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Sulphur Fertilizers

Sulphur is now formally classified as the fourth major plant nutrient in Indian fertiliser policy, alongside nitrogen, phosphorus and potassium. ICAR-IISS surveys document widespread sulphur deficiency in Indian oilseed and pulse soils, driven by decades of intensive cropping without S replenishment, declining use of Ssp Single Super Phosphate (which carries 11-12% S), and reduced atmospheric S deposition. Sulphur is supplied to crops through several carrier types.

Composition

The major Indian S fertilisers are:

  • Elemental sulphur: 90% S bentonite-coated pastilles, slow release through microbial oxidation to sulphate.
  • Gypsum (CaSO4.2H2O): ~17% S plus ~21% Ca; see Gypsum In Groundnut.
  • Ammonium sulphate ((NH4)2SO4): 21% N + 24% S; see Ammonium Sulphate.
  • Single Super Phosphate (SSP): 16% P2O5 + 11-12% S + 19-21% Ca; see Ssp Single Super Phosphate.
  • 20-20-0-13 (APS) and other S-fortified complexes; see Complex 20 20 0 13.
  • Magnesium sulphate / kieserite: 13-22% S plus magnesium.

The choice among these depends on the simultaneous need for N, P, Ca or Mg.

Mode of action

Crops absorb sulphur primarily as the sulphate anion (SO4(2-)). Sulphur is incorporated into the amino acids cysteine and methionine, into key enzymes, into glucosinolates in brassicas and into the oils of oilseed crops. Plant-available sulphate is released from elemental S through microbial oxidation (which is slow in cool, dry soils) and is directly available from gypsum, ammonium sulphate, SSP and the S-fortified complexes.

Target use and dose

  • Oilseeds and pulses: 20-40 kg S per hectare as basal, supplied through gypsum, SSP or bentonite-S.
  • Brassicas and alliums: 30-50 kg S per hectare.
  • Paddy on S-deficient soils: see Ammonium Sulphate Paddy for the substitution-for-urea practice.

For groundnut, gypsum at pegging combines S supply with the Ca needed for pod fill (Gypsum In Groundnut).

Safety and regulatory status

All listed S fertilisers are FCO-notified. Storage of elemental S requires care, as fine sulphur dust is a fire hazard.

See also: Ammonium Sulphate, Ssp Single Super Phosphate, Gypsum In Groundnut, Complex 20 20 0 13, Cms Calcium Magnesium Sulphur, Calcium Magnesium Foliar.

References

  1. Sulphur in Indian Agriculture. ICAR-Indian Institute of Soil Science.
  2. Ammonium Sulfate. Mosaic Crop Nutrition Resource Library.