Soil Organic Carbon to Clay Ratio in Different Pedoclimatic and Agronomic Conditions in Northeastern North America: A New Approach Proposed

Abstract

Soil organic C 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”. In this study, we assessed the applicability of the SOC:clay ratio as an indicator of soil carbon health 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 health. Therefore, we do not recommend using SOC:clay ratio as a metric to assess soil carbon health or degradation status. Alternatively, we tested a ratio between actual and expected SOC levels as an indicator of soil degradation. Our results confirmed that classification of soil based on the SOC:SOCexp ratio was unbiased and independent of clay content, and a positive relationship was observed between the SOC:SOCexp ratio and the soil health indicators. Furthermore, we found that SOC:SOCexp better differentiated between soil degradation classes for all the tested soil health indicators than SOC:clay. While SOC:SOCexp was found to be a better predictor of soil functionality than SOC:clay, the SOC:SOCexp thresholds were based on our dataset (a small population) and would not be appropriate across pedo-climatic zones. Overall, we conclude that SOC:clay is not an effective indicator of soil carbon health status and SOC:SOCexp might be more useful to derive baseline levels and monitor the temporal dynamics at a regional scale.

Keywords: Clay content, Production system, Soil Health, Soil organic C, soil texture