Soil Test Calibration for Cotton On An Alabama Vertisol.

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.