Ammonium sulfate

Ammonium sulfate is a crystalline salt formed from ammonia and sulfuric acid. It is typically sold with a guaranteed analysis of about 21% nitrogen and 24% sulfur, with no phosphorus or potassium content.

Chemical formula

(NH₄)₂SO₄

Nitrogen content

About 21% total nitrogen, entirely as ammonium

Sulfur content

About 24% sulfur, as sulfate

Soil reaction

Acidifying, due to nitrification of the ammonium fraction

Ammonium sulfate supplies nitrogen as ammonium, which nitrifies to nitrate in soil, and sulfur as sulfate, an anion that is directly plant-available and mobile in the soil solution in a manner similar to nitrate.

• Nitrogen is entirely in ammonium form and nitrifies over time
• Sulfate-sulfur is immediately plant-available
• Provides no phosphorus, potassium, or micronutrients

Ammonium sulfate is commonly used where a crop or field has a sulfur requirement alongside its nitrogen need, such as many oilseed and legume crops, and on soils with limited native sulfur supply from organic matter mineralisation or atmospheric deposition.

• Combines nitrogen and sulfur supply in one granular product
• Ammonium form is less prone to ammonia volatilisation than urea when surface-applied to neutral or acid soils
• Used as a component of blended fertilizer programs where sulfur is limiting

General practice accounts for both the nitrogen and sulfur supplied and for the cumulative acidifying effect of repeated ammonium sulfate use, which may call for periodic soil pH monitoring and liming where warranted.

Because its nitrogen is in ammonium form, ammonium sulfate has a lower ammonia-volatilisation risk than urea on most soils, but the nitrate produced after nitrification remains subject to leaching and denitrification, and repeated use progressively acidifies soil.