Ammonium Sulfate 21-0-0-24S Explained
The grade notation decoded — what each figure represents, how it is expressed, and the practical gap between the nominal grade and the lot certificate.
· 6 min read
On an ammonium sulfate specification the product is usually written as 21-0-0-24S. It looks like a standard fertilizer analysis, and most buyers read it as one, but the notation is being used in a slightly unusual way. The gap between the label and the certificate is small, and it decides whether two offers are comparable.
Reading the notation
The N-P-K convention describes a fertilizer by its percentage of nitrogen, phosphorus expressed as P2O5 and potassium expressed as K2O. Ammonium sulfate contains no phosphorus and no potassium, so those two figures are zero by definition rather than by analysis. What the convention has no room for is sulfur.
| Figure | Means | Typical value |
|---|---|---|
| 21 | Nitrogen (N) | 20.5–21% minimum, depending on form |
| 0 | Phosphorus (P2O5) | none |
| 0 | Potassium (K2O) | none |
| 24S | Sulfur (S) | ~24% |
The fourth figure needs explaining. The trade carries sulfur as a separate value with an S suffix, since the convention has no place for it. When a seller writes 21-0-0-24S, the description covers three nutrients — nitrogen, sulfur and nothing else.
Theoretical composition
Ammonium sulfate is (NH4)2SO4, and the composition of the pure compound is fixed by chemistry rather than by the plant. The two ammonium groups account for roughly 21% of the mass and the sulfate group for roughly 24%, which is where the headline figures come from. Pure material therefore carries about 21% nitrogen. In commercial trade, though, most of it is a by-product of another process, and the assay of what is shipped reflects the route it came from.
Why nitrogen varies
Nitrogen content follows the production route, which is the part most specification sheets leave out. Two certificates for the same grade can sit a few tenths apart without either being wrong, so the form and the source belong on the specification next to the grade. The table gives the usual figures by material, and Ammonium Sulfate by Production Source covers the route-by-route detail.
| Material | Typical nitrogen |
|---|---|
| Crystalline, caprolactam by-product | 21% minimum |
| Crystalline, other by-product routes (e.g. coke-oven) | around 19–20.5% |
| Granular (compacted from a mix of caprolactam and other by-product sulfate) | guaranteed 20.5% minimum |
Granular ammonium sulfate is made by compacting caprolactam-origin crystal together with other by-product material, and because the blend carries the lower-assay stream the guarantee is set at 20.5% minimum. Granular product that assays consistently above 21% is made solely from caprolactam crystal. So when a certificate shows 20.5% rather than 21%, the first question is what the material is and where it came from. Moisture and basis explain part of the gap; the route explains most of it.
Other effects move the figure at the margin. Results may be reported on a dry basis or as received, and as-received material holds some moisture, so its nitrogen reads lower while both figures remain correct. Ammonium sulfate is hygroscopic and picks up moisture in handling and storage, which dilutes everything else in the analysis. Laboratory methods carry uncertainty of their own, and two methods run on the same sample can return slightly different results.
The grade notation describes the product family; the certificate describes the lot, and the lot’s assay depends on its route. A buyer working against a specification should state whether the requirement is a minimum or a typical, whether it is dry basis, and what happens if the certificate falls outside it. Left open, those points are settled by a surveyor at discharge.
Confirmed per lot: the nitrogen and sulfur content, moisture, free acid and other parameters applying to your cargo are those declared on the Certificate of Analysis and Technical Data Sheet issued for the production lot shipped. We confirm the specification available for your requirement before you commit.
Sulfur convention
Sulfur can be declared as elemental sulfur, around 24% for ammonium sulfate, or as sulfate, a much larger number for the same material because the sulfate group is heavier than the sulfur it contains. Both conventions are legitimate and neither is wrong. An S figure set against an SO4 figure without conversion points to the wrong conclusion about which cargo carries more sulfur, and the two may be identical material. Confirm the convention in writing before any two certificates are compared.
Other specification items
Grade analysis alone does not define a usable product. Moisture affects storage, caking and effective nutrient content; free acid matters to handling and to some industrial processes; particle size distribution is essential for granular material going into a blend; bulk density drives volumetric handling and storage calculations. Appearance and colour are usually cosmetic, though some buyers specify them, and water insolubles matter to solution and industrial applications. None of these are fixed by the chemistry the way nitrogen and sulfur are. See the Ammonium Sulfate Specification Guide for a structured approach to writing a specification.
Comparing offers
When offers are on the table, the form decides whether they are comparable at all. 21-0-0-24S describes both granular and crystalline material, so an offer that does not state the form tells you very little. Granular is supplied to a 20.5% nitrogen minimum and caprolactam crystal to 21%. See Granular vs Crystalline Ammonium Sulfate.
A nitrogen figure below 21% is usually a production-route difference rather than a defect. Establish the source and the guaranteed minimum before price is compared, because the notation does not say which stream the cargo came from. Where the guarantee is 20.5%, the material is granular; where it is 21%, it is caprolactam crystal.
Basis matters for the same reason: dry or as received, S or SO4, since a three-tenths difference in nitrogen is often a basis difference rather than a quality one. Two materials can both be 21-0-0-24S and still differ in moisture, particle size and free acid, in ways that show up in the blender. Compare the full specification rather than the grade.
Next steps
- Ammonium Sulfate 21-0-0-24S — full product page
- Ammonium Sulfate Specification Guide
- What Is Ammonium Sulfate Fertilizer?
Send us the specification your process requires and we will confirm what the available production can meet.
Frequently asked questions
What does 21-0-0-24S mean?
It is the standard analysis notation for ammonium sulfate: nitrogen, no phosphorus, no potassium, and sulfur. The 'S' marks the sulfur figure separately because the N-P-K convention does not include sulfur. The nitrogen figure depends on the production route and form — see the nitrogen question below.
Why does my COA show nitrogen below 21%?
Most often because of the production route, not a quality defect. Crystalline ammonium sulfate from caprolactam production assays 21% minimum. By-product material from other routes, such as coke-oven production, runs around 19–20.5%, and granular ammonium sulfate — compacted from a mix of caprolactam and other by-product streams — is supplied to a guaranteed 20.5% minimum. Moisture, basis of expression (dry or as-received) and analytical tolerance also shift the reported figure. What matters is that your contract states the requirement and the basis clearly.
Is the sulfur figure on the analysis expressed as S or as SO4?
This is a common source of confusion. Sulfur can be declared as elemental sulfur (S) or as sulfate (SO4), and the numbers are different for the same material. Always confirm which convention the certificate uses before comparing two analyses.
On this page
Ammonium sulfate
(NH4)2SO4 · 21-0-0-24S · crystalline and granular