An accurate and quick soil nitrogen (N) test is necessary for developing soil-based N fertilizer recommendations for corn in the humid southeast United States. The Illinois soil nitrogen test (ISNT) has been used to predict corn responses to fertilizer in Illinois and North Carolina. This study was conducted to determine the relationships between economic optimum nitrogen rates (EONR) and ISNT to develop a fertilizer recommendation. Soil samples were acquired from 31 N response trials located in the Piedmont and Coastal Plain regions of North Carolina from 2001-2004. The ISNT was not correlated with EONR for all sites, but was strongly correlated when sites were classified as well or poorly drained (R2 = 0.87 and 0.80, respectively). A comparison of ISNT and EONR regression models across years showed no year effect which suggests ISNT can predict EONR independently of climate, although drought stressed sites were not analyzed as part of this study. Analysis showed ISNT's ability to predict EONR was not affected by different corn price and fertilizer cost ratios. Because the relationship between ISNT and EONR was robust across years and with different cost ratios, it could be used to modify or replace current N recommendation methods and foster appropriate fertilizer application.
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