The chemistry of soils and fertilizers

Preface

For se\'eral years courses of instruction have been given at the University of Minnesota to classes of young men who intend to become farmers and who desire information that will be of assistance to them in their profession. In giving this instruction mimeographed notes have been prepared, but the increase in the number of students and the volume of notes have necessitated the publication of this work. In its preparation, it has been the aim to give, in condensed form, the principles of chemistry which have a bearing upon the conservation of soil fertility and the economic use of manures.

Harry Snyder.

University of Minnesota,

April /J, iSgg.

Introduction

Early uses of manures and the explanation of their action by the alchemists ; Investigations prior to 1800 ; Work of De Saussure, Davy, Thaer, and Boussingault ; Liebig's writings and their influence ; Investigations of Lawes and Gilbert ; Contributions of other investigators ; Agronomy. Pages 1-9.

Chapter I

Physical Properties of Soils. — Chemical and physical properties of soils considered ; Weight of soils ; Size of soil particles ; Clay ; Sand ; Silt ; Form of soil particles ; Number and arrangement of

soil particles ; Mechanical analysis of soils. Soil t3'pes

Potato and truck soils ; Fruit soils ; Corn soils ; Medium grass and grain soils; Wheat soils. Relation of the soil to water ; Amount of water required for crops ; Bottom water ; Capillary water ; Hydroscopic water; Loss of water by percolation, evaporation, and transpiration ; Influence of cultivation upon the water supply of crops ; Capillar}^ water and cultivation ; Shallow surface cultivation ; Cultivation after rains ; Rolling; Sub-soiling; Fall plowino- ; Spring plowdng; Mulching; Depth of plowing; FertiHzers and their influence upon moisture content of soils ; Farm manures and soil moisture ; Permeability of soils ; Drainage ; Relation of soils to heat ; Heat from chemical reactions within the soil ; Heat and crop growth ; Color of soils ; Odor and taste of soils ; Relation of soils to electricity ; Importance of physical properties of the soil. Pages 9-48.

Chapter II

Geological Formation and Classification of Soils. — Geological study of soils ; Formation of soils ; Action of heat and cold ; Action of water ; Glacial action ; Chemical action of water ; Acvi CONTENTS

tion of air and gases ; Action of vegetation and micro-organisms ; Distribution of soils ; Sedentary and transported soils ; Rocks and minerals from which soils are derived as quartz, feldspar, mica, hornblende, zeolites, granite, apatite, kaolin, limestone, gypsum ; Chemical composition of rocks. Pages 49-59.

Chapter III

Chemical Composition of Soils. — Elements combine to form minerals ; Classification of elements ; Combination of elements ; Forms in which elements are present in soils ; Acid-forming elements, silicon, double silicates, carbon, sulphur, chlorine, phosphorus, nitrogen, oxygen, hydrogen ; Base-forming elements, aluminum, potassium, calciuni, magnesium, sodium, iron ; Classification of elements for plant-food purposes; Amount of plant food in different forms in various types of soils ; How a soil analysis is made ; Value of soil analysis ; Interpretation of the results of soil analysis ; Use of dilute acids as solvents in soil analysis; Distribution of plant food in the soil ; Composition of typical soils ; "Alkali" soils and their improvement; Organic compounds of soil ; Sources ; Classification ; Humus ; Humates ; Humification ; Humates produced by different kinds of organic matter ; Value of humates as plant food, amount of plant food in humic forms ; Loss of humus by forest fires, by prairie fires, by cultivation ; Humic acid ; Soils in need of humus ; Soils not in need of humus ; Composition of humus from old and new soils ; Influence of different methods of farming upon humus. Pages 60-101.

