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Frequently, half--and often much more--of every crop plant is invisible. This portion consists entirely or largely of roots which extend far into the soil. It is very necessary, for a proper understanding of root habits and activities, to have some knowledge of the surroundings in which they grow.
The soil is the environment of many plant activities. Plants are anchored in it, water and nutrients are absorbed from it, vast stores of food are accumulated in underground plant parts, and it is in the soil where much vegetative propagation occurs. Of the two environments in which plants grow, the soil is much the more complex. This is true whether air and soil are each considered from the physical, chemical, or biological viewpoint. The soil not only affects the development and activities of roots directly, but also, by modifying the functioning of roots, it affects the growth and yield of aboveground parts. Next to the living organisms which it supports, soil is perhaps the most complex, the most interesting, and the most wonderful thing in nature. It is not mere dirt. It is a vast chemical and physical laboratory where reactions of the greatest economic importance are constantly occurring, where various physical forces interplay while tending towards an equilibrium. It is the home of countless billions of microorganisms--bacteria, fungi, and protozoa--which throng its dark passageways. Earthworms, insects, and numerous burrowing animals delve into it for food and shelter. It consists of a complex, highly organized mixture of disintegrated and decomposed rocks, humus, water, air, dissolved substances, and microorganisms.
More than 90 per cent, by weight, of ordinary, air-dried soil consists of rock fragments. Due to the action of climatic agencies, the outer portion of the solid rock of the earth's crust has changed into a loose and disintegrated condition. This layer has performed a marvelous function, for in it, with its admixture of humus, etc., a medium for the growth of plant populations has originated. These plants, through long periods of gradual change, have evolved into the present types of vegetation. Like the life which it has supported, the soil, too, has been changing and evolving, and its present structure can be comprehended only by thinking of it as highly dynamic and continually undergoing change. In fact, to a large degree, soil is a product both of climate and of vegetation. The most obvious distinction between soil and the underlying subsoil is that of color. The soil is darker brown or almost black, because of the presence of the disintegrated and altered remains of past vegetation.
Although soils vary greatly in physical texture, chemical composition, depth, origin, and richness, as well as in many other ways, all are normally made up of a mixture of distinct components, each of which has its particular influence upon crop production. In a brief chapter, the more important characteristics of soils from the particular viewpoint of their relations to root development can merely be outlined.
As already indicated, the bulk and the basic material of the soil consists of rock particles. These small angular fragments have been derived from the underlying solid rock and possibly later transported by wind or water. During the past centuries, rocks have disintegrated and are now disintegrating by the action of such forces as alternate freezing and thawing, formation of ice in pores and crevices, erosion by wind and running water, surface scouring by glaciers, and the prying action of roots. Accompanying this fragmentation has occurred the exceedingly important process of decomposition or chemical corrosion, for plants cannot grow in pulverized rock, no matter how small the particles, unless the food materials locked in these particles as insoluble compounds have been changed chemically to water-soluble substances. The latter alone can be absorbed by the roots.
The Decomposition of Rocks.--Rock decomposition is a process constantly taking place. It is brought about chiefly by oxidation, carbonation, hydration, and solution as well as by the action of soil organisms and the roots of plants. The action of oxygen in the rusting of iron in moist air is familiar to all. In rocks containing iron compounds, this oxidizing or rusting process makes these compounds more or less soluble. Enough minerals contain iron to impart a weakness to most rocks when the iron is oxidized. As the soluble compounds dissolve, the rock becomes more porous and offers more surface for physical disintegration and chemical corrosion.
The process of oxidation is nearly always accompanied by that of carbonation, in which process carbonic acid unites with bases to form carbonates. Carbon dioxide, always present in small amounts in the air, is much more abundant in the soil atmosphere. In combination with water, it forms a weak acid which increases the dissolving power of the water many fold. By means of chemical action upon the various rock and soil bases, carbonates and the soluble bicarbonates are formed.
By the process of hydration, which is the chemical union of water with substances, rocks are decomposed into soil. This important process usually precedes or accompanies oxidation and carbonation. As a result of hydration, many rock-forming minerals, when exposed to the mechanical forces of weathering, become soft, lose their luster, and quickly break down into soil (Fig. 1).
Fig. 1.--Origin of soil from underlying limestone on top of a steep hill. Much of the soil has been transported by wind and water to the lower hillsides and valleys where it forms a layer many feet deep.
As a result of the combined forces of fragmentation and decomposition, the mineral components of the rock are simplified and slowly dissolve in the soil water.
Rôle of Plants and Animals.--The rôle of living organisms in soil formation is likewise an important one. The amount of carbon dioxide in the soil is greatly increased by the respiration of bacteria and other soil organisms as well as by the respiration of roots. The solvent action by carbonation resulting from the supply of carbon dioxide afforded by roots is beautifully illustrated by growing a corn plant in a few inches of sand in a box which has for a bottom a polished marble slab. After only 2 or 3 weeks, distinct etchings of the "root tracks" are clearly visible. The rapidity of carbonation, which takes place on a vast scale, has a direct relation to the amount of vegetation. It is chiefly through root respiration, the decay of the vegetation and the respiration of the microorganisms which it supports that carbon dioxide is supplied to the soil air. By these processes, several hundred pounds of carbon dioxide are added to an acre of soil each year.
The great dissolving power of water acidified by carbon dioxide is increased by nitric and sulphuric acids which are present in small amounts in rain water and are also supplied in small quantities to so il water by the action of bacteria. In addition to large amounts of carbon dioxide resulting from the oxidation of organic matter, organic acids are formed by the decomposition of plant and animal debris and are also excreted by living roots under conditions of poor aeration. 196 In water charged with such acids, many rock compounds are readily soluble, and all are soluble to a limited degree.
Lowly plants, such as lichens, algae, and mosses, live upon bare rock surfaces, slowly corroding them and forming a thin film of organic debris. These are followed by higher plants which exert a profound influence upon the formation and nature of the soil. Roots and rhizoids penetrate the pores and crevices, enlarging them by their solvent action and often splitting rock masses apart by the force of expansion accompanying growth.
The roots die and leaves and stems, as well as animal remains, accumulate in the surface layers. All are immediately attacked and partly decomposed by the combined action of bacteria, fungi, and other organisms which thus initiate the important process of humus formation. The resultant mass now begins to take on the character of soil.
The soil is not a simple mixture of its solid constituents moistened with water. Texture and structure are its chief physical properties. The sizes of soil particles give texture, and their arrangement results in soil structure.
Soil Texture.--Soils differ a great deal in the fineness or coarseness of the particles of which they are composed, that is in texture. This difference between a sandy soil and a clay soil is familiar to nearly everyone. There are three general groups of particles in soil. The coarse particles are called sand, those of medium size are termed silt, and the very fine particles are called clay. The relative amounts of these different grades of particles in a soil determine its texture.
While coarse sand grains are about as large as the head of a pin, clay particles are so small that it requires approximately 65,000,000 of the largest of these to equal one grain of sand (Fig. 2). 141 Such a wide range in the sizes of soil particles readily accounts for the great differences in the texture of soils. A knowledge of soil texture is important, since the agricultural value of a soil depends to a very considerable degree upon its texture.
Fig. 2.--Diagram illustrating variation in sizes of different soil particles, from the coarsest sand particles to clay. The range is so great that it is necessary to show the coarsest clay particles multiplied 200 times. (After M. F. Miller, The Soil and Its Management, Ginn and Company.)
