16 MANUFACTURE OF FERTILIZING MATERIALS 



feet in depth. The term subsoil is usually applied 

 to a layer of soil beginning at the usual depth of 

 cultivation and of a thickness of from 6 to 9 in. 

 The surface of demarcation of change of color is 

 sometimes regarded as the upper superficial 

 boundary of the subsoil. The term soil and sub- 

 soil are therefore not always used with the same 

 relative signification. The subsoil is not, as a 

 rule, so thoroughly disintegrated as the soil, since 

 it is protected in a measure by the overlying 

 material. It usually contains less organic matter 

 than the soil. There is freer circulation of air in 

 the soil than in the subsoil, and the metallic ele- 

 ments usually exists in the upper layers as higher 

 oxides. There is usually a notable difference in 

 color between the soil and subsoil, and frequently 

 a very sharp color line separates the two. 



Geologically considered, the soil is that portion 

 of the earth's crust which has been more or less 

 thoroughly disintegrated by weathering and other 

 forces from the original rock formations, or from 

 the sedimentary rocks, or from the unconsolidated 

 sedimentary material. The soil has, therefore, 

 the same essential constitution as the general mass 

 of the earth, except that this debris has been sub- 

 jected to the solvent action of water and the 

 influence of organic life. 



The chemical elements present in the soil are 

 naturally some or all of those which were present 

 in the original rocks. For purposes relating to 

 agriculture, it is not necessary to take into 



