Are Florida's Citrus BMP N Rates Sufficient for the Highest Yields?.

Profitable citrus production in Florida depends on substantial fertilizer inputs, particularly of the nutrient elements N and K. Historically, nitrate leachates have entered the shallow groundwater found below the sandy soils of the Lake Wales Ridge citrus growing area in central Florida, which prompted the development of the Nitrogen Best Management Practices (N-BMPs) for the region. For mature bearing round oranges, the BMPs state that Total N fertilizer inputs shall not exceed 269 kg/ha/year for trees > 7 years old unless justified by a prior three year production average of 70.56 Mg/ha for early and mid-season oranges and 50.4 Mg/ha for late season ‘Valencia’ oranges. In those cases a maximum N rate cap of 302 kg/ha/year is permitted. In this study, the yield response, leaf N concentrations and nitrate leachate losses of high yielding ‘Hamlin’ oranges were evaluated at five N rates ranging from 157 to 336 kg/ha/year in order to validate the maximum N rate caps for the BMPs. Optimum N rates were about 246 kg/ha/year for fruit production during two years when over 90.7 Mg/ha yields were achieved. Higher N rates resulted in diminishing yield returns and high to excessive N concentrations in the leaf tissue and soil leachates. These results suggest that the current BMP N rate caps are adequate to support the highest fruit yields and that under normal circumstances, only 81% of the maximum BMP rate would be required even for the highest yields.