Thermal admittance

Why are soils with low thermal admittance (organic) subject to extreme surface temperature fluctuations?

Let's compare a dry exposed forest floor surface (i.e. an organic soil) with an exposed dry mineral soil surface on a southwest-facing slope on a sunny summer afternoon. Solar energy absorbed at the surface is converted to heat energy. In the forest floor, as surface temperature rises, a temperature gradient develops, causing some (but slow) heat conduction downward (due to low thermal conductivity). The heat energy tends to remain near the forest floor surface. The increased heat energy content makes soil surface temperature rise very high, because the low thermal capacity (Cv) means that even a little increase in soil heat content will cause a large increase temperature. In contrast, mineral soil (having high thermal conductivity) will conduct heat rapidly away from the surface and (heaving high thermal capacity Cv) would need a great increase of heat content in the surface if it were to develop a high surface temperature.