Optimizing fertilizer applications to mitigate salt stress in olive trees (Olea europaea L. var. Arbequina)

Abstract Soil salinization is one of the most critical abiotic stresses affecting plant growth and productivity. Climate change and unsustainable agricultural practices, including excessive fertilizer use, exacerbate this issue, intensifying the adverse effects of salt stress on crops and ecosystems. This study aimed to evaluate the vegetative and physiological responses of one-year-old olive trees ( Olea europaea L. var. Arbequina) grown in 20 L pots under salt stress (100 mM NaCl) to four mineral fertilizer formulations (ammonium nitrate, urea, calcium nitrate, and potassium nitrate) and one amino-acid-based biostimulant. The plants were irrigated with half-strength Hoagland’s nutrient solution, supplemented with NaCl, and treated with fertilization treatments at a standardized rate of 30 g N per plant, applied in 28–32 doses throughout the experiment. Our results indicate that soil salinity significantly reduced vegetative growth and decreased potassium content in the olive leaves. Additionally, salinity increased Na⁺ accumulation and electrolyte leakage in the foliage. Ammonium nitrate and potassium nitrate further reduced the dry weight of vegetative biomass compared to plants irrigated with saline solution alone. Urea showed intermediate effects, with responses associated with lower leaf Na⁺ accumulation and electrolyte leakage, but without improving overall biomass production. In comparison, both calcium nitrate and foliar amino acid treatments positively affected most of the growth parameters. These treatments were associated with improved vegetative growth, higher biomass production, reduced Na⁺ accumulation in aerial tissues and lower leaf-to-root Na⁺ concentration ratios. Overall, calcium nitrate and foliar-applied amino acids were associated with more favorable growth, biomass, electrolyte-leakage, and Na⁺-distribution responses under saline conditions.