Nitrogen management for high yield and quality of citrus in sandy soils

We are in the process of developing N best management practices for citrus in sandy Entisols to increase N uptake efficiency and to minimize NO3 leaching below the rooting depth. We evaluated the effects of N sources as either dry granular form (DGF), fertigation (FRT), or controlled-release fertilizer (CRF), and rates (56-280 kg N ha(-1) yr(-1)) with optimum irrigation management on: (i) the fruit yield and qualify and leaf nutrient concentrations of bearing 'Hamlin' orange [Citrus sinensis (L.) Osbeck] trees on 'Cleopatra' mandarin (Citrus reticulata Blanco) rootstock (planted in 1974) grown in a Tavares fine sand (hyperthermic, uncoated Typic Quartzipsamments) and (ii) changes in NO3-N status in soil solution sampled from the vadose zone both within and below the root zone. For the DGF and FRT sources, 3-yr mean fruit yield increased with an increase in N rates in the range of 112 to 224 kg ha(-1) yr(-1) For further increase in N rate, the fruit yield showed a diminishing response, which was quite marked for the DGF. The N requirements for 1 mg of fruit, mean across all N rates, were 1.79,2.58, and 2.75 kg N for the CRF, FRT, and DGF sources, respectively. Leaf N concentrations were maintained within the optimal range (25-27 g kg(-1)) at N rates greater than or equal to 168 kg ha(-1) yr(-1), regardless of source of N. Various N rates and sources did not significantly affect the fruit size, nor juice quality parameters. The improved management of irrigation and fertilizer placement and timing of application contributed to minimized leaching of NO3-N below the root zone.