SOILS AND FARMING

CHAPTER II

THE NATURE OF SOIL

IF WE take up a handful of mellow soil and look at it closely, we can see only a crumbling mass of particles, intermixed with black bits of decayed and decaying vegetation. There seems to be no life in it. Put a bit of this soil on a glass slide and look at it under a powerful microscope; a scene of constant activity is now revealed. Moulds, ferments, decays, bacteria, and other organisms are constantly at work, destroying, creating, changing the structure and the agricultural value of this soil. Currents of water pass through it; waves of heat quicken it. The tiny particles of rock are ground and worn smaller each year, and the plant foods are changed from one form to another. The soil has a flora and a fauna scarcely less complex than that which clings to its surface. Little is now known about the soil as compared with other agricultural subjects; it is remarkable that the soil, the foundation of agriculture and the beginning of all wealth, should have received so little minute study. We may expect the present deep interest in soil physics and soil bacteriology to greatly increase our knowledge of this most important factor in successful farming. Some of the significant facts about the nature of the soil, according to present knowledge, are considered in the following paragraphs.

THE FINENESS OF SOIL
It was stated in Chapter I that the basis of most farm soils is rock, ground into "rock-meal" by Nature's millstones, the air, water, frost, ice and other elemental forces. At first the soil particles are very large, mere fragments of rock at the base of a cliff, but upon these wild morning-glories or mulleins may be able to grow. Some hundreds of years later these small rocks will be finer; perhaps they will average less than one-quarter inch in diameter, and they will be mixed with humus. The fining process goes on a few generations or centuries more, until the pieces of rock have been broken into such small particles that farm crops thrive upon them. Nearly every soil is constantly becoming finer. All soils that contain small rocks or pebbles receive from them each year many particles of soil by weathering, and the size of the rocks and pebbles is reduced that much. Even the rich prairie loam or alluvial clay, which is apparently all soil and contains no rocks or pebbles at all, is becoming finer. Weathering is much less active on such soil, however, than on gravelly and stony soils.

The number of individual particles in a fertile soil is astonishing to those who have not tried to count them under a microscope. A good corn soil has about 280,000,000,000 particles in an ounce, while the clay loams that are preferred for grass often have 400,000,000,000 particles in an ounce. These particles are of varying sizes and shapes, even in the same soil. Sometimes they are uniform and rounded, and pack together poorly, leaving large spaces between them, like marbles piled together. Sometimes they are uneven and jagged, packing together tightly, like the crushed rock of a macadamized road.

The spaces between the soil particles differ in size and shape, according to the size and shape of the grains. I have met a farmer who could not quite see how a soil could contain air at a depth of four feet, yet he admitted that there must be air at the bottom of his wheat bin. The trouble was he looked upon the soil as a solid mass, since he could not see the spaces between the grains with his naked eye as he could in wheat. If he would think of his soil as a bin of wheat, with the kernels about one-millionth as large, he could see how it is that air and water pass freely through all ordinary soils, and to a great depth.

It is of practical as well as of scientific interest to know about the size of the grains of a soil, and the size of the spaces between them. The value of a soil for certain crops depends quite largely upon just such factors. With the refinement of soil surveys and methods, soil experts assure us that they will be able to tell us with a fair degree of certainty that soils containing, for example, from 250,000,000,000 to 350,000,000,000 particles per ounce are adapted for potatoes; soils containing 350,000,000,000 to 450,000,000,000, for onions, and so on. At present we classify soils and judge their adaptability for certain crops in grosser terms; we say potatoes do best on a sandy loam, and that an alluvial clay loam is excellent for onions. There are limits to the practical value of this information, for the fineness of the soil is but one of many factors that determine the adaptability of a certain soil for a certain crop; yet this one point is extremely valuable to know when selecting land for special crop farming.
Fineness is Richness.—The fineness of the soil has a very important bearing upon its fertility. Other things being equal, the finer a soil is, the richer it is, because it contains more surface for the roots to feed upon. The rootlets of plants do not suck up particles of soil, as Jethro Tull supposed, in his now famous "Horse-hoeing Husbandry." They feed upon the film water upon the outside of the soil grains. This contains much plant food dissolved from the grains. The natural agencies that dissolve plant food from the soil—water, air, etc., act only on the outside of the particles. Hence the more surface there is to the grains, the greater is the "pasturage," or feeding area for the rootlets, and the more rapid is the weathering. If a small stone is broken into six pieces, the pieces have several times more surface, in the aggregate, than the unbroken stone. It has been calculated that if every particle in one cubic foot of mellow soil could have all its surface spread out flat, the aggregate surfaces of all these grains would cover about one acre.

