Ground Water Quality Irrigation- and Nitrogen-Management Impacts on Nitrate Leaching under Potato J. T. Waddell,* S. C. Gupta, J. F. Moncrief, C. J. Rosen, and D. D. Steele ABSTRACT As potato (Solanum tuberosum L.) production increases in the North-Central Region of the USA, so does the potential for deep seepage of nitrogenous compounds into the groundwater. The objec- tives of this 2-yr study were to determine howdifferent irrigation schemes(sprinkler and drip), irrigation triggers (70 and 40% of available soil water [AWl remaining), drip placement (at the soil surface or buried at 25-cm depth), andvarious N sources (urea, sulfur- coated urea [SCU], and turkey [Meleagris gallopavo] manure) and timings (three- vs. five-N splits) affect percolation andNO3 leaching. As expected, water percolation was generally higher fromthe sprin- kler-irrigation than from the drip-irrigation treatments. Withinthe sprinkler irrigation, percolation was higher whenirrigated at 70% than at 40% of AW remaining. Small but frequent irrigation in drip treatments helped reduce water percolation. Withinirrigation treat- ments, 70% AW had the most N leaching, followed by 40% AW and the drip, the last two treatmentsbeing about the same. The trend in N leaching among fertilizer treatmentswassimilar for various irrigation methods.Splitting N applications five times vs. three times reduced N leaching fromunforeseen rains. Sulfur-coated urea reduced Nleach- ing, whereas turkey manure-amended treatments showed no signifi- cant difference in N leaching compared with the urea-N treatment. In conclusion, alternatives such as 40% deficit irrigation, five-N appli- cation splits, drip irrigation, S-coated urea, and turkey manure not only reduce N leaching but also have a minimal impact on potato tuber yield and tuber quality. I N RECENT YEARS, ground water quality under sandy outwash soils of the Upper Midwest has been im- paired because of the presence of NO3 and pesticide concentrations exceeding the drinking-water standards (Klaseus et al., 1988). Myette (1984) showed that concentrations in ground water have been steadily in- creasing under intensively farmed sandy outwash soils of central Minnesota. Policymakers and producers are increasingly looking for alternatives to current fertilizer- and irrigation-management practices that will reduce ground water contamination of these sandy soils without significantly impacting crop yields. Sandy outwash regions of central Minnesota were once thought unproductive for crop production because the soils have low soil-water holding capacity and rapid drainage. In addition, precipitation in the area during crop growth is not enough to sustain crop production. Shallow ground water in the region provides an inexpen- sive source of water for irrigation. As a result, increased J.T. Waddell, S.C. Gupta, J.F. Moncrief, and C.J. Rosen, Dep. of Soil, Water & Climate, Univ. of Minnesota, St. Paul, MN 55108; and D.D. Steele, Agricultural Engineering Dep., North Dakota State Univ., Fargo, ND 58105. Received 9 Dec. 1998. *Corresponding author (waddeje@tetratech-fix.com). Published in J. Environ. Qual. 29:251-261 (2000). land has been brought under crop production in this region, especially for high-value crops such as potato and other vegetables. Typically, irrigation in the area is applied with the center-pivot overhead sprinkler system. Efforts have been underway to find irrigation- and N-management practices that can minimize NO3 leach- ing without significantly affecting crop yield. Oneac- cepted method to reduce N leaching is to apply enough irrigation to bring the soil to field capacity. Sexton et al. (1996) showed that even with the right amount irrigation, significant NO3 leaching occurs when summer thunderstorms comesoon after irrigation or fertiliza- tion. Another option within the above scenario is to allow the soil to become drier betweenirrigation events, thus providing additional soil-water storage to retain unforeseen rains (Saffigna et al., 1977). However, water stress has been shown to affect both the tuber size distri- bution and tuber yield, especially for the ’Russet Bur- bank’ potato variety (Miller and Martin, 1987). Another method to reduce N leaching is to apply irrigation water only in regions of the soil where roots exist, thus leaving space between rows to capture unforeseen rains. Drip irrigation (and subsurface drip irrigation in particular) is capable of applying water, fertilizer, or both where roots are concentrated (Phene et al., 1994; Rolston et al., 1979; Phene and Sanders, 1976). The use of drip irrigation has been studied in manyparts of the world (Camp, 1998). In general, the technology has been viewed as a means to increase crop water use efficiency in regions where water is limiting (Phene et al., 1992). In somecases, drip irrigation has also been used to manage nutrients and pesticides in crop production (Phene et al., 1979). Regardless of the end use, there are benefits of reduced fertilizer and other chemical leaching in drip-irrigated fields. How- ever, the use of drip irrigation in areas such as the Upper Midwest, where summer storms are erratic but the water is plentiful, has not been tested extensively. Besides irrigation, other methods to reduce N leach- ing include modifyingcurrent N-fertilization practices. Often, the majority of N is applied to the potato crop at planting. This maximizes the potential for NO3 leaching, not only from unforeseen rains but also from regular irrigations. Splitting N applications over the growing season such that they are synchronized with plant N demands not only fulfill plant N needs, but at the same time reduce N leaching from unforeseen rains (Joern Abbreviations: AW, available soil water; ET, evapo-transpiration; SD, surface drip irrigation; BD, buried drip irrigation; BDF, buried drip irrigation with fertilizer injected through tape; SCU, sulfur- coated urea. 251 Published January, 2000