Olive Oil Quality Decreases with Nitrogen Over-fertilization

By R. Fernández-Escobar, G. Beltrán, M. Sánchez-Zamora, J. M. García-Novelo, M. Aguilera, M. Uceda. 2006

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Mature 'Picual' olive (Olea europaea L.) trees growing in two different localities of Cordoba and Jaen provinces, southern Spain, were subjected to annual applications of 0, 0.12, 0.25, 0.50, or 1.0 kg N/tree in the Cordoba's experiment, and to 0 or 1.5 kg N/tree in the Jaen's experiment. Nitrogen was applied 50% to the soil and 50% through foliar application in Cordoba, and 100% to the soil in Jaen. Three years after the initiation of treatments, when the trees showed differences among them in nitrogen content, fruit were sampled at maturity from each experimental tree during six consecutive seasons to determine the effect of nitrogen fertilization on olive oil quality. Tree nitrogen status was always above the threshold limit for defi ciency even in control trees, indicating that most treatments caused nitrogen over fertilization. Nitrogen in excess was accumulated in fruit and, consequently, polyphenol content, the main natural antioxidants, signifi cantly decreased in olive oil as nitrogen increased in fruit. The decrease in polyphenols induced a signifi cant decrease in the oxidative stability of the oil and its bitterness. Tocopherol content, on the contrary, increased with nitrogen application, mainly by an increase in α-tocopherol, the main component in the olive oil. No effect was found on pigment content, particularly carotenoid and chlorophyllic pigments, neither on fatty acid composition. Olives (Olea europaea) are produced for extraction of olive oil for human consump- tion and for processing as table olives. Many factors may affect olive quality, including cultural practices such as fertilization. How- ever, no research has been published on the effect of mineral fertilizers on olive fruit or oil quality. Among the different essential elements, nitrogen is the most commonly applied in olive orchards, as occurs in other fruit tree species, because olives usually need N in greater amounts than other mineral nutrients (Fernandez-Escobar, 2004). With the increased value of the olive crop and the relatively low cost of N fertilization, olive growers have in- creased their application of N fertilizers based on the perception that an increase in N fertil- ization may insure high yields. This practice,

Subfields: Plant Science

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