Seat of charge in smectite layers of Vertisols of central India.

Abstract

In Vertisols, most of the intrinsic soil properties such as exchange phenomena, nutrient availability, water retention, shrink-swell behaviour and transformation of clay minerals depend upon the total layer charge as well as the origin of charge in clay smectites. Ideally, the charge in the layer silicate minerals should originate either in the tetrahedral or octahedral sheet, but it is usually observed that the charge is distributed over both the sheets. The location of charge can be ascertained by Greene-Kelly test (Hofmann-Klemen effect) which distinguishes the presence of montmorillonite and beidellite by the migration of Li+ ions into vacant octahedral sites until the charge in the octahedral layer is neutralized. In view of this, the present study on two shrink-swell soil clays, viz., Linga and Pahur from Nagpur and Yavatmal districts respectively was undertaken to determine the cation exchange capacities of soil clays before and after the Greene-Kelly test to ascertain the seat of charge. The Greene-Kelly test shows the presence of beidellie-nontronite (specifically nontronite because of high iron content) and montmorillonite type of minerals. The reduced CEC (or tetrahedral CEC) obtained by blocking the vacant octahedral sites by Li+ ions is higher in Pahur than in Linga soil clays. The CEC data show that charge is distributed in both tetrahedral and octahedral layers; the former shares higher proportion of CEC (and charge) (65%) than the latter (35%). The share of tetrahedral layers (57%) was higher even after deducting the contribution of vermiculite and chlorite minerals towards CEC. Higher tetrahedral charge may indicate that the soils may be more prone towards hydroxy-interlayering. This is evident in Pahur soils which have slightly higher tetrahedral charge than Linga soils.