Principles of agricultural chemistry

Front matter

Classv ?. 5^ -gl,

Book. ±^?I_

Copyright N"

Preface

In this book the author aims to present the fundamental principles of agricultural chemistry. The point of view is that of the chemist dealing with agricultural problems; the attempt is. made to emphasize chemical methods of investigation, and inculcate scientific habits of thought. Details are omitted so far as they are not necessary to the proper treatment of the subject. Practical applications, which are necessarily local, are left out as much as possible. , The book thus treats of agricultural chemistry rather than of chemical agriculture. It attempts to give a comprehensive view of the subject, and to prepare the student for a more detailed study of its various phases.

This book is based upon lectures given for a number of years to students in Agriculture at the Agricultural and Mechanical College of Texas. A number of references are given, some of which do not refer to the articles in which the facts were ^rst published, but to articles of interest or of value to the student, which may contain numerous references to the literature of the subject. It was not deemed desirable nor was it practical, to give references for all the statements made in the text.

The author is fully aware of the fact that there is room for differences of opinion as to what should be treated or omitted in a work of this character. He also realizes the difficulty of avoiding errors, and will be grateful to the reader who may bring errors to his attention, or oft'er suggestions for the improvement of the book.

Valuable assistance has been received from Mr. S. E. Asbury, Assistant State Chemist, and especially Dr. C. P. Fountain, Professor of English.

Agricultural and Mechanical College of Texas, Aug. 30, 1912. College Station.

Contents

CHAPTER PAGE

I. — Introduction 7

II. — Essentials of Plant Life 9

Introduction

The object of agriculture is the profitable production of useful plants and animals. Agriculture is therefore an art and not a science, since an art relates to something to be done, a science to something to be known. We may, however, speak of the science of agriculture, meaning the body of organized knowledge appertaining to this art.

Success in agriculture depends upon ability to manage men and things, to take advantage of markets and local conditions, as well as upon a knowledge of how to produce plants and animals, and also upon skill in transforming this kno^vledge into practice. That is to say, practical agriculture is a business and business methods must be followed in order to succeed in it.

More than any other pursuit, agriculture is underlaid by a body of complex scientific principles, many of which are applied, knowingly or unknowingly, by the practical farmer.

Agricultural Experiment Stations. — The importance of agriculture has been recognized by civilized governments in the establishment of agricultural experiment stations and agricultural colleges. The oldest, and the most renowned experiment station, that at Rothamsted, England, is not a State institution, but was established, conducted, and endowed by Sir John Lawes, the work having been begun on a small scale in 1828. Nearly all of the experiment stations have been established since 1870; most of those in this country date from 1876 to 1882. In addition to conducting a great variety of experiments along agricultural lines, these stations make analyses of soils, fertilizers, feeding stufifs, etc. There is at least one experiment station in each State of the United States and, in addition, the United States Department of Agriculture (U. S. D. A.) ; these agencies are doing a great deal of work for the advancement of agriculture. While our knowledge of the principles of agriculture is largely due to the work of the experiment stations, it is chiefly due, let it be said, to the work of chemists.

Agricultural Chemistry. — Agricultural chemistry is the application of chemistry and chemical methods of investigation to agricultural problems. It deals, on the one hand, with fundamental causes of phenomena, and, on the other, with practical applications to agricultural practice. The chemist was the first of the scientists to turn his attention to agriculture, and his results were so fundamental, and practically important, and the science of chemistry was capable of such broad application to agriculture, that for a long time the great body of scientific knowledge regarding agriculture was known as agricultural chemistry. The chemist has not hesitated to avail himself of the sciences of geology, mineralogy, physics, botany, or such other sciences as were needed in the solution of the problems at hand. Many of the problems of agriculture are complex, and their solution requires the harmonious cooperation of several sciences. Take, for example, the transformation of organic nitrogen to nitrates, a very important process in the soil. This is a chemical change, accomplished by means of micro-organisms, and both chemistry and biology are necessary to give a complete explanation of this phenomenon, though the explanation has been largely worked out by the chemist.

