THE SOIL AND SUBSOIL 69 



furrow slice or a little deeper. It is characterized by 

 being darker in color, and more friable and porous than 

 (b) the subsoil, which constitutes the material beneath 

 the soil in which plant roots are found. (See Fig. 13.) 

 A distinction is sometimes made between the upper 

 and the lower subsoil, the former being the layer of 

 subsoil lying between the top soil and a depth of twenty- 

 four inches from the surface, the remainder of the sec- 

 tion being the lower subsoil. 



In humid regions the subsoil is usually less productive 

 than in arid regions, owing to the greater amount of 

 leaching, and to deficient aeration consequent on the 

 movement of large quantities of water through the 

 subsoil. Plowing up the subsoil in the humid region 

 frequently results in a decreased productiveness, while 

 in an arid region the soil and subsoil may be freely 

 mixed without injury, and good crops may be grown 

 even where the top soil has been entirely removed, 

 as is sometimes done in preparing land for irrigation. 



The soil substance may be conveniently divided 

 into two groups of constituents which exhibit quite 

 different properties. These are the inorganic and the 



organic. 



INORGANIC CONSTITUENTS 



The inorganic constituents of the soil are more or 

 less modified particles of rock, varying in size from 

 boulders and coarse sand to the finest dust. Each 

 particle may consist of several minerals, but in those 

 smaller than coarse sand it is unusual to find them com- 

 posed of more than one mineral. 



