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SIGNIFICANCE OF THE FORMATION OF CALCIUM CARBONATE MINERALS IN THE PEDOGENESIS AND MANAGEMENT OF CRACKING CLAY SOILS (VERTISOLS) OF INDIA

Division of Soil Resource Studies, National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur 440 010, India

^* E-mail address of corresponding author: dkpal{at}nbsslup.mah.nic.in

Micromorphological studies were performed in order to understand the factors and processes involved in the formation of calcium carbonate (CaCO₃) in twenty three soil series of Vertisols representing sub-humid, semi-arid and arid climatic regions of Peninsular India. The study indicates that Vertisols contain both pedogenic calcium carbonate (PC) and non-pedogenic calcium carbonate (NPC) irrespective of the ecosystems to which they belong. The NPCs are part of the parent material of Vertisols. Dissolution of NPCs and recrystallization of dissolved Ca²⁺ ions are responsible for the formation of PCs. Vertisols of arid and semi-arid climates contain more PC in their soil control section (SCS) than those of sub-humid climates. Formation of PC is the prime chemical reaction responsible for the increase in pH, the decrease in the Ca/Mg ratio of exchange site with depth and in the development of subsoil sodicity. Petrographic and scanning electron microscopic (SEM) examination of quartz, feldspars and micas indicate little or no alteration, discounting the possibile formation of smectite during Vertisol formation. X-ray diffraction (XRD) analysis of clays indicates that smectites of Vertisols are fairly well crystallized and do not show any sign of transformation except for hydroxy interlayering. The preservation of the crystallinity of smectite and the lack of transformation of primary minerals thus validate the hypothesis of positive entropy change during the formation of Vertisols.

The precise cause-effect relationship between CaCO₃ of pedogenic and non-pedogenic origin, and exchangeable Mg, Na and Ca percentages (EMP, ESP and ECP) has been established in the study. This indicates that impoverishment of Ca²⁺ ions on the exchange sites of Vertisols needs to be controlled by rehabilitation methods that can replenish Ca²⁺ ions, and thus the study provides relevant information for future land resource management programmes not only on Vertisols of India but also on similar soils occurring elsewhere.

Key Words: Calcium Carbonates • Mineral Alteration • Pedogenesis • Soil Micromorphology • Vertisols

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