Iron is a micronutrient required for chlorophyll synthesis and for the cytochromes and iron-sulfur proteins that carry electrons in photosynthesis and respiration. It is immobile, so the plant cannot move it from old leaves to new ones. In solution, iron precipitates as insoluble hydroxides as pH rises, so feeds supply it as a chelate.
In practice
Deficiency appears as interveinal chlorosis on the newest leaves: bright yellow tissue with fine green veins, in severe cases nearly white. The usual cause is high root-zone pH, not a lack of iron in the bottle. Hydro feeds typically carry about 1-3 mg/L (ppm) Fe. Which chelate is used matters: EDTA-iron loses effectiveness above about pH 6.5, DTPA holds to about 7.5, and EDDHA well beyond.
Don't confuse with
Myth check
Adding more iron rarely fixes iron chlorosis when pH is the problem. Correct the root-zone pH first. See pH and nutrient availability.
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.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.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Yellowing of the leaf tissue between the veins while the veins themselves stay green; on old leaves it points to magnesium, on new leaves to iron or manganese.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.Yellowing of normally green plant tissue caused by too little chlorophyll, from nutrient shortage, poor root uptake, pH lockout, light damage, pests or normal ageing.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.An immobile nutrient stays where it is first deposited, so its deficiency shows on young leaves and growing tips first: Ca, S, Fe, Mn, B, Cu.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.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.EDDHA is a strong iron chelate that stays effective up to pH 9 or more, used for alkaline soils and hard water; it colours solutions dark red.
