Ammonium sulfate [(NH₄)₂ SO₄] was one of the first and most widely used nitrogen (N) fertilizers for crop production. It’s now less common but especially valuable where both N and sulfur (S) are required. Its high solubility provides versatility for a number of agricultural applications.
Ammonium sulfate (sometimes abbreviated as AS or AMS) has been produced for more than 150 years. Initially, it was made from ammonia released during manufacturing coal gas (used to illuminate cities) or from coal coke used to produce steel. Today, manufacturers make it by reacting sulfuric acid with heated ammonia. To get the crystal size best suited for the application, they control the reaction conditions by screening and drying the particles until achieving the desired size. Some materials are coated with a conditioner to reduce dust and caking.
Byproducts from various industries meet most of the current demand for ammonium sulfate. For example, the nylon manufacturing process produces ammonium sulfate as a co-product. In another, certain byproducts that contain ammonia or spent sulfuric acid are commonly converted to ammonium sulfate for use in agriculture.
Although the color can range from white to beige, ammonium sulfate is consistently sold as a highly soluble crystal with excellent storage properties. As described earlier, particle size can also vary depending on the intended purpose.
Growers apply ammonium sulfate primarily where they need supplemental N and S to meet the nutritional requirement of growing plants. Since ammonium sulfate contains only 21 percent N, other fertilizer sources more concentrated and economical to handle and transport often make a better choice for N-deficient fields. However, it provides an excellent source of S, which supports or drives numerous essential plant functions, including protein synthesis.
Because the N fraction is present in the ammonium form of ammonium sulfate, rice farmers frequently apply it to flooded soils, since nitrate-based fertilizers are a poor choice due to denitrification losses.
A solution containing dissolved ammonium sulfate is often added to post-emergence herbicide sprays to improve their effectiveness at weed control. This practice of increasing herbicide efficacy with ammonium sulfate works particularly well when the water supply contains significant concentrations of calcium (Ca), magnesium (Mg) or sodium (Na). A high-purity grade of ammonium sulfate often works best for this purpose to avoid plugging spray nozzles.
After addition to soil, the ammonium sulfate rapidly dissolves into its ammonium and sulfate components. If it remains on the soil surface, the ammonium may be susceptible to gaseous loss in alkaline conditions. In these situations, agronomists advise incorporating the material into the soil as soon as possible. Other options include an ammonium sulfate application before irrigation or a predicted rainfall.
Most plants can utilize both ammonium and nitrate forms of N for growth. In warm soils, microbes will rapidly begin to convert ammonium to nitrate in the process of nitrification [2 NH₄⁺ + 3O₂ → 2NO₃⁻ + 2H₂O + 4H⁺]. During this microbial reaction, acidity [H⁺] is released, which will ultimately decrease soil pH after repeated use. Ammonium sulfate has an acidifying effect on soil due to the nitrification process, not from the presence of sulfate, which has a negligible effect on pH. The acid-producing potential of ammonium sulfate is greater than the same N application from ammonium nitrate, for example, since all of the N in ammonium sulfate converts to nitrate, compared with only half of the N from ammonium nitrate that converts to nitrate.
Food companies commonly add ammonium sulfate to bread products as a dough conditioner. It’s also a component in fire extinguisher powder and flame-proofing agents. And it serves many purposes in the chemical, wood pulp, textile and pharmaceutical industries.
Source: Nutrient Source Specifics, No. 12, International Plant Nutrition Institute.