Best nitrogen and irrigation management practices for citrus production in sandy soils

Drinking water monitoring data have indicated an increase in nitrate-nitrogen (NO3-N) concentration in groundwater in some parts of the citrus production region of Florida. A proactive, incentive-based program of developing crop-specific best management practices (BMP) began with the Florida N-BMP legislation passed in 1994. A combination of careful irrigation and nitrogen (N) management is needed to improve N uptake efficiency and to minimize potential leaching of nitrate (NO3-N) to the groundwater. An improved N management practice is considered as a BMP, only if that practice is proved to decrease NO3-N leaching into groundwater in commercial groves without adversely impacting the economics of production. Therefore, long-term evaluation of horticultural responses as well as monitoring of groundwater NO3-N were conducted in five commercial groves representing different soil types, citrus variety and rootstock, tree age, and cultural practices to determine the impact of changes in N management and/or irrigation scheduling. Groundwater NO3-N, leaf nutrient concentrations, fruit yield and fruit quality were monitored for 15 months under the growers' routine management and, subsequently for 48 months, with improved N and irrigation management practices. The N management practices evaluated in this study included broadcast application of a combination of water soluble and slow release granular products, fertigation, and a combination of foliar application and fertigation. Irrigation management was improved by using tensiometer set point of 10 and 15 cbar. This article presents the fruit yield, and concentrations of N, P, K in six-month spring flush during the study period. The study showed that 5 to 8 yr old Valencia trees on Volkamar lemon rootstock produced high quality fruit in the range of 59 to 81 Mg ha(-1) with 168 kg N ha(-1) as fertigation combined with improved irrigation scheduling. Fruit yield of 36 yr old Valencia orange trees on Rough lemon rootstock was greater with application of 180 kg N ha(-1) yr(-1) as fertigation compared to that of the trees which received a similar rate of N as three broadcast applications of granular product. Fertilizer program comprising three foliar applications of N using low biuret urea to deliver 66 kg N ha(-1) yr(-1) and an additional 76 kg N ha(-1) yr(-1) as fertigation was the most effective for decreasing the surficial groundwater NO3-N while maintaining optimal fruit production and nutritional status of the leaves. This study demonstrated that economically and technically feasible N-BMPs can be developed for citrus grown on sandy soils with a combination of improved N management and irrigation scheduling.