The presence of small stones and pebbles in a soil is beneficial, making it lighter, more porous, and warmer. It would be a great calamity if all soils contained no pebbles and larger pieces of rocks. These are a store of plant food which is added to the soil from year to year. Yet the farmer should remember that, in general, fineness means richness. If a soil is lumpy, because of lack of humus or excessive moisture, its available feeding area is greatly reduced. This matter is considered more fully in succeeding chapters, where practical methods of making a soil fine and mellow are described.
This depends upon their composition and compactness. It is of interest to the farmer chiefly as an indication of the amount of vegetable matter that a soil contains, because this influences its value for cropping. The coarser the grains, the heavier the soil; humus makes a soil lighter. A heavy soil—one weighing over 80 Ibs. per cubic foot—is likely to be benefited by the addition of humus. As the term is commonly used, however, a heavy soil is one that is tenacious, and refers to texture, not to weight.

Schubler gives the average weight of a cubic foot of dry soil as follows:
Sand ......... ........................ 100 Ibs.
Garden Soil rich in humus. ..........70 Ibs.
Peat Soil. ...........................30—50 Ibs.
The weight of the soil on an acre of land is so great that if a very small percentage of it is plant food this may amount to a very large quantity per acre. An acre of clay loam, nine inches deep, weighs about 3,000,000 to 3,500,000 Ibs. Suppose this soil contains only one-tenth of one per cent, of nitrogen, which is an average amount of that plant food; the acre would contain, in the first nine inches only, 3,000 to 3,500 Ibs. of nitrogen. Compared with this amount, the 30 to 75 Ibs. of nitrogen that we apply as a fertiliser to an acre of impoverished land is a mere bagatelle.

THE MINERAL CONTENTS OF THE SOIL
The basis of most farm soils is rock that has been ground into very fine particles by frost, air, water, etc., and mixed with the remains of plants and animals. The value of decayed vegetation, or humus, in a soil is so great, and the farm practices resting upon this fact are so important, that this matter is treated in a separate chapter (XII).

The mineral contents of a soil depend upon the kind of rock from which it has been made. These rocks are of many kinds; the nature of a soil may often be determined by seeing specimens of the rocks it contains, provided the soil is south of the region that was covered by the great soil mixers, the glaciers. There is no mineral in any soil that cannot be found in the rock from which it came; there is no mineral in any plant that is not in the soil from which it sprang.

Soil is being made from many kinds of rocks, principally quartz, feldspar, mica, apatite, zeolites, hornblende; and various combinations of these, as granite, which is made of quartz, feldspar, and mica. Quartz and feldspar form the largest proportion of most soils. The chief constituent of all soils that have been made from rocks is silica (pure sand), which is the principal ingredient of quartz. This is because silica is the hardest kind of rock material and hence it is not dissolved and lost as rapidly.

The rocks of the earth, and the soils made from them, contain from 65 to 70 so-called "elements," the simple ingredients; as iron, carbon, oxygen. These elements, however, unite with one another to make innumerable " compounds," or combinations of several elements. This might be illustrated by saying that eggs, salt and milk are the elements or ingredients of a compound—omelet. It is the great number and the intricacy of these compounds that make geology and chemistry so complex.

