Summary
This study evaluated the widespread use of SOC:clay ratio thresholds for assessing soil carbon status and degradation, using 2249 soil samples from northeastern North America. The authors found the ratio to be inherently clay-biased, with fine-textured soils classified as degraded (73%) and coarse-textured soils as 'very good' (62%) regardless of actual carbon status. The authors propose an alternative metric—the ratio of actual to expected SOC levels (SOC:SOCexp)—which demonstrated less bias, independence from clay content, and stronger relationships with established soil health indicators.
Regional applicability
The findings are likely relevant to UK soil assessment practices, particularly where universal SOC:clay thresholds have been adopted. However, the authors caution that their SOC:SOCexp thresholds are dataset-specific and would require recalibration across different pedoclimatic zones, suggesting UK-specific validation would be necessary before implementation.
Key measures
Soil organic carbon (SOC), clay content, SOC:clay ratio, actual to expected SOC ratio (SOC:SOCexp), aggregate stability, permanganate oxidisable carbon (POXC), pH, soil physical/chemical/biological properties
Outcomes reported
The study assessed the applicability of SOC:clay ratio thresholds for classifying soil carbon status across 2249 soil samples from Ontario and New York, and tested an alternative ratio (SOC:SOCexp) as a less biased degradation indicator. Results demonstrated that SOC:clay classification was clay-biased and inaccurate, whilst SOC:SOCexp showed positive relationships with soil health indicators.
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