Bones can stump even the keenest zero-waster.
You always hear stories of bones being dug up years and years after they were buried, so does that mean they never break down? (thankfully, no it doesn’t!)
In this article, I explain exactly how bones and break down and tell you the best ways to compost them fast.
You can compost bones, but they’ll take a lot longer to decompose compared to other food waste and you risk attracting pests. The best way to compost bones is to hot compost them. You can also pre-process the bones by cooking them, breaking them, or putting them in a bokashi bin.
Can you compost bones
Yes, you can compost bones.
The rule is anything that was once living can be composted. Bones are 100% natural and come from a living thing. Eventually, they’ll break down and their nutrients will be released back into the ground.
The key word here being eventually. Bones are made from a much harder and more dense material than most compostable things.
Different bones take different amounts of time to compost.
Smaller bones like fish or chicken bones will disappear within a few months.
For big, in-tact bones it can take years and years for them to break down. And that’s in the right conditions (i.e a warm, damp environment). In the wrong conditions, they can last several lifetimes.
That’s why bones can be discovered hundreds of years after they were buried.
Luckily, compost provides the perfect environment for bone decomposition.
If you don’t mind waiting, composting bones is as easy as potato peel. If you’re impatient, composting bones becomes a little more complicated (but still doable).
One important consideration is pests. Bones and meat are a very attractive treats for wild animals (small and big). But as long as you take steps to protect your composting bones, you’ll be fine.
More on that later!
How do bones decompose?
Understanding exactly how bones decompose can help you compost them as efficiently as possible.
Bones are made from a mixture of collagen (a protein) and calcium (a mineral), plus a load of other organic matter, but those are the important ones.
Bacteria and fungi will happily attack collagen, but its tight linear structure makes it difficult for microbes to penetrate. Calcium is a mineral and not something that composting microbes can digest.
The calcium is bound to the collagen in the bones giving it SUPER strength and making it even harder for the microbes to attack the collagen.
The microbes need to get past the minerals, then navigate the tight structure of the collagen before they can start feeding on the bone and breaking it down.
This is hard work and takes them a long time.
Also, only certain microbes can digest collagen. So not all the microbes in your compost can work on the bones.
Eventually though, enough of the collagen will be broken down that the bone will lose its strength. The minerals will leach out of the bone and its physical structure will collapse into dust. The bone will be gone.
In simpler terms, bones break down the following way:
- Bacteria and fungi attack collagen (which is very hard to get to)
- Collagen/mineral bonds break down
- Minerals leach out of the bone
- Bone breaks down
The more microbes there are available, the faster the bones will break down. Microbes thrive in compost, so compost is the ideal place for bones to decompose. In hot, dry climates where microbes can’t survive, bones can survive for millions of years.
How to compost bones
There are a few tricks and watch-outs for you to consider before you compost bones.
Below are my top tips for composting bones.
Pre-process the bones
To speed things along, you can pre-process the bones before you compost them.
Pre-processing the bones helps make them more accessible to the microbes in your compost pile, so they’ll compost faster.
You won’t perform any miracles, but the bones should be gone in a few months rather than a few years.
You can do one, none, or all of these steps.
I always boil the bones, put them in the bokashi bin, and then break them up. The more ways you pre-process the bones, the faster they’ll compost.
Roast and boil the bones (make bone broth)
To make bone broth you roast the bones then boil them. If you’re a keen home cook, you probably already make bone broth. But you might not realize how helpful it is for composting your bones.
Roasting bones makes them more brittle (and makes your bone broth taste 10 times better).
I think about it like this.
You can’t give your dog cooked bones because they might shatter, but raw bones are fine. The cooking process dries the bone out so it can be more easily broken.
Once you’ve done that, it’s time to boil the bones.
Boiling the bones softens them (by removing collagen and minerals) and makes them easier for microbes to break down.
It’s also super healthy and nutritious. You can drink it as it is, or use it as stock in soups and stews.
Adding a splash of vinegar to the pot while you’re boiling the bones will help to extract as much collagen as possible. The more collagen you break down during the boiling phase, the weaker the bones will become and the less work there’ll be for the microbes to do.
Another positive of cooking your bones is that you cook any leftover meat on the bones. This massively reduces the risk of introducing any bad bacteria into your compost pile. Plus, cooked meat composts faster than raw meat.
For super fast composting, you can boil the bones and then put them in a bokashi bin. I explain what bokashi is in the next section. Some people even double boil their bones.
My favorite method of pre-processing bones is to use bokashi. It’s really simple, doesn’t take a lot of time or effort, and most importantly, doesn’t smell.
Bokashi is an anaerobic fermentation method. It doesn’t fully compost food waste. It ferments it. The fermented food waste is A LOT more accessible to composting microbes, so the fermented bones will be processed a lot quicker.
