IO THE SOIL SOLUTION 



them frankly empirical. For instance, Hilgard, in his classical 

 investigation of the cotton soils for the Tenth Census, treated 

 his soil samples with an excess of hydrochloric acid, evaporated 

 to dryness, extracted with water, and regarded the extracted 

 mineral constituents as available. In Germany, a method similar 

 to Hilgard's is now in common use, while in France nitric acid is 

 preferred generally because it is supposed to have peculiar sol- 

 vent powers on soil phosphates. In the United States the "offi- 

 cial method" of the Association of Official Agricultural Chemists 

 is to keep 10 grams of the soil in contact with 100 cc. of a solu- 

 tion of hydrochloric acid (specific gravity 1.115) at the boiling 

 point of water for exactly 10 hours. In England the popular 

 method is that proposed by Dyer, namely, to treat the soil with 

 a i per cent, citric acid solution, this strength of solution being 

 supposed at one time to represent the average acidity of root sap. 

 Maxwell, in Hawaii, and afterwards in Australia, claimed good 

 results for the extraction of the soil with a I per cent, solution 

 of aspartic acid, this acid being employed on the erroneous 

 ground that the organic acids of the soil are amido acids, and 

 that these are the effective agents in dissolving the soil minerals 

 and rendering their constituents "available." The Kentucky 

 Agricultural Experiment Station favors an N/5 nitric acid solu- 

 tion, 1 but does not recommend its use for soils of other localities, 

 while in a contiguous state, the Tennessee Station favors the 

 "official" method. 2 Many other methods have been proposed, 

 but the foregoing are typical and sufficient to illustrate the pres- 

 ent status of soil analysis. 



It is clear that these several methods must give differing 

 results. And it is not clear that any one of them is to be pre- 

 ferred to the others for any reasons than analytical convenience. 

 There is no reason to expect that the proportion of solvent to 

 soil required in these methods bears any relation whatever to 

 the mechanism of absorption by plant roots. And the attempts 



1 Soils, by A. M. Peter and S. D. Averitt, Bull. No. 126, p. 66 (1906). 

 "The soils of Tennessee, by Charles A. Mooers, Bull. No. 78, p. 49 

 (1906). 



