Many gardeners end up with leftover sawdust after carpentry or cutting down trees in the yard. Although sawdust can be used as animal bedding, it can also be used as one ingredient in compost.
So, how do you compost sawdust? To compost sawdust, be sure to add plenty of nitrogen-rich material (greens), such as grass clippings, to your compost pile. Keep the pile wet enough, since sawdust will dry out a compost pile. Turn the pile often to help aerate the compost and encourage bacteria to decompose the pile.
Of course, you should always remember to wear a mask when working with sawdust. Also, avoid using sawdust from chemically treated wood.
Let’s take a closer look at how to compost sawdust. Then, we’ll answer some common questions about using sawdust in the garden.
How to Compost Sawdust
The first step in composting sawdust is to make sure that your source of sawdust is safe. Do not use sawdust that comes from chemically treated wood.
Chemically treated wood includes (not an exhaustive list):
- pressure-treated wood (contain copper chemicals to preserve the wood)
- railroad ties (they are soaked in creosote, which can leach out)
- plywood (the glue used to hold plywood together contains formaldehyde, which is toxic)
If you don’t have your own sawdust, you can find some from a local sawmill or woodworking shop. Just make sure that the sawdust is from “clean” wood that excludes the materials listed above.
Once you have a safe supply of clean sawdust, it is time to start your compost pile. The next step is to find some “greens” to add to your pile.
Sawdust contains lots of carbon, which makes it a “brown” (carbon-rich) compost material. Grass clippings, on the other hand, contain lots of nitrogen, which makes them a “green” (nitrogen-rich) compost material.
You want a good mix of brown and green materials in your compost pile, so that the carbon and nitrogen levels are balanced. Too much carbon will tie up all of the available nitrogen in the pile, and if there is not enough, the pile will decompose slowly (if at all).
Some common nitrogen-rich green materials that you can add to the sawdust in your compost pile include:
- Grass clippings
- Green Leaves
- Weeds from your garden
- Tea bags
- Coffee grounds
- Crushed Eggshells
- Vegetable and fruit scraps
For more information, check out some of my articles on composting:
- Check out my article on making compost.
- Check out my article on how big your compost bin should be.
- Check out my article on how to make ericaceous compost for acid-loving plants.
- Check out my article on things you should not compost.
Remember that sawdust is very dry, so it can absorb lots of water from your compost pile. This can be helpful after a rain or if you add too much water to your compost pile by mistake.
Also, keep in mind that sawdust is very carbon-rich, so a little goes a long way. For example, your compost pile should not be made up of 50% sawdust. Even with very nitrogen-rich green materials, this would be too much carbon for the bacteria to break down the sawdust.
How Long Does It Take To Compost Sawdust?
Sawdust can be composted within 6 months. This assumes a few things, though:
- you have the proper ratio of carbon to nitrogen in the compost pile (greens, like grass clippings, to go along with the browns, like sawdust)
- you maintain the proper moisture level in the compost pile (not too wet, not too dry)
- you turn the compost pile to aerate it (to encourage bacterial growth and activity)
However, according to the University of Arkansas Extension, composting sawdust can take several years in some cases. This is due to the fact that sawdust is so carbon-rich (or, so nitrogen-poor).
In fact, the ratio of carbon to nitrogen (C to N or C: N) in sawdust can be 200:1 or even up to 500:1! A 500:1 carbon to nitrogen ratio means that sawdust with 500 pounds of carbon would contain only 1 pound of nitrogen.
How to Compost Sawdust Faster
You may not want to wait years for your sawdust to turn into compost that is ready for the garden. Luckily, there are a few steps you can take to speed up the composting process for sawdust. Let’s get into those now.
Add Finished Compost, Animal Manure, or Topsoil to Your Compost Pile
Many times, a compost pile has the proper air and moisture levels, and it has the correct ratio of green to brown materials. It just needs a little “boost” to get it started on the path to decomposition.
