CHEMICAL COMPOSITION OF THE SOIL 



169 



as a rule, the percentages of alumina, potash, and lime increases 

 as the size of the particles decrease. For example, Failyer 1 

 determined phosphoric acid, lime, magnesia, and potash in 

 separate grades of particles from a number of soils of the United 

 States by the method of complete decomposition. In the follow- 

 ing table is given the average percentage of three ingredients of 

 the sand, silt, and clay of four groups of soils : 



It is seen that the finer particles of the soil are, on an average, 

 richer in phosphoric acid, lime, and potash than are the coarser 

 particles. The percentage of each of the constituents named 

 increases with the fineness of the particles, the only exception in 

 the table being the potash in the clay of the limestone and shale 

 soils. The relative abundance of the various grades of particles 

 would determine the quantity which each contributes to the 1 soil. 

 The coastal plains soils have been so weathered and leached, that 

 they are lower in phosphoric acid, lime, and potash than the less 

 extensively weathered residual soils, and these in turn are lower 

 than the glacial soils, which consist largely of crushed rocks, many 

 of which have not been weathered to a great extent. 



Analysis by Extraction with Strong Acids. This is the method 

 usually employed in the analysis of the soil. It consists in treat- 

 ing the soil with strong acid and estimating the constituents which 

 go into solution. The extent of the solvent action depends upon 

 the nature of the soil, the kind of acid, strength of acid, tempera- 

 ture, time of contact, and ratio of soil and acid. The methods 

 used by different chemists vary. In the methods of the Asso- 

 ciation of Official Agricultural Chemists of North America 

 1 Bulletin 54, Bureau of Soils. 

 12 



