The surface of your landscape can cook or cool on a warm summer day. These photos were taken in Central Phoenix on a warm August afternoon, it was sunny and 101F. The surface of natural gravel under shade was about the same as the air temperature. The temperature of turf was not bad, 105F. The temperature of mulch was a bit warmer, 109F.
Concrete has a lightly colored surface and reflects some of the radiation from the sun. The temperature of concrete was lower than expected at 125F. The surface of natural gravel at 132F was hotter than concrete, the darker tones in the gravel adsorb more radiation than the lightly colored concrete. Brown bare earth was by far the hottest at 165F!
It was 101F outside, very pleasant in the shade but very hot on bare soil. 165F will kill most of the bacteria and other good stuff in your soil! Cover your soil with whatever you can to reduce the heat from your landscape.