Long-term soil fertility experiments (c. 1929) at 5 Alabama locations have been cropped (mostly cotton-soybean rotation) using residual soil K since 1982. Since 1957, K rates applied to crops were 0, 28, 56, and 111 kg K ha ^–1. Plow layer K has been monitored using Mehlich-1 extraction. Profile K was characterized in 1990. Most of the applied K prior to 1982 accumulated in the upper horizons except on sites with the lowest cation exchange capacity (<5 cmol kg^-1) where K accumulated in the top of the argillic horizon. Extractable plow-layer K declined very little during the 15-yr residual study. The largest decreases were in the two soils with the lowest CEC (<5 cmol kg^-1), the Benndale and Dothan soils. Critical plow-layer soil test K levels for cotton based on soil CEC as currently used by the Auburn University Soil Testing Laboratory were determined to be accurate for the cropping systems used in these studies. Including extractable profile K to a depth of 60 cm did not improve K calibration for cotton.