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Zinc Sulphate (21% / 33%)

Zinc sulphate is the dominant soil-applied source of zinc in Indian agriculture, regulated under the FCO micronutrient schedule. Zinc deficiency is among the most widespread soil deficiencies in India: ICAR-AICRP surveys report that nearly half of all Indian soils are zinc deficient, with several states showing more than 80% deficient samples.

Composition

Two FCO-notified grades are sold:

  • Zinc Sulphate Heptahydrate (ZnSO4.7H2O): 21% Zn
  • Zinc Sulphate Monohydrate (ZnSO4.H2O): 33% Zn

The monohydrate grade is the more concentrated and stable form and is now the larger fraction of Indian sales. IFFCO and major Indian companies supply both grades; the product is white to off-white crystalline solid, fully water soluble.

Mode of action

Zinc is an essential micronutrient required for enzyme cofactor activity, protein synthesis, auxin biosynthesis and chlorophyll formation. Soil-applied zinc sulphate dissolves in soil moisture to release Zn2+ ions, which are taken up by roots and translocated to growing tissues. On alkaline soils (pH 7.5-12), zinc rapidly precipitates as zinc hydroxide or zinc phosphate; co-application or banding with DAP causes Zn-P lock-up. Foliar uptake from dilute zinc sulphate sprays is rapid but is limited by the relatively low solubility at neutral spray-tank pH; chelated zinc (Edta Chelated Zinc) is preferred for foliar use.

Target use and dose

  • Paddy basal: 10-25 kg per hectare of 21% grade, or 5-10 kg per hectare of 33% grade, broadcast at puddling before transplanting (see Paddy First Dose Fertilizer).
  • Field crops: 10-25 kg/ha (21%) basal, with periodic top-up every 2-3 seasons.
  • Foliar correction: 0.5% solution of zinc sulphate plus 0.25% lime to buffer pH; chelated zinc is preferred where leaf-burn is a concern.

A critical compatibility rule: zinc sulphate should not be applied with or within five days of Dap or single super phosphate, since high phosphate concentrations precipitate the applied zinc as insoluble zinc phosphate, locking it up before root uptake.

Safety and regulatory status

FCO-notified micronutrient. Non-hazardous at field rates; routine handling precautions apply.

See also: Edta Chelated Zinc, Boron Fertilizer, Paddy First Dose Fertilizer, Multi Nutrient Fertilizer Blends, Dap.

References

  1. Zinc Sulphate Monohydrate 33%. Indian Farmers Fertiliser Cooperative.
  2. Solutions for India's Fortified Fertilizer Market. U.S. Borax India report.