THE FORMS OF SOIL-WATER 171 



its viscosity at 25° is 50 and at 30°, 45. This explains to a 

 large degree the increased rate of capillary movement due to 

 temperature rise. 1 The distance of such adjustment would, 

 however, be lessened somewhat. Salts in solution would tend 

 to check the rate of capillary movement both through in- 

 creased viscosity and the influence on surface tension. 2 It 

 would only be in alkali soils, where the concentration of soluble 

 salts is very great, that any considerable retardation would 

 occur. 



Thickness of capillary film. — It has been repeatedly noticed, 

 in the study of the capillary adjustment between two soils 

 that the lower the percentage of water, the slower is the move- 

 ment. This indicates that the thickness of the outer capillary 

 film, which connects the interstices in which lies the bulk of 

 the movable soil-water, is an important factor in the rate of 

 movement. 



The above phenomena may be empirically explained as fol- 

 lows : Let it be supposed that a withdrawal of water occurs at 

 A (see Fig. 32), the interstitial space between two of the 

 particles, the water surface being represented by the line aa'. 

 There is an immediate increase in the curvature of this sur- 

 face, and water tends to flow through the capillary film chan- 

 nel (ccV) toward this area of greater tension. If water 



1 Bouyoucos has shown that the movement in a soil column of uniform 

 moisture is from the warmer portion toward the colder. The movement 

 from a moist layer to a dryer one goes on more rapidly than when the 

 moist soil is cool and the dry soil warm. Bouyoucos, G. J., Effect of 

 Temperature on Movement of Water Vapor and Capillary Moisture in 

 Soils; Jour. Agr. Kes., Vol. V, No. 4, pp. 141-172, Oct., 1915. 



a Wollny, E., Untersuchungen uber die Kapillare Leitung des Wassers 

 in Boden. Forsch. a. d. Gebiete d. Agr.-Physik, Band 7, Seite 269-308, 

 1884. Also, Forsch. a. d. Gebiete d. Agri.-Physik, Band 8, Seite 206-220. 

 1885. 



Briggs, L. J., and Lapham, M. H., Capillary Studies; U. S. Dept. 

 Agr. Bur. Soils, Bui. 19, pry 5-18, 1902. 



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

 Tension of the Soil Solution brought about by the Addition of Soluble 

 Salts; Jour. Agr. Ees., Vol. IV, No. 2, pp. 187-192, May, 1915. 



Davis, K. O. E., The Effect of Soluble Salts on the Physical Proper- 

 ties of Soils; U. S. Dept. Agr. Bur. Soils, Bui. 82, pp. 23-31, 1911. 



