They hold little water, and thus warm much faster in the spring. The process of germination at low temperatures is slow. As temperatures increase, the speed of germination increases. If you have a garden that is well protected from cool spring winds, the amount of heat the soil collects is higher because the wind will not carry away the heat. For this reason, protected sites tend to warm much faster than open, exposed sites.
Most soils have the ability to collect heat during the day and lose it at night.
The more heat collected during the day, the longer it takes to release at night. There are a number of devices capable of increasing the amount of heat the soil is able to collect. These allow the heat to pass through them, but prevent some of it passing back out. As a result, the heat is trapped near the soil, which becomes warmer. Most garden centres sell a variety of these environment enhancing devices.