SEASONAL NITROGEN BUDGETS OF MATURE CITRUS TREES ON A SANDY ENTISOL

Approximately 30% of Florida citrus is grown on well-drained Entisols with low nutrient-holding capacity, which are prone to high nitrogen (N) leaching losses. However, increasing application frequency of N-fertilizer via multiple fertigations does not increase crop yield, whereas in agronomic crops, such an approach typically enhances N uptake efficiency. We assessed seasonal tree N tissue concentration dynamics as affected by N rate for mature fourteen-year-old 'Hamlin' orange (Citrus sinensis L. Osbeck) trees on either Carrizo citrange (C. sinsensis L. Osbeck X Poncirus trifoliata L. Raf.) or Swingle citrumelo (C. paradisi Macf. X P. trifoliata L. Raf.) rootstocks. Nitrogen was applied as ammonium nitrate in six split fertigation applications with N target values of 179 and 269 kg ha(-1)yr(-1). Leaf, twig, and branch bark tissue N concentrations decreased through the spring to minima in May and June. This time period corresponds to a period of high N demand associated with both vegetative and reproductive growth. Tissue N concentrations increased from late spring minimums to fall and winter maximum concentrations. Reduction in branch bark and wood tissue N concentrations may have been due to a redistribution of N to leaf, twig, and fruit tissues in response to low N supply. The majority of the spring N should be supplied prior to May.