The farmer or gardener ordinarily distinguishes only three general classes of soils (i.e., soils of different texture), viz., sandy soils, loam soils, and clay soils. There are other classes, however, such as sandy loam, silt loam, clay loam, etc., which are divisions of the three general classes. The class to which a soil belongs can be determined accurately only by a mechanical analysis. This consists in separating a soil into the grades of particles of which it is composed and determining the percentage of each.
Cause and Nature of Soil Structure.--Soil structure is due to the arrangement or grouping of the soil particles. The irregularity in size and shape of the rock particles. prevents a tight packing together and affords open, irregular spaces through which air and water can circulate, while their weight and mutual pressure furnish the necessary resistance for firm root anchorage. Soils of single- grain structure like sand, where the particles function more or less separately, are fairly simple. But a very complex structure is represented in clay where the soil granules or crumbs are composed of many particles bound together by colloidal or glue-like material originating from the finest clay and humus particles.
A moist soil of good structure crumbles in the hand. This granular property is an expression of its structure. It may be destroyed by shaking the soil in water or by drying it out completely.
In the former case, drying the sediment of mud does not restore it to its proper condition; in the latter, it is difficult to wet the dusty mass thoroughly, and when this has been done, only a sticky paste is produced. The soil crumbs have been destroyed, and simple wetting or drying, though it may restore the original degree of moisture, does not reconstruct the crumbs. In agricultural practice, this fact is of great importance, for a soil, if worked when too wet, becomes puddled, quite unfit for plant growth, and may require prolonged treatment before it is again fit to bear crops. 189
A rich loam usually furnishes an example of a soil with an excellent structure. It is composed of approximately half sand and half silt and clay, and often contains considerable humus. Some of the particles are large and function as individuals. Those of smaller size furnish a nucleus about which the still smaller particles aggregate to form granules. This aggregation of the smallest soil particles into groups or crumbs which act as individuals makes the soil much more porous. The larger interspaces Permit the water to drain away as they become filled with air, while the smaller ones retain the moisture. Humus has a very important effect in lightening the soil and promoting a good soil structure.
Relations of Tillage, Plants, and Animals to Soil Structure.--The chief object in tillage, aside from preventing the growth of weeds, is to make the soil more conducive to the growth of crops, an effect brought about by maintaining a good soil structure.
Cultivation results in the formation of soil crumbs or soil aggregates which give the soil a loose condition commonly known as tilth. In cultivated soils, tilth is destroyed by the deflocculating action of beating rains. Under natural conditions, it is maintained by the humus and by root penetration, deflocculation being prevented by the protection afforded by the natural plant cover.
Fig. 3.--Portions of soil taken at a depth of 5 feet showing the numerous earthworm burrows. These are 5 to 7 millimeters in diameter and extend to depths of 10 to 13 feet.
Earthworms play an important part in keeping soil in good tilth. In semiarid soils, at least, they are not confined to the surface layers but often fill the subsoil with their burrows to depths of 5 feet or more (Fig. 3). 218 In feeding, they consume decaying vegetable matter and, at the same time, take in large quantities of soil. This, in passing through their digestive tracts, is acted upon by digestive juices. The worms come to the surface to discharge their faeces or "worm casts" and, in this process, are continually bringing up well-mixed and enriched, deeper soil and exposing it to the air. They are frequently very numerous, often many thousands per acre. Darwin 52 estimated that they brought to the surface annually enough soil to form a layer 0.2 inch in depth or 10 tons per acre. Burrowing everywhere, dragging down large vegetable fragments from above, they help to aerate the soil and keep it light and of good structure. Rodents, ants, and various other animals mix and open up soil and subsoil to air and water and thus promote root penetration. 103 On the "hard lands" of the Great Plains, the work of prairie dogs and other rodents has had a profound effect upon soil structure, increasing water penetration and permitting the growth of certain deeply rooted species of plants. Insects, insect larvae, nematodes, and hosts of other organisms abound, all being instrumental in loosening the soil and thus affecting root development and crop yield.
The soil is the seat of a number of slow chemical changes by means of which the plant and animal residues are converted into the dark-colored organic matter of the soil. A good supply of this semidecomposed organic material greatly promotes productiveness. The process of the decomposition or decay of the organic residues is dependent upon various minute organisms universally distributed throughout cultivated soils.
Origin, Nature, and Effect of Humus.--Humus comprises the more or less decayed organic portion of the soil. It is mostly, but not entirely, vegetable matter, dark in color, light in weight, and more or less intimately mixed with the mineral soil components. This organic material is of great importance; indeed, without it, successful crop production would be quite impossible. Its effect in improving the physical condition of the soil is marked. It acts as a weak cement to bind sand, lightens or opens a clay soil by separating the particles, and thus increases percolation, aeration, bacterial activity, and root extent as well as the ease of tillage. Being very absorbent, it helps to retain water so that in regions of moderate rainfall crops grown in soils rich in humus are less likely to suffer from drought.
The marked effect of humus upon water conservation and storage should be emphasized. Careful estimates from experimental data show that the loss of humus through soil erosion under a precipitation of 26 inches is probably equivalent to a decrease of 6 inches in rainfall. 165 Soils that have lost humus are harder than formerly and in poorer tilth; they crack easily and expose greater surfaces to evaporation.
Based on dry weight, the organic matter of a mineral soil may constitute from less than 1 to more than 15 per cent, frequently 5 to 10 per cent being found in cultivated soil. These low percentages are due to the fact that the mineral soil matrix has a density about three times as great as that of the humus.167
Relative to volume, the humus may constitute from 4 to 12 per cent and the mineral soil components only 41 to 62 per cent, the remaining volume being pore space which is occupied by water and air. 167 The amount of humus varies with the climate. Arid soils contain less, partly because there is less vegetation from which it may form but largely because of its too rapid oxidation. Conversely, in soils that are very wet, decay is greatly retarded and plant remains may accumulate in such quantities as to constitute 85 per cent or more of the weight of the soil. This is the case in peat or muck soils.
Although much of the humus has its origin from aboveground plant parts, large amounts are formed from root decay and a smaller amount from the remains of soil organisms. The decay of the organic debris is brought about by the activities of various groups of bacteria, fungi, protozoa, and other inhabitants of the soil. Indeed, these organisms cause not only the formation of the humus but also its disappearance; for in good soils, the supply is constantly being renewed and yet it does not increase. Chemical reactions in the soil are greatly accelerated by humus decay.
The Formation of Humus.--The process of humus formation consists of a series of complex chemical changes not yet fully understood. The organic debris is finally broken down into simpler compounds, the end products being carbon dioxide, water, ammonia, marsh gas, and inorganic compounds of sulphur and phosphorus. This destructive process goes on by various stages, the end product of the activities of one group of bacteria, for example, being the raw material further reduced by another. Such reactions in all stages are continually in progress. Thus there is a close relation between the vegetation and the soil population. The latter is dependent almost entirely upon the growing crop for energy material, while the plant is equally dependent on the activities of the soil population for removing the residues of previous generations of plants and for the continued production in the soil of simple materials which are necessary to its growth. 166 The enormous importance of this process in the economy of nature is self-evident and the importance of the rôle of soil organisms can scarcely be over-emphasized. The encouragement. of their growth and activities by proper methods of tillage is an important phase of plant production. During humus formation, the material takes on the characteristic dark color. This process of decay, however, is not one of immediate simplification. The various organic acids, etc., originating as intermediary products, react upon the minerals with which they are in contact, and these may thereby be made soluble and available to the plant.
