RUSSIAN SOIL INVESTIGATIONS. 707 



of the soils. The knowledge of the laws and the forms of these 

 influences make it possible to obtain from the study of soils a ])asis for 

 the reconstitution of the recent past of the country and for sketching- 

 its recent geo-phj^sical history. The essential factors determining the 

 characteristics of natural soils are as follows: 



(1) The conditions and the factors of the origin of the given soil 

 type (the material and the organic agents); (2) the morphological 

 properties of the soil, /. t^. ,its color, depth, constitution,^ structure, 

 transition into the parent rock, etc.; (8) the physical, chemical, and 

 chemico-biological properties; (4) the moditication with the t3^pe; and 

 (5) the geographical and topographical distribution. 



The natural classification of soils can be elaborated, taking the genetic 

 principle as a starting point. In establishing the chief groups of soils 

 the existing types of formation of soils in nature must be recognized, 

 the homogeneous or similar combinations of soil-forming agents (such 

 as climate, parent rocks, organisms, relief of country, etc.) must be 

 formulated. As is well known, the weathering of the rocks alone, pro- 

 vided it takes place under similar physico-geographical conditions, 

 may efl'ace to a considerable degree the difierences which exist among 

 the rocks, and may give alluvial products of line earth more closely 

 resembling one another than the original rocks; this similarit}^ is more 

 manifest when the biological factors also tend to produce a uniform 

 result. We can, consequently, establish an ensemble of natural con- 

 ditions which will produce as a result soils, say, of the chernozem 

 group. A characteristic feature of these soils is the jjeculiar accumu- 

 lation of humus under the sod. Wherever analogous conditions pre- 

 vail soils of the chernozem type are formed. Similarly, we know 

 the climatic conditions which favor atmospheric-eolian weathering, 

 the pulverization of the soft rocks, and where these conditions obtain 

 eolian dust soils result. The soils of these groups in their principal 

 features are the natural resultant of the physico-geographical type of 

 the given continental region or zone. The soil of the difl'erent zones 

 will, of course, not be uniform, but will exhibit similarity to the extent 

 to which their content of line earth and humus reflect the analogous 

 influences of a definite and constant combination of geo-physical fac- 

 tors of soil formation. 



In this way the fir.st cltiss of zonal soils is determined. In the proc- 



^ A vertical section of a soil always shows two, three, or even iiujre horizons, 

 detailed descriptions of which are given in Russian works on soils. Of these horizons 

 the most remarkable are: (1) The upper horizon, the most tmiformly and strongly 

 colored by humus; (2) the lower horizon, distinguished from the upper by its 

 structure and color and gradually merging into the subsoil; and (3) the subsoil or 

 parent rock preserving its fundamental petrographic features. The first two horizons 

 taken together give the depth of the surface soil. Sometimes in these horizons sub- 

 horizons can be distinguished, with peculiar differences in composition, structure, 

 and tint (alkali soils, forest soils, etc.). 



