Role of Calcium Carbonate and Palygorskite in Enriching Exchangeable Magnesium to Impair Drainage of Vertisols of Semi-Arid Western India

Shrink-swell soils (Vertisols and their intergrades) as one of the major soils support high crop production in Maharashtra, Madhya Pradesh, Andhra Pradesh and northern Karnataka. Vertisols occur extensively in the states of Maharashtra, occupying 36% of the total area. Under rain-fed conditions, the yield of deep-rooted crops on Vertisols depends primarily on the amount of rainwater stored at depth in the soil profile and the extent to which this soil water is released during crop growth. Both the retention and release of soil water are governed by the nature and content of clay minerals, and also by the nature of the exchangeable cations such as sodium and magnesium. The hydraulic conductivity and clay dispersion of swell shrink soils as a function of exchangeable sodium and magnesium are two important soil properties that need to be understood in view of their concurrent pedogenic processes like clay illuviation, calcium carbonate formation and enrichment of exchangeable sodium and magnesium in the subsoils. Therefore, the present study was undertaken with thirteenVertisols of semi-arid tropical (SAT) western Maharashtra. Results of this study indicate the formation of pedogenic calcium carbonate (PC) triggers the chemical reaction for the concomitant enrichment of exchangeable sodium(ESP) and exchangeable magnesium percentage (EMP) in the subsoils and development of Sodic Haplusterts (ESP > 5) witha low value of sHC (<10 mmhr1). It was interesting to note similar low values of sHC in Typic Haplusterts endowed with palygorskite mineral. Results indicate that when the impairment of sHC in Sodic Haplusterts is primarily due to increase in ESP and EMP, the enrichment of EMP caused by the presence of palygorskite in Typic Haplusterts is equally capable of impairing the drainage (in terms of hydraulic properties) of Vertisols. This is the opposite effect from that of saturation with Ca2+ ions, which leads to blocking of small pores in the soil. This suggests that Mg2+ ions are less efficient than Ca2+ ions in flocculating soil colloids and also in creating strong plasma separation although the United States Salinity Research Laboratory grouped Ca2+ and Mg2+ together as both the ions improve soil structure. This fact assumes a great importance in the use and management of palygorskite endowed Vertisols especially when they are still classed as Typic Haplusterts, suggesting no sign of natural soil degradation in them.