THE ABSORPTIVE PROPERTIES OF SOILS 269 



parts per million of PO4 absorbed after certain intervals of 

 time are given below. (See also Fig. 49) ; ^ 



Table LVIII 

 effect of time and texture on the absorption of po4 from 



A SOLUTION OF CaH4(P04)2. 





Time 



PO4 Absorbed in Parts 

 Per Million 





Clayey 

 Soil 



Fine Sandy 

 Soil 



3 minutes 



400 

 410 

 415 

 435 

 440 

 445 



235 



40 minutes 



255 



1 hour 



260 



2 hours 



315 



4 hours 



24 hours 



335 

 370 







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- 

 ventis taken up more rapidly than the solute. Coneentra- 

 tion is thus induced. 



141. Selective absorption.^ — 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, 



^Tlie law wMcli appears to govern al»sorption of phosphates and 

 potasii by the soil may be expressed mathematieally as follows: 



||-= 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: Sehreiner, O., 

 and Failyer, Gr. H., The Absorption of PhospJiates and Fotassium hy 

 Soils; TJ. 8. Dept. Agr., Bur. Soils, Bui. 32, pp. 23-24, 37-39, 1906. 



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

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

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



