Compost Magazine

Composting tips, advice and science.

Featured image showing bulking material in hand.

Compost Bulking Agent: The Secret Key to Successful Composting

My first compost heaps were pretty awful. 

I used to put everything in a corner of the garden, and then wondered months later why I had a slimy mess. 

Later on I experimented with regular turning of my compost pile, and decided it was too much work to turn a compost heap every few days.

Fortunately, there is a way to compost that both helps your compost and reduces work. 

And that’s by using bulking agents. 


What are bulking agents?

Bulking agents are high carbon materials (browns) such as shredded straw or cardboard, sawdust, wood chips or leaves.

They are added to your compost heap along with high nitrogen materials (greens). (See Carbon:Nitrogen ratio for an explanation of these.)

Surprisingly, there’s not a huge amount of information available about bulking agents. In fact, I’ve read whole books on composting which don’t even mention them.

That’s a shame, because the correct use of bulking agents can help you achieve better, faster compost with less work. 

How bulking agents help compost


The microorganisms that turn organic matter into compost need oxygen to do their work.

A static pile without oxygen will turn from aerobic (with air) composting to anaerobic (without air) composting. That means slower composting, unpleasant odours, more harmful emissions and likely a slimy mess in the process. 

Some composting systems use regular turning to introduce oxygen.

That’s fine with a small pile, but as you compost larger amounts it can become a lot of work. Some commercial systems blow air into the windrows, but this requires more energy and increases the carbon footprint. 

A good bulking agent creates free air space inside the compost heap.

This traps air and oxygen, allowing the microorganisms in compost to do their work without the regular addition of more air. 

Absorbing water

Bulking agents can also absorb water.

This makes it ideal for use with green materials which are often high in moisture. By absorbing excess moisture, bulking agents can bring down the moisture level to the ideal level

Balancing high nitrogen compost materials

For optimal composting conditions, you need a balance of high nitrogen and high carbon. As bulking agents are usually high in carbon, they work well with high nitrogen materials. 

Adding bulk to the compost

High nitrogen material can compost down very quickly, leaving very little in its wake!

Bulking materials, as the name suggests, can add bulk to the compost, leaving you with a more substantial compost pile at the end of the process. 

Acts as a biofilter

Bulking agents also absorb bad odours from your compost, which get consumed by bacteria which lives on the bulking material. 

A study by Shao et al looked at composting municipal waste with rice straw. The optimum amount of bulking material for reducing sulphur emissions was one part straw to five parts municipal waste.

Qualities of good bulking agent


Bulking agent needs to be dry. Part of its job is to absorb moisture. As you can imagine, if you add wet material to wet material you just end up with more wet material. 

High in carbon

As bulking material is used to balance out high nitrogen materials in your compost heap, it’s also important that it is high in carbon. 


The particle size is important. You need to ensure that there is enough air space – but not too much. 

Again, experimentation is probably key here, but it’s helpful to know that one study by Gea et al found that the optimal particle size is 5mm

The challenge with bulking materials

There’s one problem I find with bulking agents, and that’s finding enough. 

In the summer, for example, I have tons of grass to compost, both from myself and my neighbours. I generally have bulking material I have saved over the winter, but that soon gets used up. 

I’ve experimented with several bulking agents.

Currently I am using sawdust, both bought and from a local woodyard. This seems to work better than the paper I used to use.

When I run out of the ideal material, I do sometimes use strips of cardboard in between grass.

This is not ideal. Cardboard should be shredded, and I seem to have more consistent results when using other materials.

However, in contrast to other bulking materials, I have a huge supply of free cardboard. 

Common bulking materials

Sawdust at a commercial compost facility.
Sawdust ready for use at a commercial composting facility.

All sorts of materials have been used as bulking materials. For example, one study examined using pine cones with straw and goat manureterial to create free air space. (The best mix, apparently, was 10% pine cones, 45% goat manure and 45% wheat straw.)

Not every potential bulking material has been studied, so it may well be the case that you need to experiment with the materials available to you locally. Here, however, are some common bulking materials used. 

Wood Chips

Wood chips provide excellent free air space and I have found they made a real difference in my HotBin. You can also buy semi-decomposed wood chips which are more likely to rot down. 

