Walk into any agro-input shop and you will see "chelated micronutrients" — Iron EDTA, Zinc DTPA, Iron EDDHA, Manganese EDTA. But most dealers and farmers don't know the critical difference. Choosing the wrong chelate is as bad as applying nothing.
Micronutrients like iron, zinc, manganese, and copper become insoluble and unavailable at high soil pH — they bind to soil particles and roots cannot absorb them. A chelate is an organic molecule that "wraps around" the metal ion and keeps it in solution, available for plant uptake.
Think of the chelate as a protective cage around the metal ion — keeping it from reacting with soil and becoming unavailable.
| Property | EDTA | DTPA | EDDHA |
|---|---|---|---|
| Full name | Ethylene Diamine Tetraacetic Acid | Diethylene Triamine Pentaacetic Acid | Ethylene Diamine Di-Hydroxyphenyl Acetic Acid |
| pH stability range | Up to pH 7.0 | Up to pH 7.5 | Up to pH 11 |
| Best for | Acidic soils, foliar spray | Slightly alkaline soils, drip | Alkaline/calcareous soils (black cotton) |
| Cost | Low | Medium | High (3–4× EDTA) |
| Available for elements | Fe, Zn, Mn, Cu, Ca, Mg | Fe, Zn, Mn, Cu | Fe only |
| Use method | Foliar + Drip | Foliar + Drip | Soil only |
| Colour | White/off-white powder | Light yellow powder | Red-brown powder (ortho-ortho isomer) |
This is the single most important thing to understand. At pH above the chelate's stability limit, the metal ion is released from the chelate and immediately converts to an insoluble hydroxide — making the product completely useless.
| Element | Recommended Chelate | pH >7.5 Soil | pH <7.0 Soil / Foliar |
|---|---|---|---|
| Iron (Fe) | EDDHA (soil), DTPA (drip/foliar) | Iron EDDHA 6% soil | Iron DTPA 11% drip/foliar |
| Zinc (Zn) | EDTA or DTPA | Zinc DTPA 7% (drip) + foliar | Zinc EDTA 12% (foliar/drip) |
| Manganese (Mn) | EDTA or DTPA | Mn EDTA 13% foliar | Mn EDTA 13% drip/foliar |
| Copper (Cu) | EDTA | Cu EDTA 15% foliar | Cu EDTA 15% foliar/drip |
| Calcium (Ca) | EDTA | Calcium Nitrate (drip) preferred | Ca EDTA foliar |
| Magnesium (Mg) | EDTA | MgSO₄ drip (no chelate needed) | Mg EDTA foliar |
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