Organisms Concerned.--The number and kinds of organisms taking part in these processes of simplification are very great. A single gram of loam from the surface soil, an amount easily held on the blade of a small pocket knife, may contain 14,000,000 to 58,000,000 bacteria 51 and in some soils, even at a depth of 3 feet, as many as 37,000 per gram have been found. 26 The number fluctuates greatly from season to season and also from day to day. Bacteria are aided in their important work by many kinds of fungi which are always abundant in soils rich in humus. The toadstools and puffballs of old pastures are the enlarged fruits of certain species whose thread-like bodies ramify the soil. Many smaller, mold-like forms occur in countless numbers, several hundred species having been identified. Some are found at depths of 3 to 4 feet. 21
The Relation to Nitrogen.--Among this vast assemblage of dwellers in the soil, certain groups deserve especial mention, since they are concerned very directly with the supply of nitrogen, a constituent of the protoplasm and a substance most indispensable to plants. The ammonia liberated in the breaking down of proteins, a process called ammonification, is oxidized to nitrites by certain kinds of bacteria. The nitrites are further oxidized by other bacteria to nitrates, which is the form of nitrogen most favorable for crops. The process of nitrification is exceedingly important in agriculture, since crops cannot use nitrogen from the abundant supply in the air but must rely entirely upon compounds absorbed in solution through the roots. Hence, the presence of mineral salts containing combined nitrogen is a factor of great importance in soil productiveness. Since the supply of nitrates is constantly diminished by leaching and cropping, its renewal is imperative for permanent agriculture.
Several species of soil bacteria functioning independently of higher plants possess the power of taking the free nitrogen from the air and incorporating it into organic compounds. In this way, much nitrogen is added to the soil. A gain of from 25 to 40 pounds per acre per year has been determined by different investigators. Both nitrifying and nitrogen-fixing bacteria thrive in the humus, and it is largely due to their activities that soils containing much humus are so rich.
Under certain conditions, however, although the production of bacteria may be stimulated, yet the nitrate content of the soil is reduced. The common agricultural practice of mulching with straw, used by gardeners, orchardists, wheat growers, and others, has been shown to retard the growth of the crop, delay ripening, and especially reduce the yield materially. 181, 2 Various bacteria use the straw as, a source of carbon and the nitrates as a source of nitrogen. Thus the nitrates are transformed into organic nitrogenous material and, for a time, are lost as available food material for the growing crop. In certain wheat-growing sections, where the addition of straw is practically the only method of maintaining the humus supply of the soil, it is very important that the effect of straw on the activities of soil organisms be fully understood. 143, 2
Soil bacteria furnish the chief food of many microscopic animals belonging to the group called protozoa. Protozoa have been shown to be common in many soils and are frequently very abundant. A gram of soil may contain anywhere from 10,000 to 2,000,000 individuals. When conditions are favorable, a too vigorous development of these organisms may seriously reduce the bacterial flora and render the soil less productive. Actual counts have shown that when certain of the protozoa called amoebae were abundant in arable land, bacteria were few, and vice versa. 51,170
Among certain plants, notably the legumes, which are very rich in nitrogenous compounds, a close relationship exists between other species of nitrogen-fixing bacteria and the roots. Here, the bacteria are found in the root nodules familiar to all on clovers and alfalfa. From 40 to over 250 pounds of nitrogen per acre may thus be added to the soil in a single season through the activities of these bacteria. This explains why the practice of growing leguminous crops and plowing them under as green manure has such a stimulating effect upon the growth of succeeding crops. However, except for the wrecking crews composed of myriads of microscopic organisms, which immediately attack the fallen vegetation and reduce it to water-soluble and, hence, usable compounds, few new crops could be grown. Otherwise, such practices as the application of barnyard manure and the plowing under of wheat stubble and other crop residues would be distinctly detrimental to the soil.
An analysis of water that has drained through a soil shows that it contains a great many substances which have been dissolved. Certain portions of the soil have gone into solution. It is from this solution that plants obtain their mineral nutrients.
Origin and Nature.--The soil solutes originate from several sources. Some are formed by the dissolving of the original rock particles; some come from decaying humus; others are built up by bacteria and other soil organisms; and still others are excreted by roots. Oxygen and carbon dioxide are important dissolved gases. The soil solution is variable in its composition, partly because of the variability of the solvent power of the water, which, in turn, depends largely upon its carbon dioxide content, and partly because of the nature and amount of soil colloids. The soil colloids of finely divided humus particles and clay retain the solutes by absorption. The amount thus retained varies with the amount of water. The more water present, the more solutes go into solution. As the water increases, however, the concentration of the solution decreases. For example, the soil solution of a Carrington loam with 15 per cent water content had over 16,000 parts per million of solutes (about 1.6 per cent concentration), but in the same soil at 38 per cent water content, the solutes were reduced to less than 500 parts per million, a decrease of over 97 per cent. 131 Due to absorption by the roots of plants, evaporation, and drainage, the water content is always changing. Moreover, crops are constantly removing nutrients, and other amounts are lost by leaching. The soil solution, although always very dilute is more concentrated in rich than in poorer soil. It seldom exceeds 0.3 per cent and is usually much less. Its concentration varies with the season even when the field lies fallow. Here, as would be expected, it is higher than in similar soils on which crops are growing, for the growth of a crop markedly diminishes the concentration of the soil solution. This effect is still evident at the beginning of the following season. 90 , 91 Thus, the soil solution is constantly changing both in composition and concentration. It contains the reserve nutrients, and as these are absorbed by crops, new supplies are liberated from the colloidal soil complexes.
Importance in Crop Production.--The importance of a rich soil solution is very great since crops absorb their nutrients from it. Although over 30 elements have been found in the ash of plants, experiments with plant cultures have shown that only a few of these are essential to normal growth. This was determined long ago by adding various soluble salts to pure distilled water or water in clean quartz sand and observing the effect upon growth and yield. Such studies have shown that plants need only the soluble salts of nitrogen, sulphur, phosphorus, potassium, calcium, magnesium, iron, and sometimes chlorine for normal growth, although certain others are not to be considered as entirely without physiological effects. 151
The amounts of these nutrients taken up by crops is exceedingly small in proportion to the size of the plant body. But in the activities of the protoplasm, each essential nutrient plays an important part, and a soil deficient in any one of them will be unable to produce crops successfully. The addition of fertilizers containing compounds of nitrogen, phosphorus, and potassium to many soils greatly increases the yield. This is also true of sulphur compounds on certain soils. The other elements are usually abundant.
Removal of Nutrients by Crops and by Leaching.--Different kinds of crops remove the various nutrients in different amounts. For example, clover absorbs much more calcium than does barley growing beside it, and barley takes up many times as much silicon as does clover. Likewise, the difference in the amounts of nitrogen, potassium, phosphorus, and calcium compounds absorbed by a crop of red clover and by one of corn is very striking. These differences are due both to the extent and degree of branching of the roots and to variation in their absorptive activity. Wheat production is greatly promoted by the addition of nitrates to the soil, the crop often being retarded in its growth in early spring owing to a too limited supply of this nutrient because of low temperature and consequent low bacterial activity. The rotation of crops is held to derive its value partly from the different demands made by different crops upon the nutrient supply of the soil.
