What is an iron chelate and why does it matter?
Free iron in solution is unstable: at typical fertigation pH it oxidizes and precipitates as iron oxides that roots can't absorb. That's why fertilizers deliver iron chelated — a carrier molecule grabs the iron ion and keeps it soluble and available. How high a pH that carrier can handle before it lets go of the iron depends entirely on the type of chelate.
EDDHA vs EDTA vs DTPA: what's the pH ceiling for each?
Not all chelates protect iron equally. Their stability drops as pH climbs — and that's exactly the trap: the moment your water gets harder and more alkaline, the cheapest chelate is the first one to fail.
- EDTA: stable only up to ~pH 6. Above that it releases the iron, which precipitates and becomes unavailable.
- DTPA: holds up to ~pH 7–7.5. Better than EDTA, but still falls short with hard water.
- EDDHA: stays stable from pH 4 to 10. Covers the entire real-world growing range, even with alkaline source water.
Why does this hit growers with hard or alkaline water so hard?
A large portion of tap water supplies in the US run alkaline (pH 7.5–8+) and moderately hard. With that water, iron chelated with EDTA starts precipitating before the plant ever gets a shot at it. The result is maddening: the label says the product contains iron, your reservoir test confirms it's there, and the plant still shows a deficiency — because the iron is present but not available.
How does an iron deficiency show up?
Iron deficiency shows up first on NEW leaves (iron is relatively immobile inside the plant): interveinal chlorosis, where the leaf turns yellow but the veins stay green, creating a net-like pattern. It's distinct from nitrogen or magnesium deficiency, both of which start on the older, lower leaves.
