Lime and Ammonium Sulfate Applied Together
A market gardener has a 10 m² plot with soil pH 4.6. To raise the pH quickly before planting cabbages, she spreads 2.0 kg of quicklime (calcium oxide) over the plot. The same morning she dissolves 500 g of ammonium sulfate fertiliser in 10 dm³ of water and waters it onto the soil. Within an hour there is a sharp smell of ammonia, and young cabbage leaves in a nearby tray turn yellow. She has read that ammonium sulfate supplies nitrogen and that lime reduces soil acidity, so she cannot understand why the two treatments together have damaged the plants. The labels state that ammonium sulfate is (NH₄)₂SO₄, M = 132 g/mol, and that quicklime reacts vigorously with water. The gardener asks whether she can still save the fertiliser by adding more lime, and what she should do differently next season.
1. Explain, with equations, why the gardener smelled ammonia and why the fertiliser lost nitrogen.
Read the worked answer
Quicklime reacts with soil water: CaO + H2O -> Ca(OH)2. The calcium hydroxide is an alkali and reacts with ammonium sulfate: Ca(OH)2 + (NH4)2SO4 -> CaSO4 + 2NH3 + 2H2O. Ammonia is a gas at soil temperature, so it escapes, carrying nitrogen away from the root zone. The same alkalinity can damage roots and leaves, helping explain the yellowing. Adding lime and ammonium fertiliser together is therefore chemically incompatible: the lime is intended to raise pH, but it also converts the ammonium ion to ammonia.
2. Calculate the maximum mass of ammonia that could be released from 500 g of ammonium sulfate. Would adding more lime recover the nitrogen? Explain.
Read the worked answer
M((NH4)2SO4) = 132 g/mol. Amount = 500/132 = 3.79 mol. Each mole gives 2 mol NH3, so 7.58 mol NH3. M(NH3) = 17 g/mol, mass = 7.58 × 17 = 129 g. This is a maximum, assuming complete reaction. Adding more lime would not recover nitrogen; it would push the reaction further to ammonia and release more gas. The nitrogen is lost to the air, and extra lime makes the soil more alkaline and may damage plants.
3. Recommend a practical sequence for next season, including timing and a test. Justify each step in terms of solubility and volatility.
Read the worked answer
First test soil pH, then apply the required lime and mix it into the topsoil. Wait several weeks (or until after rain or irrigation) so CaO hydrates, reacts with soil acids and the pH stabilises. Retest pH. Only then apply ammonium sulfate, preferably as a band or side-dress, and water it in lightly. Do not apply lime and ammonium fertiliser on the same day, because soluble ammonium ions meet alkaline calcium hydroxide and ammonia volatilises. Waiting keeps ammonium in the soil, where it is soluble and available. Use gloves and avoid breathing lime dust.