but they are made so persistently dry by the natural
drainage, that only in a wet season do they keep the
crop sufficiently supplied with water for a large crop
production. On nearly all poor soils it is impossible to
ix.] FERTILITY 235
effect much improvement by the use of manures ; in fact,
manuring will not turn bad into good land, the con-
ditions limiting the amount of crop being other than
the food supply. Of course, by the continued incorpora-
tion of humus into a light soil, its physical texture may
be improved at the same time as its richness, until it
becomes sufficiently retentive of water for the needs of
an ordinary crop, just as a heavy soil may be lightened
by similar additions of humus. It has already been
mentioned that many subsoils, especially of the heavier
loams and clays, are extremely infertile when brought
to the surface, even though they may possess a fair
proportion of phosphoric acid and potash and be arti-
ficially supplied with nitric nitrogen. Some of this
effect is due to texture, part to the very scanty
bacterial flora they possess, but it is to be noted that
in arid climates the subsoils, which are not more fine-
grained than the surface soils, do not show the same
infertility when brought to the surface.
The soils which show the greatest fertility are, as
a rule, soils of transport, uniform and fine-grained in
texture, but with particles of a coarser order than clay
predominating, so that, while lifting water easily by
capillarity, they are freely traversed by air and per-
colating water. As a rule, they also contain an appreci-
able amount of organic matter at all depths ; in Britain
they have been deposited from running water, and
represent the silt from which both the coarsest sand
and the finest clay particles have been sifted, together
with a certain amount of vegetable debris. We have
nothing comparable with the typical "black soils" of
the North American prairies or the Russian steppes,
which contain very large proportions of organic matter
to considerable depths in the subsoil : as, for example,
a soil from Winnipeg that contained 0428, 0-327, 0158,
236 CAUSES OF FERTILITY AND STERILITY [chap.
and 0107 per cent, of nitrogen in the top 4 feet of
soil successively. Many of these deep rich soils appear
to be wind-borne : in all cases they are of very uniform
texture, and represent the accumulated residues of ages
of previous vegetation in a form that is capable of
decay and nitrification so as to become available for
subsequent crops. In a peat bog there is equally an
accumulation of organic matter and nitrogen, but the
mass is infertile because of the acid character of the
humus (which causes the absence of the valuable bacteria,
such as those fixing nitrogen and nitrifying ammonia),
the deficiency of mineral plant foods, and the bad
mechanical condition which affects the supply of air
and water. In the main, then, a fertile soil is one rich in
the debris of previous vegetation, one which has been so
sorted out by running water, wind, the agency of worms,
etc., as to possess a very uniform texture, adapted to
satisfy the needs of the plant for air and water.
Mechanical texture is of fundamental importance : in
this country many soils owe their value to this property
alone ; for example, the Thanet Sand formation in East
Kent (a very fine-grained sand or silt), though it con-
tains but little plant food, yet carries some of the best
fruit and hop plantations in the kingdom, and farms on
it command a high rent
Condition.
The question of condition has equally its chemical
and its mechanical side; it is well known that on
any but the lightest soils continued cultivation makes
the texture better and renders it easier to obtain a
seed bed. On clay soils the effects of bad manage-
ment are very persistent; any ploughing, rolling, or
trampling when the soil is wet will so temper the
IX.] CONDITION 237
clay that the effect is palpable until the land has been
fallowed again or even laid down to grass. Once
protected from the action of frost, stiff soil which has
been worked ? when at all wet never seems able to
recover its texture, as may be seen by examining the
clods that are to be found on digging up an old post,
the result of the trampling when the post was originally
put in. The dependence of "condition" upon the
maintenance of a good texture is to be seen in the
custom of regarding wheat as an exhausting crop,
whereas few of our farm crops withdraw less plant
food from the soil. The popular opinion really
represents the fact that the wheat crop occupies the
land for nearly a year during which period it receives
little or no cultivation and so falls into a poor state of
tilth.
From the chemical side "condition" means the
accumulation within the soil of compounds that will
by normal decay yield sufficient available plant food
for the requirements of an ordinary crop, e.g., of
organic compounds of nitrogen which readily nitrify,
of phosphoric acid and potash compounds which readily
become " available " for the plant.
The condition of land cannot be restored all at once
by manuring ; the residues of manures left in the soil
after the first season are slow-acting, i.e., only a small
proportion of them becomes available year by year,
so that there must be a considerable accumulation of
such residues before the proportion becoming avail-
able during the period of growth is sufficient for the
crop. Per contra, the condition can be only too easily
destroyed by cropping without manure ; the unexhausted
residue left after each year is successively less and less
active, the crop falls off rapidly, till at last a sort of
stationary condition is reached, and the somewhat inert
238 CAUSES OF FERTILITY AND STERILITY [chap.
materials, still left in large quantity, liberate year by year
a fairly constant proportion of active plant food. The
plots at Rothamsted which have been cropped without
manure for more than fifty years show but little less
average production during the last twenty years than in
the twenty immediately preceding. For example, the
unmanured wheat plot shows the following crop in
bushels of dressed grain : —
First 23 years,
1852-74.
Second 23 years,
1875-97.
1898.
1899.
1900.
I4i
"I
I2j
12
12*
Condition may best be regarded as a state of equi-
librium when the land will continue to give a good return
in crop for the manure applied; as a rule, the crop
recovered by no means contains the whole of the
material applied as manure, a certain portion being
retained in a comparatively inactive form. With the
land in condition the remaining nutrient material
required for a good crop is supplied by the dormant
residues in the soil which have become active : at the
same time, these reserves are protected from depletion
by renewal from the inactive portions of the current
manuring. On the other hand, if the land is in poor
condition the crop gets little or no assistance from
the soil, but is grown from the active part only
of the manure : the rest of it accumulates and begins
to build up condition, which, however, does not tell
on the yield for some time. As a practical conse-
quence, it is noticed that only land in good condition
gives a paying return year after year for the application
of manure : yet if the experiment be made of omitting
the manure on a portion of the land for one year,
there is little corresponding reduction of yield, as
IX.] FAIRY RINGS 239
though the manure went to keep up the "condition,"
and the crop was grown out of that rather than from
the manure applied.
From the point of view of analysis the estimation of
the " condition " of a given piece of land is a difficult,
matter on which light is only just beginning to be
thrown by the determination of " available " plant food,
such as the nitrates and the phosphoric acid and potash
soluble in dilute acid solvents. By considering such
factors as these and the amount of humus soluble in
alkali, the ratio of the soil carbon to the nitrogen,
and the proportion of calcium carbonate, the agricul-
tural chemist may form an idea as to the immediate
state of the land. Doubtless, the prevalence and dis-
tribution of such necessary bacteria as those causing
nitrification are also important factors in determining
the fertility, but on this point we are without exact
information. It will be seen that "condition" is one
of the most valuable of the properties of the soil to
the cultivator; as it may be destroyed or created by
the tenant during his occupation of the land, it should
be as far as possible a tenant's asset, to be bought by
him on entry and valued to him on leaving. The
difficulty which even an experienced man finds in
putting a value on so intangible an item makes it
almost impossible to assess the condition of a farm,
but it is desirable in every way that the outgoing
tenant should be encouraged to maintain the condition
of his farm by giving him due compensation for the
unexhausted value both of manures and foods used
in the latter years of his tenancy.
Fairy Rings.
The significance of "condition" and its dependence
upon a supply of recently decayed organic matter is
240 CAUSES OF FERTILITY AND STERILITY (chai*.
well seen in the development of "fairy rings" in
pastures. "Fairy rings" are circles of dark -green
grass, common enough in poor pastures, which are
found to extend their size every year, leaving the grass
within the ring of a lighter colour and of generally
poorer aspect than that outside. On examining the
soil immediately outside a ring, it is found to be full of
the mycelium of one or two common species of fungi,
but the mycelium rarely occurs in the soil beneath the
ring itself, and never in that within the ring. The
ring appears to be dependent on the growth of
the fungus, which starts at one point and draws
upon the humus reserves contained in the soil.
Having consumed whatever humus is available, the
mycelium must proceed into the annular area of soil
immediately, round the first patch, thus from year to
year it spreads outward. After the death of the
fungus, there is left behind in the soil it has just
occupied a quantity of organic matter, which readily
decays and becomes available for plant nutrition;
thus a ring of luxuriant vegetation immediately
follows the death of the fungus. In other words,
the humus of the soil, slow to decay and nitrify in
the usual way, is changed into material undergoing
rapid change by its preliminary conversion into the
tissue of the fungus. At the same time, as the
supply of rapidly acting plant food has been solely
derived from the soil, the ultimate result is the im-
poverishment of the soil within the ring by the develop-
ment of the fungi and the subsequent luxuriant growth
of grass.
The following figures relate to the composition of
the soil (mean of five examples) within, on, and outside
fairy rings : —
IX.]
CAUSES OF STERILITY
241
Outside the ring
On the ring
Inside the ring .
Carbon per
cent.
3-30
2.99
2.78
Nitrogen per
cent.
0-28l
0266
O.247
Nitrates per
million.
2.44
11*46
1-03
It will be seen that the unchanged soil outside con-
tains the most carbon and nitrogen; the ring itself
contains an intermediate amount, and the least is
contained within the ring after the luxuriant vegeta-
tion has passed away. The soil on the ring is in high
condition, because the organic residues it contains are
recently formed and will change rapidly; after they
have been cropped out, the land is less able to support
a crop, even though there is still much plant food left
in the soil. The last column in the table (the analysis
of a single example only) shows the difference in avail-
able nitrogen ; and though in a pasture there are never
many nitrates to be detected, so rapidly are they seized
upon by the crop, still the organic nitrogen compounds
in the soil must be in a more nitrifiable condition on
the ring to yield the results there shown. Doubtless an
investigation of the nature and distribution of the
bacteria and micro-fungi in and about a fairy ring would
throw further light on the varying fertility of such closely
neighbouring areas of soil, but no data are at present
available.
Sterility of Soils,
Few soils occurring in this country can be described
as absolutely barren, yet from time to time land is met
with which yields such poor crops that it may fairly be
designated as sterile. The causes of sterility are
various; amongst them may be enumerated both the
Q
242 CAUSES OF FERTILITY AND STERILITY [chap.
want and the excess of water due to texture and
situation, deficient aeration, the absence of calcium
carbonate, and the toxic action of certain com-
pounds, such as the salts of magnesia, iron pyrites or
ferrous salts generally, and common salt itself. An
acid reaction of the soil, which is highly prejudicial to
vegetation, is generally brought about by one or other of
the causes enumerated above.
The sterility brought about by a deficiency of water
is only seen in this country when the soil is so entirely
composed of coarse sand that it possesses no retentive
power for the rainfall; even then the absolutely bare
condition does not persist long, and may be attributed
as much to the lack of nutriment as to the want of
water. Little by little vegetation is found to creep over
recent deposits of coarse sea-sand and shingle, until a
turf is established. As a rule, such deposits have perma-
nent water at a comparatively short distance below
and by this the vegetation is maintained ; but where a
coarse, open-textured sand occupies the uplands, as on
the Bagshot and Lower Greensand formations of the
south of England, or the Bunter beds of the Midlands,
the soil is kept so poor that it has largely remained
common heath land, never having been worth the
expense of enclosing. Allusion has already been made,
under the head of drainage, to the evils which ensue in
a waterlogged soil : from time to time clays are met
with of so close a texture that the vegetation suffers in
an analogous manner through deficient aeration. On
certain areas of the Oxford Clay and London Clay, and
the Boulder clays derived therefrom, pastures degenerate
after a few years into a mass of creeping rooted plants
like bent grass, and the land must be broken up afresh
in order to aerate it before any crop can be grown.
Sterility due to chemical causes is perhaps most
IX.] CAUSES OF STERILITY^ 243
generally caused in this country by the absence of
calcium carbonate from the soil. When this happens
on light sandy land it will become evident by the
tendency of black mild humus to accumulate, by the
paucity of leguminous plants in the herbage, and by the
prevalence of fungoid diseases like " finger-and-toe."
On strong lands, and when accompanied by water-
logging, black acid peat accumulates : the soil shows
an acid reaction, oxide of iron forms below the surface,
and the soil water contains soluble iron salts, as is seen
by the iridescent scum which spreads over any water
standing in the ditches.
Another source of sterility is the presence of un-
oxidised iron salts in the soil : many clay subsoils are
coloured dark blue or green by double ferrous silicates
like glauconite, or by finely disseminated iron pyrites.
Until these materials become oxidised to ferric hydrate,
the soil remains sterile: particularly is this the case
with iron pyrites, which in the form of marcasite easily
oxidises to yield both ferrous sulphate and sulphuric
acid. Voelcker has recorded three instances of soil
sterile through these causes : one was land reclaimed
from the bed of the Haarlem Lake, which contained 071
per cent, of iron pyrites and 0-74 per cent, of ferrous
sulphate, as well as some insoluble basic sulphate of
iron. Another example of land reclaimed from the
sea contained 0-78 per cent, of pyrites and 1-39 per
cent, of ferrous sulphate. Cultivation with a free use
of lime and chalk is the best means of ameliorating
such soils, which always show an acid reaction.
Kearney and Cameron in America have shown that
salts of magnesia possess, even in solutions of great
dilution, a toxic action upon plant roots, which is much
diminished if calcium salts be present at the same time.
Loew at the same time has indicated that a comparative
644 CAUSES OF FERTILITY AND STERILITY [cha*.
excess of magnesium over calcium in certain soils
results in sterility. With this may be correlated the
fact that the soils resting upon the serpentine, which
is a compound containing magnesia, are notoriously
poor, also that certain very impoverished clays on the
Wealden formation contain a high proportion of
magnesia.
Sterility caused by salt is sometimes to be seen in this
country in the marshes near the sea : more often a " salt-
ing," even where the sea has regular access, is clothed
with vegetation which is able to endure very consider-
able proportions of salt. Most farm crops will grow in
soil containing 0-25 per cent, of salt, and in the reclaiming
of the old sea lake of Aboukir in Egypt, it was found
that grasses would grow freely when there was still as
much as 1 per cent, of salt in the soil, and a scrubby
winter crop of barley was grown on soil containing more
than 1 J per cent. " With 2 per cent, of salt in the soil, a
fair crop of dineba (grass), 2 feet high, can be grown ;
with 1 per cent, it attains its full height of 4 feet, and
sells as a standing crop at from 20s. to 25s. per acre.
For l berseem,' or clover, the percentage of salt should
not exceed |, and about the same for ' sabaini (quick-
growing or seventy-day) rice/ " Much, however, depends
upon the relations between water supply and evapora-
tion, as to the amount of salt in a soil which would be
tolerable to vegetation. From time to time cases occur
in this country of crops being destroyed, and land
rendered sterile by the incursion of sea water ; the effect
is not always apparent at first, though sea water contains
as much as 2-7 per cent, of sodium chloride and 0-5 per
cent, of other soluble salts, but the permanent pasture
becomes seriously injured, and for two or three years
even the arable land yields very indifferent crops.
