THE ABSORPTIVE PROPERTIES OF SOILS 269 



parts per million of P0 4 absorbed after certain intervals of 

 time are given below. (See also Fig. 49) : x 



Table LVIII 



EFFECT OF TIME AND TEXTURE ON THE ABSORPTION OF P0 4 FROM 

 A SOLUTION OF CaH 4 (P0 4 ) 2 . 



It must not be inferred that, when a solution is brought 

 in contact with a soil, it always becomes weaker because of 

 absorption. Negative absorption may occur in which the sol- 

 vent is taken up more rapidly than the solute. Concentra- 

 tion is thus induced. 



141. Selective absorption. 2 — The fixation phenomena by 

 the soil, whether physical or chemical, is of two types: (1) the 

 absorption of molecules, the compound being taken up un- 

 changed; and (2) the absorption of ions. In the first case, 



1 The law which appears to govern absorption of phosphates and 

 potash by the soil may be expressed mathematically as follows: 



dy 



dv 



K (A— Y) 



in which K is a constant, A the maximum quantity possible for the soil 

 to absorb and y the quantity actually fixed when v, volume of the 

 solution, has percolated through. A short discussion of the mathematics 

 of this law may be found in the following publication: Schreiner, O., 

 and Failyer, G. H., The Absorption of Phosphates and Potassium by 

 Soils; U. S. Dept. Agr., Bur. Soils, Bui. 32, pp. 23-24, 37-39, 1906. 



2 A very good discussion of selective absorption is found in the 

 following: Parker, E. Gr., Selective Absorption by Soils; Jour. Agr. 

 Kes., Vol. 1, No. 5, pp. 179-188, 1913. 



