Predicting nitrogen fertilizer requirements of potatoes in Atlantic Canadawith soil nitrate determinations

Nitrogen greatly affects potato (Solanum tuberosum L.) yield, but excess N can result in environmental degradation. In this study soil nitrate (NO3-N) content was determined pre-plant to predict fertilizer N requirements of p otatoes in Atlantic Canada and in mid-season to adjust N fertilization duri ng the growing season. Soil NO3-N contents were measured to a 0.30-m depth in spring prior to planting at four on-farm sites in each of 3 yr (1995 to 1997) in the upper St. John River Valley of New Brunswick, Canada. Mid-seas on soil NO3-N contents at a 0-0.30 m depth were also determined (32-47 days after planting) at two sites in three N treatments in 1995 (0, 50, and 250 kg N ha(-1)) and in four N treatments in 1996 and 1997 (0, 50, 100, and 25 0 kg N ha(-1)). The yield response of potatoes to six rates of N fertilizat ion (0-250 kg N ha(-1)) with and without supplemental irrigation was used t o determine the economically optimum N application (Nop). The pre-plant spr ing soil NO3-N test alone could not adequately predict the N requirements o f potatoes in Atlantic Canada; the Nop and relative yield were poorly corre lated (0.07 < R-2 < 0.52) with spring soil NO3-N content. The midseason soi l NO3-N test, however, could be used to determine the need for supplemental N fertilizer; NO3-N content correlated well (0.44 < R-2 < 0.68) with the r elative yield for total and marketable yield. We suggest a critical mid-sea son value of 80 mg NO3-N kg(-1) soil for marketable yield, above which addi tional N application might not be necessary.