What Is Ammonium Sulfate Fertilizer?

Ammonium sulfate is an established nitrogen and sulfur fertilizer, traded internationally for well over a century. It is not the highest-analysis nitrogen product — urea carries far more nitrogen per tonne — yet it remains in demand because it delivers two nutrients at once, and because its nitrogen behaves differently in the soil from urea’s. The chemistry behind that difference is simple, and it explains most of how the product is bought and sold.

What it is

Ammonium sulfate is an inorganic salt. The fertilizer grade has the chemical formula (NH4)2SO4 and is described in trade by its analysis, 21-0-0-24S. That notation reads as 21 for nitrogen, 0 for phosphorus, 0 for potassium and 24S for approximately 24% sulfur.

Those figures are nominal, and the nitrogen figure depends on the production route: 20.5% minimum for granular material, 21% minimum for caprolactam-origin crystal. The analysis for the material you buy is declared on the certificate of analysis for the production lot, which is where the guaranteed figure sits. For a fuller explanation of the numbers, see Ammonium Sulfate 21-0-0-24S Explained.

Soil behaviour

Ammonium nitrogen

The nitrogen is in the ammonium form (NH4+), and it carries a lower ammonia volatilisation risk than urea. When urea is applied to the soil surface and not incorporated, it can break down and lose nitrogen to the atmosphere as ammonia gas; ammonium sulfate generally carries a lower risk under many soil conditions, which is a real advantage where incorporation is impractical. Even so, it can still volatilise, particularly when surface-applied to alkaline or calcareous soils, and rainfall or irrigation helps move it down.

Because the ammonium ion carries a positive charge, it is attracted to the negatively charged surfaces of clay and organic matter. It is therefore not immediately leached by rain the way nitrate is, though once soil bacteria convert it to nitrate the leaching risk rises. That conversion runs faster in warm, moist soil, so the period of protection is shorter in the tropics than in a cold spring.

Sulfate sulfur

The sulfur is in the sulfate form (SO4²⁻), which is what plants take up, and that is the main difference from elemental sulfur products, which have to be oxidised in the soil before the sulfur becomes available. Sulfate is mobile and can be leached, so it behaves much like nitrate. Timing an application to crop demand matters more than it does for a slow-release sulfur source.

Soil acidification

Repeated applications can contribute to soil acidification. When soil bacteria nitrify the ammonium, hydrogen ions are released and pH can fall over time, with the size of the effect depending on soil properties, application rate and management practice. On already acidic soils this needs managing, and it is one of the reasons ammonium sulfate is not the default nitrogen source everywhere.

On alkaline soils, particularly calcareous high-pH material, the acidifying effect earns its place. It can improve the availability of iron, manganese and zinc, which become locked up as pH rises. Sulfur applications in those situations are sometimes as much about the acidifying effect as about the sulfur itself.

Sulfur demand

Sulfur deficiency has become more common in many agricultural regions, for reasons that have nothing to do with ammonium sulfate. Cleaner atmospheric emissions have reduced incidental sulfur deposition from rainfall, the higher-analysis fertilizers used for decades contain far less sulfur as a by-product, and higher yields remove more sulfur from the field. Where a crop responds to sulfur, a product that supplies both nutrients avoids a separate pass, and that convenience explains more of the demand than the chemistry does.

Main uses

As a straight fertilizer it is applied directly to cereal, oilseed, forage and horticultural crops, particularly where sulfur is limiting or where pH management makes the acidifying effect useful. Beyond agriculture, the same chemistry serves industrial processing, where the specification comes from the process rather than from an agronomic grade. A fuller treatment sits in What Is Ammonium Sulfate Used For?.

As a raw material it is a standard nitrogen and sulfur carrier in NPK production and in bulk blends, which is a large share of global consumption. Blenders buy on particle size and on the nitrogen guarantee rather than on brand. That side of the business is covered in Ammonium Sulfate for Compound Fertilizer Production.

Forms and grades

Two physical forms are traded, with identical chemistry. Crystalline, also called standard or fine grade, is fine and fast-dissolving and sold largely as produced; granular is the same material processed into coarser, more uniform particles for blending and mechanical application. Fertilizer grade and technical or industrial grade are distinguished as well, though the difference lies in specification requirements rather than in the chemistry described above. If your application is industrial, take the specification from the process, not from an agronomic grade name.

Supply sources

Most ammonium sulfate is a by-product of other industrial production rather than the output of dedicated synthesis, and that shapes the market, because supply can move for reasons unrelated to fertilizer demand. A caprolactam line returning from maintenance, a steel plant changing its desulphurisation practice or a nylon producer adjusting runs all alter the tonnage on offer. Buyers planning a programme usually want to know which of those sources sits behind an offer, and the China Ammonium Sulfate Export guide sets out how the export side works.

Next steps

• Ammonium Sulfate 21-0-0-24S — specifications, applications and supply
• Ammonium Sulfate as a Nitrogen and Sulfur Fertilizer
• Ammonium Sulfate in Agriculture

Need specifications for a specific programme? Send us your application and destination and we will confirm what is available.