Calculating Charge Density of Clays : an Improvised Method

Abstract

Two benchmark Vertisols from Maharashtra, viz. Linga series (Typic Haplusterts in Nagpur district) and Pahur series (Sodic Haplusterts in Yavatmal district) of Maharashtra were chosen for the study. The fine clay fractions are dominated by smectite with small amounts of vermiculite. Reduced CECs of the fine clays were determined after the Greene-Kelly test. CEC and calculated surface charges are distributed in both the tetrahedral and octahedral layers of which the former (65%) is higher than the latter (35%). However, the proportion of octahedral CEC and charge increases (44%) with the deduction of the contribution of vermiculite and chlorite CEC from the tetrahedral (corrected) CEC. The The CEC and charge distribution exhibited fairly good relationship with some soil properties such as, fine clay, COLE, moisture retention at 1500 kPa, hydraulic conductivity and pH. The study also indicated that determination of reduced CECs from Greene-Kelley test is an effective means of measuring the octahedral and tetrahedral CECs and also for calculating the charge of soil clays. The Pahur soils could be more prone to hydroxy-interlayering owing to its higher tetrahedral charge than Linga soils. The surface area method overestimates the calculation of charge from CEC than formula weight method.In India, the black soils occupy about 116 m ha (Bhattacharyya et al., 2009) of which about 30 m ha is in Maharashtra and about 8.04 m ha is in Vidarbha region. These soils are developed in alluvium derived from weathering of Deccan basalt (Pal et al., 2006). The black soils of the Deccan Trap are rich in plagioclase feldspars and yield dioctahedral smectite as its first weathering product (Pal and Deshpande, 1987; Pal et al., 1989; Bhattacharyya et al., 1993). Earlier review on the Vertisols of India indicate that black soils of India are dominated by beidellite-nontronite type of minerals (Ghosh and Kapoor, 1982). However, fine clay smectite when subjected to the Greene-Kelly test (Greene-Kelly, 1953) by heating clays with Li and subjecting to glycerol solvation, expands to about 18 Å and contracts to ~9.5 Å indicating this to be a mixture of beidellite/ nontronite and montmorillonite in which the amount of the former is more than the latter (Pal and Deshpande 1987; Ray et al., 2003).