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Ammonium sulphate in paddy

The use of ammonium sulphate (Ammonium Sulphate, 21% N + 24% S) as a nitrogen source in transplanted paddy is a specific agronomic case where ammoniacal nitrogen is preferred over urea or nitrate fertilisers. Under continuously flooded conditions, applied nitrate is rapidly reduced and lost as N2 gas through denitrification, while ammoniacal nitrogen remains in the soil solution and on exchange sites until it is taken up.

Composition and rationale

  • Nutrient grade: 21% N + 24% S
  • N form: fully ammoniacal, stable under reducing soil conditions
  • Co-benefit: 24% sulphate-S corrects latent sulphur deficiency

In sulphur-deficient or saline-alkaline paddy soils, the sulphate fraction can revive stalled vegetative growth on its own, even when total N supply is adequate from other sources.

Target use and dose

Ammonium sulphate is used in paddy primarily in two situations:

  • as a replacement for part of the Urea requirement on sulphur-deficient or alkaline soils, applied at active tillering or panicle initiation;
  • as a corrective dressing where chlorosis and stalled growth are diagnosed as combined N-S deficiency, often in tandem with Zinc Sulphate where Zn is also low.

Rates are set against soil-test S status and the standard district paddy nitrogen schedule. Application is by broadcast in standing water followed by gentle mixing.

Limitations

The acidifying effect that benefits alkaline soils becomes a liability on acidic paddy soils, where it should be paired with lime. The S content also means ammonium sulphate is uneconomic when the only requirement is N alone, since urea remains cheaper per unit N.

See also: Ammonium Sulphate, Urea, Sulphur Fertilizer, Paddy Tillering Top Dressing, Zinc Sulphate.

References

  1. Ammonium Sulfate. Mosaic Crop Nutrition Resource Library.
  2. Ammonium sulfate. Wikipedia.