What effect does soil temperature have on plant growth?
The soil temperature permits movement of the atoms in the plant necessary for plant life. Without a soil temperature (theoretically -459.67°F. (which itself is a temperature)), plants would be in a (theoretical) state of suspended animation.
There is an optimal 'growing' temperature for each plant. With millions of different plants, there are millions of different optimal temperatures. Soil temperatures above or below the optimal for any given plant will produce less than optimal growth.
Gardeners can't always rely solely on the calendar to tell them when it's time to plant, especially when it comes to measuring the temperature of the soil. Plants that are started in cold soils may grow slowly, develop poorly, or succumb to disease.
Many vegetables, like tomatoes, beans, squash and peppers need soil temperatures to reach nearly 70ºF before their growth can really take off.