Late-sown wheat on a sodic-water farm: seed rate, gypsum dose and nitrogen timing
A 1.0 ha field in western Uttar Pradesh is to be sown to wheat on 20 December, immediately after a potato harvest, about four weeks later than the normal window. The soil is sandy loam with pH 8.9, EC 0.42 dS/m, organic carbon 0.31%, available N 118 kg/ha, Olsen P 14 kg/ha, ammonium-acetate K 210 kg/ha and DTPA-Zn 0.38 ppm. The only irrigation source is a tubewell with RSC 4.0 meq/L and EC 1.1 dS/m, and the plan is six irrigations of 7 cm each. Seed of the selected variety has 82% germination and a 1000-grain weight of 40 g; the farmer wants 250 plants/m² and expects 90% of the germinable seeds to establish. A neighbouring field irrigated from the same tubewell shows interveinal chlorosis on the younger leaves of 12-day-old seedlings.
1. Calculate the seed rate needed to reach 250 plants/m², and decide whether it should be raised further for a 20 December sowing.
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Base population: 250 plants/m². One seed weighs 40/1000 = 0.04 g, so 250 × 0.04 g = 10 g/m², i.e. 100 kg/ha of seed that actually establishes.
Correct for seed-lot quality: germinable fraction 0.82 × establishment 0.90 = 0.738. Seed rate = 100/0.738 = 135.5, about 136 kg/ha.
Late-sowing judgement: a 20 December crop loses 30–40 tillering days, so tillers per plant fall from roughly 5–6 to 3–4. That gap must be closed by plants rather than tillers, so the standard 25% uplift for very late sowing is justified. But the uplift must be capped: above about 160–170 kg/ha the canopy closes early, ears shrink, and lodging plus terminal-heat stress increase. The defensible choice is 150–160 kg/ha (about 15% over the calculated base), sown in 18–20 cm rows. Following the 136 kg/ha figure without adjustment would under-plant a late crop.
2. Compute the gypsum needed for a season of this irrigation water and state how much you would actually apply. Separately, diagnose the chlorosis in the neighbouring field.
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Water load: 1 ha-cm = 100,000 L. At 4.0 meq/L each ha-cm carries 4.0 × 100,000 = 400,000 meq = 400 equivalents of Na. Gypsum equivalent weight = 172/2 = 86 g, so per ha-cm the requirement is 400 × 86 g = 34.4 kg (the familiar constant is 8.6 kg per meq/L per ha-cm: 4.0 × 8.6 = 34.4). Season total = 42 ha-cm × 34.4 ≈ 1,445 kg, about 1.45 t/ha.
Judgement: 1.45 t is the water-neutralisation figure, not automatically the soil dose. With pH 8.9 the field is sodic, but EC 0.42 dS/m is still safe and no ESP value is given. Apply 0.8–1.0 t/ha, spread and incorporated before sowing so the first irrigation carries it in, then re-test ESP; blending with canal water, if any is available, cuts the dose proportionally.
Chlorosis: interveinal yellowing of young leaves with DTPA-Zn 0.38 ppm (critical ≈0.6) is zinc deficiency, aggravated by high pH and bicarbonate water. Nitrogen deficiency would show on older leaves. Apply 25 kg/ha ZnSO4·7H2O to soil plus two 0.5% foliar sprays.
3. Design the nitrogen programme (total dose, urea quantity and splits) for this crop.
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A late-sown irrigated wheat crop can realistically yield 4.0–4.5 t/ha, which corresponds to about 150 kg N/ha under normal sowing. Two adjustments apply here: (i) the preceding potato crop was heavily fertilised, so a residual-N credit of 20–25 kg N/ha is realistic; (ii) the short vegetative phase and terminal-heat risk argue for trimming, not raising, total N. Take 120–130 kg N/ha.
Urea: 125/0.46 ≈ 272 kg urea/ha. Pair it with a soil-test-based P dose (Olsen P 14 kg/ha is medium, so about 60 kg P2O5/ha) and only a starter K dose, since 210 kg/ha K is high.
Splits: 50% basal, 25% at crown-root initiation (25–30 DAS) and 25% at 45–50 DAS; no nitrogen after 60 DAS, because late N delays maturity into the heat spell and invites lodging. If the crop looks thin at CRI, advance the third split rather than enlarging the total - the limiting factor is season length, not nitrogen supply.