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.