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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.

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I think you are looking the problem from a wrong point of view.

Thermal insulator is not a good term and also not good way to have warmer soil.

First fact: the soil temperature. The surface temperature have huge variation of temperature (much higher that we imagine, usually we measure air temperature at 1 to 2 meters from soil). Going down in the soil, the temperature differences smooth, until around 5 to 10 meters, where the temperature of soil is equal to the average air temperature (over several years). Going down, the pressure (weight of soil above the point) start to make effect, so we will have higher temperatures). So, soil is a good insulator. The good: average temperature is usually (but in very cold regions and mountains) above freezing point.

But the main problem of root (and the cause of temperature changes) is radiation. During day sun irradiate the soil, warming it up. But during the day the soil irradiate (in infra-red) back, giving temperature back to air. Usually we want (on cold day) to reduce such irradiation, and we use two methods: greenhouse (glass has such properties, which transmit visible light, but block infra red, so it block the radiation of soil). The other method: mulching (especially the winter one). This block radiation of soil, so it keep warmer [and for summer it block also water evaporation].

To have warmer soil during night, also an other effect is used: heat storage. Stones and rocks is used to store day heat and release it during the night. This is done on some wine region. Water can also be good heat storage (region near sees and lakes have less temperature differences between high and low temperature: less continental climate). So wet soil is better. So organic, or a clay with some sand is the ideal soil.

A thermal insulating soil will not absorb heat during day, so it would not be ideal. Peat is a good insulator (and used in e.g. Ireland and Iceland to insulate roofs).

So I would try to store heat and stop irradiation of soil. My warmer garden is near a stone wall, with leaves to protect cultures.

Note: the last effect to control: wind protection. A screen/shelter helps to keep lost heat near the plants (and so it helps soil to lose less heat).

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To follow on from what Mr. Catenazzi wrote, during the winter in cold areas you WANT to keep the soil frozen so that it doesn't heave during periodic thaws. This can involve waiting until the soil freezes "hard" and then placing mulch over it - often, cut up Christmas trees. This is especially important if you planted perennials late in the season, as frost heaving could push the roots out of the ground and kill the plant. Most gardeners who use winter mulch remove it once the snow (assuming there is some) melts - typically, late March in US Zone 5.

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