Interrelationships of Sulphur with Soil Properties and its Availability Index in Some Rapeseed-growing Inceptisols of Assam

Interrelationships of forms of sulphur and its availability index with soil properties were investigated in some rapeseed-growing Inceptisols of Assam. The abundance of various forms of S was in the following decreasing order: total S>organic S>non-sulphate (SO₄) S>heat soluble S>available (SO₄)-S>water soluble S>adsorbed S. Amongst the S fractions, the highest and the lowest contribution to the total S pool was made by organic-S (70.3 to 76.3%) and adsorbed-S (0.5 to 1.74%), respectively. Significant positive correlations of organic-C and clay with all forms of sulphur indicated their dominant influence on sulphur status of these soils. Total, organic, heat soluble and non-SO₄-S showed significant positive correlation with sesquioxides. Organic-C was the most important determinant of sulphur availability index (SAI) (r=0.897**) followed by clay (r= 0.790**), CEC (r=0.705**) and available N (r=0.585**). Stepwise multiple regression equations revealed that organic-C had greater impact on total, organic, available, non-SO₄-S and SAI while pH impacted the heat soluble and adsorbed-S. About 10.1 to 20.0% of the investigated soils were found to be deficient in plant available S and needed S fertilization for sustainable production of mustard.