NITROGEN-FERTILIZATION TIMING EFFECT ON WHEAT PRODUCTION, NITROGEN UPTAKE EFFICIENCY, AND RESIDUAL SOIL-NITROGEN

Split fertilizer N application has been proposed for improving N uptake efficiency in wheat (Triticum aestivum L.) production systems, but results have been inconsistent. In this field study, 75 and 150 kg N ha(-1) was applied either all preplant or in two, three, or four split applications based on wheat growth stage (GS) to determine N timing effect on wheat yield and apparent fertilizer N recovery. Vegetative samples were also collected at Feekes GS 4, 6, and 10 to determine treatment effects on tissue N concentration during the growing season. Soil samples were collected after harvest to evaluate residual N quantity and movement in soil relative to N rate and time of application. The soil used was a Fluventic Ustochrept-Udic Chromustert intergrade. Nitrogen rate and application timing appeared to have less effect than year on yield and N parameters. Significant grain yield increases were achieved with split applications of N fertilizer when N was topdressed at GS 4 or 6 in 1989 as compared with all preplant or application at GS 10. Split N application at GS 10 produced greater grain yield than application at GS 4 or 6 in 1990. Nitrogen uptake efficiency was greatest with 75 kg N ha(-1) and split application. Soil NO3--N concentration was significantly higher at 150 kg N ha-L than at 75 kg N ha(-1). Significantly lower residual NO3--N with the preplant treatment was not accounted for by greater grain or draw N content.