EVALUATION OF SOIL AND PLANT NITROGEN TESTS FOR MAIZE ON MANURED SOILS OF THE ATLANTIC COASTAL-PLAIN

Improved 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 s oil and plant N tests as components of a manure-based N management pro gram for maize. In Phase 1 (15 sites), a factorial design was used; ma in 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 Ni trachek and Cardy meters) were used to identify N-sufficient sites in both phases. In Phase 1, we also evaluated the effect of time and dept h 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 ad ded 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 bush el(-1) maize). Average ratios for manured and nonmanured sites in Phas e 1 were 13.0 and 13.2 kg N Mg-1 maize. The PSNT effectively identifie d N-sufficient or N-deficient sites approximate to 70% of the time. De laying sampling, deeper sampling, or using NH4-N + NO3-N instead of NO 3-N alone did not significantly improve predictive accuracy of the PSN T. The stalk nitrate test was found to be effective at identifying sit es 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 th at further research should be conducted on its use as an alternative t o the PSNT. The Nitrachek meter was better correlated with laboratory measurements of soil NO3-N (r(2) = 0.87**) than the Cardy meter (r(2) = 0.60**) and should be an effective means to identify N-sufficient sites under field conditions.