There are inherent differences among soils in their capacity to supply plant-available N, which can also be markedly affected by management and cropping practices. The implication is that fertilizer practices should account for differences in soil N availability, and ideally should be implemented on a site-specific basis. This approach has only become feasible with the development of the Illinois soil N test (ISNT), and recent work has identified plant population as a key factor that must be taken into account for soil-based N management to improve N fertilizer efficiency. To investigate the interaction of soil N-supplying power (ISNT) and plant population in affecting crop N demand, a series of plot- and field-scale N-response trials were conducted in Illinois and Minnesota during 2005 and 2006, involving 50000-100000 plants per hectare. On the highly productive soils studied, economically optimum yield was usually increased with higher planting rates, which often but not always benefited economic profitability. The results suggest that the ISNT can be utilized as a basis for variable-rate planting as well as site-specific N fertilization, and have important implications for maximizing the profitability of corn (Zea mays L.) production, since plant uptake of fertilizer N will be promoted when population pressure exceeds soil N-supplying capacity. This strategy will reduce excessive N fertilization, and thereby benefit the environment.