The effects of irrigation and nitrogen management on potato tuber yield, N recovery and leaching in Northeast Florida

Nitrate leaching from agricultural fields under potato production in northeast Florida is a potential water quality concern in the St. Johns River watershed. A 3-year study was conducted to investigate the effect of an alternate seepage irrigation method and a controlled-release N source, polymer sulfur coated urea (PSCU), on potato tuber yield, crop N recovery and N leaching loss into the shallow water table. The experimental plots were arranged in a split-split-plot design. The whole plot factors were two irrigation treatments: traditional seepage irrigation (TSI) and intermittent seepage irrigation (ISI). A factorial design with two N sources (PSCU and urea) and two N rates (168 and 224 kg ha-1) constituted the split plot factors, where the subplots included two potato cultivars (Atlantic and Fabula). The average total and marketable yields in 3 years were 31.9 and 25.2 Mg ha-1, 32.1 and 28.5 Mg ha -1, and 22.4 and 16.4 Mg ha-1, respectively. Compared with the 20-year (1990 to 2009) average yield (28.3 Mg ha-1) in Florida, yield in 2007 was similar yield while yields in 2006 and 2008 were lower. In both 2006 and 2007, both N factors had little effect on tuber yields and crop N recovery. In 2008, a "leaching rainfall" (9 cm in 3 days) occurred 2 days after planting, which resulted in a lower N recovery, lower tuber yield, and higher N leaching loss compared with 2007. Higher marketable and total yield were produced with PSCU compared with urea in a single fertilizer application. There was no benefit of higher N rate in increasing tuber yield. Based on this study, we concluded PSCU has a potential to improve tuber yield, even with the occurrence of a leaching rainfall in the spring season. Also, increasing N rate from 168 to 224 kg ha-1 did not benefit tuber yields, but increased the potential of N leaching losses. Overall, the ISI system successfully reduced water use by 59%, 50% and 43% compared with TSI method in three experimental years. In 2006, the TSI had a better impact on potato tuber yield than the ISI. Potato tuber yield was maintained by ISI treatment in 2007 and increased in 2008. Irrigation strategy was critical in minimizing nitrate leaching under ISI. In the first 2 years, irrigation water was supplied at night for 12 hours whereas, the irrigation schedule was changed to supply water during the day for 12 hours in the last experimental year. Nitrate concentrations in the shallow water table were minimized by supplying irrigation during the day due to less fluctuation of the water table depth under ISI.;Another 3-year study was conducted to investigate the feasibility of fertigation method for potato production. Five N rates (0, 112, 168, 224, 280 kg N ha-1) were used as the whole plot factor in a split-plot design, while the split plot factor was two potato cultivars (Atlantic and Fabula). An average of 28.8 cm irrigation water was applied by drip throughout 3 experimental years, compared with 45 to 50 cm of the average irrigation applied with seepage irrigation systems. Water use was reduced 35 to 42% of that use by seepage irrigation. However, potato marketable yields were not maintained when the UF-IFAS recommended N rate (224 kg N ha-1) was applied. The undesirable yields were resulted from the late application of fertilizers and water through drip, which was because drip tapes could not be installed until potato emerged. Therefore, a booster dose of fertilizer at planting to meet the nutrient requirement and establishment of potato plants is probably necessary to overcome the delayed fertigation problem.