Scope of the Subject. — In a wide sense, agricultural chemistry signifies the study of all the scientific laws involved in plant and animal growth, whatever the several sciences which may be involved. We may look at this subject as a fabric in which chemistry is so interwoven in warp and woof that, if removed, the pattern would be destroyed ; if the other sciences were removed, the pattern would be very imperfect.

It shall be our object to deal with the principles ascertained in the application of chemistry to agriculture, taking up the subject from the view-point of the chemist. In particular we shall attempt to indicate the methods which have been followed in securing important results. Agricultural science is founded upon

INTRODUCTION 3

and grows by experiments. An experiment is a question put to nature. It matters not what theories or lack of theories are behind the experiment, if the question is carefully and skilfully put, and if we see with a clear eye, not dazed by prejudgment, the answer will advance our knowledge. The knowledge of how experiments have been planned helps us to plan them for ourselves ; the knowledge of how a certain problem has been solved keeps us from regarding the knowledge so secured as dogmatic, and gives us an opportunity to test it for ourselves if we so desire. The scientific investigator cannot accept the conclusions of others at their face value; he must examine the evidence oftered, and satisfy himself that the evidence justifies the conclusion.

Division of the Subject. — Agriculture falls naturally into two divisions — the production of plants, and the production of animals. Usually in the case of plants, only a portion of the plant is desired, such as the grain ot wheat or corn, the tubers of potatoes, etc. The remainder is considered as a by-product and such disposition is made of it as appears feasible. The disposition of the by-product has considerable effect upon the fertility of the soil, or the profits of agriculture. In some cases, as in the preparation of hay, the entire plant is utilized. In other cases, by-products result in the preparation required before the product can be placed on the market, such as threshing of wheat or rice, husking or shelling of corn, etc.

The study of plant production involves a study of the conditions which are favorable to plant life, the composition and properties of the atmosphere and the soil, the maintenance of soil fertility, fertilizers, methods of soil treatment, etc., as well as the composition and properties of the plant products, and a study of such chemical changes as are involved in their production or preparation for market.

The study of animal production involves a study of the principles of animal growth and nutrition, the composition and properties of feeding stuffs, their preparation or preservation, and the methods of feeding for different purposes, such as meat, milk, wool, etc.

We will begin the study of agricultural chemistry with a study of the chemical laws governing the production of plants. We must study the conditions best suited to the growth of plants ; ascertain how these conditions are filled by the air and soil in which they grow ; learn how to overcome unfavorable conditions in the soil, and how to maintain and increase its productiveness. In addition, we must study the composition of the plant.

Agriculture Primarily the Production of Organic Matter.—

Agriculture deals primarily with the production of organic matter. Organic matter, for the purpose of the agriculturalist, may be defined as the compounds of carbon which possess chemical energy. In agriculture, inorganic compounds of carbon and other bodies are caused to combine with the energy of the sun, so as to produce organic compounds containing energy, which may supply heat or energy for the use of man or other animals, which may serve as fuel, or be used for other purposes. The primary object of agriculture is thus to store up the energy of the sun. The production of organic matter is accomplished by means of plants.

Products of Plant Life. — The various soil and atmospheric agencies, acting upon the life within the seed, produce a plant built up by sunshine, water, carbon dioxide from the air, and several mineral substances from the earth. The plant is composed mostly of complex organic substances, rich in carbon, and contains a comparatively small amount of material withdrawn from the soil. It is suitable for the food of animals, while the materials from which it is built are not. If dried and heated sufficiently, the plant burns and gives ofif heat.

It has been found that the heat which is secured in the burning of plants, or which can be utilized as heat or other forms of energy by animals which consume them, comes from the sun. The energy of the sun is used to decompose carbon dioxide, water, and nitrates, and to form complex organic compounds. These bodies then contain stores of energy which can be utilized by animals or in other ways. Plants thus store up the energy of the sun, and may also be regarded as media for furnishing animals

INTRODUCTION 5

with the sun's energy. All energy utilized by plants or animals thus comes directly or indirectly from the sun.

