THE SOIL PARTICLE 



81 



same as those employed in mechanical analyses to designate 

 the soil separates. This is rather unfortunate, but it obviates 

 the increase of technical terms and a little care will prevent 

 confusion in this regard. Four fundamental groups of soil 

 are recognized: gravel, sand, loam, and clay (See Fig. 16). 

 Gravel is a soil constituent that does not often occur alone 

 and is not of great importance agriculturally because of its 

 low fertility. The other three, however, either alone or in 

 combination make up most of the arable soil. Their average 

 mechanical analyses are set forth in Table XV. 



Table XV 



MECHANICAL ANALYSES OF SANDY, LOAMY AND CLAYEY SOILS 1 



The sand group includes all soils of which the silt and clay 

 separates make up less than 20 per cent of the material by 

 weight. Its properties are, therefore, characteristically sandy 

 in contrast to the more open character of gravel and the 

 stickier and more clayey nature of the heavier groups of soil. 

 A soil to be clay must carry at least 30 per cent, of the clay 

 separate. It may even have more silt than clay but, since 

 the silt particles impart clayey characters, as long as the per- 

 centage of clay is 30 or above, the class name must remain 

 clay. 



1 Whitney, M., The Use of Soils East of the Great Plains Begion; 

 U. S. Dept. Agr., Bur. Soils, Bui. 78, p. 12, 1911. 