No one kind of rock contains all the elements, but all of the rocks from which fertile soil is made contain at least seven of them—nitrogen, potassium, phosphorus, calcium, iron, magnesium, sulphur. No plant can grow unless these seven are present in the soil; they are the "plant foods," and constitute from 80 to 90 per cent of most fertile soils. The first four of these seven are much needed by plants and so the soil is most likely to be exhausted of them by continuous cropping; while the latter three are usually so abundant that the farmer is never concerned about how he may add them to his soil. The nature and sources of these four essential plant foods, nitrogen, potash, phosphorus, and calcium, which are the necessary constituents of fertilisers, are discussed in Chapters XI to XIV.

Besides these seven elements in the soil which are absolutely necessary for the growth of plants, a number of others are frequently absorbed by the roots of plants and used by them. Of these the most common are chlorine, silicon, aluminum, and manganese. Numerous experiments have shown that plants thrive as well without these as with them, so they must be considered as accidental or unnecessary elements.

In considering the mineral contents of the soil as a supply of food for the growth of plants, we must not forget that the soil furnishes but a small part of the material out of which plants are made. We are so actively engaged in trying to keep up the fertility of our soils by checking their wastes, and by adding to them fresh supplies of the minerals that our crops have taken from them, that we are apt to think that the plant comes from the soil alone. Yet over 90 per cent, of the crops that we remove from a soil comes from the air. The air, not the soil, is the greatest storehouse of fertility. From the air plants get, through their leaves, three other foods—oxygen, hydrogen and carbon. These are all gases, the latter being combined with oxygen in the form of carbonic acid gas. The supply of these plant foods is, so far as we know, inexhaustible. A friend once remarked, "That is mighty lucky. I have a hard enough time now, trying to supply my worn out soil with enough potash, phosphoric acid and nitrogen to grow profitable crops; yet you say these are only side dishes of a plant's dinner. If I had to supply it with the main dishes, or fillers, as you might call these foods that it now gets from the air, I don't believe I could have raised my family of six on these forty rocky acres of New England soil."

HOW WATER IS HELD IN THE SOIL
All fertile soils contain many tons of water, which is present in the soil in several forms. First, and most conspicuous, is what is variously called free water, ground water, standing water or bottom water. This fills all the spaces between the particles up to a certain height, which varies with different soils, and even different parts of the same field. Free water is supplied by rainfall; it frequently comes to the surface as springs and is often the source of supply of wells. If a hole is dug in any soil water will stand in it up to a certain point, which may be several inches or many feet below the surface. This point is called the "water table." The height of the water table may be judged in a general way by the depth of surface wells, but this evidence is not always reliable. It may vary at different times during the year, according to the dryness of the season
We must consider, then, that beneath all farm soils, at some depth, is standing water; that we plow and harrow above subterranean lakes, which are no less lakes because the water is not entirely free but merely fills the spaces between the particles of soil. The importance of this fact lies in its influences upon the production of a crop. If it is only two or three feet from the top of the soil to the surface of the lake, there is not enough dry soil on top for roots to grow in and the plants drown. Such soils are said to be shallow; they are of little value for ordinary farm crops until ditched or under drained and the level of the underground lake lowered thereby. The draining of land is considered in detail in Chapter IX.

Film Water.—Water is also present in all farm soils as film moisture. Above the water table is the soil in which the roots of farm crops forage. This soil must be moist, else plants would not grow in it; but water does not fill all the spaces between the soil grains, as it does below the water table. If we look at a handful of this soil we cannot see water standing in it, but it feels moist. The water is sticking to the soil grains, covering them with a very thin film, as when small stones are dipped in water. It is held close to the grains by surface tension, or adhesion. If this soil were put in an oven and heated, the film water would be driven off as water vapour, and the soil would be left perfectly dry.

There is always a large amount of film water clinging to the grains of every soil, even in the dryest season. The dryest road dust has some film water clinging to it. The amount of water that can adhere to a single grain of soil is, of course, infinitely small, but the amount of water that can cling to all the soil grains of a field is enormous, especially when we consider the vast surface area of the grains, in the aggregate. A good farm soil often holds more than one-half its weight of film water.

