THE MINERAL CONSTITUENTS OF THE SOIL SOLUTION 49 



getter; but the concentration of the percolate with respect to 

 potassium generally lies somewhere between 25 and 30 parts per 

 million. 



The removal of a soluble constituent from the soil by per- 

 colating water appears to be described by a rate equation similar 

 to that given above for absorption. If the rate of percolation 

 be maintained constant this formula is 



dx/dt = K(B x) 



where x is the amount removed by the percolation, with time 

 t, K is a constant characteristic for the particular system under 

 consideration, and B is the total amount of the constituent 

 which may ultimately be leached out. In other words, the rate 

 in any particular soil will depend upon the amount of the con- 

 stituent still absorbed in that soil but has no necessary connec- 

 tion with the rate which would hold for the same amount of 

 the constituent in any other soil. 



Theoretically, two consequences follow from this law which 

 require consideration here. The rate at which a constituent is 

 removed gradually becomes less as percolation proceeds. If the 

 soil contains an amount of the constituent approaching the total 

 amount which it can absorb, as for instance is probably the case 

 sometimes when large applications of lime have been made to the 

 soil, the concentration of the percolating solution might be ex- 

 pected to change noticeably. Generally, however, a soil con- 

 tains nowhere near as much phosphoric acid or potassium as it 

 is capable of absorbing, so that the concentration of the percolat- 

 ing water changes but very little with respect to these constit- 

 uents. It follows from the equation that if percolation con- 

 tinues uninterrupted, the concentration of the percolate, so far 

 as it is determined by an absorbed constituent, must get less 

 and less until it becomes a vanishing quantity. This state of 

 affairs does not exist in the soil, however, for percolation by 

 pure water does not continue uninterrupted for any length of 

 time. The rise of the capillary water in the soil will, under 

 normal conditions, enable the soil to reabsorb more of the or- 

 dinary mineral constituents than is removed by percolating 



