Comprised of 40% rock particles, 25% water, 25% air, and 10% organic matter. It has 3 main uses: provides an anchorage and main support for the plant, furnishes the supply of water, and contributes mineral salts essential to the plants successful activity. Soils vary much in physical texture, chemical composition, depth, origin, richness, and other respects, but all are normally made up of a mixture of distinct components, each of which has its particular influence upon the life of plants. These components are rock-particles, water, air, humus, dissolved substances, and organisms.
Makes up the bulk and basic material of a soil and is composed of small, angular particles which have been formed by disintegration of rock. These make up 90% of the weight of ordinary good soil, furnish the necessary anchorage for the plant, and, through the substances dissolved from their surfaces, contribute the supply of available nutrient materials. The particles vary greatly in size, from those of fine clay to those of coarse gravel. They also differ in shape and chemical composition according to the type of rock producing them. The irregularity of contour which these particles display makes it impossible for them to fit very closely together, and a considerable amount of space (pore-space) is thus left between them which bay be occupied by air or water. In soils which are in good condition for the growth of ordinary plants the particles cohere in groups to form crumbs or floccules, the component grains of which are held together by water-films or by such a cementing substance as clay. One important purpose of tillage is to impart this crumb stucture of flocculation to a soil. At the soil surface, by the direct action of the rain or by other means, these crumbs may be broken into their constituent particles, which then pack closely together and on drying harden into a firm, clay-like crust.
Of vital importance, and in most cases the only one, is the rain which falls upon the soil surface. Various fates await this water. A considerable part of it may not enter the soil at all if the surface is hard or the rainfall heavy, but may drain away instead. This run-off is lost to plants, and may even do harm by washing away a portion of the soil itself. The water which does enter the soil may either percolate downward between the particles under the influence of gravity, or may be held in the soil by capillarity. Percolating or gravitational water passes downward rapidly if the soil particles are coarse, more slowly if they are finer, until it arrives at a level where all the soil spaces are filled with standing or hydrostatic water. This level is known as the water-table. Its position at any given point determines the height at which water will stand in a well dug at that point, and its distance below the surface varies from place to place and is subject to much fluctuation. A similar saturated condition occurs in the upper soil layers after heavy rains, but persists there for only a short time. When water has percolated downward to this level it is often beyond the reach of roots, and is thus quite unavailable to plants. Capillary water is water held in the soil by force of capillarity. Common observation teaches that when an object (such as ones hand) is immersed in water and then lifted out again, some water still adheres to its surface in a thin film, or “wets” the object. This is due to the fact that there is greater attraction between the surface of the object and the water than is exerted by the force of gravity or the cohesion of the water particles themselves. Any material with a large amount of surface, internal or external, which may be wetted (such as a sponge, blotting paper or coarse fabric) will therefore hold within itself, when thoroughly soaked, a great amount of water which will not drain out under gravity. For exactly the same reason, much of the water entering a soil will fail to percolate through it but will instead adhere in thin films to the surfaces, very great in total area, which are presented by the multitude of soil particles. If the amount of rainfall is small, all of it may thus be retained and none lost through percolation. Each particles in such a moist soil is covered by a thin layer of water. The films about adjacent particles coalesce, filling the minuter spaces and lining those that are larger, and a continuous film-system is thus setup. It is this film or capillary water which furnishes plants with the great bulk of their water-supply. One of the important objects in manipulating a soil is to increase, by one means or other, this water-holding capacity, and thus to prevent waste through run-off or percolation. The principle of capillarity is of further importance in determining all movements of water in the soil other than the downward one due to gravity. The familiar fact that when a narrow glass tube is placed in water, the water will rise inside the tube to a point somewhat higher than its level outside, is due to the attraction between the surface of the glass and the water, an attraction which is sufficient to lift water against gravity. The lifting force will be proportional to the exposed surface of the tube, and therefore where the volume of water is small in relation to this surface (as is the case inside the tube) the water will rise somewhat before the weight of the lifted column counterbalances the pull exerted by the surface attraction. Obviously, the narrower the tube, the higher the column of water will rise, since the volume of liquid to be lifted will be smaller in proportion to the area of the attracting surface. Thus, in any material the structure of which presents a great amount of surface surrounding small but communicating tubes, pores or other narrow spaces, as