Forms of Sulphur in Some Rapeseed-growing Soils of Assam

Relationship of different forms of sulphur and its availability index with soil properties were investigated in some rapeseed-growing Alfisols of four districts of Assam. The total S content ranged from 268 to 901 mg kg⁻¹ with a mean of 545 mg kg⁻¹ soil. Organic-S constituted the dominant form (65.9 to 79.6%) of total S followed by non-sulphate-S (14.9 to 27.4%). Water soluble S (3.37 to 6.24%), available S (5.56 to 6.25%) and adsorbed-S (0.95 to 2.95%) constituted only small fractions of the total S, respectively. All forms of sulphur gave significantly and positively correlated with organic-C and clay content. Significant negative correlation of total, organic, water soluble and heat soluble–S and positive correlation of non-SO₄-S with Fed revealed formation of S-compounds with free iron oxides upon release of S during mineralization of the S-bearing minerals. Organic-C had the most dominant influence on sulphur availability index (r = 0.629**) followed by free iron oxide content (Fed) (r = -4.439**) and CEC (r = 0.434**) in these soils. Based on the available S content, only 6.7 to 8.3% of the soil exhibited S deficiency.