How does the soil temperature affect the results? Do I need to measure the soil temperature at various depths?
Seasonal variations mainly affect the upper 10 feet (3 m) of soil, but below 15 to 20 ft (4.5 to 6 m) the soil temperature is nearly constant year round. This constant temperature value can be determined by the annual average air temperature for a given locale. This yearly average is obtainable from maps or statistics provided by the government weather service.