Nitrogen cycle
Ammonium (NH4) in soil is made by nitrogen-fixing bacteria and decomposers, bacteria and fungi that break down dead life into its parts. This process is called ammonification. Ammonium has a positive charge. It easily joins to clay and humus in the soil. Ammonia and ammonium are poisonous to fish and other animals. Sewage and other waste-water is regularly measured because of this. If ammonia levels are too high, nitrification must happen.