- A Bokashi bin (I suggest getting two because then you can fill one up while the other is fermenting)
- Bokashi bran
- Put the bones in your bokashi bin along with any other food waste you have. You can add ANY food waste. Fruit and veg waste, cooked leftovers, meat, and dairy are all fair game.
- Every two inches, add in a handful of bokashi bran. The bran is full of microbes that will work on breaking down your food. Don’t hold back with the bran, you can’t add too much. Since we know bones are hard to compost, I always add a few extra handfuls of bran when I have some bones in my bucket.
- Once the bin is full, squeeze as much air out as you can and close the bin. Don’t open the bin again for at least two weeks.
- Every day check the bin for excess liquid. Most bokashi bins have a tap so you can easily drain them.
- After two weeks (three if you have a lot of bones), open the bin and check your compost. It should be fermented.
- Try and break the bones up with a hammer or a bone cutter. They should be pretty brittle after being fermented
- Add the fermented food waste to your traditional compost bin, or bury it in the garden and wait for it to fully break down
That was my whistle-stop guide to bokashi. If you want more detail, head on over to my ultimate guide to bokashi.
As well as speeding up the bone’s decomposition, fermenting them also makes the bones less attractive to rodents. Pests hate the taste of bokashi.
However, that doesn’t stop them from being curious. Take the same precautions as you would with any other food waste. Bury the bokashi in the middle of your pile and add some brown material at the same time.
If they do come and investigate, you can rest easy knowing they won’t come back. If they’d found fresh bones… I’d be worried!
Ideally, you’d put cooked bones in the bokashi bin. They’re softer and easier for the microbes to digest. But you can also use raw bones. The anaerobic microbes will kill any potential pathogens, so you don’t need to worry about bad bacteria.
Break the bones up
Before you put anything in your compost bin the advice is to shred it, crush it, or break it up. Make it as small as you can because smaller things decompose faster.
The more surface area there is for the microbes to work on, the faster they can eat the object in question.
The same goes for bones. Only it can be a lot harder to break them up.
The easiest way to cut smaller bones is with some bone scissors. Oxo does a really good pair (Amazon link). They’re mostly used for spatchcocking chickens, but they double really well as a compost bone cutter.
Bigger bones will be much harder, or even impossible to break when they’re fresh. But after a few weeks or months in the compost, they should become a lot more brittle. While you’re turning your compost you can pick them up and try breaking them again.
This isn’t essential but will help them compost a lot faster.
If you’ve cooked, boiled, or bokashi’d the bones then you might have more luck breaking them up before you put them in the compost. A hammer could come in handy here.
Burning bones might get the police called on you, but once you explain it’s for compost they’ll leave you alone!
All jokes aside, I only recommend this option if you have a lot of land and you’re sure you won’t annoy the neighbors. It can be VERY smelly.
Burning bones is the quickest way to get rid of them. And once they’re burnt, you’ll have no vermin worries.
Get a small fire going with some wood, then add the bones. The bones will heat up and release fat which should continue to fuel the fire. Once the fire has burnt out, you’ll be able to crush the bones with your hands.
Burning the bones essentially makes bone meal (ground bones)
Bone meal is a slow-release fertilizer that provides phosphorus and protein to your plants. There’s no need to throw the crushed bones in your compost pile. You can use them straight in your garden.
Although it does work well as a compost activator!
Whatever you decide to do with the burnt bones, they’ll disappear pretty quickly.
Composting the bones
Once you’ve pre-processed your bones, it’s time to compost them.
The best way to compost bones is in hot compost. But you can also bury them. I don’t recommend cold composting bones.
An acidic environment can also increase the speed at which bones decompose.
Hot composting is the fastest method of composting and the one I recommend if you’re going to be composting bones.
It’s more involved than cold composting, but the bones will break down a lot quicker. This means the garden benefits from the nutrients quicker, and there are fewer opportunities for rodents and pests to come digging around in your compost pile.
Hot composting is very similar to traditional composting, you just need to pay more attention to the brown:green ratio and turn the compost more often. The idea is that the compost reaches at least 140° and stays there for a sustained period of time.
You can hot compost with an open pile, but it needs to be at least 3x3x3 feet in diameter. If that’s not possible for you then I recommend buying a dedicated hot composting bin. The insulted bins trap heat within your compost and make it easy to sustain the high temperatures needed.
The high temperatures will kill any potentially harmful bacteria from any raw meat, but it’s still advised to cook the bones before you compost them so they’re easier to process.
You should also follow the general composting rules such as placing the bone in the middle of the pile, and breaking them up as small as you can before adding them.
My favorite hot composter is the Jora tumbler.
It’s a two-chamber tumbler with a thick layer of polystyrene for insulation. While one chamber is curing, you can start filling up the second one.
