Rapid in situ analysis with portable XRF sensor: A pioneering study proposing fitness-for-purpose approach for performance assessment in tropical soils

• In situ XRF enables soil chemical analysis with only 5 s of scanning time; • Both low and high level of CEC and av-Ca can be reliably predicted in the field; • A fitness-for-purpose approach for evaluating sensor performance has been presented; • Portable XRF is a feasible tool for real-time soil monitoring in agriculture Recent advances in X-ray fluorescence (XRF) spectroscopy for Soil Science position it as a promising tool for in situ soil monitoring, although rapid and in situ applications of XRF sensors remain underexplored. This study evaluated the performance of a portable XRF operated in situ using a 5-second scanning time to predict plant-available (av-) Ca, av-Mg, av-K, and cation exchange capacity (CEC) in fresh soil samples (n = 128) from a Brazilian tropical field. Model evaluation was conducted using a novel fitness-for-purpose framework, which considers agronomic thresholds for acceptable prediction errors. Models were calibrated using spectral data obtained under three scenarios: laboratory with 30- and 5-second scanning times, and in the field with 5-second scanning time. We suggested maximum tolerated error thresholds based on regional soil fertility interpretations to classify model performance for semi-quantitative and qualitative applications. Results demonstrated that CEC predictions were suitable for both applications in lab and field conditions; av-Ca models were acceptable for qualitative use in both conditions and for semi-quantitative use in lab only. In contrast, av-Mg and av-K models did not meet reliable results, except for lab-based qualitative prediction of av-K. The proposed evaluation strategy allowed contextual performance assessment, overcoming limitations of conventional metrics such as R² and RPIQ. Overall, portable XRF proved to be a viable tool for rapid, on-site soil fertility diagnostics in tropical soils, particularly for av-Ca and CEC. Its use in mobile labs or directly in the field holds promise for inferring chemical attributes in precision agriculture and pedometric applications.