Ground air is in constant motion, its movements depending upon a great many factors, some among these being the winds and movements of the atmospheric air, the temperature of the soil, the surface temperature, the pressure from the ground water from below, and surface and rain water from above, etc.
=Ground Moisture.=--The interstices of the soil above the ground-water level are filled with air _only_, when the soil is absolutely dry; but as such a soil is very rare, all soils being more or less damp, soil usually contains a mixture of air and water, or what is called _ground moisture_.
Ground moisture is derived partly from the evaporation of the ground water and its capillary absorption by the surface soil, and partly by the retention of water from rains upon the surface. The power of the soil to absorb and retain moisture varies according to the physical and chemical, as well as the thermal, properties of the soil.
Loose sand may hold about 2 gallons of water per cubic foot; granite takes up about 4 per cent of moisture; chalk about 15 per cent; clay about 20 per cent; sandy loam 33 to 35 per cent; humus[10] about 40 per cent.
=Ground Temperature.=--The temperature of the soil is due to the direct rays of the sun, the physicochemical changes in its interior, and to the internal heat of the earth.
The ground temperature varies according to the annual and diurnal changes of the external temperature; also according to the character of the soil, its color, composition, depth, degree of organic oxidation, ground-water level, and degree of dampness. In hot weather the surface soil is cooler, and the subsurface soil still more so, than the surrounding air; in cold weather the opposite is the case. The contact of the cool soil with the warm surface air on summer evenings is what produces the condensation of air moisture which we call dew.
=Bacteria.=--Quite a large number of bacteria are found in the soil, especially near the surface, where chemical and organic changes are most active. From 200,000 to 1,000,000 bacteria have been found in 1 c.c. of earth. The ground bacteria are divided into two groups--saprophytic and pathogenic. The saprophytic bacteria are the bacteria of decay, putrefaction, and fermentation. It is to their benevolent action that vegetable and animal _débris_ is decomposed, oxidized, and reduced to its elements. To these bacteria the soil owes its self-purifying capacity and the faculty of disintegrating animal and vegetable _débris_.