The fact it’s a tumbler also makes it really easy to turn your compost and keep it well aerated.
The Jora tumbler is also completely pest-proof. It’s raised so ground-bound animals won’t be able to reach it, and it has a secure lock system to keep larger animals out.
Bury the bones
An alternative to hot composting is burying the bones.
This will keep them safe from pests and allows them to compost slowly without any interaction from you.
I would advise burying the bones with some other, more compostable, materials to ensure there’s a good microbial population around the bones. The further down you bury them, the less chance there is of smells and pests.
You can either plant on top of the bones while they’re decomposing, or you can wait until the bones are completely composted and then dig up the nutrient-rich soil to use elsewhere in your garden.
Burying the bones is a slow, but effortless method.
I wouldn’t recommend cold composting bones. It takes FOREVER.
Your bones will be sitting in the pile for a very long time. And the longer the bones are sitting there, the more chance there is of them attracting unwanted guests.
Increase the acidity of your compost
Composting microbes don’t feed off the minerals in our bones, so that part of the bone needs to be decomposed in a different way.
Minerals will be lost naturally over time in a process called leaching. The minerals will (VERY SLOWLY) dissolve into the water passing through your compost and be carried away.
Alkaline conditions slow down leaching, while acidic conditions speed up leaching.
Calcium phosphate (the main mineral in bone) reacts readily with acid. This is why bones decompose much faster in peaty soils than in sandy soil. Because peat is more acidic.
Enhancing the acidic nature of your active compost will help the bones decompose. Acidic ‘ingredients’ include coffee grounds, oak leaves, and pine needles. As these items decompose they create an acidic environment.
Don’t add any garden lime to the compost because this will have the opposite effect and dampen the acidity.
Once the compost has completely decomposed it will return to a neutral pH, so there’s no need to worry about acidifying your soil. If you are worried, you can always only use the compost around acid-loving plants like blueberries and hydrangeas.
If you’re burying the bones you can try burying them with a mix of garden soil and peat moss. The peat moss will increase the acidity of the soil around the bones, meaning they’ll break down faster.
Note: using peat moss is not good for the environment as it’s a finite resource and carries a significant carbon footprint. But there is no real substitute for it yet in terms of acidity.
Use a pest-proof bin
Animals have a much keener sense of smell than humans. They can smell a treat a mile off, and a bone is a very nice treat.
To prevent animals from scavenging in your bin, I suggest either getting a pest-proof bin, or building some sort of barrier around your compost.
If you’ve buried the bones, then you can put chicken wire over the top to stop pests from digging in it.
Burying the bones as deep into the pile as possible and insulting the outside will also help to keep pests away.
Can you put bones in a worm bin?
Bones will degrade in a worm bin, but it can take an extremely long time if you put the bones in whole and fresh. It’s much better to pre-process the bones first. You can make the bones softer by pickling them, boiling them, or putting them in a bokashi bin.
Once you’ve processed the bones you should be able to grind them up into a powder which will be much easier for the worms to digest.
Crushed bones also add grit to your worm bin, a vital component.
If you’ve put the bones through a bokashi bin, remember that the resultant mixture is very acidic so you should add it slowly to the bin rather than all in one go.
If the bones have any meat on them take extra care to bury them really well. Meat in a worm bin can smell really bad and attract a lot of pests
What to do with leftover bones in your compost
If the rest of your compost is ready to go but there are still some visible bones in there, don’t worry, you can still use the compost.
You just need to decide what to do with the bones. You can either:
- Sift the bones out and chuck them in your next batch of compost.
- Scatter the bones along with the compost and leave them to finish composting in your garden (if you don’t mind a few stray bones!)
After going through a whole batch of compost, the bones should be quite brittle by now and easy to break. Before you put them back in the pile, try to break them up a bit more.
Breaking the bones up will help them in their next stage of decomposition.
Frequently Asked Questions
Can I compost chicken bones?
You can compost chicken bones. Chicken bones are relatively small and thin. They will compost pretty quickly in a hot compost pile, the bones should be gone within a few months. Crushing the bones or breaking them up will increase the available surface area and speed up the composting process.
Can I compost beef or pork bones?
Beef and pork bones are thick and will take a very long time to fully decompose. But they will eventually compost. The marrow will disappear first leaving you with a hollow structure. The hollow bone structure could take years to disappear. Cutting the bones up can help.
Can I compost fish bones?
Fish bones are the fastest bones to compost. They’re very thin and you can quickly crush them up to speed the process up even more. Fish bones should be gone within a couple of weeks. Make sure to bury them well within your pile to prevent unpleasant odors and to deter pests.
Can I compost raw bones?
Yes, you can compost raw bones, but I would always recommend cooking them first. This kills harmful pathogens and makes the bones much more accessible to the bacteria and fungi that work to break them down.