Ammonium (NH) 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. 