Nitrogen best management practice for citrus trees: I. Fruit yield,quality, and leaf nutritional status

Elevated levels of nitrate-nitrogen (NO₃-N) in the surficial aquifer above the drinking water quality standard, i.e. maximum contaminant limit (MCL; 10 mg L⁻¹), have been reported in some part of central Florida citrus production regions. Soils in this region are very sandy (sand content >95%), hence are vulnerable to leaching of soluble nutrients and chemicals below the rooting depth of the trees. The objective of this research was to develop N and irrigation best management practices for citrus in sandy soils to maintain optimal crop yield and quality, and to minimize potential leaching of nitrate below the root zone. Six years of field experiment was conducted in a high productive (mean fruit yield > 80 Mg ha⁻¹yr⁻¹) >20-year-old ‘Hamlin’ orange trees [Citrus sinensis (L.) Osbeck] on ‘Cleopatra mandarin’ (Citrus reticulata Blanco) rootstock grown on a well drained Tavares fine sand (hyperthermic, uncoated, Typic Quartzipsamments) in Highland county, FL. Nitrogen rates ranged from 112 to 280 kg ha⁻¹ yr⁻¹ applied as fertigation (FRT), water soluble granular (WSG), 50:50 mix of FRT and WSG, and controlled-release fertilizer (CRF). Tensiometers were used to monitor the soil water content as a basis to schedule optimal irrigation. Fruit yield response over the entire range of N rates was greater for the FRT and WSG sources as compared to that for the WSG + FRT or CRF sources. Using the regression analysis of the fruit yield in relation to N rate, the optimum N rate appeared to be at 260 kg ha⁻¹ yr⁻¹. Based on fruit production response in this study, the N requirement for production of 1 Mg of fruit varied from 2.2 to 2.6 kg across four N sources. This study demonstrated an increased N uptake efficiency, as a result of best management of N and irrigation applications. The optimal N and K concentration in the 4–6-month-old spring flush leaves were 26–30, and 15–18 g kg⁻¹, respectively. However, fruit yield response showed no significant relationship with concentrations of P in the 4–6-month-old spring flush leaves over a range of 0.8–2.4 g kg⁻¹. The results of fate and transport of N in soil and in soil solution with application of different rates and sources of N, and components of citrus tree N budget, are reported in a companion paper.