Got a slow compost heap?
Then you might well be on the lookout for a compost accelerator or activater.
I get it – as a compost nerd, I’m always keen to speed up my own compost.
As a result, I’ve spent years researching different ways to activate the compost and trying out (mostly free!) activators.
So before you bounce out like an overexcited bunny and spend your hard-earned cash, let’s consider:
- What types of compost accelerators are there?
- Do they work?
- If so, when do they work, and which ones should you use?
- What free alternatives are there to commercial activators?
What is a compost activator?
A compost activator is a substance that is used to encourage a compost heap to start the decomposition process – or restart a compost heap that has slowed down.
They are also known as compost accelerators or starters.
Typically a compost accelerator is high in nitrogen. However, some compost activators may contain carbon or microorganisms.
These products can be sourced naturally or purchased commercially.
There are also microbial activators. These are inoculated with live microbes such as bacteria or fungi.
How do compost nitrogen and carbon activators work?
The bacteria that decompose organic material in your compost heap need both carbon and nitrogen to do their work.
The combination of these two elements is called the Carbon-Nitrogen (C:N) ratio.
While a wide range of these ratios can work, the optimum range is around 30:1 to 35:1.
To achieve this balance, you mix high carbon (Brown) and high nitrogen (Green) materials together.
A compost accelerator can be beneficial if the mixture of your compost heap is out of balance.
For example, imagine you have compost that is high in Browns, such as paper and leaves.
In this case, the bacteria won’t have enough nitrogen to decompose the compost materials.
By adding a nitrogen-rich compost activator, you rebalance the C:N ratio, and supply the microbes with the nitrogen they need to effectively decompose the material.
Similarly, if your compost heap is too high in nitrogen, a carbon-rich compost activator could help restore the carbon – nitrogen balance.
Do compost activators actually speed up composting?
Nitrogen and carbon activators can accelerate the composting process IF the carbon and nitrogen in the compost are out of balance.
However, there are often free alternatives – and these can be just as effective.
For example, one study looked at the impact of composting grass with commercial compost activators.
The researchers found that there was no improvement over easily available natural activators such as mature compost or soil.
(While the study summary didn’t mention the type of activator used, I presume it contained carbon.)
CarryOnComposting also reports that Which! magazine carried out tests on compost activators and found they did not accelerate the composting process IF the carbon-nitrogen ratio was correct.
That’s to be expected, as there’s no point in using a high nitrogen or carbon activator if the Carbon – Nitrogen ratio is right.
Do microbial compost activators work?
Activators that contain microorganisms may be effective at speeding up compost – and improving the results.
One review found that a majority of studies showed that microbial inoculants could help, especially when conditions or materials were not ideal.
The benefits included:
- prolonging the hot (thermophilic) stage of composting
- regulating microorganisms in the compost
- increasing the quantity of minerals and nutrients available to plants in the finished compost.
However, we can’t be sure that the microbial activators we buy are the same as the ones used in studies like this.
Another place to look at is professional composters, who presumably know what they are doing when it comes to composting!
In fact, professional composters often disagree on the value of microbial compost activators, with some arguing they work and others thinking they are a waste of time and/or money.
Those who are pro-inoculants sometimes add mature compost to a compost heap, believing that the microbes in it will help the process and can especially help with breaking down tough materials like lignin.
Whether this works or not, it’s free and requires little effort!
For those interested, Biocycle has an exhaustive discussion of the research and debate here, and concludes that:
“Inoculating fresh feedstocks with acclimated microbes from the composting process by either mixing in overs and/or recycling some compost is a tried and true method of bringing compost piles up to acceptable temperatures quickly.”
Do you even need a compost activator?
You probably don’t need a compost activator if you have got the ingredients of your compost pile right.
If you have a mix of Green (high nitrogen organic materials) and Brown (high carbon organic materials), your compost heap should have everything you need to get started.
In fact, if the mix is right, you could derail the composting process by adding a compost activator.
Compost activators that are high in nitrogen can add value when there are too many ‘Browns’ in the compost heap, as they can help balance the Carbon Nitrogen ratio by adding more nitrogen.
