6 THE SOIL SOLUTION 



The common procedure has been to define some arbitrary 

 percentage limit in the soil, below which the soil is supposed to 

 require fertilizers. But the amount of fertilizer to be applied is 

 suggested on the indefinite basis of "experience." Thus, Hilgard, 

 in an interesting discussion of this subject, 1 quotes Dyer as 

 showing that "on Rothamsted soils of known productiveness 

 or manurial condition, it appears that when the citric acid extrac- 

 tion yields as much as 0.005 P er cent, of potash and o.oio per 

 cent, of phosphoric acid, the supply is adequate for normal crop 

 production, so that the use of the above substances as fertilizers 

 would be, if not ineffective, at least not a profitable investment." 

 Hilgard himself sets limits as determined by strong hydrochloric 

 acid digestion; thus a soil containing upwards of 0.45 per cent. 

 (K 2 O) does not need this substance as a fertilizer, while one 

 containing below 0.25 per cent, does need it at once, and inter- 

 mediate percentages indicate that potash fertilizers would prob- 

 ably be profitable ; the corresponding upper and lower limits for 

 phosphoric acid are set at o.io per cent, and 0.05 per cent. 

 But Hilgard points out that various things, such as the content 

 of lime, or the texture of the soil, may materially alter these 

 limits. In a very interesting set of experiments in which white 

 mustard was grown in various soils, and these same soils diluted 

 with various amounts of dune sand which had previously been 

 extracted with strong hydrochloric acid, he found that the plants 

 did best when the soils had been diluted with four times their 

 weight of the extracted sand. This was the case even with a 

 pulverulent sandy loam ; and with a black adobe, the best results 

 were obtained when the diluted soil contained but 0.15 per cent, 

 potash (K 2 O) and 0.04 per cent, phosphoric acid (P 2 O 5 ). It 

 also appears that Hilgard regards soil analyses of value only in 

 the case of virgin soils or soils which have been out of cultivation, 

 and in common with other authorities, he fails to point out how to 

 determine the amount of fertilizer needed by lands. 



It is clear, therefore, that the principles underlying the practice 

 or art of soil management and crop rotation are in a state of 



1 Soils by E. W. Hilgard, 1906, p. 339, et seq. 



