PHYSIOGRAPHIC FACTORS 159 



matter. Black soils with brown or reddish calcareous subsoils. 

 Strong carbonate horizon but normal horizons indistinct. 



Br oil'?! Soils (also known as Chestnut Soils) : Bordering 

 Chernozems to the west and developed under successively 

 drier conditions, they contain successively less organic matter 

 westward and southward and become lighter in color, as in- 

 dicated by their division into Dark Brown and Brown Soils. 

 Occupy mainly the area usually called the Great Plains. 



Gray Soils : Desert and semidesert soils largely in the Great 

 Basin and southward. Gray with yellowish to reddish cal- 

 careous subsoils. Negligible organic content. Weathering 

 largely physical. 



"Within these climatically determined soil regions, are local varia- 

 tions that, because of time and topography, bear no resemblance 

 to the mature soil type. Swamps and bogs, islands and flood plains, 

 salt and alkali flats, or merely immature soils on steep slopes— all 

 are illustrations of local conditions that must be disregarded in 

 considering the broad aspects of climatic control of soil develop- 

 ment. 



The climatic classification of soils is useful because it makes 

 possible broad considerations of regional problems. It is logical 

 because it bases the major categories upon mature conditions, 

 which remain stable with the climate, and makes possible the ex- 

 planation of local variations, which represent merely stages of 

 profile development. Best of all, it has world-wide application. 

 Enough investigations have now been made to show that the same 

 general soil types are repeated in those parts of the world where 

 climatic conditions are duplicated. Thus, it has been feasible to 

 devise several schematic representations of the relationship of tem- 

 perature and moisture to soil formation that are reasonably ap- 

 plicable anywhere. A relatively simple climatic system 251 is shown 

 below, in which temperature-evaporation and precipitation-evap- 

 oration relationships are used as criteria of climatic control. It 

 serves to emphasize the importance of moisture in pedocal devel- 

 opment and grassland areas but shows that temperature is more 

 effective where pedalfers develop with the forests they support. 



Vegetation and Soil Development.— The close similarity be- 

 tween the distribution of major vegetation types and climatic soil 



