Hot composting is fun. The first time you feel the heat coming off your compost, or when you insert a thermometer and realise you could bake a potato in it, can be quite exciting!
However, hot composting does need (a little) skill and, depending on the method you use, more effort. It can also be discouraging if you think your compost needs to be hot and you fail to achieve that heat.
Let’s have a walk through both types of composting, including what’s involved, and the pros and cons of each one, so you can decide which is right for you!
- Hot Composting
- Cold Composting
- Hot and Cold Composting Compared
- Bringing it altogether: Choosing the right method for you
What is hot composting?
Hot composting is a method of quickly decomposing organic matter by controlling the temperature, moisture, oxygen levels and composition of the compost pile. When the conditions are right, the reproduction and activity of bacteria ramp up, rapidly breaking down compost material and generating the heat which gives its name to this composting method.
Do note that you don’t want compost to be too hot, as it can destroy the diversity of bacteria in compost.
One study by PF Strom suggests limiting the maximum compost temperature to 60 degrees for this reason.
The essential factors for hot composting
A Balance of High Carbon and High Nitrogen Materials
First, there needs to be a balance of carbon and nitrogen in the compost pile. To achieve this, it helps to add both green (high nitrogen materials) and brown (high carbon materials) to the compost.
|Myth: You need to get the exact ratio of greens to browns correct. |
Fact: This is impossible. Research has actually shown that you can achieve hot composting with a wide variety of CN ratios. See The Carbon – Nitrogen Ratio for more information.
To generate heat, you need bacteria to multiple. And for bacteria to multiple, you need air. So it’s important that there is sufficient oxygen in your compost.
|Myth: You need to turn your compost regularly to get air in it and generate heat. |
Fact: It’s true that turning compost regularly can be an effective way to get oxygen in your heap. However, there are other methods of including oxygen in your compost heap, such as including bulking material to ensure there is free air space. Also see: How often should you turn compost?
To get heat, you’ll need some way to keep the compost warm.
This is often done by creating a large compost heap, typically more than a metre square. The outer part of the compost provides insulation for the centre of the compost. Some of the heat from the centre seeps out to the edges of the compost, but the centre will remain the hottest part of the compost.
|Myth: You need a large compost heap to generate heat. |
Fact: A large compost heap certainly helps to generate heat. However, insulation can be achieved in other ways too. Insulated compost bins such as the HotBin or the Green Johanna are able to achieve heat without requiring large amounts of compost material. You can also help improve insulation in other ways, such as with a layer of straw.
The microbes that need air also require water to reproduce. What’s more, compost with a lack of water can also be higher in pathogens. However, too much water will force oxygen out of the compost heap, which will cool it down.
Smaller pieces of material
The bacteria which rapidly break down organic material deal better with smaller pieces of material, so it helps a lot if you chop or shred your material.
Cold composting won’t take as much time to explain as hot composting! As the name suggests, it is simply the process of composting down organic material without the use of heat.
As the conditions are not right for the rapid growth of bacteria, bacterial breakdown is slower. Over time, as oxygen runs out you are also more likely to have anaerobic (without air) bacteria than aerobic (bacteria), and it is likely that fungi will play a larger role in the breakdown.
In gardens, this often happens because the heap is not large enough, the compost materials are too large or because the greens (high nitrogen materials) and browns (high carbon materials) are not mixed in the right proportion.
Which is better? That depends on you, but to help you decide let’s compare the two more closely.
