Mixing Ammonium Sulphate with Slaked Lime
Ramesh, a paddy farmer in Nalanda district, has 50 kg of ammonium sulphate and 20 kg of slaked lime in his store. To save labour he plans to mix the two thoroughly and broadcast the mixture over his one-hectare field just before transplanting. A soil-test card from the block agriculture office shows that his field pH is 5.4, and the extension officer has advised liming. Ramesh reasons that since lime corrects acidity and ammonium sulphate supplies nitrogen, applying them together will do both jobs in one pass. He also asks whether 50 kg of urea would be a better use of money than 50 kg of ammonium sulphate. The field is irrigated, and the weather at sowing time is warm and bright, around 30 °C with moist soil.
1. Write the reaction that occurs when the two bags are mixed, identify the gas released, and calculate how much nitrogen is lost, showing which reactant is in excess.
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Reaction: (NH₄)₂SO₄ + Ca(OH)₂ → CaSO₄ + 2NH₃↑ + 2H₂O. The gas is ammonia, and it carries the nitrogen away into the air.
Molar masses: (NH₄)₂SO₄ = 132 g/mol, Ca(OH)₂ = 74 g/mol, NH₃ = 17 g/mol.
Moles of ammonium sulphate = 50 000/132 = 378.8 mol. Moles of lime = 20 000/74 = 270.3 mol.
The reaction is 1:1, so lime is the limiting reagent; only 270.3 mol of the fertilizer is decomposed and about 108.5 mol of ammonium sulphate is left unreacted.
NH₃ formed = 2 × 270.3 = 540.5 mol = 540.5 × 17 ≈ 9.19 kg of ammonia gas.
Nitrogen lost = 540.5 × 14 ≈ 7.57 kg N.
The bag holds 50 × 28/132 = 10.6 kg N, so mixing before application wastes about 7.57/10.6 ≈ 71% of the nitrogen before a single plant can take it up.
2. Compare the nitrogen value of 50 kg ammonium sulphate with 50 kg urea. Does that comparison alone decide the farmer's choice?
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Percentage nitrogen: ammonium sulphate = 28/132 = 21.2%; urea CO(NH₂)₂ = 28/60 = 46.7%.
50 kg ammonium sulphate supplies 50 × 0.212 = 10.6 kg N. 50 kg urea supplies 50 × 0.467 = 23.3 kg N - more than twice as much nitrogen per bag, and urea is usually cheaper per kilogram of nitrogen as well.
So on the nitrogen count alone urea wins clearly. But the judgement is not one-dimensional: urea is just as vulnerable to ammonia loss when broadcast on a warm, moist, limed surface, because it hydrolyses to ammonium carbonate and then loses NH₃. The real answer is that the choice of fertilizer is secondary to the method - the nitrogen must be placed in the soil and covered, or applied in splits, and never brought into contact with lime.
3. What should Ramesh do about the pH 5.4 reading and the lime he already owns, given that he also wants to apply nitrogen?
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pH 5.4 is distinctly acidic for paddy, which does best near 5.5–6.5 but takes up nutrients most efficiently closer to neutral, so liming is justified - however the lime and the ammoniacal nitrogen must be separated in time and space.
Advice:
- Do not mix. Apply lime at least 2–3 weeks before or after the nitrogen fertilizer; contact between lime and ammonium fertilizer converts NH₄⁺ into NH₃ gas.
- Incorporate the lime into the soil rather than broadcasting it on the surface; slaked lime acts quickly, so a smaller dose is needed than with ground limestone, and overdosing can push the soil alkaline.
- Place the nitrogen in the root zone and cover it, or split the dose; avoid applying it in the heat of the day or onto freshly limed, wet soil.
- Remember that ammonium sulphate is itself acidifying (nitrification releases H⁺), so continuous use will pull the pH down again. Liming must be repeated on the basis of fresh soil tests, not as a one-time operation.