Standard ammonium sulphate is the baseline crystalline product: 21 % nitrogen and 24 % sulfur in a single water-soluble salt, without reference to a particular by-product route or granulation. Most commercial material is recovered as a by-product of caprolactam, methyl methacrylate or coke-oven gas cleaning; a smaller share is made by direct neutralisation of ammonia with sulfuric acid. This page covers the nutrient and soil chemistry that applies to all of them.
Substance identity
| Chemical name | Ammonium sulfate |
| Synonyms | Ammonium sulphate, diammonium sulfate, sulfuric acid diammonium salt, AMSUL, SA; mineral form mascagnite |
| Formula | (NH₄)₂SO₄ |
| CAS No. | 7783-20-2 |
| PubChem CID | 6097028 |
| Molar mass | 132.14 g/mol |
| Appearance | Crystalline solid, white to brown depending on route |
Specification
— typical commercial ranges across routes
| Parameter | Typical range | Note |
|---|---|---|
| Total nitrogen (N), dry basis | 20.5 – 21.0 % min | stoichiometric 21.2 % |
| Sulfur (S) | 23 – 24 % | stoichiometric 24.3 %, present as sulfate |
| Moisture (H₂O) | ≤ 0.2 – 1.0 % | route and crystal size dependent |
| Free acid (as H₂SO₄) | ≤ 0.03 – 0.20 % | tight for caprolactam, loosest for coke-oven |
| Water-insoluble matter | ≤ 0.05 – 0.10 % | |
| pH, 5 % aqueous solution | 4.5 – 6.0 | |
| Bulk density | ≈ 0.9 – 1.1 t/m³ |
Nutrient declaration in most markets is on a nitrogen and sulfur basis; the applicable national or regional fertiliser standard should be the one named in the contract. In the European Union, fertilising products placed on the market are governed by Regulation (EU) 2019/1009.
Properties
Solubility. Roughly 71 g per 100 g water at 0 °C, 74–76 g at 20–25 °C, about 104 g at 100 °C. Practically insoluble in ethanol and acetone. Dissolution is mildly endothermic.
Hygroscopicity. Critical relative humidity approximately 79–81 % at 30 °C — higher than urea (roughly 72–75 %) and much higher than ammonium nitrate (roughly 59 %). In practical terms ammonium sulphate is one of the better-behaved nitrogen fertilisers in humid storage, provided it is not blended with urea, which lowers the mixture’s critical relative humidity to roughly 56 % at 30 °C.
Thermal behaviour. No true melting point. Slow ammonia loss above about 100 °C; decomposition in the 235–280 °C range to ammonium bisulfate, then further to ammonia, sulfur oxides, nitrogen and water. Not combustible, not an oxidiser.
Soil behaviour. Ammonium is adsorbed on the cation exchange complex and is comparatively immobile until nitrified. Nitrification by soil bacteria proceeds over roughly two to six weeks in warm, moist, aerated soil and much more slowly in cold or strongly acid conditions, and it releases acidity. As a planning approximation, allow about 5.4 kg CaCO₃ per kg of applied N to offset the acidification — roughly 110 kg CaCO₃ per 100 kg of product. This is a textbook figure, not a soil-specific one.
Applications
Where sulfur is limiting. Sulfate is the form roots absorb; elemental sulfur must be microbially oxidised first and does not act in the same season. Plants build cysteine and methionine from sulfate, and cereal tissue carries roughly 15 parts nitrogen per part sulfur, so nitrogen applied to a sulfur-deficient crop is used inefficiently. Sulfur deficiency has become widespread as atmospheric sulfur deposition has fallen.
Where volatilisation risk rules out surface urea. Ammonium sulphate does not depend on urease hydrolysis and does not create a high-pH reaction zone, so surface losses on neutral and acid soils are low. On calcareous soils this advantage is lost and incorporation is advisable.
Where soil pH should be lowered. Acid-preferring crops such as tea and blueberry, and alkaline soils where lowering rhizosphere pH improves iron, manganese, zinc and phosphorus availability.
Flooded rice. Ammonium nitrogen is stable in reduced soil, whereas nitrate is lost to denitrification.
Handling and storage
Dry, covered storage on pallets; keep bags closed. Do not mix with lime, basic slag, calcium cyanamide or other strong alkalis — ammonia is liberated and nitrogen is lost. Do not store pre-blended with urea. Not classified as dangerous goods for transport, but fire releases ammonia and sulfur oxides.
Notes
The salt index per unit of nitrogen is higher for ammonium sulphate than for urea, so avoid concentrated placement in direct seed contact. Choice among routes and forms is a handling and end-use decision, not an agronomic one — the nutrients are identical. See the route pages (caprolactam, MMA, steel) and the form pages (crystal, compacted granular, hydraulic granular).
Frequently asked questions
What is the practical difference between caprolactam by-product, coke-oven by-product and synthetic ammonium sulphate?
The salt is the same; the impurity profile is not. Caprolactam by-product material is normally crystalline, sometimes off-white to beige, and can carry traces of organic residue that give it a faint odour. Coke-oven (steel industry) by-product is also crystalline and may carry residual free acid, thiocyanate or trace metals depending on how the gas was cleaned. Synthetic material, made by neutralising sulphuric acid with ammonia, normally carries the lowest organic residue and the least colour of the three routes and is the usual basis for technical grades. For straightforward field use all three deliver the same nitrogen and sulphate. Where colour, odour, filterability or trace-metal limits matter, ask for the production route and a recent analysis rather than relying on the grade name alone.
