Steel-grade (coke-oven) ammonium sulphate is the workhorse of straight nitrogen-plus-sulfur field application. Its nitrogen and sulfur behave exactly as in any other ammonium sulphate; its limitations are physical — fine crystal, dust, colour and slightly higher free acid. This page is about applying it well. For route chemistry see the steel-grade page.
Substance identity
Ammonium sulfate · (NH₄)₂SO₄ · CAS 7783-20-2 · PubChem CID 6097028 · 132.14 g/mol · light grey to brown fine crystals, 0.2–1.0 mm predominant.
Specification
— typical commercial ranges
| Parameter | Typical range |
|---|---|
| Total nitrogen (N) | 20.5 – 21.0 % min |
| Sulfur (S) | 23 – 24 % |
| Moisture (H₂O) | ≤ 0.5 – 1.0 % |
| Free acid (as H₂SO₄) | ≤ 0.10 – 0.20 % |
| Water-insoluble matter | ≤ 0.10 % or contract-specific |
| pH, 5 % solution | 4.5 – 6.0 |
| Particle size | 0.2 – 1.0 mm predominant |
Properties
Nutrient chemistry as for ammonium sulphate generally. Two physical properties drive field practice. Fine particles have low ballistic mass, so spinning-disc spreaders throw them a short and uneven distance and crosswind drift is significant — effective spread width is much narrower than for 2–4 mm granular products, and overlap patterns must be set accordingly. And fine particles dissolve quickly on a moist soil surface, which is an advantage for speed of availability but gives little tolerance if the field is then not wetted in.
Applications
Broadacre nitrogen and sulfur where cost per unit of nutrient governs and where spreading equipment can be set for a fine product: pre-plant broadcast with incorporation, band placement, drilling with appropriate equipment, and rice paddy topdressing. Flooded rice is a genuinely good fit for ammonium nitrogen: nitrate applied to a reduced soil is lost to denitrification, while ammonium is stable. Ammonium sulphate is also a standard tool for lowering pH in the root zone of acid-preferring crops such as tea and blueberry, and for improving iron, manganese and zinc availability on alkaline soils.
It should not be used for fertigation, drip systems, spray-tank water conditioning, foliar solutions or retail-bagged consumer product. In each case the insolubles or the colour disqualify it.
Handling and storage
Store dry and covered on pallets; limit stack height because fines cake heavily under load. Provide dust extraction and respiratory and eye protection at transfer, bagging and blending points. Keep damp material off unprotected mild steel — the higher free acid accelerates corrosion. Away from strong alkalis.
Notes
One caution specific to flooded soils: under strongly reducing conditions in poorly drained, high-organic-matter paddy soils, sulfate can be microbially reduced to sulfide, which is toxic to rice roots. On such soils, split applications and manage drainage rather than applying large single sulfate doses. If dust and spread width are the binding constraints, the compacted or hydraulic granular versions made from the same feed solve them.