Properties, Origin and Distribution of Carbonate Nodules in Some Vertisols

Origin, properties and distribution of carbonate nodules, and particle size distribution of carbonates in some Vertisols of Karnataka were studied. White and black nodules were the two morphological types identified. White nodules were irregular in outline, had rough surface and were more porous. Black nodules were spherical, smooth and less porous. White nodules contained slightly more CaCO₃ than black ones. Al, Fe and Mn contents were distinctly higher in black nodules as compared with white ones. X-ray diffraction analysis showed that calcite was the predominant mineral with small quantity of quartz in both the types. A decreasing trend of black nodules with depth suggests that blackening of the carbonate nodules initially formed as white starts from the surface downwards. Carbonates. of the sizes of sand and silt predominate over clay size carbonates in these soils.