186 Soil-Moisture Minimum [384 



THE EXPERIMENTAL DETERMINATION OF A DYNAMIC 

 SOIL-MOISTURE MINIMUM 



By HOWARD E. PULLING 



The conditions determining the rates of water movement 

 in soils have long been recognized as of great importance 

 in plant physiology, since they not only limit the amount 

 of water a given root system may receive but also modify 

 the effects of all soil processes upon rooted plants. In aerated 

 soils water is moved by surface forces of the soil-moisture 

 films. The magnitude of these forces is dependent upon the 

 curvature of the film-air surfaces and not upon the amount 

 of water in the soil, so that a soil volume might augment 

 its water content at the expense of another contiguous soil 

 volume that contained actually less water than the first. 

 The amount of water that may be moved in unit time de- 

 pends, however, also upon the amount of water in the films 

 and a certain minimum should exist below which the quan- 

 tity of water subject to capillary movement is too small to 

 admit of any but negligible rates, regardless of the mag- 

 nitude of the surface forces. 



In aerated soils the water that responds to surface tension 

 urge is accumulated around the points of contact of soil 

 grains, so that the water adsorbed upon the surface of the 

 grains, imbibed by the soil colloids and held as water of 

 hydration by the grain constituents need not be considered 

 in the present discussion. It is apparent that the greater 

 the number of such points of contact between the soil grains 

 in a given gross volume of soil, the greater should be the 

 number of similar capillary masses of water and, conse- 

 quently, the greater should be the amount of water in the 

 soil when the rate of capillary movement becomes negligible. 

 Accordingly a complete statement of this minimum for any 

 soil may be represented not by a point but by a curve, in 



