3 Best Compost Thermometers for Accurate Readings (2021 Review)

How to Choose The Best Compost Thermometer

Monitoring the temperature of your compost is the best way to ensure efficient composting and quality compost.

By knowing when your compost is heating and cooling, you’ll be able to turn it at the appropriate time during its heating cycle to achieve optimum results.

Compost thermometers also indicate when your compost has reached certain stages of its development, so you’ll know the exact moment when your compost is ready to use.

Here you’ll find more information about thermometers and composting in general, to help you find the best compost thermometer and get the most out of your compost:

Checking if Your Compost is Too Cold

You might think that everything is going well with your compost – you’ve prepared the heap, got loads of material on it, and are patiently waiting for that quality compost yield to come.

The problem is, without using a thermometer, you can’t really know what’s going on with your compost and if it’s working effectively.

The optimum temperature for compost, when hot composting, is between 55-72°C (131-161°F). If you don’t add any more organic matter, the compost can remain at this temperature for up to several weeks. This is ideal for efficient composting.

However, sometimes the compost might be colder than this, which can be checked using a thermometer.

If you’ve identified that your compost is not heating up, these steps should help you get it warm again:

1. Compost can fail to heat up if the compost pile isn’t large enough. If your compost thermometer shows that the core of the compost it too cool, consider the size of your compost heap. Hot-composting heaps need a minimum biomass of 3 m³ – this can be achieved in a composting area that is 1 x 1 x 1 m. The aerobic bacteria in the compost heap will get to work and heat up the contents, but, without enough surrounding biomass for insulation, this heat it will get lost quickly.
2. Another thing to consider after getting a low temperature reading on your compost thermometer is whether you are turning the heap frequently enough (or too frequently!). The heat is in the core of the heap, so it’s important to flip it regularly to make sure the ‘shell’ (outside) of the heap gets some time in the centre. Plus, turning the pile aerates it as well. This should be done approximately every 3 days.
3. Your compost pile might be too dry, which will result in it being too cool to work properly. This might be something you can’t tell without a getting a ‘cold’ temperature reading from a compost thermometer. Moisture is needed to help microbes survive, and they’re what create the heat. Compost should feel damp, not dry, with a moisture level of 40-60%. You can add water to your compost heap if it’s too dry, but avoid adding too much. A balance needs to be found between the compost having access to enough oxygen and enough water (so you want to avoid filling all of the airways with water).

You can also use a compost thermometer to make sure your compost is correctly following through the different stages of composting. Which leads us onto the next section:

The Different Stages of Composting

It’s really important to know what stage of composting your compost heap is at. This can dictate what action you need to take with it, and also let you know when it’s ready to use.

These are the different stages:

Heating Up

If all of the correct conditions are met (see information above in “Checking if Your Compost is Too Cold”), compost should start warming up within a couple of days. Different microbes get to work at different temperatures, so at each heat stage different activity will be taking place in the pile.

Optimum Heat

When the pile reaches a temperature of 55-72°C, this is the optimum temperature. It will stay at this temperature for a couple of days whilst the microbes get to work, then, when this ‘hot temperature’ bacteria dies off, the compost will start to cool.

Regularly checking the temperature of your compost will allow you to keep an eye on when it reaches its maximum temperature, and when the temperature starts to drop.

Turning the compost every three days whilst it’s in this ‘optimum heat’ phase should help it maintain this temperature for around 10 – 15 days. If, in this time, the temperature drops below 55°C or rises above 72°C, this is another indication that the heap should be turned.

Cooling Down

Eventually there comes a time when most the bacteria that works at the hottest temperature dies off completely, and the pile will cool down.

When it cools down enough, if there is more composting to be done, the bacteria which work at lower temperatures will come back and kick start the cycle again.

Identifying When Compost is ‘Ready’

When compost has gone through all of the above stages and is no longer producing heat, it’s likely ready to use. Finished compost has an ‘earthy’ ‘dirt’ smell, and there won’t be any of the original organic matter visible. The pile will generally be half the size that it started out as, and the ‘compost’ will be dark and crumbly.

