Ammonium sulfate is a chemical compound. Its chemical formula is (NH4)2SO4. It contains ammonium and sulfate ions. It is slightly acidic because ammonia is a weak base. It is made by reacting ammonia with sulfuric acid. It breaks down when heated. It is used in fertilizers to provide both sulfur and nitrogen to plants. It is banned in some parts of the Middle East because it was being used to make explosives. It rarely occurs in the ground.