Interveinal chlorosis in a lime-rich banana field at Raver
Sunita Patil has 1.2 ha of Grand Naine banana at Raver, Jalgaon, planted in February at 1.5 m x 1.5 m. The soil is a deep black soil with 11% free lime, pH 8.3 and EC 0.18 dS/m. Her 0-30 cm soil test reports organic carbon 0.52%, available N 210 kg/ha, Olsen P 31 kg/ha (rated high), available K 340 kg/ha, DTPA-Zn 0.34 ppm, DTPA-Fe 3.1 ppm and DTPA-Mn 4.0 ppm. Since planting she has applied, per plant per month, 250 g DAP and 150 g urea through fertigation for three months, and nothing else. In the fourth month the youngest three leaves show interveinal chlorosis with a green midrib, and the lamina looks thin and papery. Older leaves are dark green and glossy. There is no wilting, and the excavated roots are white and healthy.
1. Diagnose the disorder and explain how it can occur although the soil contains plenty of iron and zinc.
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Diagnosis: iron deficiency, with zinc deficiency in the same package. The clue is which leaves are affected. Chlorosis on the youngest leaves with a green midrib points to an immobile nutrient, i.e. one the plant cannot move out of old leaves: Fe, Zn, Mn or Cu. The soil test supports Fe (DTPA-Fe 3.1 ppm, below the ~4.5 ppm critical level used for banana) and Zn (0.34 ppm against a critical level near 0.6 ppm). Healthy white roots and no wilting rule out waterlogging and root rot; Mn deficiency would normally show necrotic speckling, which is absent.
Why deficiency on a soil full of iron: in a black soil with 11% free lime at pH 8.3, iron is present but in the ferric (Fe3+) form as oxides and carbonates, which is essentially insoluble. Bicarbonate dissolved from calcium carbonate keeps the soil solution alkaline and oxidizing, so Fe3+ stays precipitated at the root surface and cannot be reduced and taken up. Zinc behaves the same way, adsorbing onto and precipitating with carbonate and phosphate. The soil test measures available (DTPA-extractable) nutrient, not total, so a high total content is perfectly consistent with a deficiency. A second aggravating factor is her very high phosphate application, which forms insoluble zinc phosphate and induces Zn deficiency and, indirectly, Fe chlorosis.
2. Critique the fertilizer programme she has followed and show with numbers why it cannot be corrected by adding more DAP.
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At 1.5 x 1.5 m the density is about 4,444 plants/ha, so 1.2 ha carries roughly 5,330 plants.
Phosphorus: 250 g DAP supplies 18% N = 45 g N and 46% P2O5 = 115 g P2O5 per plant per month. Over three months that is 345 g P2O5 per plant, or about 1,840 kg P2O5/ha - several times what even a heavy banana crop needs. This is exactly why Olsen P has climbed to 31 kg/ha (high).
Nitrogen: 150 g urea x 3 = 450 g N, plus 135 g N from DAP, i.e. about 585 g N per plant, or roughly 3,100 kg N/ha in four months. That is grossly excessive: it leaches as nitrate, raises rhizosphere salinity, and pushes vegetative growth at the cost of the bunch.
Potassium: zero has been applied, although banana is the most potassium-hungry field crop and a Grand Naine crop removes several hundred kilograms of K2O per hectare. Her 340 kg/ha K is only adequate on paper.
Why more DAP makes it worse: phosphate and zinc are antagonistic in soil and in the plant. Extra phosphate precipitates zinc phosphate at the root and blocks Zn translocation, so the chlorosis deepens. Adding DAP also adds more bicarbonate-driven alkalinity at the root zone. The corrective action is to cut DAP hard, not raise it.
3. Give a 30-day corrective plan, naming products and explaining why the soil route and the foliar route are both used.
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Stop DAP immediately and hold urea; the plant has more than enough N and P.
Foliar (fast, 7-10 day response):
- FeSO4 0.5% (5 g/L) plus 0.1% citric acid as an acidifying carrier, sprayed on the newer leaves, 2-3 rounds at 10-day intervals.
- ZnSO4 0.5% on a separate day or in a separate tank, since Fe and Zn salts compete and can precipitate with phosphate carriers.
- Do not tank-mix these with phosphate fertilizers or with alkaline products.
Soil (slow, corrects the root zone):
- Fe-EDDHA chelate at 5-10 g/plant in the fertigation or as a root-zone drench. EDDHA is the correct chelate here precisely because it stays stable above pH 8; FeSO4 put into this soil would be oxidized and fixed within days.
- ZnSO4 at 10-20 g/plant worked into the wet root zone.
- Muriate of potash to start meeting K demand in the split doses already scheduled.
- Switch part of the N to ammonium sulphate, which acidifies the rhizosphere, and add 5-10 kg FYM or vermicompost per plant to build organic carbon and chelate micronutrients naturally.
Monitoring: repeat DTPA-Zn and DTPA-Fe after the next soil test cycle, and use leaf/petiole analysis rather than soil values alone for Fe. Long-term, elemental sulphur or green manuring will lower pH slowly; there is no quick fix for a calcareous soil.