DIRECT MEASUREMENT OF PETIOLE SAP NITRATE IN POTATO TO DETERMINE CROP NITROGEN STATUS

Rapid determination of crop nitrogen (N) status in the field would facilitate the use of plant tissue testing as a tool for N management in potato (Solanum tuberosum L). The Cardy portable nitrate (NO3) meter may provide the means for on-site determination of petiole sap NO3 (PSN) concentration. Our objective was to establish the relationship between Cardy meter readings of PSN and laboratory measures of petiole dry matter NO3 (PDMN). In a two-year replicated field study in western Montana, 'Russet Burbank' potatoes were grown under a line-source sprinkler irrigation system with N fertilization treatments of 0 N or a rate of 270 kg N/ha applied in four different timing schemes. This created a wide range in crop N and water status. Petiole samples were collected at five or six dates each year. Subsamples were analyzed for PSN in expressed sap using the Cardy NO3-Specific electrode or for PDMN with a benchtop NO3-specific electrode after extraction with a solution of aluminum sulfate [Al2(SO4)3]. Petiole sap NO3 levels were highly correlated (r = 0.956** in 1990 and 0.971** in 1991) with PDMN. No significant differences in intercept or slope of the regression equations were detected between the two years. The combined regression is: where both variables are expressed on a NO3-N basis. Independent calibration from Minnesota agreed very closely with these results. Criteria for the use of PSN in potato N management can therefore be calculated readily from established PDMN criteria.