Abstract
Several research groups have evaluated the Illinois soil N test (ISNT) as a tool for identifying the N responsiveness of corn (Zea mays L.). These studies have shown mixed results. Research in New York improved the ISNT accuracy by adjusting the critical ISNT value (0–20‐cm depth) for organic matter estimated by loss‐on‐ignition (LOI), resulting in an ISNT × LOI curve above which corn tended to be nonresponsive. We conducted 34 N rate trials (2005–2007) to further evaluate the accuracy of the ISNT × LOI curve in predicting the N responsiveness of corn in rotation with grass–legume sod or soybean [Glycine max (L.) Merr.]. First‐year corn did not respond to sidedress N (16 trials), yet the ISNT × LOI curve predicted that sidedress N was needed for six sites. For second‐, third‐, or fourth‐year corn after sod and corn after soybean, the ISNT × LOI curve accurately predicted N responsiveness for 15 of 18 sites (eight responsive and 10 nonresponsive sites). The ISNT values of first‐ and second‐year corn were closely correlated (ISNTYear2 = −12.8 + 1.1 ISNTYear1, r2 = 0.96, P < 0.001). The results of laboratory and field trials on ISNT dynamics from spring sod plowdown through sidedress showed an ISNT peak 1 to 4 wk after plowdown, indicating that samples taken at or after sidedress do not reflect sod N credits. We conclude that the ISNT × LOI curve accurately identified nonresponsive sites as long as the sample was not collected within 5 wk of sod plowdown and that sod N credits need to be taken into account for first‐year corn.
Published Version
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