THE FOEMS OF SOIL-WATER 163 



obtained may be fairly accurate for a sand, they are certainly 

 much too high for heavy soils. Comparisons with field soils 

 have shown the data obtained by the above method to be from 

 30 to 130 per cent, too high.^ 



91. The capillary capacity of soils.— As might nat- 

 urally be expected, the factors that tend to vary the amount 

 of capillary water in a soil are several and their study is 

 rather complex due to the secondary influences that they may 

 generate and to the variable nature of the capillary moisture. 

 These factors may be discussed under four heads: (1) surface 

 tension, (2) texture, (3) structure and (4) organic matter. 



Any condition that will influence surface tension will ob- 

 viously influence the forces active in the outer portion of the 

 capillary water. A rise in temperature, for example, if the 

 soil is capillarily saturated, will allow some of the water to 

 become gravitational. A lowering of temperature would cause 

 an opposite change. This theory has been verified by certain 

 experiments by King,^ in which he found, other conditions 

 being constant, a very decided influence on capillary water 

 through change of temperature. "WoUny ^ has shown that a 

 depression of .65 per cent, in sand to as high as 3.7 per cent. 

 in kaolin may occur from a rise in temperature of twenty 

 degrees. While surface tension may be greatly varied by the 

 presence of salts in solution, the soil-water is generally so 

 dilute that the condition is not very important ^ in determining 



^Alway, F. J., and McDole^ G. R., The Belation of Movement of 

 Water m a Soil to tts Hygroscopicity and Initial Motstness; Jour. Agr. 

 Res., Vol. X, No. 8, pp. 391-428, 1917. 



Israelson, 0. W., Studies on Capacities of Soils for Irrigation 

 Water; Jour. Agr. Ees., Vol. ZIII, No. 1, pp. 1-36, 1918. 



^King, F. H.J Fluctuations in the Level and Bate of Movement of 

 Ground Water; U. S. Bept. Agr., Weather Bur., Bui. 5, pp. 59-61, 

 1892. 



® WoUny, E., TJntersuchungen uter die Wasserlcapacitat der Bodenarten; 

 Forsch. a. d. Gebiete der AgrL-Physlk, Band 9, Seite 361-378, 1886. 



^Karraker, P. E., Effect on Soil Moisture of Changes in the Surface 

 Tension of the Soil Solution Brought About By Addition of SolubH 

 Salts; Jour. Agr. Res., Vol. 4^ No. 2, pp. 187-192, May, 1915. 



