A colorimetric procedure is described for the determination of small amounts of potassium. The soil is extracted with Bray's extracting reagent and the available potassium is precipitated as cobaltinitrite. The determination of potassium precipitated involves the diazotization of sulphanilic acid, using the nitrite groups, of the cobaltinitrite precipitate, and subsequent coupling with dimethylaniline to give methyl orange. The colour is measured with a Spekker absorptiometer. The method is applicable to the routine determination of potassium in soil extracts.