ROCKS AND SOILS 



81 



Figure 5.5 World distribution of the principal zonal soil groups, (f-rom Yearbook of Agriculture, ^94^, 



deep, dark organic horizon above a compact, hard 

 B horizon, calcium carbonate hardpan. The dark 

 layer is the basis for this soil sometimes being called 

 black earth. Chernozem regularly grades into Gray- 

 Brown Podsolis, the transition area constituting the 

 Prairie Soils with their tall grasses. True Prairie. 

 Chestnut and Brown soils have more carbonate but 

 less organic material than Chernozems. These two 

 soil types have grassy steppe or brushy vegetation. 

 Sierozem and Red Desert soils contain no organic 

 layer but support a desert vegetation. Sierozem gen- 

 erally has bunchgrasses and Red Desert, desert 

 shrubs and cacti. 



Zonal soils, except Tundra, may be placed in two 

 categories. Podsols and Lateritic soils are sometimes 

 grouped as Pedalfers, a term emphasizing their alum- 

 inum and iron content; and Chernozems, Chestnut, 

 Brown, Sierozem, and Red Desert Pedocab to empha- 

 size their calcium content. 



Halomorpbic Soils. These are the saline and alkali 

 soils of poorly drained, arid regions and of marine de- 

 posits. They are light-colored and sometimes display 

 visible evidence of their salt or alkali content. Their 

 vegetation consists of a sparse cover of grasses, forbs, 

 shrubs, and trees that can withstand the high mineral 

 content of the soil. 



Hydromorphlc Soils. Although these soils are 

 usually found in cool, humid to tropical climates, they 

 can be in any situtation where fresh water accumu- 

 lates. These soils are usually dark-colored, but they 

 can be light. Their vegetative cover varies, consisting 

 of grasses, sedges, and flowering plants; swamp 

 forest; broad-leaved forests; and tropical forests. 



Calomorpbic Soils. These soils are in cool to hot, 

 humid to dry climates but reflect a large amount of 

 limestone in their parent materials. They tend to be 

 dark-colored and have a plant cover of grasses and/or 

 broad-leaved trees. 



INTRAZONAL SOILS 



Because these soils are dependent upon local condi- 

 tions, it is best to treat the three types separately. 



AZONAL SOILS 



These have no true profile development; however, 

 there may be some indication of such zonation. Soils 



