Mature 'Picual' olive (Olea europaea L.) trees growing in the field were subjected to two different experiments, with the aim to optimize the annual practice of nitrogen fertilization in olive orchards. In one experiment, the effect of the amount of N applied and the form of application (soil vs. soil and foliar application) on tree growth and yield was studied. In the second experiment, the application of N fertilizer was based on the previous season's leaf N concentration. After three years of experiment, the preliminary results indicate that increasing the amount of N applied from 0 to 1 kg of N per olive tree did not result in an increase of yield, fruit size, oil content or vegetative growth. However, leaf N concentration seems to increase when N was applied to both soil and leaves, compared with only soil applications. Results also indicate that annual application of N fertilizer to olive orchards is not necessary to obtain good productivity and growth when leaf nitrogen is above the sufficiency threshold. Instead, annual diagnosis of tree N status by leaf analysis may be the best guide for N fertilization.