Under natural conditions, the materials removed from the soil by growing vegetation are ultimately returned in plant remains and animal excretions. Those washed down into the subsoil are often brought to the surface again through absorption by deeply penetrating roots. Certain soil constituents, such as nitrogen compounds and calcium carbonate, are easily leached out and large amounts lost annually in drainage water, but those of potassium and phosphorus are almost entirely retained in place by the absorptive. action of the soil colloids. Thus, soils of and and semiarid regions, occupied largely by grassland or desert vegetation, are potentially richer than well-leached soils in more humid regions.
Alkali Soils.--In and regions where drainage is very slight, the soil salts may accumulate to such a degree, especially in lowlands, that they are distinctly harmful to most plants. These accumulations of soluble salts are termed alkali. Even sodium nitrate, an important constituent of fertilizers, if in excess, produces an alkali soil.
Soil alkali is harmful to plants in a number of ways. A concentrated soil solution may delay seed germination either temporarily or indefinitely by hindering water absorption. if germination is successful, a later concentration of salts may cause the movement of water from the root hairs to the soil. This gives rise to a condition of plasmolysis; absorption is inhibited and wilting and death may result. The bark of plants may be corroded at the soil surface by alkali salts, especially by the carbonates, concentrating in the surface soils during drought. In this way, the bark on plants in orchards and vineyards may be so thoroughly destroyed that the passage of food from the leaves to the roots is prevented. Moreover, alkali carbonates may affect soil structure detrimentally, at least to most plants, by dissolving out the humus and deflocculating the clay. Many alkali soils are underlaid by a hardpan produced in this manner, which is impervious to both water and root penetration. The most satisfactory method of correcting soil alkali is by drainage. 78
In using the natural vegetation for determining the possibilities of alkali lands for crop production, the root habits of the native species should be regularly taken into account. 116
Acid Soils.--In humid regions, the soil is frequently acid. The causes of soil acidity are complex, but in general, acidity is due to the absence of sufficient calcium and magnesium bases to counteract the acids of whatever origin. The decrease in the amount of bases is brought about by the continued leaching of these soluble compounds. The strength of an acid solution is not dependent upon the total quantity of acid present in it but rather upon the number of hydrogen ions present in a certain volume of the solution, i.e., on the hydrogen-ion concentration. Consequently, the degree of soil acidity is often expressed as hydrogen-ion concentration (PH values). 227
An acid soil solution may affect plant growth by checking the work of nitrifying bacteria and all forms of nitrogen-fixing bacteria, by preventing the normal decay of humus and promoting the accumulation of resulting toxic organic substances, as well as by limiting the availability of potassium and other soil salts. Furthermore, plants need lime, which occurs in too small amounts in acid soils, since it is a necessary nutrient and also acts as a neutralizing and precipitating agent within the cell sap. 210
Acidity in soils may be corrected and such soils made more productive to some crops by the addition of some form of lime. This practice, like the addition of manure and other fertilizers, is a method by which the soil solution can be modified. Tillage of all kinds is of benefit in so far as it makes the soil solution and the soil environment more favorable for root activities.
Cultivated plants obtain their entire water supply from the soil. Water is important to the plant in many ways. It is a component of protoplasm and, with carbon dioxide, is essential in building plant foods. It usually constitutes 70 to 90 per cent of the weight of herbaceous plants. All substances which enter plants must do so in solution. Water is the great solvent. It serves as a medium of transport of food materials and foods from place to place, since their transport can take place, for the most part, only in solution. It keeps the cells turgid or stretched, a condition essential for their normal functioning. It also serves to prevent excessive heating of the plant, acting as a buffer in absorbing the heat generated by the multitudinous chemical reactions taking place in the plant. A large quantity of heat
energy is absorbed when liquid Water is transformed into the vapor formed during transpiration. In corn, a transpiring leaf is uniformly cooler than a dead one. A difference of 8.5°F. has been found in the sun when transpiration was high, and 4.2°F. at the same time in the shade. 118 Thus, it is clear that the greatest sources of danger which the plant has to meet are insufficient absorption and excessive transpiration.
Relation between Precipitation and Water Content.--Only a very general relation exists between the amount of precipitation and the water content of a soil. Frequently during light showers, a growing crop will intercept practically all of the rainfall which evaporates without reaching the soil. Much precipitation is thus lost. Even during heavier rains, 20 per cent or more may be intercepted. 96 Where torrential rains occur, especially if they fall upon close-textured soils, large amounts of water are lost by run-off and do not aid in replenishing the moisture of the soil upon which they fall. This phenomenon is very pronounced in the Great Plains where as much as 20 to 50 per cent of a rain may be lost in this way. 183 Not only is the water lost to the soil and growing crops, but in hilly lands, much damage may be done by the erosion of the surface layers, which are the richest part of the soil. A similar precipitation on coarse sandy soil results in little run-off.
Under hot, arid, and semiarid conditions, the drying power of the air is so great that additional large quantities of water are quickly lost by evaporation. This largely accounts for the fact, for example, that wheat can be profitably grown under a rainfall of only 20 inches in North Dakota, but in Texas, on a similar soil with an equal precipitation, agriculture is confined to the most drought-resistant and drought-enduring crops. 22 Much of the water that enters the soil and is not lost by evaporation may percolate downward so readily in soils of coarse texture that it gets far beyond the reach of the roots of crops. Hence, the best method of ascertaining the water available for plant growth is to determine it from the soil directly.
Kinds of Soil Water.--After a heavy rain or irrigation, much of the water drains or sinks away under the force of gravity. This is called gravitational water. But large amounts are retained in the minute spaces between the fine soil particles, as films surrounding the particles, and by absorption by the soil colloids. All but a small percentage of this water is available to the plant. Even air-dry soil contains appreciable amounts of water, as may be shown when, by heating dust in a closed container, drops of water are deposited on the lid. The relatively small amount of water absorbed by dry soil from the atmosphere is termed the hygroscopic water. It is held so tenaciously by the soil colloids which coat the rock particles that it is unavailable to plants. The hygroscopic coefficient is a term used to designate the maximum hygroscopic water that a soil will hold, i.e., the percentage of water absorbed by a dry soil from a saturated atmosphere. Since it is unavailable for plant growth, it is usually subtracted from the actual water content of the soil to obtain the available water content. The total amount of water that is held against the forces of gravity and capillarity and does not drain through the soil is termed the water-retaining capacity. It is expressed in percentage of the dry weight of the soil. It includes the hygroscopic water as well as the much larger quantity which the soil holds besides, commonly called capillary water. The amount is very different in different soils; a coarse sand under field conditions may retain only 12 per cent of its dry weight of water, but a silt loam may retain 35 per cent. The smaller the particles of a soil, the more film surface it will present for the retention of water. Likewise, the greater the proportion of colloidal constituents, clay and humus, the more water there will be held. The high absorptive power of soil colloids for water is due to the extremely large surface exposed by matter in the colloidal state.