Carbon activators could also help when there is too much nitrogen.
However, compost activators can’t resolve other problems.
If your compost is too wet, too dry, or doesn’t have enough air, composting will slow down and could become compact and smelly.
Natural compost activators
There’s generally no need to buy a compost activator, as there are free and DIY options available. These include:
Urine is high in nitrogen, and research by the Rodale Institute* has found it can be effective in accelerating decomposition.
Challenges can include:
- getting enough urine
- the disgust some people feel about using wee
- avoiding getting the compost heap too wet if you do have a large quantity.
(On the other hand, that final factor also makes it very valuable if your compost heap is too dry.)
Side note: The National Trust in the UK introduced pee bales for their employees. Instead of going to the toilet, staff pee on bales of hay which are later spread in the fields.
If they are peed on enough, the bales of hay should have a great carbon-nitrogen ratio!
High nitrogen plants
Any plant high in nitrogen will help activate a compost heap with a high carbon level.
Examples include nettles, comfrey, and legumes such as peas and beans.
One option is to plant green manure such as field beans, which can extract nitrogen from the air.
You can plant them in the autumn, harvest them in the spring before they flower and mix them with the compost heap in the spring.
(Side note: I once tried this, but they looked so healthy I ended up letting most of them crop and eating the beans! They were so good I’ve saved some beans for replanting every year since.)
As with all compost materials, it’s best to chop or shred these high nitrogen materials to maximize their surface area.
I personally find grass clippings are very effective at getting a compost heap hot.
In one experiment, when I mixed grass into my homemade trash bin composter, the heat increased by 10 degrees Celsius in 2 days.
However, grass clippings need to be well mixed in with the compost and/or dry brown material, or they will soon become a soggy, compacted mess.
Fresh manure is high in nitrogen, and as such is an excellent compost activator.
I’ve found in the past that adding chicken manure to my insulated compost bin increased temperatures, while a combination of cow manure, grass, and sawdust produced temperatures of around 65 degrees Celsius in my pallet bin.
The amount of nitrogen in each type of manure varies greatly – you can see a breakdown of the nutritional values of different manures here.
Coffee grounds and tea leaves
Both coffee and tea leaves are high in nitrogen and can be used as an activator.
However, it can be a challenge to find enough of them – unless you have a good relationship with a local cafe!
(Or you could try what I do, and strategically place kitchen compost bins in kitchens at work!)
What’s more, tea usually comes in teabags which can contain microplastics.
They can be opened to release the tea leaves, but the process is likely to be too time-consuming to get the quantities needed.
Related: How to Compost Tea Bags
Mature compost and soil may not add much in terms of nutrients, but they do have beneficial microbes.
These may or may not help the decomposition process, but adding a few handfuls of compost is cheap and virtually effortless, so you don’t have much to lose.
I often scoop a few shovelfuls from a mature compost bin in between layers of other materials in a new compost pile.
Compost and soil are also high in carbon, which can help balance high nitrogen piles.
Used Cooking Oil
One recent study found that cooking oil can increase the heat of a compost pile.
That wasn’t the only benefit either, as the final compost had 20% more nitrogen than compost made without cooking oil.
However, this has only been shown in one study so far. Ideally, I’d like to see other studies confirm that adding used cooking oil is effective.
What’s more, when I spoke to the researchers, they weren’t sure if it would also increase the speed of the composting process.
For more information, including tips from the researchers and how much oil to use, see Cooking Oil, Compost Heat, and Nitrogen.
Blood, fish, and bone
Blood, fish, and bone are high in nitrogen as well as other essential nutrients.
However, in contrast to many of the other items here, you do need to purchase it, and you may find it is better to use it as a fertilizer.
If you really don’t have the time or energy to find natural compost activators, you might want to buy a commercial one.
One thing to bear in mind, though, is that while research suggests microbial compost starters work, we don’t necessarily know if the commercial activators we can buy are the same ones used by the researchers.
