The Berthelot reaction has been applied to an AutoAnalyzer method for the analysis of ammonium‐nitrogen. This method is suitable for the determination of total nitrogen in soils and plant material, and sufficiently stable and sensitive to determine levels of NH+4‐N in natural waters. Amino acids interfere, but only contribute a relatively small fraction (11%) in soil extracts and were not present in the soil solution examined. The inclusion of the generally used disodium hydrogen phosphate buffer was found to enhance interference from amino acids, and precipitate calcium phosphate from calcareous soil extracts.