222 SOILS: PROPERTIES AND MANAGEMENT 



Fig. 36. — Diagram show- 

 ing the mechanics of the 

 capillary movement of 

 water in soil. The read- 

 justment takes place in 

 the direction of (.4) due 

 to the high tension devel- 

 oped by the sharp film 

 curvature at this point. 



to become equal, due to the pulling force developed by 

 ♦he angle of curvature between the particles. It is evi- 

 dent that differences in curvature must be the motive 

 force in the capillary movement 

 of soil water. Let it be supposed, 

 for convenience, that three equal 

 spheres when brought in contact 

 contain unequal amounts of water 

 in the angles of curvature (see 

 Fig. 36). In this case the greater 

 pull would exist at A, since the 

 angle here is more acute. Conse- 

 quently water must move through 

 the connecting film until the pull 

 at A and that at B become the same. Such an adjust- 

 ment might go on over a large number of films, and if 

 one end of the column was exposed to an evaporation 

 of just the right rate and the other end was in contact 

 with plenty of moisture, large quantities of water would 

 be pumped by capillarity. 



This capillary movement may go on in any direction in 

 the soil, since it is largely independent of gravity; yet 

 under natural field conditions the adjustment tends to 

 take place very largely in a vertical direction. When 

 a soil is exposed to evaporation the surface films are 

 thinned and water moves upward to adjust the ten- 

 sion. This explains why such large quantities of soil 

 water may be lost so rapidly from an exposed soil. 

 Capillary adjustment may go on downward, also, as is 

 the case after a shower. Here the rapidity of the ad- 

 justment is aided by the weight and movement of the 

 water of percolation. 

 The capillary adjustment in a soil may go on under 



