48 THE SOIL SOLUTION 



absorbed, x amount of solution that has passed, and A is the 

 total amount which can ultimately be absorbed by that particular 

 soil from that particular solution. K is also a characteristic 

 constant. If the percolation be maintained at constant rate, then 

 /, time, can be substituted for x and the equation becomes 

 dy/dt = K(A y), the ordinary rate equation for a mono- 

 molecular reaction of the first order, whether chemical or phys- 

 ical. 



With such absorptions as are involved in soils, a clay exposes 

 a greater amount of absorbing surface than does a loam or sand, 

 and it will show the greatest absorption towards any particular 

 solution, other things being equal. The curve showing the con- 

 centration of percolate would lie lower for a clay than for a 

 loam, or for a sand. This is illustrated in the accompanying 

 sketch diagram, where y represents concentration of percolate 

 and t represents time. 



Fig. i. 



If after percolation has proceeded for some time (in some 

 experiments for several weeks and until the soil contained I 

 or 2 per cent, of phosphoric acid) pure water be passed through 

 the soil, then, as soon as the previously used phosphate solution 

 has been displaced, the concentration of the percolate drops and 

 continues practically constant for an indefinite period. More- 

 over, no matter what the soil may be as to texture or compo- 

 sition, the same concentration of percolate is obtained, namely, 

 6-8 parts per million, the concentration which the soils yielded 

 prior to treatment with the phosphate solution. Similar ex- 

 periments when the soils were treated with salts of potassium 

 have given like results, although the curves obtained from pass- 

 ing pure water through the soils do not lie quite so close to- 



