Ammonium Sulfate 21-0-0-24S Explained

On an ammonium sulfate specification you will see the product described as 21-0-0-24S. It looks like a standard fertilizer analysis, but it is being used in a slightly unusual way — and the differences matter when you are comparing offers or checking a certificate.

Decoding the notation

The N-P-K convention describes a fertilizer by its percentage of nitrogen, phosphorus (as P2O5) and potassium (as K2O). Ammonium sulfate contains no phosphorus and no potassium, so those two figures are zero.

The fourth figure is the interesting one. Standard N-P-K notation has no place for sulfur, so the trade adds it as a separate value with an S suffix. When you see 21-0-0-24S, you are looking at a three-nutrient description: nitrogen, sulfur, and nothing else.

Where the numbers come from

Ammonium sulfate is (NH4)2SO4. The theoretical composition of the pure compound is fixed by chemistry and can be calculated from the formula:

• Nitrogen: the two ammonium groups account for roughly 21% of the mass
• Sulfur: the sulfate group accounts for roughly 24% of the mass

Pure ammonium sulfate therefore carries about 21% nitrogen. In commercial trade, however, most ammonium sulfate is a by-product of other processes, and the assay of the material actually shipped reflects the route it came from.

Why the nitrogen figure varies — and it is not just moisture

This is the point most specification sheets leave out. Ammonium sulfate nitrogen content depends on the production route — see Ammonium Sulfate by Production Source for the detail:

Granular ammonium sulfate is made by compacting a blend of caprolactam-origin crystal and other by-product ammonium sulfate into granules. Because the blend contains the lower-assay material, the guaranteed figure 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 to ask is what the material is and where it came from — not just how it was expressed. Moisture and basis differences explain part of the gap, but the production route explains most of it.

Other factors still contribute at the margin:

Basis of expression. Results may be reported on a dry basis or on an as-received basis. Material as received contains some moisture, so as-received nitrogen is lower than dry-basis nitrogen for the same material. Both figures can be correct while describing the same cargo.

Moisture content. Ammonium sulfate is hygroscopic and picks up moisture in handling and storage. A higher moisture content dilutes everything else.

Analytical tolerance. Laboratory methods have uncertainty, and different methods can give slightly different results on the same sample.

The practical rule: the grade notation describes the product family; the certificate describes the lot — and the lot’s assay depends on its production route. If you are buying against a specification, state whether the requirement is a minimum, a typical, a dry-basis figure, and what happens if the certificate falls outside it.

S or SO4 — check which convention is used

This is one of the most common misunderstandings in ammonium sulfate documentation.

Sulfur can be declared as:

• Elemental sulfur (S) — around 24% for ammonium sulfate
• Sulfate (SO4) — a much larger number for the same material, because the sulfate group is heavier than the sulfur it contains

Both are legitimate, and neither is wrong. But if you compare an analysis expressed as S against one expressed as SO4 without converting, you will draw the wrong conclusion about which material has more sulfur. They may be identical.

Before comparing two certificates, confirm the convention.

What else belongs on a specification

Grade analysis alone does not define a usable product. A specification worth buying against also covers:

• Moisture — affects storage, caking and effective nutrient content
• Free acid — relevant to handling and to some industrial processes
• Particle size distribution — essential for granular material going into a blend
• Bulk density — affects volumetric handling and storage calculations
• Appearance and colour — usually cosmetic, but some buyers specify it
• Water insolubles — relevant to solution and industrial applications

These are producer- and lot-specific. They are not fixed by the chemistry the way the nitrogen and sulfur figures are. See the Ammonium Sulfate Specification Guide for a structured approach to writing a specification.

Using the notation when comparing offers

Three practical rules:

1. Check the form. 21-0-0-24S describes both granular and crystalline material. If the offers do not state the form, you are not comparing like with like. Granular is supplied to a 20.5% nitrogen minimum; crystalline from caprolactam to a 21% minimum. See Granular vs Crystalline Ammonium Sulfate.
2. Check the grade and the source. A nitrogen figure below 21% is usually a production-route difference, not a defect. Establish the source and the guaranteed minimum, not just the notation.
3. Check the basis. Dry or as-received, S or SO4. A three-tenths difference in a nitrogen figure is often a basis difference rather than a quality one.
4. Check the specification, not just the grade. Two materials can both be 21-0-0-24S and still differ in moisture, particle size and free acid — in ways that matter to your process.

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.