An automated method is described for the determination of ammonium in 2 N potassium chloride soil extracts. The determination is based on the indophenol blue method following a dialysing step. As little as 0·03 mg 1–1 of ammonium-nitrogen can be determined. The recoveries have been investigated for a number of different soil types and satisfactory results were obtained. Determinations can be carried out at the rate of thirty samples per hour.