We have used the term "organic" in connection with the compounds formed in plants. It was believed in the beginning of the 19th century that organic bodies, such as starch, sugar, urea, etc., differed greatly in chemical nature from inorganic bodies, and could only be formed under the influence of mysterious lifeforces. But in 1820 VVohler prepared a product of animal life found in the urine, called urea, from a purely inorganic body, ammonium cyanate. The sup]iosed barrier between organic and inorganic substances was thus broken down ; great numbers of organic compounds have since been prepared, some of which occur in nature, and the chemist now hardly places bounds to the possibilities of organic synthesis in the laboratory. It is well known that organic and inorganic bodies obey the same laws, though on account of the size and complexity of the subject, organic chemistry is still treated separately.

As far as agricultural chemistry is concerned, there is a wide difference between organic and inorganic bodies. Organic compounds may serve as food for animals, but inorganic do not. On the other hand, inorganic bodies serve as food for plants ; but to only a very limited extent, if at all, do plants make use of organic bodies. With the aid of light, plants build up organic bodies which possess chemical energy, from inorganic bodies which do not possess chemical energy. For the student of agriculture, organic compounds are compounds of carbon which possess chemical energy, and they are usually the products of plants or animals.

Conditions of Plant Life. — The conditions necessary for the production of organic matter by green plants may be summed up briefly as follows :

(1) Light.

(2) Favorable Temperature.

(4) Certain elements in certain forms of combination. If any of these conditions are unfavorab'e, the plant will suffer

O PRINCIPLES OF AGRICULTURAL CHEMISTRY

and perhaps die. The varying needs of different kinds of plants and their varying powers of satisfying these needs, permit plants to flourish in nature under a great diversity of conditions, as in the tropics, or in arctic regions, in shade or in sunshine, in water or in deserts. The conditions of temperature, light, or water favorable to cultivated plants are more limited than those of wild plants, but still the range is wide.

The simple conditions we have named are rendered more complex by the varying degree in which different classes of plants require them, and the varying powers they have of supplying their needs. The varying powers of soils to supply the needs of the plant growth thereon, and the necessity of maintaining the fertility of the soil, render the matter still more complex.

Relation of the Plant to the Atmosphere. — I'he atmosphere has its part in supplying some of the conditions for the growth of plants, the more important being light, heat, carbon dioxide, and, indirectly, water. The atmospheric conditions are less susceptible to control than soil conditions; nevertheless, sometimes a partial control is established, as temperature and humidity in green houses, light in the growth of plants by artificial light or under shade, and the prevention of frost by smoke clouds. The atmosphere indirectly supplies the plant with small quantities of combined nitrogen, through the soil.

Relation of the Plant to the Soil. — The relation of the plant to the soil is more complex than its relation to the atmosphere. The functions of the soil are primarily to support the plant, supply it with water and certain necessary elements, and maintain a favorable temperature. These are, however, fulfilled in a very complicated manner.

Methods of Experiment. — The methods of studying the problems of agricultural chemistry must be varied to suit the end in view. At various points we shall bring in experimental evidence in support of certain views, thus illustrating by example some of the more important methods. The earnest student is advised to study

INTRODUCTION 7

the original papers that mark important steps in agricuUviral science.

The problems of agricultural chemistry are often so complex and interrelated as to render their solution very difficult. In the study of them, one should endeavor to vary one factor at a time and keep the others constant. Let us take, for example, the essential elements in plants. By chemical analysis we can ascertain that plants contain certain elements. Which of these are necessary to the plant and which are not? The solution of the problem is obtained by growing the plants under the most favorable conditions, with an ample supply of all the elements found in the plant except one. If the plant does well, then this one is not needed. If it does very poorly, and all the conditions are most favorable, then the element is necessary. The difficulties in following out this method of experiment will be presented later.

