The ammonium sulfate or sulfate of ammonia ((NH4)2SO4; sulfate is also spelled sulphate), sometimes called ammosul or further abbreviated as AS or AMS , is a chemical compound that is primarily used as a nitrogen fertilizer with other minor uses. It occurs in crystals with particle size that is variable, colored white to beige (IPNI 2012). It is a two-in-one synthetic fertilizer that supplies both the essential elements nitrogen and sulfur. Also interchangeably called 21-0-0 or 21-0-0-24S, it contains 21% nitrogen and 24% sulfur.
All of the nitrogen that is supplied by the ammonium sulfate fertilizer is in the positively-charged ammonic form (NH4+, ammonium ion) just like in urea (CO(NH2)2, 46% N), anhydrous ammonia (NH3, 82% N), and aqua ammonia or ammonium hydroxide (NH4OH, 20-41% N) . But the sulfur is in the oxidized sulfate form carrying a negative electrical charge. Upon conversion into ammonic nitrogen in the soil, it behaves according to its properties regardless of fertilizer source.
This fertilizer can be applied directly to the soil either singly or mixed with some other fertilizers. It can also be dissolved in water and applied as soil drench, or distributed with irrigation water, or applied as foliar spray.
Ammonium sulfate was one of the first nitrogenous fertilizers. It has been continuously produced since more than 150 years ago. It is produced by reacting sulfuric acid with heated anhydrous ammonia but is also a byproduct in various industrial processing. It is a byproduct in coal-based manufacturing of nylon and steel (IPNI 2012).
AS used to dominate the market for nitrogenous fertilizers together with ammonium nitrate until it was unseated by urea. It reached its peak production of 15.3 million tons in 1980 but dwindled steadily and was soon overtaken by urea. In 1998, it had a total production of 12.5 million tons while urea had 86.13 million tons (Soh 2006).
Some reasons that contributed to the downtrend in AS production and utilization are: (1) reduction in the processing of industrial products in which it is a byproduct (Soh 2006); and (2) preferential use of other fertilizers, mainly urea which has so much higher nitrogen content (46% N) and therefore has lower cost of transport per unit of nitrogen.
1. It contains nitrogen and sulfur. It is therefore a valuable fertilizer in soils that are deficient in both elements. In such condition, it may have real advantage over other nitrogen fertilizers in terms of cumulative cost of nitrogen and sulfur.
2. Among the major nitrogenous fertilizers, it has the least tendency to absorb atmospheric water. This characteristic favors longer storage (Thorup 1984).
3. Compared to urea, it is more resistant to ammonia volatilization. In neutral to acid soils, it can be broadcasted or otherwise applied without soil incorporation. However, on alkaline soils (above pH 7.0) with high moisture and under high temperature, the possibility of volatilization increases with soil surface application where it lies exposed for several days. Under this condition, the applicable rule is soil incorporation like in urea (Thorup 1984).
4. It can improve the efficacy of post-emergence herbicidal sprays. When water has high content of calcium, magnesium, or sodium, the addition of dissolved AS in the herbicide solution will prevent plugging of spray nozzles (IPNI 2012).
(Ben G. Bareja Jan. 2013)