512 STATE BOARD OF AGRICULTURE. 



different for various soils, lliat it decreases with a rise in moisture con- 

 tent, and tliat it is completely satisfied at a rather high moisture con- 

 tent, then our present views concerniuo- the movement of capillary water 

 in moist soils needs modification. The present theory regarding the 

 capillary movement of water consists of an analogy from the rise of 

 water in capillary tubes. The interstitial spaces of a soil mass are con- 

 sidered as forming channels analogous to capillary tubes, and are often 

 designated as bundles of capillary tubes. The capillary water is be- 

 lieved to exist as surface films around the particles, and as capillary films 

 in the capillary spaces between the particles, and its movement is said 

 to depend entirely upon the curvature of the capillary films. When a 

 dry soil, for instance, is well moistened and brought to equilibrium, the 

 water films are thick and the curvature of the capillary films small, and 

 there will be no further capillary attraction of water if this soil is 

 brought in contact with water. If now this soil is allowed to dry at 

 the top the surface films become thinner and the force of the capillary 

 films increases in direct ratio with their degree of curvature, hence, there 

 will be a pull of water from the thicker surface filrus and less curved 

 capillary films below, towards the surface. 



It is obvious that this theory of capillary movement of water attributes 

 the whole cause of the capillary motion of water in a moist soil to the 

 curvature of the capillary films between the particles, and considers the 

 moist soils as being passive, inactive, and exerting no influence whatever 

 upon the motion of water. Indeed, Briggs^^ and Lapham in trying to ex- 

 plain the differences in capillary action in dry and moist soils, make the 

 following statement: "In a moist soil, however, we have quite another 

 condition. A film of the liquid covers all the surfaces of the soil grains. 

 Since this film, once established, is maintained in a saturated at- 

 mosphere, it follows that the soil-air and solid-liquid surfaces forces no 

 longer r>lay any part in the capillary movement, which if? produced en- 

 tirely by the air-liquid surface force and is opposed onlv by the weight of 

 the liquid column." In view of this general belief Bri^as, as well as 

 other investigators, has tried to alter the properties of the soil water, 

 by increasing its surface tension, etc., with the object in view of increas- 

 ing its capillary action. 



If it were true that as long as a thin film of water is maintained in a 

 damp or slightly moist soil, the soil material itself exerts no longer any 

 influence upon the movement of capillary water, then the preceding 

 theory might be true. But we have seen in postulate (1) page 14, that 

 the soils, and especially those rich in colloidal material, possess a very 

 great attractive power for water, that this attractive power is satisfied 

 only at a rather high moisture content, that as long as it is not satis- 

 fied the soils will continue to take up water, and that they hold the 

 Avater Avith a force of great magnitude. In view of tJie considerations 

 presented in this postulate and in tiew of the fact that the precedinr/ 

 thermal movement of icater appeals to he largely controlled hy the at- 

 tractive forces of the soil for water, it seems wrong to consider the soil 

 material in moist condition as a static, passive, inactive and irresponsive 

 slceleton upon which the liquid plays its role. The soil material, in 

 moist condition, short of saturation, is di/namic and not static in respect 

 to moisture movement. Hence, the capillary movement of water should 



"U. S. Dept. of Agr. Bureau of Soils Bulletin No. 19, p. 2S (1902). 



