This page covers off-white caprolactam-route ammonium sulphate specifically as an agricultural nitrogen and sulfur source. The tint comes from trace organic material carried through from the caprolactam process liquor; it has no bearing on nutrient content, on nutrient availability, or on crop response.
Substance identity
Ammonium sulfate · (NH₄)₂SO₄ · CAS 7783-20-2 · PubChem CID 6097028 · 132.14 g/mol · cream to pale beige coarse crystals, 2–4 mm predominant.
Specification
— typical commercial ranges
| Parameter | Typical range |
|---|---|
| Total nitrogen (N) | 21.0 % min |
| Sulfur (S) | 24 % min |
| Moisture (H₂O) | ≤ 0.2 – 0.5 % |
| Free acid (as H₂SO₄) | ≤ 0.05 % |
| Water-insoluble matter | ≤ 0.05 % |
| pH, 5 % solution | 5.0 – 6.0 |
| Particle size | 2 – 4 mm predominant |
| Colour | off-white / cream |
Properties
Both nutrients are in directly plant-available form: ammonium-N and sulfate-S. Ammonium is held on the soil cation exchange complex and is therefore less mobile than nitrate in the days after application, then nitrified to nitrate over roughly two to six weeks in warm, moist, well-aerated soil, and much more slowly in cold or strongly acid soil. Nitrification releases acidity: plan roughly 5.4 kg CaCO₃ per kg of applied N as a first approximation of the liming offset. Critical relative humidity around 79–81 % at 30 °C means the coarse crystal stores well in most climates without special conditioning.
Applications
A straight N+S fertiliser for broadacre and horticultural use. Two agronomic cases favour it over urea specifically:
- Sulfur-responsive crops. Sulfur is built into cysteine and methionine and therefore into every protein the crop makes. Where soil sulfate is low — which is now common because atmospheric sulfur deposition has fallen sharply in industrialised regions — nitrogen applied alone is used inefficiently. Oilseed rape and other brassicas, onions and garlic, alfalfa, tea and milling wheat are the usual responders.
- Surface application without incorporation. Unlike urea, ammonium sulphate does not depend on urease hydrolysis and does not generate a high-pH reaction zone, so ammonia volatilisation loss from the soil surface is low on neutral and acid soils. The exception is calcareous soil, where free carbonate reacts with the ammonium and volatilisation can be significant — on those soils, incorporate or irrigate in.
The coarse crystal of this route spreads more consistently and dusts less than fine coke-oven crystal, which is the practical reason to pay for a caprolactam-route product for direct field use.
Handling and storage
Dry covered storage, pallets rather than bare floor, bags closed between uses. Do not blend with lime, basic slag or calcium cyanamide — ammonia is liberated and nitrogen is lost. Do not blend with urea for storage: the published critical relative humidity of a urea/ammonium sulphate mixture falls to roughly 56 % at 30 °C, well below either component alone, so a blend that will be stored rather than spread promptly will absorb moisture and set.
Notes
Salt index per unit of nitrogen is higher for ammonium sulphate than for urea, so avoid heavy rates placed in direct contact with seed. For colour background, see the off-white colour page; for route chemistry, see the caprolactam grade page.