Film water is far more important in farming operations than free or bottom water, for it is the direct supply of plants. No common farm crops can thrive in free water, but all must have a large area of soil that is moist with film water. Much of this supply of film water, however, is drawn from the natural reservoir of free water below.

Water absorbed from the air.—Under certain conditions the soil absorbs a small amount of water from the air. The air that fills the spaces between the particles of soil usually contains much water vapour; if the soil becomes very dry it may absorb some of this. The surface soil may also absorb water vapour from the air, especially when there are heavy fogs. This "hydroscopic" water, however, is not of much importance as a means of supplying plants with water, except in a time of great drought.

THE TEMPERATTJKE OF THE SOIL
The soil must be warm in order to produce crops. Most farm soils of the United States are not likely to become too warm for ordinary crops; there is far greater likelihood that they may be too cold. This is especially true in the Northern States, where the season is short, and it is very often desirable to make the soil warmer, particularly in early spring. The seeds of most cultivated plants will decay before they have had time to germinate if the temperature of the soil is below 45°; the colder the soil, the slower the seeds germinate. Only after the soil has reached a temperature of 65° to 70° do most crops grow well in it. The soil temperature that is considered most favourable for the germination of barley has been determined by experiment to be 61° to 70° F.; of clover, 77° to 100° F.; of pumpkins, 100° F.; of tomatoes, 100° F.

The growth of a crop after germination is influenced fully as much by the temperature of the soil as is the sprouting of the seeds. The farmer knows that certain crops, as onions, barley, turnips, parsnips, peas and potatoes, are "cool plants"; they can be sown early when the ground is cold, and thrive in the coolness of spring. Others, as corn, tomatoes, melons and squashes, are "hot plants"; seeds of these do not sprout well if sown very early, and the plants do not begin to grow satisfactorily until there have been summer days to warm the soil thoroughly and deeply.

The Temperature of Different Soils.—The temperature of a soil depends upon many factors, most of which are beyond the control of the farmer, but some of them he can regulate by comparatively simple means. The temperature of every soil varies widely with the season, and from day to night. The surface soil becomes warm on a hot day and cools several degrees at night, but this fluctuation rarely extends below two and one-half feet. At a depth of thirty feet the soil temperature changes little if any throughout the year, even in the Northern States. Much also depends upon the materials of which the soil is composed. The coarser it is, the warmer it gets, and the better it holds the heat; hence gravelly and sandy loams are among the earliest and warmest of soils. In Europe, gardeners sometimes put loose gravel around grape vines to keep them warm during the night. But a soil in which the particles are very small, as in clay, warms much faster than sand because the particles lie so close together that the heat passes more readily from grain to grain than in sand where the grains lie loosely. For the same reason a clay soil loses more heat by radiation than a sandy soil. Moreover, a clay soil holds more water than a sandy soil and so loses more heat because of the larger amount of evaporation. Hence, fine-grained soils, though they absorb more heat than coarse-grained soils, are colder. Sandy soils are "warm," clay soils are "cold."
Draining a Soil Warms it.—The warmth of a soil comes chiefly from the sun and incidentally from the fermentation and decay of the vegetable matter and other refuse that it contains. The temperature of a soil is modified most by the amount of water it contains. Wet soils are cold. The wetter a soil is the colder it is, at least during the summer, when warmth is needed most. It is the coolness as much as the excess of moisture and lack of air that makes corn with "wet feet" grow poorly. The chief reason for this is that it takes a large amount of heat to evaporate the excess water from a soil, and also much heat to warm the wet soil that remains, water being a poor conductor of heat, the evaporation of one pound of water from a cubic foot of clay soil makes it 10 degrees cooler. There may be a difference of 7° to 10° in the temperature of a well-drained loam and a poorly drained soil of the same character. There is one exception to the statement that the wetter a soil is the cooler it is.
In early spring we frequently have warm rains that raise the temperature of the surface soil several degrees. It is after these rains that "things just jump."

