THE FORMS OF SOIL-WATER 



165 



film, tending to give more water at the 

 base of the column. 



The condition may be explained em- 

 pirically as follows: If a number of par- 

 ticles carrying maximum capillary films 

 are brought together vertically, the weight 

 of a large portion of the conducting film 

 is thrown momentarily on the surfaces at 

 the top. The capillary spaces at this point 

 immediately lose water downward, so that 

 they may assume a greater curvature and 

 thus support this extra weight thrown on 

 them. This curvature must be sufficient to 

 balance the curvature pressure of the par- 

 ticles below plus the weight of the water 

 in the connecting films. The particles be- 

 neath are at the same time undergoing a 

 similar adjustment with a set of particles 

 farther below, losing water in order to 

 allow a change of curvature. The action 

 continues in this manner in an attempt to 

 establish equilibrium, thus giving more 

 water at the bottom of the column. If the 

 amount of capillary water is too great to be 

 supported, enough is lost by gravity to 

 bring about an equilibrium (see Fig. 29). 



The above illustration, however, does not 

 apply strictly to soil conditions, since only 

 part of the capillary water is in a true film 

 form and free to move with extreme ease. 

 Moreover, rain water is applied from 

 above, where also occurs rapid evaporation. 

 Thus at any particular time the moisture 

 content of a field soil might be higher near 

 the surface than farther down in the soil 



Fig. 29. — Diagram 

 showing in a con- 

 ventional way 

 the adjustment 

 tendency of the 

 outer capillary 

 water in a long 

 column and the 

 appearance of 

 free water if the 

 weight is too 

 great. 



