Do Soil Tests Help Forecast Nitrogen Response in First‐Year Corn Following Alfalfa on Fine‐Textured Soils?

Core Ideas ISNT and PSNT concentrations did not differ among alfalfa stand ages in five long‐term trials. Only PSNT concentration increased during first‐year corn in three of five long‐term trials. The PSNT was less related to soil N mineralization and more related to available N than the ISNT. The PSNT more accurately predicted corn response or nonresponse to fertilizer N than the ISNT. The product of alfalfa stand age and PSNT separated N response in 87% of 15 independent site‐years. Improved methods are needed to predict grain yield response to fertilizer N for first‐year corn (Zea mays L.) following alfalfa (Medicago sativa L.) on fine‐textured soils. Data from 21 site‐years in the North Central Region of the United States were used to determine how Illinois soil nitrogen test (ISNT) and pre‐sidedress soil nitrate test (PSNT) concentrations change during alfalfa–corn rotations and how they relate to mineralized N, available N, and grain yield response to fertilizer N in first‐year corn. In five long‐term crop rotation trials, ISNT and PSNT taken in the alfalfa phase did not change with alfalfa stand age, but PSNT concentration in first‐year corn increased with alfalfa stand age in three of five trials. Across 21 site‐years, the PSNT more accurately predicted corn response or nonresponse to fertilizer N than the ISNT (67 vs. 52% of site‐years). The interaction of PSNT with alfalfa stand age resulted in 87% accuracy when validated with 15 independent site‐years; no N response was likely in first‐year corn when the product of PSNT × stand age exceeded 42. Incubation of soil from first‐year corn showed that the ISNT explained over twice as much variation in soil nitrate‐N accumulation after 17 wk than the PSNT (30 vs. 13%). Overall, the PSNT was less related to soil N mineralization and more related to aboveground corn N uptake, total available N, and grain yield response to fertilizer N in first‐year corn than the ISNT. This research provided a simple method for predicting when N response is likely in first‐year corn after alfalfa, but further research is needed to predict the size and mineralization rate of soil N pools after alfalfa termination to estimate the economically optimum nitrogen rate when fertilizer N response is likely.