THE SOIL 9 



a proportion of vegetable matter that the mineral 

 elements are often in insufficient quantit}'. The 

 presence of large quantities of lime in a soil gives it 

 some characteristic peculiarities. The name of cal- 

 careous soils is applied where more than ten per cent 

 of this substance is present. It usually happens 

 that the surface layers of the soil contain more de- 

 composed vegetable matter and so are darker colored 

 than the part that lies below. This upper portion 

 is called the top soil or surface soil, while the deeper 

 layers are spoken of as the subsoil. The mechanical 

 composition of the two is often quite different, and 

 the line of demarcation between them may be clearl}^ 

 marked. The top soil may vary in depth from 

 a few inches to as many feet. A deep surface soil is 

 usually an indication of fertility. In some cases, as 

 in the loess soils at some points along the Mississippi 

 River and the red cane lands of the Tertiary period 

 in the West Indies, there is no proper subsoil, since 

 the surface soil extends downward practically un- 

 altered for a great depth. 



Biological Activities in the Soil. — So far only the 

 chemical and physical properties of the soil have been 

 considered. It is also the place for innumerable 

 biological activities. Instead of being the inert, 

 lifeless mass that it seems, it is literally swarming 

 with untold myriads of minute living beings whose 

 life processes profoundly affect its composition and 

 properties. Without the aid of these active but 

 infinitesimal workers the soil as we know it to-day 

 could never have been built up ; and if their activi- 

 ties should suddenly cease, its fertility would be 



