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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Illinois Soil Nitrogen Test with Organic Matter Correction for Predicting Nitrogen Responsiveness of Corn in Rotation

^a Nutrient Management Spear Program, current address: Cornell Cooperative Extension of Lewis County, 5274 Outer Stowe Street, P.O. Box 72, Lowville, NY 13367
^b Nutrient Management Spear Program, Dep. of Animal Science, Cornell Univ., 323 Morrison Hall, Ithaca, NY 14853
^c Nutrient Management Spear Program, Dep. of Animal Science, Cornell Univ., Ithaca, NY 14853
^d Dep. of Crop and Soil Sciences, Cornell Univ., Ithaca, NY 14853
^e PRO-DAIRY, Cornell Univ., Ithaca, NY 14853

^* Corresponding author (qmk2{at}cornell.edu).

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 x 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 x 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 x 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 x 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 (ISNT_Year2 = –12.8 + 1.1 ISNT_Year1, 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 x 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.

Abbreviations: DM, dry matter • EONR, economically optimum nitrogen rate • ISNT, Illinois soil nitrogen test • LOI, loss-on-ignition • PSNT, presidedress nitrate test