1919] Pendleton: A Study of Soil Types 373 



This development of the mapping of soils as an outgrowth of areal 

 geology in France and Germany may be contrasted with the develop- 

 ment of soil classification from other viewpoints, such as that of the 

 Russian school. In Russia there is not the predominance of residual 

 and shallow soils which characterize much of western Europe and 

 which in France especially have led to the adoption of the geologic 

 basis of classification. Dokoutchayev and Sibirtzev have been the 

 chief proponents of a classification of soils based upon the "conception 

 of a soil as a natural body having a definite genesis and a distinct 

 nature of its own. ' ' 7 



The genetic conditions of the formation of natural soils include 

 the following variable factors which cause variation : 



(1) The petrographie type of the parent rock; (2) the nature and intensity of 

 the processes of disintegration, in connection with the local climatic and topo- 

 graphic conditions; (3) the quantity and quality of that complexity of organisms 

 which participate in the formation of the soil and incorporate their remains in it; 

 (4) the nature of the changes to which these remains are subjected in the soil, 

 under the local climatic conditions and physico-chemical properties of the soil 

 medium; (5) the mechanical displacement of the particles of the soil, provided 

 this displacement does not destroy the fundamental properties of the soil, its geo- 

 biological character, and does not remove the soil from the parent rock; and (6) 

 the duration of the processes of soil formation. 



Upon this genetic basis there has been developed a series of soil zones, 

 ranging from the laterite soils in the tropics to the tundras in the 

 Arctic regions. The outstanding and controlling factor in the scheme 

 proposed is the relation of these zones to climate. For this reason the 

 statement usually seen is that climate is the basis of the classification. 8 

 There are nearly as many groups of intra-zonal and azonal soils as 

 of those belonging to the zones proper. The former include alkali, 

 marshy, alluvial, and other soils. 



Hilgard, while actively interested in the genetic viewpoint of soil 

 classification, was the foremost proponent of a classification upon 

 the basis of the natural vegetation growing upon the soil. 9 This 

 criterion is not always available, though some groups of plants, as the 

 alkali tolerant ones, are almost invariably present where the condi- 



- Exp. Sta. Record, vol. 12 (1900), p. 704. 



See also Sibirtzev, Cong. Geol. Intern., 1897, pp. 73-125; abstract in Exp. Sta. 

 Rec. vol. 12 (1900-01), pp. 704-12, 807-18. 



Tulai'koff, N., The Genetic Classification of Soils, Jour. Agr. Sci., vol. 3 (1908), 

 pp. 80-85. 



s Coffey, U. S. Bur. Soils, Bull. 85 (1912), p. 32; Jour. Amer. Soc, Agron., 

 vol. 8 (1916), p. 241. 



9 Hilgard, E. W., Soils (New York, Macmillan, 1906), pp. 487-549. 



