Effect of levels of sulphur from various sources on sulphur transformations in onion growing soils

Sulphur (S) deficiency is widespread in Indian soils, and it has been emerging as a major limitation in Onion production. Onion being a sulphur loving crop, sulphur fertilisation to onion is important, and, therefore, to assess the supply of native sulphur from soil to onion crop requires an understanding about the effect of various sulphur fertilizers and their transformation in soils. With this view of understanding the soil S transformations, an incubation experiment was carried out for the period of 3 months in S deficient black soil (Vertic Ustropept) which was sandy loam in texture with available S content of 7 mg kg-1. Ammonium sulphate, Single super phosphate, Gypsum and Elemental Sulphur were the four sources of sulphur applied at the levels of 0, 20 and 40 kg S ha-1 along with recommended doses of N, P and K (60:60:30 kg ha-1) for onion. The treatments were laid out in CRD with 3 replications. The soil samples were analysed for different S fractions at fortnight intervals throughout the period of incubation. The mean values of different fractions of S namely total S ranged from 110.57 to 252.11 mg kg-1, organic S ranged from 51.7 to 176.79 mg kg-1, occluded S ranged from 32.57 to 66.54 mg kg-1, Exchangeable S ranged from 3.56 to 12.57 mg kg-1, Water soluble S ranged from 3.08 to 18.57 mg kg-1 and Cacl2 extractable S ranged from 7.11 to 32.55 mg kg-1. On an average, out of total sulphur, at 90 DAI, 4.95% got incorporated into water soluble sulphur, 4.07% into exchangeable sulphur, 10.45% into CaCl2 extractable sulphur, 22.39% into occluded sulphur and 58.14% into organic sulphur. Comparing the sources of sulphur applied ammonium sulphate at both 20 and 40 kg S ha-1 and SSP @ 40 kg S ha-1 resulted in highly significant pattern on soil easily available sulphur fractions release over other sources.