320 NATURE AND PROPERTIES OF SOILS 



doubt a potent factor in the decomposition of the mineral 

 matter, it has been proposed to use a solution of carbon diox- 

 ide as a solvent in soil analysis. The amounts of soil con- 

 stituents taken up by this solvent are much less than are taken 

 up by any of the others heretofore mentioned, but all mineral 

 substances used by plants are soluble in it to some extent. 

 The amount of phosphoric acid is so small as to make its 

 detection by the gravimetric method difficult. Like other 

 methods employing very weak solvents, this is open to the 

 objection that much of the material dissolved cannot be re- 

 moved because of the absorptive power of the soil, and as this 

 varies with the character of the soil, adequate comparisons 

 cannot be made. Water charged with carbon dioxide has been 

 very largely replaced by pure water in making such extrac- 

 tions. 



When soil is digested with distilled water, all the mineral 

 substances used by plants are dissolved from it, but in very 

 small quantities. It has been proposed to employ this extract 

 for soil analysis on the ground that it is a natural solvent 

 and dissolves only those nutrients in a condition to be used 

 by plants. By determining the moisture content of the soil 

 and using a known quantity of water for the extraction, the 

 parts per million of the extracted nutrients may be expressed 

 on the basis of the dry soil or of the solution. The aqueous 

 extract does not by any means contain the entire quantity of 

 nutrients which were in the soil solution and is not an exact 

 measure of the fertility in this form. Absorption holds back 

 an undetermined and variable quantity of the important con- 

 stituents and thus vitiates the method, especally for compar- 

 ing different soils. The method, however, is very valuable for 

 comparing the same soil at different times, especially as re- 

 gards the nitrates. The nitrate radical is not absorbed to any 

 great degree by the soil and presents a very fair measure of 

 the concentration of the soil solution as far as this constituent 

 is concerned. 



