The Movement of Soil Water. 65 



are no water films to react on the structure and make it looser. 

 The structure tends to be the closest possible, and a soil to be 

 persistently permeable must be composed of fairly large parti- 

 cles. An approach toward uniformity is also favorable. The 

 presence of much smaller material means a clogging of the inter- 

 spaces and a decrease of permeability. 



Evaporation from soils. The loss of water 

 from soils by evaporation takes place practically altogether 

 from the surface of the soil. The atmosphere of a moist soil is 

 always saturated with water or nearly so, but experiment has 

 shown that diffusion of water vapor through it, or out of it 

 into the open air, is very slow. * It is only at the actual soil 

 surface, or within a few inches of it, that any appreciable evap- 

 oration takes place. Evaporation has therefore a very close 

 dependence upon capillarity. The evaporation of a little water 

 from, say, the surface inch of a soil decreases the water content 

 of that inch, the capillary pressure increases correspondingly 

 and water is drawn from lower and moister layers to restore the 

 equilibrium. Continued evaporation must, then, be accom- 

 panied by continued capillary supply. The motive power lies, 

 of course, in the evaporation, the energy being supplied from 

 without as the heat necessary for vaporization. 



Obviously evaporation will be shut oflf if the capillary supply 

 is interrupted and this is the secret of the so-called "dry mulch." 

 This consists of a layer of dry material of any sort, even dry soil, 

 spread on top of the soil. It constitutes an interruption to the 

 continuity of the capillarity films, breaks, as it were, the pipe 

 through which the water has been coming to the surface, and 

 prevents its further rise. Its efficacy as a water conserver has 

 been amplv demonstrated in arid region agriculture and the 

 much talked of "dry farming" methods owe most of their utiHty 

 to its employment. A mulch may, however, be formed by 

 purely natural processes. When evaporation is intense and 

 capillary supply less so, water may be lost from the surface soil 

 so rapidlv that it is not replaced from below and the surface 

 soil dries to the point where the continuity of the water film 

 svstem is destroved. This dry layer then acts as a mulch, 

 prevents any further capillary supply, and stops evaporation. 

 There is an apparent inconsistency between this statement and 



*See Buckingham. — Bull. 34, Bureau of Soils, V. S. Dept. of Agriculture (1907). 



