First principles of soil fertility

Front matter

Class _A5^L_

Cop)TightN^_

COPYRIGHT DEPOSIT,

Preface

This little book is intended primarily for home reading. It is written, however, largely from lecture notes used by the author in a course in soil fertility given to winter course students, and, therefore, will be found to be a suitable text for short courses. It is an attempt to present in non-technical language a subject of great scientific and practical interest to the farmer. The subject of Soil Fertility is so large, that each phase of it could of necessity be only briefly considered in a small book, and the writer did not pretend to make the discussion of any topic exhaustive. For this reason some things have been omitted which might have been discussed with profit in a larger work. Only the more important facts concerning the maintenance of fertility have been presented, but it is believed that the statements so far as they go are in accord with the best scientific thought and practice of the day.

A certain amount of repetition has been purposely used in the text ; for it is felt that in this way only can the more important facts be given the emphasis which they deserve, but it is thought that this method of treatment has not been carried to the point of monotony. It is hoped that this book may find a little place in agricultural literature which is not already filled, and that it may be the means of leading a few earnest minds to a more extended study of this important subject of the fertility of the soil.

Alfred Vivian.

Ohio State Uiiii'crsitv.

Columbus, Ohio.

Contents

CHAPTER PAGE

I. Introductory 3

II. Atmosphere as a Source of Plant Food . . 12

Chapter I

Farming is a business, and the successful farmer must be first of all a business man. He follows his vocation primarily for the money he can make, and like other business men should aim to get the greatest possible returns for the money and labor involved. It is not enough simply to grow crops, but they must be so produced as to yield a profit on the capital invested. To succeed, he must be thoroughly acquainted with every detail of his occupation, and must strive to stop all leaks and prevent needless waste. At the same time, he must bear in mind that it is a good business principle to spend a dollar whenever he can see that it will come back to him with interest.

Agriculture is not merely a business but an art as well : the art of producing plants and animals that are useful to man. A real knowledge of farming necessitates a knowledge of the principles upon which the art of agriculture is founded; for an understanding of these principles is essential to an intelligent and rational practice. A few years since, "anyone could be a farmer." It was only necessary to sow and reap, for Nature dealt lavishly with man, and gave to him freely of the fertility she had been storing up for countless ages. A system of extravagant and unbusinesslike farming, however, has so impoverished the soil, in some parts of our country, that many farms are already abandoned, having ceased to be profitable ; and that too, in localities where the land once commanded high prices. This fact is the more lamentable, because the exhaustion of the soil might have been prevented by an intelligent foresight on the part of our earlier farmers. The farming of the future, therefore, must be done by men of broad training which should in-

An abandoned farm. Many farms have been abandoned because the soil is said to be exhausted. This exhaustion of the soil might have been prevented and the soils in many cases can be restored to their original fertility.

elude, among other things, some knowledge of such sciences as geology, chemistry, botany, zoology and physics. These sciences have done much to explain how the fertility of the land may be conserved, and it is the aim of this little book to present in a brief manner the latest views of agricultural investigators and farmers on this important subject. The intention is to make the treatment of the subject thoroughly practical, and for this reason the minimum of theory and the maximum of demonstrated facts will be given, and with the least possible use of technical language. Before taking up the subject of manures and fertilizers it is desirable to devote a short time to the consideration of plant food in general, explaining what it is and its source of supply.

Plants of First Importance to the Farmer. — All agriculture depends upon the growth of plants, and

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Plants are of first importance to agriculture, The production of animals and animal products is merely one way of marketing the crop consequently the profit that accrues to the farmer depends primarily upon the value of the crops his farm produces. In some kinds of farming the profit comes from the sale of crops that are useful in providing food, fuel, or raiment for man, while in others the direct gain comes from the sale of animals or animal products. Even in the latter case the feeding crops that can be grown upon the farm determine its earning power, for the sale of animal products is simply an indirect method of marketing the crops.

