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.
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