The compound
Ammonium sulphate, (NH4)2SO4, is one of the oldest manufactured nitrogen fertilisers still in routine commercial use. Its formula weight is 132.14, from which its two headline numbers follow directly: two nitrogen atoms give 21.2 % nitrogen, and one sulphur atom gives 24.3 % sulphur. Those figures are stoichiometric, not commercial claims — any material sold as ammonium sulphate is judged against how close it comes to them.
The product is a white to off-white crystalline salt, freely soluble in water, and stable in normal storage when kept dry.
Two nutrients, two behaviours
The distinguishing feature of ammonium sulphate is that both of its ions are nutritionally useful, and they behave very differently once in the soil.
The ammonium ion (NH4⁺) carries a positive charge, so it is held on the cation exchange sites of clay and organic matter. It is therefore comparatively resistant to leaching in the short term. Soil bacteria progressively oxidise it to nitrate through nitrification, a process whose speed depends on temperature, moisture, aeration and pH — fast in warm, moist, well-aerated soil, slow in cold or waterlogged conditions. Until that conversion happens, the nitrogen stays broadly where it was placed.
The sulphate ion (SO4²⁻) carries a negative charge and is not retained by most temperate topsoils, so it moves with soil water in much the same way nitrate does. It is, however, immediately plant-available — no microbial oxidation step is required, unlike elemental sulphur.
Why the acidifying effect matters
Nitrification releases hydrogen ions, and because ammonium sulphate delivers a relatively low proportion of nitrogen per tonne of salt, it is the most acidifying of the common nitrogen carriers per unit of nitrogen applied. This is a liability on soils that are already acidic, where it accelerates the need for lime and can worsen aluminium or manganese availability. It is an asset on calcareous and alkaline soils, where a mild localised drop in pH around the fertiliser band improves the availability of phosphorus and of micronutrients such as zinc, iron and manganese. The same property is exploited in crops grown deliberately on acid soils, such as tea and blueberries.
Any long-run programme built on ammonium sulphate should include a liming plan on acid soils. This is a manageable, well-understood cost, not a reason to avoid the product — but it should be budgeted rather than discovered.
Volatilisation behaviour
Surface-applied urea can lose substantial nitrogen as ammonia gas, because urea hydrolysis raises pH sharply at the granule surface. Ammonium sulphate does not carry that mechanism and is generally far less prone to volatilisation on neutral and acid soils. On free-lime calcareous soils the picture changes: surface-applied ammonium sulphate reacts with calcium carbonate, and appreciable ammonia loss can occur if the material is left on a dry surface. Incorporation or irrigation after application addresses this.
Physical forms
Ammonium sulphate reaches the market as fine crystal, standard crystal, compacted or granulated product. The chemistry is identical; the particle size determines whether it can be blended with other granular fertilisers without segregating, and whether it spreads evenly. This is discussed in detail in the article on physical form and demand segments.
Where it fits
Ammonium sulphate is chosen when a grower needs nitrogen and sulphur together, when soil pH is high enough that acidification is neutral or beneficial, when volatilisation risk from urea is a concern, or when sulphur-hungry crops — oilseeds, brassicas, alliums, forage grasses, tea, sugarcane — are in the rotation. It is not the cheapest way to move nitrogen over long distances, for reasons covered in the article on nutrient density and freight, and it is rarely used as the sole nitrogen source in high-rate cereal programmes. It is best understood as a dual-nutrient specialist rather than a bulk nitrogen commodity.