Chapter IV

Nitrogen of the Soil and Air, Nitrification and Nitrogenous Manures. — Importance of nitrogen as plant food ; Atmospheric nitrogen as a source of plant food. Experiments of Boussingault, Ville, and Lawes and Gilbert ; Result of field trials ; Experiments of Hellriegel and Wilfarth and recent investigators ; Composition of root nodules ; Amount of nitrogen returned to soil by leguminous crops and importance to agriculture ; Nitrogenous compounds of the soil ; Origin ; Organic nitrogen ; Amount of nitrogen in soils ; Removed in crops ; Nitrates and nitrites ; Ammonium compounds ;

CONTENTS vn

Ammonia in rain and drain waters ; Ratio of nitrogen to carbon in the soil ; I^osses of nitrogen from soils ; Gains of nitrogen to soils ; Nitrification ; Former views regarding ; Workings of an organism ; Conditions necessary for nitrification ; Influence of cultivation upon these conditions ; Nitrous acid organisms, ammonia-producing organisms, denitrification, number and kind of organisms in soils ; Inoculation of soils with organisms ; Chemical products produced by organisms ; Losses of nitrogen by fallowing rich prairie lands ; Deep and shallow plowing and nitrification ; Spring and fall plowing and nitrification ; Nitrogenous manures ; Sources ; Dried blood, tankage, flesh meal, fish scrap, seed residue, and uses of each ; Leather, wool waste, and hair ; Peat and muck ; Leguminous crops as nitrogenous fertilizers ; Sodium nitrate, ammonium salts ; Cost and value of nitrogenous fertilizers. Pages 102-137.

Chapter V

Fixation. — Fixation a chemical change, examples of; Due to zeolites ; Humus and fixation ; Other compounds of soil cause fixation ; Soils possess different powers of fixation ; Nitrates do not undergo fixation ; Fixation of phosphates ; Fixation a desirable property of soils ; Fixation and the action of manures. Pages 138- 140.

Chapter VI

Farm Manures. — Variable composition of farm manures ; Factors which influence composition of manures ; Absorbents ; Relation of food consumed to manures produced ; Bulky and concentrated foods ; Course of the nitrogen of the food during digestion ; Composition of liquid and solid excrements ; Manurial value of foods ; Commercial valuation of manure ; Influence of age and kind of animal ; Manure from 3'oung and old animals ; Cow manure ; Horse manure ; Sheep manure ; Hog manure ; Hen manure ; Mixing manures ; Volatile products from manure ; Human excrements ; Preservation of manures ; Leaching ; Losses by fermentation ; Different kinds of fermentation ; Water necessary for fermentation ; Composting manures ; Uses of preservatives ; Manure produced in sheds ; Use of manures ; Direct hauling to field ; Coarse manures may be injurious ; Manuring pasture land ; Small

Vlll CONTENTS

piles of manure in fields objectionable ; Rate of application ; Most suitable crops to apply to ; Comparative value of manure and food ; Comparative value of good and poor manure ; Summary of wa3-s in which manures may be beneficial. Pages 141-171.

Chapter VII

Phosphate Fertilizers. — Importance of phosphorus as plant food ; Amount of phosphoric acid in soils, amount removed in crops ; Source of soil phosphoric acid ; Commercial forms of phosphoric acid ; Phosphate rock ; Calcium phosphates ; Reverted phosphoric acid ; Available phosphoric acid ; Manufacture of phosphate fertilizers, acid phosphates, superphosphates ; Commercial value of phosphoric acid ; Basic slag phosphates ; Guano ; Bones ; Steamed bone ; Dissolved bone ; Bone black ; Use of phosphate fertilizers ; How to keep the phosphoric acid of the soil available. Pages 172- 185.

Chapter VIII

Potash Fertilizers. — Potassium an essential element ; Amount of potash removed in crops ; Amount in soils ; Source of soil potash ; Commercial forms of potash ; Stassfurt salts, occurrence of ; Kainit ; Sulphate of potash ; Other Stassfurt salts ; Wood ashes, composition of ; Amount of ash in different kinds of wood ; Action of ashes on soils ; Leached ashes ; The alkalinity of ashes ; Coal ashes ; Miscellaneous ashes ; Commercial value of potash ; Use of potash fertilizers ; Joint use of potash and lime. Pages 186-195.

