Vertisols of the Blackland Prairie of Central Alabama and Mississippi were once the heart of the North American cotton belt in the 19^th Century.  Although cotton is not as widely grown as it once was, farmers who do grow cotton on these soils are seeing more problems with K deficiencies in spite of the fact that these soils often test “high” or “very high” in K.   In 2005, a comprehensive soil fertility experiment with cotton was established on a Vaiden clay (very-fine, smectitic, thermic Aquic Dystruderts) on the Black Belt Research and Education Center in west-central Alabama.  The objectives of this experiment were to determine optimum N rates and to provide soil test calibration for P and K for this soil.  Treatments included 6 N rates,  5 P rates, and 6 K rates on a soil that initially tested low in P using the Mississippi-Lancaster extraction procedure (16 mg P kg^-1), very high in K (180 mg K kg^-1) and high in Mg (60 mg Mg kg^-1).  Six years of non-irrigated cotton yields strongly indicate a response to K fertilization and no response to P fertilization.  These results suggest major changes are needed in our current soil test calibration for P and K for cotton.  Current N recommendations appear to be satisfactory for non-irrigated cotton.