It is obvious that if two variables are present, it would be impossible to tell which one produced a given effect, or what part each had in it. The conditions of agricultural experimentation are sometimes such that it is difficult to reduce the experiment to a variation in one variable, and sometimes proper precautions are not taken to eliminate other variables. Take, for example, a field experiment on corn, or any other crop. Variables are weather, insects, seed, soil, etc. We attempt to eliminate them by subjecting the entire field to the same conditions, but it is very difficuli to make all conditions uniform.

Agricultural investigations must be brought to the test of actual conditions. Conditions in the laboratory or in pot experiments, are often radically dift'erent from those which prevail in practice. The gap between the two must be bridged by experiment, rather than by theory.

Observation and Experience. — Agricultural knowledge is largely based upon observation and experience. General agricultural practice is based upon experience, passed on from one generation to another. Experience is, indeed, based upon experiments, though the experiments are not always consciously

8 PRINCIPI^ES OF AGRICULTURAI, CHEMISTRY

made, \ery often imperfectly planned and often very expensive. Sometimes the trial is made intentionally, and sometimes the trials arc more or less unintentional and due to ignorance. Usually the trials are very limited in scope, and therefore differ widely from consciously organized experiments dealing with definite problems.

Chapter II. Essentials Of Plant Life

Prior to 1840, comparatively little was done to apply chemistry to the solution of agricultural problems. Much information was collected regarding the chemical composition of soils, plants, and animals, and a few books discussing the 1 elation of chemistry to agriculture were published, those of Sir Humphrey Davy and Thaer being perhaps the most important, but the fundamental principles of plant and animal nutrition were not recognized, and the books offered little of practical importance to the farmer.

At that time the prevailing theory was that plants feed upon the organic matter, or humus, of the soil, just as animals feed on organic matter. According to this theory, the soil should be kept full of vegetable matter to feed the plant. The ash or mineral matter of the plant, whose presence was known and could not be ignored, was thought to act as a stimulant, and not as food. Indeed, Thaer, and perhaps others, held that mineral matter could be created by plants.

The great German chemist, Justus von Liebig, in 1840 published a little book entitled "Chemistry in its Application to Agriculture and Physiology," which developed an entirely new theory of plant nutrition. Plants, he said, do not secure their organic matter from the soil, but from the air. He showed by calculations that there is not enough organic matter in the soil to produce average yields of farm crops. The material of importance which comes from the soil, he said, is the mineral matter. Supply the soil with a sufficiency of mineral matter, and it will remain fertile, regardless of its content of organic matter. Such, in brief, was Liebig's mineral theory of plant nutrition. This was a practical theory and easily tested. Liebig himself, as an object lesson, transformed a barren, sandy piece of land near Giessen, Germany, into a beautiful garden, by means of his mineral manures. Mr. John Lawes was incited to begin field experiments at his manor of Rothamsted, England.

These experiments led to the discovery of the process of mak- 2 lO PRINCIPLES OF AGRICULTURAL CHEMISTRY

ing acid phosphate by treating phosphate rock with sulphuric acid. The process, which was patented, became the foundation of the fertiHzer industry. The Rothamsted experiments, probably the most famous field experiments yet instituted, have been carried on with the same applications of fertilizers and manure since 1852. Sir John Lawes endowed the Rothamsted Experiment Station, and made provision for continuing its work indefinitely. The great fertilizer industries, and the era of agricultural experimentation, may be said to date from the application of chemistry to agriculture made by von Liebig in 1840.

Evidence for the Mineral Theory. — As evidence that the organic matter of the soil is not necessary to plants, plants have been grown to full maturity in soils from which all the organic matter had been burned out. Plants were also grown in pure water containing no organic matter, or carbon, to which certain mineral salts had been added. A prize was offered by the University of Gottingen for the solution of the question, whether the ash of plants was taken from the soil or created by them. Such prizes are still offered in Europe. This prize was won by Weigmann and Polstorff.^ In their first series of experiments, they grew a number of plants of different kinds, upon sand from which the soluble materials had been removed, as far as possible, by extraction with strong acids. One set of plants received only distilled water, the other set received a mixture of the mineral salts found in the ash of plants, and nitrates. The plants which received distilled water hardly grew at all, but the others grew well. This was evidence that the mineral matter was necessary for the growth of the plants. The plants grown on the sand with distilled water alone, when burned, were found to contain slightly more ash than was present in the same quantity of seed from which they were grown. Weigmann and Polstorff thought that this gain came from the sand. They accordingly instituted further experiments and grew plants in a platinum dish on platinum scraps with distilled water, the seed being weighed. Upon incineration, the quantity of ash in the plants was found to be exactly equal to ^ Dissertation, 1842, cited Meyer's Agricultur Chemie.

