Many studies in Iowa have shown that the optimal rates of N fertilization for com following soybean tend to be less than the optimal rates for com following com. This conclusion is supported by data presented in Figure 1, which summarizes the results of 35 pairs of N-response trials having 10 rates of fertilizer N applied. Quadratic-plateau yield response curves were fit to the data from each site and analyzed to determine economic optimum rates of fertilization. The mean economic optimum rates of fertilization were 103lb N/acre for com following soybean and 163lb N/acre for com following com.