Impact of Alternative Citrus Management Practices on Groundwater Nitrate in the Central Florida Ridge I. Field Investigation

A research project was conducted to evaluate the impact of alternative citrus nitrogen and water management practices on groundwater nitrate concentrations beneath the vulnerable sandy soils in the ridge citrus region of Central Florida. Fifteen months of baseline data indicated that groundwater nitrate-nitrogen concentrations were above the Environmental Protection Agency’s Maximum Contaminant Level (MCL) beneath mature groves on the Central Florida Ridge. Data from beneath a flatwoods grove off the ridge showed groundwater nitrate-nitrogen levels well below the MCL, and data from beneath a native vegetation site on the Central Florida ridge showed virtually no detectable nitratenitrogen in groundwater. After the baseline monitoring period the following site-specific best management practices (BMPs) were implemented: (1) application of a combination of slow release and dry soluble fertilizer at a rate of 180 kg N/ha/yr split into three applications; (2) application of 18 doses of liquid fertilizer at a rate of 180 kg N/ha/yr applied through a fertigation system; (3) application of 18 doses of liquid fertilizer at a rate of 168 kg N/ha/yr applied through a fertigation system; (4) application of a combination of 18 doses of liquid fertilizer at a rate of 78 N kg/ha/yr through a fertigation system and three applications of foliar spray fertilizer at a rate of 64 kg N/ha/yr (total 142 kg N/ha/yr); and (5) use of irrigation scheduling based on tensiometer measurements to minimize excess leaching. Analysis of 52 months of post-BMP monitoring data indicated that all of these BMPs produced statistically significant downward trends in nitratenitrogen concentration, and all have the potential to meet the EPA MCL for groundwater. The average downward trends ranged from –0.4 to –4.6 mg NO3-N/L-yr, and were greatest for the fertigation/foliar spray BMP, which represented the largest reduction in total N applied.