The amounts of C, N, and P stored in soils represent the balance between inputs and outputs of these elements to and from terrestrial ecosystems. Soil management and unintentional perturbation of element cycles (e.g., inputs of pollutants) may profoundly influence soil organic matter and associated nutrients. Reliable information on changes in C, N, and P stored in soils is essential to prevent ecosystem degradation. Agricultural land in Ontario is managed intensively, but quantitative information about the influence of management on element storage is lacking. Our main objectives were to quantify the masses of soil C, N, and P stored in soils from the agricultural regions of Ontario and to assess the impacts of agriculture by comparing soils under cropland and adjacent forests or woodlots. These assessments are valid to the extent that adjacent forests are appropriate 'ecological references', such that differences between soils under cropland and forest represent the cumulative effects of clearing and subsequent agricultural management. Mass of C, N, and P per unit area stored in surface and subsurface layers of cropland and adjacent forests was measured for 15 sites. The influence of two or more cropping practices was assessed at six of the sites. Compared with the forest soil, the surface layers of the cultivated soils (averaged for the 15 sites) had 34% less C, 19% less N, and 24% more P. Decreases in C storage were attributed to reduced C inputs and enhanced rates of plant litter decay. Changes in N storage were dependent on management of N fertility, and cultivation-induced narrowing of C/N ratios indicated preferential maintenance of N relative to C storage. Increases in P storage were attributed to fertilization. These quantitative comparisons indicated that decreases in C storage were less severe than frequently suspected and that the increases in P storage may have an equally important influence on ecosystem function.