THE FORMS OF SOIL- WATER 173 



He found, for example, that a six-inch column would deliver 

 six times more water to its surface in a given time than a 

 thirty-inch column operating under the same conditions. 



In air-dry soil it is obvious that, before capillarity may 

 function, a continuous film must be present. Such a condi- 

 tion is impossible unless some of the more active capillary 

 moisture is in the soil. The water content in a soil must often 

 be rather high before capillarity is a noticeable phenomenon. 

 This condition is taken advantage of in the use of soil-mulches, 

 where a loose dry layer of soil on the surface may check 

 evaporation by impeding capillary rise. The presence of oily 

 substances on the soil grains may also be of some importance 

 in this respect. 



Texture. — 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. Both the greater exposure 

 of surface and the increased pressure serve to raise the fric- 

 tion 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 distance to which water may be drawn by the effective 

 capillary power of a soil, equilibrium being established, de- 

 pends on the number of interstitial angles. The greater the 

 number of angles, the greater is the total pulling power of 

 the films. As a silt soil contains a larger number of such 

 angles, its capillary pull is greater than that of sand, and con- 

 sequently the ultimate movement would be of greater scope. 

 The finer the texture, then, the slower is the rate of capillary 

 movement but the greater is the distance. 



The relation of texture to rate and height of capillary move- 

 ment in air-dry soil is shown by the following unpublished 



