Evaluation of Soil and Plant Nitrogen Tests for Maize on Manured Soils of the Atlantic Coastal Plain

Abstract

AbstractImproved N management programs for fertilizers and manures are needed in the Atlantic Coastal Plain because of concerns about agriculture's role in groundwater contamination with nitrate‐N. We conducted a 3‐yr study (1990–1992) at 56 locations in Delaware to evaluate the use of soil and plant N tests as components of a manure‐based N management program for maize. In Phase 1 (15 sites), a factorial design was used; main plots were either poultry manure (PM, at 4.5 Mg ha−1) applied at planting or starter fertilizer (SF, at 30 kg N ha−1) and split plots were six rates of sidedress N. Phase 2 was conducted on 41 farms and evaluated the response of maize to sidedress N (0 to 268 kg N ha−1). The pre‐sidedress soil nitrate test (PSNT) and two quicktests (the Nitrachek and Cardy meters) were used to identify N‐sufficient sites in both phases. In Phase 1, we also evaluated the effect of time and depth of PSNT sampling and two plant N tests: the leaf chlorophyll meter (LCM; 1992 only) and the late‐season stalk nitrate test. Maize response to sidedress N was rare at manured sites, and the ratio of total N added as manure and/or fertilizer to maize yield was consistently lower than the commonly used value of 17.8 kg N Mg−1 maize (1.0 pound N bushel−1 maize). Average ratios for manured and nonmanured sites in Phase 1 were 13.0 and 13.2 kg N Mg−1 maize. The PSNT effectively identified N‐sufficient or N‐deficient sites ≈70% of the time. Delaying sampling, deeper sampling, or using NH4‐N + NO3‐N instead of NO3‐N alone did not significantly improve predictive accuracy of the PSNT. The stalk nitrate test was found to be effective at identifying sites that were nonresponsive to sidedress N; our critical range was 0.7 to 2.0 g NO3‐N kg−1. The LCM results were encouraging and suggest that further research should be conducted on its use as an alternative to the PSNT. The Nitrachek meter was better correlated with laboratory measurements of soil NO3‐N (r2 = 0.87***) than the Cardy meter (r2 = 0.60***) and should be an effective means to identify N‐sufficient sites under field conditions.