Why do specifications differ between 20.5% N minimum and 21% N minimum, and what else should I read?
Pure (NH4)2SO4 contains about 21.2% nitrogen and roughly 24% sulphur by mass, so the declared nitrogen figure is really a purity statement. A 21% minimum implies material close to theoretical; 20.5% leaves room for moisture, insolubles and additives. Read the specification as a set rather than one line: moisture, since nitrogen is usually declared on a dry basis and wet material costs more per real kilogram of N; free acid as H2SO4, which corrodes bags, spreaders and pumps and indicates incomplete neutralisation; water-insoluble matter, which is gypsum or organic carry-over that blocks fertigation filters; and particle size distribution. Ask which basis is used and whether values are guaranteed minima or typical figures.
Crystal, granular or compacted — which physical form should I buy?
Fine crystal dissolves fastest and is the least processed of the three forms, but it dusts, packs in the bag and segregates in a bulk blend, which suits fertigation, solution preparation and industrial use, but it dusts, packs in the bag and segregates badly in a bulk blend. Granular or compacted material is sized to sit alongside urea, MAP, DAP and potash so a blend stays homogeneous in the hopper and throws an even spread pattern. If you are blending or broadcasting mechanically, ask for the particle size distribution and, if the supplier reports them, size guide number and uniformity index, then match those to the other blend components. If the product will be dissolved anyway, a granulation premium buys nothing, and coatings slightly dilute the analysis.
How do I compare an ammonium sulphate offer against urea on a like-for-like basis?
Compare cost per unit of nutrient delivered to the field, not price per tonne. Ammonium sulphate carries roughly 21% nitrogen against about 46% in urea, so a tonne of product moves less than half the nitrogen; freight, handling and spreading cost per kilogram of N rise accordingly, which is why it rarely competes as a straight nitrogen source over long distances. The comparison only balances when the sulphate has value: price the sulphur against whatever you would otherwise buy for it, such as elemental sulphur, ammonium thiosulphate or gypsum, and subtract that credit before comparing nitrogen prices. Also normalise for moisture, packaging and the eventual liming cost of the acidification.
What should ammonium sulphate never be blended or stored with?
Keep it away from alkaline materials. Lime, basic slag, calcium cyanamide and similar products liberate ammonia from the ammonium ion on contact, so the blend loses nitrogen and smells; the same reaction argues against spreading it immediately onto freshly limed ground without an interval. Calcium nitrate is a second problem: double decomposition yields calcium sulphate and ammonium nitrate, so the mixture goes damp, cakes and changes character. Urea blends are physically workable, but the mixture’s critical relative humidity falls below that of either component, so a urea blend picks up moisture and sets far sooner than either product alone — blend close to application. As with any ammonium salt, store it apart from strong oxidisers, chlorates and nitrites.
When is ammonium sulphate the wrong choice?
Several situations argue against it. On acid, weakly buffered sandy soils it is the most acidifying of the common nitrogen carriers per unit N — the figure usually quoted in liming tables is around five kilograms of calcium carbonate equivalent per kilogram of nitrogen — so repeated use without lime pushes pH down and can raise aluminium and manganese availability. Where soil or irrigation water already supplies sulphur, you are paying freight on a nutrient you do not need. In continuously flooded soils sulphate can be reduced to sulphide, which is toxic to roots. In fertigation with hard, calcium-rich water it can precipitate gypsum and foul emitters. And if nitrogen alone is the requirement, urea or ammonium nitrate usually wins on cost per unit N.
How should it be stored, and how does its behaviour compare with ammonium nitrate?
It is hygroscopic but less so than ammonium nitrate or urea: critical relative humidity is around 80% at ordinary warehouse temperatures, so it stays free-flowing in conditions that would already be spoiling other nitrogen products. Caking usually comes from humidity cycling, pressure in tall stacks and moisture in the original material rather than from the salt itself. Store on pallets in a dry, ventilated building, keep bags closed, limit stack height and rotate stock. It is not an oxidiser and is not normally handled as a dangerous good in transport, unlike ammonium nitrate. On strong heating it decomposes rather than melting cleanly, releasing ammonia and sulphur oxides, so keep it clear of hot work.
Related substances
- Urea and ammonium sulphate lose nitrogen by entirely different routes – urease-driven ammonia volatilisation versus nitrification-driven acidification – which decides which one suits a calcareous field and which suits an already acid one.
- Ammonium chloride supplies the same ammoniacal nitrogen without sulphate, provided the crop tolerates chloride and sulphur is supplied from elsewhere and sulphur is being supplied from elsewhere.
- Where sulphur rather than nitrogen is the real target, magnesium sulphate monohydrate delivers sulphate without adding ammonium, which matters because ammonium competes with magnesium and potassium at the root surface.
- Both products acidify, but ammonium sulphate acidifies the whole rooting zone over weeks while MAP acidifies only the few millimetres around each granule – a distinction worth reading before choosing a starter for high-pH soil.