Abstract
A survey of drinking water wells conducted in Florida between 1988 and 1991 found that nitrate nitrogen (NO3-N) concentrations in surficial well water near citrus groves on Entisols in central Florida exceeded maximum contamination levels (MCL) of 10 mg L-1. The proportion of wells in Florida contaminated with NO3-N was similar to that of a nation-wide survey; however, the proportion of wells contaminated above MCL was an order of magnitude higher. Eighty-nine percent of wells contaminated above MCL were located in the central Florida counties of Lake, Polk, and Highlands. Citrus in these counties is grown on Entisols which are uncoated sands with low water holding capacities of 0.04 to 0.09 cm3 cm-3. Such sands are particularly vulnerable to nutrient leaching. To reduce potential ground water contamination due to citrus production, best management practices were established and UF/ IFAS publication SP169 was written with a recommended nitrogen (N) fertilizer application upper limit of 240 kg ha-1 yr-1 for mature bearing citrus trees. Prior to publication of SP169, the UF/IFAS annual N rate recommendation was 18 kg ha-1 per 10 Mg ha-1 fruit production based on Bulletin 536D. Publication SP 169 is currently being revised and updated. Based on controlled experiments conducted since 1990, arguments are given for an annual N rate recommendation of 17 kg ha-1 yr-1 per 10 Mg ha-1 yr-1 based on a 5 yr running average of orange [Citrus sinensis (L.) Osbeck.] fruit yield and 24 kg ha-1 yr-1 per 1 Mg ha-1 yr-1 based on soluble solids yield. N rate studies on grapefruit (C. paradisi Macf.) were also reviewed, however, insufficient evidence exits for a size or yield based annual N rate recommendation.
