A yield plateau in a twelve-year rice-wheat rotation
For twelve years a farmer in the Indo-Gangetic Plain has grown rice-wheat continuously on 4 ha, applying 150 kg N, 60 kg P2O5 and 40 kg K2O per hectare to rice and 120:60:40 to wheat, with no organic manure and no residue incorporation; the straw is burned. Rice yield has stayed at 5.4 t/ha and wheat at 4.2 t/ha for four years despite the recommended fertilizer. Soil test: pH 5.9, organic carbon 0.45%, available P 22 kg/ha, available K 95 kg/ha, DTPA-Zn 0.32 ppm, hot-water-soluble B 0.28 ppm, available S 8 mg/kg. At 30 days after transplanting, rice shows stunted growth and brown spots on older leaves; wheat shows pale patches that disappear after tillering. The farmer blames the seed and wants to change varieties. Labour is scarce and he is unwilling to stop burning straw unless convinced of a yield benefit within one season. Irrigation is from a tube well and drainage is poor in kharif.
1. Diagnose the most likely constraints and rank them. Support your ranking with an approximate nutrient budget for potassium and zinc.
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The plateau is a nutrient-balance problem, not a variety problem.
Potassium: a rice crop of 5.4 t/ha grain with straw removed removes roughly 20-25 kg K2O per tonne of grain, i.e. about 110-130 kg K2O/ha. Wheat at 4.2 t/ha removes about 90-110 kg. Total removal is roughly 200-240 kg K2O/ha/year against 80 kg applied (40 + 40). The deficit is about 120-160 kg K2O/ha/year. Soil K at 95 kg/ha is in the medium range but is being mined; this is the clearest long-term constraint.
Zinc: continuous flooding, no Zn input and straw burning that volatilises and removes Zn have driven DTPA-Zn to 0.32 ppm, below the 0.6 ppm critical limit. The brown spots on older rice leaves at 30 days after transplanting fit Zn deficiency. Boron at 0.28 ppm is also low, and S at 8 mg/kg is marginal against a 10 mg/kg critical level.
Ranking: (1) K depletion; (2) Zn and B deficiency; (3) declining organic carbon and residue burning, which worsen N and micronutrient supply; (4) poor drainage, which affects Zn availability and N losses. P is adequate at 22 kg/ha because applications exceed removal.
2. Design a three-year integrated nutrient management plan that fits the farmer's labour constraint and his demand for a first-season response.
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Year 1, aimed at a visible response: apply 25 kg/ha zinc sulphate at puddling in rice (once in two years is enough) and 1 kg/ha borax; include 100 kg/ha single super phosphate in the wheat cycle to cover S and P; raise K to 60 kg K2O/ha in rice and 40 in wheat, still below removal but a clear step up. These give a same-season response, which is what will earn the farmer's confidence.
Straw: do not demand full incorporation immediately. Retain the wheat straw as surface mulch on the rice crop or incorporate it with 10 kg N per tonne of straw to avoid immobilisation, and burn nothing. Explain that the first-season yield effect may be small but the K recycled is worth more than the K fertilizer saved.
Year 2: add a green manure (dhaincha) in the kharif fallow or 5 t/ha FYM before rice, and split N into four applications. Train the farmer to use the leaf colour chart to avoid excess N.
Year 3: move to soil-test-based K, maintain Zn every second year, and correct drainage with shallow field channels in the kharif season.
Each component must have a purpose: K stops mining, Zn and B correct the visible symptoms, organic matter rebuilds carbon, and drainage reduces losses. Avoid a blanket 'apply FYM' recommendation that ignores labour; hire a straw chopper on a custom-hire basis.
3. The farmer wants proof within one season. What would you measure, and what first-season response is realistic?
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Set up four strip plots of 0.1 ha each: farmer practice, farmer practice + Zn/B, farmer practice + K, and all three. Replicate each twice in a staggered layout across the field to avoid a single soil gradient.
Measure grain yield from three 1 m2 quadrats per strip, dry to 14% moisture and weigh; record straw yield because K and Zn are retained in straw. Take soil samples before and after the season for K, Zn and organic carbon. Record the cost of each input and the price of paddy to compute a partial budget.
Realistic expectation: Zn and B can lift yield by 5-10% in the first season where deficiency is confirmed, and the brown spots should fade within two weeks of spraying or soil application. K response is slower on a soil still rated medium; expect little in year 1 and a benefit that accumulates. Straw incorporation rarely lifts yield in the first season.
Be honest with the farmer: the trial is designed to show whether the diagnosis is right, not to promise a large immediate gain. If Zn application produces no greening, revisit the diagnosis and check for nematodes, herbicide carryover or drainage-related chlorosis.