Published on Organic Gardening (http://www.organicgardening.com)
A soil thermometer is the most accurate way to take your soil's temperature. Simply stick the probe into the soil and wait to see consistent readings for a few days. Plant when the soil reaches your crop's ideal temperature. A soil thermometer with an 8-inch probe costs about $9.
Links:
[1] http://www.organicgardening.com/successful-sowing-know-soil-temperature-380
[2] http://www.organicgardening.com/learn-and-grow/10-easy-soil-tests
[3] http://www.organicgardening.com/learn-and-grow/seed-starting-glance-0