Seeds need certain conditions to germinate. Temperature is one of them. If your soil temperature is too low, your seeds will not germinate no matter what. But, there is something we can do to change that. Minihooper (From New Dimension Seed) is a great device that can be used to modify soil temperature, and allow your the pride and joy of an early harvest. It will raise the soil temperature by 10 degree, and allow you to plant many things that you otherwise couldn't plant.
Get a instant dial thermometer and take a few readings around your garden. I did mine this morning. The soild temperature is between 40-50F. I sowed my spinach and peas last week, they are already up.