Developing Nitrogen Best Management Practices to Reduce Risk to Water Quality in Organic Carrot in Florida.

Industry leaders and farmers estimate that within the next three years, carrot acreage in the Suwannee Valley region in north central Florida will exceed 3,200 – 4,000 ha. The carrot industry is currently 1,214 ha, of which 405 ha are certified organic. Pursuant to the Florida Watershed Restoration Act, a two year study of organic carrot was initiated to develop N BMPs that would retain N in the root zone and minimize risk to water quality. The University of Florida’s (UF/IFAS) N recommendation for conventional carrots is 196 kg ha^-1 N. No N recommendations exist for organic systems, specifically. Five N treatments ranging from 168 kg ha^-1 N to 393 kg ha^-1 N in 56 kg ha^-1 increments were randomized and replicated four times in the certified organic unit at the UF/IFAS Suwannee Valley Extension Center near Live Oak, FL. Prior to field preparation, barrel lysimeters were installed under each plot to a depth of 0.81 m to monitor nitrate leaching. A commercially-available poultry litter fertilizer (3%N, 2%P₂O₅, and 3% K₂O) was used. Boron (B) at 2.24 kg ha^-1 was added per pre-plant soil test recommendation. All B and 50% of the N were pre-plant incorporated. Remaining N was applied in two equal applications at 5 and 7 weeks after planting (WAP) by banding between rows and lightly incorporating to a depth of 3.8 cm. A cello-type carrot [Daucus carota L. var. sativus (Hoffm.) Arcang] ‘Choctaw’ was seeded November 14, 2016 on 18.3 m beds (four 16.5 cm row spacing) by mechanical seeder to 459,200 seed ha^-1. Center pivot irrigation was scheduled to meet evapotranspiration losses calculated by a 183 cm Sentec probe (Waterford MI) installed in the center of the trial area. One day prior to harvest, soil samples and aboveground weed biomass were collected from each plot, and lysimeters were pumped yielding samples from four plots. On April 13, 2017 (12 WAP) carrots were harvested within a 3 m section of each plot, graded, and weighed. Carrot root and shoot subsamples were analyzed for root:shoot weight and total N; and together with weed biomass, lysimeter and soil solution data, will be used to estimate the distribution of N within each N rate treatment. Carrot yield was similar among treatments and ranged from 47,636 kg ha^-1 in TRT 2 (224 kg ha^-1 N) to 56,872 kg ha^-1 in TRT 5 (392 kg ha^-1 N) and supported our visual observations of similarities among treatments prior to harvest. Yield in this experiment was similar to commercial growers in our area. While these results are preliminary, evidence suggests that the amount of N required in organic carrot systems is less than published recommendations for conventional carrot systems, and additional N may pose a risk to water quality.