Fertiliser plan from a constrained soil test on alkaline wheat land
Ramesh farms 1.5 ha in a semi-arid tract where wheat follows rice. His soil test report shows pH 8.4, EC 0.18 dS/m, organic carbon 0.32%, available N 112 kg/ha (low), P 14 kg/ha (medium), K 310 kg/ha (high), and DTPA-Zn 0.36 mg/kg (deficient). He has 4 t of well-rotted FYM, 100 kg DAP, 150 kg urea, and 25 kg zinc sulphate in store. He plans to broadcast all urea with the first irrigation at crown-root initiation and to skip zinc because 'the soil is already alkaline and zinc will be fixed.' The wheat variety is a 125-day irrigated type, and the field has had no legume for three years. The district averages 420 mm rainfall, mostly before sowing. Ramesh wants maximum yield but cannot buy more fertilizer this season, and he has labour for only two fertilizer operations.
1. Using only the materials on hand, prepare a season-long fertilizer and amendment plan for the 1.5 ha. State what you would omit and why, and show the approximate N, P and Zn supplied.
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Omit potassium fertilizer. Soil K is 310 kg/ha, well above the critical level for wheat; a 4 t/ha crop removes only about 30-40 kg K2O/ha, so no response is expected. He has no K fertilizer anyway, so this constraint costs nothing.
Basal operation (before sowing): Spread 4 t FYM (about 2.7 t/ha) and incorporate it. Apply all 100 kg DAP in bands 2-3 cm below and beside the seed, not broadcast: this gives about 31 kg P2O5/ha and 12 kg N/ha. Place the 25 kg zinc sulphate with the FYM on the most alkaline 1 ha; the remaining 0.5 ha should get foliar zinc (see next task).
Nitrogen: 150 kg urea contains 69 kg N. Split it: about 40 kg urea/ha at crown-root initiation and 60 kg urea/ha at first node/booting. Do not broadcast all urea at the first irrigation; splitting reduces leaching and volatilization and matches crop demand. FYM adds roughly 13 kg N/ha.
Total supplied: approximately 71 kg N/ha, 31 kg P2O5/ha and 13 kg K2O/ha, plus zinc on part of the field. This is below the 120 kg N/ha and 60 kg P2O5/ha normally recommended, so yields will be input-limited. Priority should go to the CRI and booting top-dresses, and no further area should be sown without fertilizer.
2. Evaluate Ramesh's claim that zinc will be fixed in alkaline soil. Explain how to apply the 25 kg zinc sulphate for the best response.
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His claim is only partly correct. At pH 8.4, zinc is adsorbed on calcium carbonate and clay surfaces, so soil availability falls; that is exactly why deficiency appears. But zinc sulphate applied correctly still works, and foliar sprays bypass soil fixation entirely.
With only 25 kg zinc sulphate for 1.5 ha (16.7 kg/ha), soil application at the full 25 kg/ha rate is possible on only 1 ha. Options:
- Soil application on the worst 1 ha: mix 25 kg zinc sulphate with 2-3 t FYM or compost and band it before sowing. Organic matter reduces fixation, and the acidifying effect of DAP in the root zone helps. Do not simply broadcast on the surface.
- Foliar spray on the whole field: dissolve 500 g zinc sulphate in 100 L water (0.5%) with a few drops of sticker and spray at tillering and again at booting. Two sprays need about 5 kg/ha, or 7.5 kg for 1.5 ha, so the 25 kg stock is more than enough. Foliar zinc is efficient when the soil stock is small and fixation is high.
- Combination: soil-apply 25 kg on the most deficient 1 ha and foliar-spray the remaining 0.5 ha. This uses the stock where it gives the greatest response.
Avoid mixing zinc sulphate with phosphatic fertilizer in the same band, because precipitation reduces both. Apply zinc in a separate operation or with FYM.