Response of Young and Maturing Citrus Trees Grown on a Sandy Soil to Irrigation Scheduling, Nitrogen Fertilizer Rate, and Nitrogen Application Method

This study examined the effect of irrigation rates, nitrogen (N) fertilizer rates, and methods of applying N on growth and productivity of young (3 to 5 years old) and maturing (8 to 10 years old) citrus trees. A long-term study was conducted with the following objectives: 1) to measure the main effects of N rate, N application method, and irrigation on citrus tree growth and production from planting to maturity; 2) to establish growth and production relationships for long-term N rates and irrigation on well-drained sandy Entisols; and 3) to determine the effect of split fertilizer applications at two soil moisture regimes on citrus growth and production for two tree age classes as trees mature. Irrigation was applied using two selected ranges of soil moisture tensions and annual N rate varied by tree age as percentages of recommended. Methods of applying N included a dry granular fertilizer (DGF) containing soluble N applied four times annually or a controlled-release fertilizer (CRF) applied once per year and fertigation applied either four (FG04) or 30 (FG30) times annually. Canopy size and yield were higher with the moderate irrigation rate compared with the low rate for both young and maturing trees. Critical N rates for both canopy volume and yield were between 178 and 200 kg·ha −1 . The CRF and FG30 treatments produced larger trees and higher yields compared with FG04 and DGF in the young tree study, indicating that younger trees benefitted from frequent split fertilizer applications. As the trees matured and filled their allocated space, the two irrigation rates were continued and N was applied at six rates using either DGF or FG30. For these 8- to 10-year-old trees, critical values of N application rates were 210 and 204 kg·ha −1 for DGF and FG30, respectively. The absence of a significant interaction between N rate and application method indicated that N uptake efficiency was similar for all application methods tested. DGF and FG30 treatments resulted in similar maturing tree yields and fruit total soluble solids. Canopy volumes, for the same trees, were significantly greater all 3 years with the FG30 treatment compared with DGF. Thus, if increase in tree size is desired, increased number of split applications will likely promote tree growth; however, little increase in fruit yield may be obtained.