Maize Population, Fertiliser Bags and a Wet Forecast
Mr Banda plants 2 ha of maize on a sandy loam with 1.0% organic matter and 700 mm annual rainfall. He uses rows 0.9 m apart and 0.5 m between stations with one plant per station, and harvested 2.8 t/ha last season against a potential of 6 t/ha. The extension recommendation is 45,000 plants/ha, 100 kg N, 30 kg P2O5 and 25 kg K2O per hectare. He has Compound D (7% N, 14% P2O5, 7% K2O) and urea (46% N). Because labour is short, he plans to place all the fertiliser in the planting furrow on the day of planting. The long-range forecast shows 60 mm of rain within 48 hours of his planned top-dressing date, and his soil test shows potassium at 0.30 cmol/kg, a moderate level.
1. Calculate the plant population his current spacing gives and work out how he can reach the recommended 45,000 plants/ha, given that he is short of labour.
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Current population. Area per station = 0.9 m × 0.5 m = 0.45 m². Stations per hectare = 10,000 ÷ 0.45 = 22,222. With one plant per station, the population is 22,222 plants/ha, almost exactly half the target. A halved population alone can explain a large part of the gap between 2.8 t/ha and a 6 t/ha potential, because each cob has fewer neighbours to share light and water with, but the field cannot compensate beyond a point.
Option A - closer stations. Required area per plant = 10,000 ÷ 45,000 = 0.222 m². In-row spacing = 0.222 ÷ 0.9 = 0.247 m ≈ 25 cm, one plant per station.
Option B - two plants per station, spacing unchanged. 22,222 stations × 2 plants = 44,444 plants/ha, within 1% of the target, with no change to row or station spacing.
Judgement. Because labour is the binding constraint, Option B is the better first move: dropping two seeds per station and thinning to two costs little extra time and keeps his ridges. Option A needs more precise planting and more stations to be drilled per row length. Either way, thin to the final stand before six leaves so the plants do not compete.
2. Calculate the Compound D and urea he needs for the 2 ha and state when each should be applied. Comment on whether he needs to buy potassium fertiliser as well.
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Basal - Compound D at planting. 200 kg/ha × 2 ha = 400 kg (eight 50 kg bags). That supplies per hectare: N = 0.07 × 200 = 14 kg; P2O5 = 0.14 × 200 = 28 kg; K2O = 0.07 × 200 = 14 kg. Phosphorus at 28 kg/ha is close enough to the 30 kg/ha recommendation, and phosphorus must be placed at planting because it hardly moves in the soil.
Nitrogen - urea in splits. N still required = 100 − 14 = 86 kg/ha. Urea needed = 86 ÷ 0.46 = 187 kg/ha, which is 374 kg for 2 ha (about seven and a half 50 kg bags). Split it: half at the six-leaf stage and half at tasselling, banded beside the row and covered.
Potassium. The crop receives 14 kg K2O against a 25 kg/ha recommendation, a shortfall of 11 kg/ha. Muriate of potash (60% K2O) would supply this at about 18 kg/ha, or 36 kg for the field. Since the soil test puts potassium at a moderate 0.30 cmol/kg and maize on a sandy soil responds weakly to potassium when nitrogen is the first limit, this is a low priority compared with getting the nitrogen splits right. Buy it only if cash remains after the urea.
3. Explain why his plan to place all the fertiliser at planting is risky on this soil, and advise him on what to do about the 60 mm of rain forecast after top-dressing.
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Why all-at-planting fails here.
- Leaching. A sandy loam with 1.0% organic matter has a low cation exchange capacity and little water-holding capacity. Urea converts to ammonium and then to nitrate, which moves freely; a single large dose at planting is below the root zone before the crop's demand peaks between six leaves and tasselling.
- Denitrification and volatilisation. In saturated soil nitrate is lost as gas, and urea left on the surface after rain loses ammonia.
- Salt injury. A concentrated band of urea next to germinating seed burns the radicle and reduces the stand, which is the last thing a crop already at half population needs.
- Timing of demand. The crop takes up only a small fraction of its nitrogen in the first three weeks; the bulk is needed later, so a single early dose cannot match demand.
The 60 mm forecast. Do not top-dress 48 hours before a downpour of that size on this soil: much of the nitrogen will move past the roots. Wait until the rain has passed and the soil has drained, then band the urea beside the row and cover it lightly with soil. If the rain arrives early anyway, do not double the next dose blindly; check crop colour and consider a smaller third application. Splitting into three smaller doses rather than two is the safest response on a leachable sand.