Chapter IX

Lime and Miscellaneous Fertilizers. — Calcium an essential element ; Amount of lime removed in crops ; Amount of lime in soils ; Different kinds of lime fertilizers ; Their physical and chemical action ; Action of lime upon organic matter and correcting acidity of soils ; Lime liberates potash ; Aids nitrification ; Action of land plaster on some "alkali" soils; Quicklime and slaked lime ; Marl ; Physical action of lime ; Judicious use of lime ; Miscellaneous fertilizers ; Salt and its action on the soil ; Magnesium salts ; Soot ; Sea-weed ; Strand plants ; Wool washings. Pages 196-204.

CONTENTS IX

Chapter X

Commercial Fertilizers. — History of development of industry ; Complete fertilizers and amendments ; Variable composition of commercial fertilizers ; Preparation of fertilizers ; Inert forms of matter in fertilizers; Inspection of fertilizers; Mechanical condition of fertilizers ; Forms of nitrogen, phosphoric acid, and potash in commercial fertilizers ; Misleading statements on fertilizer bags ; Estimating the valne of a fertilizer ; Home mixing ; Fertilizers and tillage ; Abuse of commercial fertilizers ; Proper use of ; Field tests ; General principles ; Preliminary experiments ; Verifying results ; Deficiency of one element ; Deficiency of two elements ; Will it pay to use fertilizers ? Amount to use per acre ; Influence of excessive applications ; Fertilizing special crops ; Commercial fertilizers and farm manures. Pages 205-224.

Chapter XI

Food Requirements of Crops. — Amount of fertility removed by crops ; Assimilative powers of crops compared ; Way in which plants obtain their food ; Cereal crops ; General food requirements ; Wheat ; Barley ; Oats ; Corn ; Miscellaneous crops ; Flax ; Potatoes ; Sugar-beets ; Roots ; Turnips ; Rape ; Buckwheat ; Cotton ; Hops ; Hay and grass crops ; Leguminous crops. Pages 225-237.

Chapter XII

Rotation of Crops. — Object of rotating crops ; Principles involved in crop rotation ; Deep and shallow rooted crops ; Humus-consuming and humus-producing crops ; Crop residues ; Nitrogen-consuming and nitrogen-producing crops ; Rotation and mechanical condition of soil ; Economic use of soil water ; Rotation and farm labor ; Economic use of manures ; Salable crops ; Rotations advantageous in other ways ; Long- and short-course rotations ; Problems in rotations ; Conservation of fertility ; Necessity of manures ; Uses of crops ; Losses of fertility with different methods of farming ; Problems on income and outgo of fertility from farm. Pages 238- 254-

References ; Experiments ; Review Questions ; Corrections. Pages 255-272.

THE CHEMISTRY OF

INTRODUCTION

Prior to 1800 but little was known of the sources and importance of plant food. Manures had been used from the earliest times, and their value was recognized, but the fundamental principles underlying their use were not understood. It was believed that they acted in some mysterious way. The alchemists had advanced various views regarding theii action ; one was that the so-called "spirits'' left the decaying manure and entered the plant, producing more vigorous growth. As evidence, the worthless character of leached manure was cited. It was believed that the spirits had left such manure. The terms 'spirits of hartshorn', 'spirits of niter', 'spirits of turpentine', and many others reflect these ideas regarding the composition of matter.

Before the composition of plant and animal bodies was established, it was believed that one substance, like copper, could be changed to another substance, as gold. Plants were supposed to be water transmuted in some mysterious way directly into plant tissue. Van Helmont, in the seventeenth century, attempted to prove this. " He took a large earthen vessel and filled it with 200 pounds of dried earth. In it he planted a wallow weighing five pounds, which he duly watered wdth rain and distilled water. After five years he pulled up the willow and it now weighed one hundred and sixty-nine pounds and three ounces."' He concluded that 164 pounds of roots, bark, leaves, and branches had been produced b}' the direct transmutation of the water.

