The moisture in soil is derived from two sources--the rain and the ground-water.
Above the level of the ground-water the soil is kept moist by capillary attraction and by evaporation of the water below, by rainfall, and by movements of the ground-water; on the other hand, the upper layers are constantly losing moisture by evaporation from the surface and through vegetation.
When the ground-water rises it forces air out of the soil; when it falls again it leaves the soil moist and full of air.
Above the level of the ground-water all soils contain air, varying in amount with the degree of looseness of the soil.
The greater portion of this region is an open steppe, sandy in places and in others dotted with low volcanic hills, but with occasional ground water and in favourable seasons furnishing support for a considerable pastoral population.
