Uneven maize stand, all fertilizer at planting, and only two bags of urea left
Mr Banda farms 2.0 ha of rainfed maize in Kasungu District at about 1,200 m elevation. He planted hybrid SC 403 on 28 November, three days after the first planting rains, at 75 cm between rows and 25 cm within rows, one seed per station. A three-week dry spell followed and germination was uneven, so that two or three stations in a row are now empty. All his fertilizer went in at planting: two 50 kg bags of NPK 23:21:0+4S and one 50 kg bag of urea over the whole field. At five weeks the crop is pale green, with yellowing starting on the oldest leaves and moving upwards; on the sandy upper slope some young plants have purple leaf margins and thin stems. The lower slope holds water for a day or two after rain. He still has two 50 kg bags of urea and about MK 60,000. Rains normally end in mid-April and SC 403 takes about 130 days.
1. State the most likely cause of the yellowing and of the purple stems, and list the field checks you would make before advising him.
Read the worked answer
Yellowing from the oldest leaves upwards points to a deficiency of a mobile nutrient, above all nitrogen. The fertilizer budget supports this. Two bags of NPK over 2 ha supply roughly 23 kg N/ha and 21 kg P2O5/ha, and the one bag of urea adds about 23 kg N/ha - around 46 kg N/ha in total, against the 90–110 kg N/ha that a 130-day hybrid needs for 5 t/ha or more.
The purple stems and thin stems fit very low phosphorus, but purple colouring is equally a symptom of cold soil, compaction or waterlogging, so it cannot be read as P deficiency on colour alone.
Checks before advising:
- Dig several plants: are roots shallow, flattened or rotting, especially on the lower slope?
- Split stems for stalk borer and grub around roots for cutworm and termite damage.
- Confirm what was actually applied and how - broadcast urea on dry soil is largely lost.
- Test pH with a field kit; strongly acid patches stunt plants in a way that mimics deficiency.
- Look for herbicide carryover or drift from a neighbour.
Diagnosis by leaf colour alone is not safe; the checks decide whether he is treating a nutrient problem or a root problem.
2. Using plant population, season length and his cash position, advise whether he should replant or gap-fill in January.
Read the worked answer
His spacing gives about 53,000 stations/ha. If roughly 40% are empty, the field is carrying about 32,000 plants/ha - a thin stand, but not a failed crop. Well-fed, well-weeded maize at 32,000 plants/ha can still produce 3–4 t/ha, so the existing crop is worth investing in.
Replanting the whole field in mid-January is poor economics here. SC 403 needs about 130 days, which puts maturity in late May, after the rains normally end in mid-April; grain filling would depend on residual moisture and the risk of a total loss is high. Gap-filling with the same variety merely adds plants six to seven weeks behind the main crop, which compete for water and nutrients and contribute little.
Recommendation:
- Count empty stations in ten random rows. If fewer than about 30% are empty and the gaps are scattered, do not replant - spend the MK 60,000 on weeding and top dressing.
- If there are large bare patches, fill only those, immediately, with a short-season variety of 90–100 days that can mature by end April. Do not buy more SC 403.
- Never sow the long-duration hybrid again at this date; late planting of a long-season variety is the main avoidable risk in this field.
3. He asks where the two remaining bags of urea should go and how they should be applied. Give a reasoned allocation.
Read the worked answer
Two bags is 100 kg of urea, about 46 kg N. Spread over 2 ha that is only 23 kg N/ha - too little to change much; concentrated on 1 ha it is 46 kg N/ha and can shift yield noticeably. Fertilizer should go where the water supply makes a response likely.
Allocation and method:
- Put the urea on the middle and lower-mid slope, where moisture is adequate, and not on the sandy upper slope, where leaching and drought risk are highest and the chance of a return is lowest.
- Avoid the waterlogged bottom corner: nitrogen applied there is lost by denitrification and the plants are already root-damaged.
- Band it about 10 cm from the plant and cover with soil. Urea left on a dry surface loses nitrogen as ammonia; broadcasting is only acceptable if rain is imminent.
- Apply when the soil is moist and rain is forecast within a day or two; do not apply to wet foliage.
- Split what he has: half now at knee height, half at tasselling if the rains continue, rather than a single dose that can be leached by one heavy storm.
- Weed before applying. Weeds take up the nitrogen faster than a stunted crop does.
He should also be told plainly that the small total quantity, not the timing, is the main limit on this crop.