Monitoring the temperature of your compost is the best way to ensure efficient composting and quality results.
By knowing when your compost heap is heating and cooling, you’ll be able to turn it at the appropriate time during its heating cycle. This is how to achieve optimum results and wonderfully nutritious compost.
Therefore, a thermometer is a valuable piece of compost equipment. Compost thermometers also indicate when your compost pile 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 the composting process in general, to help you find the best compost soil 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 pile – you’ve prepared the heap, got loads of organic waste 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 a compost pile 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 pile 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:
- 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-compost piles 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 pile will get to work and heat up the contents, but, without enough surrounding biomass for insulation, this compost heat will get lost quickly.
- 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.
- 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 pile 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 pile 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:
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.
When the pile reaches a temperature of 55-72°C, this is the optimum temperature and known as the ‘thermophilic stage’. Compost 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 pile 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 pile 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.
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 key features to look for are durability and rust resistance.
The best compost thermometers have a stainless steel body; this is an ideal material for moist conditions as it’s rust resistant. Of course, a plastic compost 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. This can make it difficult to take a temperature reading. Some dials are hermetically sealed, meaning they’re airtight. The best compost thermometers will have hermetically-sealed dials and an ‘ingress protection’ rating of IP55 or more.
A rating of IP55 means the compost thermometer is sealed tightly enough to not allow dust in, and is also water resistant (it’s just not resistant to immersion in water). This helps prevent fogging.
Compost 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 close to the surface of large piles won’t give an accurate 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 pile before buying a smaller thermometer. A 50 cm or 40 cm probe is generally optimal for taking readings from the middle of the pile.
You may occasionally come across compost thermometers with a much shorter stem than this and wonder what they’re used for. Shorter stems, around 20 cm, can be used for checking temperatures in shallower worm composting trays. Other thermometers are too long to be useful in this context. Worms like compost temperature to be between 12 – 26°C, so there are different requirements compared to a hot compost pile.
Displayed Temperature Range
How the temperature is displayed on the compost 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 of the best compost thermometers come with colour coded sections, so you can see whether the pile is ‘warm’, ‘active’ or ‘hot’. This makes the compost thermometer easy to read quickly, as you’ll be able to see at a glance where your compost pile 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 compost thermometer which shows both Celsius and Fahrenheit, make sure it’s clear to read and understand. Some thermometers that display both still 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 avoid fogging 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 compost thermometer as dry as possible and not leaving it outside. Purchasing a compost thermometer with a ‘water resistance rating’ (e.g., IP55) should also reduce the amount of moisture that is able to creep in.
Dials which are ‘hermetically sealed’ are better at preventing condensation and water ingress. A hermetically sealed dial is designed to keep things completely airtight.
When it comes to the dial itself, a dial which is 50 mm 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.
Some thermometers have dials with coloured zones to quickly show whether the compost is ‘warm’, ‘active’ or ‘hot’. This makes an ideal compost thermometer for reading from a distance without needing to see specific numbers.
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