Why Use a Compost Thermometer?As we saw in how compost forms, an active compost pile heats up to about 110 to 140 degrees F in the middle after 4-5 days, and is the most productive when it has heated up well. Conversely if it gets too hot it may leach nutrients, and start to smell. So the temperature gives us valuable information:
Have we balanced the mix of materials properly so that it's heating up?
If it doesn't heat up right away after we finish it off, then we need to look at what went into the mix, and if we can jump start it with some other materials (usually it will need more nitrogen), or adding moisture, or trying to jump start it with a compost activator.
Has the middle of the pile started to cool, indicating that it's time to turn the pile?
We want the pile to break down was quickly as possible, but by the same token, we want to minimize the amount of work that we have to put into it. If we let the pile cook just until it start to cool, turning after it cools to less than 100 degrees or so, we can get compost in short order, while doing a minimal amount of work.
What to look for in a Compost Thermometer?
Just a few things to be aware of in a compost thermometer:
Length of the probe: Obviously you want a probe that can reach the center of your compost pile. Typical thermometers have probe lengths varying from 12 to 30 inches. A common size useful for the home gardener is 20 inches, good for a 3x3 foot pile. Large windrow compost piles will obviously need larger, commercial grade thermometers. ReoTemp makes a good line of commercial grade compost instruments, including thermometers and moisture meters.
Fast reading: Some traditional style thermometers use a mechanical bimetallic type element to give a temperature reading, so can take a minute or 2 to give a stable reading. This varies with the type of element , so be aware of it. The newer electronic meters can give an almost instantaneous reading.
Temperature range: Not too critical, but be sure it goes to at least 200 F