Baseline geochemistry and PTE contamination assessment in agricultural soils of Onaizah, central Saudi Arabia

Establishing domain-specific agrogeochemical baselines is essential for interpreting cultivated soils in arid environments, where lithogenic controls, irrigation, and field management jointly influence topsoil chemistry. This study developed a robust agrogeochemical baseline for cultivated topsoils in Onaizah, central Saudi Arabia, with emphasis on the dominant date-palm management domain. A total of 33 agricultural topsoil samples were analyzed, and baseline characterization was based on robust central descriptors together with percentile-based upper background limits (UBLs), expressed as the empirical 95th percentile (P95) and its one-sided bootstrap upper 95% confidence bound (P95-UCL95). Non-palm fields were retained as a comparative layer to test crop-type contrasts in management-sensitive elements. The soils showed marked inter-element variability. Conservative mineral-framework indicators such as Fe and Ti displayed comparatively coherent behavior, whereas management-sensitive elements, particularly P and S, showed broader upper-tail responses. Within the date-palm domain, P had a median of 500 mg/kg, P95 of 2660 mg/kg, and P95-UCL95 of 3780 mg/kg, while Fe showed a median of 10,800 mg/kg, P95 of 20,120 mg/kg, and P95-UCL95 of 26,400 mg/kg. Spatial analysis indicated that elevated P values were strongly localized, whereas Fe exhibited smoother and more coherent patterns. Crop-type comparison showed that S was substantially higher and more variable in non-palm soils. Potentially toxic element assessment indicated that exceedances were infrequent and spatially restricted. Average Igeo values were negative for all evaluated elements, and pollution load index values ranged from 0.09 to 0.53, indicating an overall unpolluted status. The study demonstrates that a date-palm-specific baseline provides a defensible framework for monitoring cultivated arid soils while distinguishing localized enrichment from domain-wide contamination.