In fact, as I was writing this, I went and had a look at a few random compost activators on Amazon.
I couldn’t see any which listed the bacterial formulations they used. That could be because they don’t want competitors to know their formula – but it also means we can’t look up the formulation.
How do you make your own compost starter?
None of the activators above need any special mixing it, but there is one recipe for making a compost starter – drunken composting.
Drunken composting doesn’t involve getting drunk! Instead, it involves adding beer, soda, and ammonia and then spraying the mixture over the compost.
Here’s a video that explains the process – although do note you can replace the soda with sugar.
Do note I can’t vouch for it, as when I tried to split-test it in small bins the compost dried out!
To bring it all together, let’s recap on the questions at the start.
Do compost activators work? Yes, IF the Carbon-Nitrogen ratio is out of balance.
Do you need to buy a compost activator? No. While they could help, there are free alternatives readily available.
Do microbial inoculants work? Probably. It certainly won’t do any harm to add some finished compost to your compost heap. However, we can’t be sure if commercially available activators work.
I’ve never tried commercial activators myself, as I usually have a range of free or cheap alternatives to hand.
However, I do know a very experienced gardener and composter who does use compost starters.
For decades he got excellent compost.
Of course, we can’t be sure if that’s cause or correlation (i.e. he may have got good compost because of other factors).
But I am inclined to agree with Carry on Composting – if you already use activators and get great results, maybe you should carry on doing so.
Or, even better, set up two compost heaps, one with an activator and one without, and see which one works best!
(When you find out, don’t forget to let me know in the comments below!)
Q. How do I know when to add a compost activator?
A good time to add an activator would be when your compost pile isn’t decomposing as quickly as you’d like. If it has a lot of brown materials, and other factors such as moisture and aeration are correct, then it could be a good idea to add a nitrogen-based compost activator.
A compost thermometer can help you monitor the temperature and gauge the activity level in your pile – you can see how to use one in our guide to compost thermometers.
Q. Can I use beer as a compost activator?
A: Beer is rich in yeast and should help stimulate microbial activity in your compost pile. However, one study that looked at yeast in composting found that the temperature had to be controlled to gain benefits from the yeast, as high temperatures kill the yeast.
One popular composting method, ‘drunken composting’, combines beer, sugar, and ammonia to speed up the process. However, I haven’t seen any split tests or research that proves it works.
Do note that beer can also attract fruit flies to your compost bin.
Q. Is pet waste a good compost activator?
Pet waste can be high in nitrogen, and therefore should work well as a nitrogen activator. However, it’s best only to use pet waste from carnivorous animals if you are an experienced composter, and can be sure you will get the high temperatures that will kill pathogens. Droppings from pets that prefer plants – such as rabbits and guinea pigs – are safer to use, even in cooler piles.
Q: Can I make my DIY own compost activator?
A: Yes, you can make your own compost activator. Materials rich in nitrogen, like fresh grass clippings, coffee grounds, or aged manure, can be used as a nitrogen-rich compost activator. You don’t need to mix these together – just put them in layers in the compost when you turn it, or make holes in the compost with a rebar or compost aerator and put the materials down the holes. (This is also a good opportunity to add water if the compost is too dry.)
Q: How often should I add a compost activator to my compost pile?
A: This depends on the state of your compost pile. If your compost pile is cool, isn’t breaking down efficiently, or if you have added a lot of ‘browns’ to your pile, you might need to add a nitrogen compost activator. Monitor your compost pile regularly to determine when it needs a boost – again, a compost thermometer is a good tool to help you do this. However, if the compost has been through the hot phase, and has already started to break down, it is best to leave it to complete the maturation phase.
Compost Activators, Accelerators, Inoculators and Makers (Carry on Composting): Detailed discussion of compost activators which references a number of studies and also discusses specific examples of commercial activators.
Compost Accelerators, Starters and Activators (Garden Myths): Author and gardener Robert Pavlis shares a skeptical take on compost activators.
*While it doesn’t seem to be available online, the research was mentioned in the Rodale Book of Composting (1992 edition).