THE MINERAL CONSTITUENTS OF THE SOIL SOLUTION 39 



Consequently it is to be expected that variations in the concen- 

 tration of the natural soil solution would be less than in aqueous 

 extracts, when there is employed a constant and relatively large 

 proportion of water to soil. These considerations are of great 

 theoretical importance since they appear to negative the possi- 

 bility of getting, with present experimental resources, any exact 

 knowledge of the concentrations of the mineral constituents in 

 the soil solution when the soil is in condition to grow the com- 

 mon crop plants. Moreover, they furnish a guide to the limi- 

 tations which must be recognized in attempting to postulate 

 what these concentrations may be on the basis of analytical data 

 obtained from aqueous soil extracts. 



Many attempts have been made to extract the solution natur- 

 ally existing in the soil and to analyze it. The results obtained 

 have not been very satisfactory, owing mainly to the mechanical 

 difficulties involved. As pointed out above, the solution in a 

 soil under suitable conditions for crop growth is held by a force 

 of great magnitude. Nevertheless, by using powerful centri- 

 fuges, with saturated soil, it has been possible to throw out the 

 excess of solution over the critical water content of the soil. 

 In this way small quantities, generally a very few cubic centi- 

 meters at a time, have been obtained. The analysis of a few cubic 

 centimeters of a very dilute solution is in itself difficult, involv- 

 ing necessarily more or less uncertainty as to the absolute value 

 of the results. Nevertheless, the concentration of the soil solu- 

 tions thus obtained, with respect to phosphoric acid and potash, 

 varied but little for soils of various textures from sands to 

 clays, and the variations observed could not be correlated with 

 the known crop-producing power of the soils. The average 

 concentrations of the soil solutions thus obtained lies in the 

 neighborhood of 6-8 parts per million (p.p.m.) of solution for 

 phosphoric acid (P 2 O 5 ) and 25-30 parts per million for potash 

 (K.,0). 1 In the following table are given the results obtained 

 1 In this connection it is interesting to note that recent investigations 

 on the proportions of phosphoric acid, potassium and nitrates in cultural 

 solutions best adapted to the growth of wheat, give the same ratio of 

 phosphoric acid to potassium as the figures just cited show to exist nor- 

 mally in the soil solution. 



