The soil : an introduction to the scientific study of the growth of crops

Front matter

22102044630

THE SOIL

First Edition . . . March 1903.

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 consider 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 complex laboratory, peopled by many types of lower organisms, whose function is in some cases indispensable, 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

VI

INTRODUCTION productivity of the land : agriculture is the oldest and most widespread art the world has known, the application 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

Vll 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 Rothamsted 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 researches 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 beforehand ; 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, ix

Contents

PAGE

In the Scientific Study of Soils, Chemical, Physical, and Biological Considerations are involved . . . i

CHAPTER I 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 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 . . . . . . .32

Chapter III

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 . . 60 xi xii

CONTENTS

Chapter IV Tillage And The Movements Of Soil Water

PAGE

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 . . . . . . .89

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. . . . . . . .120

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” 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

FAGK

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 Texture of the Soil — The Amelioration of Soils by Liming, Marling, Claying, Paring and Burning— The Reclamation of Peat Bogs . . . 233

Chapter X

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

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

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-

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

1 1. 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 from Rothamsted

Wheat Plots ... . . 226

16. Nature and Distribution of Alkali Salts . . . 246 xv

THE SOIL

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 worthless for general agricultural purposes, may be valuable in the neighbourhood of a large town, because its earliness 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.

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 suitable 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 transforming 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 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 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 landr 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 operations 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 combination, 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 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 permanently 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

Sedentary Soils, and Soils of Transport — Weathering — The Composition 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 io 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 bedding planes it is evident that water makes its way, for the edges of the cracks are slightly altered and discoloured. Nearer the surface, the cracks and lines of

Chap. I.]

SEDENTARY SOILS 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 embedded 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

[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-

WEATHERING ij

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 sedimentary 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 alternations of temperature, frost, wind, rain, and glacial ice — and chemical, the complex effects of solution and oxidation 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

IO 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 alternations 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 considerably 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 sufficient to form a thin turf over a bare shingle bed.

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 — 1 0 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.