Anomalous Potassium Release and Adsorption Reactions: Evidence of Polygenesis of Tropical Indian Soils

Abstract

Pedological and mineralogical research in recent years indicate that major soil types of tropical Indian environment, experienced climate change from humid to semi-arid in the geological past. Dominant low charge di-octahedral smectite in shrink-swell soils (Vertisols and Vertic intergrades) formed at the expense of plagioclase feldspar in previous humid tropical (HT) climate is being preserved in the present day semi-arid (SA) climate, which favoured the transformation of almost fresh biotite mica to high charge vermiculite. Red ferruginous (RF) soils of southern India though dominated by kaolin formed at the expense of low charge di-octahedral smectite in previous HT climate, preserved kaolin, and favoured almost unweathered biotite mica to transform to high charge smectite or low charge vermiculite in the prevailing SA climate. Presence of kaolin in soils of the Indo-Gangetic Plains (IGP)also indicate its genesis in previous HT climate but these soils in the present SA climate are dominated by clay mica consisting of both muscovite and biotite. In SA climate biotite transforms to high charge smectite or low charge vermiculite. All these soils contain pedogenic calcium, which is formed in the present SA climate. Therefore, major tropical soils have unique combination of non- silicates and layer silicate minerals, which are climate specific. Therefore these soil types are polygenetic and contain di- and trioctahedral mica, high and low charge smectite and kaolin. As biotites are almost fresh to weakly weathered, K release in relation to soil their particle size may or may not follow the pattern of specimen mica. Kaolinites are of no significance in K adsorption /fixation reaction, while vermiculites are converted to mica by layer contraction by K. Low charge di-octahedral smectites do not possess this property and they do not adsorb K selectively unless the charge density is high like in high charge smectite or low charge vermiculite. It is thus envisaged that the polygenetic nature of tropical Indian soils can be comprehended following their K release and adsorption behaviour because of their unique combination layer silicates that control such reactions.