Any practice which enables the rains to sink quickly to considerable depths should be adopted by the dry-farmer.  This is perhaps one of the great reasons for advocating the expensive but usually effective subsoil plowing on dry-farms.  It is a very common experience, in the arid region, that great, deep cracks form during hot weather.  From the walls of these cracks evaporation goes on, as from the topsoil, and the passing winds renew the air so that the evaporation may go on rapidly.  The dry-farmer must go over the land as often as needs be with some implement that will destroy and fill up the cracks that may have been formed.  In a field of growing crops this is often difficult to do; but it is not impossible that hand hoeing, expensive as it is, would pay well in the saving of soil moisture and the consequent increase in crop yield.

How soil water reaches the surface

It may be accepted as an established truth that the direct evaporation of water from wet soils occurs almost wholly at the surface.  Yet it is well known that evaporation from the soil surface may continue until the soil-moisture to a depth of eight or ten feet or more is depleted.  This is shown by the following analyses of dry-farm soil in early spring and midsummer.  No attempt was made to conserve the moisture in the soil:—­

Per cent of water in Early spring  Midsummer
1st foot             20.84         8.83
2nd foot             20.06         8.87
3rd foot             19.62         11.03
4th foot             18.28         9.59
5th foot             18.70         11.27
6th foot             14.29         11.03
7th foot             14.48         8.95
8th foot             13.83         9.47
Avg                  17.51         9.88

In this case water had undoubtedly passed by capillary movement from the depth of eight feet to a point near the surface where direct evaporation could occur.  As explained in the last chapter, water which is held as a film around the soil particles is called capillary water; and it is in the capillary form that water may be stored in dry-farm soils.  Moreover, it is the capillary soil-moisture alone which is of real value in crop production.  This capillary water tends to distribute itself uniformly throughout the soil, in accordance with the prevailing conditions and forces.  If no water is removed from the soil, in course of time the distribution of the soil-water will be such that the thickness of the film at any point in the soil mass is a direct resultant of the various forces acting at that particular point.  There will then be no appreciable movement of the soil-moisture.  Such a condition is approximated in late winter or early spring before planting begins.  During the greater part of the year, however, no such quiescent state can occur, for there are numerous disturbing elements that normally are active, among which the three most effective are (l) the addition of water to the soil by rains; (2) the evaporation of water from the topsoil, due to the more active meteorological factors during spring, summer, and fall; and (3) the abstraction of water from the soil by plant roots.