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C urrently recommended N management in North Carolina and elsewhere in the humid southeastern USA is based on realistic yield expectations (RYE). In North Carolina, N recommendations based on RYE are made from a database of RYE by soil map unit or based on a grower's documented historic yields (the average of the best three yields during a 5-yr period; North Carolina Nutrient Management Workgroup, 2003). The recommended N rate is calculated using the RYE and a soil-and crop-dependent N application factor (18-22.5 kg N Mg −1 corn grain yield). These N fertilizer recommendations are based on decades of fi eld response trials (Kamprath et al., 1973; Kamprath, 1986), but can result in under-or overapplication, in part because they do not consider soil N (Mulvaney et al., 2001). Soils under certain management practices (e.g., manure applications or legume cover crops) or soils with high organic matter content may have suffi cient residual and potentially mineralizable N for optimum corn growth and need little or no N fertilizer. Overapplication of N fertilizer has been linked to high levels of NO 3 in shallow groundwater underneath agricultural fi elds in North Carolina and elsewhere (Gambrell et al., 1974; Jacobs and Gilliam, 1985). The Neuse River Basin in North Carolina has experienced fi sh kills linked to nonpoint sources of NO 3 pollution (North Carolina Division of Water Quality, 1996, p. 44). Agricultural N pollution could be reduced by more effectively determining accurate fertilizer rates, so less N is susceptible to leaching and runoff (Khan et al., 2001). A soil N test to predict corn N need or corn responsiveness to N fertilizer could be used to more accurately predict fertilizer recommendations. The challenge is to develop a soil N test that is accurate, timely, and cost effective for predicting corn yield response to N fertilizer or corn fertilizer N requirement. The climate in the southeastern USA complicates soil N test development because of warm temperatures and rainfall that exceeds evapotranspiration, resulting in rapid mineralization, immobilization, leaching, and denitrifi cation. The preplant and presidedress NO 3 tests have had limited success in estimating corn N need in the humid southern USA (Bundy et al., 1992; Grove, 1992). Soil N tests that measure mineralizable N under aerobic conditions are not practical for predicting yield response to fertilizer because they require a long incubation period (30-60 d; Bundy and Meisinger, 1994). Potential soil tests for predicting fertilizer response in corn would be an anaerobic IRNT (Bundy

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Precise management of nitrogen (N) using canopy color in aerial imagery of corn (Zea mays L.) has been proposed as a strategy on which to base the rate of N fertilizer. The objective of this study was to evaluate the relationship between canopy color and yield response to N at the field scale. Six N response trials were conducted in 2000 and 2001 in fields with alluvial, claypan and deep loess soil types. Aerial images were taken with a 35-mm slide film from C1100 m at the mid-and late-vegetative corn growth stages and processed to extract green and red digital values. Color values of the control N (0 kg N ha -1 ) and sufficient N (280 kg N ha -1 applied at planting) treatments were used to calculate the relative ratio of unfertilized to fertilized and relative difference color values. Other N fertilizer treatments included side-dressed applications in increments of 56 kg N ha -1 . The economic optimal N rate was weakly related (R 2 B 0.34) or not related to the color indices at both growth stages. For many sites, delta yield (the increase in yield between control N and sufficient N treatments) was related to the color indices (R 2 B 0.67) at the late vegetative growth stage; the best relationship was with green relative difference. The results indicate the potential for color indices from aerial photographs to be used for predicting delta yield from which a site-specific N rate could be determined.

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