Tulip Test Gardens

What's Happening Underground?
Soil Temperature Investigation

Teacher Background
One winter, Journey North staff dug through the snow and stuck in a soil thermometer. When the air temperature was 2°F. below zero, the soil two inches down was 27°F. above zero! What was going on?

In the winter, soil temperatures tend to be higher than air temperatures. (See B, below. "Ground surface" is the air temperature.)
Soil can hold heat better than air does. It is also insulated by soil above it, vegetation, and snow. In fact, the deeper you go, the more insulation and the higher the average temperature, to a point. From about 30 to 200 feet below the surface, the soil temperature is relatively constant (about 55 degrees F.).

Air temperatures (and shallow soil temperatures) also fluctuate more than temperatures in deeper soils over the course of a day or year. (See A and B, below.)
It takes deeper soil layers longer to respond to changes in the surface that are caused by daily and seasonal changes in the amount of the sun's energy reaching the earth. By the time you reach about 30 feet deep, soil temperatures are fairly constant.

In the summer, the whole cycle reverses! (See B, below.)
In the spring, air and soil temperatures both rise, but the soil warms up more slowly. So by the time summer comes, the soil temperature is lower than the air temperature. And the deeper you go in the summer, to a point, the cooler the soil is.

A. Soil temperatures over three days at different depths. At shallow depths, soil temperatures fluctuate a great deal. The deeper you go in the soil, the more constant the temperatures.
B. Soil temperatures during a year at different depths. (Click for a larger image.) In the winter, the deeper the soil, the warmer the temperature. This reverses in the summer. In general, deeper the soil, the more constant the temperatures.

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