General agricultural chemistry

Contents

I Introduction 7

II The Atmosphere 23

III The Soil o5

Preface

Since the time of Liebig, agriculture in its many phases has profited from the science of chemistry, A store of useful information has been made available through the study of the elements and compounds fundamentally concerned in the art of agriculture. It is reasonable to expect that this art will in the future be enriched from the same source.

This little book was written in the interest of the young farmer and the student beginning the study of agricultural chemistry. No extended knowledge of chemistry is required for its understanding. It makes no special appeal to the chemist. It is a survey of the general field of chemistry applied to agriculture, with the emphasis always placed on the applied side.

Throughout the book we have striven to express safe views rather than to echo the most recent. Hypotheses and theories have not been discussed. We have attempted to give, in general, only well tested and established principles. Formulas and recipes have been avoided as far as possible. While we recognize their helpfulness, nevertheless, they are as yet but imperfect expressions of relations not fully understood.

The authors have drawn freely from various publications, endeavoring to bring together from scattered sources the materials essential to such a work on agricultural chemistry. In this regard we are especially indebted to the works of Ingle, Warington, Storer, Voorhees, Vivian, Jordan and others, all of which have aided greatly in the preparation of this work.

Introduction

Agricultural chemistry concerns itself with the chemical composition of the food of plants and animals and with the chemical changes involved in the processes of life. It has to deal with the composition of soil, air, and water, of the bodies of plants and animals, of manures and commercial fertilizers and with the chemical changes which these substances undergo.

Before beginning the study of the soil or air or the plant it will be necessary for the student to learn something of the important elements concerned in agriculture and the meaning of some of the common terms used in chemistry.

The whole earth, so far as is known, is made up of about eighty-one elements, a large proportion of which play little or no part in the ordinary processes of plant and animal life. Indeed a considerable proportion are found only in extremely small quantities and are but curiosities to the student of chemistry. Prom the standpoint of the farmer they possess no interest. They are called elements for the reason that they are the simplest substances known, and cannot by any means yet discovered, be separated into simpler or different substances. Iron, gold, silver, zinc, lead, and sulphur are examples of elements.

The bodies of plants and animals are built up of compounds of the following elements and these, therefore, become of the first importance to the farmer:

Oxygen Phosphorus Sodium

Hydrogen Calcium Iron

Carbon Magnesium Chlorine

Nitrogen Potassium Silicon Sulphur

A short account of these elements will be given at this place. Oxygen (0) is the most abundant and most important of the elements. It forms about half the weight of the solid crust of

8 Agriculiwal Chemistry,

the earth, eight-ninths of the water, and about one-fifth of the weight of the air. In the first and second instances the oxygen is in a combined state. That which is held in chemical combination in the soil takes no part in the formation of plant tissue. In the atmosphere it exists as a free element, merely mixed Avitb the other constituents. Oxygen in the interstices of the soil is an active agent in bringing about many chemical changes, as oxidation of the organic matter and disintegration of the soil particles. It also forms about fifty per cent of the compounds found in plants and animals.

Oxygen is a colorless, odorless gas and veiy slightly soluble in water. It shows great tendency to combine with other substances and the act of union is usually attended by the production of much heat. liurning or combustion is nearly always due to the heat produced by the combination of the substance burned with the oxygen of the air. Any substance, which will burn in air (containing about twenty-one per cent of free oxygen) will bum with increased brilliancy in pure oxygen.

It is possible, with suitable apparatus, to measure the quantity of heat a substance will produce Avhen burned. The vmit of heat here employed is the "calorie," which represents the quantity of heat required to raise one gram (about 1-28 of an ounce) of water from 0° to 1° on the scale of the centigrade thermometer. A large Calorie, one thousand times larger than the above, is employed for the expression of large quantities of heat and will be employed here.

