The Illinois State Water Survey operates a network of 19 sites that report daily soil temperatures at 4 and 8 inches.
Soil temperatures in the fall are critical for the application of nitrogen (N) fertilizer. According to the Agronomy Handbook (University of Illinois), “fall N applications should be done when daily maximum bare soil temperature at 4 inches is below 50 degrees.”
The daily maximum and minimum soil temperatures are measured over grass. In addition, the values are adjusted using regression to estimate the bare soil temperatures, to better represent the soil temperatures in a cultivated field. Grass tends to insulate the soil so the daily temperature swing is a little smaller than over bare ground. In addition, soil temperatures within a particular field may vary due to soil color, soil moisture, and crop residue on the surface.
You can see the daily data and maps at http://www.isws.illinois.edu/warm/soiltemp.asp