64 THE SOIL SOLUTION 



of any outside disturbing influences. The law governing the 

 rate of absorption by soils has not therefore possessed any great 

 practical interest and has not been studied from a quantitative 

 point of view, although it is known qualitatively that the rate 

 is increased by increasing the concentration of the solution, 

 or by increasing the amount of the absorbent or at least its 

 effective surface. Two rate equations are of interest in this con- 

 nection, and have been carefully studied. The rate at which a 

 salt or other dissolved substance will advance into an absorbing 

 soil from a solution is given by the same equation as that describ- 

 ing the rate of advance of the water itself, y n = kt where y is 

 the distance and t the time. 1 The constants n and k for the 

 slower moving dissolved substance are different from those for 

 the water. This equation has probably little importance for 

 ordinary agriculture, for absorption by the soil from a large (and 

 relatively illimitable) mass of solution is unusual. That it may 

 have considerable importance in seepage, irrigation, and some 

 soil engineering problems, seems quite likely. 



The rate at which a soil will absorb a dissolved substance from 

 a percolating solution is given by the equation dx/dt = K(A 

 x}, as has been pointed out above. 2 More interesting and 

 important, however, is the fact that this same equation describes 

 the rate at which an absorbed substance is removed from the 

 soil by leaching. In the case of soils in humid areas dx/dt 

 rapidly becomes exceedingly small as x approaches A, that is, 

 when the amount of soluble material in the soil becomes small, 

 and is practically constant under such conditions, as has been 

 pointed out above when describing the removal of potassium 

 and phosphoric acid from soils by percolating waters. This 

 formula has a special interest in considering the reclamation of 

 alkali lands by underdrainage, a problem to which reference 

 will be made later. 



Both percolation experiments, as those cited above, and direct 

 absorption experiments made by shaking up soils with solutions 



1 See formula, page 28. 

 * See formula, page 47. 



