If you’ve constructed your compost heap well, it won’t be long before it starts heating up. And if you’re like me in my early days, curiosity will soon start to gnaw at you.
What’s the temperature? Is it too cool or too hot? When do you need to turn it? What happens if I add this or that material to the heap?
If that is you, it might be time to consider a compost thermometer.
We’ve covered the best current thermometers in another article. Here we’ll take a more in-depth look at how to choose them, as well as how to use one when you have it.
What is a compost thermometer?
A compost thermometer is a device used to measure the internal temperature of compost. It usually consists of a dial attached to a metal or plastic probe. The probe has a temperature sensor at its tip which is inserted into the compost.
Do you actually need a compost thermometer?
That depends. You certainly don’t need a compost thermometer if you have a small compost pile or bin that doesn’t get hot.
However, if you are interested in hot composting, a thermometer is highly useful. That’s because:
- It allows you to take action if your compost does not get hot enough or indeed if gets too hot. That helps in several ways. For example, by taking action to get the compost warmer, you can get compost faster and ensure you kill pathogens. Plus, by keeping it from getting too hot you can get better quality compost.
- A compost thermometer is also a learning tool. When you use different materials and techniques, you can measure the results and learn what works for you, your materials and your climate.
- Curiosity! If you’ve built a good compost heap, it won’t be long before you are dying to know what temperature it is. The temperatures will also indicate which type of compost bacteria are active, and which stage the composting is at.
Can you just use any thermometer?
You could certainly use other thermometers to measure the heat.
However, compost thermometers come with specific features which are designed for the composter.
These can include a longer stem, hermetic sealing to reduce fogging, and a robust design to withstand the elements.
Types of compost thermometer
There are two main types of compost thermometers: digital and analog. Most home composting devices are analog, with a needle indicating the temperature.
Choosing a compost thermometer
Compost thermometers come with widely varying stem lengths. The best stem length depends on the type of composting you need:
If using a hot bin or compost bin
With many compost bins, organic material is added to the top of the bin over time and taken out from the bottom of the bin.
That often means that the bacteria which generate heat are most active at the top of the bin.
Here it’s ideal to have a shorter thermometer so you can monitor the activity at the top part of the pile.
If using a compost heap/large bin
In contrast to a bin, the heat in a pile is often hottest near the center.
That’s especially the case with a compost heap which is built in one go rather than constructed over a long time.
Here a longer stem is ideal, as it can reach further into the pile. The exact length, of course, will depend on the size of your pile.
I currently have two thermometers. The first is a short-stemmed one that came with my HotBin, and has lasted several years, while the second is a 16-inch REOTEMP FG16P-C43.
(Mine is in celsius, so if you are based in the US you may prefer the fahrenheit version.)
Many compost thermometers claim to be fog proof, but even those can be vulnerable to fogging up at times.
I’ve mostly found that a foggy compost thermometer is a nuisance rather than a disaster, as you can usually still read the temperature despite the fog. What’s more, you can clear the fogging (more on that later!)
However, some thermometers are vulnerable to damage from consistent fogging. To minimize the risk of it happening, look for a hermetically sealed design (i.e. a thermometer screen so tightly closed that no air or water can leave or enter it.)
Material, construction and stem thickness
Compost isn’t always soft and crumbly!
At the early stages of composting, especially if you have lots of tough, brown materials in your compost, it can be quite thick. A sturdy stem will ensure you can push it into the heap without breaking it.
If you want to leave your compost thermometer in the compost, you also want to ensure it is rust-proof. It needs to withstand the elements and the dampness. (Especially if you’re like me, and you occasionally accidentally bury your thermometer!)
Above: Measuring the accuracy of a compost thermometer. Note when I do the actual measuring I hold the thermometer to ensure it does not touch the bottom or sides.
An accurate reading can help you know when your compost is too hot. For example, you might want to keep your compost below 66 degrees to ensure you don’t kill off beneficial microbes.
Having an accurate compost thermometer can let you know precisely when you need to take action!
Here’s how to measure the accuracy of your compost thermometer.
- Fill a glass with ice.
- Add water to the glass until it reaches the top of the ice.
- Stir the ice water mixture for a few seconds to ensure uniform temperature.
- Wait for three minutes.
- Place the thermometer in the glass, ensuring the probe is fully submerged in the ice water mixture but not touching the sides or bottom of the glass.
- Monitor the temperature, which should read 32°F/0°C if the thermometer is accurate.
Right temperature measurement for you
There are essentially three ways a thermometer can measure temperature – Fahrenheit, celsius, or a simple cold/cool/warm/hot reading. (Some thermometers combine all two or three measures.)
If you are inexperienced or have limited vision, you might find that a color guide to show you ideal temperature ranges is really helpful.
However, I would recommend that you also select a device that combines those colors with exact temperature measurements. As your composting knowledge and experience grow, you’ll start to appreciate precision!
You also need to decide if you want numbers displayed in celsius or Fahrenheit. If you think in celsius, for example, make sure that your thermometer includes that way of measuring temperature.
If you do like to read guides to composting, it might be handy to have both celsius and Fahrenheit so you don’t have to convert temperature from one to the other.
Roetemp dominates the compost thermometer market. They are commonly used by industrial composters but do have great (and reasonably priced) options for the home composter too.
They have been manufacturing compost thermometers for nearly 60 years and are a soil builder member of the US Composting Council.
While they’re not the only good brand out there, they are my first choice when it comes to thermometers.
How do you use a compost thermometer?
It’s really simple! Just insert the thermometer and leave it to do the work. Do note that some thermometers take time to come up to temperature, so it’s worth leaving it in the compost for a few minutes.
I generally have a thermometer in my compost at all times, so I can quickly check the temperature. However, if you are absent-minded like me you may occasionally end up burying the device!
Dealing with fogging
Fogging will usually disappear if you place the thermometer in a dry place. Alternatively, you can clean and dry the thermometer and place it on a radiator. This will usually de-steam the device.
Can you calibrate a compost thermometer?
Usually, only high-end compost thermometers can be calibrated. For instructions on how to do so, the following resources are useful:
REOTEMP: Calibration Instructions for Dial Compost Thermometers
Johny’s Seeds: Soil Thermometer Instructions