Ammonium sulphate

Ammonium sulphate stands as the earliest industrially produced nitrogen fertiliser. It occurs as white crystals with high water solubility. Its nitrogen content reaches roughly 21%, entirely in ammoniacal form, whilst the sulphur fraction amounts to 23–24%, all present as sulphate^[1]. The lack of any metallic cation in its make-up accounts for its soil-acidifying nature^[1]. Its physical properties include a bulk density of 720 to 1,040 kg/m³ and an angle of repose of 32 to 33 degrees^[1].

Manufacture and processes

Industrial production arises from the neutralisation of ammonia with sulphuric acid. The source notes that various production methods exist^[1]. Phosphogypsum can be recovered in certain processing routes, notably for manufacturing sulphuric acid and ammonium sulphate^[2]. Within the urea sector, some plants add ammonium sulphate to molten urea liquor ahead of the solidification stage, yielding compound products^[3].

Agronomic uses

Applied during tillage or as a top dressing, this product supplies both nitrogen and sulphur, each being immediately available to the crop owing to their water solubility^[1]. It suits sulphur-deficient soils and crops with high sulphur demand particularly well. The substrate acidification it brings about can be turned to advantage on land with high lime reserves, though closer monitoring is required in already acidic conditions. The absence of nitrate nitrogen delays immediate leaching, although the subsequent biological conversion of ammonium to nitrate cannot be prevented. Combining this fertiliser with raw phosphate rock or urea is not recommended^[1].

Market and trade

In the United States, ammonium sulphate ranks amongst the principal derivatives of ammonia, with 2025 output tonnages placing it behind urea, ammonium nitrate, nitric acid and ammonium phosphates^[4]. Within the nitrogenous fertiliser range, it appears alongside urea, ammonium nitrate, CAN and UAN solutions in industry comparison tables^[5]. Its share of international trade remains constrained, however, by its modest nitrogen content, which handicaps its freight cost per unit of plant nutrient relative to concentrates.

Regulation and specifications

Placing on the European market falls under Regulation (EC) No 2003/2003^[6]. Technical standards drawn from the Indian reference framework set the following thresholds: maximum moisture 1.0%, minimum ammoniacal nitrogen 20.6%, minimum sulphur 23.0%, maximum free acidity 0.025% (0.04% for materials derived from ammonia and by-product gypsum), maximum arsenic 0.01%^[1]. The classification of fertilisers containing ammonium nitrate as oxidising substances under UN transport regulations does not apply to this product, which contains no nitrate^[6].