Dymond has attributed this after effect to the injurious
ix.] ALKALI SOILS 245
action of the sea water on the texture of the soil, due to
the attack of the sodium chloride upon the double
silicates of the soil, lime in particular being displaced
by soda. The result is the deflocculation of the clay,
which will not settle down for many weeks when sus-
pended in water. The sodium chloride of the sea water
would also interact with any calcium carbonate in the
soil, giving rise to sodium carbonate, the deflocculating
effect of which upon the clay has already been noticed.
Biological effects may also be surmised : it is always
seen that the earth worms are killed in the land which
has been flooded with sea water, and in view of the
known unfavourable effect of chlorides on nitrification,
it is possible that the rate of production of nitrates in
the inundated soil is seriously lessened.
Alkali Soils.
In arid climates the rainfall is often insufficient to
produce percolation through the soil and subsoil into the
underground water system ; in consequence, the salts
produced by the weathering of the rocks tend to
accumulate in the subsoil, and may be brought to the
surface by capillary rise so as to cause almost entire
sterility. Such bad lands are known in America as
" alkali soils," but they are well known in India and in
Egypt, and indeed are common to all countries possess-
ing a small rainfall and great evaporation. In its most
aggravated form alkali land, particularly at the end of
the dry season, shows an actual white efflorescence of
salts at the surface ; all vegetation is destroyed, except
one or two plants which seem tolerant of large quantities
of saline matter, such as " greasewood," Sarcobatus sp.,
or the Australian " saltbushes," Atriplex setnibaccatum,
etc. In some cases the alkali is chiefly located at a
slight depth in the soil, and only effloresces on spots a
246 CAUSES OF FERTILITY AND STERILITY [chap.
little below the general level, where the subsoil water
comes to the surface. A heavy rainfall may be followed
by a rise of alkali, because a connection is then
established between the saline subsoil water and the
evaporating surface, whereupon a continuous capillary
use of salts takes place, followed by their crystallisation
at the surface. Per contra, the establishment of a soil
mulch, and shading the ground with a crop, so that
evaporation only takes place through the leaves, will aid
in keeping the alkali down. The composition of the
salts varies; as a rule, sodium chloride predominates,
with some sulphates of sodium, magnesium, and calcium,
in which case the material is known as " white alkali."
Under other conditions the material is really alkaline,
containing carbonate and bicarbonate of soda ; the
saline solution then dissolves some of the humus present
in the soil, and also causes the resolution of the clay
material into its finest particles, so that the soil forms
an intensely hard black pan when dry, which is known
as "black alkali." The carbonates are far more
injurious to vegetation than the neutral salts ; few
plants can bear as much as o I per cent, of sodium
carbonate, but are tolerant of 0-5 to 1 per cent, of
the other salts.
Though the alkali salts are sometimes chiefly
sulphates, more commonly sodium chloride is the
main constituent, together with the products of its
action in mass upon calcium carbonate and sulphate.
The diagram (Fig. 16), due to Hilgard, shows the dis-
tribution with depth of alkali salts in this type of soil
at Tulare, California ; the greatest accumulation of salts
takes place at a depth of 30 inches, the point to which
the annual rainfall penetrates. One of the most difficult
features presented by the cultivation of land in arid
regions where alkali occurs in the soil, comes from the
iches, 3
FlG. 16. — Nature and Distribution of Alkali Salts (Hilgard).
ix.] EVILS DUE TO IRRIGATION 247
tendency of the sterile spots to spread and the alkali to
be brought to the surface as soon as irrigation water is
employed, for without irrigation agriculture is hardly
possible. Many districts, which at first carried good
crops and were even laid down in fruit or vines, have been
ruined through the rise of alkali to the surface brought
about by irrigation ; in fact, in all these arid regions it
becomes exceedingly dangerous to raise the water table
in the land anywhere near the surface, because capillarity
then causes a rise of the salt changed water, and evapora-
tion concentrates it on the top. Just as some of the worst
alkali land occurs where rain falling upon the surround-
ing mountains finds its way by seepage through the
subsoil rich in salts and then rises to the surface in the
dry basin areas below, so the introduction of irrigation
canals pouring large volumes of water upon the land,
may equally establish the capillary connection between
the subsoil salts and the surface. The following extract
from Bulletin No. 14, U.S. Dept. of Agric, Div. of Soils,
dealing with alkali soils in the Yellowstone Valley,
shows the evil effects of incautious irrigation : —
" Irrigation has been practised for twelve or fifteen
years. The water for the main ditch supplying the
valley is taken out of the river nearly 40 miles above
the town of Billings. When the country was first settled,
and indeed above the ditch at the present time, the
depth to standing water in the wells was from 20 to 50
feet, and there was no signs of alkali on the surface of
the ground. Under the common practice of irrigation,
however, an excessive amount of water has been applied
to the land, and seepage waters have accumulated to
such a degree that water is now secured in wells at a
depth of from 3 to 10 feet in the irrigated district, while
many once fertile tracts on the lower levels are already
flooded, and alkali has accumulated on them to such an
248 CAUSES OF FERTILITY AND STERILITY [chap.
extent that they are mere bogs and swamps and alkali
flats, and the once fertile lands are thrown out as ruined
and abandoned tracts."
Nor is it necessary that the subsoil be charged with
salts for irrigation to produce alkali land; the mere
continual evaporation of ordinary river or spring water
may cause such an accumulation of saline matter at the
surface as is harmful to vegetation. This is well seen in
Egypt, where perennial irrigation is practised with the
Nile water, and the following quotation from Willcock's
Egyptian Irrigation will explain the action that takes
place :—
" The introduction of perennial irrigation into any
tract in Egypt means a total change in crops, irrigation,
and indeed everything which affects the soil. Owing to
the absence of rain, the land is not washed as it is in
other tropical countries, unless it is put under basin
irrigation.
" An acre of land may receive as many as twenty
waterings of about 9 cm. in depth each, i.e., a depth
of water of i-8o metre per annum, which is allowed
to stand over the soil, sink about half a metre into
the soil, and then be evaporated. Since the Nile
water, especially in summer, has salts in excess, these
salts accumulate at the surface, and if not eaten down
by suitable crops, soon appear as a white efflorescence.
While the spring level is low, capillary attraction
cannot bring up to the surface the spring water, which
generally contains a fair proportion of salts, but where
the spring level is high the salt-carrying water comes
to the surface, is there evaporated, and tends to further
destroy the soil. In old times the greater part of
the cultivation land was under basin irrigation, and
was thoroughly washed for some fifty days per annum ;
while the rest, consisting of the light sandy soils near the
IX.] IRRIGATION NECESSITATES DRAINAGE 249
Nile banks, was protected by insignificant dykes, which
dykes were burst every very high flood, and thus allowed
to be swept over by the Nile and washed once every
seven or eight years. All this is at an end now in
the tracts under perennial cultivation, and other remedies
have to be found."
The only remedy for the evils attending irrigation
is the introduction of drainage channels at a lower
level than the canals bearing the irrigation water; in
this way the percolation through the soil, which in
humid climates naturally removes the salts not taken
up by the crops, is effected artificially; there is some
apparent loss of water, but this is absolutely necessary
to maintain the land free from injurious salts. As
an example, the following passage may be quoted
from one of Major Hanbury Brown's reports on
Egyptian Irrigation : —
" It has been ascertained that the blessing of
improved water supply which has resulted from the
barrage having been made to do its duty, has been
attended in some localities with the evils due to
infiltration and want of drains. The remedy, as pointed
out in last year's Report, is to remove the want of
drains by digging them, and to provide the means of
washing out the salt brought to the surface, by infil-
tration in the shape of a liberal supply of water, by
which the salt would be carried away in solution along
the drains, or be forced down below the surface of the soil
to a depth at which it would be harmless. The liberal
water supply is not to be obtained except by the con-
struction of a storage reservoir at Aswan or elsewhere."
It was the neglect of drainage, when irrigation
canals were introduced, that led to so widespread a
deterioration of land in Egypt. To quote from Lord
Milner's England in Egypt ; —
25o CAUSES OF FERTILITY AND STERILITY [chap.
"But perhaps the worst feature of all was the
neglect of drainage, which was steadily ruining large
tracts of country. Even where drains existed, they
were frequently used also as irrigation channels, than
which it is impossible to conceive a worse sin against
a sound principle of agriculture. In some cases these
channels would be flowing brimful for purposes of
irrigation, just when they should have been empty
to receive the drainage water. Elsewhere the salt-
impregnated drainage water was actually pumped back
upon the land.
" It was the want of drainage which completed the
ruin of the Birriya, that broad belt of land which
occupies the northern and lowest portion of the Delta,
adjoining the great lakes. There are upwards of
1,000,000 acres of this region, now swamp, or salt
marsh, or otherwise uncultivable, which in ancient
times were the garden of Egypt."
It has been the business of the English irrigation
officers since the occupation to restore and improve
the drainage system, and to begin the reclamation of
the salted areas by cutting drainage canals and passing
enough of the abundant winter flood water through
the soil to wash out the salts into these drains.
Hilgard in California has also indicated that it is
impossible to wash the salts from the soil, even by
leaving the water to stand upon the surface for some
time, unless provision is made to remove the salted
water by underdrainage. In the case of black alkali,
however, the soil has become too impervious to
allow water to percolate at all; the first remedial
measure is to incorporate considerable quantities of
gypsum with the soil ; this will interact with the
sodium carbonate, producing sodium sulphate and
calcium carbonate, at the same time precipitating the
IX.] FERTILISERS DESTROYING THE TEXTURE 251
humus in a flocculent form. If now underdrainage be
brought into practice the soluble salts can be washed
through, and a very fertile soil results, owing to the
presence of the finely divided humus and calcium car-
bonate. Where underdrainage is hardly practicable
because of the expense, irrigation water should be
used in as limited amounts as possible, and every
care should be taken to keep the surface tilled and
under crop, so as to minimise evaporation from the
bare ground. In humid countries like our own,
damage due to the accumulation of salts are rare;
the author has, however, seen one case where the
vegetation of a lawn was destroyed during a hot dry
spell of weather by continuously applying water in
quantities which never washed down into the subsoil,
but evaporated every day. An efflorescence practically
identical with white alkali is sometimes seen on green-
house borders, which are constantly watered, but never in
sufficient quantities to cause percolation ; and gardeners
again are familiar with the check of growth which
sometimes occurs in the case of plants long in one
pot and constantly watered with hard water. The
remedy is to water from time to time so heavily as
to cause abundant percolation and thus wash all the
salts out.
Closely related to some of the phenomena presented
by alkali soils are certain secondary effects upon the
texture of the soil which are produced by the action of
some of the salts used as artificial manures. A good
friable texture in a heavy soil depends upon the clay
particles being generally flocculated and gathered
together into little aggregates, which give the soil a
coarser grain until they are resolved into their ultimate
particles by incautious working when the clay is in a wet
state. It has already been shown that acids and most
252 CAUSES OF FERTILITY AND STERILITY [chap
soluble salts, particularly those of calcium, possess a
strong flocculating power, whereas the soluble alkalis —
the carbonates and hydrates of sodium, potassium, and
ammonium — are active deflocculators, causing the clay
particles to separate into their most fine-grained condition.
It has long been recognised that large or frequent
dressings of nitrate of soda had an injurious action upon
the tilth of the soil, causing it to remain very wet, and
then to dry into hard, unkind clods. Since nitrate of
soda is very hygroscopic, the wetness induced in the
land was attributed to the absorption of moisture from
the atmosphere by the nitrate of soda, but when it is
considered what a very small proportion the water
absorbed by as much as 5 cwt. of nitrate of soda would
bear to the hundred tons which is the approximate
weight of an acre of soil an inch thick, it is obvious that
the difference in water content so induced would not be
sensible.
Clay soils, in fact, which have been treated with
nitrate of soda, do not show any excess of water ; but
they are very much deflocculated, as may be ascertained
by comparing the appearance after standing of a jar of
distilled water rendered turbid by shaking up in it a
gram of the soil, with a second jar in which the water
has been shaken with a gram of the same soil in its
normal condition. But nitrate of soda itself possesses
flocculating powers even when concentrated, hence the
observed deflocculation can not be due to the direct action
of the fertiliser upon the clay. However it has been
found that when plants feed upon a nutrient solution
containing nitrate of soda, an excess of the nitric acid
is withdrawn by the plant, and part of the soda is left
in the medium combined with the carbon dioxide
secreted by the plant The existence of this soluble
alkali after the growth of the plant can be verified by
IX.] BAD TILTH DUE TO FERTILISERS 253
experiments with water cultures, it can also be extracted
from the soil of the Rothamsted plots which have for
many years been manured with nitrate of soda. Small
as the amount may seem to be, it is quite sufficient to
account for the deflocculation of the clay and the
defective tilth observed on heavy land after nitrate of
soda has been used.
The bad repute of nitrate of soda as exhausting or
scourging the land, is less due to any sensible diminution
in the stock of plant food in the soil that follows its use,
than to the deflocculation it sometimes induces, and the
consequent deterioration of the texture of the soil.
As a remedy lime is not effective, since it is an alkali
itself; instead the nitrate of soda should be used in
conjunction with acid flocculating manures like super-
phosphate, or a mixture of nitrate of soda and sulphate
of ammonia should be used as a nitrogenous manure,
because the two manures will act upon the soil in
opposite ways, the nitrate of soda as an alkali and the
sulphate of ammonia as an acid. Dressings of soot are
also effective; not only does it assist the soil mechanically,
but also the small percentage of sulphate of ammonia it
contains possesses some power of flocculating the clay.
Other fertilisers which give rise to an alkaline
reaction in the soil are sulphate of potash, common
salt, and other soluble salts of sodium and potassium,
which as has already been noticed (p. 217) interact
with calcium carbonate in the soil, and give rise to a
little soluble alkaline carbonate. The injurious effects
of sulphate of potash upon the tilth of the heavy soil
at Rothamsted is very evident on the mangold field,
where the plots receiving this fertiliser every year
become excessively sticky and clinging in wet weather,
and dry, with a hard caked surface. It has often been
noticed that applications of potash salts and common
254 CAUSES OF FERTILITY AND STERILITY [chap.
salt have depressed instead of increasing the yield ; this
may probably be set down to the deterioration of tilth
that ensues when the soil is heavy and also contains
calcium carbonate. Some fertilisers, on the contrary,
aid in the flocculation of clay soils, the most effective
being superphosphate, which is acid and contains gypsum,
an effective flocculating agent. The ammonium salts,
which give rise to free acids, in consequence of the
withdrawal of ammonia by the moulds, etc., living in
the soil, act as very potent flocculators, and at
Rothamsted, for example, give rise to an open and
friable soil, as compared with the neighbouring plots
receiving nitrate of soda. Lime, which is the chief
flocculating agent employed in practice, is only effective
when it has become dissolved as bi-carbonate in the
soil water.
