Polygenetic Vertisols of the Purna Valley of Central India

The Purna Valley, a part of the Payanghat Plain, is an oval basin covering an area of about 1.9 Mha in central India. The soils are deep, calcareous, clayey and very dark greyish brown to dark brown in colour, and meet criteria for the Vertisols order of Soil Taxonomy. Cracks extend down to the slickenside zones in soils of the northeastern part of the valley, but cut through the slickenside zones in the soils of the southwestern part. Soils of the southwestern part are strongly alkaline with exchangeable sodium percentages (ESPs) of 5–26, whereas those of the northeastern part are moderately alkaline and have ESP values <5. Despite their similar coefficients of linear extensibility, volumetric shrinkage potentials, clay contents and amounts of fine clay smectite, the plasmic fabric of the slickenside horizons in soils of the northeastern part is porostriated, whereas in soils of the southwestern part, it is stipple-speckled to mosaic-speckled, indicating weak plasma separation. The soils have both pedogenic and nonpedogenic calcium carbonate, but those of the southwestern part have more pedogenic carbonate than those in the northeastern part. The semiarid climate removes Ca2+ ions from the soil solution by precipitating carbonate, and also causes the ESP and sodium absorption ratio to increase with depth. The lack of soil water in soils of the southwestern area is thought to be the reason for weak swelling of smectite, for larger amounts of pedogenic carbonate and for cracks cutting through the slickenside zones. The Vertisols of the southwestern part of the of the Purna Valley are, therefore, polygenetic because they exhibit distinctly different soil properties resulting from increasing aridity in the valley during the late Holocene.