Choosing a Well-Designed, Durable Thermometer

Compost thermometers are designed to be used in warm and moist environments, so it’s important that they are durable and, most importantly, rust proof.

Most thermometers are made from stainless steel; this is good because the metal won’t rust. A plastic thermometer won’t rust either, but it may get brittle or break when inserted into more compacted compost.

A common problem with compost thermometers is condensation getting into the dial. A sealed dial can prevent water from seeping in, although the best compost thermometers will have an IP55 rating.

A rating of IP55 means the thermometer is tightly sealed enough to not allow dust in, and is also water resistant (it’s just not resistant to immersion in water).

Thermometer Probe Length

The length of the probe is important, as you’ll need to take the temperature from as close to the middle of the pile as possible. While your measurements don’t need to be right at the centre, taking the temperature too close to the surface won’t give a good reading of what’s really going on at the core.

A probe of around 50 cm long is suitable for large piles of compost; some thermometers have a probe of 40 cm or less, but it’s advisable to check the size of your compost heap before buying a smaller thermometer. A 50 cm or 40 cm probe is generally optimal for taking readings from the middle of the pile.

Displayed Temperature Range

How the temperature is displayed on the thermometer can vary between models. Most thermometers measure in 1 or 2 degree increments and the best models will have a range of 0 – 100°C. Cheaper models may have a smaller temperature range.

Some compost thermometers come with colour coded sections, so you can see whether the pile is ‘warm’, ‘active’ or ‘hot’. This makes the thermometer easy to read quickly, as you’ll be able to see at a glance where your compost is in its heat cycle.

You may have a preference when it comes to reading temperatures in Celsius or Fahrenheit. Some thermometers are exclusively in Celsius or Fahrenheit, whilst others display both units. If you’re looking at a thermometer which shows both Celsius and Fahrenheit, make sure it’s clear to read and understand. Some thermometers that display both still seem to favour one unit over the other and can be hard to interpret quickly.

A Clear, Easy-Read Dial

Trying to find a dial that doesn’t mist up when in contact with humid compost can be difficult.

Some have an anti-fog coating which helps stop the dial misting up on the outside.

Condensation on the inside can also cause problems and make the dial hard to read. The best way to avoid this is by keeping the thermometer as dry as possible and not leaving it outside. Purchasing a thermometer with a ‘water resistance rating’ (e.g., IP55) should also reduce the amount of moisture that is able to creep in.

When it comes to the dial itself, a dial which is 5 cm in diameter is generally easy to read from a distance. If the numbers are too small, or of a lighter colour, they can be hard to read, so large numbers that contrast with the background colour are optimal.

The coloured ‘zones’ that some thermometers have to show ‘warm’, ‘active’ and ‘hot’ are also very simple to read without needing to see specific numbers.

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Compost Thermometer FAQs

How do I use a compost thermometer?

Simply insert the compost thermometer into the pile as far as it will go. Wait until the hand on the dial stops moving and make a note of the temperature. It is advisable to check the reading in several areas of the compost heap as there is not always a uniform temperature across the pile and you may occasionally find cold or hot spots.

How often should I turn my compost and what temperature should my compost be at when I turn it?

Ideally, for a hot compost pile, once the temperature reaches between 55°C and 72°C you should turn the pile every three days. It should maintain this temperature for around two weeks. If the temperature drops below 55°C in this time, or goes above 72°C the pile should also be turned.

After this couple of weeks the temperature will drop and the process will start from the beginning. You will need to continually check the temperature of the compost until it starts to rise again.

How do I know when my compost is ready for use?

Compost that is ready to use looks dark and has a crumbly texture. It smells ‘earthy’ and has no residue of any of the original organic matter that you fed to it.

Whilst finished compost can still retain a little heat, the pile won’t be particularly warm. The temperature of ‘ready’ compost will be approximately 21°C.

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