It is evident from the preceding example that aii}-- thing like an adequate idea of the growth and composition of plant bodies could not be gained until the composition of air and water were established.

The discovery of oxygen by Priestley in 1774, of the composition of water by Cavendish in 1 781, and of the role which carbon dioxide plays in plant and animal life by DeSaussure and others in 1800, form the nucleus of our present knowledge regarding the sources of matter stored up in plants. It was from 1760 to 1800 that alchemy lost its grip and the way was prepared for the development of modern chemistry.

The work of DeSaussure, entitled "Recherches sur la Vegetation," published in 1804, was the first systematic work showing the sources of the compounds stored up in plant bodies. He demonstrated,

INTRODUCTION 3

quantitatively, that the increase in the amount of carbon, hydrogen, and oxygen, when plants weie exposed to sunlight, was at the expense of the carbon dioxide of the air, and of the water of the soil. He also maintained that the mineral elements derived from the soil were essential for plant growth, and gave the results of the analyses of many plant ashes. He believed that the nitrogen of the soil was the main source of the nitrogen found in plants. These views have since been verified by many investigators, and are substantially those held at the present time regarding the fundamental principles of plant growth. They were not, however, accepted as conclusive at the time, and it was not until nearly a half century later, when Boussingault, Liebig, and others repeated the investigations of DeSaussure, that they were finally accepted by chemists and botanists.

From the time of DeSaussure to 1835, scientific experiments relating to plant growth were not actively prosecuted, but the scientific facts which had accumulated were studied and attempts were made to apply the results to actual practice. Among the first to see the relation between chemistry and agriculture was Sir Humphry Davy. In 181 3 he published his " Essentials of Agricultural Chemistry", which treated of the composition of air, soil, manures, plants, and of the influence of light and heat upon plant growth. About this same period, Thaer published an important work entitled " Principes Raisonnes d' Agriculture". Thaer believed that humus determined the fertility of the soil, that plants obtained their food mainly from humus, and that the carbon compounds of plants were produced from the organic carbon compounds of the soil. This gave rise to the so-called humus theory, which was later shown to be an inaccurate idea regarding the source of plant food. The writings of Thaer were of a most practical nature, and they did much to stimulate later investigations.

About 1830 there was a renewed interest in scientific investigations relating to agriculture. At this time Boussingault became actively engaged in agricultural research. He was the first to establish a chemical laboratory upon a farm and to make practical investigations in connection with agriculture. This marks the establishment of the first agricultural experiment station. Boussingault's work upon the assimilation of the free nitrogen of the air is reviewed in Chapter IV. His study of the rotation of crops was a valuable contribution to agricultural science. He discovered many important facts relating to the chemical characteristics of foods, and was the first to make a comparative study of the amount of nitrogen in different kinds of foods and to determine the value of foods on the basis of the nitrogen content. His study of the production of saltpeter did much to prepare the way for later work on nitrification. The

INTRODUCTION 5

work of Boussingault covered a variety of subjects relating to plant growth. He repeated and verified much of the earlier work of DeSaussure, and also secured many additional facts relating to the chemistry of crop growth. As to the source of nitrogen in crops, he states that : "The soil furnishes the crops with mineral alkaline substances, provides them with nitroofen, bv ammonia and bv nitrates, which are formed in the soil at the expense of the nitrogenous matters contained in diluvium, which is the basis of vegetable earth ; compounds in which nitrogen exists in stable combination, only becoming fertilizing by the effect of time." As for the absorption of the gaseous nitrogen of the air by vegetable earth, he sa}'s : " I am not acquainted with a single irreproachable observation that establishes it ; not onlv does the earth not absorb gaseous nitrogen, but it gives it off."^

