A chelate is a complex in which an organic molecule, the chelating agent, grips a metal ion at two or more points, like a claw (the word comes from the Greek for claw). The grip protects the metal from reacting with hydroxide, phosphate or carbonate and dropping out of solution. Iron, manganese, zinc and copper are the metals most often chelated in fertilisers.
In practice
Without chelation, iron in particular precipitates within hours once pH rises above about 6. Synthetic chelates differ in stability: EDTA is fine for typical hydro pH, DTPA holds higher, and EDDHA survives alkaline conditions. Natural compounds such as amino acids, humic and fulvic acids and citrate also bind metals, but more loosely and less predictably. Labels list which elements are chelated and with what agent.
Don't confuse with
- Complexing agents: a broader class; not all complexes are true chelates.
- Biostimulant: a marketing category; a chelate is a defined chemical form.
- Sequestered: often used loosely on labels to mean chelated or complexed.
Myth check
"Chelated" on a label does not make a product better in itself. The chelate type has to match the root-zone pH. See pH and nutrient availability.
Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.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.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.Fulvic acid is the small, soluble fraction of humic substances that stays dissolved at any pH and binds metal ions; its benefit in cannabis is largely unproven.A biostimulant is a product claimed to improve plant growth, uptake or stress tolerance by means other than supplying nutrients, such as seaweed extract, humics, amino acids or microbes.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.An amino acid chelate is a metal bound to amino acids such as glycine; it is a gentler, biodegradable alternative to synthetic chelates, often sold for foliar sprays.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.
