Land managers are concerned with maintaining crop, forest, and range production. If erosion losses from moderately deep or shallow soils occur faster than the soil is replenished by weathering from underlying bedrock or consolidated sediments, productivity will decline. Therefore, rates of soil formationf can be considered constraints on tolerable soil losses from relatively shallow soils; their long-term soil-loss tolerances can be equated with their rates of formation from bedrock or consolidated sediments. There are few data for determining rates of soil formation, however. Data from small, nonagricultural watershed studies in which the inputs and outputs of silica and the major cations have been measured can be utilized for this purpose. Rates of soil formation, based on these data from 18 watersheds with noncarbonate lithologies, range from 0.02 to 1.9 Mg/ha yr. The rates are most highly dependent on (1) the volume of run-off water and (2) the soil-to-rock ratio, where this ratio is a mass of soil divided by the mass of bedrock weathered to produce that mass of soil. An equation developed to predict rates of soil formation from these two parameters has a very high coefficient of determination,R2= 0.94.