ammonium sulfate ((NH₄)₂SO₄)

Ammonium sulfate ((NH₄)₂SO₄) is a nitrogen-and-sulfur-bearing inorganic fertiliser produced industrially by reacting ammonia with sulfuric acid. The pure compound contains 21% nitrogen as ammonium-N and 24% sulfur as sulfate-SO₄. It is a white crystalline solid, highly water-soluble (744 g/L at 20°C), and chemically stable at room temperature with a long shelf life when stored dry.

In Indian agriculture, ammonium sulfate is a workhorse fertiliser for sulfur-demanding crops — oilseeds (groundnut, mustard, soybean), pulses, alliums (onion, garlic), and cole crops (cabbage, cauliflower). Sulfur deficiency now affects an estimated 40% of Indian arable land, particularly intensively cropped rice-wheat systems where decades of high-analysis NPK fertilisers (urea, DAP, MOP) supplied no sulfur at all. Ammonium sulfate addresses both deficiencies simultaneously. The ammonium-N form is also slower-acting than urea — it is held on soil cation-exchange sites and converted by nitrification to nitrate over 2–4 weeks, reducing leaching loss and volatilisation. The sulfate-SO₄ form is the only sulfur form plants take up directly.

In biogas plants, ammonium sulfate appears in two contexts. First, it is the end product of ammonia stripping from liquid digestate — a process where digestate is heated and pH-raised to drive off NH₃ vapour, which is then absorbed in a dilute sulfuric acid scrubber to produce a 30–40% ammonium sulfate solution. This recovery route concentrates the nitrogen value of digestate into a low-volume, high-value liquid fertiliser that justifies long-distance transport (where raw digestate cannot). Second, it appears in flue-gas scrubbers downstream of biogas-fired CHP engines where NOₓ control occasionally produces ammonium sulfate as a co-product. The commercial price in Indian markets is approximately ₹15–25 per kg, against ₹6–8 per kg of urea per kg of N — but the sulfur content and slower release add agronomic value. Caution: ammonium sulfate is acidifying in soil (it releases hydrogen ions during nitrification), so repeated application on already-acidic soils requires lime correction every 3–5 years.