80 THE PRINCIPLES OF SOIL MANAGEMENT 



the soil and the subsoil. In humid regions, the large 

 rainfall and consequent seepage through the soil is 

 associated with a greater degree of fineness in the sub- 

 soil than in the soil. On the other hand, in arid regions 

 where there is not this large rainfall, and consequent 

 leaching, the subsoil is not finer than the soil, and, in 

 fact, is inclined to be more coarse. 



28. Some properties of soil separates and classes. 

 As has been indicated, the justification for a study of 

 individual soil particles from an agricultural standpoint 

 is in their fundamental relation to the management of 

 the soil. Every farmer is well acquainted with the 

 striking difference in crop relations and tillage properties 

 of sand and clay. He well knows that they must be 

 managed differently and are suited to different crops. 

 He knows sand to be better suited to early maturing 

 crops, like truck, than to late crops and the grasses. 

 He knows that one does not withstand dry weather, 

 while the other will carry a crop through a long 

 period of drought. The cause traces back to the 

 size and consequent properties of the soil units. This 

 will appear more clearly in the discussion of soil 

 moisture. 



29. Number of particles. Since soil particles run 

 to very small diameters, the number in any given mass 

 or volume is very great. This is shown in the following 

 table, which gives the number of particles in 1 gram 

 (1 lb. equals 453.6 gr.) of each of the fine earth separates, 

 considering the particles to be spheres of mean diameter 

 and of specific gravity 2.65. 



If the particles of a soil are assumed to be spheres 



