58 jenny. ORIGIN OF SOILS [Ch. 2 



In the lower part of the subsoil, the high electrolyte concentration 

 reflocculates the clay particles. As colloids settle out, the zone of 

 illuviation becomes denser, the pores smaller, and additional migrating 

 clay particles may be retained by mere sieve action, even in absence 

 of excess electrolyte. The surface soil becomes depleted of clay, and 

 the accumulation zone (B horizon) becomes thicker. The resulting 

 extremely slow passage of water preserves the claypan over very long 

 periods of time. 



Lateritic Soils and Laterization 



More than 100 years ago, in 1830, Buchanan described the red 

 soils of India which are used locally for making bricks ("later"). In 

 ensuing years the word "laterite" was applied to all red soils occurring 

 in tropical regions; in fact, sometimes to all red soils anywhere. In 

 recent years, especially under the influence of Pendleton, the concept 

 of laterite has tended to be restricted to specific soil strata rich in 

 sesquioxides, possibly formed under the influence of ground-water 

 conditions. 



The literature on laterites and lateritic soils is very voluminous. 

 Among pedologists the prevailing viewpoint stresses laterization as a 

 widespread soil-forming process of humid regions in which silica and 

 bases are lost from the soil profile. However, only in extreme cases 

 would laterization produce an actual laterite as defined by Buchanan 

 and Pendleton. 



According to Wiegner (1926), laterization is the direct result of 

 normal weathering of rocks in absence of acid humus. In accordance 

 with the ideas on rock decay presented in a previous section, one may 

 write schematically 



colloidal silica 



Feldspar 



+ HOH 



colloidal aluminum hydroxide 

 colloidal iron hydroxide 



KOH.NaOH.Ca (0H) 2 -(highpH) 



Since colloidal silica is negatively charged, alkaline reaction produced 

 by the free bases will disperse it, and silica will leave the soil profile. 

 Often silica is found precipitated in lower soil strata as chalcedony. 



Iron and aluminum hydroxides form positively charged colloids, and 

 they are flocculated at high pH. Accordingly, the sesquioxides remain 

 in the soil. The preferential leaching of silica tends to limit the 