Factors Determining Water-retaining Power.--The water-retaining power of a soil is determined by a number of factors. Most important among these are soil texture or sizes of particles, soil structure, i.e., the arrangement and compactness of the particles, and the amount of organic matter. A soil with many fine particles will retain more water than a coarse soil, and a plowed soil in good tilth, more than a hard compact one. Organic matter has a high absorptive capacity especially when well decayed.
By mechanical analysis, soil particles may be separated into groups according to their diameters.
Table 1 gives the mechanical analyses of soils at several depths from eastern Nebraska, north central Kansas, and the short-grass plains in Colorado. These are of especial interest because of the extensive work done here on the root development of both native, and cultivated plants. The diameters of the particles vary from 1 to 2 millimeters in fine gravel, from 0.1 to 0.05 millimeter in very fine sand, but the clay particles are only 0.005 millimeter or less in diameter.
TABLE 1.--MECHANICAL ANALYSES OF SOILS Very Depth of Coarse Fine Coarse Medium Fine Fine Silt, Clay, sample, gravel, gravel, sand, sand, sand sand per- per- feet percent percent percent percent percent percent cent cent From Lincoln, Nebr. 0.0-0.5 0.0 0.0 0.3 0.5 1.6 19.8 48.6 29.2 0.5-1.0 0.0 0.0 0.2 0.6 1.3 16.7 52.4 28.8 1-2 0.0 0.0 0.1 0.2 0.8 16.7 55.6 26.6 2-3 0.0 0.0 0.1 0.1 0.5 19.0 57.9 22.3 From Phillipsburg, Kans. 0.0-0.5 0.0 0.0 0.3 0.2 1.2 43.5 35.8 19.0 0.5-1.0 0.0 0.0 0.0 0.2 0.5 44.4 32.8 22.1 1-2 0.0 0.0 0.0 0.2 0.3 39.7 34.0 25.8 2-3 0.0 0.0 0.0 0.3 0.6 41.2 31.9 26.0 3-4 0.0 0.0 0.0 0.1 0.2 37.5 31.4 30.8 From Burlington, Colo. 0.0-0.5 0.0 0.0 0.0 0.1 2.6 48.6 33.4 15.3 0.5-1.0 0.0 0.0 0.0 0.1 2.2 49.1 32.5 16.1 1-2 0.0 0.0 0.0 0.2 1.9 46.7 32.0 19.3 2-3 0.0 0.0 0.0 0.1 1.5 45.5 31.0 21.9 3-4 0.0 0.0 0.0 0.1 0.9 42.2 34.2 22.6
The chief component of the eastern Nebraska soil is silt, and because of its desirable proportions of sand and clay, and good humus content, it is designated as a silt loam. That from Kansas is a very fine sandy loam, the, very fine sand predominating at all depths. As in the preceding soil, the other constituents are also quite uniform throughout. The Colorado soil is also a very fine sandy loam. There is a gradual increase in the amount of clay from 15 per cent in the surface soil to 23 per cent in the fourth foot with a corresponding decrease in the amount of very fine sand. This concentration of the colloidal clay below the second and third foot has been brought about by the light precipitation and vigorous root absorption, water rarely penetrating beyond a depth of 2 to 4 feet. With the penetration of water year after year, the colloidal clay has been carried down and, upon drying, aided by calcium carbonate likewise brought from above, has profoundly modified the soil structure. This soil layer is cemented into a "hardpan," 8 to 24 inches thick, which is very different from the soil above and below. The non-sandy soils of the Great Plains are underlaid with a hardpan or carbonate layer which greatly influences the root habits of both native and cultivated plants. 224
All these fine-textured soils have a high water-retaining capacity, once the water enters, and because of this property, under light precipitation, they keep the water near the surface and promote shallow-rooting habits among crops. Where more sand occurs, greater penetration results, plants root deeper, and crop production is more certain during drought. 222 Where rainfall increases to about 28 inches, the subsoil is constantly moist to great depths.
Water Content During the Growing Season.--The amounts of water in these several soils in excess of the hygroscopic coefficient or maximum hygroscopic water during a growing season of rather normal precipitation are given in Table 2.
TABLE 2.--WATER CONTENT IN EXCESS OF HYGROSCOPIC COEFFICIENT, 1920 Date 0-0.5, 0.5-1, 1-2, 2-3, 3-4, foot foot feet feet feet Lincoln, Nebr. Apr. 10 17.6 16.1 14.1 10.0 8.6 Apr. 21 17.5 22.4 20.3 May 5 20.5 19.1 19.7 June 9 18.7 20.3 June 16 5.9 12.5 14.8 15.6 June 23 4.9 7.1 9.6 July 14 22.0 16.8 9.1 13.2 14.7 July 28 7.2 3.9 6.3 Aug. 5 2.5 4.0 5.4 9.7 11.6 Aug. 12 8.7 5.7 4.8 Aug. 19 26.3 7.7 3.2 Aug. 31 Continued heavy rains, no samples taken Hygroscopic coefficient 9.5 8.7 8.6 7.1 6.2 Phillipsburg, Kan. May 7 14.7 15.3 12.5 14.7 14.0 June 2 4.8 6.6 11.6 11.8 13.0 June 10 7.6 9.7 9.0 June 24 6.6 3.1 5.0 9.0 July 1 1.9 4.1 2.8 July 9 1.5 2.8 1.6 July 21 -1.4 0.1 -0.2 2.3 5.3 Aug. 4 0.7 7.7 -0.3 0.5 3.5 Aug. 18 12.6 12.3 5.4 Aug. 26 4.0 4.1 0.4 2.0 5.5 Hygroscopic coefficient 10.6 10.6 10.9 10.6 10.7 Burlington, Colo. Apr. 15 16.7 13.7 11.1 4.9 1.8 June 3 2.3 5.2 7.3 6.9 2.9 June 12 -1.8 -0.1 2.8 June 25 7.4 2.5 1.8 1.4 0.0 July 2 -1.6 -0.5 0.0 July 8 -2.9 -1.3 0.0 July 20 -0.7 -2.7 0.0 0.1 0.1 Aug. 5 4.6 2.7 0.0 0.0 0.1 Aug. 19 -3.1 -3.1 0.0 Aug. 24 -0.8 -2.1 0.0 0.0 0.0 Hygroscopic coefficient 10.9 10.9 12.2 12.0 11.4
It may be seen that at Lincoln, in eastern Nebraska, sufficient water was available at all depths to promote good growth. At Phillipsburg, in north central Kansas, the soil and subsoil were very dry during July and parts of August; but at Burlington, in eastern Colorado, a drought occurred in June, and water was very scarce thereafter. The effects upon plant growth were very pronounced (Fig. 4). 45
Amount of Water Absorbed.-Plants absorb water in large amounts but lose nearly all of it by transpiration. From 200 to over 800 pounds of water are required to produce a pound of dry matter aboveground. 23 Thus, a single corn plant, during its period of growth, may absorb 35 to 50 gallons of water from the soil. 118 More is required from very dry or very wet soils and from poor soils than from rich ones., The amount actually
used depends, furthermore, upon the dryness of the air, which exerts a marked influence upon I transpiration.
Fig. 4.--White Kherson oats grown at Burlington, Colo. (left), Phillipsburg, Kan. (center), and Lincoln, Nebr. (right) during 1920 (cf. Table 2).
