A mineral ecological cropping system (MECS) that uses mineral fertilizers but no synthetic pesticides has been tested in field trials in Germany over 6 yr. Uncertainty persists over whether mineral nitrogen favors crops or weeds under MECS, and this issue was examined in a greenhouse pot study. The study investigated how corn and spring barley and selected weed species (common lambsquarters and barnyardgrass for corn; blackgrass and common chickweed for spring barley) responded to mineral nitrogen. The experiment included crop:weed combinations of 1:0, 1:2, 1:5, and 0:1 for corn, and 1:0, 1:5, 1:15, and 0:1 for spring barley, representing two monocultures, evaluated under four nitrogen doses (0N, ½N, 1N, and 2N). Data on root and shoot development of weeds and crops were determined at harvest. Increasing the dose of nitrogen increased the biomass of both corn and weeds, while weeds benefited more from the higher nitrogen doses than corn. Competitiveness of spring barley against weeds varied with increasing nitrogen doses. Blackgrass benefited more from higher nitrogen levels than spring barley, while common chickweed was better suppressed by spring barley when nitrogen levels were high. Weed:crop biomass ratios confirmed the observed shifts in competitive balance. The results highlight the importance of effective multitactic weed control in MECS.