276 NATURE AND PROPERTIES OF SOILS 



a factor to be considered. Obviously the concentration and 

 composition of the soil solution is first of all a function of 

 the absorptive capacity of the soil complexes, modified by the 

 rate of solution and the magnitude of crop and leaching 

 activities. 



144. Absorption and the soil solution. — In a bare moist 

 soil, where there is no evaporation or leaching to disturb 

 equilibrium tendencies, the soil presents a three-phase 

 system. The phases are: (1) the solution surfaces, 1 

 (2) the absorptive or colloidal surfaces, and (3) the 



SOLUTION 

 SURFACES 



ABSORPTION *- SOIL 



COMPLEXES -^ SOLUTION 



Fig. 50. — Diagram showing the equilibrium tendencies that exist between 

 the solution surfaces, the colloidal complexes and the soil solution. 



soil solution itself. When solution takes place, the con- 

 stituents so affected are acquired in part by the soil mois- 

 ture as a solute and in part by the absorptive complexes. 

 There is a constant attempt at equilibrium, which of course 

 is never attained as long as solution continues. Under field 

 conditions, many other disturbing factors enter. The rate 

 of solution may vary, and the capacity and character of the 

 absorbing colloidal complexes are always changing. Moreover, 

 the amount of water in the soil is never constant, due to 

 drainage and evaporation. The feeding of the plant, as re- 



1 This term refers to the soil surfaces from which solution takes 

 place. 



