PHYSIOGRAPHIC FACTORS 



157 



When the water is removed by plants or evaporation, the carbon- 

 ates are left behind, at or above the dry layer, depending upon the 

 depth of penetration of the moisture at the time. 



Climatic Soil Types.— On the basis of absence or presence of a 

 lime carbonate layer formed by calcification, the mature profiles 

 of all soils of North America fall into two groups : pedalfers, with- 

 out the layer; pedocals, with the carbonate layer. The two condi- 

 tions occur regardless of the nature of parent material or its geo- 

 logical origin, and their distribution is obviously controlled by 

 climate. Soils of eastern North America are all pedalfers, for the 

 unfavorable balance between rainfall and evaporation necessary to 

 carbonate deposition does not occur here. West of about the 99th 

 meridian (a line through the center of the Dakotas to the pan- 



FlG. 81. General distribution of the important zonal soil groups of the 

 United States. After Kellogg, 136 from Klages, Ecological Crop Geography, by 

 permission of The Macmillan Company, publishers. 



handle of Texas), where annual precipitation is normally less than 

 twenty inches a year, mature profiles almost invariably show 

 pedocal characteristics except where climatic conditions are vari- 

 able, notably in the mountains and in parts of California. Climate, 

 vegetation, and soil have corresponding distributions. The pedal- 



