Below are a few factors to consider in answering these questions. Clay soils have the ability to hold more water than sandy soils. Water has a high specific heat - meaning it takes a large amount of energy to raise the temperature of water. Therefore, clay soils, with their high water-holding capacity, require a lot of heat to warm them. Clay soils thus do not warm early in spring.
Sandy soils are very much the opposite of clays.
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.