The major use of ammonia is not for the production of nitrogen containing chemicals for further industry use, but for fertilizers such as urea or ammonium nitrates and phosphates. Fertilizers consume 80% of all the ammonia produced. In the USA in 1991, for example, the following ammonia-derived products were consumed, mostly for fertilizers: urea, ammonium sulfate, ammonium nitrate and diammonium hydrogen phosphate. Chemical uses of ammonia are varied. The Solvay process for the manufacture of soda ash uses ammonia, though it does not appear in the final product since it is recycled. A wide variety of processes take in ammonia directly, including the production of cyanides and aromatic nitrogen-containing compounds such as pyridine. The nitrogen in many polymers (such as nylon or acrylics can be traced back to ammonia, often via nitriles or hydrogen cyanide Most other processes use nitric acid or salts derived from it as their source of nitrogen. Ammonium nitrate, used as a nitrogen-rich fertilizer, also finds a major use as a bulk explosive.