Liming Acid Soil Without Losing Fertilizer Nitrogen
A farmer has a 0.4 hectare plot with soil pH 5.2. Tomato seedlings grow best at pH 6.0–6.8. The soil test also shows low nitrogen. The farmer has 200 kg of hydrated lime, Ca(OH)2, and 100 kg of ammonium sulphate fertilizer. He plans to broadcast both on the same day and transplant two days later. An agriculture officer warns that this plan can waste nitrogen and injure young roots. The farmer can irrigate, has pH paper, and can wait up to three weeks before transplanting. Previous season seedlings were stunted and yellow. He wants a practical sequence that raises pH without over-liming and still supplies nitrogen. He also asks why mixing the two materials causes a smell of ammonia.
1. Explain why mixing hydrated lime and ammonium sulphate wastes nitrogen. Include a word equation or chemical equation.
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Hydrated lime is alkaline, while ammonium sulphate contains the ammonium ion. In alkaline conditions ammonium is converted to ammonia gas, which escapes. The reaction can be written: ammonium sulphate + calcium hydroxide → calcium sulphate + ammonia + water, or (NH4)2SO4 + Ca(OH)2 → CaSO4 + 2NH3 + 2H2O. The smell is ammonia. The nitrogen that should have fed the crop is lost to the air. Fresh lime can also be caustic and injure young roots if seedlings are transplanted too soon. This is why the two materials should not be broadcast together on the same day.
2. Recommend a practical sequence for liming and fertilizing, including waiting and testing. What should the farmer avoid?
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First apply the lime uniformly and incorporate it into the soil, using the soil-test recommendation rather than blindly spreading all 200 kg. Irrigate to help it react, then wait about two to three weeks. Retest the pH with the pH paper; the target is 6.0–6.8, not higher. Only after the pH has moved into range should ammonium sulphate be applied, preferably in split doses and incorporated or watered in. Avoid mixing lime and ammonium sulphate, avoid transplanting into freshly limed soil, and avoid over-liming above 6.8. If the pH is already near 6.8, stop adding lime.
3. Suppose the pH after liming reads 7.8. What should the farmer do next, and why is adding more ammonium sulphate not a simple fix?
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At pH 7.8 the soil is already too alkaline for the target, so no more lime should be added. Broadcasting ammonium sulphate on a strongly alkaline soil is risky because more of its ammonium nitrogen can be lost as ammonia gas; the fertilizer would be wasted and could injure seedlings. The farmer should retest to confirm the reading, add organic matter, avoid further alkaline amendments, and seek a local recommendation for acidifying material if the pH must be lowered; gypsum is not a reliable pH-lowering agent. The key judgement is to stop adding lime and not to treat fertilizer as a quick correction for over-liming.