How Soil Nitrogen Tests Relate to Nitrogen Mineralization in First-Year Corn Following Alfalfa.

First-year corn (Zea mays L.) following alfalfa (Medicago sativa L.) usually does not require fertilizer N, but sometimes high amounts are needed. The pre-sidedress and Illinois soil N tests (PSNT and ISNT, respectively) may help predict yield response to fertilizer N in first-year corn following alfalfa. These tests predict corn yield response to fertilizer N by measuring specific fractions of soil N and relating them to a critical level. Although these tests have demonstrated potential for predicting corn responsiveness to fertilizer N under some conditions, they often recommend unneeded N when significant mineralization occurs after the test is taken. Thus, a controlled-environment experiment was used to determine whether PSNT and ISNT concentrations at the six leaf collar corn stage (V6) from 17 field experiments in the Upper Midwest relate to cumulative net soil N (NO₃-N + NH₄-N) mineralization at weekly or bi-weekly intervals through corn maturity. Results show that the PSNT and ISNT did not relate well to cumulative net soil N mineralization from V6 through corn maturity. Future research should identify reliable methods by which soil N mineralization can be estimated prior to fertilizer N application.