This paper deals with the determination of potassium in fertilizers by the volumetric method involving the use of sodium tetraphenylborate. The potassium ion is precipitated with a slight excess of a standard solution of sodium tetraphenylborate. The excess of sodium tetfdphenylborate is titrated with a solution of zephiran chloride (alkyldimethylbenzylammonium chloride), after the filtration of the precipitate of potassium tetraphenylborate. The determination of potassium with five replications in potassium fertilizers like potassium chloride and potassum sulfate and in three fertilizers mixtures, were performed in order to study the precision and the accuracy of the method. The data obtained allow to conclude that the method for determining potassium in fertilizers by the volumetric method, usng sodium tetraphenylborate, is very rapid and afford a very good precision and accuracy.