Wben one gram of the following dry substances is burned in oxygen, the quantity of heat produced, expressed in large Calories, is as follows :

Charcoal «.0

Hydrogen 34 . 4

Wood 2.8

Coal 7.5

Coke 7.0

Fat of sheei> !».4

Fat of butter 9.2

Cane sugar 4.0

Cellulose 4.1

Jnlrod'uction. 9

In ordinary cases of burning, the evolution of heat is readily evident, but in some cases the combustion is so slow that the heat evolved is carried away as fast as produced and very slight or no elevation of temperature is apparent. In some cases of slow combustion where the escape of heat is hindered from any cause, the temperature may rise so as to be perceptible or even dangerous. It may, under particularly favorable conditions, rise sufficiently to start a rapid combustion with oxygen and flames then result. Such cases of '^spontaneous combustion" frequently occur. Drying oils, as linseed or cotton-seed oil, especially when spread on cotton waste, and fermentation changes in vegetable matter as hay and tobacco are notable examples of these conditions.

Hydrogen (II). This element is rarely found in a free state in nature, but is combined with carbon and ox^^gen as in animal and vegetable matter, with oxygen to fonn water, and in a few cases with some of the base elements to form hydroxides. It is not found in large amounts in the soil and that which forms a part of the tissues of plants and animals comes largely from the hydrogen in water. It is a colorless, odorless gas and characterized by its lightness. This fact has led to its use for filling balloons, although coal gas is now more generally employed but is not nearly so efficient. In a free state it has been found in the gases escaping from volcanoes.

Carbon (C) is the element most closely associated A\ath plant and animal life. It forms a large proportion of the solid matter of all living beings; and the chemical processes of animal and plant life are mainly those in which carbon plaj^s an important part. It exists in the combined state in many minerals as the carbonates of calcimn, magnesium, iron, zinc, and also in a small but very important constituent of the air, carbon dioxide. The carbon of the soil, M'here it exists as the main constituent of organic bodies, takes no direct part in forming the carbon cora- ];ounds of the plant. It is not necessary to apply carbon fer-

]0 AgncnUvral Chemistry.

tilizers to produce the carbon compounds of the plant, because the carbon dioxide of the air is the source for crop production. It is estimated that there arc about thirty tons of carbon dioxide in the air over every acre of the earth's surface.

This element occurs in three distinct forms: (1) as the diamond, (2) as graphite and (3) as charcoal, lamp black, etc. The diamond is crystalline and transparent; graphite is crv'stalline but opaque; while lamp black and charcoal are non-crystalline. The black carbon which is produced when animal or vegetable substances are strongly heated without access of air (charring) is due to the separation of free carbon from the various carbonaceous compounds present.

Nitrogen (N) is much less abundant in nature than the elements already described. A peculiarity of its occurrence is that it appears to be present only in the outermost portion of the earth, the greater portion being free in the air. No true minerals containing it are known except those which owe their origin directly to plant or animal life, as coal, and Chili salt-petre. All living matter, however, contains it as an essential constituent. In its free state it is a colorless, odorless gas, showing little tendency to combine with other elements. It constitutes about seventy-nine per cent of the atmosphere and over each acre of land there is consequently about thirty thousand tons.

Although in the free state it is so inert, the nitrogen compounds, as a rule, possess great chemical activity and many are very important substances. Some powerful drugs and poisons as quinine, strychnine, and prussic acid contain nitrogen, while most explosives, as nitro-glycerine and gun cotton are also nitrogen compounds. It is an absolutely essential ingredient in the food of hotli animals and plants. It must be supplied to animals in compounds in which it is combined with carbon, hydrogen, oxygen, and certain other elements and which are known as proteins. while phmts acquire it generally from nitrates, which are simple compounds of oxygen, nitrogen, and some base, as calcium, soidum, and j)ota.ssium. Only under very special conditions can

Introduction. 11

some species of plants obtain their necessary nitrogen from the air. It will be seen in the later chapters that, although plants are surrounded by air, rich in free nitrogen, combined nitrogen is one of the essential and most valuable constituents of manures. A large part of the nitrogen in the food consumed by man and animals is eliminated as simple compounds in the excreta and unfortunately, especially in our cities, sent down the sewers and rivers and finally discharged into the sea. To agriculture this valuable combined nitrogen is therefore wasted. This element is the most expensive of those necessary for plant growth and is among those liable to be most deficient in our soils. No other element takes such an important part in agriculture or in life processes.

