Yellowing Maize at the Stream Edge
At a 2-hectare maize field in the derived savanna, six weeks after planting, lower leaves are pale yellow and plants are stunted. The yellowing started at the field edge next to a stream after three days of heavy rain and has moved inward. The farmer applied two 50 kg bags of NPK 15-15-15 per hectare at planting and nothing since. A soil test reports pH 5.1, organic matter 0.8%, available phosphorus 4 mg/kg, exchangeable potassium 0.12 cmol/kg, and 1,200 nematode juveniles per 200 g of soil. The field has no drainage furrows. Neighbouring farms planted with the same seed and fertilizer look healthy, but their fields are on higher ground. The farmer wants to buy four more bags of urea and apply them immediately.
1. Which factors most likely explain the yellowing and stunting? Rank them and justify each using the data.
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Ranking and reasoning
Waterlogging and root anoxia (most likely trigger). Yellowing began at the stream edge after heavy rain and moved inward; the field has no drainage furrows. Saturated soil blocks root respiration, kills fine roots, and lets denitrifying bacteria convert nitrate to nitrogen gas. Lower leaves yellow first because nitrogen is mobile in the plant.
Nematode damage (strong contributor). 1,200 juveniles per 200 g is high. Root-knot nematodes produce galls that impair water and nutrient uptake, so the plant shows deficiency even when fertilizer is present. This helps explain why the same fertilizer worked on neighbouring higher fields only partly.
Nitrogen deficiency (real but partly secondary). Two bags of NPK at planting supplies some N, but 15% of 100 kg is 15 kg N/ha, which is modest for maize and was applied once. Leaching and denitrification after rain would reduce it further. Adding urea before correcting drainage would be wasted.
Acidity and low phosphorus. pH 5.1 reduces P availability; available P at 4 mg/kg is low. Exchangeable K at 0.12 cmol/kg is also low. These are longer-term constraints, not the immediate cause of the edge pattern.
The pattern, timing, and soil data together point to a waterlogging-nematode complex, with nitrogen deficiency as a consequence. A single-cause answer is not supported.
2. Design a simple strip trial the farmer could run in the next season to separate waterlogging from nutrient deficiency, using only two or three treatments and no laboratory.
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Use the field's natural wet-to-dry gradient as blocks. Mark three strips of 10 m × 5 m from the wet edge to the dry interior, and repeat each treatment three times in a randomised block design.
Treatments:
- A: control - current practice, no extra fertilizer.
- B: drainage only - open shallow furrows 30 cm deep at 5 m spacing, leading away from the field.
- C: drainage + split NPK - same furrows plus 100 kg/ha NPK 15-15-15 at two weeks and again at six weeks.
Do not add lime in this trial; lime changes pH slowly and would confuse the result. Measure plant height, leaf colour score (1–5), and grain yield per plot. Also pull five plants per plot at four weeks and score root galls on a 0–5 scale. If B improves growth over A, waterlogging is confirmed. If C improves over B, nutrient supply is limiting after drainage. If neither improves, nematodes or acidity are more important. Keep the trial simple enough for the farmer to manage.
3. Recommend an immediate action and a next-season plan; state one action the farmer should avoid.
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Immediate: Do not apply urea now. Open drains immediately along the wet edge and between rows; re-route runoff away from the field. Urea on waterlogged soil is lost and may worsen acidity. Take a foliar sample if extension advice is available, but do not spray at midday.
Next season:
- Land-form drainage first: broad beds or furrows, and a grassed waterway.
- Apply lime based on a buffer-pH test, not a guess; incorporate it four to six weeks before planting.
- Use a nematode-tolerant maize variety and rotate with a non-host legume such as cowpea or groundnut.
- Apply well-decomposed poultry manure or compost to raise organic matter from 0.8%.
- Split NPK: one-third at planting, two-thirds at four to six weeks, and add K where the soil test is low.
Avoid: doubling the nitrogen dose to compensate. It increases cost, acidifies the soil further, and does not fix root damage. Also avoid planting the same susceptible maize on the wet edge next season.