The profit from the farm is dependent not only upon the total crop produced but also, and to perhaps a still larger degree, upon the yield per acre. It stands to reason that if the crops now produced on two hundred acres could be grown upon one hundred, the returns would be greater, provided the labor and other expense involved were not materially increased ; for in the latter case the interest on the money invested in one hundred acres of land would be clear gain. On the other hand, it is apparent that nothing is gained by increased production per acre if the larger crop is obtained at a total expenditure in excess of that required for the smaller yield. As a matter of fact, our most successful farmers have demonstrated that the present average of crops can be doubled, and that at a cost per acre scarcely more than is now required for the halfcrop. To accomplish this necessitates a broader knowledge of the food requirements of plants than is possessed by the majority of our farmers. This knowledge being fundamental, it seems strange that more efforts have not been made to acquire it by those vitally interested. Strange as it may seem, it is a fact that while he has reasonably clear ideas on feeds for animals, the average farmer has only very vague and often false notions on the subject of plant food and plant nutrition. A thorough understanding of these subjects on the part of our forerunners in agriculture would have rendered it unnecessary to deal with the matter considered in the next paragraph.

Exhaustion of the Soil. — It is a matter of common experience that continued cropping results in a loss of fertility. The experiences of the older sections of our country teach some lessons by which the newer parts may profit. In the beginning the productiveness of the rich virgin soil seemed unlimited. For years large crops were produced with apparently no decrease in fertility. Sooner or later, however, the crops began to diminish in size, gradually, to be sure, but unceasingly, until at last the yield became so small that it no longer paid for the cost and labor of cultivation. This state of affairs came about more rapidly if the same crop was grown continuously on the same field, as was often done with wheat. The soil was now said to be exhausted, and in many cases the farms were abandoned. An exhausted soil in this sense means one that will no longer yield profitable returns, and not necessarily one that will produce no crop. As a matter of fact a soil can not become exhausted, if by exhaustion we mean total inability to produce a crop.

At the experiment station at Rothamsted, England, barley grown continuously on the same plot for fortythree years without the use of fertilizers of any kind yielded in the forty-third year lo bushels of dressed grain per acre; the average for the last eight years being ii^ bushels. Wheat grown in the same way for fifty years produced in the fiftieth year 9^ bushels of grain per acre ; the average for the last eight years being 11^ bushels. In these cases the soil seems capable of keeping up the yield indefinitely, as the average for the last twenty years is practically the same as the average given above for the last eight years.

While these facts indicate that the soil can never be completely exhausted, it is exhausted for all practical purposes when the crop produced ceases to be profitable. The first question that naturally suggests itself is, Why does the productive power of the soil diminish?

The Plant Removes Something from the Soil. — It is evident that the virgin land must have contained large quantities of some substance or substances that were necessary to vigorous plant growth and that these materials were removed from the soil when the crop was harvested. It is not possible to explain the rapid decrease in fertility on any other basis ; for it can not be ascribed to any changes in climatic conditions. The change in the physical condition of the soil has been suggested as a possible explanation for its decreased productive power, but even this is not an adequate explanation. It is apparent also, that plants vary in their power to extract these substances from the soil ; for it is well known that a soil may be unfertile for one class of plants and still produce a luxuriant growth of another. To ascertain what these materials are that the plant removes from the soil, it is necessary to analyze the plant and then to determine the sources of the ingredients found there. For the purpose of this study the corn, or maize, plant is chosen, as it is perhaps the most important of all plants to the American farmer. Before presenting the analysis it is advisable to devote a moment to a few preliminary considerations.

Elements and Compounds. — Chemistry teaches that all matter is composed of simple substances called elements. Between 70 and 80 of them are known. They are called elements because they are the simplest substances known, and can not by any means yet discovered, be separated into simpler or different substances. Iron, gold, silver and sulphur are examples of elements. Two others, both gases (i. e. oxygen and nitrogen) make up the bulk of the air.

