Some seeds and seedling can germinate in cooler soil and their seedlings can tolerate cold air temperatures (even snow!). Like peas and other cool season crops. Others need to be planted later when things are warmer. Here is a post I wrote about planting seeds and seedlings.
I tried an experiment in late February to see if black plastic over the soil for a few weeks would heat it up. After 3 weeks I tested the soil temperature and I couldn't see any difference.
My uncle bought a groovy soil thermometer and it came with an awesome chart showing soil temperatures for seed germination.
|Soil Thermometer for Spring Seed Planting|