On fine-textured soils, first-year corn (Zea mays L.) following alfalfa (Medicago sativa L.) responds to fertilizer N about one-half of the time, but reliable methods to predict responsiveness are lacking. In 14 on-farm experiments across Minnesota, five rates of N fertilizer were applied at planting or as a sidedressing at the six leaf-collar stage in first-year corn following alfalfa on fine-textured soils. In the seven experiments where corn grain yield was increased with applied N, the economically optimum fertilizer N rate (EONR) at a 5.6 price ratio (US$ kg^-1 N/US$ kg^-1 grain) at planting was 80 to 115 kg N ha^-1 in five experiments, 140 kg N ha^-1 in one experiment, and ≥200 kg N ha^-1 in another. At four of these N-responsive sites (57%), applying N as a sidedress reduced the EONR by 32 to 46% without reducing grain yield; at the other sites, timing of N application did not alter the EONR. Soil-test results from samples collected to a depth of 30 cm at sidedressing produced inconsistent predictions of N responsiveness. The pre-sidedress soil nitrate test (PSNT; critical level set at 21 mg kg^-1) correctly identified all seven N-responsive experiments and the lack of responsiveness at two sites (nine sites correctly identified), but also predicted five false positives, which would have led to unnecessary N application 36% of the time. The Illinois soil nitrogen test (ISNT) identified a similar number of sites correctly – 10 using a critical level of 225 mg kg^-1 and eight using a critical level adjusted for soil organic C. However, the ISNT predicted three false negatives using either critical level, which would have led to economic losses from omitting N application on 43% of N-responsive sites. These results confirm that fertilizer N requirements for first-year corn following alfalfa may be reduced with sidedressing and that soil testing may help identify the need for supplemental N on fine-textured soils.