in blotting paper, lamp-wicks, and the like, water will evidently be carried to a considerable distance in all directions by capillarity. Just such a material as this is the soil. The multitude of its tiny particles, packed closely together, form a capillary system which is able to carry water far. This water tends to surround each particle in a thin, capillary film, but if the soil particles are very coarse the film cannot pass around them, and under such conditions the ascent of water necessarily stops. Water moves readily within the films and when those at the top of the ascending column are thinned through evaporation or through the attraction exerted by still higher and unwetter surfaces, the films below are drawn upon, and water passes upward through the whole system. This movement continues until the weight of the water lifted balances the surfaced attraction at the top of the column. The height to which water will rise by capillarity is dependent chiefly on the size of the soil particles; for the smaller the particles, the larger will will be their surface in proportion to the spaces between them, and thus the higher will be the rise of water. In ordinary soil this rise varies roughly from two to twenty feet. It is evident, therefore, that water which has percolated very far below the surface ordinarily cannot be made available to plants again through capillary ascent. In most soils there is a capillary movement of water toward the soil surface, where it evaporates. If the particles at the surface are very close together, as they are where the soil has been packed down or where a crust has been formed, a very efficient capillary system is produced there which connects the soil surface with deeper water-holding layers, and thus greatly hastens the loss of water by drawing it up to a point where it may be evaporated. An important purpose of tillage is to prevent such waste of water by breaking up the capillary system at the surface and forming there a layer of loose, coarse material called a mulch. Capillary movement of water is by no means always vertical but may take place in all directions within a soil, just as ink spreads in all directions in a piece of blotting paper. This movement tends to continue until the water films are of equal thickness throughout the entire soil mass, causing it to be uniformly moist. When water is removed at any particular point, as by surface evaporation or root absorption, it is therefore darawn thither from all other points until equilibrium is restored. In soils which have lost all their capillary water by evaporation, there still remains around each particle an exceedingly thin film of hygroscopic water, which clings so tenaciously that it may be driven off only by subjecting the soil to a high temperature. When the air is very dry, this water is present in minimum amount, but when humidity rises, more water may be taken up directly from the air, or hygroscopically. This type of water is removed with such difficulty from the soil particles, however, that the plant is able to obtain little or none of it.
Since oxygen is essential for the healthy growth of ordinary plant roots, the presence in the soil of a plentiful supply of air is a matter of vital importance. If the spaces between the soil particles become filled with water, most of the air is necessarily driven out, and when this condition of saturation is long maintained, ordinary plants suffer. We have seen, however, that such excess of water normally passes downward by percolation, and as it does so the soil spaces fill again with air. In most cultivated soils, from 20-35% of the volume consists of air spaces. The composition of the air which fills these is often somewhat different from that of the atmosphere, the proportion of carbon dioxide being relatively high. Plowing tends to increase greatly the air content of a soil, since the structure of the whole mass is loosened and the crumbs are more widely separated. A soil in this condition is said to be in good tilth. Where water occurs only in capillary form much air is present in the larger spaces, and such a state of the soil is therefore clearly the most favorable for plant growth since then, and then only, is a plentiful supply of water combined with a plentiful supply of oxygen.
All rich soils contain a considerable amount of material derived from the dead bodies of organisms, particularly plants. Roots which die and remain in the soil, and leaves and other plant parts which fall on the soil surface, are the sources from which this organic matter is mainly derived in nature. In the practice of agriculture it is increased in amount by various artificial means. After entering the soil it soon begins to undergo decomposition, and for the most part is finally broken down into simple end-products - carbon dioxide, water, and ammonia. As this organic material decays it becomes characteristically dark in color and undergoes a series of complex chemical changes. In this condition it is known by the general name of humus. Humus is of importance to plants in many ways. It improves the physical condition of the soil, for because of its coarse and fragmentary character it tends to separate the particles and thus increase materially the air-content of the soil. Since humus absorbs water readily, its presence also adds to a soils water-nutrient materials, notably an abundant supply of nitrogen compounds, which ultimately become available to plants. Humus is also the seat and food-supply of the soil bacteria, minute organisms which are indispensable in plant nutrition. Any treatment of the soil which will increase its humus content will therefore tend to increase its productivity, and whatever decreases the humus content will impoverish the soil.
