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 diminisli 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. 