Amendments of the Soil.
Many soils, without being absolutely sterile, carry
very poor crops until their physical character has been
altered by the admixture of some considerable quantity
of one or other of the constituents of a normal soil
that may happen to be originally wanting. These
amendments of the soil by the mixture of other soils
date from the time that enclosures first began to be
made ; they were perhaps at their height during the
early years of the nineteenth century, after the middle
of which they rapidly diminished as it began to be
less and less remunerative to "make" land, until at
the present time the fall in the prices of produce and
the rise in the cost of labour have put an end to all
such operations. Among other causes of this neglect
may perhaps be set down the increased use of artificial
manures ; men began to take too exclusively a chemical
view of the functions of the soil, and shirked expendi-
IX.] WARPING 255
ture which did not seem to add directly any food for
the plant. However, it is probable that with modern
facilities for moving earth on a large scale by steam
power, the improvement of much poor land might even
now be profitably undertaken.
The operations which may be grouped under the
head of " amendments of the soil," comprise — drainage,
which has been dealt with elsewhere ; the marling and
claying of light sands ; the reclamation of peat bogs ;
the improvement of clay soils by liming and chalking,
or by paring and burning ; and lastly, the creation of
new alluvial soils by warping.
Warping.
The operation of "warping," or u colmetage," is
only possible in the vicinity of tidal estuaries, where
lands exist below the level of high water, and is in
this country practically confined to the estuaries of
the Humber and Ouse. Warping is carried out by
the construction of a wide drain protected by sluices
from the tidal river to the low land, which is first
divided by embankments into compartments of various
sizes up to 150 acres. When the embankments have
become consolidated, the flood tide, heavily charged
with suspended matter which is really fine earth
brought down by the river, is admitted into the
compartment, where it deposits most of its silt and
is allowed to run off when the level of the water in
the river has fallen during the ebb. The operation
is repeated until a layer of silt has formed 1 to 3
feet thick over the land, which is then dried and
brought under crop. As the chief deposit is always
near the mouth of the drain, where the velocity of
the silt-bearing current is first checked, the position
of the inlet must be shifted about to secure a
256 CAUSES OF FERTILITY AND STERILITY [chap.
uniform deposit all over the land and to distribute
the valuable fine silt which settles furthest from
the inlet. In some cases the sluice gates are auto-
matic, and water is admitted and drawn off at
every tide, but in others only every other tide is
admitted, thus giving time for the deposit of the finer
particles, and greatly improving the character of the
resulting land. As a rule, only the spring tides are
utilised, because the suspended matter is then at its
maximum, and the process is confined to the summer
months, to avoid danger from flooding when there is
much land water about. In exceptional cases land
may be warped 2 or 3 feet deep in one year — from
January to June — in other cases, where the water is
less charged with sediment, or the land is at a higher
level, an efficient warping, which should not be less
than 18 inches deep, requires three or four years.
When finished, the land is allowed to dry and con-
solidate, drainage grips are then thrown out, and a
light crop of oats, in which are sown clover and rye-
grass, is taken; after the seeds have been down two
years the land is generally ready to carry wheat.
Warp soils are, as a rule, fertile, and noted for
growing seed corn of high quality; they are to all
intents and purposes artificial alluvial soils, composed
entirely of the finer sands and silts without much
clay material, and are comparatively rich in organic
debris and other plant food, except perhaps potash.
The fertilising of the Egyptian land by the red Nile
flood water, the formation and improvement of river
meadows by winter flooding, are both analogous to
the process of M warping."
Marling and Claying.
Many light and blowing sands, almost too pure to
IX.] MARLING 257
permit of any vegetation, have in their immediate
neighbourhood a bed of marl or clay which can be
easily incorporated, practically creating a soil where
there was none before. Among the New Red Sand-
stones of Cheshire and the Midland Counties beds of
true marl occur and were at one time enormously
worked, so that every farm and almost every field shows
its old marl pit; the sandy Lower Greensand soils in
the Woburn district have been extensively marled from
the adjoining Oxford clay, and many of the Norfolk
soils have been made out of blowing sands, by bringing
up the clay which immediately underlies them. The
earlier volumes of the Journal of the Royal Agricultural
Society contain numerous accounts, showing how much
land was brought into cultivation by these means in
the first half of the nineteenth century.
" He that marls sand may buy the land,
He that marls moss shall suffer no loss,
But he that marls clay flings all away."
The usual practice in Norfolk was to open pits down
to the marl or clay, dig and spread it at the rate of
50 to 150 loads to the acre on a clover ley or turnip
fallow. In some cases trenches were opened all along
the field, and the clay thrown out on either side. By
the action of the weather, drying and wetting, followed
by frost, the clay comes into a condition to be harrowed
down, after which it can be ploughed into the ground.
The effect of marling or claying is more evident after
a year or two than at once, because the fine particles
become each year more thoroughly incorporated with
the soil. The effects are to be seen in increased crops, the
production of better leys and pastures, greater resistance
to drought, and particularly an increased stiffness in the
straw where manures are used to grow the crop.
R
$5^ CAUSES OF FERTILITY AND STERILITY [chaP.
Marl containing carbonate of lime is always far more
valuable than clay; pure clay is so little friable, and
so sterile itself, that it effects an improvement only
slowly; marl not only ameliorates the texture but
adds at once a supply of carbonate of lime, potash
compounds, and in some cases phosphoric acid also.
Clay and marl both have a tendency to sink, and
eventually require renewing, but if well done will last
for thirty to fifty years, because the accumulation of
humus and fibrous root-remains, due to the increased
crops, itself binds the soil together.
At the present day the need of marling or claying
on a small scale is often seen in old gardens,
particularly in old town gardens which are situated
upon gravel soils, initially very short of the finer soil
particles. The constant breaking of the surface by
cultivation, and the use of large quantities of stable
manure, which decays and leaves the soil open, result
in a continual washing down of the finest particles, until
the remaining soil loses all power of cohesion and of
resisting drought, falling into a dusty powder immedi-
ately on drying. A coating of clay in the early autumn,
or, better still, of good marl, is the only method of
giving consistency to such a soil and soon remedies
its worst defects, such as susceptibility to drought and
rapid fluctuations of temperature, and tendency to
produce soft vegetation, veiy liable to disease.
Reclamation of Peat Land.
One of the earliest methods of bringing peat land
in the Fens and similar districts into cultivation was,
to dry the land by means of open drains and break
up the surface with the breast plough ; the clods were
then gathered together, and burnt when dry, after-
wards the ashes were spread and a crop of rape taken.
ix.] RECLAMATION OF FEN LAND 259
The fire was never allowed to burn too fiercely, the
object being to obtain charred residues rather than
white ashes. The effect of burning the peat was to
provide a certain amount of ash rich in saline matters
and particularly in alkaline carbonates, thus correcting
the two great faults of the remaining peat, its deficiency
in mineral matters, and its sour reaction. At the same
time the weeds and other coarse vegetation occupying
the surface were destroyed, and a clean seed-bed prepared
for the crop. However, the process of burning is a
very wasteful one, involving the loss of the combined
nitrogen contained in the accumulated organic matter,
and after a few repetitions the land was found to be
seriously depleted of its reserves of humus. Burning
became replaced in the Fens by a marling process,
especially where the peat was of a sandy nature ; trenches
were opened to the bed of marl or clay always found
beneath the peat, and the clay thrown out and spread
at the rate of 100 loads or so per acre, the burning
process being reserved for the first reclamation, when
a mass of surface vegetation had to be got rid of. In
other districts, where marl is less available, peat has
to be brought into cultivation by draining the land with
open cuts, allowing some considerable time to elapse
during which the peat dries, shrinks, and consolidates,
and then correcting the acidity with lime. It is desir-
able to use large dressings of mineral manures like
basic slag and kainit to compensate for the deficiency
in mineral matter, especially where the peat is initially
of an acid character. In the Fens the peat is some-
times found to be mild humus containing lime ; this
does not respond to liming, and gives better crops with
superphosphate than with basic slag.
26o CAUSES OF FERTILITY AND STERILITY [chap.
Paring and Burning,
When the poorer clay soils were first taken into
cultivation, a beginning was generally made by " paring "
the surface with the breast plough, and " burning " the
clods as soon as they were sufficiently dry. The clods
were made up into heaps a yard or so in diameter with
the brushings of the hedges and all the rough surface
vegetation, together with as much clay as was judged
prudent. Each heap was then allowed to burn slowly
and char the clay, without permitting the heat to rise
sufficiently to vitrify the clay or dissipate such valuable
material as the alkalis of the ash. The resulting ashes
effected a great improvement in the soil : the clay was
partially dehydrated, or at least coagulated, thus pro-
viding a certain amount of coarse material to ameliorate
the texture ; in the charred clay also, some of the potash
was rendered more available, while the plant residues
provided mineral salts and alkalis to promote nitrifica-
tion. The drawback to the process is the inevitable
loss of nitrogen to the soil; but any one who has
noticed how freely crops grow on the patches of
arable land where couch heaps have been burnt the
season before, will see that, for the time being, the
fertility of the soil is increased by the process. Other
advantages of burning lie in the destruction of weeds
and insect life of all kinds, and although it has been
almost wholly discontinued at the present day, the
older writers on agriculture are unanimous as to its
beneficial effects in bringing poor clay land into cultiva-
tion. Recent investigations also show that heating the
soil to low temperatures, such as that of boiling water,
bring about a great increase in its productivity, probably
owing to a rearrangement of the bacterial flora of the
soil, for complete sterilisation is only effected at higher
IX.] LIMING 261
temperatures. The subject is still obscure, but these
effects of heating the soil may well be a factor in the
value of such processes.
A variation on the old process of " clod burning "
consists in " border burning," in which clay is dug from
one corner of the field and burnt by means of the couch
and other weeds cleaned off the land, the hedge trim-
mings, etc. ; the burnt clay is then spread over the
surface to improve the texture of the soil. Without
doubt the latter process might still be profitably adopted
where heavy clay land is under the plough ; if every
year some clay were added to the fires made from the
weeds and hedge trimmings, valuable material for
lightening the soil would be obtained without wasting
too much soil nitrogen.
Of course the incorporation of any large-grained
material will improve the texture of clay soils ; in some
cases sand has been dug and spread with advantage ;
road scrapings, town refuse, and even coal ashes help
to lighten the soil, though, in the case of gardens, coal
ashes should be avoided.
Liming and Chalking.
Of all the methods of improving the soil, other
than actual manuring or cultivation, none is more
important than the incorporation of lime or chalk.
It has already been indicated that many soils exist,
chiefly clays and sands, containing less than 1 per cent,
of carbonate of lime ; on all such land liming produces
very pronounced effects, both on the physical texture of
the soil and on the character of the resulting vegetation.
It is on the clays and other strong soils that lime
produces the greatest alteration in texture; its effect
in coagulating and causing the finer particles to form
into aggregates, which remain loosely cemented by
262 CAUSES OF FERTILITY AND STERILITY [chap.
the carbonate of lime, has already been discussed.
The soil becomes much less retentive of water, perco-
lation is increased so that the limed land is drier
and warmer, admits of cultivation at an earlier date
in the spring, and is far more friable when dry.
In fact, the liming gives a coarser texture to the clay
soil, and all the effects pertaining to the coarser texture,
such as diminished capacity for retaining water and
consequent greater warmth, less shrinkage and tendency
to cake on drying, are all manifest after the application
of lime. It does not, however, follow that the crop
will mature more readily though the season is made
earlier through liming ; in many cases in dry seasons
crops upon clay ripen prematurely, because the drying
up and shrinkage of the impervious clay cut the
roots off from all access of moisture. The liming,
by opening up the soil to the motion of water by
surface tension, keeps the plant growing for a longer
period ; at the same time, the increased amount of plant
food rendered available also tends to prolong the dura-
tion of growth. On very light soils the addition of lime
acts to a certain extent as a binding material, and in-
creases the cohesion and water-retaining power of the
soil, but it is not so effective in this respect as humus.
Besides its physical effect upon the texture of stiff soils,
lime has a very powerful chemical effect, liberating freely
the reserves of plant food of all kinds in the soil and
rendering them available to the plant ; so that on soils
naturally deficient in carbonate of lime, manures of all
kinds can only find their proper value if lime be also
used from time to time. On soils that have been
under intensive cultivation for a long time immense
reserves of plant food have been accumulated, which
only require the addition of lime to bring them into
action. As an example may be quoted the result of
IX.]
ACTION OF LIME
263
applying lime to an old hop garden at Farnham,
Surrey, where the soil consisted of an alluvial loam,
very deficient in carbonate of lime, and heavily dressed
with organic manures for many years previously. The
plots chosen for comparison received a complete artificial
manure with or without 1 ton of lime per acre; the
figures for the crops in the following table have been
reduced to percentages to eliminate the great fluctua-
tions due to season.
Year.
Artificial Manures.
With Lime.
Without Lime.
189S
1896
1897
1900
1901
IOO
IOO
IOO
IOO
IOO
70
84
80
81
90
Of course, as lime itself supplies no food to the
plant, but only sets in action the dormant residues
already present in the soil, the forcing of crops by the
aid of lime alone soon results in the exhaustion of the
land. Hence the old saw : —
" Lime, and lime without manure,
Will make both land and farmer poor."
The exact effect of lime in promoting fertility
depends upon the plant food in question. We have
already seen that all the decay processes which result
in the oxidation of the humus are promoted by the
presence of a base to combine with the organic acids
produced by the decay, and, in particular, that the
presence of an easily attacked base is necessary for
nitrification. As a nett result, the oxidation of the
humus and the formation of nitrates is much increased
264 CAUSES OF FERTILITY AND STERILITY [chap.
by a dressing of lime, which, indeed, is the first
indispensable step towards rendering available the rich
organic residues accumulated in a sour soil. As regards
the mineral constituents, lime has a very marked power
of bringing potash into a soluble state ; the double
hydrated silicates of potash and alumina, etc., which
result from the partial breaking down of felspars and
are the sources of the potash of our soils, are de-
composed, lime being substituted for the potash going
into solution. It is a case of mass action, where the
addition of one soluble constituent to the soil will
increase the amount that goes into solution of all
the other constituents which are capable of being
replaced by the base added ; the extent of the action
is therefore dependent upon the amount of lime used.
The fact that more potash has been rendered avail-
able in limed soils is clearly seen in the character of
the vegetation, e.g., in an increased proportion of
clovers in the herbage of pasture or hay land. The
action of lime as a liberator of potash is illustrated
by the effect of a dressing of chalk applied in 1881
to part of the permanent grass plots at Rothamsted;
by 1884 differences began to be manifest, the chalk
caused a change in the herbage of those plots which
had been receiving potash each year for twenty-five
years previously, increasing the production as a whole,
and particularly increasing the proportion of leguminous
plants in the herbage. On the plots, however, which
had been receiving no potash, and therefore contained
no recently accumulated reserves of this material, the
chalk had practically no effect, either in the weight or
character of the crop.
