Petiole nitrate content of Maine-grown Russet Burbank and Shepody potatoes in response to varying nitrogen rate

Russet Burbank and Shepody potatoes were grown with at-planting nitrogen fertilizer rates varying from 0 to 270 kg ha^?1 following small grains and red clover. Petiole samples were collected from the 4th and 5th leaflets at four to six dates each during the 1986 to 1989 growing seasons. The samples were dried and analyzed for NO ₃ ^? N content. Petiole NO ₃ ^? N levels were strongly related to nitrogen rate regardless of cultivar, growing season, and cropping system. Differences among treatments and NO ₃ ^? N content varied substantially as the growing season progressed with petiole NO ₃ ^? N levels declining rapidly in underfertilized plots as the plants aged. Petiole NO ₃ ^? N levels were higher at midseason following red clover than following small grains. Sampling 50 to 60 days after planting (DAP) is most appropriate as a tool for scheduling supplemental nitrogen applications. At 50 DAP, critical petiole NO ₃ ^? N levels were 1.6 and 1.7% for Russet Burbank and Shepody, respectively. Petiole NO ₃ ^? N levels above 2.2% at 50 DAP resulted in lower yields of Russet Burbank than when petiole NO ₃ ^? N levels were in the 1.6 to 2.2% range. Petiole NO ₃ ^? N testing should be particularly useful as a diagnostic tool in management strategies which make maximum use of previous crop residues, organic amendments, and soil reserves as nitrogen sources.