Nitrogen BMP for citrus to minimize potential nitrate contamination of groundwater

The overall objective of this project is to develop best management practice for the fertilization of bearing and non-bearing citrus trees to minimize nitrate contamination of groundwater without adversely affecting the fruit production. Specific objectives include: (1) Determine the N requirement for optimal tree growth and fruit production under best management practices of scheduling and application of fertilizer and irrigation. (2) Determine the relative proportion of total N requirement for mature citrus trees contributed from the N mineralized from decomposition of leaf and root debris under the canopy. APPROACH: Field experiments will be conducted on non-bearing and bearing citrus trees with various N rates, sources, and delivery system. Irrigation scheduling will be based on tensiometers to restrict the depth of wetting within the rooting depth. Suction lysimeters will be used to sample soil leachate at various depths within and below the predominant depth of rooting to evaluate the leaching of nitrate below the rooting depth. Tree growth, fruit yield, and juice quality response will be evaluated. In situ N mineralization experiments will be conducted to determine the rate of N mineralization in two predominant citrus production systems. — PROGRESS: Field experiments are continuing to develop nitrogen best management practices (BMP) for bearing orange trees in sandy Entisols which are vulnerable for nutrient and chemical leaching. The nitrogen BMPs have to be economically feasible and satisfy water quality consideration, i.e., minimize nitrate leaching into groundwater. Under optimal irrigation scheduling and precision placement and timing of fertilizer application, the results up to this point show that the nitrogen requirement for highly productive grove conditions was 2.50 kg per 1 Mg of fruits (fresh weight basis). This is equivalent to 202 kg N/ha/yr for groves withfruit yield potential as high as 80.6 Mg/ha. Leaf to nitrogen concentration was maintained within the optimal range (25 to 27 g/kg) with nitrogen application greater than 168 kg/ha/yr. A parallel field experiment also showed that nitrate concentration in the groundwater was lowered byadaptation of fertigation with optimal irrigation management.