Foliar application of nano-NPK and calcium-boron during the ‘off-year’ season enhances yield, improves oil quality, and mitigates biennial bearing in ‘Picual’ olive cultivar

Abstract Background Global olive cultivation is often hindered by the physiological phenomenon known as alternate bearing, which results in inconsistent yields from year to year, thereby diminishing economic returns and challenging effective orchard management. While foliar nutrition is acknowledged as a viable approach to boost reproductive performance in olive trees, there is limited research exploring the combined use of NPK and calcium boron nano fertilizers (NPK-NF and CaB-NF) during the ‘off-year’ season to concurrently enhance flowering, fruit set, oil composition, and reduce the severity of alternate bearing. Results Field experiments conducted during the 2023 and 2024 seasons on fifteen-year-old ‘Picual’ olive trees demonstrated that foliar application of NPK-NF and CaB-NF significantly enhanced reproductive performance and oil quality while reducing alternate bearing severity. Trees receiving NPK-NF (4 mL/L) + CaB-NF (2 mL/L) exhibited maximum flowering parameters, including 9.31 inflorescences per shoot and 68.51% perfect flowers, alongside the highest initial fruit set (37.37%) and final fruit set (26.95%). This treatment produced the greatest ‘off-year’ yield (0.79 tonnes per acre) while reducing fruit drop to 27.83% compared to 51.77% in controls. Leaf and fruit mineral analysis revealed substantially enhanced nitrogen, phosphorus, potassium, and calcium concentrations in treated trees. Virgin olive oil maintained extra virgin classification with improved oleic acid proportions, reaching 67.85%, and increased pigment concentrations. Notably, the combination of NPK-NF (4 mL/L) + CaB-NF (3 mL/L) reduced alternate bearing severity to 31.06% compared to 88.08% in control trees. Conclusion These findings suggest that the deliberate foliar application of NPK-NF and CaB-NF during the ‘off-year’ season can significantly boost flowering, fruit set, and oil quality, while also alleviating the severity of alternate bearing in ‘Picual’ olive cultivar. This approach offers a viable nutritional strategy to enhance both yield stability and productivity in commercial olive orchards.