Critical nitrogen curve and nitrogen nutrition index for potato in easternCanada

Plant diagnostic methods of N deficiency should be based on the definition of a critical N concentration, that is the minimal N concentration required to achieve maximum growth. The critical N concentration (N-c) of potato wa s determined from six on-farm field trials in which two cultivars (Russet B urbank and Shepody) were grown under three or four N rates with and without irrigation. Dry matter yield and N concentration of shoots and tubers were determined approximately weekly during the growing season. The N-c was det ermined by selecting data points for which the highest total biomass compri sed of shoots and tubers was obtained, and by expressing N concentration (N ) as a function of total biomass (W). The N nutrition index (NNI), calculat ed as the ratio between measured N concentration and predicted N-c during t he growing season, was then related to the relative yield measured at harve st. Under non-limiting water conditions, the allometric relationship betwee n N, and total biomass for Russet Burbank (N-c = 4.57W(.0.42)) was differen t from that of Shepody (N-c = 5.04W(.0.42)), indicating that Shepody had a greater N concentration than Russet Burbank. The N concentration of Shepody was also greater than that of Russet Burbank under limiting water conditio ns. These results suggest that there is intra-specific variability in potat o for the relationship between N concentration and biomass. The NNI ranged from 0.50 to 1.4. The relation ship between relative yield and NNI expresse d by a quadratic function accounted for a greater proportion of the variabi lity with irrigation (71% for Russet Burbank and 82% for Shepody) than with out irrigation (65% for Russet Burbank and 32% for Shepody). Our results su ggest that the NNI could be a reliable indicator of the level of N stress d uring the growing season, particularly under non-deficient water conditions .