A number of different diagnosis methodologies have been proposed in the past for the interpretation of plant analysis data. In this paper, a new index (DOP: Deviation from Optimum Percentage) is introduced as a plant nutrition diagnosis tool. The DOP index provides information on the nutritional status both under a quantitative (concentration) and qualitative (balance) viewpoints. For each analysed nutrient the DOP index is calculated by a simple mathematical expression as (Cx100)/Cref)-100; where (C) is the nutrient concentration in the sample to be studied, and (Cref) is the nutrient concentration considered as optimum, both values given on a dry matter basis. The optimal nutritive status for one nutrient or for a nutrient data set is defined by a DOP equal to zero. The DOP index makes possible to know the limitation order among a set of nutrients analyzed. Furthermore, negative and positive DOP indexes will be originated from a defect or an excess, respectively, of the nutrient being studied. The DOP index can be potentially applied for the diagnosis of the nutritive status of any plant species.Trabajo financiado por los proyectos CA-11/86 del CONAI (Diputación General de Aragón) y AGR89-0502 del Programa Nacional Agrícola.Peer reviewed