NITROGEN MANAGEMENT FOR VEGETABLES AND CITRUS - SOME ENVIRONMENTAL CONSIDERATIONS

Much of Florida's water-quality emphasis for plant nutrients has cente red around phosphorus, though nitrate-N has been reported at high leve ls throughout traditional citrus areas of the state. This paper review s the magnitude of nitrate-accumulation trends beneath some peninsular Florida croplands. In one state-wide domestic-wellwater survey, 85% o f the high-nitrate wellwaters were located in the two central-Florida counties with the greatest historical citrus production. Findings of t he Lake Manatee Demonstration Project for shallow-watertable (Flatwood s) sites consistently demonstrate rapid dissipation of any nitrate-N ' 'pulses'' which reach the watertable following fertilization and/or ra infall events (in the case of non plastic-covered beds). For deeply-ro oted citrus, on the other hand, high nitrate-N levels commonly persist , to considerable depths within the watertable. Denitrification is sho wn to be a feasible mechanism for explaining such site-specific differ ences, by comparing the dissipation rates at shallow watertable sites with values reported in the literature for similar denitrifying system s. Best Management Practices (BMPs) to assist with nitrate-loss reduct ions for both vegetables and citrus are discussed. Watertable control at suitable depths is a key ingredient for vegetable BMPs, in part to minimize specific-gravity-driven fertilizer ''dropout''. Increased num bers of annual fertilizer-application ''splits,'' plus ability to cont rol fertilizer application from tree to tree in groves with appreciabl e numbers of young-tree resets, are key factors with respect to citrus BMPs.