Two Maize Fields, Two Chloroses: Diagnosing Mg and Fe Deficiency on Calcareous Soil
A maize crop in Chitwan shows yellowing between the veins of the oldest leaves while the younger leaves stay green. A second field 3 km away, sown with the same hybrid and irrigated from the same canal, shows interveinal yellowing of the youngest leaves with the veins remaining green. Both fields received 120 kg N/ha as urea. Soil test, field A: pH 7.8, organic matter 1.2%, exchangeable Mg 40 ppm (critical 50 ppm). Soil test, field B: pH 8.1, calcium carbonate 12%, DTPA-extractable Fe 3.5 ppm (critical 4.5 ppm). Leaf tissue, field A: Mg 0.12% (sufficient 0.15–0.25%). Leaf tissue, field B: Fe 38 ppm (sufficient 50–100 ppm). The farmer can afford one intervention per field before the next irrigation and has on hand Epsom salt (MgSO₄·7H₂O), ferrous sulphate, elemental sulphur and gypsum. He asks which single product to use on each field and whether a spray or a soil application will work faster.
1. Identify the deficiency in each field and explain, from phloem mobility, why the symptoms appear on old leaves in one field and young leaves in the other.
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Field A - magnesium. Tissue Mg 0.12% is below the 0.15% sufficiency threshold, exchangeable soil Mg is under the critical 50 ppm, and the symptom pattern is interveinal chlorosis of the oldest leaves. Mg is phloem-mobile: when supply is short, the plant reticulates Mg out of mature leaves and loads it into growing tissue, so the old leaves bleach first. Mg is also the central atom of chlorophyll and a cofactor for the ATPase that drives phloem loading, which is why chlorosis is interveinal - the veins stay green as long as the transport system is intact.
Field B - iron. Tissue Fe 38 ppm is below 50 ppm, DTPA-Fe is 3.5 ppm, and the pattern is interveinal chlorosis of the youngest leaves. Fe is essentially phloem-immobile, so the plant cannot move it from old tissue to new; the growing leaves are the last to be supplied and the first to show chlorosis. Fe is required for chlorophyll biosynthesis and chloroplast protein assembly, so the tissue turns yellow while the veins, which have some apoplastic supply, remain green.
The contrast in position of the symptom is the diagnostic discriminator: mobile nutrient → old leaves; immobile nutrient → young leaves. The nitrogen history is not the explanation, since both fields received identical urea and N deficiency would cause uniform, whole-leaf yellowing starting on old leaves.
2. Choose one product and one method for each field, and justify the choice on soil chemistry grounds. Why would soil-applied ferrous sulphate fail in field B?
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Field A: foliar Epsom salt, 2% MgSO₄·7H₂O, 2–3 sprays 7–10 days apart. The soil Mg is marginal rather than absent, and foliar Mg bypasses the root-uptake competition from Ca²⁺ at pH 7.8 (Ca–Mg antagonism). Greening of the old leaves should appear within about a week, which also serves as confirmation. Soil-applied MgSO₄ would work but more slowly, and dolomite or lime must be avoided because they raise pH further and worsen both fields.
Field B: foliar ferrous sulphate, 0.5–1% with a sticker, 2–3 sprays. At pH 8.1 with 12% calcium carbonate, iron applied to soil is rapidly oxidised from Fe²⁺ to insoluble Fe³⁺ oxides and hydroxides; the 'available' DTPA-Fe fig of 3.5 ppm already shows this. Soil FeSO₄ would disappear chemically within days and could not be taken up - the plant-available pool, not total soil Fe, is the limitation.
Why not the other two products? Elemental sulphur does acidify soil, but microbial oxidation to sulphuric acid takes months to a season - useless for this crop. Gypsum (CaSO₄·2H₂O) supplies calcium and improves sodic soil structure but does not lower pH, so it cannot mobilise Fe. The correct judgement is: foliar rescue now, and an acidifying programme (ammonium sulphate, organic matter, or Fe-EDDHA chelate, which stays available at high pH) as the long-term fix.
3. The farmer wants to spray the whole of field B the same afternoon. What confirmation step and what precautions would you insist on, and what would you conclude if there is no greening after 7 days?
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Confirmation first. Spray a marked strip or a few rows and leave an unsprayed control strip beside it. Deficiency diagnoses built on symptom position plus one soil and one tissue number can be wrong; a spray-response trial converts the reasoning into evidence within a week. For Fe, the new leaves should begin to green 5–7 days after spraying; for Mg, the older leaves.
Precautions. Ferrous sulphate sprays scorch readily, especially in strong midday sun and at high concentration - trial 0.5% on a few plants before committing to 1%, spray early morning or evening, add a sticker, and do not tank-mix with alkaline products such as Bordeaux mixture or lime sulphur, which would precipitate the iron. Acidify the spray water if it is hard.
If there is no greening after 7 days, reconsider the diagnosis rather than escalating the dose: manganese and zinc deficiency also give interveinal chlorosis (Mn on younger leaves, but with necrotic flecks), herbicide carry-over and root damage from waterlogging or nematodes mimic nutrient chlorosis, and a high-CaCO₃ soil can induce several micronutrient deficiencies at once. The next step is a multi-element tissue analysis, not a stronger spray. Also note the economics: foliar sprays cost little, but a severely affected field has already lost yield potential, so effort is better spent on the less damaged area and on next season's soil programme.