HOW SOILS DRY OUT 147 
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 expen- 
diture 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 hygro- 
scopic 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 sum- 
mer, 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. 
