It was found that sodium tetraphenylboron, NaB(C6H5)4, can be used as a reagent for determination of K in acid sodium acetate extracts of soils. After extraction and filtration, sodium hydroxide and formaldehyde are added prior to the reagent to avoid interference of ammonium. At low concentrations of K in the extracts (<30 mg K/1) the white precipitate of potassium tetraphenylboron, KB(C6H5)4, is colloidal and the concentration of K can be determined turbidimetrically with a simple instrument in the field or on the farm.