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NITROGEN MANAGEMENT

Diagnostic Tests for Site-Specific Nitrogen Recommendations for Winter Wheat

Dep. of Soil Sci., 1525 Observatory Drive, Univ. of Wisconsin, Madison, WI 53706-1299

^* Corresponding author (lgbundy{at}wisc.edu).

Received for publication September 16, 2003. 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 (R² = 0.53). The combination of soil NO₃–N content at GS 25 plus N uptake at GS 30 was also strongly related to EONR (R² = 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.

Abbreviations: EONR, economic optimum nitrogen rate • GS, Zadoks growth stage

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