THE SOIL AS AN ASSET 



to such absorptive substances as filters. This 

 power is not equally potent for all substances. 

 The soil exercises a selective property, as, for 

 instance, it has a greater avidity for potash 

 than for chlorine. 



The property is founded on the fact that 

 the surface of solids, such as soil grains, has 

 a tremendous attraction for solutions. Sup- 

 posing a metal ball to be composed of an in- 

 finite number of molecules, each exerting an 

 attraction for each other, the forces acting 

 on the molecules in the center of the ball are 

 equal on all sides, therefore the molecule can 

 move about freely. Those molecules on the 

 surface, however, are being drawn only toward 

 the center of the ball, as the attraction of the 

 surrounding air is much less than the interior 

 of the ball. We are not accustomed to regard 

 a film of water on a plate of glass as possess- 

 ing unusual dynamic qualities, yet it is being 

 attracted to the glass by a great pressure. 



Thus, the film of soil moisture surrounding 

 a soil grain is held as a tight skin around the 

 grain under a pressure that is variously esti- 

 mated at between 6,000 and 25,000 atmos- 

 pheres. If it were 10,000 atmospheres, the 

 pressure exerted would be (Bulletin 55, 



