Maize Tasselling Before the Rains End
A smallholder in the sandveld has 1.2 ha of maize planted on 20 November. The field is a sandy loam with about 1.1% organic matter and a hardpan at 25 cm from years of shallow discing. Plants are now at tasselling and silking. The farmer applied 200 kg/ha of 2:3:2 (22) at planting and nothing since. Leaves below the ear are pale yellow from the tip inward along the midrib, older leaves show a V-shaped yellowing, and the crop is showing drought stress on a field that received only 18 mm in the past three weeks. The long-range forecast suggests the rainy season is ending early. The farmer has 50 kg of urea in the shed and can hire a knapsack sprayer, but no irrigation.
1. Diagnose the most likely nutrient problem and decide whether applying the 50 kg of urea now is a sound decision. Justify your reasoning with attention to timing and soil conditions.
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The symptom pattern - yellowing beginning at the tip and running down the midrib of older leaves, with younger leaves still green - is the classic mobile-nutrient pattern for nitrogen deficiency. Because N is mobile in the plant, the crop moves N from old leaves to the growing point, so older leaves show symptoms first. The 200 kg/ha of 2:3:2 (22) supplied only about 44 kg N/ha at planting, which on a sandy soil with low organic matter and likely leaching is insufficient for a maize crop targeting even 4 t/ha.
However, applying urea at tasselling is mostly wasted. Maize takes up the bulk of its N before tasselling; by silking, demand has peaked and uptake is falling. A surface application of urea also needs moisture to dissolve and move into the root zone, and the field has had only 18 mm in three weeks with no irrigation available. Urea on dry sandy soil will sit undissolved or volatilise as ammonia, especially if not incorporated. If the season is ending early, the crop's yield is already largely determined by the number of kernels set at silking.
Judgement: Do not spend the urea on this crop as a rescue. Better options are to keep the urea for the next crop, or if any rain is forecast within days, band and incorporate a small amount rather than broadcasting. The real lesson is that N should have been split: basal at planting plus a top-dressing at the 6–8 leaf stage.
2. Explain how the hardpan and the low organic matter interact to increase the crop's vulnerability in a short season.
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The 25 cm hardpan restricts rooting depth, so the maize root system is confined largely to the topsoil. That has two consequences. First, the plant cannot reach subsoil moisture, so it depends entirely on rainfall stored in the shallow layer - which is why the 18 mm deficit has hit so hard. Second, the shallow root zone holds less total water and fewer nutrients, so the crop runs out of both sooner.
Low organic matter (about 1.1%) compounds this. Organic matter is the main reservoir of exchangeable nutrients and the main contributor to aggregate structure and water-holding capacity in a sandy loam. With little organic matter, the soil holds less water per unit depth, crusts more easily, and leaches nitrate readily. Together, the hardpan and low organic matter mean the crop has a small, leaky bucket of resources in a season that is being cut short. The correct response is a break-crop/ripping strategy and long-term organic matter building (manure, residues, cover crops), not more fertiliser on a compromised root system.