What is a Compost Thermometer?

A compost thermometer is a device used to monitor the core temperature of a compost pile. By maintaining the optimum temperature of the pile, refuse will decay and be composted more quickly. The use of a compost thermometer will allow this temperature to be monitored and regulated, most often by watering or turning the pile.

Organic waste such as vegetable peelings, dead leaves and grass clippings decompose naturally. Microorganisms consume the material over time, releasing small amounts of heat in the process. By using a compost pile for this refuse, the process can take place much more quickly. There is more decomposing material that has been insulated by additional layers of refuse, so more heat is generated and captured in a compost pile. The heat stimulates the bacteria, causing them to multiply more quickly, accelerating the process.

Ideally, the core temperature of a compost pile should be between 120 degrees and 150 degrees Fahrenheit (about 50 degrees to 65 degrees Celsius). Colder temperatures slow the decay, causing the material to take longer to decompose. High temperatures can kill off the bacteria responsible for decomposing the pile.

By monitoring with a compost thermometer, the gardener can intervene when the temperature moves away from this ideal. Optimum temperature can then be restored, most often by watering or turning the compost. Depending on the pile’s content, carbon- or nitrogen-rich materials might need to be added to give the bacteria needed energy and nutrients. Heavy or frequent rain can reduce the temperature as well, and a cover might be required in some climates.

Typically, a compost thermometer has a dial with a long metal probe extending from its back. Probe lengths vary, with most home garden models between 20 and 24 inches long (about 50 to 60 cm), but some models are as long as 6 feet (nearly 2 meters) or more. When using a compost thermometer, the reading must be taken from the center of the pile, making it important to select a thermometer with a probe of the correct length.

As an alternative, a metal pole can be used as a substitute compost thermometer to gauge the core temperature of a compost pile. When the end of the pole is driven into the pile, it should begin to heat up. After a moment, it can be lifted out and should be warm to the touch. Although no temperature reading is available with this method, many gardeners find it sufficient for monitoring the temperature inside the compost pile.