Fortunately the means of controlling this factor is largely in the hands of the farmer. The excess water may be removed, and the soil warmed by draining it. The draining of land by deep plowing, ditching, tiling and other methods is considered in Chapter IX.

Influence of Exposure on Warmth of Soil.—The "lay of the land" with reference to the compass, and the steepness of the slope, have an important influence on the warmth of the soil. The soil on a northern slope—which receives about one-third less sunshine than a southern slope, depending upon its steepness—may average 7° to 10° cooler in summer than the soil on a southern slope. The soil of a gentle southern or western slope may be 3° to 5° warmer than the same kind of soil is on a level. In the northern part of the United States the sun is always more or less in the south, so that its rays never strike level soil squarely. It is farthest in the south when the need of greater soil warmth is most likely to be felt. In early spring a slope of 12 to 15 feet in a hundred will catch the largest number of the sun's rays, being most nearly at right angles to them. Many of the rays glance off from the level land because they strike it obliquely. The practical conclusion is that a moderate slope to the south or southwest is the best site for a crop when earliness is desired; which is what husbandmen, especially fruit growers and gardeners, have known and practiced for centuries.

Dark-coloured Soils Absorb More Heat.—The colour of a soil is often some index to its agricultural value and has an important influence on its temperature. A dark-coloured soil is usually warmer and earlier than a light coloured soil. All dark substances absorb more of the sun's rays than light substances. That is why we wear light-coloured clothes in summer, and partly why snow melts faster on the dark-coloured, plowed ground than on the meadow. In Switzerland farmers sometimes hasten the disappearance of the snow by strewing it with black, powdered slate. Gardeners sometimes sprinkle a light coloured soil with peat, charcoal and bog mould; these are called "sun traps." Melons are ripened in Saxony with the aid of a layer of coal dust. But although colour has an important influence on the power of a soil to absorb heat, it has not ability to retain heat. Schubler states that, other things being equal, a dark coloured soil is about 8° warmer near the surface than a light coloured soil.

This difference in the temperature of soils, due to colour, may have a marked influence upon the growth of a crop, especially on its germination. When earliness is a prime consideration, as it is with most market-garden crops, the colour of a soil may become very important. Dark, sandy loams, rich in humus, are preferred by market gardeners. Light coloured soils may be made dark by filling them with humus. Two or three green manuring crops plowed under will darken a fight-coloured soil quite noticeably. I have a neighbour who, in three years, has transformed a poor, yellow soil into a black, retentive and productive loam by plowing under four inches of composted manure every fall. Another neighbour, under similar circumstances, has accomplished nearly as good results by plowing under muck drawn from a near by swamp. The chief reason for adding humus to a soil is to improve its texture, but another benefit, and one that is often quite important, is to improve its colour.

The buff yellow and yellowish-brown colours of soils are usually due to the presence of iron oxides. These soils are most common south of the glaciated part of the United States, particularly in the southern Appalachian states.

The Influence of Tillage on Soil Temperature.— The way in which a soil is handled has much to do with its warmth. Uneven, ridged soil, like that left by fall plowing, loses more heat than smooth, level soil. However, ridging may warm the soil by drying it, and this usually more than counterbalances the loss of heat because of the greater surface exposed. Rolling land in fair and warm weather makes it warmer, but rolling it in cloudy and cold weather, especially if it is wet, makes it colder. Deep plowing makes the soil cooler, because loose soil is a poor conductor of heat. The decay and fermentation of farm manure plowed into a soil may raise its temperature several degrees; it produces as much heat in the soil as it would if burned in the open air. Manured soil is usually about 2° warmer in spring than unmanured soil. Thorough tillage, especially in the preparation of a seed bed, has a marked influence on soil temperature; it prevents the evaporation of soil moisture and hence keeps in the soil the large amount of heat that it takes to evaporate water. Good tillage saves heat, then, as well as water, especially in early spring. This means that the soil for early crops should be plowed early and tilled often.