The investigations of DeSaussure and Boussingault, and the writings of Davy, Thaer, Sprengel, and Schiibler prepared the way for the work and writings of Liebig. In 1840 he published "Organic Chemistry in its Applications to Agriculture and PliA'siology". Liebig's agricultural investigations were preceded by many valuable discoveries in organic chemistry, which he applied directly in his interpretations of agricultural problems. His waitings were of a forcible character and were extremely argumentative. They provoked, as he intended, vigorous discussions upon agricultural problems. He assailed the humus theory of Thaer, and showed that humus was not an adequate source of the plant's carbon. In the first edition of his work he showed that farms from which certain products were sold naturally became less productive, because of the loss of nitrogen. In a second edition he considered that the combined nitrogen of the air was sufficient for crop production. He overestimated the amount of ammonia in the air, and underestimated the value of the nitrogen in soils and manures. A study of the composition of plant-ashes led him to propose the mineral theory of plant nutrition. De- Saussure had shown that plants contained certain mineral elements, but he did not emphasize their importance as plant food. Liebig's writings on the composition of plant-ash, and the importance of supplying crops with mineral food led to the commercial preparation of manures, which in later years has developed into the commercial fertilizer industry. The work of Liebig was not conducted in connection with field experiments. It had, however, a most stimulating influence upon investigations in agricultural chemistry, and to him we owe, in a great degree, the summarizing of previous disconnected work and the mapping out of valuable lines for future investigations.

Liebig's enthusiasm for agricultural investigations may be judged from the following extract : " I shall be happy if I succeed in attracting the attention of men

INTRODUCTION y

of science to subjects which so well merit to eneao-e their talents and energies. Perfect agriculture is the true foundation of trade and industry ; it is the foundation of the riches of states. But a rational system of agriculture cannot be formed without the application of scientific principles ; for such a system must be based on an exact acquaintance with the means of nutrition of vegetables, and with the influence of soils, and actions of manures upon them. This knowledge we must seek from chemistry, which teaches the mode of investigating the composition and of the study of the character of the different substances from which plants derive their nourishment. "3

Soon after Liebig's first work appeared the investigations at Rothamsted by Sir J. B. Lawes were undertaken. The most extensive systematic work in both field experiments and laboratory investigations which have ever been conducted have been carried on by Lawes and Gilbert at Rothamsted, Eng. Dr. Gilbert had previously been a pupil of Liebig, and his association with Sir J. B. Lawes marked the establishment of the second experiment station. Many of the Rothamsted experiments have been continued since 1844, and results of the greatest value to agriculture have been obtained. The investigations on the non-assimilation of the atmospheric nitrogen by crops, published in 1 861, were accepted as conclusive evidence upon this much-vexed question. The work on manures, nitrification, the nitrogen supply of crops, and on the increase and decrease of the nitrogen of the soil when different crops are produced, has had a most important bearing upon maintaining the fertility of soils.

'' The general plan of the field experiments has been to grow some of the most important crops of rotation, each separately, for many years in succession on the same land, without manure, with farmyard manure, and with a great variety of chemical manures, the same kind of manure being, as a rule, applied year after year on the same plot. Experiments with different manures on the mixed herbage of permanent grass land, on the effects of fallow, and on the actual course of rotation without manure, and with different manures have likewise been made." "^

In addition to Davy, Thaer, DeSaussure, Boussingault, Liebig, and Lawes and Gilbert, a great many others have contributed to our knowledge of the chemistry of soils. The work of Pasteur, while it did not directly relate to soils, indirectly had a great infiuence upon soil investigations. His researches upon fermentation made it possible for Schlosing to prove that nitrification was the result of the w^orkings of living organisms which have since been isolated and studied by Warington and Winogradsky.