Most materials with which we are familiar are complex and are combinations of two or more elements. Such bodies are called compounds. While the number of elements is small there are many thousands of compounds. This is due to the fact that the same elements can combine in many different ways, each combination forming a different compound. Alcohol, sugar, starch, fats and acetic acid, for example, are substances very unlike in their properties and yet all consist of the three elements carbon, hydrogen and oxygen, but these elements are present in different proportions. Plants are composed of a large number of compounds, and an ideal analysis would first separate the plant into its compounds and then these compounds into the elements of which they are composed. Approximately such an analysis can be made.

Chemical Composition of the Corn Plant.— If a quantity of green corn is allowed to wilt in the sun it loses a large percentage of its weight by the evaporation of the water which it contains. If the remainder is now heated in an oven at 212° F. it again decreases in weight, but finally reaches a point where the weight does not change, because all the water is driven off. Water is composed of the two elements hydrogen and oxygen. What remains after expelling the water is called the dry matter of the plant. The dry matter burns on being ignited and a very small amount of lO FIRST PRINCIPLES OF SOIL FERTILITY

mineral matter remains, which is called ash. The part that burned, and completely disappeared is known as organic matter. The organic matter is composed of four classes of compounds known as fat, crude fiber, carbohydrates and protein. The first three of these compounds are made up of the elements carbon, oxygen and hydrogen; and the protein contains in addition to

Corn or mai^>3 13 one of the most important crops for the American farmer. It removes large quantities of plant food from the soil, however. The analysis of the corn plant is given on page 1 1 .

these the element nitrogen. The ash contains the elements' potassium, phosphorus, calcium, magnesium, iron, sulphur, chlorine, sodium and silicon. The following table shows the ingredients found in i,ooo pounds of the matured corn plant, i. c. when the plant is in condition to be cut for shocking.

From what has been said it will be seen that of the elements known only thirteen are found in plants ; for what is true of the corn plant holds true of all other plants. It will be shown that of these thirteen, three are probably not necessary to plant growth, leaving only ten elements that are essential. The table shows

II that three elements (i. c. hydrogen, oxygen and carbon) make up 98^ per, cent of the entire composition of the plant, the remaining elements constituting only i>2 per cent.

COMPOSITION OF THE CORN PLANT

CORN PLANT 1,000 lbs.

r Water j Ilydfogen 88.1 i Oxygen 704.9

Dry Matter

Organic

Matter

Ash 12

Protein Fat 5.

Fiber 50.

Carbohydrates \22.

Chlorine 0.4 Potash 4.0 Pnosphoric Acid 1.2 Lime 1.6 Magnesia 1.4 Iron Oxide 0.3 Sulphuric Acid 0.3 Soda 0.4 Silica 2.4 r Nitrogen 2.9 I Carbon 90.5 I Oxygen 88.9 I Hydrogen 12.7

(Note. — All of the elements mentioned above as occurring in the ash, with the exception of chlorine, are combined with oxygen. In the table the names under "ash" represent these combinations, i. c. potash is composed of potassium and oxygen; phosphoric acid is phosphorus and oxygen; lime is calcium and oxygen, etc.)

CHAPTER II Importance of Water to the Plant. — One of the

most striking points brought out by the chemical analysis is the large proportion of water that enters into the composition of the plant. A reference to the table shows that nearly 800 of the 1,000 pounds of the matured corn plant consist of water in a form that can be driven off at a heat not above the boiling point. In the organic matter is found 12.7 pounds of hydrogen and 88.9 pounds of oxygen which practically all came originally from water, making a total of nearly 900 pounds derived from this source. These figures represent but a small part of the water actually required by the crop. Water is being continually given off into the air by the plant-leaves. This exhaled, or *'transpired," water is in the form of a vapor and is invisible, but that it actually exists can be proved by a simple experiment.

