Yellow Lower Leaves in a Waterlogged Maize Field
On a smallholding near Kasungu, a farmer planted 2 ha of hybrid maize. At three weeks, the lowest leaves on many plants are pale yellow, starting at the leaf tip and running along the midrib; the upper leaves are dark green. The field received 100 kg/ha of urea at planting, followed by 80 mm of rain in 24 hours. The soil is compacted and water stands in wheel ruts for two days. A neighbour planted the same variety on a slope and applied the same amount of urea; that field is uniformly green. A third plot on the farm received 8 t/ha of cattle manure and is also greener. The farmer has a bag of urea in the store and proposes to broadcast another 100 kg/ha immediately. An extension officer warns that this may waste money and could worsen the problem.
1. State the most likely cause of the yellowing and justify your answer with two observations from the field.
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The most likely cause is nitrogen deficiency, made worse by waterlogging.
Evidence: (1) the yellowing begins on the oldest, lower leaves while the upper leaves remain dark green, which is typical of a mobile nutrient such as nitrogen; (2) the field received heavy rain (80 mm in 24 hours) after urea application and water stood in the ruts, so nitrate could be leached or lost by denitrification in waterlogged soil; (3) the manured plot is greener, consistent with extra nitrogen; (4) the neighbour's sloping field with the same urea is green, suggesting that the problem is site drainage rather than the seed or variety.
Sulphur deficiency would normally affect the youngest leaves first, so it is less likely.
2. Explain why nitrogen deficiency appears first in the older leaves.
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Nitrogen is part of chlorophyll, proteins and nucleic acids. It is mobile in the plant. When supply from the roots is low, the plant breaks down proteins and chlorophyll in older leaves and transports the amino acids through the phloem to the growing shoot and younger leaves.
The older leaves therefore lose chlorophyll and turn pale yellow first, while the young leaves remain green. This is a re-translocation strategy that allows the plant to keep its growing points supplied, but it reduces photosynthesis in the lower canopy.
3. Evaluate the farmer's plan to broadcast another 100 kg/ha of urea immediately. Recommend two better actions.
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Applying more urea immediately is risky. If the soil is still waterlogged, the urea will be converted to nitrate but oxygen-deficient soil promotes denitrification, converting nitrate to nitrogen gas. More rain could also leach nitrate below the root zone. A large surface application can also volatilise ammonia.
The farmer should first check drainage and wait until the soil is moist but not saturated. Better actions: (1) open shallow drains or use contour ridges to remove standing water and improve aeration, so roots can absorb nutrients; (2) apply nitrogen in split doses after drainage, incorporating it or placing it near the roots rather than broadcasting on the surface; (3) add cattle manure or compost to improve soil structure and water-holding capacity, as the manure plot shows. A leaf tissue test or small test strip could confirm nitrogen deficiency before more fertilizer is bought.