Sulphur (S) is found both free and combined in nature. The free element is found in volcanic districts, while in the combined state it occurs as hydrogen sulphide in mineral waters and as sulphides of many metals, as for example iron, lead, and zinc. The sulphide of iron, known as iron pyrites, is often mistaken for gold because of its yellow color ; sulphur also occurs as sulphate of calcium, in which form it is very widely distributed in soils, and is the main source of the sulphur for crops.

The element sulphur (brimstone) is a yellow, brittle substance and very inflammable. It bums in air -with a pale blue flame, forming the suffocating gas, sulphur dioxide. Such forms of sulphur are very poisonous to plants and animals, while sulphates are not only harmless, but necessary. Sulphur is present in the proteins of both plants and animals and when putrefaction of these substances occurs is often liberated as hydrogen sulphide. This substance is perceptible by its disagreeable odor as one of the chief products of the decay of eggs.

There is generally less than 0.10 per cent of sulphur trioxide. as sulphates in ordinary soil, and it is now known that the amount required by crops is considerable ; for this reason it may be necessary to use certain sulphates occasionally as fertilizers and as sources of sulphur for the growing crops.

Phosphorus (P) always occurs in a state of combination. Phosphorus compounds, chiefly phosphates, are very widely distributed, but in small proportion, in the rocks of the earth. Deposits of calcium phosphate occur in certain localities and are one of the chief sources of our phosphate fertilizers. All fertile soils contain small quantities of phosphates, which are taken up by plants and through plants find their way into animals, where they accumulate in the bones or other hard parts, as teeth and shells.

The element phosphorus, as usually prepared, is a yellowish waxy substance, Avhich has the power of emitting a faint light when exposed to the air. This property was the origin of its name, Avhich is derived from the Greek and means "the light bearer." The emission of light is due to slow combination with the oxygen of the air, resulting in the production of heat.

Phosphorus is a violent poison. -It is largely used in the manufacture of lucifer matches and rat-poison. For the farmer its chief importance lies in the use of its compounds, the phosphates, as fertilizers, and its occurrence in certain fats and protein compounds of feeding stuffs and in the bodies of animals.

Soils are quite liable to be deficient in phosphates, as the latter are largely drawn upon by many crops, particularly grain crops, where the phosphorus accumulates in the seed and is sold from the farm.

Calcium (Ca) is veiy abundant in nature, always occurring in a combined state. Calcium carbonate is found in enormous quantities, as chalk, limestone and marble, and contains the three elements, calcium, carbon and oxygen. It also occurs as gypsum, a compound of calcium, sulphur and oxj-gen. The element itself is an easily oxidisable metal, difiicult to prepare, and of no importfince to the fanlier. Its oxide, or a compound of calcium and oxygen, is the important substance, quick lime. This is made by burning limestone, whereby the carbon and part of the oxygen are removed as a gas. Calcium is an essential constituent of

Introduction. 13

plant food and in the soil is present in a variety of forms, as calcium carbonate, calcium sulphate and calcium phosphate.

Potassium (K) occurs in many minerals. It will be found in many silicates, as orthoclase or mica, which are complex compounds of potassium, silicon, aluminum, oxygen and other elements. It also occurs in sea water, from which sea weeds accumulate large quantities of potassium compounds. The immense salt deposits at Stassfurth, Germany, furnish a large proportion of the potassium used in our potash fertilizers.

The element is a lustrous metal, very soft, and so susceptible to change in the air that it must be kept away from contact with air or moisture by immersion in naphtha. By contact with water it reacts violently, producing much heat and floating on the surface of the water with a hissing sound.

Potassium compounds are of the greatest importance in agriculture and are necessary constituents of all fertile soils. They are intimately associated with the growth and increase of plants and are always found in greatest abundance in the twigs, young leaves and other rapidly growing portions. In some plants the potassium is in combination with certain organic acids, as citric and tartaric acids. In the ash of plants — that which is left after burning — it generally occurs as a carbonate. Potassium salts are very soluble in water, but are absorbed and retained by certain constituents of the soil, so that their loss by drainage from soil is little to be feared.

