AbstractDuring summer of 1961 bulk samples at 4‐inch to 30 inches depth increments were collected at 15 random locations within an apparently uniform single acre of forest soil, Hartsells loam, under oak‐hickory sawtimber on the Cumberland Plateau in Tennessee. Net radioactivity, total potassium content, and mechanical fractions were determined in the laboratory. After samples were stored in sealed containers for 7 days, activity was counted again and found to be increased. Linear regressions of initial activity on silt‐plus‐clay and on K, K on silt‐plus‐clay, and 7‐day activity on clay, on K, and on 1‐day activity were highly significant. Distributions of K, and of initial activity in the soil profile were more closely related to the profile of total fines than to the clay curve. Initial activity was believed to be due largely to K40, but activity gain during storage was probably due to daughters of U238.