As the temperature rises in spring, with a decrease in the relative humidity, and an increase in direct sunshine, evaporation from the soil surface increases greatly.  However, as the topsoil becomes drier, that is, as the water fihn becomes thinner, there is an attempt at readjustment, and water moves upward to take the place of that lost by evaporation.  As this continues throughout the season, the moisture stored eight or ten feet or more below the surface is gradually brought to the top and evaporated, and thus lost to plant use.

The effect of rapid top drying of soils

As the water held by soils diminishes, and the water film around the soil grains becomes thinner, the capillary movement of the soil-water is retarded.  This is easily understood by recalling that the soil particles have an attraction for water, which is of definite value, and may be measured by the thickest film that may be held against gravity.  When the film is thinned, it does not diminish the attraction of the soil for water; it simply results in a stronger pull upon the water and a firmer holding of the film against the surfaces of the soil grains.  To move soil-water under such conditions requires the expenditure of more energy than is necessary for moving water in a saturated or nearly saturated soil.  Under like conditions, therefore, the thinner the soil-water film the more difficult will be the upward movement of the soil-water and the slower the evaporation from the topsoil.

As drying goes on, a point is reached at which the capillary movement of the water wholly ceases.  This is probably when little more than the hygroscopic moisture remains.  In fact, very dry soil and water repel each other.  This is shown in the common experience of driving along a road in summer, immediately after a light shower.  The masses of dust are wetted only on the outside, and as the wheels pass through them the dry dust is revealed.  It is an important fact that very dry soil furnishes a very effective protection against the capillary movement of water.

In accordance with the principle above established if the surface soil could be dried to the point where capillarity is very slow, the evaporation would be diminished or almost wholly stopped.  More than a quarter of a century ago, Eser showed experimentally that soil-water may be saved by drying the surface soil rapidly.  Under dry-farm conditions it frequently occurs that the draft upon the water of the soil is so great that nearly all the water is quickly and so completely abstracted from the upper few inches of soil that they are left as an effective protection against further evaporation.  For instance, in localities where hot dry winds are of common occurrence, the upper layer of soil is sometimes completely dried before the water in the lower layers can by slow capillary movement reach the top.  The dry soil layer then prevents further loss of water, and the wind because of its intensity has helped to conserve the soil-moisture.