Iron Chlorosis in a Calcareous Vineyard: Treating the Soil or the Leaf?
A 1.5 ha Thompson Seedless vineyard on Dogridge rootstock in Niphad taluka, Nashik, has been under drip fertigation for six years. Bunch weight has fallen from about 350 g to 240 g over three seasons. From 30-45 days after pruning, the youngest leaves show interveinal chlorosis while the veins stay green; shoots are short and the canopy is visibly thinner on the western rows. The grower sprayed FeSO4 0.5% twice last season; leaves greened slightly for about ten days and then relapsed. Soil test (0-30 cm): pH 8.7, EC 0.42 dS/m, free CaCO3 16%, organic carbon 0.31%, available N 168 kg/ha, P2O5 22 kg/ha, K2O 340 kg/ha, DTPA-Fe 2.4 ppm, DTPA-Zn 0.38 ppm, DTPA-Mn 3.1 ppm. Borewell water used for irrigation has EC 1.4 dS/m, bicarbonate 6.2 meq/L and pH 8.3, and white crusting is visible on the emitters. The grower wants a permanent fix before the next pruning and asks whether increasing foliar iron to four sprays would solve it.
1. Explain the cause of the chlorosis and why the foliar FeSO4 response was so short-lived. What do the soil and water figures add to the diagnosis?
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Diagnosis: bicarbonate-induced iron chlorosis, not a simple soil iron shortage.
- pH and lime: pH 8.7 with 16% free CaCO3 keeps iron as insoluble oxides and hydroxides; only a very small fraction of soil Fe is plant-available.
- DTPA-Fe 2.4 ppm is below the usual critical level of about 4.5 ppm, confirming deficiency, and DTPA-Zn 0.38 ppm (critical ~0.6 ppm) means zinc is also limiting. Zinc shortage alone can produce a similar interveinal pattern, so both must be corrected.
- Irrigation water: 6.2 meq/L bicarbonate at pH 8.3 raises root-zone pH and directly inactivates iron at the root surface - the classic bicarbonate effect. The white emitter crust is the visible sign of this precipitation.
- Why foliar iron failed: iron is essentially immobile in the phloem. Only the sprayed leaves green up; the next flush emerges chlorotic within 8-10 days. In addition, FeSO4 in hard alkaline water oxidises in the spray tank before it is absorbed.
Conclusion to give the grower: the deficiency originates in the root zone and the irrigation water. Four foliar sprays will repeat the same short-lived greening at higher cost. The permanent fix is acidification of the irrigation water plus a stable chelate placed in the wetted root zone.
2. Set out a corrective programme for the next two seasons - water treatment, amendment, fertiliser source, dose and placement - and state the order in which you would act and why.
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Order of action matters: water first, then root zone, then tissue testing.
- Acidify the irrigation water (highest priority). Neutralising 6.2 meq/L bicarbonate needs roughly 0.3 kg of concentrated H2SO4 per 1,000 L; target water pH 6.0-6.5 and residual bicarbonate near 1 meq/L. Acidify continuously through the drip, then flush lines and clean the crusted emitters. Targeting water before soil is essential because every irrigation is currently re-loading the root zone with bicarbonate.
- Use the right chelate. Fe-EDDHA (ortho-ortho isomer) at 25-40 g/vine/year in three splits (at pruning, 30 days after fruit set, post-harvest), placed in the wetted zone, not broadcast. Fe-EDTA is ineffective above pH 7.5, so buying it would waste money.
- Correct zinc: ZnSO4 10 kg/ha banded in the root zone plus two foliar sprays of 0.5% ZnSO4 with 0.1% citric acid at 15-day intervals.
- Build buffering capacity: 20 t/ha of well-rotted FYM or vermicompost plus a mulch or green-manure crop to reduce pH fluctuation and improve Fe availability. Avoid heavy phosphorus doses; high P antagonises Zn and Fe uptake.
- Monitor: soil pH and EC at 0-30 and 30-60 cm quarterly; petiole analysis at bloom (not leaf blade alone); record bunch weight row by row.
Cost check: about 5,000 vines x 30 g Fe-EDDHA is 150 kg, roughly Rs 1.6-1.8 lakh per season. A 30% bunch-weight loss on the current crop is worth several lakh rupees, so the programme is cheaper than continuing the loss.
3. How would you verify that the programme is working, and how would you persuade the grower that the chelate cost is justified before the whole vineyard is treated?
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Verification design
- Treat the western rows as a demonstration block and leave five adjacent rows under the grower's current practice as an untreated control. Compare bunch weight, shoot length and leaf greenness (SPAD meter) at 45 and 90 days after pruning.
- Confirm the mechanism, not just the colour: sample soil at 0-30 cm in the wetted zone and show pH falling below about 7.5 and DTPA-Fe rising; take petioles at bloom and compare with the control.
- Keep the yield record per row; the vineyard already has three seasons of bunch-weight data, so a 240 g to 300 g shift is measurable against that baseline.
Persuading the grower (partial budget, not a lecture)
- State the loss in his own units: 1.5 ha at roughly 20 t/ha is about 30 t; a 30% shortfall is 9 t; at Rs 60/kg that is about Rs 5.4 lakh a season, against a chelate and acid bill of about Rs 2 lakh.
- Show that four foliar sprays cost roughly Rs 40,000-50,000 a season and return nothing beyond ten days of greening - the money is already being spent, just on the wrong tool.
- Propose a phased start on 0.3 ha for one season, with the decision to scale up based on the control-block comparison. This protects him from a large upfront commitment while giving the evidence needed before the next pruning.