Release of potassium from fixed forms in an illite, montmorillonite, and vermiculite and from the native forms of an illite, biotite, and muscovite was investigated. Methods used in studying potassium release were chemical, electrochemical, and biological. Changes in minerals following potassium release were followed with X‐ray diffraction and chemical techniques. Results of the study indicated the potassium “fixed” by vermiculite and montmorillonite to be available for plant growth. The native potassium of biotite was a great deal more available for plant growth than that of an illite or muscovite.