FACTORS AND PROCESSES OF SOIL DEGRADATION IN VERTISOLS OF THE PURNA VALLEY, MAHARASHIRA, INDIA

The Vertisols of the Purna Valley, which cover the districts of Amrava ti, Akola and Buldhana in the state of Maharashtra, India, lack any pe rceptible evidence of salt efflorescence on the soil surface which wou ld indicate the presence of salt, but the drainage conditions are poor . The limited data available indicate that the adverse physical condit ion of the soils is due to their poor hydraulic conductivity (HC), whi ch is impaired by sodium in the exchange complex. However, the factors and processes that are inherently related to the development of sodic ity in these shrink-swell soils are not yet understood. In order to es tablish the cause-effect relationship, eight Vertisol pedons from meth odically selected sites were studied morphometrically in the field, an d for their sodicity-related physical and chemical properties in the l aboratory. The soils are deep, calcareous, clayey and very dark greyis h brown to dark brown in colour. Cracks extend up to the slickenside z ones in soils of Pedons 1-3 in the northeastern area of the valley, wh ile they cut through the slickenside zones in soils of Pedons 4-8 in t he southwest. The slickenside faces were larger in the soils of the so uthwest than in those of the northeast. All these soils meet the speci fications of the Vertisols order of soil taxonomy. Saturation extracts of the soils had very low electrical conductivity (ECe less than or e qual to 2 ds m(-1)). They ranged from moderately alkaline in the north east, Pedons 1-3, to strongly alkaline in the southwest, pedons 4-8. I n soils from the northeast the exchangeable sodium percentage (ESP) wa s less than 5 throughout the depth of the pedons, whereas in other soi ls it was up to 6 in the surface horizons and between 7 and 26 in the subsoil horizons; four of these soils qualified as sodic according to the criteria of the United States Salinity Laboratory. The inherently low hydraulic conductivity was due to the dispersion of clay particles caused by a high percentage of exchangeable magnesium (EMP) in the hi ghly smectitic soils, and also to a slight increase in ESP (greater th an or equal to 5). The results of this study suggest that ESP 5 should be used as the lower limit for sodic subgroups of Vertisols, rather t han ESP 15 as given in Keys to Soil Taxonomy (Soil Survey Staff, 1994) . This is because there are severe limitations to the use of such soil s owing to the development of adverse physical conditions even at such a low ESP. The authors emphasize the need to keep this fact in mind d uring future land resource management programmes on the soils of this valley as well as on similar soils occurring elsewhere. The developmen t of sodicity in the soils of the southwestern part of the valley has been attributed to the semi-arid climatic conditions that have induced the pedogenetic process of depletion of calcium ions from the soil so lution in the form of calcium carbonate, thereby resulting in an incre ase of both the sodium adsorption ratio (SAR) and the ESP with pedon d epth. This chemical degradation, which affects the sodicity of Vertiso ls, appears to be a basic process that needs to be recognised in the f uture along with those already described as natural processes of soil degradation.