Nitrogen budget and leaching risk after compost application
A 1.0-ha field of sandy loam will be planted to lettuce with a recommended N requirement of 120 kg N/ha. Soil organic matter is 1.2%, and the expected soil N supply is 20 kg N/ha. The previous crop was faba bean, credited at 30 kg N/ha. The farmer applies 2.0 t/ha of compost (fresh weight) containing 1.8% N, with 25% of compost N expected to be available in the first year; the compost C/N ratio is 28. Irrigation water has EC 2.1 dS/m, and winter rainfall is 450 mm, often as intense storms. The farmer plans to apply all compost and all fertilizer N in a single pass before the winter rains, then plant lettuce in spring. The field is slightly sloping and drains to a ditch that feeds a village pond. The farmer asks how much fertilizer N is needed, whether the compost will release or tie up N, and whether the timing creates an environmental risk.
1. Calculate the first-year available N from compost and the fertilizer N requirement. State any assumptions.
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Compost total N = 2,000 kg/ha × 0.018 = 36 kg N/ha. First-year available compost N = 36 × 0.25 = 9 kg N/ha. Fertilizer N = crop requirement - compost available N - legume credit - soil N supply = 120 - 9 - 30 - 20 = 61 kg N/ha. Assumptions: the 2.0 t/ha is applied uniformly and incorporated; the 25% availability factor already accounts for the first-season release; the legume credit and soil N supply are reliable for this field; and no N is lost before uptake. The calculation is a budget, not a guarantee: if the compost is immature or poorly incorporated, less than 9 kg N/ha may be available.
2. Assess the immobilization risk from a C/N ratio of 28 and recommend how the farmer should manage the compost timing and any extra N.
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A C/N ratio of 28 is above the roughly 25:1 zone where net mineralization is usually expected, so there is a real risk of temporary immobilization, especially if the compost is immature and incorporated shortly before planting. Soil microbes may take up available nitrogen while decomposing the carbon-rich material, reducing the N available to young lettuce. The farmer should apply and incorporate the compost several weeks before planting, not just before sowing, and should monitor the crop. If the compost is immature, add about 20-30 kg N/ha as a starter or split application to cover immobilization, and reduce it later if the crop shows adequate growth. The 61 kg N/ha calculated from the budget should be split rather than applied all at once; starter N near planting and the rest at sidedress reduce the chance of loss and match crop demand.
3. Explain the environmental risk of applying all compost and fertilizer N before intense winter rains on this sloping sandy loam, and propose a safer timing strategy.
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Sandy loam has limited water-holding capacity, and 450 mm of winter rain, often in intense storms, can move nitrate below the root zone before spring planting. Nitrate is mobile and is not held strongly by soil particles. Applying all 61 kg N/ha plus compost before winter therefore risks nitrate leaching into the ditch and the village pond, where it can contribute to eutrophication. The slope increases runoff and the chance of transport off-site. A safer strategy is to keep the compost application in the pre-planting period but delay most fertilizer N until spring, then split it according to crop demand and rainfall. Maintain a cover crop or residue cover over winter to take up residual N and reduce erosion. Avoid applying fertilizer just before forecast heavy rain, keep a vegetated buffer strip along the ditch, and monitor soil nitrate and crop N status.