Invert a wide mouthed bottle or fruit jar over a small plant, and after a short time the inner surface of the bottle will be found to be covered with moisture. The earth around the plant should first be covered with a piece of oil cloth or oiled paper to make sure that the water does not come from the soil. If the underside of a leaf is examined with a magnifying glass or microscope it will be found that the surface is not entire, but is perforated by numerous small openings. These Openings are called ''stomata" (little mouths), and it is through these that the water is exhaled. This power of transpiration continues during the life of the plant, the water being obt a i n e d from the ground through the roots. Very large quantities of water are used in this way.

Amount of Water Required by Crops.

— European experiments have shown that approximately 300 pounds of water passes through the plant for each pound of dry matter produced, so that 1,000 pounds of corn use at least 30 tons of water

during its growing period. As this quantity of corn can be raised on one-thirtieth of an acre, it follows that to mature an acre of corn the crop must be supplied with 900 tons of water, or an amount that would make a layer over the acre about 8 inches deep.

This again takes no account of the quantity of water lost from the land by percolation or drainage. It has been estimated that this amount is at least equal to that used by vegetation, so that one acre of corn probably requires a precipitation of at least 1,800 tons of

Experiment to show that water is given off from the leaves of plants. The bottle on the left has been over the plant for some time and is cloudy from the moisture which has collected on the inside. The one on the right has just been placed over the plant and is transparent.

(Drawn from photograph)

water. These statements show clearly the necessity of carefully conserving the moisture of the soil, a point that can not be too strongly emphasized.

King found in investigations made at Wisconsin that the amount of water used by the crop was from 300 to 500 times the weight of the dry matter. His results are summarized in the following table.

AVERAGE AMOUNT OF WATER USED TO PRODUCE ONE TON OF DRY MATTER

Tons of water for Crop grown one ton dry matter

Barley 461. i

Clover 576.6

Potatoes 385.1

From this and other data he calculated the minimum amount of available water necessary to produce the various yields of the more common grain crops. These interesting figures are given below.

LEAST AMOUNT OF WATER PER ACRE REQUIRED TO PRO- DUCE DIFFERENT YIELDS OF GRAIN

Yield Acre Inches of Water Required

per acre Wheat Barley Oats Corn

15 20 30 40 50 60

4-5

2.35

2.52

6.0

4.28

3-14

3.36

9.0

6.42

5-04

2.0

8.56

6.27

6.72

5-0

10.70

7.84

8.40

8.0

12.84

9.40

10.08

Functions of Water. — Water is important to the plant in several different ways. It is first of all the most essential plant food, in the sense that it composes about 80 per cent of the mature crop. It also supplies the hydrogen and oxygen found in the dry matter, which amounts to lo per cent more, making a total of 90 per cent of the weight of the plant which is derived directly from the water.

Water is necessary to dissolve the plant food in the ground, and enable it to enter the plant, as will be noted later. It is needed to give stiffness or rigidity to the more succulent parts of the plant. This fact is shown by the drooping or wilting of plants during the hot hours of the day when the water is not furnished by the roots with sufficient rapidity to replace the loss by evaporation from the leaves. It is probable that water performs an important function in controlling the temperature of the plant. The chemical processes in the plant cells produce heat, and the excess of heat is removed by transpiration of water through the leaves, just as it is removed from the human body by the transpiration (perspiration so-called) through the skin. Water is also necessary for the nfovement of food within the plant. The food materials absorbed by the roots, and that manufactured by the leaves can be transported to the different parts of the plant where they are needed only when in solution in water.