Many of the more recent investigations relating to the chemistry of soils are reviewed in the following chapters. Our knowledge regarding the chemistry,

INTRODUCTION 9

physics, geology, and bacteriology of soils is at the present time far from complete, but many facts have been discovered which are of the greatest value to the practical farmer. Of late years investigations relating to the chemistry of soils and fertilizers have become so extensive that the term 'agronomy' has been used to designate that part of agricultural chemistry.

In soil investigations it has frequently happened, owing to imperfect interpretation of results and to the presence of many modifying influences, that the results and conclusions of one investigator appear to be directly contradictory to those of another. This is well illustrated in the investigations relating^ to the assimilation of the free atmospheric nitrogen, in which seemingly opposite conclusions now form a complete theory.

Chapter I Physical Properties Of Soils

1. Soil. — Soil is disintegrated and pulverized rock mixed with animal and vegetable matter. The rock particles are of different kinds and sizes, and are in various stages of decomposition. If two soils are formed from the same kind of rock and differ only in the size of the particles, the difference is merely a physical one. If, however, one soil is formed largely from sandstone, while the other is formed from limestone, the difference is both physical and chemical. Hence it is that soils differ both physically and chemically. It is difficult to consider the physical properties of a soil without also considering the chemical properties. The chemical and physical properties of a soil, when jointly considered, determine largely its agricultural value.

2. Physical Properties Defined. — The physical

properties of a soil are :

1. Weight.

2. Color.

3. Size, form, and arrangement of the soil particles.

4. The relation of the soil to water, heat, and cold.

5. The relation of the soil to electricity.

6. Odor and taste.

PHYSICAL PROPERTIES OF SOILS n

3. Weight. — Soils differ in weight according to the composition and size of the particles. Fine sandy soils weigh heaviest, while peaty soils are lightest in weight. But when saturated wdth water, a cubic foot of peaty soil weighs more than a cubic foot of sandy soil. Clay soils weigh less per cubic foot than sandy soils. The larger the amount of organic matter in a soil the less the weight. Pasture land, for example, weighs less per cubic foot than arable land. Weight is an important property to consider when the total amounts of plant food in two soils are compared. For example, a peaty soil containing i per cent, of nitrogen and weighing 30 pounds per cubic foot has less total nitrogen than a soil containing 0.40 per cent, of nitrogen and weighing 80 pounds per cubic foot.

(i) The weight of soils per cubic foot is approximately as follows : ^

Pouuds.

Clay soil 70 to 75

Fine sandy soil 95 to 1 10

Loam soil 75 to 90

Peaty soil 25 to 60

Average prairie soil -r

Uncultivated prairie soil 65

Figures for the w^eight per cubic foot or specific gravity of soils are on the basis of the dry soil. When taken from the field the weight per cubic foot varies with the amount of water present.

(2) The volume of a soil varies w^th the conditions

12 SOILS ANI> FERTILIZERS »

to which it has been subjected. Usually about 50 per cent, of the volume of a soil is air space. A cubic foot of soil from a field which has been well cultivated weighs less than from a field where the soil has been compacted. Hence it is that soils have both a real and an apparent specific gravity. The apparent specific gravity of a soil is sometimes less than half of the real specific gravity. The specific gravity of different soils as given by Shoen is as follows : ^

Specific gravity.

Clay soil 2.65

Sandy soil 2.67

Fine soil 2.71

Humus soil 2.53

4. Size of the Soil Particles. — The size of the soil particles varies from those hardly distinguishable with the microscope to coarse rock fragments. The size of the particles determines the character of the soil as sandy, clay, or loam. The term ' fine earth ' is used to designate that part of the soil wdiich passes through a sieve with holes 0.5 mm. (0.02 inch) in diameter. Coarse sand particles and rock fragments which fail to pass through the sieve are called skeleton. The amount of fine earth and skeleton is variable. Arable soils, in general, contain from 5 to 20 per cent, of skeleton.

