724 soils: FROPBBTmS AND MAJSTAGJEMENT 



exceedingly heterogeneous in nature and are without 

 sorting or stratification except as the action of wind and 

 water may have combined with the action of the ice; 

 (6) gravity, or the slow creep of material on slopes, which 

 is a minor agency of soil formation (see Chapter II). 



623. Climate. — Soils owe their origin to the operation 

 of one or more of the forces named above. Usually some 

 one of these agencies is predominant and gives specific 

 character to the soil. The elements of climate have been 

 used in the practical classification of soils to only a small 

 degree, since the inherent properties of the material in 

 these divisions are usually distinct enough to make sepa- 

 ration easy. The excessive accumulation of the soluble 

 salts known as alkali is associated with a low rainfall, 

 and other chemical and physical properties are correlated 

 with aridity. Three main divisions in humidity and pre- 

 cipitation may readily be made, namely, (1) humid, 

 (2) semiarid, (3) arid. The exact precipitation limits of 

 these divisions depend on the temperature relations and 

 the time and manner of occurrence of the precipitation. 



In a world system of soil classification the temperature 

 relations of the soil would be recognized, but this division 

 is seldom important in any single country. 



624. The practical classification of soils in the United 

 States. — As practiced in the United States, the classi- 

 fication of soils 1 has disregarded the climatic factor and 

 has usually combined the kind of rock and the agencies 

 of formationas a single basis of separation of soils, desig- 

 Bating the division resulting therefrom as a soil province. 

 In some areas one element of formation is dominant and 



1 Marbut, C. F., Bennett, H. H., Laphana, J. E., and Lap- 

 liam, M. H. Soils of the IJnited States. U» S, D. A., Bur. 

 Soils, Bui. 96, p. 891. 1913, 



