Relating Soil Organic Carbon to Elevation and Landform for Separation of Humods and Orthods Separation in Vermont and Maine.

Soil Taxonomy separates Humods and Orthods based on soil organic carbon (SOC) present in a 10cm layer  within the Spodic horizon: Humods have  > 6% SOC, Orthods < 6%. The Spodic horizon can include all or part of various genetic horizons (e.g,,  Bhs, Bh and Bs horizons). The distinction between Orthods and Humods is important as the representative SOC assigned to a soil series is used in land management decisions, modeling, and planning.  In Essex County, Vermont  Orthods and Humods are closely intermingled and  have been separated based on elevation and landform breaks.  The objective of this study was to examine the relationship between SOC, elevation, and other soil and landform properties for three datasets: Essex County, VT (N=66), Camels Hump, VT (N=35) and sites throughout Maine (N=78). Data values were grouped by genetic horizon designations in the Spodic horizon, the soil series assigned at the time of sample collection (sampled as groups), aspect, soil depth class, and parent material.  SOC did not have a normal distribution so transformed data (logSOC) and non-parametric tests were used. Regressions between elevation and logSOC were weak (r²=0.02, p=0.09). The sampled as groups were significantly different (p=0.06), but most groups had samples above and below the 6% SOC criterion. Horizon designation, aspect, and parent material groups were not significantly different. When individual datasets were analyzed, there was no improvement in the correlation of SOC with elevation. In both the Maine and Camels Hump datasets  soil depth and parent material groups had significantly different SOC. Neither soil depth nor parent material were  significantly different in the Essex County dataset. More importantly, there were no data groups with more than 60% of samples above or below 6% SOC. Elevation or landform breaks do not meaningfully separate Humods and Orthods in Essex County, Vermont.