Cotton Fertiliser Gap on a High-pH Basaltic Soil
Ramesh Patil farms 2.0 ha of irrigated Bt cotton in Yavatmal district. A soil test from the KVK laboratory reports: pH 8.3, EC 0.21 dS/m, organic carbon 0.38%, available N 210 kg/ha, available P2O5 18 kg/ha, available K2O 340 kg/ha, and DTPA-Zn 0.42 ppm. The recommended dose for his yield target is 120 kg N, 60 kg P2O5 and 60 kg K2O per hectare. At sowing he applied 100 kg DAP and 50 kg urea per hectare through a drill. The crop is now at square formation; newer leaves show interveinal chlorosis and are smaller than normal, while the lower leaves look healthy. His borewell water has EC 1.1 dS/m and the field has a history of heavy basaltic soil. He plans one more urea top-dress and asks whether he should buy MOP and zinc sulphate. He has 1,500 kg of urea and 200 kg of SSP in the shed and wants to avoid unnecessary purchases.
1. Calculate the fertiliser quantities still needed to meet the recommended NPK dose, showing the contribution of the already-applied DAP and urea. State whether MOP is justified.
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DAP is 18-46-0 and urea is 46% N, so the sowing dose supplied:
- 100 kg DAP × 0.18 = 18 kg N and 100 × 0.46 = 46 kg P2O5
- 50 kg urea × 0.46 = 23 kg N
- Total applied: 41 kg N, 46 kg P2O5, 0 kg K2O
Against 120-60-60, the gap is 79 kg N, 14 kg P2O5 and 60 kg K2O. For P, 14 ÷ 0.16 = about 87.5 kg SSP, which also supplies about 9.6 kg S and some Ca. For N, 79 ÷ 0.46 = about 172 kg urea. MOP is not automatically justified: available K2O is 340 kg/ha, rated high, and cotton removal for this yield level is roughly 60–80 kg K2O/ha. A maintenance dose of 50 kg MOP (30 kg K2O) is defensible because no K was applied, but a full 100 kg MOP is unnecessary. The priority is to correct P, split the N, and treat the zinc deficiency.
2. Explain the chlorosis and advise a correction, including why the lower leaves are unaffected.
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Zinc is poorly mobile in the phloem, so a deficiency appears first on new leaves; the older leaves retain zinc and stay green. The soil pH of 8.3 and the heavy basaltic clay reduce Zn availability, and the high P applied through DAP can aggravate Zn deficiency through P–Zn antagonism. DTPA-Zn of 0.42 ppm is below the usual critical range of about 0.6 ppm, so this is a real deficiency, not just a colour variation. Correct it with foliar ZnSO4·7H2O at 0.5% plus 0.25% lime to reduce spray burn, two or three sprays at 10–15 day intervals. For a longer-term correction, apply 25 kg/ha ZnSO4·7H2O with irrigation and add 5 t/ha FYM to improve availability. Do not rely on more DAP to cure the chlorosis.
3. The farmer wants to apply 100 kg urea in one dose at flowering. Evaluate this plan.
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100 kg urea supplies 46 kg N, which is only about 58% of the 79 kg N still required. Applying it all at flowering also concentrates the risk of ammonia volatilisation and leaching, and it does not match the crop's continuing demand through boll development. A better plan is about 75 kg urea now at square formation and the remaining 97 kg urea at early boll development, each incorporated or washed in with irrigation. Keep the SSP application separate and apply it before the next irrigation. If heavy rain is forecast, delay the urea and split it again. The one-dose plan is therefore not wrong because urea is a bad source, but because the quantity and timing are inadequate.