Sodium (Na) is very widely distributed in nature and is a constituent of many silicates. In the form of sodium chloride — a compound of sodium and chlorine — it is very plentiful as rock salt and as the largest saline constituent of sea-water.

Its properties resemble those of potassium. Sodium compounds are largely used in the arts and the preparation of sodium carbonate is one of the largest and most important of chemical industries.

Sodium is found in the ash of most plants, but, except in the case of certain plants, does not appear to be essential to their development. A striking difference between sodium and potassium compounds, which are so much alike in most of their properties, is in their behavior towards the soil when applied in solution. The potassium salts are retained by the clay and organic matter in an insoluble form, but the sodium salts are more easily washed out by water and escape into the drains. Although like potassium in its chemical properties it cannot take its place in agriculture.

Magnesium (Mg) is widely met in nature as carbonate and silicate. The element itself is a bright, §ilvery metal, and capable of burning in air with an intense and dazzling white light. IMagnesium is found in the ash of plants and is required by all crops. It is particularly abundant in the seeds. There is generally in all soils an amount sufficient for crop purposes and it is not necessary to consider this element in connection with fertilizers.

Iron (Fe) occurs in a large number of compounds. Haematite, a compound of iron and oxygen, magnetite, a similar compound, but Avith a different proportion of oxygen, and spathic iron ore, a compoimd of iron, carbon, and oxygen; the above are all abundant minerals and valued as ores of iron. The element occurs in two states of combination with oxygen, one a relatively small gmount and called ferrous iron, the other a relatively larger amount and designated ferric iron. The former yields salts which are white or green in color, while those of the latter are red or yellow. Ferrous compounds are often present in rocks or minerals deep under ground, but when brought to the surface they combine with the oxygen of the air to form ferric compounds. The change of the state of iron is indicated by a change in color, often from green or gray to red or yellow. Only ferric compoimds should exist in good soils. Iron is essential to plants, but a small quantity is all that is required and most soils contain from one to four per cent, an abundant supply.

Chlorine (CI) is very abundant, especially in combination with sodium, as rock salt in the sea and in spring water. Other compounds of chlorine also occur as minerals. The element chlorine

Introduction. 15

is a yellowish green gas with an irritating and suffocating smell, very soluble in water and of great chemical activity. The properties of chlorine, which are most valued in the arts, are its bleaching, disinfecting and deodorizing powers. It readily destroys most coloring matters and is largely employed in bleaching vegetable textile fabrics, as cotton or linen. It cannot be used for woolen or silk fabrics, as it injures the fibres themselves. Chlorine only bleaches in the presence of water and it really acts by decomposing the water, with formation of oxygen, which is the active agent. Its action as a disinfectant is probably due to the same process, the oxygen of the water combining with the organic matter and micro-organisms and destroying them.

Chlorine is present in all soils, generally in combination with sodium, as sodium chloride. It is present in all plants, although its necessity for plant growth may be questioned. Crops have been brought to maturity in its entire absence. Chlorine with sodium, as common salt, is sometimes used as an indirect fertilzer.

Silicon (Si) is extremely abundant in the rocks of the earth's crust, and though it forms a very important ingredient in soils and occurs in most plant ashes, it does not appear to be absolutely essential as a plant food. Some recent work, however, has shown that soluble silica in a soil enables a plant to subsist in the presence of a smaller quantity of phosphoric acid than would be necessary without the silica.

The element itself is a bro\\Ti solid and at one time was difficult to prepare in any quantity. At present, with the electric furnace, it is easily produced and its price per pound has been greatly reduced.

The oxide, called silica, is a compound of silicon and oxygen and is a very abundant substance, occurring free as quartz, flint and sand ; in combination with metals the very numerous and important substances called silicates, are produced. It has been estimated that nearly half of the solid mass of the earth's crust consists of silica.

IG Agricultural Chemistry.

DEFINITIONS.

It now becomes necessary to define, in a fragmentary way, some of the commoner terms used in chemistry.

