Urea, Lime or Both? A Maize Field on an Acid Sandy Loam
Ramesh cultivates 1.2 ha of maize on a sandy loam in a sub-humid tract. A laboratory report gives pH 5.4, organic carbon 0.38%, CEC 7.5 cmol(p+)/kg, available N 180 kg/ha, available P2O5 22 kg/ha and available K2O 110 kg/ha. At sowing he broadcast 150 kg DAP and 100 kg urea on the dry surface and incorporated them shallowly with a cultivator. Within 24 hours, 45 mm of rain fell. At 25 days after sowing, the crop is uneven: older leaves are pale yellow and firing at the tips, some stems are purple, and growth on the upper slope is worse. Ramesh plans to broadcast another 100 kg urea immediately. A neighbour advises applying 2 t/ha lime before the next crop. Ramesh has only enough cash for either the urea top-dress or a small lime purchase this season, not both at the full rates.
1. What is the most likely set of causes for the uneven yellowing, and why is a single 'more urea' response risky? Explain the roles of the rain, the acid soil and the method of application.
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The pattern points to several overlapping constraints rather than a simple nitrogen shortage. The 45 mm rain soon after broadcasting can move urea-N below the shallow root zone on the sandy loam and can also cause runoff where the slope is steeper; the upper-slope patch being worst supports this. Urea left on the surface can also lose N as ammonia, especially if the soil is warm and moist. The purple stems and pale older leaves are consistent with P deficiency, but the DAP was broadcast and only shallowly incorporated, so much of the P may have been fixed by the acid soil (pH 5.4) and by iron and aluminium oxides before roots reached it. At pH 5.4, aluminium activity can restrict root growth, making the crop less able to use whatever N and P are present; that is why adding more urea alone may produce only a temporary colour change. Low organic carbon (0.38%) and low CEC (7.5 cmol/kg) mean the soil holds little N and is easily leached. The uneven stand could also include sulphur or zinc deficiency, common on sandy acid soils, so a blanket 'more urea' recommendation is risky: it may hide the real constraint, increase lodging and leaf disease, and waste scarce cash.
2. Recommend a season plan with the limited cash, including fertilizer timing, placement, lime decision and a simple check to confirm the diagnosis.
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With limited cash, I would not spend everything on one large urea top-dress. First, apply a modest urea dose-say 40–50 kg/ha-when the soil is moist but not saturated, and place it in a band or incorporate it lightly rather than broadcasting on a slope; keep the rest for a second dose at knee-high. Band the remaining P fertilizer rather than broadcasting it, because banding reduces contact with fixing soil. For lime, the correct rate must come from a lime-requirement test, not from the neighbour's figure; 2 t/ha may be reasonable for this pH and texture, but it is a capital investment that should be incorporated 3–4 weeks before the next crop, not mixed with urea at top-dressing. With limited cash, I would put lime on a small, clearly marked strip or a priority 0.2 ha and compare it with the untreated area. That gives a local response check instead of gambling the whole farm. Also take paired soil samples from good and poor patches and, if possible, a plant tissue test for Zn and S. The immediate priority is to protect the existing crop and generate evidence for the next season.