Potassium deficiency of crops on southern U.S. Coastal Plain soils has been known since the l880s.  Long-term soil fertility studies have been conducted with K since 1911 on several Coastal Plain and related Hapludults, Paleudults, and Kandiudults in Alabama.  Data from these experiments allow us to answer questions such as (1) How much K can accumulate in Coastal Plain soils under cropping and fertilization? (2) Is subsoil testing necessary to obtain an accurate soil test calibration for K? (3) How much K does a crop remove from the subsoil? (4) How long does it take to deplete soil test K? (5) Does soil test K vary during the year?