THE FORMS OF SOIL WATER 229 



oils on the particles, and a consequent exposure of clean 

 surfaces for capillary movement. These authors found 

 also that concentrated solutions reduced the rate of 

 capillary movement. Davis, 1 in working with a silt loam, 

 obtained variable results, some salts depressing and some 

 accelerating capillary rise. Potassium acid phosphate 

 caused the maximum retardation, while ammonium ni- 

 trate most markedly increased the rate. Since only one 

 soil was used and the greatest observed capillary rise was 

 less than twelve inches, additional data must be presented 

 before it is clear that the concentration of salts may be- 

 come a very important factor in humid soils. In alkali 

 soils, in which the concentration of the salts is very great, 

 there is no doubt that considerable retardation may occur. 



154. Effect of texture on capillary movement. In 

 soils of fine texture, not only is the amount of film surface 

 exposed greater than in coarse soils, but the curvature of 

 the films is also greater, due to the shorter radii. The 

 effective pressure exerted by the films is consequently 

 much higher in fine-grained soil. The greater exposure 

 of surface and the increased pressure both serve to raise 

 the friction coefficient and retard the rate of flow. The 

 finer the texture of the soil, other factors being equal, the 

 slower is the movement of capillary water. Water should 

 therefore rise less rapidly from a water table through a 

 column of clay than through a sand or a sandy loam. 



The height to which water may be drawn by the effec- 

 tive capillary power of a soil, equilibrium being estab- 

 lished, depends on the number of interstitial angles. The 

 greater the number of angles, the greater is the total 



1 Davis, R. O. E. The Effect of Soluble Salts on the Physi- 

 cal Properties of Soils. U. S. D. A., Bur. Soils, Bui. 82, pp. 

 23-31. 1911. 



