Olive Yields and Tree Nutritional Status during a Four-Year Period without Nitrogen and Boron Fertilization

Nitrogen (N) and boron (B) are mobile elements in soil. Therefore, the application of these nutrients is typically performed annually, as a single dose, or even split into several fractions in the case of N. In olive (Olea europaea L.), however, controversial literature has suggested that yearly application of N may not be required. In the case of B, some authors indicated that one single application is sufficient for 3 or 4 years. Thus, the effects of these elements on olive yield, leaf N and B concentrations, as well as soil available N and B were investigated during a field trial performed in an olive orchard located in northeast Portugal, in which N and B were not applied for four consecutive growing seasons. Fertilizer treatments consisted of the following: the control, which was a complete fertilization plan where N and B were included (N + B treatment); -N treatment, with N excluded from the fertilization plan; and -B treatment, with B excluded. Available soil N and B were estimated from a pot experiment with Italian ryegrass (Lolium multiflorum L.) and from chemical laboratory extractions. Olive yield decreased significantly in the -N treatment in comparison to the control. A slight yield reduction in the -B treatment in comparison to the control was also observed. Leaf N and B concentrations decreased significantly in the -N and -B treatments, respectively, in comparison to the N + B treatment. Soil available N and B at the end of the experiment were significantly lower in the -N and -B treatments, respectively, in comparison to the N + B control. The results showed a continuous decrease in olive yield and leaf N and B concentrations, which reflected the reduction in soil-available N and B in the treatments lacking the respective nutrient. Therefore, it seems prudent to recommend adjustments to the rates of N and B every year to prevent reduction in tree crop performance and improve nutrient-use efficiency.