Iron Chlorosis: Why Crops Turn Yellow and How to Correct It

Iron chlorosis is the yellowing of new leaves caused by a plant's inability to take up enough iron, almost always because high soil pH has locked the iron into insoluble forms rather than because the soil is short of iron. Match the correction to your pH: foliar ferrous sulphate for a quick rescue, an EDTA chelate on acidic to near-neutral ground, and an EDDHA chelate where the soil runs above pH 7.5. Confirm the diagnosis with a soil and tissue test before you spend on product, because rates and thresholds vary by crop, soil, and lab method.

How to recognize it

The signature is interveinal chlorosis on the youngest leaves: tissue between the veins fades to pale green, yellow, or in severe cases nearly white, while the veins stay distinctly green. Iron is immobile in the plant, so the shortage shows up on new growth at the tips first. That detail drives the diagnosis. Nitrogen and magnesium deficiencies start on older, lower leaves, so when the top of the plant is yellow and the bottom stays green, iron is the prime suspect. Tissue testing helps separate iron from manganese, zinc, and sulfur problems.

Sensitive crops make the pattern easy to spot. Soybean, sorghum, citrus, grapes, blueberry, peach, and pin oak chlorose readily on the wrong soil, often patchy across a field, following the high-lime or poorly drained spots. UF/IFAS describes citrus iron chlorosis on calcareous soils as young leaves turning light yellow to nearly white with greener veins; in acute cases leaves are small and thin, trees die back at the top, and fruit set falls.

Why it happens: pH, not total iron

Most soils hold plenty of iron. The problem is availability. Iron is most soluble in acidic soil and becomes progressively locked up as pH climbs above neutral. NC State Extension puts the comfort zone at pH 6.0 to 6.5, with availability dropping off above that. Colorado State notes that in alkaline soils above pH 7.0, iron is rapidly fixed into insoluble solids roots cannot absorb, and chlorosis is common in calcareous soils above pH 7.5. UF/IFAS describes calcareous soil as high in calcium carbonate with a pH around 8.3, where iron may be present but only slightly available.

Field conditions make the chemistry worse. High-bicarbonate irrigation water, prolonged wet or cold soil, poor drainage, excess salts, and heavy phosphorus all aggravate the problem. Utah State, working in soils that run pH 7.2 to 8.3, flags free lime as the single most important predisposing factor. Corteva's soybean agronomy is more specific: calcium carbonate equivalent above 5% and soluble salts above 0.5 mmho/cm signal a high probability of iron deficiency chlorosis, while soil iron content itself is a poor predictor.

For confirmation, a DTPA soil test in the 2.5 to 10 ppm range is generally considered adequate for vegetables, and healthy leaf tissue typically runs 50 to 200 ppm depending on crop. Treat these as starting reference points and defer to your local lab's crop-specific guidance.

How to correct it

There are three practical levers, and the right one depends on pH and how fast you need a response.

Foliar iron for a fast rescue. A dilute spray greens up existing leaves within days. Utah State uses about 0.1% ferrous sulphate; NC State cites foliar FeSO₄, FeCl₂, or Fe-chelates at roughly 2.7 to 4.5 lb/acre. UF/IFAS gives chelated-Fe rates of 0.4 to 1.0 lb in 100 gal water per acre for vegetables and 0.1 to 0.2 lb in 25 gal per acre for deciduous fruits. The catch: iron doesn't move into leaves that emerge later, so foliar work is a touch-up, not a cure, and usually needs repeating. In soybean, university and seed-company trials have repeatedly found that foliar iron improves color but does not reliably raise yield. Citrus is a special case — UF/IFAS does not recommend foliar Fe there, citing patchy greening and burn risk. Crop and label matter.

Soil-applied chelate matched to pH. On acidic to near-neutral soils, FeSO₄ or an EDTA chelate can supply iron economically. But chelate choice is pH-driven. UF/IFAS lists effective ranges of pH 4.0 to 6.5 for Fe-EDTA, 4.0 to 7.5 for Fe-DTPA, and 4.0 to 9.0 for Fe-EDDHA. The stability data tell the same story: at pH 7.5, the fraction of iron held in chelate is about 1.0 for EDDHA, 0.5 for DTPA, and only 0.025 for EDTA. Above pH 7.5, ortho-ortho Fe-EDDHA is the chelate that keeps iron in solution, which is why it earns its higher cost in citrus, grapes, and nursery crops on calcareous ground. Plain ferrous sulphate is not a cure-all on alkaline soil — UF/IFAS notes it transforms quickly to iron oxide and fails to supply enough available iron for citrus in either acid or alkaline soils. For soybean on problem soils, Corteva reports in-furrow Fe-EDDHA at about 3 lb/acre lifting yield substantially where chlorosis is severe.

Manage the root environment for the long run. Fix drainage, ease compaction, check bicarbonate in irrigation water, and avoid over-liming and heavy phosphorus. On permanent plantings, pick iron-efficient varieties or rootstocks; UF/IFAS notes citrus rootstocks differ markedly in iron uptake. On free-lime soils, acidifying to lower pH is usually impractical, so genetics and chelate strategy carry the load.