Esse;ntials of plant life ii

the quantity of ash in the seed planted. Hence the plants did not create any ash, and the ash gained in the previous experiment must have come from the sand. This work is an example of the importance of continuing experiments until only one possible conclusion is indicated, and also shows the danger of formulating conclusions upon insufficient data.

The plants in the platinum dish after reaching the height of only two or three inches, began to turn yellow, and died. This showed that the ash in the seed was sufficient for only a limited development of the plant, and, taken in connection with the preceding experiment, showed the mineral matter was necessary to the growth of plants.

Finding the Essential Elements. — An essential element is an element whose presence is absolutely necessary to the full growth and maturity of the plant. A useful element, though not essential, may be serviceable to the plant.

The following fourteen elements are invariably present in plants : —

The eight non-metals: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, silicion and chlorine.

The six metals: potassium, calcium, magnesium, iron, sodium and manganese.

Other elements are sometimes found in plants.^ Iodine occurs constantly in sea-weeds and sponges, being present as an organic compound in the latter. It is prepared from the ash of sea-weeds. Fluorine, arsenic, boron, rubidium, bromine, lithium, barium, aluminum, thallium, lead, zinc, titanium and copper have also

Water Culture Experiments. — Experiments to ascertain which been found in plants, in minute quantities.

elements are essential or useful to plants are made by growing

plants in pure water, to which the salts to be tested are added.

This is known as the zvater culture method, and is used because it

is comparatively easy to secure pure water and salts, but almost

impossible to secure a sand or soil from which plants do not ex-

tract some substance. Only the fourteen elements invariably found in plants need be considered, and as two of these (hydrogen and oxygen) are in the water, and one (carbon) is supplied from the air, there remains eleven to be tested. Twelve solutions are prepared. One contains all eleven elements, and is used as a check. If the plant does not thrive in it, something is wrong with the experiment. Each of the other solutions contain salts of ten elements, one being left out of each solution. For ex-

Fig. I. — Buckwheat grown in complete nutrient solution (O) and with CI, K, etc., absent.

ample, sodium is left out of one, potassium left out of another, and so on. Seeds are germinated on moist filter paper, and the strongest seedlings are supported by a cleft cork in the neck of the bottle containing the nutrient solution, so that the roots are immersed, but the cotyledons are above the surface. The plants would probably die if the cotyledons were immersed. The vessel is covered with black paper to exclude light and so prevent the growth of green algae, which interfere vrith the success of the experiment. The nutrient solution is replaced by a fresh solution every few days, to avoid injury to the plants by change in the chemical composition of ihe solution. Absorption of excess of acid or basic radicals by the plants, leaves the liquid acid or alkaline, according to its previous composition.

Where the plant grows well, reaches a good size, and produces seed, the element absent from the solution is not essential. Where the plant makes a very poor and imperfect growth, and produces only a few seed, or none at all, the element absent from the solution is essential. Since the seed always contain a certain quantity of the essential elements, some growth of the plant is to be expected.

The results of one series of experiments, are not, of course, considered final. In experimental work, the work of one man has usually to be confirmed by that of others, before it receives general acceptance.

The method of experiment known as "water culture"^ is suitable for experiments in which the material supplied to the roots must be accurately controlled. It is impossible to secure a soil or a sand from which plants will not obtain some mineral material. Weigmann and Polstorff, as we have seen, found that plants extracted ash from sand which had been exhausted with strong acids, and other workers have had similar experiences. By using pure water and pure salts, we know exactly what material is presented to the roots of the plants. The plant may take up a small portion of the silica from the glass v/hen the experiment is conducted in glass vessels, but this is not usually of importance. If necessary, vessels of platinum or of paraffin, can be used.

