Soil organic carbon to clay ratio in different pedoclimatic and agronomic conditions in northeastern North America

Soil organic carbon levels are strongly influenced by pedoclimatic and agronomic environments; hence, establishing universal threshold values for SOC to differentiate soils into healthy and degraded classes is nearly impossible. Previously, SOC:clay ratio thresholds were used to classify soils into categories of “degraded”, “moderate”, “good”, and “very good”. Here, by comparing the percentage of soils under these categories, we assessed the applicability of the SOC:clay ratio as an indicator of soil carbon status on 2249 soil samples collected from Ontario (Canada) and New York (USA). Our results confirmed that the classification of soil based on SOC:clay ratio was highly clay biased, inaccurate, and not a true representation of soil degradation status. Fine-textured soils had a high percentage of degraded soils (73 %) whereas coarse textured soils had a large percentage (62 %) of soils with “very good” soil carbon status. Therefore, we do not recommend using SOC:clay ratio as a metric to assess soil carbon or degradation status. Alternatively, as originally proposed by Poeplau and Don (2023), we tested a ratio between actual and expected SOC levels (SOC:SOCexp) as an indicator of soil degradation. Linear regression between SOC and clay content for the pasture systems was used to calculate SOCexp. Our results confirmed that classification of soil based on the SOC:SOCexp ratio was less biased, independent of clay content, and had a positive relationship soil health indicators (aggregate stability, permanganate oxidizable carbon (POXC), pH). Furthermore, we found that SOC:SOCexp better differentiated between soil degradation classes for all the tested soil physical, chemical, and biological properties than SOC:clay. While SOC:SOCexp was found to be a better predictor of soil carbon status than SOC:clay, the SOC:SOCexp thresholds were based on our dataset (a small sample size relative to population) and would not be appropriate across pedo-climatic zones. Overall, we conclude that SOC:clay is not an effective indicator of soil carbon status and SOC:SOCexp might be more useful to assess soil condition and derive baseline soil carbon levels at a regional scale.