The fine earth is composed of six grades of soil particles. The names and sizes are as follows :

PHYSICAL PROPERTIES OF SOILS I3

Millimeters. Inches.

Medium sand 0.5 to 0.25 0.02 to o.oi

Fine sand o. 25 to o. i o.oi to 0.004

Very fine sand o. i to 0.05 0.004 to 0.002

Clay 0.005 ^iid. less 0.0002 and less

Soils are mechanical mixtures of various sized particles. In most soils there is a predominance of one grade, as clay in heavy clay soils, and medium sand in sandy soils. No soil, however, is composed entirely of one grade. The clay particles are exceedingly small ; it would take 5000 of the larger ones, if laid in a line wath the edges touching, to measure an inch, w^hile it would take but 50 of the larger medium sand particles to measure an inch.

5. Clay. — The term clay used physically denotes those soil particles less than 0.005 nin^- (0.0002 inch) in diameter, without regard to chemical composition. As used in a physical sense clay may be silica, feldspar, limestone, mica, kaolin, or any other rock or mineral which has been pulverized until the particles are less than 0.005 ^^^- i^ diameter. Chemically, however, the term clay is restricted to one material, as wall be explained in another part of the work. The physical properties of clay are well knowm. It has the power of absorbing a large amount of water, and will remain suspended in water for a long time. The roiled appearance of many streams and lakes is due to the presence of suspended clay particles. The amount in agricultural soils may range from 3 to 50 per cent. Clay soils, if worked when too wet, become puddled ; then percolation cannot take place, and the accumulated surface water must be removed by the slow process of evaporation.

6. Silt. — The silt particles are, in size, between sand and clay. Many of the western prairie subsoils, clay-like in nature, are composed mainly of silt. The silt imparts characteristics intermediate to sand and clay. While a clay soil is nearly impervious to water, and when wet works with difficulty, a silt soil is more permeable, but is not as open and porous as a sandy soil. When a soil containing large amounts of clay and silt is treated with water, the silt settles slowly, while the clay remains in suspension. The fine deposit in ditches and drains, where the water moves slowly, is mainly silt.

7. Sand. — There are three grades of sand. The characteristics, as permeability and non-cohesion of particles, are so well known that they do not require discussion. A soil composed entirely of sand would have little, if any, agricultural value. Sandy soils usually contain from 5 to 15 per cent, of clay and silt. The relative sizes of sand, silt, and clay are given in the illustration.

8. Form of Soil Particles. — Soil particles are ex-

PHYSICAL PROPERTIES OF SOILS tremely varied in form. When examined with the microscope they show the same diversity as is observed in larger stones. In some soils the particles are spherical, while in others they are angular. The shape

Fig. I. Medium Sand X 150. Fig. 2. Fine Sand X 150. Fig. 3, Very Fine Sand X 150. Fig. 4. Silt X 3?5. Fig- 5- Fine Silt X 325. Fig. 6. Clay X 325-

of the particles is determined by the way in which the soil has been formed and also by the nature of the rock from which it was produced.

The form and arrangement of the particles are important factors to consider in dealing with the water content of soils. In the wheat lands of the Red River Valley of the North, the particles are small and spherical, being formed largely from limestone rock, while the snbsoil of the western prairie regions is composed largely of angular silt particles, which are intermingled with clay, forming a mass containing only a minimum of inter soil spaces. The silt particles being angular and imbedded in the clay, the soil has more the character of clay than of silt. While these two soils are unlike in physical composition, the form and arrangement of the particles give each nearly the same water-holding power. On account of a difference in the form and arrangement of the soil particles two soils may have the same mechanical composition, and yet possess materially different physical properties. In some soils lo per cent, of clay is of more value agriculturally than in other soils. Ten per cent, of clay associated with 60 or 70 per cent, of silt, makes a good grain soil, while 10 per cent, of clay associated largely with sand makes a soil poorly suited to grain culture.