| A series of field experiments was conducted to evaluate three plant tissue tests and three soil testing approaches for estimating the need for supplemental nitrogen (N) fertilization of 'Tyler' winter wheat (Triticum aestivum L. em Thell) in Pennsylvania. Delayed N fertilizer applications, used to facilitate tissue testing, were also evaluated in eight of these experiments. Tissue test parameters that were measured between Feekes growth stage (GS) 3 and 6 included stem nitrate concentration, whole-plant Kjeldahl-N concentration, and crop N uptake. Soil testing approaches evaluated were deep (100 cm) soil nitrate levels, determination of an aerobic N mineralization potential, and a mass balance approach. Of the plant tissue tests, whole-plant N concentration was the best predictor of yield-reducing N deficiency, had the lowest spatial variability, and appeared to be best suited for the current wheat management system in Pennsylvania. The whole-plant N concentrations necessary to prevent yield reductions, or critical levels, were 39.0, 35.0, and 26.5 g N kg('-1) for GS 4, 5, and 6, respectively. The stem nitrate tissue test was to be most sensitive to short-term changes in the soil N supply and appeared best suited for early season crop monitoring systems. The critical level for the stem nitrate test, 2.25 g NO(,3)('-)-N kg('-1) did not fluctuate more than 10% between growth stages. The N uptake test was the poorest predictor of N deficiency, had the largest spatial variability, and was affected by factors that limited crop dry matter production. Soil nitrate was a poor indicator of N requirements because overwinter leaching apparently caused substantial nitrate losses. The N mineralization potentials were correlated with crop N uptake but were laborious to estimate. The mass balance approach appeared to have the most potential for improving current recommendations. Several important components in a mass balance equation for winter wheat in Pennsylvania were quantified in this study: a 130 kg ha('-1) crop N uptake at maximum yields, a fertilizer efficiency of 0.50, and estimates of soil N-supplying capabilities for the three current N recommendation categories. Early (GS 3) fertilizer applications produced higher yields and were more profitable, on the average, than GS 5 applications. |
