The physical properties of soils depend largely upon the relative proportions of stones, gravel, sand, clay, lime, and organic matter pres ent. Sandy soils are as a rule light colored, drv, warm, of low adhesive power, have little absorptive capacity for moisture and fertilizing matter, and are generally poor. Clayey soils are more adhesive and have a stronger absorp tive power for water and fertilizing matter. They are as a rule wetter, cooler and more dif ficult to work than sandy soils, but are naturally more fertile. Soils containing a considerable amount of humus are dark colored and light in weight and have a strong absorptive power. Hence, humus soils are frequently wet soils. Lime improves the structure of both sandy and clayey soils, hastens the decay of organic mat ter, corrects acidity and promotes nitrification and other beneficial bacterial activities.
The color of soils is determined in most cases by the proportions of organic matter and oxide of iron they contain and is of little im portance except that it probably affects in some degree the temperature of the soil (see below).
Soils vary widely in weight, as already inti mated. Hall gives the following figures: is drawn from the soil, thus lowering its tem perature.
Relation of Soils to The develop ment of plants is impossible without a sufficient supply of water in the soil at all periods of growth. Water is not only an important con stituent of plant tissue, but it is of the greatest importance as a solvent and carrier of food in the soil and in the plant. A comparatively small part of that taken into the plant is used to build tissue, the larger part passing out through the leaves by transpiration, which is, however, essential to the healthy growth of the plant. It is estimated that for each pound of dry matter produced in the crop from 250 to 500 pounds of water is drawn from the soil. All soils are capable of absorbing and retaining moisture, but the extent to which they do so varies widely, a fact which is of great import ance in determining the mutual adaptability of crops and soils. As a rule the capacity of soils to hold water is proportional to the size of the soil particles, the finer the particles the larger the total surface area of the particles and the greater the capacity for water, although this is not invariably tru,:. King has computed the pore space and surtace area of the particles of different kinds of soils as follows: Texture and Structure of The pro ductiveness of a soil depends to a considerable extent upon its texture (fineness of particles) and structure (arrangement of particles). These properties determine largely the circulation of water and gases, the solution and retention of plant food and the growth of roots of plants. Good texture and structure thus enable a poor soil to produce better crops than a more fertile soil of which the texture and structure are not so good. One of the main objects of tillage is
to promote better soil texture and structure. These properties are affected to a considerable extent by various fertilizers. For instance, lime has the power of flocculating the soil particles and thus renders soils more porous, while sodium nitrate and many other substances have a tendency to puddle the soil, that is, to keep the particles separate and thus prevent the open floccular structure. Soils containing large amounts of clay and other fine particles are most injured by puddling and hence most bene fited by flocculating by means of lime.
Relation of Soils to The tempera ture of the surface soil is subject to the same changes as that of the air, but these changes occur more slowly. The variations decrease as the depth increases until they finally disappear. There are several modifying influences affecting the temperature of the soil. The first of these is color. A dark colored soil is usually warmer than a light colored soil. A soil containing much sand or gravel is as a rule warmer than one containing much clay or humus, as already intimated. Soils exposed so as to receive a large amount of the direct rays of the sun are warmer than those not having such exposure. Probably the most important factor determining the temperature of a soil is its water content. A wet soil is cold. Evaporation is a cooling process, and the heat necessary to carry it on Coarse sand allows water to run through freely, retaining relatively little, while fine clay absorbs and retains a large amount. The pro portion of organic matter is also an important factor in determining the water-holding ca pacity of soils, the larger the proportion of or ganic matter the greater the capacity of the soil for holding water. Healthy root development is not possible in a soil saturated with moisture. The most favorable amount of water for plant growth is stated to be from 40 to 75 per cent of that which the soil is capable of holding when completely saturated. Another important factor to be taken into consideration is that soils vary in the readiness with which they give up water to growing plants when the amount becomes limited. Crops can utilize a larger proportion of the water in open, coarse-grained sandy soils than of that in compact fine-grained clay soils. In other words, plants will suffer from drought on compact clay soils containing a proportion of moisture which would be entirely sufficient for their needs on sandy soils. Thus, while sandy soils have a much smaller total storage capacity for water than clay soils, the available water is more nearly equal in the two cases than would at first be supposed. Sachs found that tobacco plants began to wilt when the water content of sandy soil was reduced to 1.5 per cent, of clay soils to 8 per cent and of sand and humus to 12.3 per cent.