Planting by the calendar disregards weather variations from year to year. Timing planting based on soil temperature helps assure plants get a good start. Plants that have been planted in soil that is too cold exhibit symptoms similar to those planted in soil that have poor nutrient values. Once the soil warms up, they begin to grow vigorously, if they have survived the cold conditions.
Insert the thermometer to a maximum depth of 2" to 3" (5-8 cm.) in the soil. Check the temperature recorded after 3 minutes.