Acid. A substance generally possessing a sour taste and the property of changing vegetable blues, as blue litmus, to red. As types of acids, we have sulphuric acid, commonly used for the Babcock test, and acetic acid, the principal acid in vinegar. The possession of a sour taste and the power of changing vegetable blues to red is indicated by saying that the substance has an acid reaction.

Alkali. A substance opposed in its properties to an acid, capable of neutralizing and destroying the characteristics of an acid, forming in so doing, a salt. The most important alkalies are soda, potash, lime, and ammonia. A substance is said to have an alkaline reaction if it turns certain vegetable colors, as red litmus, to a blue color.

Organic matter, strictly speaking, is matter which has been produced by organisms, such as plants or animals, but the term is used in a wider sense in chemistry for any compound of carbon, whether produced by life processes or artificially. Almost all fonns of organic matter, when strongly heated out of contact with air, blacken, owing to the liberation of carbon. With free access of air, combustion occurs, and carbon dioxide and other products are formed.

Oxidation and Reduction. By oxidation, literally speaking, is meant union with oxygen, but in a chemical sense the term is given a wider significance, that is, combination with more oxygen or with some sulistance playing the part of oxygen.

Reduction is used in exactly the opposite sense. A substance which brings about oxidation, is called an ''oxidizing agent" while one Avhich removes oxygen is called a "reducing agent." Common oxidizing agents are air, nitric acid, nitrates and chlorine ; common reducing agents are easily oxidizable metals, as zinc, and many forms of decaying organic matter.

Inb^odudlon. 17

Fermentation. A process of decomposition, often accompanied by the oxidation of carbonaceous matter, and produced by the life processes of bacteria, yeasts and molds. When the process occurs out of free access of air and bad smelling gases are formed, the process is called puirefaciion.

The constituents of plants. All agriculture depends upon the growth of plants and consequently all profit for the farmer depends upon the value of the crop his farm produces. This is true whether the crop is sold directly from the farm or whether it is fed to animals and the products such as live stock, beef, pork, wool, eggs, or milk, used as the source of revenue. If the crops now produced on two hundred acres of land could be grown on one hundred without a great increase of labor and other expense, the profit would be greater. Successful farmers have demonstrated that the present average of crops can be doubled, and that at a cost per acre scarceh' more than is now required for the one-half crop.

To accomplish this requires a broader knowledge of the food requirements of plants than is possessed by most of our farmers. A thorough understanding of the subject of plant food and plant nutrition by our forerunners in agriculture would have rendered it unnecessary to emphasize constantly the relation of the constituents of the plant to soil exhaustion.

It is common experience that continued cropping results in a loss of fertility. The productiveness of a virgin soil seems unlimited, for large crops are produced from year to year with no apparent decrease. But sooner or later they begin to diminish in size, gradually to be sure, but unceasingly, until at last the yield becomes so small as to make the cost and labor of production unprofitable.

At the Experiment Station at Rothamsted, England, barley grown continuously on the same plot for forty-three years without the use of fertilizers of any kind, yielded in the forty-third year 10 bushels of dressed grain per acre, the average for the laijt eight years being 11% bushels. Wheat grown for fifty years

I in the same way produced in the fiftieth year 9% bushels of grain per acre, the average for the last eight years being IIV2 bushels. The soil seems capable of keeping up the yield indefinitely, but the amount of crop produced ceases to be profitable.

It is evident that the virgin soil must have contained largo amounts of some substances that were necessary for vigorous plant growth and that these were removed by the successive crops when harvested. The rapid decrease in fertility finds its most rational explanation on this basis. Changes in climate and physical condition of the soil are inadequate as explanations for this decreased productive power.

A description of the elements important to agriculture has already been given and the very reason for their importance to the farmer lies in the fact that they are the elements which constitute the compounds of plants and are removed from the soil when the crop is hai*vested.

Source of elements. However, not all of the elements described have come from the soil. Plants obtain the elements of which they are built up partly from the soil and partly from the atmosphere. From the soil they obtain by means of their roots all their ash constituents, all their sulphur and phosphorus, and in most cases, nearly the whole of their nitrogen and water. From the atmosphere they obtain, through the instrumentality of their leaves, the whole or nearly the whole, of their carbon. There are exceptions, especially in regard to nitrogen, which is obtained from the atmosphere by certain plants, such as alfalfa, clover, vetch, pea and bean, under certain conditions to be described later.

