Diagnostic tests for site-specific nitrogen recommendations for winter wheat

Wheat (Triticum aestivum L.) yields can be limited by both inadequate and excessive N availability. This study evaluated several diagnostic tests for predicting the economic optimum N rate (EONR) for winter wheat at 21 site-years. Tests included soil NO3 (90 cm) at three sampling times [preplant, predormant, and at Zadoks Growth Stage (GS) 25-five tillers], UV absorbance of NaHCO3 soil extracts (30 cm) at preplant, and plant N concentration, N uptake, and chlorophyll meter measurements taken at GS 30 (pseudo stem initiation). All samples were obtained from the control plots (no N fertilizer). Yield response to applied N fertilizer was positive for 13 site-years (EONR of 34 to 168 kg ha(-1)), negative for three site-years, and not significant for five site-years. Nitrogen additions to sites with high soil NO3 levels resulted in yield reductions up to 30%. The strongest relationship among diagnostic tests and EONR was preplant soil NO3 content at the 90-cm depth (R-2 = 0.53). The combination of soil NO3-N content at GS 25 plus N uptake at GS 30 was also strongly related to EONR (R-2 = 0.58). Nitrogen rate recommendations for wheat adjusted for preplant soil NO3 contents >56 kg ha(-1) avoided excessive N applications at 11 of 21 sites, thereby increasing economic gains by $1 to $110 ha(-1) (median $17) due to reduced fertilizer costs and avoiding yield reductions due to excessive N availability. Wheat N recommendations based on preplant soil NO3-N measurements in the 0- to 60-cm depth have potential for avoiding reduced profits, lower yields, and environmental risks associated with excess N use.