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. Its composition is fixed by chemistry, and the theoretical content 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

This is why the grade is stable across the industry. It is not a formulation choice — it is what the compound is.

Why the certificate can show a different figure

If the chemistry is fixed, why does a certificate of analysis sometimes show nitrogen at 20.5% rather than 21%?

Several legitimate reasons:

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.

Impurities. Depending on the production route, small quantities of other substances may be present, which displaces some of the ammonium sulfate by mass.

The practical rule: the nominal grade describes the product; the certificate describes the lot. If you are buying against a specification, make sure the contract states 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. See Granular vs Crystalline Ammonium Sulfate.
2. Check the basis. Dry or as-received, S or SO4. A three-point difference in a nitrogen figure is often a basis difference, not a quality difference.
3. 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.