Residual Soil Nitrate after Potato Harvest

Nitrogen loss by leaching is a major problem, particularly with crops requiring large amounts of N fertilizer. We evaluated the effect of N fertilization and irrigation on residual soil nitrate following potato ( Solanum tuberosum L.) harvests in the upper St‐John River valley of New Brunswick, Canada. Soil nitrate contents were measured to a 0.90‐m depth in three treatments of N fertilization (0, 100, and 250 kg N ha −1 ) at two on‐farm sites in 1995, and in four treatments of N fertilization (0, 50, 100, and 250 kg N ha −1 ) at four sites for each of two years (1996 and 1997) with and without supplemental irrigation. Residual soil NO 3 –N content increased from 33 kg NO 3 –N ha −1 in the unfertilized check plots to 160 kg NO 3 –N ha −1 when 250 kg N ha −1 was applied. Across N treatments, residual soil NO 3 –N contents ranged from 30 to 105 kg NO 3 –N ha −1 with irrigation and from 30 to 202 kg NO 3 –N ha −1 without irrigation. Residual soil NO 3 –N content within the surface 0.30 m was related ( R 2 = 0.94) to the NO 3 –N content to a 0.90‐m depth. Estimates of residual soil NO 3 –N content at the economically optimum nitrogen fertilizer application (N op ) ranged from 46 to 99 kg NO 3 –N ha −1 under irrigated conditions and from 62 to 260 kg NO 3 –N ha −1 under nonirrigated conditions, and were lower than the soil NO 3 –N content measured with 250 kg N ha −1 We conclude that residual soil NO 3 –N after harvest can be maintained at a reasonable level (<70 kg NO 3 –N ha −1 ) when N fertilization is based on the economically optimum N application.