Interveinal Chlorosis in Drip-Fertigated Tomato
An irrigated tomato crop of 0.8 ha near Sangli is 55 days old and in full flowering, with a target yield of 60 t/ha. The farmer applied 10 t of FYM before transplanting and, through the drip system over the first 50 days, 250 kg MOP, 300 kg SSP and 200 kg urea. Soil pH is 7.9 and EC 0.5 dS/m; the irrigation water has an EC of 1.4 dS/m. Leaves in the lower and middle canopy show interveinal chlorosis with the veins remaining green; in severe cases the margins turn brown while the petioles stay green. The youngest leaves look normal and dark. Leaf analysis reports Mg 0.22% (critical range 0.30-0.35% at flowering) and K 4.6%. Fruit set on the affected plants is poor. The farmer is convinced this is a micronutrient problem and wants two foliar sprays of a chelated micronutrient mix costing ₹1,800 per kg, at 500 g per spray.
1. Is the micronutrient diagnosis correct? Diagnose the disorder from the symptom pattern, the leaf analysis and the fertilizer history, and explain why the youngest leaves are normal.
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The micronutrient diagnosis is wrong, and the spray would waste about ₹1,800 plus labour.
- The symptoms are on the older and middle leaves, which means a mobile nutrient is being moved out of them to the growing point - that points to magnesium, nitrogen or potassium, not to iron, manganese, zinc or copper, which are immobile and appear first on young leaves.
- Interveinal chlorosis with green veins, followed by marginal browning, is the classic magnesium pattern. Nitrogen deficiency would give uniformly pale older leaves, not green veins.
- Leaf Mg of 0.22% is below the 0.30-0.35% critical range, confirming the visual reading.
- Leaf K of 4.6% shows luxury potassium uptake. The fertilizer history explains why: 250 kg MOP on 0.8 ha is about 187 kg K2O/ha, on top of already-high soil potassium and roughly 50 kg K2O/ha from the FYM. Potassium and magnesium compete for the same uptake sites, and the saline irrigation water (EC 1.4) adds further stress.
So the cause is induced magnesium deficiency driven by excess potassium, and the correction is to lower K and supply Mg, not to add a trace-element mix.
2. Give a corrective plan with doses, method and approximate cost for the 0.8 ha, and state how the potassium programme should change.
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Immediate (this week): foliar magnesium sulphate at 1% - 1 kg per 100 litres - at 500-600 L/ha. For 0.8 ha that is about 400 L per spray, i.e. 4 kg MgSO4 per spray, three sprays at 7-10 day intervals = 12 kg. At roughly ₹30-40/kg the material costs ₹400-500, plus spraying labour of about ₹300 per round. Add a small amount of sticker; spray in the early morning.
Soil: band or fertigate 25 kg/ha of MgSO4 (20 kg for the field). At pH 7.9 sulphate of magnesium is still effective; dolomitic lime is not appropriate because it raises pH.
Potassium: crop removal for 60 t/ha of tomato is about 200-250 kg K2O/ha. He has already applied roughly 187 kg K2O/ha as MOP plus about 50 kg from FYM, so stop MOP entirely unless leaf K falls below 3.5%. If K is still needed, use sulphate of potash rather than MOP, because the irrigation water already carries chloride at EC 1.4. Keep the K:Mg ratio in the fertigation solution near 3-4:1.
Monitor: re-sample the fourth leaf from the top 10-12 days after the first spray. Because the crop is at flowering, a response in fruit set should be visible within two weeks; if it is not, look for thrips or leaf curl virus before spending more.
3. The farmer wants to raise potassium further because fruit set is poor. Using the numbers, show what that will do and state the maximum justifiable K dose for the remaining 55 days.
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Adding K will make the problem worse, not better.
- The limiting nutrient here is magnesium, not potassium: leaf K is already 4.6%, well above sufficiency, while leaf Mg is at 0.22% against a critical 0.30-0.35%. More MOP would widen the K:Mg ratio, increase the antagonism, and deepen the chlorosis in exactly the leaves that are feeding the fruit.
- The extra MOP would also add salt to a field already irrigated with water of EC 1.4 dS/m, raising the risk of marginal scorch and blossom-end rot.
Maximum justifiable dose: total K2O removal for a 60 t/ha crop is about 220-250 kg/ha. He has already put on ~187 kg K2O/ha as MOP plus ~50 kg from FYM - about 237 kg. So no further MOP is justified; the ceiling is essentially reached. If a later leaf test shows K below 3.5%, apply 10-15 kg K2O/ha as sulphate of potash, and no more.
Before any further purchase, check the actual cause of poor fruit set: night temperatures above 30 °C, poor pollination, thrips damage or tomato leaf curl virus all depress set and none of them responds to potassium.