Do soil tests help forecast nitrogen response in first-year corn following alfalfa on fine-textured soils?

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.