268 THE PRINCIPLES OF SOIL MANAGEMENT 



to plants, as drainage water and the ash of crops carry 

 off relatively large amounts of these substances each 

 year; but in spite of this loss, the soil is able to provide 

 at least some plant-food material for each crop, when 

 called upon by the plant. 



II. SOLUBILITY OF THE SOIL IN VARIOUS SOLVENTS 



For purposes of analysis intended to show the 

 amounts of mineral plant-food materials in the soil any 

 one of several different solvents may be used. These 

 solvents differ in strength, and consequently the per- 

 centages of the various constituents obtained from 

 samples of the same soil are different for each solvent. 

 A chemical analysis of a soil is a determination of the 

 amounts of the constituents that have been dissolved 

 in the solvent used. Therefore it will readily be seen 

 that the interpretation of a chemical analysis must 

 depend largely upon the nature of the solvent, and, 

 unless the solvent is equivalent in its action to some pro- 

 cess or processes in nature, the result must be entirely 

 arbitrary. The solvents used have generally been in- 

 tended to show some definite relation of the soil to the 

 food requirements of crops. Upon the accuracy with 

 which this is accomplished depends the value of the 

 chemical analysis. 



127. Complete solution of the soil. By the use of 

 hydrofluoric and sulfuric acids, the entire soil mass may 

 be decomposed and all of its inorganic constituents 

 determined. Such an analysis shows the total quantity 

 of the plant-food materials except nitrogen, which 



