Ch. 2] 



PODSOLS AND PODSOLIZATION 



59 



formation of clays to those having narrow silica-alumina ratios. Ac- 

 cordingly, laterization is characterized by preferential accumulation 

 of sesquioxides and -hydroxides (gibbsite, limonite) and kaolinitic 

 clays. Soil colloids extracted 

 from lateritic soils have silica- 

 alumina ratios of 2 and less, 

 as illustrated in Fig. 11. 



According to Wiegner, lat- 

 erization takes place, on a 

 minute scale, in frigid zones, 

 above the zone of vegetative 

 growth. Its prevalence in trop- 

 ical regions may be the result 

 of a combination of warm hu- 

 mid climate, which favors hy- 

 drolysis and leaching, and 

 length of weathering periods 

 embracing the entire Pleisto- 

 cene and a considerable part 

 of the Tertiary. Laterization 

 should be especially preva- 

 lent in deep horizons where infiltration of humic acids is neg- 

 ligible. 



PODSOLS AND PoDSOLIZATION 



The podsol profile (Fig. 1) occurs under a great variety of environ- 

 mental conditions in cold, temperate, and tropical regions. It consists 

 of the following horizons: 



10 20 30 40 



Depth of soil in inches 



50 



Fig. 11. Illustration of lateritic and 

 podsolic weathering and soil formation. 

 Silica-alumina ratios of clay colloids iso- 

 lated from various soil horizons. 



A : <1 to 5 inches. Partially decomposed forest litter and leaf mold. 



A\\ 1 to 6 inches thick. Dark brown humus layer (raw humus) mixed with 



mineral soil; strongly acidic (pH 4). 

 Ai\ 1 to 8 inches thick. Bleached horizon, grayish white; relatively rich in 



silica. 

 B\,B<l\ 2 to 15 inches thick. Rust-brown horizon with accumulations of 



iron hydroxide and some humus; also known as ortstein. 

 C: parent material, usually of sandy or loamy texture. 



The characteristic chemical feature of the podsol profile is displayed 

 by the trend of the silica-alumina ratio of the colloidal clay fraction 

 isolated from various horizons (Fig. 11). The curve reaches a maxi- 

 mum in the A 2 horizon and a minimum in the B horizon. 