One great way to give your compost pile the boost it needs is to add materials that already contain plenty of the bacteria that you want. A few things that already contain bacteria that help to break down compost include:
- Finished compost – this is compost that has completely decomposed. It looks and smells like fresh earth, and there is no detectable trace of the materials that were originally part of the pile.
- Animal manure – this includes waste from animals (such as chickens, cows, horses, etc.) and also their bedding (sawdust, wood shavings, straw, and hay). In addition to containing bacteria, animal waste contains plenty of nitrogen (you can tell by the smell!)
- Topsoil – healthy topsoil from your garden should contain bacteria that will help to break down the materials in a compost pile. If you are lucky, your topsoil may also contain some worms, which can help to break down a compost pile even faster.
For more information, check out this article on sawdust composting from the Cooperative Extension.
Keep Your Compost Pile Moist (But Not Wet!)
In order for the bacteria in a compost pile to do their work of decomposing the materials in the pile, they need the proper moisture levels. It should not be too wet, and it should not be too dry.
Unfortunately, sawdust is extremely dry, and adding a lot of it to your pile can absorb a lot of water and leave the bacteria without enough moisture to do their work. If you find that your compost pile is dry after adding sawdust, you can add some water with a hose to keep it moist enough.
Generally, you can tell that the bacteria are not working if the pile is cool or warm, rather than hot. A compost pile can get up to 140 degrees Fahrenheit when the bacteria are really working!
Don’t worry if the compost pile gets too wet after a heavy rain, or if you added too much water by mistake. In that case, simply add some more sawdust to absorb the extra water.
Just make sure to add enough extra green compost materials (like grass clippings) to offset the extra sawdust you added (remember, it’s all about balancing the carbon and nitrogen in the pile!)
Aerate Your Compost Pile by Turning It
In addition to moisture, the bacteria in your compost pile need air (oxygen) in order to work properly. If your pile just sits there, it may not get the air circulation it needs.
One way to aerate your compost pile is to simply turn it over with a shovel or pitchfork. When you do this, you should see steam coming up from the hot compost if the bacteria are working properly.
You should turn your compost pile anywhere from 1 to 2 times per week to give it the proper aeration.
Another option is to use a compost tumbler to store your pile. This will allow you to protect the pile from getting too much rain.
More importantly, it will allow you to turn the tumbler by hand without using a shovel or pitchfork. The best part of all is that you can put a wheelbarrow right under the tumbler and empty the compost for easy transport when it is fully decomposed.
Is Sawdust Bad For Soil?
Yes, pure uncomposted sawdust by itself is bad for soil. There is too much carbon in sawdust, and not enough nitrogen.
As a result, the carbon in sawdust ties up the nitrogen in soil as it decomposes. This makes the nitrogen in the soil unavailable for plants.
According to the Texas A&M University Extension, this lack of nitrogen can last anywhere from several months to several years. The time frame depends on the amount of sawdust added to the soil and the soil conditions (moisture, aeration, etc.)
If you have already added sawdust to your soil, you can counter the problem by adding finished compost to your soil. The nitrogen in the compost will offset some of the carbon in the sawdust, and the bacteria in the compost will speed up the process of decomposing the sawdust.
Can Sawdust Be Used As Fertilizer?
No, pure uncomposted sawdust should not be used as a fertilizer. As mentioned above, sawdust does not contain a significant amount of nitrogen.
Plants need nitrogen in large amounts for proper growth. A lack of nitrogen will lead to chlorosis (yellow) leaves, stunted growth, and other problems in plants.
For more information on nitrogen and other soil nutrients, check out my article on NPK ratio and what it means.
Will Sawdust Kill Grass and Weeds?
Yes, if you put sawdust over grass and weeds, it will kill them by smothering them. This works just like any other mulch would – a thick layer of mulch will cause a lack of air and sunlight, which will kill grass and weeds eventually.
As mentioned above, sawdust in soil can leach nitrogen when mixed in. However, this is not such a problem if it is used as mulch over the top of soil and not mixed in.
By now, you have a much better idea of how to compost sawdust. You also know about its other uses, and how not to use it in the garden.
I hope you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about composting sawdust, please leave a comment below.