THE MINERAL CONSTITUENTS OF THE SOU, SOLUTION 47 



The results given in the foregoing tables were obtained under 

 great difficulties, and in some parts the variations they show are 

 undoubtedly due to inevitable inaccuracies of analytical work 

 done under such circumstances. Some of the variations may 

 also be due to the disturbing influences in the soil referred to 

 above. Experience has shown, however, that the preparation 

 of an aqueous extract of the soil of any particular field is by no 

 means a simple matter. Extracts made from samples taken 

 within a few feet of one another frequently show variations of 

 the same order as with samples from entirely different fields, or 

 even soil types. Differences in the preliminary drying out of 

 the sample before the addition of the water, seems to result in 

 the same order of differences as obtained between different soils. 

 In consequence of these facts, and of the further fact that an 

 arbitrary aqueous extract of a soil cannot be assumed to repre- 

 sent in any definite way the natural soil solution, the results of 

 the field examination are inconclusive as to the concentration of 

 the soil solution in situ. It is more necessary, therefore, that 

 other lines of evidence should be sought as to the mineral char- 

 acteristics and concentration of the soil solution. Such a line 

 of evidence is found in certain percolation experiments. 1 



If a solution of a soluble phosphate be percolated through a 

 soil, a part of the phosphate will be removed from the solution 

 and absorbed by the soil; that is, there will be a redistribution 

 of the phosphate between the soil and the water. As the pro- 

 cess continues, however, relatively less and less phosphate is 

 absorbed by the soil and the concentration of the percolate be- 

 comes more and more nearly that of the added solution. This 

 absorption takes place more or less closely in accordance with 

 the simple law that the absorption of phosphates by the soil, 

 per unit of solution which is percolating, is proportional to the 

 total amount of phosphate which the soil may yet take from 

 that solution if percolated indefinitely. This law is expressed 

 by the equation dy/dx = K(A y) where 3; is the amount 



1 The absorption of phosphates and potassium by soils, by Oswald 

 Schreiner and George H. Failyer, Bull. No. 32, Bureau of Soils, U. S. 

 Dept. Agriculture, 1906. 



