The effect of foliar vs. soil application of urea to olive trees

Most of olive orchards growing in Spain receive excessive applications of N fertilizers (primarily soil application) without any criterion, contributing to groundwater (NO3- leaching) and atmospheric pollution. The experiment was conducted in order to study the effect of the amount and method (soil vs. soil and foliar) of urea application on leaf N concentration, yield, fruit size, tree growth, and oil content of mature olive trees (Olea europaea L.) cv. Picual. After six years of experiment, the results indicate that increasing the amount of N applied annually from 0 to 1 kg of N per tree did not result in an increase of yield, oil content or vegetative growth. However, leaf N concentration was higher when N was applied to both soil and leaves rather than only soil. Results support the hypothesis that annual applications of N fertilizer to olive orchards are not necessary in order to obtain good productivity and growth when leaf N is above the deficiency threshold. Also, N use efficiency seems to increase when urea is applied to leaves.