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Variable Rate Technology As a Tool to Reduce Nitrate Leaching Potential in Irrigated Potato Cropping Systems

Whitley, K.M. 2004. M.Sc Thesis, Washington State University

Abstract

Potato (Solanum tuberosum L.) is a widely grown, high value crop in Washington state Columbia Basin. The combination of coarse textured soils (sandy loam to sand) and hilly topography in this region as well as the high nitrogen requirement of potato have lead to interest in nitrogen management. Topography contributes to the movement of nitrogen (N) in the field by affecting water infiltration and the direction of flow. The landscape is typically hilly in the large (ca. 55 ha) center pivot irrigated fields. The movement of the water, thus nitrate- N, results in a wide range of nitrogen values across a field. Potato growth and development can vary with the nitrogen across the field and can be affected by the excess or shortage of nitrogen created. Site Specific Crop Management (SSCM) and Variable Rate Technology (VRT) are potential tools to develop strategies for nitrogen fertilizer management in these systems. In this two-year study, two adjacent potato fields were selected each year (1999 and 2000). One field was fertilized with variable nitrogen rate while the other was conventionally fertilized, applying a uniform rate across the field based on the field average. Sites for monitoring seasonal changes were selected based on landscape iv position (knoll, side-slope, valley) and soil test organic matter content. In season petiole and soil samples were gathered at two key phenological stages, tuber initiation and tuber bulking. Whole plant samples were collected near the end of the season to determine plant biomass and nitrogen partitioning. Soil moisture was also monitored throughout the growing season. The objectives of this research were to evaluate pre-plant variable rate N fertilizer application as a tool to decrease whole field NO3- N leaching in potato and to assess the affect on potato yield, quality, and N utilization. The results were different by year due to varietal differences. VR pre-plant N fertilizer application showed a reduction in NO3- N leaching potential with the shorter season potato variety. Tuber quality and N utilization were also improved under VR. The longer season varieties likely require in-season and pre-plant VR N applications to prevent variability from redeveloping.