To some extent lime seems able to act as a liberator
of phosphoric acid in the soil. As pointed out by
Thdnard, lime is able to act upon the very insoluble
ix.] LIMING 265
phosphates of aluminium or iron which are present in
many soils, and, by converting them into phosphate of
lime, renders the phosphoric acid more available for
the plant.
Besides its specific actions in thus rendering more
soluble the soil constituents which nourish the plant,
lime exerts a very beneficial action by maintaining
the neutral reaction of the soil ; it neutralises the acids
produced by the decay and nitrification (see p. 174) of
the organic matter in the soil, or those due to the
oxidation of materials like iron pyrites in other soils
(see p. 243). Again, as has been shown already, it
is necessary as a base to satisfy the requirements of
artificial manures like sulphate of ammonia, superphos-
phate, and kainit (see p. 216), or to prevent the soil
being invaded by such organisms as the destructive
fungus causing " finger-and-toe " (see p. 209). It must,
however, be clearly realised that lime is wanted as a
base, not as a compound of calcium, necessary though
calcium itself may be to the economy of the plant ; and
that only carbonate of lime (chalk, limestone, etc.) or
quicklime and slaked lime, which promptly become
carbonate of lime when incorporated with the soil, are
capable of acting as the required base. Other calcium
compounds, as superphosphate of lime or sulphate of
lime (gypsum), or phosphate of lime in bones, etc., are
either acid or neutral, and do not supply the base
required to effect the beneficial actions set out above;
they cannot replace lime or chalk — in fact, they do not
contain any " lime " in the farmer's sense. Unfortunately,
it has been too often supposed that the use of artificial
manures, such as superphosphate of lime, removed the
necessity of a periodical liming of the soil, and some
of the neglect into which this all-important operation
has fallen may be set down to the unfortunate confusion
266 CAUSES OF FERTILITY AND STERILITY [chap.
hanging round the word lime. However, as will have
been gathered from a consideration of the effects of
sulphate of ammonia in depleting the Woburn soil of
carbonate of lime, the use of artificial manures generally
demands an increased rather than a lessened attention
to the periodical liming of the land.
The method of liming which was formerly in vogue
consisted in applying very large quantities of quicklime
at comparatively long intervals, ioo to 150 bushels
per acre ( = 2 to 4 tons) every eight or ten years, or
an initial dressing of 100 bushels, with a further dressing
of 50 bushels per acre every third year. The reason
for this interval lies in the fact that the best effects
of lime are to be seen after the lapse of a year or
two; the material becomes carbonate, which, being
insoluble, is incorporated with the soil and passes into
solution as bicarbonate but slowly. The immediate
effect of lime may even be a diminution of the crop
if it be used on very rich land, or in actual contact with
fresh dung ; under these conditions there appears to be
some loss of ammonia by volatilisation. Of course the
effect of lime is not very persistent, and the dressing
must be repeated ; as the farmers say, the " lime sinks in
the land," i.e. y carbonate of lime is removed from the
surface soil by solution as bicarbonate.
In carrying out the operation of " liming," the aim
should be to ensure as fine a division as possible, so as
to incorporate the material intimately with the soil. In
some cases the lime is thrown out in heaps on the
stubbles in autumn, and slaked by pouring on water,
the hot slaked powder into which the quicklime falls
being immediately spread over the land. This method
only answers with "fat" limes, which slake and fall
readily to a dry powder ; a better method is to lay up
the quicklime in heaps and cover the heaps with soil ?
ix.] LIME 267
in which case the lime slakes gradually to a fine powder
that can be spread before the plough. It is not wise to
spread the quicklime over the land, as much of it,
after slaking and becoming carbonated, remains in
lumps which cannot be reduced to a powder.
The expense of liming in this fashion is consider-
able, and as the action is not immediate, owing to the
difficulty of getting the material mixed with the soil,
it is desirable to replace it, if possible, by a cheaper
process. This has been attained by the use of ground
lime, which is at the present time prepared by most
lime works for the use of builders ; 5 cwt. of ground
lime per acre, distributed by a manure barrow or by
one of the artificial manure distributors now manu-
factured, will be found more effective for one or two
seasons than ten or twelve times as much applied in
the old-fashioned method. Of course such a small
dressing of ground lime requires renewing more fre-
quently; but, as the expense is comparatively trifling,
both for labour and material, as compared with the
older process, it may be hoped that on many soils this
all-important operation will assume its old prominence
in the routine of farming.
Considerable differences are to be seen in the
character of lime made from the various calcium car-
bonate rocks burnt for lime in the British Islands; in
the main a distinction may be drawn between the white
11 fat " limes made from the White Chalk, the Mountain
Limestone and other comparatively pure deposits of
calcium carbonate, and the " thin " grey or stone limes
made from less pure and more argillaceous limestones.
The " fat " limes are the purer, slake readily and swell
considerably in the act, forming afterwards a bulky
white powder ; the " poor " or " thin " limes slake with
comparative difficulty and do not increase much in
268 CAUSES OF FERTILITY AND STERILITY [chap.
bulk. The "thin" limes partake somewhat of the
nature of a cement, setting after mixture with water,
and are more esteemed by builders than the "fat"
limes, which harden with extreme slowness and are
chiefly employed for plastering and kindred work.
Naturally the " fat " limes are preferable from an agri-
cultural point of view, both for their purity and the
finer condition into which they fall; unfortunately few
of the lime works grind the white lime in the ordinary
course of trade, as they do the builders' lime.
The lime made by burning the Magnesian Lime-
stone which occurs in Durham, Yorkshire, Derby-
shire, and Notts, is disliked by farmers and regarded
as injurious rather than beneficial to the land. It
contains 50 to 80 per cent, of lime and 4 to 40 per
cent, of magnesia, which latter constituent may be
the cause of the ill effects.
The following analyses show the mean composition
of several samples of "fat" and "poor" lime, being
"white" and "grey" lime respectively, made from the
Upper and Lower Chalk of the North Downs : —
White Lime.
Grey Lime.
Caustic Lime • < • •
Carbonate of Lime • •
Magnesia • •
Oxide of Iron . . •
Alumina • •
Silica as Soluble Silicates • •
Insoluble Residue • •
Water, Alkalis, etc. . •
90.20
2*40
o-35
0-52
1*70
2»6o
•25
1.98
74.OO
2-66
0-38
I'OO
7.60
8.60
•94
4.82
ioo-oo
IOO-OO
In place of lime, chalk may often be used with
advantage when it is readily accessible ; for example, on
ix.j CHALKING 269
one side of the Chalk formation the Gault and the upper
beds of the Lower Greensand, and on the other side the
London Clay and the Bagshot Sands, are generally in
need of lime, and are never very remote from the out-
crop of the chalk. The superficial clays and sands
lying on the Chalk itself are often deficient in lime, and
may be readily chalked by sinking shallow pits. It
must be remembered that much larger quantities of
chalk than of lime are needed to produce a given effect ;
not only is the chalk equivalent chemically to about
half its weight of lime, but in practice it can never be
reduced to so fine a state of division as lime obtains by
careful slaking. In chalking, it is desirable to obtain
the soft upper white chalk from a pit, so that it is
saturated with quarry water ; if then spread over the land
in autumn it gets frozen while still full of water, and
becomes reduced to a comparatively fine powder which
can be ploughed in if on arable soil or spread with
a harrow on the pastures. Very large quantities of
chalk are used, up to 100 loads to the acre; naturally
the effect of such treatment is more permanent than
the usual liming.
The custom of "chalking" was very extensively
practised during the seventeenth and eighteenth centuries
in Hertfordshire on the high plateau land on which the
Rothamsted estate is situated. There the "clay with
flints" and the "boulder clay," though not, as a rule,
more than 10 to 12 feet thick, and resting on the chalk
rock from which they have largely been derived, have
been completely decalcified by the solvent action of the
rain water, and no longer contain more than a trace of
carbonate of lime. It was customary to sink bell pits
through the clay until the chalk was reached ; this was
then dug out, hauled to the surface in baskets, and dragged
out on to the fields in sledges. Sixty to a hundred or
rjo CAUSES OF FERTILITY AND STERILITY [chap. ix.
even a hundred and fifty loads per acre were spread,
and from time to time the process was repeated. The
amount of chalk thus spread upon the surface was con-
siderable ; the surface soil of the arable fields on the
Rothamsted estate now contains from 3 to 5 per cent,
of carbonate of lime, which is equivalent to 30 to 50 tons
per acre ; and since none has been spread for the last
seventy years at least, and solution in the rain water has
constantly been going on, there must have been nearer
100 tons per acre at the beginning of the nineteenth
century.
The result has been practically the creation of a soil
fit for arable farming, for some of the Rothamsted fields
which had never undergone the operation have had to
be laid down to grass, so difficult did their cultivation
prove in wet seasons.
Chalk is perhaps more suited than lime to very
light sandy soils like the Lower Greensand or the
Bagshot beds, for on such dry hot soils the application
of quicklime is apt to result in too rapid a decay of the
organic reserves of the land ; on clay soils, however,
quicklime is preferable, as it is a much more effective
agent in coagulating and improving the texture of the
clay
CHAPTER X
SOIL TYPES
Classification of Soils according to their Physical or Chemical
Nature — Geological Origin the Basis of Classification — Vege-
tation Characteristic of Various Soil Types : Physical Structure,
Chemical Composition, Natural Flora and Weeds character-
istic of Sands, Loams, Calcareous Soils, Clays, Peat, Marsh,
and Salt Soils — Soil Surveys, their Execution and Application.
Perhaps the question of the greatest practical import-
ance in connection with the scientific study of soils
is their classification into certain types defined by
their physical or chemical properties, and the alloca-
tion of these types to their appropriate areas, so as to
obtain a soil map of any given district. Despite
disturbing factors, to which allusion will be made later,
certain types of soil persist over wide stretches of
country, and are characterised not only by a general
resemblance in chemical or physical constitution, but
by a corresponding similarity in the natural flora they
bear, and their appropriateness to certain crops. The
constancy of the soil types is the result of a common
origin from the same kind of rock, and the difficulty
lies less in recognising the types than in drawing
boundary lines, so imperceptibly does one class shade
off into another. The only classification that can be
at all general, is one based upon the physical structure
271
272 SOIL TYPES [chap.
and texture of the soil, viz., into sands, loams, and clays,
with subdivisions dependent on the presence or absence
of calcium carbonate, and upon the situation, as causing
the accumulation or otherwise of humus.
In attempting to review the vegetation appropriate
to different types of soil it will be found that two
distinct factors must be taken into account — the
relations of the soil to water, and its chemical con-
stitution — which factors often interact in a complex
fashion, different causes producing the same effect. A
plant, for example, may be found upon sand because
of its dryness, or, because of the absence of calcium
carbonate usually associated with sand; another
plant, having adapted its structure to use very small
quantities of water, may equally well be found on a
dry sand, or on a clay which holds so much water
as to be injurious to the ordinary plant.
Plants have adapted themselves to conditions of
dryness in very diverse ways; in some cases, as in
gorse or broom, the leaf surface is much restricted ; in
others, the thickness of the cuticle has been increased,
or the surface of the leaf is thickly clad with hairs ; in
other cases the leaves possess special tissues for storing
water. Such plants are known as " xerophytes," and
are found on soils which appear to differ very much
from one another, for a soil may contain plenty of
water and yet be physiologically dry, because of the
presence of some other constituent hindering the
absorption of water by the plant.
The areas on which xerophytic plants are found
include not only the true desert areas, where great
heat and intense illumination occur during the larger
part of the year, but also the pervious sandy soils re-
taining very little moisture — sand dunes, shingle flats,
and the like. Again, the plants of alkali soils and of
x.] PHYSIOLOGICAL DRYNESS 273
salt marshes invaded by the sea, develop a xerophytic
structure, because they would be injured if they absorbed
large amounts of saline soil water. Peaty areas also
act in the same way, for it is found that the humic acids
in such soils withhold the water from the plant very
obstinately. Exposed elevated regions with a low
temperature, by reducing the power of the roots to
absorb moisture, render it necessary that the plant
should lose little by transpiration; hence we see
certain conifers flourish both on dry sandy soils and
wet elevated moors.
As regards the chemical side of the question, the
most important soil constituent affecting vegetation is
calcium carbonate; a large number of plants seem
absolutely intolerant of lime in the soil, while others
are rarely seen off limestone and chalk areas. Even
among the humus-loving plants a different flora is found
on the acid peaty areas from that prevalent on the
mild humus areas where the soil water contains calcium
bicarbonate in solution.
But, however characteristic the general aspect of
the vegetation may be upon the different types of soil,
it is rare to find cases of plants entirely intolerant of
a different kind of soil from that which they habitually
frequent; many plants show a preference for one soil
or other without being exclusively confined to it For
example, the common primrose is undoubtedly a clay
lover, yet it will be found widely distributed over all
the English soils; the beech and the yew are typical
trees of the chalk, good oak and hornbeam of the clay ;
Spanish chestnut, and many conifers like the Scotch
fir, are sand lovers ; yet each of these trees will be
found commonly enough on other kinds of soils. It will
rarely be found that plants will absolutely refuse to grow
or even to flourish on soils of which they are naturally
S
274 SOIL TYPES [chap.
intolerant ; a lime-hating plant like gorse, for example,
will grow freely enough on a calcareous soil in a garden
where it is protected from competition. But in nature
all plants are subjected to severe competition, and a very
small depression of their vitality brought about by the
presence or absence of some constituent of the soil, may
so turn the scale against them that they are almost
invariably crowded off areas of such soil, the exceptions
being due to some other favourable factor coming into
play locally.
Sands.
The typical sandy soils of this country are either
alluvial flats in the lower levels of our rivers, passing
into dunes where the sand accumulates near the sea,
or are directly derived from some of the many coarse-
grained sandy formations developed in England. The
Bagshot beds and the Lower Greensand form wide
areas in the south-east ; the sandy beds of the Oolite
produce similar soils in Northamptonshire and the
East Midlands ; further west and northward the Bunter
beds give rise to other very coarse-textured soils, as
does the Millstone Grit in more elevated areas in the
North.
As these coarse-grained sands have been laid down
in rough water, they consist in the main of silica, which
alone is able to resist the degree of weathering and
attrition to which the original material has been
subjected. In consequence, the rock is initially with-
out much calcium carbonate or other material which
will yield soluble salts on further weathering ; the open
texture of the material also results in a very free
movement of soil water, and this continues the removal
of anything soluble. Occasionally a sandy rock is
found which has been largely formed by the disin-
X.] SANDY SOILS 275
tegration of shells, so that it is rich in calcium
carbonate ; but, as a rule, sandy soils are characterised
by poverty in this material. Most soils of this sandy
type seem to possess considerable amounts of oxide
of iron ; the actual proportion present may not be so
large as in ordinary soils, but, being spread over the
comparatively small surface offered by the large grains,
it is more in evidence. The phosphoric acid, which
is rarely present in any quantity, generally about 01
per cent, is chiefly combined with the oxides of iron.
