Compost Thermometer Guide: All You Need for Temperature Perfection

If you’ve constructed your compost heap well, it won’t be long before it starts heating up. And if you’re like me, 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 pile?

If that is you, it might be time to consider a compost thermometer.

What is a compost thermometer?

A compost thermometer is simply 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. 

Now the inevitable definition is out of the way, let’s get on with the meat of the post!

Do you actually need a compost thermometer?

Not really! And you certainly don’t need a compost thermometer if you are cold composting or trench composting.

That said, if you are a keen composter, it does have some benefits:

It helps you avoid temperature extremes

A compost thermometer helps you take action if the compost isn’t at the right temperature.

That helps in two ways. First, by taking action to get the compost warmer, you can get compost faster and ensure you kill pathogens.

However, I sometimes have the opposite problem—my compost gets too hot, which can kill beneficial microbes. By monitoring the temperature, I can know when to take steps to reduce heat.

It helps you learn

One of the things I love most about a compost thermometer is the insights it gives you. As you add different materials, you can observe how your compost reacts.

In the long term, that makes you a better composter!

If you’ve built a good compost pile, it won’t be long before you are dying to know what temperature it is. It’s also super interesting to know what stage the compost is at, and which compost bacteria are likely to be active.

Learn more about the differences between hot and cold composting.

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 simple needle indicating the temperature. 

Choosing a compost thermometer

With compost thermometers, it’s not a case of one size fits all. Instead, the thermometer you choose depends on your compost set up. With that in mind, let’s take a look at factors that might influence your choice.

Compost thermometers come with widely varying stem lengths. There’s no single right or wrong length – it all depends on the type of composting you are doing:

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 that 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 pile

/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.

Fog proof

Many compost thermometers claim to be fog proof, but in my experience, most still fog up to some degree.

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, there are ways to remove the fog if it annoys you (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.

Material, construction and stem thickness

Compost isn’t always soft and crumbly!

At the early stages of composting, especially if you have a lot of tough, brown materials, the compost can become quite dense. A sturdy stem will ensure you can push it into the heap without damaging the thermometer. 

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!)

Accuracy

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 test the accuracy of your compost thermometer.

1. Fill a glass with ice.
2. Add water to the glass until it reaches the top of the ice.
3. Stir the ice water mixture for a few seconds to ensure uniform temperature.
4. Wait for three minutes.
5. 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.
6. 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 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 frequently read composting guides, having both Celsius and Fahrenheit can save you from having to convert temperatures.

Brand

Reotemp 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?

Measuring

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 clear the condensation. 

FAQs

How often should I check my compost’s temperature?

There are no hard and fast rules, but I do the following:

1. Check 2-3 days after constructing a new heap, so I can check to see if the pile is activating.
2. Every other day (or daily if I have time) during the hot phase. That’s partly because I am curious, but also in case the compost gets too hot.
3. After it’s cooled down, if I turn the compost again, I’ll monitor it to see how much it heats up again.
4. Once it’s cooled down again and is in the maturing phase, I don’t bother to check the temperature again.

That’s what I do, but it’s at least partly driven by intrigue – adjust according to your schedule and curiosity levels!

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 
Johnny’s Seeds: Soil Thermometer Instructions

Read more…