The Ortho-Ortho Percentage in Fe-EDDHA: The Number That Decides Value

If you have collected quotations for Fe-EDDHA, you will have noticed something odd: several suppliers offer what looks like the same product — 6% iron, EDDHA chelated, micro-granular — at prices that differ substantially.

The explanation is almost always the ortho-ortho isomer content. It is the single most important figure on an Fe-EDDHA specification, it is the reason two apparently identical products behave differently in the field, and it is the number some suppliers prefer not to lead with.

What the isomers are

EDDHA is the abbreviation for ethylenediamine-N,N’-bis(2-hydroxyphenylacetic acid). When it is manufactured, the chemistry produces several structural variants — isomers — depending on where the hydroxyl group sits on each aromatic ring. The three that matter commercially are:

• Ortho-ortho (o,o-EDDHA) — hydroxyl groups in the ortho position on both rings
• Ortho-para (o,p-EDDHA) — one ortho, one para
• Para-para (p,p-EDDHA) — both para

All three appear in the finished product, and all three contribute to the total iron figure printed on the bag. They do not, however, contribute equally to what you are buying the product for.

Why only one of them really works

The ortho-ortho isomer forms a hexadentate complex — it binds the iron ion at six points, wrapping it completely. That structure is extremely stable and holds iron in solution across a very wide pH range, including the alkaline, carbonate-buffered conditions where you would use EDDHA in the first place.

Ortho-para forms a weaker complex. It holds iron at moderate pH but is far less reliable at the high pH values that make EDDHA necessary. Para-para is weaker again and contributes very little useful iron chelation at high pH.

So the practical position is this: on a calcareous soil, essentially only the ortho-ortho fraction does the job you are paying for. The rest adds to the total iron number without delivering proportionate performance.

How the arithmetic works out

Consider two products, both labelled 6% iron, EDDHA chelated.

Product B contains roughly a third of the effective iron of Product A. If it is priced at, say, 60% of Product A, it is not a saving — it is substantially worse value, and you would need to apply nearly three times as much to achieve the same result.

This is not a criticism of low-o-o products as such. They have legitimate uses in less demanding conditions and at appropriate prices. The problem is comparing them on the 6% figure alone, which makes them look equivalent when they are not.

What ortho-ortho levels are typical

Commercial Fe-EDDHA products span a wide range. As a rough orientation:

• 4.2% to 4.8% o-o — high specification, the standard for severe chlorosis on strongly calcareous soils
• 3.0% to 4.0% o-o — mid specification, workable in moderately alkaline conditions
• Below 2% o-o — low specification; performance at high pH will be limited

A well-documented product states both the total iron and the ortho-ortho content explicitly on its specification sheet. Our Fe-EDDHA 6% lists both.

How to compare quotations properly

1. Ask for the ortho-ortho figure in writing

If a specification sheet quotes total iron and total chelated iron but not the o-o percentage, ask directly. A supplier who cannot or will not provide it is telling you something. It is a routine analytical figure, and any manufacturer producing a serious product knows it.

2. Calculate cost per kilogram of ortho-ortho iron

This is the only comparison that means anything. The arithmetic is straightforward:

cost per kg of o-o iron = (price per bag) ÷ (bag weight in kg × o-o% ÷ 100)

Run it for every quotation on the table. The ranking frequently reverses once you do.

3. Check what analytical method was used

Ortho-ortho content is determined by HPLC. Ask which method and, if you are buying at volume, whether a certificate of analysis for the specific batch is available.

4. Watch for figures quoted on different bases

Occasionally o-o content is expressed as a percentage of the chelating agent rather than of the finished product, which produces a much larger and entirely non-comparable number. Confirm what the percentage refers to.

When you do not need high ortho-ortho

Being fair about this matters. High o-o content is worth paying for when the conditions demand it:

• Soil pH above 7.5, particularly with free carbonate present
• Irrigation water with high bicarbonate
• Severe, persistent chlorosis in perennial crops

It is not worth paying for when:

• Root-zone pH is below about 6.5 — you should not be buying EDDHA at all. Fe-EDTA costs far less and works.
• You are running a pH-controlled hydroponic system — same conclusion.
• The chlorosis turns out to be caused by waterlogging or root damage — no chelate will fix that. See the iron chlorosis guide.

Buying a premium chelate for conditions that do not require it is the mirror image of the mistake this article is about.

Questions worth asking your supplier

1. What is the ortho-ortho content, as a percentage of the finished product?
2. Can you provide a certificate of analysis for the batch supplied?
3. What analytical method was used to determine it?
4. Is the total iron figure fully chelated, or does it include non-chelated iron?
5. What is the recommended pH range for this specific grade?

Those five questions separate suppliers who understand their product from suppliers who are reselling something. Our broader guide on evaluating a fertilizer supplier extends the same approach to the rest of a purchasing decision.

Summary

• Total iron on an Fe-EDDHA label does not tell you how the product will perform at high pH
• The ortho-ortho isomer is the fraction that holds iron in alkaline, carbonate-buffered conditions
• Compare cost per kilogram of ortho-ortho iron, not per bag or per unit of total iron
• Ask for the figure in writing, with the analytical method
• If your pH is below 6.5, you do not need EDDHA at all

Send us the specification sheets you are comparing and we will work through the numbers with you — including telling you when a competing quotation is the better buy.