Of such consequence to vegetation is the w^ater supply that some investigators claim that the question of fertility is wholly one of having present in the ground the proper amount of moisture, and that it is independent of the chemical composition of the soil, except as this composition affects its power to furnish the plant with water. This view is undoubtedly extreme, and is not generally accepted. There is no doubt, however, that the proper condition of moisture is the most important single factor in determining the fertility of the land, and that more soils fail to produce good crops for lack of it than for any other cause. It seldom happens that the water supply is sufficient to grow the maximum crop of which the soil is capable, as is demonstrated by the fact that a large increase in yield can be obtained by irrigation even in sections of very heavy precipitation. While the amount of water that falls on the land can not be controlled, much can be done to save the water so as to tide over the periods of scanty rainfall; a fact that will be emphasized throughout this book. Too much stress can not be laid upon the importance to the plant of an adequate supply of water in the soil, and the knowledge that certain methods increase the amount of moisture available to the crop should be sufficient reason for their adoption. The reader is asked to carry this thought with him as he reads the following pages.

Part of the Oxygen from the Air. — A small quantity of the oxygen in the plant probably comes from the air. One-fifth of the volume of the air is oxygen, and the plant uses this to some extent. Plants breathe in much the same manner that animals do, for all cells must have a supply of oxygen in order to live. The oxygen of the air combines with the materials in the cells one of the resuUs being the production of heat, just as the oxidaltion taking place in the animal body produces ilieat, T^^at heat is evolved by the living vegetable cell can easily be proved experimentally by confining the plant in such a way as to prevent radiation. The rapid heating of silage in the silo is doubtless due to the breathing process of the cell, the heat in this case being unable to escape.

Carbon in Plants Derived from the Air. — Nearly one-half of the dry matter in the ])lant consists of the element carbon, all of which is derived from the carbonic acid gas which is present in the atmosphere. Carbonic acid is the colorless gas which is formed when a piece of coal or charcoal is burned, and is a compound containing the two elements carbon and oxygen. Charcoal is a good exami)le of nearly pure carbon. Carbonic acid gas is found everywhere in the atmosphere, although present in very small quantities, constituting only four one-hundredths of one per cent of the volume of the air, or about four parts in 10,000. It seems strange to think that so large a proportion of the solid material of the plant should be formed from this gas, but it is well known that green plants have the i)ower to decompose this gas, retaining the carbon and setting free the oxygen. This process is known as the ^'fixation (sometimes assimilation) of carbon," and takes place chiefly in the leaves. The power to fix carbon is dependent in some way on the presence of the green coloring matter (chlorophyll), so that it is only those plants having green leaves that can use the carbonic acid. Such ]:)lants as mushrooms and other fungi, for instance, can not obtain their carbon in this manner but must procure it through the decomposition of organic matter, or in other words, must have their food jireviously prepared for them. Green plants, on the other hand, can manufacture their own food from the inorganic materials of the soil and atmosphere.

Sunlight Necessary to Carbon Fixation. — The decomposition of carbonic acid by the plant, and the assimilation of the carbon take place only during the daytime. A certain amount of energy is necessary to break apart the carbon and oxygen of carbonic acid, and this energy is furnished by the sunlight. The stronger the light the faster the fixation of carbon. This explains the commonly observed fact that most plants grow more vigorously in full sunlight than in shade or diffused light. The plant has not the power to use carbonic acid in the absence of light, so that this process ceases during the night. It is well known that seeds will germinate in the dark, and produce a feeble spindling growth of pale foliage, but that the plants so produced soon cease to develop. Such plants grow until they exhaust the food stored in the seed, but have no power to use the food in the air and soil, and analysis shows that the plant contains less dry matter than w^as present in the seed. In the presence of light, however, the plant absorbs the carbonic acid of the air by means of its leaves, and causes the carbon to combine with the water and mineral matter taken in through its roots to form carbohydrates, proteids and the other complex compounds of which the plant is composed.

