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

Split fertilizer N application has been proposed for improving N uptak e efficiency in wheat (Triticum aestivum L.) production systems, but r esults 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 a pplications based on wheat growth stage (GS) to determine N timing eff ect on wheat yield and apparent fertilizer N recovery. Vegetative samp les were also collected at Feekes GS 4, 6, and 10 to determine treatme nt effects on tissue N concentration during the growing season. Soil s amples were collected after harvest to evaluate residual N quantity an d movement in soil relative to N rate and time of application. The soi l used was a Fluventic Ustochrept-Udic Chromustert intergrade. Nitroge n rate and application timing appeared to have less effect than year o n yield and N parameters. Significant grain yield increases were achie ved with split applications of N fertilizer when N was topdressed at G S 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 appli cation at GS 4 or 6 in 1990. Nitrogen uptake efficiency was greatest w ith 75 kg N ha(-1) and split application. Soil NO3--N concentration wa s significantly higher at 150 kg N ha-L than at 75 kg N ha(-1). Signif icantly lower residual NO3--N with the preplant treatment was not acco unted for by greater grain or draw N content.