Because of the general lack of finer particles — in the
main clay derived from the weathering of felspar —
soils of this type are notably deficient in potash.
Despite their warmth and free aeration, sandy
soils often accumulate considerable amounts of humus,
an effect probably due to the absence of calcium
carbonate. Where depressions occur in the general
level of the ground, a layer of impervious ferric
hydrate or "pan" forms below the surface and holds
up the drainage water, which waterlogged condition
is at once followed by an accumulation of peat.
On these sandy areas cultivation is a very artificial
affair, and the soil has practically to be created. The
first necessity is a supply of lime and mineral salts,
to remedy the lack of nutriment ; then as much humus
as possible must be obtained, by turning in or fold-
ing green crops, or even from their roots and stubble
only. The humus binds the soil together, creates a
reserve of manurial material, and much increases the
retentive power of the soil both for water and mineral
salts.
Being so dry, the specific heat of sandy soils is
exceptionally low; in consequence these soils are
early, and as they also recover quickly from rain, so
that cultivation is not forced to wait much on the
276 SOIL TYPES [chap.
season but can be proceeded with very readily, they are
especially suited to market gardening, wherever situ-
ated sufficiently near to a large town to enable large
quantities of manure to be obtained cheaply. Where
the water table is close to the surface, sandy soils can
become very fertile, roots range freely in them, and appli-
cations of manure have their full effect. Like all light
soils, they are apt to become very weedy. Of the crops
suitable to soils of this type, spring wheat is often better
than the autumn-sown variety; the quality of wheat
is, however, generally inferior on sandy soils; barley
is better than oats, and maize is worthy of attention
as a fodder crop. Swedes, cabbages, and the cruciferous
crops generally, are subject to " finger-and-toe," in conse-
quence of the poverty of the soil in lime and soluble
mineral constituents.
With certain exceptions, leguminous plants do
not grow well on sandy soils, and require considerable
supplies of lime and mineral manures ; there are, how-
ever, some leguminous plants which are characteristically
calcifuges — i.e., intolerant of lime in the soil — lupins,
serradella, and gorse belong to this class. Allusion has
already been made to the reclamation of sandy land in
Prussia by means of lupins, and probably more use
might be made of the crop in this country on similar
soils. Experiments made with gorse on the coarse
sandy soil of the Royal Agricultural Society's farm
at Woburn, indicate that it may become a profitable
fodder crop on such soils.
Potatoes are perhaps the best crop on the sandy
soils, but require considerable expenditure of manure,
including large dressings of potash, to do well. Carrots
are another crop particularly appropriate to sandy soils,
as they need a deep, fine tilth.
The manuring of sandy soils must be based upon a
X.] SANDY SOILS 277
liberal use of lime, frequently renewed because of the
ease with which water percolates and removes the
calcium carbonate. Marling and chalking, wherever
such materials are available, are better for the land
than the use of quicklime, which is apt to induce too
rapid an oxidation of the organic matter. Nitrogen is
best supplied in its organic forms, as in well-rotted dung,
the guanos, fish or meat manure, rape cake, etc. ; nitrate
of soda is apt to induce too rapid a growth, and also to
be washed away. Sulphate of ammonia is unsuitable,
owing to the lack of lime in the soil. Of phosphatic
manures, superphosphate is unsuitable owing to its
acid nature. Basic slag is also unsuitable as a rule,
owing to the small quantities of water retained by
the soil, but it answers well on sands where the
water table is near the surface; on the whole,
neutral easily available phosphates like phosphatic
guano and steamed bone flour give the best results
on these soils. Potash manures are much needed,
and either kainit or sulphate of potash may be used.
Gypsum is often used with good effect on such soils
in the Wealden area, acting as a liberator of what
little potash may be in the soil.
The natural flora of the sandy soils is of a double
character — in part xerophytic, and associated with the
prevailing dryness of the soils; in part calcifuge,
and dependent on the absence of calcium carbonate.
Plants with mycorhiza are abundant, owing, as already
explained, to the comparative poverty of these soils
in both water and soluble salts.
The characteristic sand trees are the Spanish
chestnut, birch, holly, and many conifers; of these
the Spanish chestnut and some of the firs, like
Pinus pinaster, are particularly intolerant of calcium
carbonate.
278 SOIL TYPES [chap.
Many of the Ericaceae, such as common heather and
the heaths, cultivated species like the rhododendrons
and azaleas of our gardens, are similarly intolerant of
lime and associated with sandy soils ; at higher levels
various species of V actinium and kindred plants are
common. Gorse ( Ulex europceus and U, nanus), broom
{Cytisus scoparius), Genista anglica, Ornithopus, and
several vetches like Vicia cracca, are characteristic legu-
minous plants of sandy soils. The foxglove {Digitalis
purpurea), sorrel {Rumex acetoselld), and in undrained
situations the sundews {Drosera sp.) are intolerant of
lime and are common plants on sandy soils, as also
are the common bracken {Pteris aquilina), and wavy hair
grass {Air a flexuosa).
Characteristic weeds of sandy soils are, spurrey
{Spergula arvensis), and sandwort-spurrey {Spergularia
rubra), corn marigold {Chrysanthemum segetum), and
knawel {Scleranthus annuus and perennis) ; Papaver
dubium and Centaurea cyanus are also common on
such soils. The bulbous buttercup {Ranunculus bul-
bosus) is very frequent on dry pastures, whether sandy
or chalky, as is the small bindweed {Convolvulus
arvensis) of similar soils under cultivation ; the silver-
weed {Potentilla anserina), though generally indicative
of winter flooding, is to be found on all kinds of
poor, light land.
The Loams,
The sandy soils pass by imperceptible stages into
the loams — free-working soils containing enough sand
to be friable and to admit of percolation, yet retain-
ing sufficient water near the surface to withstand
short spells of dry weather. If the sandy fractions of
the loam are mainly fine grained, the soil is apt to
run and become very sticky in wet weather, afterwards
X.] ALLUVIAL LOAMS 279
drying to hard clods; an admixture of coarser sand
results in a better texture. The loams are typical
soils of arable cultivation and are suitable to all crops ;
their manurial requirements vary with the origin of
each soil, and are largely conditioned by its poverty
or richness in calcium carbonate. While no special
flora can be associated with the loams, there are several
weeds generally taken as indicative of good fertile soils
of this class; such are chickweed (Stellaria media),
groundsel (Senecio vulgaris) , fat hen {Chenopodium
album)) stinking mayweed (Antkemis Cotula), and
the Sow thistle (Sonchus oleraceus). Other weeds
of cultivated land, which only occur when the soil
is capable of carrying fair crops, are goose grass
{Galium aparine), the speedwells {Veronica agrestis,
etc.), pimpernel {Anagallis arvensis), henbit (Lamium
amplexicaule\ wild poppy {Papaver rhaeas\ and the
small spurges like Euphorbia Peplus. The alluvial
soils which border the rivers and pass into con-
siderable marshes at their mouths, must, by their
texture, be classed among the loams, and present no
specific features, except where they are waterlogged
and marshy, or near the sea where the subsoil water
becomes so rich in salt as to alter the character of
the vegetation. The marshy patches accumulate, as
a rule, what has already been described as " mild
humus," owing to the presence of bicarbonate of lime
in the soil water; it is generally accompanied by
deposits of pulverulent ferric hydrate. The presence
of rushes, of sedges like the carnation grass, or orchids
like 0. maculata and O. latifolia y are characteristic of
these spots requiring drainage. Lousewort (Pedicularis
palustris) is said only to occur in marshes where the
water contains lime.
The salt marshes possess a characteristic vegeta-
2 8o SOIL TYPES
tion of what are termed halophytes, plants capable
of resisting a considerable quantity of salt in the
medium in which they grow. Among cultivated
plants, mangolds, asparagus, and crucifers like cabbage,
are most tolerant of salt, and the two former are true
«
halophytes.
Many halophytes live by acting as xerophytes, and
taking very little water up; they are also able to
store away in their tissues quantities of saline matter
which would be toxic to the majority of plants. The
ash of Armeria maritima shows 12 to 15 per cent, of
chlorine in the ash; in Aster tripolium the proportion
rises to over 40 per cent, in the ash of the leaves, and
to 50 per cent, in that of the stem; yet although the
plants habitually contain these large amounts of salt,
they will grow perfectly well in ordinary soil where
they can get but little.
The Australian salt-bush {A triplex semibaccatum),
which has already been mentioned as tolerant of a
large amount of alkali in the soil, also removes much
soluble matter — the dry plant containing as much as
20 per cent, of ash, so that the salt content of
the soil may be materially reduced by cropping with
this plant. The halophytes seen in the salt marshes
of this country consist of various species of Atriplex y
Beta (the source of the cultivated beets and man-
golds), and other Chenopodiacese, Statice armeria.
Aster tripolium, Frankenia, and a number of crucifer-
ous plants like Crambe, and Cakile, with umbellifers
like Crithmum. Some plants show a great dislike to
salt, even in small proportion, e.g., the Rosaceae,
Orchidaceae, and the Ericaceae.
x.] CALCAREOUS SOILS 281
Calcareous Soils.
It is difficult to draw an exact line of demarcation
between the loams and calcareous soils, so variable is
the proportion of carbonate of lime, owing to its con-
tinual removal by the percolation of water containing
carbonic acid. Even on the chalk and limestone, where
the thickness of the soil layer is to be measured in
inches, the surface soil may have its calcium carbonate
almost wholly removed, and, again, where the deeper
soils of calcareous origin accumulate in the valleys,
there is nothing to distinguish them from ordinary
loams. However, the general aspect of the calcareous
soils containing from 5 to 60 per cent, of calcium
carbonate is characteristic, and the natural flora always
indicates the presence of much lime. The texture of
the calcareous soils may vary within any limits, accord-
ing to the formation from which they have originated.
On the one hand, extremely fine-textured heavy marls
exist; for example, the soils derived from the strata
at the base of the Chalk and upper beds of the Gault
in the south and east of England; on the contrary,
fairly coarse sand may form a considerable proportion
of the soil, rendering it light in texture, as is the case
with many of the soils resting on the chalk of the
North Downs. In all cases these calcareous soils are
typically sticky when wet, and easily cake on the surface
when dried. Such soils, again, lose their organic matter
very rapidly by decay ; in farming them it is desirable
to use every means to increase the proportion of humus
by adding farmyard manure, by folding roots on the
land, or by ploughing in green crops. Slowly acting
nitrogenous manures, like rape dust or shoddy, are
valuable ; again, there is always enough calcium
carbonate present naturally to render to sulphate of
282 SOIL TYPES [chap.
ammonia its full value as a source of nitrogen. The
lighter calcareous soils require a free use of nitrogenous
manures to get good crops. Calcareous soils are
generally well provided with phosphoric acid, owing to
the organic origin of the calcium carbonate; the rule
is, however, by no means universal, the upper Chalk,
for example, yields soils with less than 01 per cent,
of this constituent. Superphosphate is undoubtedly
the best source of phosphoric acid for such soils,
basic slag is almost without action. The propor-
tion of potash present in these soils is generally
reflected in their texture; if light and near the
unaltered rock, they are as a rule very deficient in this
constituent, and require its addition for the growth
of any of the root crops. Salt is generally bene-
ficial as an addition to manures on the calcareous
soils.
The calcareous soils are generally warm, dry, and
healthy for stock; when deep and sheltered they are
extremely fertile; the thinner soils are rather subject
to certain insect pests, like the turnip flea. The abund-
ance of worms in chalky pasture is worthy of note.
The lighter calcareous soils are notoriously weedy. In
addition to the usual weeds of light land, Fumitory
{Fumaria officinalis). Geranium tnolle, and kindred
species, are almost confined to soils with a consider-
able proportion of calcium carbonate. Two crops
are very characteristic of calcareous soils wherever
the climate will admit of their growth, viz., sainfoin
and lucerne, which flourish excellently, and provide
abundant and valuable fodder even on the driest
chalk soils.
The natural flora of these calcareous soils includes
the beech, yew, and wild cherry, among trees; the
juniper, box, mealy guelder rose {Viburnum lantana\
X.] CALCAREOUS SOILS 283
beam tree {Pyrus aria), dogwood (Cornus sanguined),
and Clematis vitalba, among shrubs.
The vegetation is characteristically rich in flower-
ing plants: amongst the Leguminosae, the horse -shoe
vetch {Hippocrepis comosa), bird's-foot trefoil {Lotus
corniculatus), kidney vetch (Anthyllis Vulnerarid), are
everywhere abundant; milkwort (Polygala), bladder
campion {Silene inflatd), Spircea filipendula, burnet
(Poterium sanguisorbd), wild parsnip {Pastinaca sativd)*
sheep's scabious {Scabiosa columbaria), chicory (Cicho-
rium intybus), and certain of the Gentianaceae, as G,
amarella and Chlora perfoliata, the viper's bugloss
(Echium vulgare), and a number of labiates like Ori-
ganum, are characteristic of the pastures and waste
places on chalk and limestone. Amongst grasses,
A vena pubescens, A. flavescens, Bromus erectus, and
Brachypodium pinnatum, are common.
While it has been indicated that many plants are
intolerant of lime, others show the effect of any excess
in the soils by a stunted development of the plant,
often accompanied by a reduced size of the leaf, and
a sickly yellow or even white colour. This unhealthy
condition of "chlorosis" is particularly noticeable on
the stiff marls, which are but little aerated but contain
much calcium carbonate; on the Continent it often
affects vines, particularly when grafted on American
stocks.
Clay Soils.
It has already been indicated that clay soils are
those in which the finer fractions of sand, silt, and
clay predominate ; the presence of any considerable pro-
portion of coarse sand causes the soil to become friable,
and would class it with the loams. The texture of
284 SOIL TYPES [chap.
clay soils naturally varies very much; the heaviest
clays occur in dry climates, where the percolation
has not been sufficiently great to wash away many
of the finer particles ; in the east and south-east of
England the Oxford and the London Clay, with the
Boulder clays derived therefrom, give rise to the
most stubborn and intractable clays. On these soils
the old practice of an occasional bare fallow is still
carried out, and is almost necessary to maintain the
soil in good cultivation. As a rule, the strong clay
soils have of late years been laid down to permanent
pasture; the cost and the difficulty of arable culti-
vation (for much wet weather in autumn or spring
may render it impossible to put horses on the land
for long periods), and the great fall in prices of both
wheat and beans, the staple crops of such land, have
rendered it necessary to resort to a cheaper method
of farming.
Most clays carry good permanent pasture, because
the soil retains enough water to keep the grass growing
through any but the longer periods of drought ; in very
dry years, however, clay suffers severely from the
drought; the surface cracks and the subsoil dries
through the cracks ; the resistance also offered by the
close texture of the soil to the capillary rise of soil
water renders the winter rainfall le$s available to the
crop than on soils of lighter texture. The benefits
accruing from drainage, in making the soil dry more
quickly after rain and resist drought better, have already
been discussed. Certain clay soils may be found too
close textured to carry good pasture; the soil sets so
firmly that aeration becomes very defective, and the
vegetation degenerates into surface rooting, stoloniferous
grasses like Agrostis alba.
