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Measuring the nitrogen nutrition status of plants is useful for nitrogen fertiliser management. As nitrogen is one of the main structural components of chlorophyll, its nutrition status is highly correlated with the greenness of leaves. This paper proposes and evaluates a new low-cost method to estimate the N-nutrition status of plants using digital colour image analysis. A method has been developed in which principal component analysis is applied to digital images to calculate a greenness index using RGB components of the colour image, which yields an estimate of the amount of N in the plant. To evaluate its quality, we calculated the correlation between the index and measurements obtained with a SPAD-502 chlorophyll meter, normally used in decision-making in fertiliser management. The performance of the proposed index is better than that of others previously investigated. Furthermore, the capacity of our index …