黄化麦田：氮素不足还是渍害
Ramesh farms 0.8 ha of wheat (cv. GW-366) sown on 18 November in a sandy loam at pH 7.9 with organic carbon 0.31%. Soil test values are 165 kg/ha available N, 9 kg/ha available P2O5 and 240 kg/ha available K2O; the recommended dose is 120 kg N and 60 kg P2O5 per hectare. At sowing he broadcast 100 kg DAP and 40 kg urea per hectare and applied no other nitrogen since. Fifty-five days after sowing, the lower leaves of most plants are pale yellow starting from the leaf tip, with no lesions or fungal growth. Four days ago 65 mm of rain fell and about one-fifth of the field still has standing water; plants there are stunted and their roots are brown with a rotten smell, while roots in the drained area are white but sparse. Day temperatures are 16–20 °C. Ramesh now plans to broadcast another 100 kg urea per hectare over the whole field, and he has 140 kg urea and 30 kg DAP in stock.
1. Diagnose the yellowing. Is it nitrogen deficiency, waterlogging injury, or both? Give the evidence for each possibility and one field check that would separate them.
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Both are present, but in different parts of the field.
Nitrogen deficiency (most of the field): lower leaves yellow first and the colour loss begins at the tip and moves down the midrib - nitrogen is mobile and is remobilised to the growing point. There are no lesions, no fungal growth and no distinct patch pattern. The soil supports this: organic carbon 0.31% in a sandy loam means little mineralisable N, nitrate leaches easily, and only about 36 kg N/ha has been applied against a 120 kg/ha target.
Waterlogging injury (the one-fifth with standing water): brown, rotten-smelling roots mean oxygen starvation, not nitrogen shortage. Under anaerobiosis roots cannot absorb nutrients and nitrogen is lost as N2/N2O by denitrification, so these plants look worse than the rest.
Field check: pull 10 plants each from the drained area and the waterlogged patch and compare root colour, root hair density and shoot weight. Then apply urea to a 1 m × 1 m drained strip only; greening within 5–7 days confirms N as the limiting factor there.
2. Calculate the N and P2O5 already applied, and the fertilizer still needed to meet 120 kg N and 60 kg P2O5 per hectare over the whole 0.8 ha. Is his stock sufficient?
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Already applied (per ha):
- DAP 100 kg × 18% N = 18.0 kg N; × 46% P2O5 = 46.0 kg P2O5
- Urea 40 kg × 46% N = 18.4 kg N
- Total N = 36.4 kg/ha; total P2O5 = 46.0 kg/ha
Balance per ha: N = 120 − 36.4 = 83.6 kg; P2O5 = 60 − 46 = 14 kg.
Correcting for the extra N carried by DAP: 14 kg P2O5 needs 14 ÷ 0.46 = 30.4 kg DAP/ha, which supplies 30.4 × 0.18 = 5.5 kg N. Urea N still required = 83.6 − 5.5 = 78.1 kg, i.e. 78.1 ÷ 0.46 = 170 kg urea/ha.
For 0.8 ha: urea 170 × 0.8 = 136 kg; DAP 30.4 × 0.8 = 24.3 kg.
His stock of 140 kg urea and 30 kg DAP is just sufficient - but only if the whole 136 kg is not dumped at once (see next task).
3. Give a corrected application plan for the next 10 days, covering drainage, timing, splitting and placement, and explain why broadcasting 100 kg urea immediately over the whole field would waste money.
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- Drain first. Open shallow drains along the lowest corner and the field boundary so the standing water leaves within 24–48 hours. Urea applied onto standing water is lost by denitrification and leaching and the crop cannot take it up anyway.
- Do not apply the full 83.6 kg N in one dose at 55 days. Split it: about 45–50 kg N/ha now (urea ≈ 100–110 kg/ha) once the surface is workable, and the remaining 30–35 kg N/ha (≈ 70 kg urea/ha) at 75–80 days, before flag-leaf emergence. A single heavy dose on a sandy loam is leached by the next rain and produces rank growth that lodges.
- Place, don't broadcast, where possible. Band urea 5 cm deep and 5 cm to the side of the row or apply with the first post-drainage irrigation; broadcasting on a wet surface leaves much of the N on the soil surface where it volatilises.
- Apply the 24 kg DAP now in a band, since phosphorus is immobile and should go near the root zone; do not top-dress P.
- Leave the waterlogged strip out of the first dose until the roots recover; a plant with dead roots cannot use the N and it is simply lost.