However, I do find that the wood chips don’t rot down as fast as other material due to their high lignin content. That can lead to woody compost. You can sieve the compost to retrieve (and indeed re-use) the wood chips, but ideally you want to minimise the work needed. 

Woodchips themselves are very cheap, but the decomposed wood chips provided for use in the HotBin are much more expensive. You can choose to decompose them yourself. 


Cardboard is very high in carbon and soaks up liquid. It also rots down a lot more quickly than wood chips. You may find you have some on the outside of your compost heap, but in the centre it rots away nicely. 

I have seen some sites suggesting it becomes too wet. Personally, though, I haven’t found that to be an issue (obviously it will depend on the overall moisture content of your heap).

Ideally, though, it should be shredded. If you have a large compost pile that can be a problem for people who don’t have shredder.

However, as mentioned above, I have managed to compost effectively with strips of cardboard. 

Cow manure

Being a manure, cow dung does not seem like a likely candidate.

Still, a number of studies have shown that it is an effective bulking agent. That might be because cow manure is lower in nitrogen than many other manures


Sawdust works well as a biofilter, and also absorbs bad odours. 


Paper is often used alongside other bulking agent to soak up water. However, waste paper has been used as a primary bulking material in a study by Zorpas et al to produce high quality compost.

I used to use a lot shredded paper in my own compost heap, but I’ve now switched

Straw (both rice and wheat)

Dry straw is high in carbon.

Ideally it should be shredded before being used as a bulking material – but manure with straw always seems to work even thought the straw is not shredded!

Zeolite and perlite

One recent study tested several bulking agents, and found that the best ones were Zeolite and Perlite.

However, these are not always easily available in bulk and at a reasonable price in my area, so I haven’t tried them yet!

How much bulking agent should you add?

Layering compost.
In the above picture, I am layering grass with dried paper and dry manure and straw. The outer layer is a straw insulation I was experimenting with.

Compost is both an art and a science.

You’ll see varying advice on how much bulking agent to add, and even varying results from study.

For example, the study on goat manure we looked at previously suggested using 55% bulking agent to 45% goat manure. However, a study on municipal waste found that 20% rice straw was the optimum ratio. 

Ultimately, there are many factors which can affect the composting process, such the material you use and the climate you are in. 

For example, if you are using very wet materials and you’re in a damp environment, you’ll need to add more bulking agent. If you are using dryer materials, and you are in a dry environment, you’ll need less. 

Unless you’re an advanced compost facility able to measure the volume and moisture levels of your compost material, you’re never going to get it exactly right. However, time and experience will help you learn what works with the conditions and material you use.

A good starting point is a roughly 50:50 mix. That should also produce a reasonable C:N ratio

Tips for using bulking agents

Mix or layers?

Ideally you’’d mix your bulking material with your greens. However, there’s always a play off between best practice and time required!

When I use my hot bin, I am processing smaller amounts of waste, and I use a tool to mix the top layer.

However, when I compost large amounts of material I simply add the different materials in layers for speed. 

Thick or thin layers?

Some sources suggest using layers of up to four inches thick.

As I compost a lot of grass, I prefer to use thinner layers. Grass can quite easily deteriorate into a slimy mess if left in a thick layer. 

Again, there’s a trade off between time and optimum results.

If you have the time, then use very thin layers. If you are short of time, you may choose to use thinner thicker? layers. 

Plan ahead

If you have a large garden, you can very easily generate large quantities of high nitrogen material in a short period of time.

The temptation then is to pile it on to your compost heap without adding bulking material. 

To avoid this, it’s a good idea make sure you have plenty of bulking material ahead of time.

Green Mountain Technlogies recommends ensuring that you always have a large quantity of sawdust on hand for compost emergencies!

Storing bulking materials

As we’ve seen, bulking materials need to be dry. So it’s a good idea to store bulking materials inside if possible. If you can’t, it’s worth covering them over to keep them dry. 

Wrapping up

Given the importance of bulking material, it’s surprising it doesn’t get more attention in compost literature.

Getting enough bulking material is a challenge during the growing season. But as it dramatically improves results, it’s well worth putting the effort in to obtaining it. 

Read more

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