You can also have a temperature cycle of 600C with dirt. Water is limited to 50-70C so the dirt loses a factor of 4 in specific heat, but wins back a factor of 10 due to delta T.

The net is dirt wins by a factor of 2.5.

The thermal conductivity of rocks (and, I presume, soils) also typically declines as temperature increases, falling by a factor of 2 or 3 at 600 C.