74 



THE PRINCIPLES OF SOIL MANAGEMENT 



pebbles constitute a very different condition from large 

 boulders in all phases of tillage. 



The relative dimensions of the particles in the groups 

 may be illustrated graphically by the following diagram. 



Fig. 20. Diagram illustrating the relative size of the groups of particles, made 

 in mechanical analysis by the Bureau of Soils Classification 



26. Agricultural classes based on texture. Ob- 

 viously, no natural soil is composed entirely of material like 

 any one of these groups, but a soil may contain a large 

 proportion of material of any one size. Thus, a sandy 

 soil is one containing a large proportion of sand par- 

 ticles, and the coarser the sand or the larger its propor- 

 tion the more sandy the soil appears. A clay soil is one 

 containing a large proportion, but not necessarily a 

 larger quantity of clay than of material of any other 

 size. A given amount of fine particles has a larger 

 effect on the properties of the soil than the same amount 

 of coarse particles. The presence of silt particles in 

 addition to clay serves to make a soil more heavy than 

 if the same quantity of sand were substituted for the silt. 



