Usually the temperature of the ground is the hottest and driest at the top of the soil, around here in Colorado that's about 6". After 6" depth even if you have pretty scrub ground you should start seeing moisture retention which would support soil microbes. At that moisture point, even if the top 1" of the soil is a scorching 100*F+, that soil at 6" depth would be below 86*F by far. If the farmers were utilizing soil microbes, the soil would be even better ameliorated and would generally be cooler mainly due to the fact it is retaining water better.
Soil that has microbes in it supports everything better, including itself. Heat will not kill all microbes. Same goes with drought. While some of the beneficial bacteria, mycorrhizae, and trichoderma fungi die off, as soon as water is present again they are rehydrated and begin to re-proliferate. Keep in mind most of the applied beneficial organisms are in the form of dust or dry granules.
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8 hours after beneficials application to container potatoes. Media is virgin coco coir with nothing added.
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Close up of above.
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I cut some clones, soaked the plugs in beneficials solution, and added the clones. This is 8 hours after application. You can see hyphae growing vigorously. These are Root Riot cubes, the only starting media I recommend unless you can afford Jiffy Pre-forma. (have to custom order in pallet size)
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I do the same for my seeds. I find the best result that when a live inoculant is already applied that even before pre-emergence and extension of the root radicle there are already symbiotic "roots" helping the plant out. Also, because of the mixture I use, there is no chance of damping off, mildew, downy or powdery mold, anthracnose, fusarium, pithium, etc etc etc.