Significance of soil modifiers (Ca-zeolites and gypsum) in naturally degraded Vertisols of the Peninsular India in redefining the sodic soils

Earlier hypothesis on the factors and processes of natural degradation in Vertisols and also their evaluation for crops on the basis of only hydraulic properties was developed on limited soils by the Division of Soil Resource Studies (DSRS) of the National Bureau of Soil Survey and Land Use Planning (NBSS & LUP), Nagpur, India. In order to validate the above similar soils occurring in major states of the Peninsular India and also to document the extent of modification by gypsum and Ca-zeolites therein, the present study on twenty-six benchmark Vertisols representing a climosequence from sub-humid moist to arid dry climate was undertaken. The results of the present study validates the hypothesis that formation of pedogenic calcium carbonate (PC) at the expense of non-pedogenic calcium carbonate (NPC) is the prime chemical reaction for the natural chemical degradation realized in terms of impairment of hydraulic properties of soils mediated through the development of subsoil sodicity. Presence of gypsum and Ca-zeolites, on the other hand, prevented the rise of pH, decrease in Ca/Mg ratio of exchange sites and improved the hydraulic properties amidst an exchangeable sodium percentage (ESP) > 15. The improvement in saturated hydraulic conductivity (sHC) (> 10mm h(-1)) of zeolitic sodic soils does commensurate fairly well with the performance of rainy season crops. Thus characterization of sodic soils in terms of sHC < 10 mm h(-1) (weighted mean in 0-100cm depth of soil) instead of any ESP or sodium adsorption ratio (SAR) emerges as a robust criterion that stands for a universal acceptance for the better use and management of such naturally degraded soils, not only in the Indian semi-arid tropics, but also in similar climatic and geologic areas elsewhere. (c) 2006 Elsevier B.V. All rights reserved.