The temperature of field soils shows rather definite changes at different depths and at different seasons of the year. The changes are determined by the amount of the radiant energy that reaches the soil surface and by the thermal properties of the soil. The amount of radiant energy reaching the soil surface is determined by the angle at which the sunlight strikes the earth and by the nature of the atmosphere. Only the part of the energy that is absorbed causes changes in soil temperature.

Dark-colored soils having a low reflecting power capture a much higher proportion of radiant energy than do light-colored soils. Thus a fallow Brunizem absorbs nearly 80 percent of the radiation that reaches the soil surface. A grass-covered soil and quartz sand absorb, respectively, 65 percent and 30 percent.

The energy that has been adsorbed by the soil surface is disposed of in one or more of the following ways: By re-radiation to atmosphere as longwave radiation, by heating of air above the soil by convection, by increasing the temperature of the surface soil, or by conduction to the deeper soil layers.

During daylight hours there is normally a net influx of heat to the soil surface. This situation is reversed during the night. Consequently the temperature of the exposed soil surface shows wider daily fluctuations than those of the air.

Soil temperatures vary in a characteristic manner both on a daily and seasonal basis. The fluctuations in both instances are greatest at the surface and decrease in size at lower depths. A well-defined time lag, which increases with depth in the soil, also occurs in both the daily and seasonal variations.

7. Soil temperatures in temperate regions show characteristic seasonal variations, which decrease with depth.

The observed fluctuations in soil temperature are related to the heat capacity and to the thermal conductivity of the soil. Both of these properties are strongly affected by the proportions of the total soil volume that are occupied by the solid, liquid, and gaseous soil constituents.

The thermal conductivity of water is greater than the corresponding value for the solid soil particles, which in turn is higher than that of air. Consequently the rate of heat flow in a soil is increased when the volume occupied by air is decreased. Such a reduction in the volume fraction of air can be obtained either by compacting the soil or increasing its moisture content. Modifications of this kind, however, cause a reduction in the temperature change that will result from the flow of a given amount of heat into or out of a unit volume of soil.

The foregoing relations are modified in frozen soils because the thermal conductivity of ice is nearly three times that of water, whereas its specific heat is only one-half that of water.

The reaction of soil to physical manipulation, such as is involved in tillage, is primarily an expression of the properties of cohesion, adhesion, and plasticity. These properties are largely determined by the size, shape, and arrangement of the soil particles and by the nature of the water films surrounding them.

8. Soil temperature fluctuates widely during a 24-hour period, but the variations become smaller below the soil surface.

Water molecules, being highly dipolar, are strongly adsorbed and oriented on the negatively charged surfaces of soil particles. Also, water itself exhibits strong cohesion, as shown by its high surface tension. Therefore, as a result of drying, adjacent soil particles are drawn together with considerable force and the soil develops greater cohesion and mechanical strength.

Orientation of the particles into a tighter state of packing results in further shortening and strengthening of the water bonds between the particles. This results in shrinkage of the soil.

Soil loses its cohesion and becomes plastic in the presence of increasing amounts of water. This phenomenon is also associated with the water films and is most strongly shown by soils of high specific surface having platelike particles. Orientation of the soil particles parallel to the direction of the deforming force occurs. The soil then has little or no shearing strength and is readily deformed.

At a very high moisture content, soil loses its cohesive strength and its plastic properties and approaches a fluid in its mechanical properties. In this condition, the interparticle distances are so great that the effects of the oriented water molecules on the surfaces cannot influence the behavior of the particles.

The amount and kind of clay minerals, the nature of the exchangeable cations, and the content of organic matter are factors of major importance in all the foregoing rheologic reactions of soil. The phenomena of cohesion, plasticity, and shrinkage are the most strongly expressed in soils containing a large amount of clay of the montmorillonite type. Such properties are also enhanced by the presence of monovalent cations on the exchange complex.

Many important soil-management problems involving physical properties are associated with the kinds and the amounts of the clay and exchangeable bases in the soil. Increases in bulk density resulting from mechanical manipulation, such as occurs in tillage, are affected strongly by the amount of moisture in the soil. This relation is described by the compaction curve, which relates bulk density resulting from a given compactive effort to the moisture content.

Compaction curves of soils that vary in their content of clay, exchangeable cations, or organic matter content show marked differences in the critical moisture percentage and the maximum bulk density that are obtained.

Soils high in organic matter are less susceptible to compaction and exhibit less well-defined critical moisture percentages. Tillage on such soils therefore is less likely to result in excessive compaction even when performed over a wide range of moisture content.

Soil crusting is another form of consolidation important in soil management. Soil crusts are formed by the desiccation of a thin layer of dispersed surface soil. Such crusts, which vary in thickness from a few millimeters to several inches, frequently have considerable mechanical strength they therefore may keep seedlings from emerging or cause injury to the stems of established plants.

Crust formation involves three processes dispersion, segregation, and desiccation.

Dispersion accompanies the breakdown of the aggregates in the surface soil, which may result from excessive tillage or through the impact of raindrops. Because aggregate stability is low in the presence of excess water, raindrop impact is a major factor leading to the dispersion of unprotected surface soil.

Following the destruction of the aggregates, some segregation of particles of different sizes occurs in the dispersed surface soil. This segregation results in a closer packing of the soil particles and an increase in bulk density.

Mechanical strength of the dispersed soil is greatly increased by drying. Appreciable shrinkage accompanies the drying in soils high in clay or organic colloids. The cracking that results because of shrinkage reduces the mechanical impenetrability of the crust. Therefore the problem of crust formation on soils high in clay or organic matter is usually less acute than on silty soils.

All the physical properties discussed above affect plant growth through their relation in a qualitative sense to the root environment in the soil.

Of equal importance are the quantitative aspects of the soil as a habitat for plant roots. It is necessary that nutrients, air, and water are present in optimum concentrations for normal root development and plant growth. It is necessary also that enough of all of them be present throughout the growing season to meet plant needs.

It is important therefore to emphasize the depth of soil available for rooting as a major physical factor that influences soil management. Severe limitations are placed on the ways in which the soil can be used, in situations where the rooting volume is restricted by bedrock, cemented layers, a high water table, or other root barriers. Deep, permeable soils through which plant roots can develop extensively provide much wider possibilities for use in crop production.