Using a probe-type thermometer is an accurate method for testing soil temperature prior to planting seeds and small plants into the garden. Probe-type thermometers with at least 3 inches of metal stem may be the best device for measuring soil temperature. The long stem allows the thermometer to be placed deeply into the soil or at varying depths. Available at most home and hardware stores, they resemble a meat thermometer but have a lower temperature range, typically from 0 to 180 degrees F.
Cultivate garden soil thoroughly before testing soil. Using a powered rototiller or hand cultivator, work the soil two weeks to a month before planting.
Level the soil after cultivation. Remove any large clumps of soil or rocks, which may interfere with the soil being heated by the sun. If the soil or garden area is extremely wet, let it completely dry out. Testing wet soil can give the thermometer inaccurate results, and heavy moisture may cause evaporative cooling along the length of the thermometer.
Use the pencil to make a quick sketch of the garden area in the notebook. It does not have to be extremely accurate, just a rough outline of the space. The soil temperature should be taken every 10 feet in the garden. Tally all readings to create an average temperature for the garden space.
Insert the thermometer probe into the soil for approximately five minutes. Take a reading. Record this on the soil temperature map you made in the notebook.
Continue to perform the tests until all areas have been recorded. Add all the readings together and divide this number by the number of readings taken to find the garden bed's average temperature. If total temperature readings equaled 695 degrees F with a total of 10 readings, divide 695 by 10 (695/10) for an average garden temperature of 69.5 degrees F.
Consult the seed germination and plant transplanting chart for your particular area. The average temperature found in step 5 will aid you in optimizing seed germination and transplanting. Some bean and melon seeds require soil temperatures at a particular degree reading. Failure to plant these types of seeds under the correct soil temperatures will lead to rot.