Chelate: a Molecule That Keeps a Metal Dissolved in the Nutrient Solution

A chelate is a metal bound to an organic molecule, which keeps it dissolved in the nutrient solution. In fertiliser you meet it mainly with iron, as iron DTPA or iron EDDHA. The reason: unbound iron precipitates out of the solution at higher pH and is then lost to the root, even though it is still in the pot.

Why it is needed

Iron, manganese, zinc, and copper precipitate out of the nutrient solution above roughly pH 6 unless they are present as a chelate. The plant then shows a deficiency even though the fertiliser contains enough, with iron typically as yellowing between the veins of the youngest leaves.

How a chelate works

The chelate grips the metal like pincers, hence the name from the Greek word for a crab’s claw. Bound this way, the metal stays dissolved even where, free, it would react with other substances in the solution to form an insoluble precipitate. An iron chelate is therefore iron in a form the root can still reach at a somewhat higher pH.

Which chelate at which pH

Chelates also only hold up to a limit. Iron DTPA is sufficient up to pH 6.5 and stays stable up to pH 7. Iron EDDHA holds up to pH 9, iron HBED up to pH 12. For coco, rockwool, and hydroponics, which are run at roughly 5.5 to 6.5, DTPA is therefore enough. EDDHA is only needed when the pH at the root stays above 6.5.

What to do first with iron deficiency

Measure the pH at the root. If it is too high, lower it; as long as it stays too high, extra iron without a suitable chelate precipitates as well. Only if the pH cannot be brought into the target range do you switch to a more stable chelate. Retailers sometimes give other pH limits for individual chelates, which contradict each other; the figures here come from a handbook for greenhouse crops.

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