Field Evaluation of a Portable Multi-Sensor Soil Carbon Analyzer Under Real-World Farming Conditions: Performance, Precision, and Limitations

Soil organic carbon (SOC) monitoring is central to carbon-farming Monitoring, Reporting and Verification MRV, yet high laboratory costs and sparse sampling limit its scalability. We present the first independent field validation of the Stenon FarmLab multi-sensor probe across 100 temperate European arable-soil samples, benchmarking its default outputs and a simple pH-corrected model against three laboratory reference methods: acid-treated TOC, temperature-differentiated TOC (SoliTOC), and total carbon dry combustion. Uncorrected FarmLab algorithms systematically overestimated SOC by +0.20 % to +0.27 % (SD = 0.25–0.28 %), while pH adjustment reduced bias to +0.11 % and tightened precision to SD = 0.23 %. Volumetric moisture had no significant effect on measurement error (r = –0.14, p = 0.16). Bland–Altman and Deming regression demonstrated improved agreement after pH correction, but formal equivalence testing (accuracy, precision, concordance) showed that no in-field model fully matched laboratory standards—the pH-corrected variant passed accuracy and concordance yet failed the precision criterion (p = 0.0087). At ~ €3–4 per measurement versus ~ €44 for lab analysis, FarmLab facilitates dense spatial sampling. We recommend a hybrid monitoring strategy combining routine, pH-corrected in-field mapping with periodic laboratory recalibrations, alongside expanded calibration libraries, integrated bulk-density measurement, and adaptive machine-learning to achieve both high resolution and certification-grade rigor.