THE VENTILATION OF THE SOIL
The spaces between the soil grains are filled either with water, or air, or both. This soil air is somewhat different from the free air above the surface, containing less oxygen, more carbonic acid gas and more ammonia gas. Part of its oxygen is used by the plant roots; the other gases are absorbed from the vegetable matter decaying in the soil.

Practically all of our farm crops need a well ventilated soil. The roots of plants, except certain bog, marsh and water plants, must have air to breathe. If it is denied them, because the interspaces of the soil are filled with water, the plants will die. Corn is "drowned out" in low, wet places, chiefly because the roots cannot breathe. Furthermore, air is needed in the soil to make more plant food. The air penetrates deeply into the soil and its oxygen, carbonic acid and ammonia dissolve the minerals and make the soil more fertile. The nitrogen of this air may be used as a food by certain plants (See Chapter XII), The oxygen of the soil air combines with the nitric acid produced by the decay of plants, making it a nitrate, which is a plant food. Manure which is piled loosely, so that air penetrates it readily, heats quicker and stronger than tightly packed manure; likewise a soil that is well drained and open, so that air passes into it freely, has more life, fermentation and fertility in it than a close-grained, air-tight soil. Air may penetrate the soil to a depth of many feet, depending upon its openness. Soil air changes in temperature like surface air, and continually passes up and down in currents.

Methods of Improving the Ventilation of Soil.— Any kind of tillage which stirs and loosens the soil, like plowing and harrowing, promotes a better aeration, or ventilation, of the soil. Plowing under farm manure, green manure or stubble also has the same desirable effect, since the humus thus produced separates the particles of soil and renders it more porous, hence more open to the downward passage of air. Under draining, however, is the chief means of ventilating a heavy soil. Remove the water and the air will rush in. When the water table is lowered two or three feet, as it may be by under-draining, the roots of plants grow deeper; when they decay, they leave little channels in the soil and through these air penetrates. Earthworms and ants still further deepen and aerate the soil by following these channels.

When land is tile-drained, the tiles themselves provide a system of underground ventilation of far reaching influence. The soil of a tile-drained field is ventilated much more thoroughly than the soil of another field of the same character in which the water table stands naturally at the same height. The air in tile drains is largely surface air.

The roots of most farm crops deepen and aerate the soil, but the roots of leguminous plants, especially of clover and alfalfa, are particularly useful in soil ventilation. This is partly because clover roots are large and bore straight down into the subsoil for several feet, leaving much larger and more effective channels for the passage of air and water than the roots of grains; and also, in a very slight measure, because these plants absorb nitrogen from the soil air, thus making it necessary for more surface air to be forced into the soil to replace that which is lost.

Fortunately for the farmer, most soils are able to absorb various gases, notably ammonia, which is very valuable for the nitrogen which it contains. Advantage is taken of this fact when decaying animal matter is buried to remove the offensive smell, and when sandy loam is used behind cows in the stable. The soil acts much like a charcoal filter which is used to remove objectionable odors from water.

THE ELECTRICITY OF THE SOIL
Weak currents of electricity continually pass through the soil and through the plants it nourishes. In recent years the effect of soil electricity on plant growth has been studied quite thoroughly. The practical value of passing moderate currents of electricity over and through the soil by means of wires has been demonstrated in several European and American fields. For this specific purpose Messrs. R. & B. Bomford, near Evesham, England, have 19 acres of land with wires suspended 16 feet above the ground so as not to interfere with steam plowing. The current discharged from the wires is generated by a dynamo. This treatment is said to increase the yields of barley and wheat 25 per cent, and give a still larger increase of straw. It makes the plants germinate quicker and grow lustier. The current is turned on morning and evening until harvest. In our own country, several small fields and greenhouse soils have been treated with electricity from wires sunk in the soil, with decidedly beneficial results. The U. S. Department of Agriculture is making a special study of this matter.

