Several plant-based diagnostic methods have been developed over the last 20 years. These methods use either whole plants or specific plant parts (e.g. leaf or petiole). The concept of a critical N curve (Nc), based on the N concentration of whole plants for increasing plant biomass during the growing season, has been successfully applied in eastern Canada to annual crops including potatoes. The critical N curve is then used to calculate the N nutrition index (NNI) as the ratio between the measured N concentration of the plant biomass and the predicted N_c. A major difficulty in using the NNI at the farm level, however, is the need to determine the actual plant biomass and its N concentration. For this reason, it may be more practical to use the NNI as a reference for calibration of simpler procedures (e.g. petiole nitrate concentration, light transmittance or reflectance) to determine the potato N status. Petiole nitrate concentration is one of the most widely used diagnostic tools to assess potato N sufficiency. Critical values or ranges of petiole nitrate concentrations have been suggested for potatoes in several producing areas of the world and they were mostly established using the relationship with tuber yield. A novel approach to defining critical petiole concentrations was proposed in which the NNI is used as a reference. Measurements of light transmittance (e.g. SPAD-502 meter) or reflectance can also be used to quantify plant N sufficiency with the main advantage of not requiring plant sampling. Using the NNI to calibrate those light measurements might provide a valuable approach to determine critical values. This has never been tested in potatoes but examples of this approach in corn and wheat will be presented.