Carbonic Acid the Sole Source of Carbon. — It has been thoroughly demonstrated that the green plants derive their carbon solely from the carbonic acid of the atmosphere, and are not dependent in any way

Upon the carbonaceous matter in the soil. In fact, that they are incapable of using carbon except in the form of carbonic acid gas. The quantity of carbonic acid in the air seems so insignificant that it might be feared that the supply might some day be exhausted. The bulk of the atmosphere is so enormous, however, that the total amount of carbonic acid is very large. Johnson calculated that the atmosphere when taken to its entire height contains not less than 3,400,000,000,- 000 tons of carbonic acid. This amounts to about 28 tons over every acre of the earth's surface, and as only one-fourth of the earth's surface is land he estimated that the carbonic acid in the air is sufficient, without renewal, for a hundred years of growth. As a matter of fact, the supply of this gas is being constantly renewed, and at such a rate that the proportion found in the air remains about constant. When w^ood or coal or any other substance containing carbon is burned, carbonic acid is formed and passes into the atmosphere. This gas is produced during all kinds of decay and fermentation. Animals live directly or indirectly on plants, and breathe out the carbon they consume in the form of carbonic acid gas. It will thus be seen that the carbon is continually in circulation, being combined by the plant into complex compounds only to be broken down, and returned again to the air through these various agencies. In all probability that now present in the air has been many times built up into organic matter, only to be again set free by decomposition.

Numerous experiments have proved that the supply of carbon in the air is ample for the largest crops. To be sure, in certain pot experiments a larger yield was obtained by increasing the carbonic acid in the air, but under field conditions the yield is limited by other factors, and never by the supply of carbon.

Carbon Costs the Farmer Nothing. — The point of practical importance brought out by this study of the fixation of carbon is that the carbon is furnished free of cost. In other words the carbon compounds produced in the crop result in no impoverishment of the soil. Hence, there is no need of supplying strictly carbonaceous manure to the field, as the crop does not use the carbon in the soil. It will be shown later that such manures may be indirectly beneficial to the plant, however.

Chapter III Nitrogen As A Plant Food

Nitrogen the Most Costly Plant Food. — A reference to the table given in Chapter I shows that only abont lYz per cent of the dry matter of the corn plant consists of nitrogen. Some plants contain more nitrogen than this, but the amount rarely equals 3 per cent of the dry matter, or six-tenths of one per cent of the green plant. In spite of the small quantity of nitrogen in the crop it is the most important of all plant foods from the practical point of view. In fact the solution of the problem of the maintenance of fertility depends upon an economical method of conserving and renewing the nitrogen supply of the soil. This does not imply that nitrogen is more necessary to vegetation than are the other constituents, but that it is the most expensive element to be furnished by means of fertilizers, and is also, unfortunately, the element most easily lost and wasted.

The Nitrogen of Most Plants Comes from the Soil. — Most of the crops raised by the farmer are entirely dependent upon the soil for their supply of nitrogen. The greater part of the nitrogen present in the soil is locked up in the insoluble organic matter, and in this form is not available to plants. Some of the nitrogen exists in simple compounds called nitrates, which consist of nitric acid combined with one of the mineral elements of the soil. The majority of farm crops can use only that part of the nitrogen in the soil that is present as nitrates, so that so far as the nitrogen is concerned, the fertility of the land depends upon its nitrate content. The nitrate present in the soil at any one time is exceedingly small, but under proper conditions the supply may be renewed with sufficient rapidity to meet the needs of the plant.

Source of the Nitrogen of the Soil. — A small part of the nitrogen in the soil is derived directly from the atmosphere. Minute traces of ammonia (a compound of nitrogen and hydrogen) are always found in air, and during electrical storms small quantities of the nitrogen and oxygen in the atmosphere are combined to form nitric acid. These substances are dissolved in the rain water during showers and are carried into the soil. The quantity received by the soil from this source is very small, amounting only to from 3 to 8 pounds an acre a year, the maximum amount being less than one-tenth of that required by a crop of corn. Nearly all of the nitrogen in the soil is present in the more or less decayed organic matter left behind by the plants that it has previously produced. Plants build up the nitrogen into complex protein compounds, and, under ordinary conditions, when they die these substances in connection with the other constituents of the plant become a part of the soil. As long as the nitrogen remains in this form it is of no value to the new generation of plants, for the organic matter must first be decomposed, and the nitrogen changed into the form of nitrates.