Owing to their fine division, their origin from the
x.] CLAY SOILS 285
compound silicates of primitive rocks, and the reduced
percolation which they permit, all clays are compara-
tively rich in soluble mineral salts. Many of them show
crystals of selenite (CaS0 4 2H 2 0) in the subsoil com-
paratively near to the surface ; magnesium sulphate is
often abundant, and strongly impregnates the water
obtained from the wells or the occasional springs to be
found in the clays. In the Weald of Kent the shallow
wells in the clay yield water that is almost undrinkable,
containing, as it does, from 150 to 450 parts of dissolved
matters per 100,000, consisting chiefly of the sulphates
(with some chlorides) of magnesium and calcium. The
sulphates often originate from the oxidation of finely
divided iron pyrites. The presence of ferrous salts
and other unoxidised iron compounds has already
been alluded to as a source of sterility in clay soils
particularly where the subsoil has been incautiously
brought to the surface. In the cultivation of all land
it is important to keep the surface soil on the top, and
to attempt to deepen the staple with care ; but this is
particularly the case with clays, where the land may
easily be injured for years by over-deep ploughing. No
soils show more marked change than the clays do in
passing from soil to subsoil, both in chemical com-
position and physical texture.
Many clay soils, especially when undrained, possess
a great tendency to accumulate hydrated ferric oxide
some few inches below the surface, at about the level
to which the soil is ordinarily aerated. This deposit
sometimes forms a continuous layer or " pan " ; in drier
climates it becomes a kind of " crowstone " gravel, made
up of little nodules of hydrated oxide of iron, contain-
ing also manganese. This material frequently forms
a serious obstacle to cultivation, and requires to be
broken up with a crowbar or a subsoil plough before
286 SOIL TYPES [chap.
any deep-rooting crop can be properly grown. Its
origin is perhaps not entirely explained as yet; the
respective shares of the iron bacteria of Winogradsky,
or the purely chemical actions of solution and reduc-
tion by the organic matter and carbonic acid, followed
by redeposit on evaporation, is a matter requiring
further investigation. The formation of the material
is only noticed in clays very poor in calcium carbon-
ate and liable to waterlogging through insufficient per-
colation.
Owing to their coolness, their retention of moisture,
and comparative impermeability to air, humus tends to
accumulate in the clay soils ; both arable and pasture
soils show a higher proportion of organic matter and
of humus than is found, as a rule, on the lighter
lands; the effect of manures like farmyard manure is
also more lasting. The use of more slowly acting
nitrogenous manures is therefore not so desirable
on the clay soils; on the other hand, sulphate of
ammonia is often unsuitable because of the want of
calcium carbonate, and nitrate of soda, which often
gives the best returns, is apt to affect the texture
injuriously.
The clays are very generally deficient in calcium car-
bonate, often to an extreme degree, much to the detri-
ment of the texture of the soil. The use of lime is of
the utmost value to all clay soils, improving the texture,
making them drier and therefore warmer and earlier,
and rendering available the supplies of nitrogen and
potash with which they are often liberally endowed.
The excess of magnesia and unoxidised iron com-
pounds which also characterise many clays is corrected
by the use of lime.
Many clay soils also show a considerable deficiency
of phosphoric acid, and respond freely to dressings
x.] CLA y SOILS 287
with manures containing this substance. Superphos-
phate may be used with advantage wherever there is
enough calcium carbonate in the soil, but basic slag
is the typical phosphatic manure for the strong soils
which retain sufficient water to render the phosphates
active. While supplying phosphoric acid, it also
contains free lime in a fine state of subdivision, and
so liberates in a soluble state the reserves of nitrogen
and potash in the soils. It should, however, not be
forgotten that as the basic slag only supplies one
element of plant food, the phosphoric acid, the soil
may easily be exhausted by continual cropping and
manuring with basic slag alone.
Potash is always present in large amounts in clay
soils, 05 per cent, soluble in strong hydrochloric acid
is often to be found, while the proportion which can
be extracted after completely breaking up the soil
with hydrofluoric acid may rise to 2 per cent. Clay
soils are late, and their crops grow sltfwly and ripen
tardily except in specially dry seasons, when the clay
shrinks so much as to cut off all access of moisture
from the subsoil, and prematurely ends the period of
growth ; on the other hand, the quality of crops grown
on the clay is often high. The typical crops of strong
land are wheat, beans, and mangolds; owing to the
closeness of the texture of the soils, weeds are much
less in evidence on the clays than elsewhere, though
some few are exceedingly troublesome.
On the poorer pastures, the spiny form of
rest harrow (Ononis arvensis), the wild teazel
(Dipsacus sylvestris)) Ranunculus arvensis, and Genista
tinctoria, are characteristic and often troublesome
weeds ; on cultivated land the " black-bent " grass
(Alopecurus agrestis), and field mint (Mentha arvensis)
are difficult to deal with. Other plants characteristic
2&8 SOIL TYPES [chap.
of strong soils are the primrose {Primula vulgaris),
and the wild carrot {Daucus carota).
Peaty Soils.
The accumulation of humus to form peaty soils has
already been discussed, and is associated with water-
logging, which cuts off the access of air and so sets
up an anaerobic fermentation of the residues of the
vegetation growing upon the surface. There is always
a deposit of ferric hydrate accompanying the accumu-
lation of peat, as explained before. As the reclama-
tion of peaty soils has already been dealt with, it will
be sufficient here to indicate that their great character-
istic is a deficiency in soluble mineral constituents,
notably salts of lime and potash. It has also been
mentioned that, as a consequence of the acid nature
of the medium, the bacteria of nitrification are absent
or few in number. All attempts at the cultivation
of peaty soils begin with drainage, and must then
proceed on the basis of neutralising the organic acids
with lime and providing a sufficiency of mineral food
for the plant, thus also inducing nitrification to render
available the large quantities of nitrogen which have
accumulated. Of the common crop plants, oats and
potatoes are perhaps the most tolerant of extreme
amounts of acid humus. The normal vegetation of
peaty soils is a mixture of xerophytic and calcifuge
forms; the Conifers, the Ericaceae, Drosera, Rumex
Acetosella, Pedicularis sylvatica, Sphagnum moss, and
many sedges and rushes, are characteristic of sour,
peaty soils. Other characteristic plants are more pro-
perly Northern or Arctic species, and, occurring only in
the uncultivated uplands, need not be considered here.
x.] SOIL SURVEYS 389
Soil Surveys.
To render the scientific study of soils properly
available for the service of the agriculturalist, more is
required than the examination of single samples of
soil, representing, at the best, only the land dealt with
by one person. Over any wide district, not only would
such work become expensive and practically endless,
liable also to many sources of error through local
and accidental variations of the soil on the spot from
which the sample was drawn, but each analysis would
lose the greater part of its value if it could not be co-
ordinated and brought into line with others drawn from
soils of the same type. A general soil survey of a
district, so as to be able to lay down a plan of the
distribution of the various soil types, accompanied by
a discussion of the broad characteristics of each,
should be the basis upon which the interpretation of
the analysis of the soil of any particular field is to
be founded. Only by comparison with the type can
the analysis of any particular soil be properly inter-
preted — e.g., the fact that a soil from a given arable
field contains 015 per cent, of phosphoric acid takes
a very different aspect when it is known that the
soils of the same type contain as a rule 018 to 0-20
per cent, of phosphoric acid, particularly if, also, the
response of that kind of land to phosphatic manures is
known by field trials, or from the accumulated experience
of farmers. The first question which requires settle-
ment is how far a soil survey is possible; to what
extent can the boundaries of soil types be traced ; are
the various types sufficiently constant over a wide area
to render this mapping feasible ? In many cases there
seems to be little but confusion, even in the soils on a
single farm ; field differs from field, and great variations
T
296 soil Types (chap.
may be manifest even within the confines of a single
field. But, in the main, each soil type has a well-defined
area, within which it presents a reasonably constant
composition and texture, and though the boundaries
cannot be laid down with the precision of the outcrop
of a stratum, the zone of transition from one type to
the other may be indicated with approximate accuracy.
The basis upon which any soil survey must be
constructed is the origin of the soils; each geological
formation, for example, will give rise to a distinct type
of soil if it has been formed in situ; should the
weathered material have further undergone transport
by water, two or more types may have been constructed
by the sorting action of the water. It is also well
known that a geological formation may change very
considerably in lithological character in passing from
the lower to the upper portions of the bed. For
example, stiff as the London Clay is, the upper beds
become increasingly sandy in character, so that it is
not easy to draw a line of demarcation between the
soil arising from these beds and those due to the Bag-
shot Sands above. The lower beds of the Gault Clay
are also very pure, and give rise to a stiff clay deficient
in calcium carbonate; the upper beds become marly,
and form soils indistinguishable from those due to the
contiguous Chalk Marl. Similarly, a geological forma-
tion may show a progressive change of character in
passing into a different area, which change will be
reflected in the soils derived from them. For example,
the Great Oolite limestones of the Cotswolds shade
off into sandstones in Northamptonshire, and the Hythe
beds of calcareous sandstone in East Kent become pure
coarse-grained sandstones in West Surrey. However,
in the main, geological origin may be taken as the
basis of a soil survey, to which must be added the
X.] MAPPING SOILS 291
further subdivisions due to the causes enumerated
above, or to the local movements of rain-wash or ill-
defined drift, that may alter the character of the soil
without being of any particular geological importance.
A soil map will consist of a " drift " map, with some
further details showing the superficial formations occupy-
ing the surface of the ground, and notes regarding the
local variations in the type of soil derived from each
particular stratum. The aim of a soil survey is to
carry further the work of the geological survey as
regards the superficial formations; the only classifica-
tion which can be adapted will be one based upon the
physical texture of the soil, and indicated by such
conventional terms as clays, clay loams, sandy loams,
marls, etc. At the same time, the map must indicate
the various origin of the different loams which may
be found in the area under survey, and, by reference
to the accompanying text, should give those details
of physical structure and chemical constitution which
characterise the soil, but which cannot be set out
except by an over-elaborate classification.
The field portion of the work of a soil survey con-
sists in the exploration of the subsoil by means of
an auger, aided by any natural sections which may
be displayed. The boundary between two soil types
may generally thus be laid down by the aspect of the
soil and subsoil ; from time to time, however, samples
must be retained for more detailed examination in
the laboratory, whenever the look and touch of the
material are not sufficient for a decision in situ. An
immediate examination with the microscope, the
behaviour of the soil with acid, or a rough sifting in
a stream of water, will, as a rule, be sufficient to
refer a given example of subsoil to one type or another.
Complicated cases arise from time to time, especially in
292 SOIL TYPES [chap.
the river valleys, where alluvium of varied epochs and
rain-wash of not very different origin may be hope-
lessly intermingled. In many cases, however, where
the outcrop of the originating formations is broad, and
where the gradients of the country are slight, the soil
may be extremely constant in composition over a wide
area, so that the survey has only to notice such minor
variations as the grading from a lighter to heavier type
as one descends a slope, or the occasional influx of
drift material by creeping from a neighbouring area.
The further work of a field survey will be the selection
of typical samples of soil and subsoil for detailed
examination and analysis in the laboratory, the col-
lection of such data as the distance to, and nature of
ground water, and any particulars which may be
available locally, as to special features in the working
of each type of soil, or in the growth of its crops;
the nature and character of such deposits as "brick
earth" in each district can also be reported. The
samples for detailed analysis should be taken where a
general survey of the district indicates the soil as most
likely to be typical of the formation, and free from
admixture with drift and other accidental intrusions.
At the same time, since the soil in the main will by no
means be so pure as the typical samples, a much larger
number of samples should be taken and subjected to
a less detailed examination, by way of ascertaining
within what limits the normal variation of the soil is
confined.
The number of type samples to be taken must be
entirely decided by an examination of the circum-
stances; in continental areas, where deposition has
been very uniform over wide districts, one or two
samples may be sufficient to characterise an extensive
soil type ; in other cases the local variations may be so
X.] PURPOSE OF SOIL SURVEYS 293
much in evidence that the "typical soil" can only be
constructed by putting together the results of many
separate determinations.
But the practice of constructing a typical soil for
analysis by mixing together equal fractions of many
samples drawn in the area in question, is not to
be recommended. Not only may an entirely foreign
or accidentally impure sample be introduced without
detection, but, further, the limits of variation normally
to be expected in individual soils of the same type is
just as important as the composition of the type itself.
Again, the existence of unsuspected systematic varia-
tions is entirely obscured by any process of mixing
samples. The character of the information which
should accompany the soil maps must largely depend
on the purpose of the survey, whether it is concerned
with the agriculture of an old and settled country, or
whether it partakes of the nature of an exploration, and
aims at showing the capacities of the land for new
crops and industries. In the United States, for
example, the latter form of soil survey is exemplified ;
in many parts of the country agriculture is so recent
that there is no accumulation of experience as to the
crops most suited to each kind of land; hence the
survey, by comparisons of the texture of the soil, the
climatic conditions, and the depth to ground water,
with the conditions prevailing in better known areas,
can directly tell the settler with what crops he is most
likely to succeed. The cultivation of special crops like
tobacco and sugar beet, to take two examples of special
interest at the present time in the United States,
can be extended into new districts possessing suitable
soils, with a minimum of the risk which must always
attend the introduction of a novel form of culture. The
suitability of other classes of land for irrigation, the
294 SOIL TYPES [chap.
nature and extent of already existing alkali patches,
and the most promising methods of reclamation, are
also prominent features in the work of the United
States soil survey. As the crops in a new country of
this kind are in the main grown by the aid of the
natural fertility of the soil alone, and fertilisers are
little used, the chemical examination of the soil becomes
of less importance than the mechanical.
In a settled country like our own, the character of
the information to be derived from a soil survey is of a
different order ; the land has been under cultivation so
long that a great mass of local information, based upon
experience, exists as to the character even of individual
fields.
Hints as to the cultivation, based upon the texture
of the soil as determined by analysis, would be too
general to be of any service ; indeed, it is rather to
be hoped that by collating many mechanical analyses
with the information derived from men possessing long
experience of the soil, further light can be shed upon
the connection between physical structure and the finer
points of tillage. The suitability of the different types
of soil to new crops — as, for example, the extension of
the area under fruit — can be ascertained, and many
expensive mistakes due to planting on unsuitable land
could be saved to the farmer. Suggestions can also be
made as to the amelioration of the soil by drainage, or
by the incorporation of materials like clay, chalk, or
marl, occurring in the vicinity. Fifty years ago, no
department of British agriculture was more carefully
attended to than the improvement of the texture of
the soil, and great tracts of what is now fertile land
were practically created ; lower values to-day have
caused this important matter to be almost entirely
neglected.
