60 jenny. ORIGIN OF SOILS [Ch. 2 



To comprehend the formation of a podsol profile, or, in other words, 

 the process of podsolization, we may resort to ideas expressed by 

 Wiegner (1926). He stresses the role played by the acid humus col- 

 loids in bringing about the reversal of the process of laterization. 



Wiegner reasons as follows: From the thick organic A and A t hori- 

 zons, acid humus colloids enter the mineral soil and neutralize the 

 bases as rapidly as they are being formed. This important step shifts 

 the weathering process from lateritic to podsolic. The soil solution, 

 being acid, flocculates negative colloidal silica, which thus remains in 

 the surface soil. Acidity disperses positive colloidal aluminum and 

 iron hydroxides, a process which is encouraged by the protective action 

 of the acid humus colloids. Accordingly, colloidal iron and alumina 

 are removed from the surface soil. They are reprecipitated in the sub- 

 soil (B horizon) where electrolyte concentration and pH are relatively 

 high. 



In contrast to mere clay migration, as it occurs in claypan soils, 

 podsolization involves a differential behavior of colloidal silica and 

 colloidal sesquioxides. Colloids extracted from A 2 horizons have high 

 silica-alumina ratios. Those in the B horizon have low ones (Fig. 11). 



Podsol formation postulates the presence of acid colloidal humus. 

 Accordingly, it can occur wherever large amounts of acid humus are 

 formed, be it as a result of low temperature, as in northern regions, 

 or as a result of high rainfall, as in certain parts of the tropics. 



REFERENCES 



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 Harradine, F. F. (1949). The variability of soil properties in relation to stage 



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 Haseman, J. F., and Marshall, C. E. (1945). The use of heavy minerals in 



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 Hilgard, E. W. (1912). Soils: The Macmillan Co., New York, 593 pages. 

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