276 



THE PRINCIPLES OF SOIL MANAGEMENT 



factor in the decomposition of the mineral matter, it has 

 been proposed to use a solution of carbon dioxide as a 

 solvent in soil analysis. The amounts of soil constitu- 

 ents taken up by this solvent are much less than by any 

 of the others heretofore mentioned, but all mineral 

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

 The amount of phosphorus is so small as to make its 



Fig. 97. The cut-out disc harrow, adapted to hard or stony soil. 



detection by the gravimetric method difficult. Like 

 other methods employing very weak solvents, it is open 

 to the objection that the extraction fails to remove a 

 considerable portion of the dissolved matter that is 

 retained by absorption, and, as this will vary with soils 

 of different texture, it makes impossible a fair com- 

 parison of such soils by this method. 



140. Extraction with pure water. When soil is 

 digested with distilled water, all of the mineral substances 



