Grades and Chemical Composition under FCO
Zinc is the most critical agricultural micronutrient in India, with over 40% of agricultural soils testing deficient due to intensive multi-cropping.
The Fertilizer Control Order (FCO) recognizes two standardized grades of mineral zinc sulphate for soil and foliar application.
- Zinc Sulphate Heptahydrate (ZnSO4·7H2O): Contains a minimum of 21% elemental zinc (Zn) and 10% sulfur (S) by weight
- Zinc Sulphate Monohydrate (ZnSO4·H2O): Highly concentrated grade containing 33% elemental zinc (Zn) and 15% sulfur (S) by weight
- High Water Solubility: Completely soluble in water, making both grades suitable for basal soil broadcasting, fertigation, and foliar sprays
Recognizing Zinc Deficiency: Khaira Disease and Stunting
Zinc is an essential enzyme co-factor required for tryptophan synthesis, which precedes the natural plant growth hormone auxin (IAA).
Deficiency leads to stunted internodes, severe interveinal chlorosis, and rusty brown leaf discoloration.
- Khaira Disease in Paddy: Appears 2 to 3 weeks after transplanting; young leaves develop reddish-brown rusty spots that coalesce, severely stunting tillers
- White Bud in Maize: Emerging young terminal leaves turn pale yellow to bleached white, with stunted stalks
- Little Leaf in Cotton & Citrus: Leaves remain abnormally small, narrow, and cup-shaped, reducing boll set
Application Methods and Antagonism Precautions
Zinc can be applied as a basal soil dressing or as a quick-acting curative foliar spray when symptoms appear.
Never mix zinc sulphate directly with phosphatic fertilizers like DAP or SSP in the same fertilizer hopper, as insoluble zinc phosphate forms, rendering both nutrients unavailable.
- Basal Soil Application: Broadcast 10 kg of 33% zinc sulphate or 20 kg of 21% zinc sulphate per acre before final harrowing
- Foliar Spray: Dissolve 5 grams of zinc sulphate plus 2.5 grams of agricultural lime per liter of water to prevent leaf scorching
- Phosphate Separation: Maintain a gap of at least 3 to 5 days between DAP and zinc applications to avoid chemical fixation
Official source
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Ministry of Agriculture - Fertilizer Control Order ↗Related on KisanPe
Common questions
What is the difference between 21% and 33% zinc sulphate?
21% zinc sulphate is heptahydrate with 7 water molecules; 33% zinc sulphate is monohydrate, offering a more concentrated form requiring lower application rates per acre.
What disease does zinc deficiency cause in rice?
Zinc deficiency causes Khaira disease in paddy, characterized by rusty brown spots on leaves and severely stunted plant tillering.
Can zinc sulphate be mixed with DAP fertilizer?
No, mixing zinc sulphate with DAP causes chemical precipitation into insoluble zinc phosphate, which plants cannot absorb.
General guidance, not personal advice. For your farm, ask your local Krishi Vigyan Kendra or agriculture officer.
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