According to Roberts (2008), the origin of the 7 degree difference in summer temperatures was a study in Alaska involving a forested site with an O horizon and very little seasonal amplitude in temperatures versus an open area lacking an O horizon. The Roberts (2008) study, in contrast, shows that the difference is not always this great in Maine, and that cover vegetation type can have an effect independent of soil.
There were proposals to change the summer temperature criteria, either raising one, or lowering the other, so that they are the same for both O and non O soils. Yli, Mokma, and Starr (2001) recommended that the temperature difference be reduced to 4 degrees to satisfy their needs in Finland.
It seems that soil climate is just a proxy for atmospheric climate wherein the insulative properties of substrates and vegetation affects the rate which heat flows to and from the soil. Soil can be warmed independent of the air if heated directly by the sun, but ultimately, the amount is still reflected in atmospheric climate. Exceptions to this is where geothermal heating is involved, driving soil temperatures independently higher than atmospheric climate would predict. The only soil temperature regime threshold that makes sense as a soil property is where permafrost is involved. For ecological and agricultural “use and management” purposes, it makes sense to focus on the growing season temperatures and hardiness zones, not soil temperatures. Yes, soil temperatures affect the rate of decay and seed germination, but the driver is atmospheric climate with the various modifications to the soil surface that can change the outcome.
Roberts, W.J., 2008. Frigid forests vs. cryic fields: soil temperature regimes of two different cover types in Aroostook County, Maine. Soil Survey Horizons, 49(2), pp.44-47.
Yli‐Halla, M., Mokma, D. and Starr, M., 2001. Criteria for frigid and cryic temperature regimes. Soil Survey Horizons, 42(1), pp.11-18.
