Impact of nitrogen management practices on nutritional status and yield of Valencia orange trees and groundwater nitrate

A field experiment was conducted using 36-yr-old Valencia orange trees (Citrus sinensis) planted in an Astatula fine sand (hyperthermic, uncoated, Typic Quartzipsamments) to evaluate the effects of two different N management practices on the mineral nutrition status of the trees, fruit yield and quality, and nitrate (NO;) loading into groundwater, The N treatments included 180 kg ha(-1) yr(-1) as either (i) three broadcast applications per year of a combination of dry soluble form (DSF; 135 kg N ha(-1) yr(-1)) and controlled-release form (CRF; 45 kg ha(-1) yr(-1)); or (ii) fertigation in 18 equally divided applications per year. Nitrate-N was measured in groundwater sampled at 3-wk intervals using four monitoring wells in each of the two blocks over 2 yr. Fruit yield was greater by 2.7 to 8.2 Mg ha(-1) in the fertigation compared to that in the DSF/CRF broadcast treatment. Leaf nutrient concentrations or fruit quality parameters were not significantly influenced by the different N management treatments, Most of the leaf nutrients were within the recommended optimal ranges. Mean groundwater NO3-N concentrations for the DSF/CRF treatment ranged from 6.5 to 16.5, and 9.4 to 14.1 mg L-1 during 1995 and 1996, respectively. The corresponding values for the fertigation treatment ranged from 7.0 to 14.6 and 1.4 to 6.9 mg L-1. Fertigation with 18 applications per year decreased NO3-N loading into groundwater as compared to three broadcast applications of the same rate of N using DSF/CRF granular blend.