The Essential Elements. — It has been found by experiments such as those described above, that the following elements are, without doubt, essential to the life and growth of plants :

The four metals : potassium, calcium, magnesium, and iron.

The three non-metals: nitrogen, sulphur, and phosphorus. ■ Knop, Sachs and others, Jahresber. Agr. Chem., 1861, pp. 126, 136.

The non-metals: carbon, hydrogen and oxygen, secured from the air and water, as already stated, are essential. A soil, to be fertile, must supply to the plant an abundance of potassium, calcium, magnesium, iron, nitrogen, sulphur, and phosphorus.

Plants can be grown to full maturity by means of the following solution : —

One gram calcium nitrate,

0.25 gram potassium nitrate,

0.25 gram potassium sulphate,

0.25 gram magnesium sulphate,

0.20 gram ferrous phosphate.

1,000 cc distilled water.

Chlorine Essential but Unimportant. — Experiments to ascertain whether or not chlorine is essential to plants were at first conflicting. Three independent investigators grew plants in water culture to full and complete maturity without chlorine. Knop grew corn, buckwheat and cress ; Wagner grew corn ; and Birner grew oats.

On the other hand, Nobbe and Siegert' found that, although buckwheat grew well in water culture without chlorine up to the time of flowering, a little later the tips of the stalks died off, the leaves became brittle, spotted, and fluffy, starch accumulated in the stems and no seed were produced. The diseased condition was remedied by the addition of chlorine. Chlorine thus appeared to be essential to the formation of the seed of buckwheat. Leydhecker- also found that buckwheat would not seed in absence of chlorine, and Nobbe later confirmed previous results by a second series of experiments.

Thus one group of investigators finds chlorine not essential, the other group finds that it is essential. These contradictory results are explained by the work of Bayer. ^ Bayer grew oats in water culture, with and without chlorine, all the other essential elements being, of course, present. With chlorine 12.5 grams seed were

^ Landw. Versuchs-stat., 7, p. 370.

ESSENTIALS OE PLANT UEE secured, without chlorine 7.5 grams of seed. Apparently chlorine was not essential. 1 he seed grown in the absence of chlorine were found, on analysis, to be nearly free from chlorine. These seeds were used in another experiment in water culture as before, with and without chlorine. With chlorine, they made seed ; without chlorine, they failed to produce seed. Chlorine, therefore, is essential. The seed used in the first series of experiments conr^jfe:

\ \ J A n

Fig. 2. — Pot experiment showing soil deficient in nitrogen and in phosphoric acid. Texas Station.

tained a sufficient qtiantity of chlorine for the full development

of the plant, but those used in the second experiment did not.

There are a number of instances in which apparently contradictory results of different workers have been reconciled by further investigation.

The opposite results of the investigators referred to above are due, in one case, to the presence of sufficient chlorine in the seed used ; in the other case, to insufficient chlorine in the seed. We may conclude that chlorine is essential to the plant, but the minute quantity required may be present in the seed. Chlorine is l6 PRINCIPLES OF AGRICULTURAL CHEMISTRY

needed in such small quantity that it need hardly be considered as a plant food by the agricultural chemist.

Silicia not Essential but Useful. — Silica is present in all plants grown under normal conditions, and makes up a considerable proportion of the ash of some plants. The ash of cereal straws contain 20 to 40 per cent. It was formerly thought that silica was essential to tl.'C strength of cereal straws. Plants have been grown to maturity without silica, though they secured traces from the glass vessels in which the solutions were contained.^ The plants attained a normal development, and produced seed well. Silica is, therefore, not essential to plant life. Though not essential, silica is useful. In certain water culture experiments by Kreuzhaga and Wolfif- with oats, the presence of the silica increased both the number and the weight of the seed. The silica appeared to aid the plant to mature and form seed. The following table shows the results of an experiment in which all conditions were constant except the silica: —