Crop management influences the quantity and quality of organic matter in agricultural soils. A crop rotation study established at Lethbridge, Alberta in 1951 was sampled in September 1992 to determine the effect of crop management on total, light fraction and mineralizable (10-wk) organic matter contents. Spring wheat was the dominant cropping system; treatments examined include fallow frequency, forage hay production in rotation, manure amendment, N fertilizer application, and native grass. The two latter treatments were introduced in 1985. Total and light fraction organic matter did not vary among phases of the rotation whereas mineralized C tended to be lowest during and shortly after a fallow phase. When averaged across rotation phases, total, light fraction, and mineralized organic matter were enhanced by reduced fallow frequency, manure additons, hay production and native grass. Highest concentrations of total and labile organic concentrations in the 0- to 7.5-cm soil depth were generally found in the continuously-cropped wheat and native grass treatments. Hay production significantly increased soil organic matter in the 15- to 30-cm soil depth. Nitrogen fertilization did not increase soil organic matter in this study, likely because of minimal yield response over the treatment period. Sensitivity of the various indicators to treatment [(highest–lowest)/lowest] was 0.2 for total, 2.5 for light fraction, and 1.5 for mineralized soil organic C. Light fraction organic matter was the most robust indicator of management-induced effects on soil organic matter. Key words: Labile organic matter, fallow frequency, forage hay production, manure, native grass