Olive & Oil 31/01/2016

How, when and why to fertilize the olive tree with nitrogen and potassium

Nitrogen and potassium are the two strategic minerals for growth and good fruiting olive tree but it is necessary to know how to give them and especially when to make the most of their power nutritious

Two experiments were conducted for 13 years in two olive groves of southern Spain to study the long-term effect of nitrogen (N) fertilization on trees and soil. In the first experiment, 12-year-old ‘Picual’ olive trees were arranged in a split plot design with method of N application (soil versus a 50% soil:50% foliar combination) as the whole plot factor, and amount of N applied annually (0, 0.12, 0.25, 0.5 or 1.0 kg N tree−1) as the subplot factor. In the second experiment, N application to 50-year-old ‘Picual’ trees was based on the previous season's leaf N concentration.

Urea was the source of N in both experiments. During the last 4 years, soil samples were taken at 0–20, 20–40, 40–60, 60–80, and 80–100 cm depth to evaluate the effect of N application on soil eutrophication. Fertilization with N had no significant effects on yield, fruit characteristics, and growth of olive trees for the 13 years of study, even when leaf N concentration increased with the amount of fertilizer N applied. Combining soil and foliar application may reduce the amount of fertilizer N necessary to correct a possible N deficiency because our experiments showed this practice to be more effective in increasing leaf N that applying N only to the soil. Our results question the established deficiency threshold of 1.4% of N in dried leaf because no reduction in yield or growth was observed for lower concentrations. However, leaf N concentration did not drop below 1.2% after 13 years with no N application, probably because of N inputs from rainfall and the mineralization of organic N. Whereas under natural conditions of the non-fertilized treatments NH4+–N represented the dominant fraction of mineral N in soil, accumulation of high amounts of NO3−–N in the soil profile occurred in the fertilized plots, which represents a high risk of N leaching from soil.

All these results suggest that annual applications of fertilizer N are unnecessary to maintain high productivity and growth in olive. Applying N only when the previous season's leaf analysis indicates that leaf N concentration is below the deficiency threshold, is thus a recommended practice to optimize N fertilization in olive orchards and to reduce N losses by leaching.

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Experiments under both greenhouse and field conditions were performed to study the influence of the type of salt applied (KCl or K2SO4) and to compare the effectiveness of their method of application (soil versus foliar) on leaf K concentration and yield in olive trees established under rainfed conditions.

In the experiment performed in the greenhouse, mist-rooted 3-month-old ‘Picual’ olive plants growing in 2-l pots containing perlite were irrigated with a complete nutrient solution containing either 0.05 or 2.5 mM KCl during 128 days after transplanting (DAT). A group of plants received one K soil application at a dose equivalent to 1 kg of K per tree at 63 DAT. Another group received four foliar applications at 63, 78, 93 and 108 DAT with a K concentration of 10.5 g/l. The experiment under field conditions was developed with 80-year-old ‘Hojiblanca’ olive trees. Soil application was performed in March every year at a rate of 1 kg K/tree by injecting K fertilizer around the trees. Foliar sprays were carried out in April, March, June and July every year with the same K concentration used for the greenhouse experiment. The results showed that, under both field and greenhouse conditions, either KCl or K2SO4 increased K concentration in leaves.

Also, both K salts had a positive effect in increasing K content in fruits under field conditions, although the intensity of the increase varied among years.

Tree water stress and low K status seemed to influence K uptake.

The method of K application, soil or foliar, did not affect the effectiveness of K fertilization.


R. Fernández-Escobar, L. Marin, M.A. Sánchez-Zamora, J.M. García-Novelo, C. Molina-Soria, M.A. Parra, Long-term effects of N fertilization on cropping and growth of olive trees and on N accumulation in soil profile, European Journal of Agronomy, Volume 31, Issue 4, November 2009, Pages 223-232, ISSN 1161-0301

H. Restrepo-Diaz, M. Benlloch, C. Navarro, Ricardo Fernández-Escobar, Potassium fertilization of rainfed olive orchards, Scientia Horticulturae, Volume 116, Issue 4, 20 May 2008, Pages 399-403, ISSN 0304-4238

di R. T.