Most of the beneficial effect of electricity is probably due to the fact that it makes some of the plant foods more soluble; perhaps, also, it enables the plants to take some nitrogen from the air. Only weak currents can be used; a strong current kills the plants. It is quite doubtful whether the benefit derived from the use of a weak current will make it profitable to use electricity in general crop production, for the expense of wiring a field is large; but it may be useful in greenhouses.

GERM LIFE IN THE SOIL
No soil has exactly the same composition from year to year, or even from month to month. It is constantly receiving additions of new soil from the weathering of rocks, from the decay of plants, the deposits of winds and other sources. It is constantly losing by leaching, by erosion and by the demands of plant growth. It also has numerous activities within itself that exert a most potent influence on its fertility. Some of these activities are physical, some are chemical and some are due to germ life. A few are already known and understood, but only the merest beginning has been made in the study of soil life.

Nitrogen-Fixing Germs.—One of the most interesting phases of soil life is the process called "nitrification," due to the activity of very minute germs or bacteria, and sometimes called the "nitric acid ferment." This is somewhat like the ferment that sours milk, and the bacteria in yeast that raise bread by their growth. Although the air contains vast amounts of nitrogen, this is not used by any plants, so far as is known, except to some extent by the "legumes," of which clovers, alfalfa and vetch are examples. (See Chapter XII.) Most farm crops get their nitrogen, which they need in considerable quantities, solely from the soil. This nitrogen enters into their structure, and is returned to the soil when the plants decay, but not in the same form. It enters the plant as a salt of nitrogen—a nitrate; it returns to the soil in combination with many other substances, and is called by the chemist "organic nitrogen." The important point about this is that plants cannot use organic nitrogen, because it will not dissolve in water, and all the food that plants get from the soil must be taken in liquid form. It must first be separated from its partners in the compound, and then changed into a nitrate before the soil water can dissolve it, and the roots of plants absorb it.

The work of transforming valueless organic nitrogen into valuable nitrates, which are plant food, is performed by our tiny helpers, the "nitrogen-fixing germs." They are found in all fertile soil in inconceivable numbers, busily engaged in making plant food out of all vegetation that is returned to the soil, provided the conditions are right. One essential condition is that they have plenty of food. All these ferments may be considered very minute plants; they must have food like other plants. One food of the nitrogen fixing germs is phosphoric acid, which is also one of the most important foods of ordinary farm crops. If a soil has very little phosphoric acid in it, the transformation of humus into plant food is apt to take place very slowly. The principal food of the germs, however, is humus itself. This they can use only after the leaves, stems, or other vegetation has been thoroughly incorporated with the soil and is rotted.

These minute plants need moisture and a medium temperature in order to thrive and do their work, as the yeast ferment needs moisture and a certain temperature in order to multiply and as a corn plant needs water and hot weather in order to bring forth its increase. The growth of these microscopic soil plants is checked in very dry weather as much as the growth of the larger plants above ground. Furthermore, they do not thrive in a very wet soil. The temperatures most favourable for their growth have been found to be 54° to 99° F. In the Southern States they grow the year around. Another essential condition is a plenteous supply of oxygen, such as would be had if the soil were well drained and hence well ventilated.

It will be seen, therefore, that the conditions that favour the growth of these useful workers are those that are most necessary for the growth of farm crops—a moist, well-drained soil and thorough tillage. Given these conditions, a multitude of the germs attack the rotten leaves, stems or stubble lying in the soil, or the clover, rye or cow peas that have been plowed under, and soon change the useless organic nitrogen into a nitrate. In order to do this, however, the soil must contain a sufficient quantity of some "base," as lime, to combine with the nitrogen and so make it a nitrate. If the soil is at all acid, or sour, (see Chapter XIV), the germs cannot complete their work.
Germs That Waste Nitrogen.—It is interesting to know that there are also at work in some soils bacteria that accomplish a result exactly opposite to that of the nitrogen-fixing germs. The process is sometimes spoken of as "denitrification," and the germs may be called "nitrogen-wasting" germs. They feed upon the nitrates, and set free the nitrogen gas, which may then escape into the air and so be lost to the soil. These germs are abundant in wet soils; under-draining benefits the soil in more ways than by merely removing water. Thus these two, the nitrogen-saving and the nitrogen wasting bacteria, are pitted against each other; the one is a blessing to the soil, the other may be a detriment. It is wise farming to encourage the growth of the former by providing the conditions most favourable for them—thorough tillage and excellent drainage.

