EDDHA, ethylenediamine-N,N'-bis(2-hydroxyphenylacetic acid), is a synthetic chelating agent used almost entirely for iron. Fe-EDDHA products typically contain about 6% Fe. The quality of EDDHA products depends on the share of iron held in the more stable ortho-ortho form, which labels may state.
In practice
Fe-EDDHA is the fix for iron chlorosis in alkaline soils and with high-pH, hard water, where EDTA and DTPA fail. It costs more per gram of iron. It stains water, reservoirs and runoff a deep wine-red, which alarms growers the first time. For hydro kept in the usual pH 5.5-6.5 range, it is rarely necessary; DTPA usually covers drift. Using it as a short-term rescue for iron deficiency in high-pH soil is a common practice.
Don't confuse with
- EDTA: cheaper, works only below about pH 6.5.
- DTPA: middle ground, good to about pH 7.5.
- Red runoff from organic matter: tannins or humics, not EDDHA.
A chelate is a metal ion held by an organic molecule at several points, keeping micronutrients such as iron, zinc and manganese dissolved and available across a wider pH range.An iron chelate is iron bound to an organic molecule (EDTA, DTPA or EDDHA) that keeps it dissolved and available; each type holds up to a different pH.General hardness (GH) is the calcium and magnesium dissolved in water, expressed as mg/L CaCO3 or German degrees (dGH); hard water supplies some Ca and Mg.EDTA (ethylenediaminetetraacetic acid) is the most common synthetic chelating agent in fertilisers, effective for iron up to about pH 6.0-6.5.DTPA (diethylenetriaminepentaacetic acid) is a synthetic chelating agent that keeps iron available up to about pH 7.0-7.5, a common choice for coco and hydro.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Iron (Fe) is an immobile micronutrient needed to make chlorophyll and run electron transport; deficiency yellows new leaves between green veins.