X.] APPLICATION OF SOIL SURVEYS 295
But the chief application of a soil survey in this
country lies in the information that can be afforded
as to the use of manures ; enormous economies might
be effected in the bills of almost every farmer using
artificial manures, if the latter were properly adapted
to his soils and crops. Farmers are often recom-
mended to carry out manurial trials upon their own
farms until they have ascertained the peculiarities and
specific requirements of the soil, but advice of this
kind treats altogether too lightly the somewhat delicate
business of conducting field experiments. Putting aside
the mechanical difficulties attending a trial of this kind,
and the overpowering effect of minor inequalities of the
ground and other accidental conditions which so often
nullify the experimental treatment, it is rarely that
the farmer will be found able to arrange a scheme of
experiment likely to give information of permanent
value. If one may judge from the published accounts
of many field experiments carried out in this country
by public bodies, which so often show a misappre-
hension of the points really at issue, there is every
probability that the individual farmer will be as often
misled as guided by the results of his own experiments.
The design and conduct of field experiments must be
left to the expert, who surveys the subject from a wider
standpoint, who can compare various trials, and is in a
position to continue them for a period of years, rejecting
at any early stage a considerable proportion which are
inevitably vitiated by some concealed local peculiarity.
A body of experts conducting a soil survey and field
experiments simultaneously in the same area and co-
ordinating their results, can give advice of the most
definite character as to the scheme of manuring to be
adopted for each soil type. The fundamental factor
requiring consideration in this matter, and brought out
296 SOIL TYPES [chap.
in the soil survey, is the proportion of lime normal to
each soil type ; knowing this factor, and the retentivity
of the soil for moisture under ordinary conditions of
rainfall, one can decide upon the character of the
manures for most loamy soils. Soils of more specific
character, like the sands or clays, present more character-
istic deficiencies of some of the manurial constituents, so
that for many crops the use of manures like phosphates
and potash is wholly determined by the soil and not the
crop.
It is not too much to say that the information as
to the manuring which is being accumulated at many
experimental centres throughout the \ country can only
be rendered properly available by the execution of
a soil survey in the district under consideration. In
many countries a soil survey has been made part of
the national service for the agriculturist; the mag-
nificent publications of the Division of Soils of the
United States Department of Agriculture form a case
in point. In Prussia, the maps and reports of the
Laboratorium fur Bodenkunde at Berlin may be con-
sulted as models of the thoroughness and refinement
to which work of this kind can be pushed ; the
Gembloux Station in Belgium is executing a system-
atic chemical survey of the Belgian soils. In France,
the work rests with the local authorities of each
Department, but in parts is being carried out,
as witness the beautiful maps due to the single-
handed work of M. Gaillon, Director of the Station
Agronomique de TAisne at Laon. In Britain the
great initial want is the publication of drift maps
of the geological survey on the 6-inch scale; the
I -inch to the mile survey, which alone has been
published, or even executed in most districts, is too
small to admit of necessary detail. It is also very
X.J SOIL MAPS 297
often laid down on an early cadastral survey, which
makes the identification of the modern boundaries a
matter of difficulty. If the country were in possession
of a series of "drift" maps on the scale of 6 inches to
the mile, the work could be rapidly supplemented by
soil surveys and analyses executed by the local agri-
cultural colleges and research institutions, until every
farmer could be put in possession of that exact
knowledge of the soil which is fundamental to all
farming operations.
APPENDICES
APPEN
CHEMICAL ANALYSES
Percentages on the
Number .
•
1
2
3
4
5
6
District
• {
Wisley, Surrey.
woodnesborough,
Kent.
Teynham,
Kent.
Formation .
• •
Bagshot Sand.
Oldhaven Beds.
Thanet Sand.
Naturb • «
-(
Very poor, light
land, much of it
in waste.
Very Light Sand,
valuable for Market
Gardening, but not
for General
Farming.
Light loam of
great repute for
fertility.
Arable.
Arable.
Arable.
Soil.
Subsoil.
Soil.
Subsoil.
Soil.
Subsoil.
Moisture • •
• •
O.79
i-oi
I«05
204
1*62
I.71
Loss on Ignition
• •
3*32
2.18
3-33
1-93
3-46
2.49
Nitrogen .
• •
O'lO
0*07
CI2
0-08
o«i6
0-04
Potash •
• •
031
0.17
o-33
0.38
o-35
O.47
Potash, soluble in
cent. Citric Acid
: pei :}
0*02
• •■•
0018
...
0*019
.»«
Lime •
CIO
• »•
0*31
.».
0.58
. . .
Magnesia •
CI2
• • •
0.23
...
0-26
...
Alumina • .
O.92
...
1.74
...
2-34
. • .
Ferric Oxide .
0-57
...
1.38
...
2.10
...
Oxide of Manganese
OO4
...
0-05
...
0-14
...
Calcium Carbonate
OOI
0-04
0*02
O-OI
o-33
0-05
Phosphoric Acid
C05
003
0O6
0.05
O'lO
007
Phosphoric Acid, soluble ^
m I per cent.
Acid • .
Citric V
0*012
....
O.OI7
...
0*044
...
Sulphuric Acid
• .
0O3
...
0-05
...
0*01
• »•
300
DIX I.
OF TYPICAL SOILS.
Fine Earth— Air-dried.
7
8
9
10
U
12
13
14
15
16
Sutton by
Dover, Kent.
Bentley,
Hants.
Marden,
Kent.
Wanborouqh,
SURREV.
woodchurch,
Kent.
Chalk.
Upper Green sand.
Alluvial.
London Clay.
Weald Clay.
Light loam,
" sheep and
barley land."
Good loam,
noted
for fertility.
Heavy loam,
rather poor.
Heavy loam,
of fair repute.
Very heavy clay,
of little value.
Arable.
Arable.
Arable.
Arable.
Arable.
Soil.
6.76
9-28
0-25
o-43
0-018
0-69
6.45
4.27
0.16
18.1
0*19
O-OOI
0.09
Subsoil.
4.18
7-37
0.13
0.60
•••
»•-.
•••
11.4
0.17
• *•
Soil.
3-23
4-60
0*19
0'6o
0-025
2-6l
0.38
9.87
2-25
0-12
0.47
0.27
0»i6
0-06
Subsoil.
4*22
4-03
0*12
0.63
0-031
2.61
0*48
2- 06
0'i6
3-48
0.15
0-08
0*12
Soil.
2.40
6.11
0.18
0-90
0-006
0.99
o-37
8-44
3-92
0-03
0-64
0-12
0-009
006
Subsoil.
3-i6
3-78
••*
••*
•••
Ma
• ••
»••
•••
...
0-18
0-06
•••
•••
Son.
3-91
4-38
0-19
o-33
0-017
o-35
4.14
2.47
0-71
0-06
0-06
0014
0*04
Subsoil.
2.94
3-88
0-06
0-67
»—*
•»•
•••
••«
•••
o-o8
0-03
SoU.
4.07
8-73
0*26
1*03
0-012
0-70
C3I
6-45
8-8i
0-08
0-08
O-II
0-004
0-08
Subsoil.
3-62
5-37
0*12
I'll
• ••
• ■•
• •■•
••4
• •*
. ••
0-03
0-05
. «•
.»•
301
APPENDIX II
BIBLIOGRAPHY
The following short list of references will take the student to
some of the more important original sources of information on
each of the subjects treated of in this book. They have been
selected from among the great mass of papers that exist, not
because they are necessarily the most important, but as suggestive
in themselves, and likely to lead the student to make further
acquaintance with the methods as well as with the results of
research. Several of the papers also contain a series of references
to other workers in the same field.
The Origin of Soils.
i. Risler, E. — Ghlogie Agricole, 4 vols., Paris, 1884-97.
2. Merrill, G. P. — Rocks, Rock Weatherings and Soils, New
York, 1897.
Analysis and Composition of Soils.
3. Launfer and Wahnschaffe.— Mit, a. d. Laboratorium fur
Bodenkunde, B. III., h. 2, Berlin.
4. Petermann.— Recherches de Chimie le, III., Brussels, 1898.
5. Hilgard, U.S. Dep. of Agric, Div. of Chem., Bull. 38, 1893.
6. Hilgard, U.S. Dep. of Agric, Div. of Soils, Bull. 4, 1896.
7. Hilgard. — Soils, The Macmillan Company, New York, 1906.
8. Mitscherlich, E. A. — Bodenkunde fiir Land u. Forstwirte,
1905.
9. Lawes and Gilbert.— -Rothamsted Memoirs, V., No. 19.
10. Dyer.—/. Chem, Soc, 1894, 65, 115.
11. Dyer. — Phil, Trans., 194, B. (1901), 235.
12. Dyer.— U.S. Dep. of Agric, Bull. 106, 1902.
13. Hall, Plymen, and Amos.— Trans. Chem. Soc, 1902, 81,
117 ; 1906, 89, 207.
14. Hall.— "Analysis of Soil by the Plant,"/ Agric, Sci., 1905,
1,65.
802
APPENDIX 11 303
Soil Physics.
15. WARINGTON. — Physical Properties of Soils, Oxford, 1900.
16. King. — Physics of Agriculture, Madison, Wis., 1901.
17. Hilgard. — U.S. Dep. of Agric, Weather Bureau, Bull. 3,
1892.
18. Whitney. — U.S. Dep. of Agric, Weather Bureau, Bull. 4,
1892.
19. Hellriegel. — B. z. d. nat. Grundlagen des Ackerbaues,
Braunschweig, 1883.
20. Lawes and Gilbert. — Rothamsted Memoirs, Vol. III., No.
II j Vol. V., No. 5.
21. BRIGGS. — U.S. Dep. of Agric. Year-Book, 1900, 397.
See also Hilgard, No. 7 ; Wollny, No. 20.
Soil Organisms.
22. Wollny. — Zersetzung der Organise/ten Stoffe, Heidelberg,
1897.
23. Omeliansky. — Compt. Rend., 121 (1895), 653 ; 125 (1897),
97o.
24. WiNOGRADSKY.— Compt Rend., 121 (1895), 742.
25. Lafar. — Technische Mycologie, 2nd ed., Fischer, Jena, 1904.
Fixation of Nitrogen.
26. Lawes and Gilbert.—/^//. Trans., II., 431, 1861 ; B. I.,
1889.
27. Lawes and Gilbert.—/. R. Ag. Soc, E., 3rd s., II., 657,
1891.
28. Hellriegel and Wilfarth. — D. Land. Vers. Stat., 460,
1887.
29. Nobbe and Hiltner. — D. Land. Vers. Stat., 1899.
30. Winogradsky. — Co?npt. Rend., 118 (1894).
31. Maz£. — Ann. de VInst. Pasteur, 11 (1897).
32. jACOBlTZ.~Cent.fi/rBakt., II., 1901, 7, 783.
33. Beijerinck. — Cent, fur Bakt. y II., 1902, 9, 3.
Nitrification.
34. Schloesing and Muntz.— Compt. Rend., 80 (1877), 1250.
35. Warington.— /. C. S., 1878, 1879, l88 4> 1891.
36. Lawes, Gilbert, and Warington.—/. R. Ag. Soc, E. t ,
2nd s., 19 (1883).
3 o4 APPENDIX II
37. Winogradsky. — Compt. Rend., 113 (1893), 116.
38. Deh£rain. — Ann. Agron., 19, 409.
39. King. — Wisconsin Ag. Exp. Station Annual Reports, 17
(1900), 18 (1901).
40. Boullanger and Massol. — Ann. Pasteur, 1903, 17, 492.
Denitrification.
41. Gayon and Dupetit. — Compt. Rend., 95 (1882), 644.
42. Wagner. — D. Landw. Presse, 1895, 123.
43. Warington.— J. R. Ag. Soc, E., 3rd s., 8.
44. Stoklasa. — Cent.f. Bakt, 7 (1901), 260.
Mycorhiza.
45. STAHL.—Ja/ir&.f. wiss. Botanik., 34.
Absorption of Salts by Soil.
46. Way.—/. R. Ag. Soc, E., 1st s., 11, 323 ; 13, 123.
47. Voelcker.— /. R. Ag. Soc, E., 1st s., 21 (i860), 93; 25
(1864), 333.
48. Voelcker.—/. R. Ag. Soc, E., 2nd s., 10 (1874), 132.
49. Lawes, Gilbert, and Warington.—/. R. Ag. Soc, E.,
2nd s., XVII. (1881), 241 ; XVIII. (1882), 1.
50. Hall and Gimingham, Trans. Chem. Soc, 91 (1907), 677.
Alkali Soils.
51. Hilgard.— U.S. Dep. of Agric. Year-Book, 1895, 103.
52. Hilgard.— Univ. of California Ag. Exp. Sta. Bull. 128 (1900).
53. Willcocks. — Egyptian Irrigation, 2nd ed.
Soil Surveys, etc.
54. Schimper, A. F. Vt.—Pflanzen Geographie, Jena, 1898.
55. U.S. Dep. of Agric, Div. of Soils, "Field Operations," 1901.
56. Whitney.— "Tobacco Soils," U.S. Dep. of Agric. Bull. 11,
1898.
57. Veitch.—" Maryland Soils," Maryland Ag. Exp. Sta. Bull
70, 1901.
See also Nos. 3 and 4,
INDEX
Absorption of salts by soli, 211.
Acid soils, 44, 204, 242.
Aerobic bacteria, 1 71.
Alinit, 174.
Alkali soils, 245, 272.
Alluvial soils, 279.
Alluvium, 12.
Altitude, effect of temperature, 133.
Alway, soil- water required for crops,
86.
Ammonia, absorption by soil, 214;
formation from urea, 172 ; salts,
removal of lime by, 216, 224.
Anaerobic bacteria, 174.
Analysis, available constituents, 158 ;
chemical, 1 39 ; interpretation of,
151, 165 ; mechanical, 32, 50; of
soil by plant, 155.
Anbury in turnips, 204, 208.
Angus Smith on denitrification, 197.
Anticyclones, reversal of tempera-
ture in, 134.
Apatite, 24.
Arid soils, characteristics of, 46.
Assimilation and transpiration, 90.
Atwater on fixation of nitrogen, 179.
Augite, 21.
Autumn cultivation, gain of water
by, 95.
Available plant food in the soil, 157,
164.
Azotobacter chroococcum, 187.
805
BACTERIA in soil, 4, 168 ; denitri-
fying, 197 ; nitrifying, 191 ; iron,
203 ; nitrogen fixing, 174.
Bailey-Denton on effect of drainage
on temperature, 132.
Bare fallows, 86, 112.
Basalt, weathering of, 21.
Beaumont, E. de, on antiquity of
soil, 29.
Beijerinck on Azotobacter, 187
Benetzungs-warme, 88.
Berthelot on fixation of nitrogen,
179, 186 ; and Andre on nitrogen
in humus, 47.
Black soils, 235.
Bog iron ore, 25.
Boussingault on source of nitrogen
in plants, 175, 177.