Absorption Of Water.--It has already been pointed out that plants cannot absorb all of the water from a soil. The reasons for this can best be understood after recalling the principles governing absorption. Water and all substances dissolved in it, before entering the plant, must pass through the walls of the cells of the root epidermis, many of which are elongated into. root hairs, and also through the layer of cytoplasm which lines the cell walls. The cell wall will permit the soil water and solutes to enter unhindered. It is very permeable. But the cytoplasm is only partially so, or semipermeable. It does not permit the solutes such as sugars, organic acids, etc., dissolved in the cell sap to pass out. Normally, the total concentration of solutes in the cell sap is greater than that of the soil solution. Under this condition, the water moves into the root hairs from the soil solution. The rate of this inward water movement depends upon the concentrations of the two solutions, decreasing as these become more nearly equal. But the greater concentration of the cell sap in the root hairs is maintained by the constant removal of water through a similar process of osmosis towards the interior of the root, from whence it ascends to the shoot. Hence, this movement of water tends to go on just as long as the concentration within is greater than that outside. It moves in under the pulling forces of osmosis as well as those of imbibition of the cell wall and the cytoplasm. The pectic cell walls of root hairs, like the cytoplasm, are gelatin-like or colloidal in nature. They have a great water-absorbing power and are well adapted to remove water from the soil particles by imbibition.
When the soil becomes quite dry, water is absorbed slowly, and if the day is warm and the transpiration high, the plant will partially wilt. Under such conditions, corn rolls its leaves. But at night, when transpiration is low, even this slow rate of absorption may furnish sufficient water to permit recovery from wilting, and the stem and leaves may again become turgid. If the soil gets still drier, a point will be reached where the plant cannot recover even in a saturated atmosphere, since it is unable to absorb any more water from the soil. The plant is permanently wilted. The forces of absorption are in equilibrium with the forces of capillarity and colloidal imbibition retaining the water in the soil. Just how much water thus unavailable for growth will be left in the soil depends upon the composition of the latter and perhaps also upon the character of the plant. In coarse sand, a plant may be able to get all but 1 or 2 per cent, while in clay, 15 per cent or more may be withheld. Although a great deal of experimental work has been done on the amounts of water left in soils at permanent wilting, further study is needed. Contrary to common opinion neither water nor nutrients move great distances toward absorbing surfaces. It seems clear that crops with much-branched, deeply penetrating, and widely spreading roots, thus coming in contact with a large soil surface, would have a decided advantage in dry habitats.
Many other factors, such as greater density of cell sap or the presence of cell colloids with a greater hydration capacity, may make some plants more efficient absorbers than others. Under field conditions, it appears, as already stated, that the amount of water unavailable for growth approaches the hygroscopic coefficient. 221, 6
Absorption of Nutrients.--As regards the absorption of soil salts, such as sodium and potassium nitrate, calcium phosphate, potassium chloride, and magnesium carbonate, they can enter the plant only in solution. This does not mean, however, that they enter at the same rate as the water. Every solute moves into the plant quite independently of every other solute and of water. Whether or not it will be absorbed, assuming that the cytoplasm will permit it to pass, depends upon its concentration within the cell sap compared to the soil solution. Potassium nitrate, or more strictly speaking, K(+) and NO3(-) ions will be absorbed rapidly, provided they are being rapidly removed into the tissues or elaborated into other compounds. At the same time, another salt may not be absorbed, but later, both may enter at an equal rate, which may be either faster or slower than that of the water. This phenomenon accounts for the fact, previously mentioned, that clover absorbs more calcium but less silicon than does barley.
Thus, absorption is affected by the rate of transpiration, the extent of the root system, the concentration of the soil solution, and the rate of movement of the soil water.
The activities of plants are profoundly affected by temperature and are practically suspended below a certain point, which is about 40°F. for most cultivated crops. The temperature of the soil affects not only the vegetative development of plants but also the germination of seeds and bacterial and chemical activities in the soil, as well as growth and other functions of roots.
Relation to Activities of Higher Plants.--The rate of absorption, as with all the physical and chemical processes taking place within the roots, is decreased by a lowering of the soil temperature. A low temperature permits only a slow rate of water absorption. Even in the latitude of southern Arizona, the conditions of soil temperature for most favorable water absorption do not prevail in winter, and the effect is a limitation to the development of both root and shoot of winter annuals. 29 This also explains the damage often done to trees, shrubs, winter wheat, and other plants in early spring by warm weather and high winds when the soil is still cold, if not frozen. Under these conditions, transpiration exceeds absorption. Winterkilling is perhaps more often due to drying than to freezing.
Favorable soil temperatures promote rapid seed germination and seedling establishment and are necessary for vigorous root growth. The warmer the soil in spring, the more rapid are germination and growth. Plants vary a great deal in regard to soil temperature requirements for germination. This is shown in Table 3, where crops requiring low, medium, and high soil temperatures are given.
TABLE 3.-TEMPERATURES FOR GERMINATION 74 Minimum, Optimum, Maximum, Crop degrees degrees degrees Fahrenheit Fahrenheit Fahrenheit Wheat 40 84 108 Maize 49 93 115 Pumpkin 52 93 115
It has been found that if other conditions are favorable, staple crops will grow when the temperature of the soil is as low as 40°F. and as high as 120°F. The most favorable temperature for growth is between 65° and 70°F. 190 Roots in the deeper soils grow vigorously at much lower temperatures. This, of course, varies with the species. Certain cacti make their best root growth. at a temperature of 93°F., although the rate of growth is also correlated with the length the root has already attained. 34 The roots of many native and cultivated plants grow vigorously, especially in the deeper soils, at temperatures much lower than 65°F. The soil temperatures at which the roots of oats, wheat, and barley, described in later chapters, grew in 1921 are shown in Table 4.
TABLE 4.--SOIL TEMPERATURES IN DEGREES FAHRENHEIT, 1921 April 28-30 May 19-21 June 9-10 June 22 June30 Depth Site Site Site Site Site Site Site Site Site Site Site Site in feet 1 2 3 1 2 3 1 2 3 1 2 3 0-0.5 59.7 75.2 53.6 69.8 68.0 72.0 72.5 70.5 67.6 70.2 73.8 84.6 0.5-1 55.4 73.5 51.8 66.2 64.0 59.0 71.2 68.4 65.8 70.0 73.3 79.0 1-2 53.6 59.0 50,9 56.1 61.5 57.2 68.4 65.8 62.2 68.2 70.7 75.0 2-3 53.6 57.2 50.9 55.6 59.0 54.3 65.5 63.0 60.4 65.8 70.2 71.2 3-4 51.4 53.6 50.0 53.6 56.3 53.2 64.0 60.8 59.0 63.3 68.9 68.0 Site 1 = Lincoln, Nebr. Site 2 = Phillipsburg, KS. Site 3 = Burlington, Colo.
