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Nitrogen is the most important nutrient in growing potatoes (Solanum tuberosum L.) because either a deficit or excess in its supply can affect yield and quality, resulting in economic losses. To make a rational use of this nutrient, it is necessary to monitor N status during crop development. The green index (IV) could be a valuable method to assess N sufficiency. In experiments with variable N supply, the IV was assessed during crop growth and an N sufficiency index was derived (ISN-IV). The relationship between tuber yield and the IV and ISN-IV was established, as well as the association between the IV and ISN-IV and N concentration in the following varieties: Innovator, Bannock Russet and Gem Russet. During the vegetative growth stage, the IV and ISN-IV were respectively above 40 SPAD units and 93%; while N concentration in the plant was above 4% in the three varieties, which suggests the N supply was sufficient. During the tuber-bulking stage, the necessary IV and ISN-IV thresholds to reach maximum yields were 40.6 and 35 SPAD units and 93.7 and 91% at 89 and 103 days after planting, respectively. We concluded that the chlorophyll meter is an appropriate tool to determine the nutritional status in potato crops. The IV and ISN-IV values showed that the crop achieved maximum yields when N levels are above the threshold values found in this research.