Composition of the plant. The most abundant ingredient of a living plant is water. Many succulent vegetables, as the turnip and lettuce contain more than ninety per cent of water. The green com plant contains eighty-five to ninety per cent of water.

Combustible part of plants. If a stalk of com is dried and burned the greater part is consumed and passes away in the form of gas. But there is always left behind a small quantity of white

IntroducUon. 19

ash, coiTespondiug exactly to the ash left in the stove after a stick of timber is burned.

The constituents which form the dry matter of plants may be divided into two classes — the conibustihle and the non-cornbustible part. The combustible part of plants is made up of six chemical elements — carbon, oxygen, hydrogen, nitrogen and sulphur, with a small amount of phosphorus. Without these no plant will grow. Carbon generally forms about one-half of the dry combustible part of plants. Nitrogen seldom exceeds four per cent of the dry matter and is generally present in much smaller amounts. Sulphur and phasphorus are still smaller in quantity. The remainder is made up of oxygen and hydrogen. The carbon, hydrogen and oxygen form the cellulose, starch, lignin, gummy matters, sugars, fats and vegetable acids which plants contain. The same elements united -with sulphur and nitrogen form the very important proteins, which are the life centers of the plant. "When all the above elements are united to phosphorus, we have additional important groups of plant compounds, called nucleins and lecithins.

Non-combustible part of plants. The non-combustible or ash constituents form generally but a small part of the plant. A fresh, mature corn plant will contain about 1.2 per cent of ash, while the corn grain when dry, contains about 1.5 per cent. In the straw of cereals the ash constitutes 4-7 per cent and cereal grains 2-3 per cent of the dry matter. In hay 5-9 per cent will be found. We find in leaves, especially old leaves, the greatest proportion of ash. In the leaves of root crops the ash will amount to 10-25 per cent of the dry matter.

Essential elements. The non-combustible ash always contains six elements — potassium, magnesium, calcium, iron, phosphorus and sulphur. It was once thought that these ash elements were accidental, simply dissolved in the soil water and absorbed by the plant and that they were not essential to its development. Liebig proved that they were necessary; seeds were planted in pure quartz sand contained in a series of pots to one of which nitrogen

Agricultural Chemistry.

(ioinpoimds alone were added, and to the others, nitrogen componnds phis a small amount of plant ash. The plants in the pots which received the ash grew to maturity, while those in the other pots made only a feeble growth.

4 3 12 5

Water cultures of buckwheat. This method of experimental culture, "Which is known as water culture, has been of the greatest service in determining which elements are essential for plant growth. No. 1. Plant grown in normal solution.

No. 2. Plant grown in normal solution without potassium. No. 3. Plant grown in normal solution with sodium instead of

potassium.

No. 4. Plant grown in normal solution without calcium. No. 5. Plant grown in normal solution without nitrogen.

Non-essential elements. Besides the elements just named an ash will generally contain sodium, silicon, chlorine, and frequently manganese, and perhaps minute traces of other elements. These elements just named sometimes form a considerable portion

Introduction. 21

of the ash. For the reason that plants have been brought to maturity in their absence, it has been generally accepted that they are non-essential. However, it is necessary to remember that such experiments have generally extended over a single generation and that it is possible that an attempt to grow the crop through successive generations from its own seed in a soil devoid of sodium, chlorine, silica, or manganese might meet with failure.

How ash elements occur. The ash elements named above occur in part in the plant as salts, being combined with phosphoric, nitric, sulphuric and various vegetable acids of which acetic, oxalic, malic, tartaric and citric acids are the most common. It is also very certain that part is in combination with the organic or combustible part of the plant. Sulphur occurs partly as sulphates and also as a constituent of proteins. Phosphorus as a phosphate in the stem and root of the plant, but in organic form in its seeds. In addition, such ash elements as potassium, magnesium, calcium, iron and silicon are very probably in part constituents of the organic compounds of plants.