156 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 



trees is notably low, and decomposition products of the litter they 

 produce always give an acid reaction. 



Laterization is characteristic of tropical conditions with high 

 temperatures and abundant rainfall. It is essentially the leaching 

 of silica from the surface soil. The low acidity produced by de- 



.. .. 



FlG. 80. The layer of calcium accumulation in a pedocal soil under sage- 

 brush desert as shown in a road cut in Nevada— Photo by W. D. Billings. 



composition of tropical litter promotes the solution of silica as 

 well as alkaline materials. After laterization, the surface soil is high 

 in iron and aluminum, which are not removed by the process. 



Calcification may occur anywhere but is most important in 

 regions with low rainfall unevenly distributed throughout the 

 year and with temperatures producing a relatively high rate of 

 evaporation. Under these conditions, a permanently dry stratum 

 may develop in the profile below the depth to which rainwater 

 penetrates. Carbonates produced by carbonation in the surface 

 layers, as well as those that may be present in the original soil ma- 

 terial, are carried downward in solution toward this dry layer. 



