Iron Fertilizer Sources: Ferrous Sulphate vs EDTA-Fe vs EDDHA-Fe — How to Choose

Why soil pH — not iron content — usually decides the outcome

It is tempting to rank iron fertilizers by their guaranteed iron percentage: ferrous sulphate heptahydrate carries the most iron per kilogram (about 19–20% Fe), EDTA-Fe around 13%, and EDDHA-Fe only about 6%. On that basis EDDHA-Fe looks like the weakest product. In the field the ranking often reverses.

The reason is chemistry. As soil pH climbs above roughly 6.5, iron rapidly converts to insoluble ferric oxides and hydroxides that roots cannot take up. In calcareous, alkaline soils — pH in the 7.4–8.5 range, with elevated bicarbonate — this lock-up is severe, and it is one of the most common causes of iron-deficiency chlorosis worldwide. So the question is not "how much iron is in the bag," but "how much of that iron stays soluble and available at my soil pH."

Ferrous sulphate: cheap, high-iron, but pH-limited

Ferrous sulphate heptahydrate (FeSO4·7H2O) is the classic, low-cost iron source. It dissolves readily and delivers both iron and sulphur. RunziChem supplies it at a typical ~19–20% Fe (heptahydrate), a level confirmed per batch on the certificate of analysis.

Its weakness is durability in soil. Once dissolved in a neutral-to-alkaline, calcareous soil, the ferrous iron quickly oxidizes and precipitates, so a soil application is often short-lived or ineffective. University extension guidance is blunt on this: iron sulphate can give good results on acid soils or turf, but soil-incorporated iron sulfate is generally ineffective on alkaline, high-pH soils (Utah State Extension notes it is "not effective in most Utah soils"). That is why ferrous sulphate is used most successfully as a foliar spray, where dilute iron is delivered straight to the leaf and bypasses the soil chemistry. Foliar iron acts fast but the response is frequently incomplete and temporary, and repeat sprays are needed as new leaves emerge.

EDTA-Fe: a soluble chelate for neutral to slightly acid conditions

Chelates protect iron inside an organic "claw" so it stays soluble across a wider pH band than a raw salt. EDTA-Fe (RunziChem typical ~13% Fe, confirmed per batch) is fully water-soluble and convenient for foliar and fertigation programs. But EDTA is a relatively weak chelate for iron at high pH: it strongly holds iron up to about pH 6, and by pH 7 almost none of the iron remains plant-available. Practically, EDTA-Fe is a sound, economical choice on acid to near-neutral soils and in hydroponic or soilless systems where pH is managed — but it is the wrong tool for a calcareous field.

EDDHA-Fe: the high-pH specialist

EDDHA-Fe is the strongest of the commonly used iron chelates. Extension guidance puts its effective range past about pH 9, and peer-reviewed stability work shows it holds iron across roughly pH 4–9 (most stable near neutral) — well into the calcareous range where sulphate and EDTA-Fe have already failed. That stability is why EDDHA-Fe is the go-to soil and fertigation product for citrus, stone fruit, olive, avocado and vines on limestone soils.

Not all EDDHA-Fe is equal. Total iron is typically ~6%, but the agronomically active fraction is the ortho-ortho (o,o) isomer — the configuration with the binding geometry that keeps iron chelated at high pH. RunziChem EDDHA-Fe is specified with an ortho-ortho content around 4.8% (typical, confirmed per batch COA). When comparing EDDHA-Fe products, the o,o percentage — not just the "6%" headline — is the number that predicts field performance.

Foliar vs soil / fertigation

Application method matters as much as the molecule:

• Foliar: Fast, corrective, and where ferrous sulphate and EDTA-Fe earn their keep. Good for a quick green-up, but expect partial and temporary results and plan repeat sprays. Spray in cool conditions to reduce leaf-burn risk.
• Soil / fertigation: Slower to show but longer-lasting, and the right route for chronic chlorosis on high-pH ground. On calcareous soils this is EDDHA-Fe territory; EDTA-Fe and sulphate applied to the same soil are largely wasted.

A common, cost-aware pattern is EDDHA-Fe through the drip line to fix the season-long soil-availability problem, with an occasional foliar top-up for rapid correction.

Cost: match the product to the pH, not the other way around

On a per-kilogram-of-iron basis, ferrous sulphate is cheapest, EDTA-Fe is moderate, and EDDHA-Fe is the most expensive — typically several times the cost per kg of iron of EDTA-Fe (varying widely by grade, o,o-content and order volume), reflecting its more complex synthesis and superior stability. The economically correct approach is to spend the minimum that works at your pH: use low-cost ferrous sulphate or EDTA-Fe where soil pH is below ~6.5–7, and reserve the extra cost of EDDHA-Fe for calcareous, high-pH soils where nothing cheaper stays available. Buying EDDHA-Fe for an acid soil wastes money; buying ferrous sulphate for a calcareous soil wastes the whole application.