A lot of plants are supposed to do better with warmer roots. So, it stands to reason that they would do better with soil with poor insulation qualities during the growing season, and that they might have difficulty with soils that insulate well (and stay cooler underneath). Perhaps good insulators might be better during the winter, to help keep the roots warmer during the coldest periods. Perhaps good insulators would be somewhat of an advantage in areas with cool nights (to keep the soil warmer at night), although they might keep the soil cooler in the day. I'm thinking roots might venture deeper in soils with poor insulation (during the warm season, anyway).
Anyway, I'm wondering: Which soil types make the best and worst thermal insulators?
My personal guess is that clay would make the best insulator, and a more porous soil (although I'm not sure which) might be a worse one, but I don't know. I know our clay loam soil feels pretty cool if you dig in it, especially if you go very deep.
I'm not sure that any sources exist on the topic of soil insulation, but if we knew the answer, I think the knowledge could help gardeners and farmers a lot. Maybe I just didn't find them.
This could be particularly important for peppers and watermelon, although I'm sure most sources will already tell you that they don't prefer clay. I imagine they don't normally prefer insulators generally. Also, it might be useful for areas where plants die because of the heat; perhaps cooler roots would help those.