Brick earth, 13, 15.
Brown on energy absorbed in
assimilation, 90.
Brownian motion, 38.
Calcareous soils, 281.
Calafuges, 276, 288.
Calcium carbonate, 22, 40 ; aids
decay of organic matter, 173 ;
determination of, 144 ; in soils
subject to " finger-and-toe," 209;
removal by manures, 216.
Capacity of soil for water, 67.
U
306
INDEX
Capillarity, 72 ; supply of water to
crop by, 97, 106.
Carbon, determination of, 143.
Carbon to nitrogen in soils, ratio
of, 46.
Carbonic acid in soil gases, 13 ;
excreted by roots, 158 ; and water,
action on rock minerals, 13.
Catch crops, no.
Chalk, 41; "pipes" in, due to
weathering, 23; soils, 281.
Chalking, 261.
Chemical analysis of soil, 139.
China clay, 19, 35.
Chlorosis, 283.
Citric acid as solvent in soil analysis,
159.
Clay, colloid, 35 ; flocculation of,
38 ; impermeability to water, 34,
71 ; nature of, 36, 52 ; origin of,
19 ; shrinkage on drying, 34, 284 ;
soils, 30, 283.
Claying, 256.
Cleopatra's Needle, weathering of,
n.
Climate and situation, 136.
Clostridium Pastorianum, 187.
Club root in turnips, 204, 208.
Cohesion due to surface tension, 82.
Colloid clay, 35.
Colmetage, 255.
Colour of soils, 27; and temperature,
127.
Condensation of water by soil, 87.
Condition of land, 233.
Conventions necessary in chemical
analysis of soil, 141.
Crops, plant food removed by, 152.
, Crops, water required by, 91, 108.
** Cultivation, effect of, on water con-
tent of soil, 95, 101, 104.
rounded by running water, 17 ;
weathering of felspar, 19.
Deflocculation, 39, 252.
Defoe, definition of manure, 154.
Deherain on available phosphoric
acid in soils, 158 ; nitrates in
drainage waters, 228 ; nitrifica-
tion, 195.
Deherain and Maquenne on denitri-
fi cation, 198.
Denitrification, 197.
Density of soils, 63.
Drainage, 92 ; required with irriga-
tion, 249 ; warms the land, 1 32 ;
waters, composition of, 222.
Drains, flow of, 77.
Drain-gauges, 78 ; flow greater than
rainfall, 87.
Drift, glacial, 15 ; maps, 297; soils, 8.
Drought of 1870 at Rothamsted,
108.
Drought, susceptibility of different
soils to, 99.
Drying effect of crops, 86, 108.
Dunbar, potato soils of, 1.
Dung, effects on water in soil, 117;
and denitrification, 201.
Dyer on determination of available
phosphoric acid and potash, 1 59 ;
potash and phosphoric acid re-
tained by Rothamsted soils, 218.
Early and late soils, 135.
Ebelmar on weathering of basalt, 2 1.
Egypt, irrigation in, 248.
Eremacausis, 170.
Evaporation, cooling effect of, 1 30 ;
losses of water by, 97.
Exhausting effect, of nitrate of soda,
253 ; of wheat, 237.
DARWIN on action of earthworms,
14.
Daubree on size of sand grains not
Fairy rings, 239.
Fallows, bare, 86, 112.
Felspar, 18.
INDEX
307
Fen land, potatoes from the black
soils of, 2 ; reclamation of, 258.
Ferrous carbonate in soils, 145.
Fertility, 233.
Film of water surrounding soil
particles, 73.
"Finger-and-toe," 204, 208, 243,
276.
Fixation of nitrogen, 1 74.
Flint, 18.
Flocculation of clay, 38, 40, 253.
Flow of water through soils, 70.
Frank on mycorhiza, 205.
Frosts in valleys, 135 ; killing effect
of, 124.
Fruit trees in grass land, ill.
Fuller's earth, 37.
Fungi in the soil, 168, 204.
Gayon and Dupetit on denitrifica-
tion, 198.
Germination, temperatures of, 124.
Glacial drift, 1 5.
Glauconite, 25.
Granite, weathering of, 19.
Gravel, 15.
Green manuring, 185.
Greenstone, weathering of, 21.
Grey wethers, 1 7.
Gypsum, 24.
Halophytes, 280.
Hanamann on weathering of basalt,
22.
Heat received by the soil, 120, 126 ;
required for evaporation, 130.
Heavy soils, 64.
Heinrich on hygroscopic moisture,
85-
Hellriegel on fixation of nitrogen,
176, 179 ; on optimum proportion
of water for growth, 69 ; on water
transpired by crops, 89.
Hilgard on alkali soils, 246 ; on dis-
tillation of water from subsoil, 88 ;
on mechanical analysis of soils,
50 ; on method for determination
of water capacity, 67.
Hilgard and Jaffa on nitrogen in
humus, 46.
Hiltner, cultivation of nodule organ-
isms, 183.
Hoeing, 102.
Hornblende, 21.
Humic acid, 44.
Humus, 24, 27, 42, 118, 168, 171,
174, 243.
Humus, soluble, 44, 165.
Hygroscopic moisture, 84.
Ignition, loss on, 143.
Inoculation of soil, 183.
Interpretation of soil analysis, 151,
165.
Irrigation, 246.
Iron bacteria, 203.
Iron pan in soils, 24, 275, 285.
Iron pyrites, 26 ; causing sterility,
243.
Kaolinite, 19, 34, 36.
King on area of surface of soil
particles, 66 ; on bare fallows,
113 ; on effect of cultivation on
water content of soil, 96, 101, 103;
on effect of slope on temperature
of soil, 133 ; on flow of water
through sand, 70 ; on nitrates in
soil, 195 ; on percolation, 76 ; on
transpiration water, 89 ; on water
in soils too dry for growth, 85.
Kossowitsch on fixation of nitrogen,
186.
Kriiger and Schneidewind, 186, 200.
Laking, 213.
Langley on solar radiation, 126.
3o8
INDEX
Late soils, 135.
Laurent on fixation of nitrogen,
181.
Lawes and Gilbert on fixation of
nitrogen, 176 ; on ratio of carbon
to nitrogen in soils, 46 ; on tran-
spiration water, 89. (See also
Rothamsted.)
Leguminous plants, fixation of
nitrogen by, 180 ; green manur-
ing with, 185.
Liebig, mineral theory, 175.
Light soils, 64.
Lime, effect on temperature of soil,
127; composition of, 268 ; floccu-
lation of clay by, 40; for " finger-
and-toe," 209 ; in drainage waters,
223 ; removal from soil by use of
ammonium salts, 216, 224.
Liming, 41, 209, 261.
Limonite, 24.
Loams, 31, 278.
Loess deposits, 10.
Lois-Weedon husbandry, 114.
Lucerne requiring inoculation, 184.
Lysimeter, 78.
Molecular forces, 71.
Molisch on iron bacteria, 203.
Moor-band pan, 25.
Moore, cultivation of nodule organ-
isms, 183.
Mould, 171.
Mulches, 102, 246.
Muller on nitrification, 191.
Mycorhiza, 205, 277.
Natrolite, 26.
Nile water, 256.
Nitragin, 183.
Nitrates, determination of, 147; for-
mation of, 112, 194; in drainage
waters, 226 ; in soil, effect of
cultivation on, 195 ; produced by
bare fallow, 112, 196.
Nitrification, 191.
Nitrogen, determination of, 144 ;
fixation of, 174 ; proportion re-
covered in crop, 199, 202.
Nobbe on nitragin, 183.
Nodules on leguminous plants, 180.
Magnesia in drainage waters, 225 ;
in soils, 146, 243, 285.
Manures, absorption of, by soil, 21 1;
effect on texture of soil, 251;
original signification of, 154; re-
covery of, in crop, 164 ; time of
application of, 229.
Marcasite, 26.
Marling, 256.
Marls, 31, 257.
Mechanical analysis of soil, 32, 50.
Mica, 20.
Mineral theory, Liebig's, 175.
Minerals, composition of rock-form-
ing, 16.
Mitscherlich on benetzungs-warme,
88.
Oemler on specific heat of soil,
129.
Olivine, 22.
Optimum proportion ot water in
soils, for growth, 69 ; tempera-
tures for growth, 123.
Orchids, germination of, 208.
Osborne on mechanical analysis of
soils, 50.
Osmosis, entry of plant food by,
157.
Packing of soil particles, 6r.
Pan, iron, in soils, 24, 275, 285.
Paring and burning, 259.
Parkes on temperatures of drained
and undrained land, 132.
INDEX
309
Peat, origin of, 43, 174 ; reclamation
of, 258 ; soils, 288.
Percolation, 75, 115; through
gauges at Rothamsted, 78.
Phosphoric acid, determination of,
146 ; retention by soil, 219.
Pipe clays, 35.
Pipes in chalk rock, 23.
Plasmodiophora brassicae, 204, 208.
Plasticity of clay, 34, 40.
Ploughing, damage done by deep,
29, 285.
Pore space, 60.
Potash, determination of, 146, 150;
origin in soils, 19, 28 ; retention
by soil, 217.
Potatoes grown on different soils,
value of, t.
Pseudomonas radicicola, 180, 182.
Qqality of produce from early and
late soils, 137.
Quarry water, 1 1.
Quartz, 17.
Quincke on range of molecular
forces, 71.
Radiation, 126.
Rainfall and crops, 91.
Rainfall and drainage, 115.
Recovery of manures in crop, 164.
Red lands of Dunbar, value of
potatoes from, 1.
Richthoven on loess deposits, 10.
Rock-forming minerals, 16.
Rolling, effect of, 105.
Roots, solvent action of, 158 ;
weathering action of, 14.
Rothamsted, available constituents
in soil of, 162 ; bare fallows at,
114 ; composition of drainage
waters at, 222 ; composition of
soil, 37. S 8 I drain gauges at, 78 ;
drought of 1870 at, 108 ; investi-
gations on fixation of nitrogen at,
176, 178, 189 ; lime in soil of, 216;
losses of nitrogen in manure at,
201; method of sampling soils at,
48 ; nitrates in soil at, 196 ; phos-
phoric acid in soil of, 162, 22 1;
potash in soil of, 218 ; weights of
soil at, 64.
Rutile, 34.
Sachs on solvent action of roots,
158; on temperature and tran-
spiration, 124 ; on water in soil,
when plants wilt, 84.
Salfeld on soil inoculation, 185.
Salt, causing sterility, 244 ; marshes,
vegetation of, 279.
Salts, soluble in soils, 147.
Sampling of soils, 47.
Sand, origin of, 17, 33 ; velocity of
current required to carry, 33.
Sandy soils, 30, 242, 274.
Saturation of soils by water, 69, 77.
Saussure" on theory of nutrition, 175.
Schloesing on colloid clay, 35 ; on
mechanical analysis of soils, 50.
Schloesing and Muntz on nitrifica-
tion, 191.
Schloesing/?/* on fixation of nitrogen,
181.
Schneidewind on denitrification, 200;
on fixation of nitrogen, 186.
Schultz on green manuring, 185.
Sea, effect on climate, 1 36.
Sedentary soils, 7.
Selenite, 24, 26, 285.
Senft on decomposition of felspar,
19.
Serpentine, 22.
Sewage farms, crops on, 92 ; purifi-
cation of, by soil, 213.
Shingle, growth of vegetation on,
10.
3io
INDEX
Shrinkage of clay on drying, 34,
284.
Silica, 17.
Silt, 53.
Soil, alkali, 245, 272 ; analyses of
typical, Appendix I.; description
of, 7, 29 ; drift, 8 ; gases, 13 ;
inoculation, 183 ; mulch, 107; of
transport, 8 ; relation to subsoil,
7, 26, 194 ; sedentary, 7; surveys,
289 ; temperatures, 122 ; typical,
55, 149, 271; worn-out, 28, 257.
Soot, effect on temperature of soil,
128 ; effect on texture of soil, 253.
Specific heat of soils, 128, 275.
Spring on impermeability of clay,
71.
Spring cultivation, gain of water by }
97 ; frosts, 135.
Stahl on mycorhiza, 205.
Sterility of soils, 241.
Stoklasa on alinit, 173.
Stones, reputed growth on arable
land, 12 ; retention of moisture
by, 105 ; warming effect of, 131.
Subsoil, description of, 7, 26, 194 ;
as regulator of water supply, 100 ;
packing, 107.
Surface of soil particles, area of, 65 ;
rise of salts to the, 245.
Surface tension, 71; cohesion due
to, 82 ; variations in, 81 ; water
lifted by, 97, 106.
Surveys, soil, 289.
Symbiosis, 169.
Talc, 22.
Temperature, effect of colour on,
127; effect of exposure on, 133 ;
required for growth, 120, 123 ;
weathering due to alternations
of, 9.
Texture of soil, 32, 60, 251.
Thanet sand, soils on, 236.
Tillage and soil water, 89.
Tilth, 97, 253.
Transpiration water, 89.
Typical soils, chemical analysis of,
149 ; mechanical analysis of, 55.
UREA, conversion into ammonia,
172.
Valley frosts, 134.
Velocity of currents carrying sand
grains, 33.
Ville on fixation of nitrogen, 179.
Vineyards, stones on surface of, 105.
Virgin soils, nitrogen in, 189.
Voelcker on absorption of manures
by soil, 212 ; on analysis of drain-
age waters, 222 ; on fixation of
nitrogen, 179; on "finger-and-
toe " in turnips, 209.
Wagner on denitrification, 199.
Warington on determinations of
nitrates in soils, 147; on denitri-
fication, 198 ; on nitrification, 191 ;
on wetness of land in February,
88.
Warp soils, 255 ; growth of potatoes
on, 2.
Water capacity of soil, 67.
Water table, 77.
Way on absorption of salts by soil,
212.
Weathering, 7, 8 ; due to alterna-
tions of temperature, 9 ; due to
frost, 11; due to water and car-
bonic acid, 13 ; due to roots, 14 ;
of granite, 19 ; of basalt, 21.
Weight of a cubic foot of soil, 64.
Wheat as an exhausting crop, 237.
Wilfarth on fixation of nitrogen, 1 80.
Wilting of plants in soils containing
water, 84.
INDEX
3ii
Wind, transport of, soil by, 10.
Winogradsky on iron bacteria, 203,
286 ; on nitrification, 192 ; on
nitrogen-fixing bacteria, 186.
Woburn, lime in soil at, 204 ;
residues left by ammonium salts
at, 232 ; weight of cubic foot of
soil at, 64 ; fruit farm, growth of
trees planted with grass, 112.
Wollny on composition of soil gases,
13 ; on effect of slope on tempera-
ture, 133 ; on oxidation of humic
acid, 173 ; on water required by
crops, 89.
Worms, fine soil brought to surface
by, 14.
Worn-out soils, 28, 257.
Xerophytes, 272, 277, 288.
Yeasts in the soil, 1 70, 204.
Zeolites, 25, 39, 214.
Zircon, 34.
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