Soil water is by no means pure water but carries dissolved within it a great variety of substances. Anything which is to be taken in by the roots of plants must be in solution, and it is consequently obvious that these dissolved substances are the only portion of the soil, aside from water itself, which is directly available as nutrient material for plants. Their origin and chemical composition are therefore of much importance botanically. The solvent power of soil water is increased by the presence within it of carbon dioxide, liberated in the respiration of plant roots and of the lower organisms. Thus reinforced, water not only attacks the surfaces of the rock particles but absorbs any soluble material which may appear in the humus or as a product of bacterial activity. There is a great variety of substances present in the soil solution, and we know from chemical analysis of the ash (reside left after complete combustion of the plant tissues) of plants that very many of them may be taken into the plant body. Compounds of nitrogen, sodium, potassium, calcium, magnesium, iron, manganese, aluminium, phosphorus, sulphur, chlorine, and silicon are commonly absorbed by the roots, and many others may be taken up occasionally. Certain of these elements are far more important to the plant than others, however, and it has been clearly proven by experiment that seven are essential for normal plant growth: sulphur, phosphorus, calcium, magnesium, potassium, iron, and nitrogen. The actual amount of these mineral nutrients taken up by the plant is exceedingly small in proportion to the size of the plant body, but in the activities of protoplasm each plays a very necessary part, and a soil which is deficient in any one of them will be unable to support vegetation successfully. The removal of large amounts of nutrient materials from agricultural soils, in the form of crops and in other ways, reduces the available supply of certain chemical elements, notably nitrogen, phosphorus, and potassium, to such an extent that a fresh supply must be returned to the soil if abundant plant growth is to be maintained permanently thereon. This necessitates the common practice of adding to the soil various types of fertilizers which renew the supply of essential salts there available to plants.
Aside from its service as a medium for the root growth of higher plants, the soil provides a dwelling-place for a great variety of other organisms, whose activities have a profound effect on the composition of the soil and on the processes which go on therein. Rodents, insects, and angleworms all modify the physical character of the soil by their abode within it. Those most minute and lowly of living things, however, which we group together as micro-organisms are of far greater importance, for experiment has shown that without their presence the soil would soon become unfit to support a vegetation of higher plants. Most notable among these micro-organisms are the bacteria, tiny, single-celled plants which lack the green pigment chlorophyll. Many of these - the bacteria of decay - decompose the complex organic substances found in humus into such simple end-products as carbon dioxide, water, and ammonia, thus releasing great quantities of nutrient materials which would otherwise be locked up and useless in dead bodies of animals and plants. Still other bacteria in the soil cause chemical changes of various sorts there, the results of which are of great moment to the higher plants. Notable among these are bacteria concerned with the transformations of nitrogen and its compounds, for through their activity alone is the available supply of this necessary element maintained in the soil. The continual circulation of nitrogen through its various successive stations in organism, air, and soil is known as the Nitrogen Cycle. Complex nitrogenous substances returned to the soil in the bodies of dead animals and plants are broken down by the bacteria of decay into simpler compounds, which are finally reduced to ammonia. Since most plants can use nitrogen only when it occurs in the form of nitrate salts, however, this ammonia is not directly available to them but must first be converted into nitrate salts through the process of nitrification. This is carried on by two types of nitfifying bacteria; the nitrite bacteria, which change ammonia to nitrites, and the nitrate bacteria, which in turn converts nitrites into nitrates. In this form nitrogen is readily absorbed and assimilated by plants, and is ultimately returned to the soil again in the bodies of plants or animals, thus completing the cycle. Through the activity of another group of these minute organisms, certain of the seed plants are also able to take advantage of the enormous supply of nitrogen in the atmosphere, which is ordinarily quite unavailable. These are the nitrogenfixing bacteria. They are present in most soils and cause the development of the tubercles or nodules usually found on the roots of plants belonging to the Legume family, which inclues beans, peas, clover, alfalfa, and similar plants. These bacteria are able to absorb the free gaseous nitrogen of the air and to build it into nitrogenous compounds in their bodies, whence it ultimately becomes available to the particular plant on the roots of which the bacteria grew. Without drawing at all upon the nitrogen compounds in the soil, a leguminous plant is consequently able to aquire an abundant supply of this important element. In the case of many species of plants, particularly those which grow in forests or other situations rich in humus, thread-like filaments of fungi are intimately associated with the smaller roots, entering their outer tissues and surrounding the root with a web-like jacket of fungus threads. These very largely take the place of root-hairs and aid the plant in absorbing water and nutrient material from the soil; and the fungus, as well, is evidently benefited by such relationship. This root-fungus association is known as mycorrhiza. Certain plants have become so dependent in this way upon particular species of fungi that they cannot thrive when these fungi are absent.
In conclusion, we may emphasize again the extreme complexity of the soil and the vital significance to plants of its composition and of the changes which go on within it. The study of this remarkable material has required the collaboration of almost all of the sciences, but we still lack a precise knowledge of many of its aspects and fail to understand clearly the manner in which it affects the life of plants growing in it.