It has been found in the case of peas in water cultures that, if insolation is not excessive, the amount of daily fluctuation of root temperature within a range of 40°F. (44°F. to 84°F.) affects growth but little. But where root temperatures are high, a slight decrease in insolation may promote better growth of shoots. 20
That root growth occurs in certain native species at temperatures so low as to be distinctly unfavorable for other species has been fully demonstrated. The shallow-root habit of certain desert plants is thought to result from subsoil temperatures too low to promote root growth. The general distribution of many cacti seems closely related to the response of the roots to the temperature of the soil, although the effect of aeration is also a contributing factor. 31, 32, 33
Root systems superficially placed are subject to maximum temperature changes quite unlike those experienced by more deeply seated ones. That soil temperatures have an influence on general plant growth is shown by the florists who resort to bottom heat for certain plants. Cuttings often require definite soil temperatures for rooting. In some cases, considerable vegetative growth can be produced, despite unfavorable atmospheric conditions, by maintaining a soil temperature favorable for root development. 33
Relation to Soil Organisms and Soil Reactions.--Many desirable biological and chemical soil reactions are retarded or stopped by unfavorable soil temperatures. Most soil bacteria do not become active until temperatures of 45° to 50°F. are attained. Temperatures of 65° to 70°F., which afford good root growth, also promote such changes as the decomposition of organic matter with the production of ammonia and the formation of nitrate nitrogen. Likewise, the fixation of atmospheric nitrogen depends upon similar favorable temperatures.
The rapidity of rock weathering in the tropics illustrates the fact that chemical changes in the soil are greatly accelerated by high temperatures. Temperature also exerts a marked effect upon such physical changes as rate of percolation, evaporation, diffusion of gases, vapors, and salts in solution. The aspirating effect brought about by a small change in soil temperature is often so marked as to result in thorough renewal of the oxygen supply of the soil to a depth of several inches.
Factors Affecting Soil Temperature.--Among the factors that directly affect soil temperature are the color, texture, structure, water content, and amount of humus in the soil and the slope of the soil surface with respect to the sun, as well as the presence or absence of a cover of vegetation. Of all these factors, water content is the most important for the reason that water has a specific heat about five times greater than that of the solid constituents of soil. 31 This explains why wet soils are colder in spring than drier ones and why a heavy rain in summer lowers the temperature of the soil. Fluctuations in soil temperature, like those in moisture, are much slower and less marked than those of the air. Roots have a far more constant environment than shoots. The surface soil layers vary more or less with the air temperature and, therefore, exhibit greater fluctuations than the subsoil. Daily fluctuations are not marked, even in dry soils, except in the surface layers. As shown in Table 5, however, seasonal variations of temperature are considerable even in the deeper soil.
TABLE 5.--AVERAGE TEMPERATURE IN DEGREES FAHRENHEIT AT LINCOLN, NEBR., 1891-1902 199 Depth in inches Season Air 1 3 6 12 24 36 Winter 25.9 28.8 28.8 29.5 32.2 36.3 39.1 Spring 49.9 54.8 53.6 51.7 48.5 45.7 44.3 Summer 73.8 82.0 80.9 79.1 73.8 69.0 66.2 Autumn 53.9 56.4 57.6 57.1 57.2 59.3 60.3
The most important factor in the control of soil temperature is the maintenance of an optimum moisture supply. This can be promoted by drainage or irrigation, by proper methods of tillage to produce a good soil structure, and by maintaining sufficient organic matter in the soil.
Relation to Disease in Plants.--Aside from its direct and indirect influences upon root growth, the soil temperature has a marked effect upon the development of disease-producing organisms. These may greatly limit successful crop production.
Frequently, the influence of environmental factors on the host seems to be the fundamental cause of susceptibility to disease. 111 Proper soil and other conditions for a vigorous development of root and shoot are desirable. Corn and wheat seedlings, for example, are sheathed at the outset with protective coverings through which invasions of soil organisms may rather easily take place. But in normally balanced seedling development, the cell membranes of the protective coverings pass quickly to a condition of maturity. Because of chemical changes in the cell walls, the tissues which were subject to invasion change rapidly so that they become relatively resistant. Soil and air temperatures which promote this normal balanced seedling development vary with the crop. In wheat, for example, they are low, but in corn, much higher.111, 55, 56
It has been clearly demonstrated that many soil-borne diseases are conditioned in their occurrence upon the factor of soil temperature. Cabbage yellows, 70, 112, 113 flax wilt, 207, 208 tomato wilt, 42 and tobacco root rot 107,109 are examples of diseases caused by soil-inhabiting fungi gaining entrance to the host plant through the root system. In each case, the disease has been experimentally developed in destructiveness ranging consistently from 0 to 100 per cent of the crop by changing the single factor of soil temperature. The temperature ranges employed were well within those reasonably congenial to the respective hosts. 111 For example, flax wilt, which, like the wilt of cabbage and tomato, is favored by a high temperature, gives extreme development of disease at 24° to 28°C. but none above 38°C. or below 14°C. 115 Conversely, tobacco root-rot development is favored by lower soil temperatures, the most favorable range being between 17° and 23°C. 108
The regional distribution of a plant disease is sometimes determined by temperature. The presence of onion smut, for example, is dependent upon the soil temperature during the seedling stage of the growth of the host. Infection and development of smut are favored by relatively low temperatures and inhibited by high ones, with 29°C. as approximately the critical point. Hence, although common in the North, it is almost unknown in the South. 215 It has been found in the Pacific Northwest that soil temperatures of 0° to 5°C. are decidedly unfavorable to successful infection of wheat by stinking smut. This holds, also, for temperatures higher than 22°C., while 15° to 22°C. are optimum for its development. Wheat growers in this region, where smut produces heavy losses after infecting the crop in the early seedling stage, have found that by sowing their winter wheat either very early (warm soil) or very late (cold soil), they can reduce the loss from smut to an almost negligible percentage.110 Of course, this is only one factor affecting the yield of wheat, and the time of seeding should not be entirely controlled by it.
These examples, which might be greatly multiplied, illustrate the importance of soil temperature as indirectly influencing crop production. Other soil factors, such as water content, aeration, available plant-food material, acidity, and toxicity, are often of great importance, and their effects, like those of temperature, exercise a strong influence upon the development and expression of disease. 98
The pore spaces of a soil are filled with air and water. After a heavy rain or irrigation has filled the soil interspaces with water and forced the air out, a fresh supply enters as the gravitational water sinks away. Soil has a very porous structure. In fact, only about one-half of its bulk is solid matter. Many cultivated plants thrive best in a soil which contains approximately only 40 to 50 per cent of its maximum water-retaining capacity. The rest of the interspace, about 20 per cent of the volume of a soil in good tilth, is filled with air. Dry soils contain much more air. Frequently, cultivated soils, during periods of drought, are too loose and dry for proper root development, and the plant is thus deprived of nutrients which the soil contains. Conversely, water-logged soils have no air except that dissolved in the water, but certain plants grow well even under this condition. The pore space increases with fineness of texture, degree of granulation, and abundance of organic matter. Thus, the total pore space of a sandy soil may be only 30 per cent of its volume, that of a loamy clay 45 per cent, but a heavy clay may have over 50 per cent. Soil in good tilth is filled with air spaces which are more or less continuous from the surface to the subsoil. Cracks, burrows, and spaces left by decayed roots, as well as the removal of water by absorption, promote gaseous exchange among the different soil horizons. There seems to be a slow but constant movement of the soil atmosphere, brought about chiefly by differences in water content and temperature changes, and to a less degree by diffusion and fluctuations in atmospheric pressure.
