Micronutrient Chelate & Match It to Your Soil pH

It depends on the metal and the pH limit of each chelate. EDTA holds zinc, manganese and copper to about pH 6.5; HEDTA to about 7.0; DTPA to about 7.5 — the workhorse on neutral soils; and EDDHA holds iron to about pH 9, the only chelate that works in calcareous soils. Below the limit the chelate keeps the metal in solution; above it the metal is lost to calcium competition and hydrolysis. Enter your metal and pH and the tool ranks them and picks the stable one.

Inorganic sulphates release the metal directly into the soil solution, where above about pH 7 zinc, iron and manganese rapidly precipitate as hydroxides, carbonates or phosphates and become unavailable. A chelate wraps the metal in an organic cage that resists this precipitation up to its pH limit, keeping it available to roots. On acid soils sulphate is fine and far cheaper; on calcareous soils it is largely wasted, which is exactly when a chelate earns its cost.

EDDHA. It is the only common chelate that holds iron stable up to about pH 9, so on high-lime calcareous soils where iron chlorosis is the problem, EDDHA-Fe is effectively the only soil-applied chelate that works. EDTA-Fe and even DTPA-Fe lose their iron to calcium well below pH 8, so they fail on the soils that need iron most. EDDHA is expensive, but the tool shows it as the clear pick at high pH because nothing else delivers.

Divide the target metal rate by the product's metal percentage. To supply 3 kg of iron per hectare with EDDHA-Fe at 6% iron, you apply 3 ÷ 0.06 = 50 kg of product per hectare. The tool does this for the recommended chelate and shows the typical soil and foliar metal-rate ranges so you can sanity-check the rate before you buy.

They are different chelating molecules with different binding strengths and pH stability. EDTA is the cheapest and works to about pH 6.5; DTPA binds more strongly and holds to about pH 7.5; EDDHA binds iron extremely strongly and holds it to about pH 9 but carries iron only. Higher stability costs more, so the trick is to match the chelate's pH limit to your soil — paying for EDDHA on an acid soil, or using EDTA on a calcareous one, both waste money.

Yes, and on high-pH soils where even the best chelate is stressed, foliar is often the smarter route because it bypasses soil tie-up entirely. Foliar rates are much lower than soil rates — typically a fraction of a kilogram of metal per hectare — and may need repeating. The tool switches the typical rate range when you select foliar, and the recommendation leans toward foliar when soil stability is poor.

EDDHA is built to bind ferric iron specifically and is not used commercially to deliver zinc, manganese or copper. So when you select Zn, Mn or Cu the tool greys EDDHA out and ranks EDTA, HEDTA, DTPA and sulphate; when you select Fe, EDDHA appears and dominates at high pH. This mirrors the real product market, where EDDHA-Fe is the dedicated calcareous-soil iron chelate.

It is the modelled fraction of the chelated metal that remains protected and plant-available at your soil pH, based on the chelate's stability around its pH limit. Near 100% means the chelate is comfortably within its working range; around 50% means you are at its pH limit; below about 40% means it is failing and the metal is precipitating. The tool uses this fraction to flag a good, marginal or unstable choice.

Yes — on acid soils, below about pH 6.5–7 depending on the metal, the inexpensive sulphate keeps the metal available and there is no agronomic reason to pay several times more for a chelate. The tool deliberately prefers the cheaper sulphate when its stability is essentially equal to the chelates, so you only buy a chelate when your pH actually requires one.

The pH limits and the stability behaviour follow Norvell's classic chelate-equilibria work and extension micronutrient-chelate guidance, so the ranking — which chelate to use and roughly the share that stays available — is reliable for product selection. The exact stable percentage depends on your soil's calcium, phosphate and the specific formulation, so treat it as a sound selection guide and confirm the rate against the product label and a tissue or soil test.

Almost certainly a pH difference. An iron chelate that holds fine at pH 7 can lose most of its iron at pH 8.3, so the same EDTA-Fe or DTPA-Fe product succeeds on the neutral field and fails on the calcareous one. Run each field's pH through the tool: where it flags the chelate as unstable, you need EDDHA-Fe or a repeated foliar, not a higher rate of the failing product.

No — a test tells you which micronutrient is actually deficient and how much to supply; this tool tells you which chelate form to buy for your soil pH and how much product that target rate needs. Use them together: diagnose the deficiency and rate with a test, then select the stable chelate and product dose here. Guessing the deficiency without a test risks correcting the wrong nutrient.