The Movements of Soii^ Water. 6 1 



points of union and that these forces are extremely important 

 in the determination of soil structure. But they have another 

 and not less important influence, namely, that on the movement 

 of soil water. In addition to the pulls exerted on the soil parti- 

 cles to which the films are attached (as explained in the previous 

 paper) there is also a hydrostatic tension exerted upon the water 

 itself and tending to draw more water into the film. The phe- 

 nomena are quite analogous to the well know rise of water in 

 capillary tubes, except that in the latter case there is only one 

 curved water-air surface, namely, that at the top of the column 

 of water. In the soil the whole water-film system is bounded by 

 these surfaces and the total forces concerned are correspondingly 

 greater. Water will rise much farther in a soil column than in 

 a caj illary tube, but in both cases it rises until the total surface 

 tension of the capillary system is just balanced by the weight 

 of the water which has been lifted. Were there no opposing 

 force of gravity the capillary movement of water into a soil would 

 continue to saturation. The individual films of the system tend 

 to thicken because of the tensions on their bounding surfaces, 

 and the whole film system tends to extend itself (by virtue of 

 the "hygroscopic forces," otherwise adhesion) to those particles 

 which had not previously been wetted at all. * There exists, 

 then, because of surface tension or "capillarity," an ever present 

 tendency for the movement of water into soils, a tendency the 

 intensity of which is measured by the total hydrostatic tension 

 of the water films. This tension w^e shall hereafter call the 

 capillary pressure. J 



But the capillary pressure, like the structural forces of the 

 preceding paper, varies with the thickness of the water-films 

 and hence with the water content of the soil. The thicker the 

 individual water films the less is the average curvature of their 

 bounding surfaces, and hence the less is the capillary pressure. 1 1 

 In any given soil, therefore, the capillary pressure is greater the 

 less the water content already present in the soil. It follows 

 that in a soil of uniform mechanical composition but varied water 



♦The first step in this extension of the water system to dry particles is probably a conden- 

 sation or absorption of w;iter out of the soil atmosphere onto the dry particles. Of 

 course the forces producing this abforption may be included among the hygroscopic forces 

 above mentioned. On the absorption of vapors bv soils «ee: Patten and Gallagher. — 

 Bull. 51; Bureau of Soils, U. S. Dept. of Agriculture, (1907), and the literature there 

 cited. 



Jin calling this force a pressure we must remember that it is applied in a negative direction . 

 I |The surface tension on a curved water-air surface varies inversely with the radius of curva- 

 ture of the surface. 



