Above about pH 7.5 an iron chelate lets go and the iron precipitates out of reach. Why EDDHA holds where EDTA and amino-acid iron do not, and what to read on a specification.
Iron chlorosis on high-pH, calcareous ground is a familiar sight: interveinal yellowing on new growth while a soil test says there is plenty of iron in the profile. Both things are true at once, and the reason is chemistry rather than agronomy. The failure happens to the chelate, not to the crop.
What actually fails
A chelate is a ligand holding a metal ion inside a cage of bonds. How tightly it holds depends on pH, because the hydroxide ion competes for the same metal. Above about pH 7.5, EDTA and amino-acid chelates lose their grip on iron: the iron comes off the ligand and precipitates as ferric hydroxide. In that form it is still iron, still in the soil, and no longer available to the plant.
So the yellowing is not a dosing failure. You can raise the rate of an EDTA-chelated iron on calcareous ground and change very little, because the limit is not how much iron went out — it is how much of it was still chelated by the time roots could reach it. The iron percentage on the bag was never the problem.
Where it shows up in Australia
Two conditions push the same way, and much of Australian horticulture has both. The first is carbonate in the soil: the limestone sands and calcareous soils of the Swan Coastal Plain around Perth, the South Australian Mallee and Riverland, and the light country of Sunraysia all buffer strongly alkaline. The second is the irrigation water. Bore water across much of Western Australia is high in bicarbonate, so every watering nudges root-zone pH upward regardless of what the soil started at.
Tree crops and viticulture carry it most visibly, macadamia and avocado in particular — perennial plantings on alkaline ground, where a season of chlorosis is a season of lost growth rather than a crop you can re-sow.
Why EDDHA holds
EDDHA is the most stable of the common iron chelating agents across the alkaline range, and stays in solution to around pH 9. That is the whole reason it exists in a catalogue that already has EDTA: it covers the pH window where the cheaper ligand has already let go.
But the total iron figure on an EDDHA label does not tell you how much of that stability you are buying. EDDHA exists as several isomers, and it is specifically the ortho-ortho isomer that holds iron at high pH. More ortho-ortho means better stability. A product can carry 6% total iron and very little of it ortho-ortho chelated, which is why two 6% irons can behave nothing alike on the same block.
- →Read the ortho-ortho content, not the total iron — it is the figure that predicts performance on alkaline ground.
- →Check whether it is quoted as a minimum or a typical value, and whether the certificate of analysis reports it per batch.
- →Check the iron band. A narrow specified range tells you more about batch consistency than an open minimum does.
- →Check the solution pH. An EDDHA product should give a clear solution and sit alkaline in its own right.
Our EDDHA Fe-6 is specified as a 5.6–6.2% w/w band rather than an open minimum, and typically carries 4.8% w/w ortho-ortho against a 3.0% minimum. The ortho-ortho figure is typical, and the certificate of analysis reports it per batch.
This is not a replacement for amino-acid iron
It is worth being plain about where the line sits, because the two products are often set against each other and they should not be. An amino-acid chelated iron does its job, and does it economically, on neutral to mildly acidic ground. EDDHA is the extending arm for the high-pH, calcareous ground where an amino-acid chelate can fail. Same shelf, different pH range — and for a blender, two grades rather than one argument.
The same logic applies to the EDTA-chelated micronutrient mixture: excellent value for six elements across a working pH window of roughly 5 to 8, and the wrong tool if the iron in it has to survive a calcareous profile.
What to send us
If you are formulating for alkaline country, tell us the grades you sell, the soil pH and bicarbonate level they go out on, and whether the material is going into a dry blend or a soluble grade. We can generate compatibility test data on request, so a trial answers how the material behaves in your blend rather than only what it is. The full Grow range is published with specifications.
All Grow materials are supplied as raw materials for blending and formulation. Values are typical and do not constitute a specification for any finished product; the composition, registration and labelling of finished fertiliser products remain the responsibility of the blender.