Interrelationship of Forms of Sulphur with its Availability Indices and Soil Properties in Entisols of Assam

Forms of sulphur and their relationships with soil properties and sulphur availability indices were investigated in some Entisols under rice-mustard cropping sequence in four districts of Assam. The total-S content in the soils ranged from 217 to 672 mg kg⁻¹ with mean value of 399 mg kg⁻¹. The mean values of organic-S, non-SO₄-S, adsorbed-S, sulphate-S, heat-soluble and water-soluble S were 306, 77, 2.60, 15.47, 31.2 and 12.56 mg kg⁻¹, respectively. All the forms of S showed significant and positive correlation with organic carbon indicating S as the integral part of soil organic matter. Positive correlations of clay and silt fractions with forms of S indicate that appreciable quantity of sulphur was associated with finer fractions of the soils. Negative correlation was observed with soil pH, free and crystalline Fe oxides. Sulphur availability indices (SAI) was significantly and positively influenced by organic-C, clay and CEC whereas it was negatively affected by pH, sand, free and crystalline Fe oxides. The step-wise multiple regression analysis revealed that organic-C had maximum impact on different forms of S except heat soluble and water-soluble S. Organic-C was the dominant factor governing SAI followed by pH. About 26.4% of the Entisols investigated were deficient in the available sulphur.