Hot and Cold Composting Compared
At a glance
|♨️Hot Composting||❄️Cold Composting|
|Speed||Can be ready in weeks (but you still need to leave it mature).||Can take a year or two.|
|Materials||Some balance of browns and greens is required.||While a mixture of browns and greens can help, this is less important.|
|Maintenance levels||Can be high, depending on the method used||Low|
|Size required||If composting in the open, a large size is required. However, insulated bins can be used to compost smaller materials.||Any size|
|Odour||Reduced odours.||More likely to smell.|
|Pathogen elimination||Eliminates pathogens both in the hot and cooling stages.||Pathogen elimination takes place in the cooling down stage.|
|Weed seeds||Killed if temperatures rise high enough for long enough||Weed seeds survive.|
|Herbicides and pesticides||Some, but not all, are broken down in the hot stage||Fewer are broken down|
|Moisture level||Can get too dry in the hot stage||Can get too wet|
|Tools||It helps if you have a thermometer!||Only garden tools are needed.|
|Distribution of finished compost||Finished compost is distributed throughout the heap/bin.||More likely to have unfinished materials in the outer areas of the compost heap.|
Impact on Speed
First, in hot composting bacteria breaks down the organic material super quickly. This can take many months off the composting process. Cold composting can easily take a year or more.
However, do bear in mind that the compost is not ready to be used after the heat has dissipated. Claims you see of compost being made in 18 days refer to the first part of the process – not the curing stage. See 5 Reasons to Mature Your Compost for Longer for more info.
Variety of materials
You can compost a lot of things with a hot compost heap. (I’ve even composted dead chickens – when I turned the heap, the only thing I found were a couple of bones.)
In contrast, you have to take more care with cold composting, as the material won’t be broken down as quickly. Compost the wrong things and you run an increased risk of smells, pathogens and vermin. As we’ve seen, it also helps hot composting if you break down materials – this is not required with cold composting.
Cold compost heaps can also give off more smells (which can be compounded by using the wrong materials or by an anaerobic composting process). This is caused by increased emissions of VOCs (Volatile Organic Compounds) i.e. compounds that are emitted as gases.
Weed seed elimination
A major benefit of hot composting is weed seed elimination. Weed seeds can survive for many years (sometimes even centuries!) If you get your compost hot enough for long enough, you will kill many or all of these seeds. That makes for much easier garden maintenance!
Hot composting also kills a lot of pathogens quickly! However, it’s worth remembering that compost controls pathogens in a number of ways – and heat is only one of them.
For example, the beneficial microbes in compost compete for resources with pathogens. They release antibiotics that attack microbes, and they can penetrate and kill germs. (See The Pathogen-Destroying Power of Compost for a dive into this fascinating area.)
Herbicides and pesticides
If you are using materials from outside your garden and kitchen, herbicides and pesticides can be a problem. Hot composting can really speed up the process of getting rid of some. However, even hot composting doesn’t get rid of all pesticides and herbicides.
Difficulty level and effort
Hot composting can be more tricky than composting. As we’ve seen, you don’t need to turn your compost every two days – but you do either need a larger pile or a compost bin, and to put some thought into its construction.
I find moisture is one of the trickier things to get right with hot composting. The heat that is generated often means the initial moisture gets used up, especially in dry summers. It’s one of the reasons why I do turn my compost heaps at least once, as I often find I need to spray layers of the compost to introduce more moisture.
This is much less of a problem with cold composting – which is instead likely to get too wet, increasing the chances of bad odours.
Distribution of finished compost
Hot composting, especially if you combine it with turning, tends to lead to finished compost throughout the heap. On the other hand, with cold composting, you have a greater likelihood of unfinished material on the outside.
Other than the garden tools you already have in your shed, you don’t really need any tools for either method. That said, hot composting does become easier when you have composting tools such as a compost fork or a thermometer.
A thermometer lets you know if your compost heap is not hot enough – or indeed if it is too hot, and action is required. That’s something you don’t need to worry about with cold composting.
Bringing it altogether: Choosing the right method for you
If you have a decent-sized garden, or access to plenty of compost material, I’d suggest hot composting. With a little bit of investment of either time or money, you can generate high-quality compost in less time – and have fun doing so at the same time.
However, if you have a small garden, and are lacking in time or enthusiasm, cold composting might be the way forward. It may take longer, but you will end up with compost in the end – and it will still be full of nutrients and beneficial microbes.
Whichever you choose, the most important thing is to compost. Ultimately, whatever method you use, you are turning a waste product into a resource which is incredibly valuable for your soil, your plants and your garden.
The Five Rules of Hot Composting