Other Soil Bacteria.—These two kinds of bacteria are but a very small part of the germ life of the soil. Adametz has calculated that there are 50,000 germs of various kinds in a single gram of fertile soil. Many are beneficial, most of them are harmless, some are injurious. When the roots and stubble of a certain crop decay in the soil, a certain kind of "ferment," which is bacterial growth, is produced. If the crop is grown for several years on the same soil, after a while the soil may become crowded with the particular kind of ferment that the decay of the crop produces. The result may be that eventually the soil will no longer produce satisfactory crops of this plant, but it will produce larger crops of some other. This is the explanation, in many cases, of "clover-sick" and "flax-sick" soils and other soils that fail to respond as they used to. The practice of inoculating soils with certain beneficial bacteria is discussed in Chapter XII, with particular reference to leguminous crops.

The limits of the practical value of soil bacteriology can only be surmised at this time; but it seems not improbable that the farmers of some future generation may be able to inoculate their soils with different beneficial bacteria and secure specific and valuable results, much as the butter maker of to-day secures certain flavours with certain cultures. The field of study opened before us by recent investigations in soil bacteriology is extremely interesting and it may yield extremely important results.

CHEMICAL CHANGES IN THE SOIL
The chemical changes that are constantly taking place in every farm soil are no less numerous and no less important than the changes resulting from the work of bacteria. The elements of which the soil is composed are always shifting and changing. The compounds, which are merely combinations of several elements, are continually dissolving partnership and the elements join themselves together in new bonds, according to affinity. The nitrogen released from a nitrate by the nitrogen-wasting germs may be instantly seized by some near-by hydrogen to make ammonia. The ammonia may then be attacked by the nitrogen-saving germs and made into nitrous acid; which, in turn, may soon become a nitrate, or it may escape into the air and be lost to the soil, until brought down by rain. The phosphoric acid that the farmer applies in superphosphate or bone meal is at once seized by hungry elements and enters into several partnerships. Some of it is readily soluble in water and might leach away were there not some lime or sodium handy to catch it. That part of it which is not used by plants the first year or two may get locked up so strongly in partnerships with other elements that it becomes valueless to plants. When a potash fertiliser, as ashes, is applied to the soil, the plant food it contains would mostly dissolve in the soil water and wash away were it not that it unites with some of the "bases" of the soil and becomes "fixed." In fact, the plant food in most fertilisers applied to soils would e quickly leached or washed away, if these chemical changes did not occur and hold it until the roots of plants can use it. Plants feed, not upon the materials that we apply to the soil—ashes, bones, phosphates, guano, and the like—but upon the chemical compounds formed in the soil by them.

These and other chemical changes that all fertilisers pass through before they are absorbed by the roots of the plants illustrate what takes place with each and every constituent of the soil, whether it is essential to the growth of the plant or not. The soil is a great chemical laboratory. Numberless reactions, or new adjustments of the partnerships between the elements, occur every hour. No chemist holds the beaker or fires the great retort; the changes take place in obedience to natural laws, quietly and methodically, yet with results so far reaching that we can hardly grasp their significance. It is the business of the chemist and the bacteriologist to explore this laboratory and report how its chemical changes are effected by the different methods of handling the soil. It is the business of the farmer to keep the soil laboratory in excellent working order, by a wise and varied husbandry; and especially by giving careful attention to those principles of good farming that we already know make it run smoothly—thorough tillage, excellent drainage, and a rotation of crops.