Composition of the Soil Atmosphere.--Because of its proximity to roots and microorganisms, both of which constantly give off carbon dioxide and absorb oxygen, soil air is very different in composition from the ordinary atmosphere. It often contains much more carbon dioxide, 0.2 to 5 per cent, or even more depending upon depth, amount of organic matter including manure, abundance of roots, etc. Moreover, the proportion of oxygen is less, and moisture content much greater and more constant than in the air aboveground. Thus, the soil air is not static but, like the soil itself, constantly undergoing change. 44
Soil air is either in direct contact with roots and microorganisms or separated from them only by a thin film of water or colloidal matter. Within these films, the oxygen supply is very limited and the carbon dioxide content very high, as much as 99 per cent having been found. 171 The significance of highly acidulated water in bringing nutrients into solution has already been discussed.
The rôle of oxygen in the process of breaking down insoluble minerals into a soluble form and the consequent enriching of the soil solution has already been pointed out. This gas is no less important in the transforming of plant and animal remains into a condition where their nutrient materials become soluble and may be absorbed by plants. Biochemical oxidation proceeds rapidly, when conditions are favorable, and much oxygen is incorporated in the compounds produced.
Relation to Root Growth and Other Biological Activities.--Oxygen is also necessary for the germination of seeds, root growth, root-hair development, and absorption by roots. Without it, nitrification would stop, and earthworms and most other soil organisms would cease their activities. A few microorganisms could get their oxygen supply anaerobically by breaking down valuable compounds, such as nitrates, and would thus decrease the soil productivity.
Even roots can carry on respiration for a time without free oxygen, i.e., anaerobically. Since the anaerobic respiration of plant roots, bacteria, molds, etc., gives rise to organic acids, alcohol, and other toxic substances, aeration is fundamentally connected with the production of soil toxins. 44
The dependence of root development of most plants upon aeration is clearly shown by water-logging the soil. In a few days, the usual cultivated plants turn yellow, show wilting, and may ultimately die. But they may survive submergence for weeks, provided the water is kept well aerated. 16 Even cranberries and blueberries, which will stand submergence for months when inactive, are harmed by water-logging the soil only 3 or 4 days in summer. 48 Rice, too, gives a better growth when the soil is frequently irrigated and drained. 24, 25
In nutrient solutions, plants grow best where constant and thorough aeration is given. The superiority of roots grown in aerated cultures is shown not only by their greater weight but also by their greater extent and degree of branching. 4 The living parts of bog plants are superficially placed either because they assume a horizontal position above the water level, the taproot if present being ephemeral and replaced by horizontal laterals, or because the roots, if all vertical, die at the water surface. 58 In plants like cattails, bulrushes, tall marsh grass, etc., whose roots may be submerged for long periods, special anatomical adaptations promote gaseous exchange.
Exclusion of oxygen from the roots of most plants interferes with the respiration of the protoplasm of the root cells, resulting in its death and the consequent failure of the roots to function as absorbers for the plant. The cessation of water intake is soon followed by the progressively decreasing turgor of the shoot and leaves and finally by wilting and death. 129 Different roots respond somewhat differently to variations in the composition of the soil atmosphere. Roots of the mesquite (Prosopis) continue growth for a considerable period of time in a soil atmosphere containing only 2.67 per cent oxygen, while those of a cactus (Opuntia) promptly cease growing. 34, 35 An increased air supply to the roots of certain species favors root branching and probably accelerates root growth. 38, 39
Plants growing naturally in well-drained soils are much more sensitive to the composition of the soil atmosphere than those in poorly drained and poorly aerated habitats. Certain deeply rooted species like alfalfa are able to grow in an atmosphere containing only 2 per cent oxygen. 36 It seems probable that one of the beneficial effects of good rains, especially in heavy soils, is the increased oxygen supply to the roots, for rain water, although displacing the soil gases, is a solution highly charged with oxygen and has a markedly stimulating effect upon growth. 118
Orchard trees have been known to die as a result of the "puddling" of the soil and resulting deficient aeration. Also, trees are sometimes killed by cattle tramping and packing the ground about them to such a degree that the air supply to the roots is largely cut off. 159 In heavy soils, such large quantities of carbon dioxide may be given off by a sod of grass roots growing under fruit trees that the trees and fruits do not develop normally. 99 It has been demonstrated experimentally that many plants respond to an increased carbon dioxide content of soil by developing roots which are much shallower and more widely spreading In the surface soil. It is believed that carbon dioxide content of garden soils is sometimes so high as to be detrimental to the root development of some common garden species. 148,149
A deep, well-prepared seed bed is essential for good aeration, especially in humid regions. It not only promotes plant growth directly by lightening and warming the soil and conserving moisture, but also indirectly by promoting various biological activities, especially ammonification and nitrification. The preparation of a good seed bed, as against no seed bed, resulted in an increase in the yield of corn of 14.5 bushels per acre in Illinois. 142 When soil is plowed, the bottom of the furrow slice comes into direct contact with the aboveground atmosphere, and aeration in the furrow slice itself is greatly increased. Listing has less effect upon soil aeration and is not practiced in heavy soils of humid regions. The effects of irrigation in changing the soil atmosphere have already' been mentioned. Underdraining has a very beneficial effect, since large quantities of air move into the interspaces formerly occupied by water. This has a pronounced effect upon both the chemical and biological activities taking place in the soil, as well as upon root growth.
The soil exerts such a marked effect upon the form, distribution, and activities of roots that a knowledge of it is necessary for an understanding of root behavior and the resultant effect upon crop production. By slow physical, chemical, and biological processes acting through the ages, the outer crust of the earth's surface has been transformed from rock to soil. These same slow soil-forming processes are still actively at work maintaining the soil in a condition of tilth and productivity. The texture of the soil resulting from the differences in the sizes of the soil particles, its structure which depends upon the arrangement of these particles, the amount of humus, and the activities of the organisms which the soil supports, all exert a marked effect upon the nutrients, water content, and air content, as well as upon the temperature of the soil. All these affect root behavior and crop yield. A knowledge of the physical and chemical relations of the soil particles is necessary for an understanding of the interrelation between soil and roots and for proper soil management. Biological activities in the soil result in the decay of plant and animal debris, thus liberating for use vast quantities of food materials. The inert nitrogen of the atmosphere is also made available to plants through the work of soil organisms. These and many other chemical changes, especially of the mineral soil constituents, provide the nutrients of the soil solution. Although very dilute, this solution is usually of a suitable concentration and contains the nutrients in the proper proportion for plant growth. They are removed by root absorption and by leaching. Under poor management soils may become depleted. Water content is an important and often limiting factor in crop production. The optimum moisture of a soil, which leaves sufficient interspaces for aeration, is generally near the optimum condition for root activities and plant growth. Such a water content affords very favorable conditions for the activities of the soil flora and fauna. The large quantities of water absorbed and transpired by plants bear little relation to the absorption and use of soil nutrients. Every solute enters the plant according to physical laws of diffusion and quite independently of any other solute or of water. Absorption, root growth, and many chemical and physical soil processes are retarded by low temperatures and many soil-borne, disease-producing organisms are conditioned in their growth by the temperature of the soil. The volume and movement of soil air is determined largely by the water content. As a result of the respiration of roots and soil organisms and other oxidative processes, soil air, contains less oxygen than the ordinary atmosphere. Aeration exerts a marked effect upon many soil processes and biological activities, including root growth. This complex, highly organized mixture of disintegrated and decomposed rocks, humus, water, air, dissolved substances, and microorganisms is the environment in which roots grow and from which they absorb the water and nutrients necessary for crop production.