Assessment of Boron Phytotoxicity Risk and Its Relationship with Sodicity and Major Cations in Irrigation Groundwater from the La Yarada Los Palos Coastal Agroecosystem, Caplina Basin, Tacna, Peru

Across ten months of monitoring (1 October 2024–1 July 2025) at three drilled irrigation wells in the La Yarada Los Palos coastal aquifer, this study evaluates boron phytotoxicity risk and its interaction with salinity and sodicity in a hyper-arid coastal agroecosystem. Groundwater samples (n = 10 per well; n = 30) were analyzed for boron, major cations (Ca2+, Mg2+, Na+, K+) and EC. Salinity–sodicity indices (EC-based classes, SAR, Kelly Index, %Na, Mg/Ca ratio) were computed, and relationships among boron, cations, and EC/TDS were examined using correlation analysis and principal components. Boron concentrations ranged from 1.18 to 2.47 mg/L; all samples exceeded the FAO guideline for sensitive crops (0.7 mg/L), and 56.7% were ≥1.5 mg/L. Southern Border exhibited the highest boron (mean ≈ 2.10 mg/L), Ashlands intermediate (≈1.65 mg/L), and Bio Garden Los Palos the lowest (≈1.35 mg/L). EC remained ≈1–1.5 dS/m at Southern Border and Bio Garden Los Palos but reached ≈3–4 dS/m at Ashlands; all SAR values were <9, indicating low sodicity risk. Spearman correlations revealed weak associations between boron and EC/TDS, but moderate positive correlations with Ca2+ and Mg2+, highlighting partly decoupled controls on boron and salinity. For tolerant crops such as olive and orange, and more sensitive ones such as oregano and quinoa, these conditions imply risks that require combined management of salinity, boron, and cation balance. A risk-based monitoring scheme centered on EC, SAR, boron, and Ca–Mg–Na ratios is proposed to support irrigation decisions in La Yarada Los Palos and similar hyper-arid coastal agroecosystems.