Fertiliser Decision for a Maize Field with Yellowing Leaves
A farmer grows maize on 1 hectare. At the six-leaf stage, the older leaves are turning yellow from the tip inward, while the newer leaves remain green. A soil test reports 8 mg/kg of nitrate-nitrogen in the top 30 cm, which is below the 20 mg/kg target for maize at this stage. The farmer has three fertiliser options: urea (46% N, 50 dollars per 50 kg bag), ammonium sulphate (21% N, 45 dollars per 50 kg bag), and a compound NPK 15-15-15 (60 dollars per 50 kg bag). The field has a pH of 5.2, which is acidic. The farmer needs to apply 60 kg N/ha. Rainfall is expected within two days.
1. Which fertiliser should the farmer choose to supply 60 kg N/ha most economically? Calculate the amount and cost of each option and recommend one.
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Urea: 60 kg N ÷ 0.46 = 130.4 kg urea. Each 50 kg bag costs 50 dollars, so cost per kg = 1 dollar. Cost = 130.4 × 1 = 130.40 dollars. Ammonium sulphate: 60 ÷ 0.21 = 285.7 kg. Cost per kg = 45 ÷ 50 = 0.90 dollars. Cost = 285.7 × 0.90 = 257.13 dollars. NPK 15-15-15: 60 ÷ 0.15 = 400 kg. Cost per kg = 60 ÷ 50 = 1.20 dollars. Cost = 400 × 1.20 = 480 dollars. Urea is the cheapest source of nitrogen at 130.40 dollars, followed by ammonium sulphate at 257.13 dollars. NPK is the most expensive and also supplies unnecessary phosphorus and potassium unless the soil test shows deficiencies. The recommendation is urea, provided it is applied correctly, because it gives the required nitrogen at the lowest cost.
2. Considering the acidic soil pH of 5.2 and the expected rain, judge whether urea is still the best choice or whether ammonium sulphate is preferable. Explain your reasoning.
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Urea is still the better choice overall, but the farmer must manage two risks. First, urea converts to ammonium and then to nitrate, releasing hydrogen ions and acidifying the soil slightly. Ammonium sulphate is more acidifying because it adds sulphate and has a stronger acidifying effect per kg of nitrogen. On a soil already at pH 5.2, repeated use of ammonium sulphate would worsen acidity, which can reduce nutrient availability. Second, urea is prone to ammonia volatilisation if left on the surface, especially in warm, moist conditions. With rain expected within two days, the farmer should apply urea just before the rain or incorporate it into the soil so it dissolves and moves into the root zone rather than being lost as gas. The recommendation is urea applied and incorporated before rain, plus a plan to add lime to raise pH over time.
3. The farmer applies urea but two weeks later the yellowing has not improved. Suggest two possible reasons and one diagnostic action for each.
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Reason 1: Nitrogen loss by volatilisation. If urea was broadcast on the surface and rain did not arrive as expected, much of the nitrogen may have been lost as ammonia gas. Diagnostic action: check application records and rainfall data, and inspect for a white residue or a strong ammonia smell shortly after application. Reason 2: The yellowing is not caused by nitrogen deficiency. At pH 5.2, phosphorus may be locked up and aluminium toxicity can damage roots, or waterlogging may have caused denitrification. Diagnostic action: dig up a few plants to inspect roots for damage and take a fresh soil test for pH, phosphorus, and aluminium. A third possibility is sulphur deficiency, which also causes yellowing of older leaves, but the soil test pointed to nitrogen. The judgement is to diagnose before adding more fertiliser, because adding nitrogen blindly may waste money and worsen acidity.