Response of microsprinkler-irrigated navel oranges to fertigated nitrogen rate and frequency

Microsprinklers allow precise control of irrigation water applications and offer the potential for higher efficiency of water and fertilizer use compared with flood irrigation. A field experiment was conducted during 1999-2002 in central Arizona (AZ) to evaluate effects of various N rates and fertigation frequencies on fruit yield and quality, leaf N concentration, and residual soil N of 'Newhall' navel oranges (Citrus sinensis) on 'Carrizo' citrange (Porcirus trifoliata x Citrus sinensis) rootstock (planted in 1997) grown in a Gilman (coarse-loamy, mixed, superactive, calcareous, hyperthermic Typic Torrifluvents) fine sandy loam. The experiment included nonfertilized control plots and factorial combinations of three fertigation frequencies (27, 9, and 3 applications annually) and three N rates (68, 136, and 204 g N tree^-1 yr^-1). Maximum yields occurred at N rates of 105 to 153 g N tree^-1 yr^-1 for the fourth to the sixth growing seasons. The yield-maximizing N rates were equivalent to 17 to 34% of currently recommended N rates for citrus grown in AZ. Fruit and juice quality did not show significant response to N rate or fertigation frequency. Leaf N concentrations at yield-maximizing N rates were above the critical leaf tissue N range of 25 to 27 mg g^-1, indicating that this range may be too low for these 'Newhall' navel orange trees. During all three seasons, higher residual soil NO₃ concentrations resulted from the highest N rate. Our results suggest that optimum N rates for microsprinkler-irrigated 'Newhall' navel oranges in AZ are lower than currently recommended N rates.