Sawdust for Composting
First I would make sure the wood has not been chemically treated.
Check a cross section of the wood for the distinctive ring of green color around the first half inch or so.
If it has been chemically treated, it will contain chemicals like arsenic, chromium, and copper — not suitable for composting.
Make sure sawdust (and other carbon-rich "brown" materials) isn't more than about 80% of the material you are composting.
That's just a basic guideline.
You need some nitrogen-rich "green" material (living organics like leaves, fresh grass clippings, and other kitchen scraps) for decomposition.
Ideally, add the green materials and then spread sawdust on top of them.
Sawdust for Mulch Sawdust is sometime recommended as an effective mulch for acid-loving plants (e.g. rhododendrons, begonias, impatiens, blueberries, etc), but for anything else, you will have to manage the acidifying effect as the sawdust decomposes.
Wood chips and saw dust will rob soil of nitrogen as it decomposes, but since saw dust will decompose a lot faster, you may have to compensate the addition of nitrogen.
Keep in mind that saw dust can also compact severely over a single season, so you have to be sure to break it up periodically.
Some people recommend a combination of straw and saw dust, but you have to watch for excessive water runoff nonetheless.
gardening.stackexchange.com/users/85/Robert Cartaino
I have used it as a mulch for the path ways.
A thick layer does a good job of keeping out grass and weeds, if they do grow they are easily pulled off the top of the sawdust.
If used on pathways you don't have to worry about it taking nitrogen from your plants.
gardening.stackexchange.com/users/2685/T.Smith
I wouldn't use the sawdust if the wood has been treated with any chemicals.
I wouldn't use it in a food garden anyway.
It won't hurt the plants, but who knows what chemicals are on the wood and if they will get in your food.
Otherwise, you can use them however you want to use them.
In compost is fine or on the ground as mulch or mixed into the soil.
If you're worried about sawdust competing with plants for nitrogen, then compost them first or use as mulch.
gardening.stackexchange.com/users/2404/Randy
I read something recently about using wood chips as mulch that might be concern for some people.
Since sawdust is just really, really small wood chips I thought it might be worth pointing out in case the same applies.
It seems wood chips used as mulch can attract shotgun
fungus which can shoot these tar like spores on nearby houses and cars.
The spores are very difficult to remove.
One university webpage I read on it compared them to super-glue.
I've never run into or even heard of this problem before.
I've only used bark mulch.
Just something to consider.
More info: Wood mulch, rubber mulch and other bad ideas Artillery Fungus -
Frequently Asked Questions Update:
Was looking for other info and came across this page that talks about sawdust (among other materials) as mulch.
http://www.weekendgardener.net/garden-plants/mulch-060806.htm
Cliffs: Sawdust can be acidic
so it's good for acid loving plants.
As sawdust decomposes it can use up the nitrogen in the soil
so you need to make allowances for that.
gardening.stackexchange.com/users/2448/OrganicLawnDIY
Beware!
If you use any walnut sawdust and your tomatoes probably won't fruit.
My brother-in-law had his garden real close to a walnut tree and couldn't figure out what was up with his tomatoes.
When he researched it he moved his garden; problem solved.
Do walnut and cedar sawdust have the same effect?
I can only say that it would be best to avoid it near gardens.
On the other hand, this makes these sawdusts better for paths and areas where we want weed suppression.
gardening.stackexchange.com/users/11351/Dale
After sawing a lot of wood, we're left with quite a lot of sawdust. I have been using it during composting to ensure that the compost doesn't get too green and wet, but is there something else I can use it for in the garden - and is it ok to use the sawdust in the compost, as I've been doing?  The sawdust is from natural and untreated wood.
What can I use sawdust for in a garden?
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<compost><mulch>
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gardening.stackexchange.com/users/1011/Danger Fourpence
You can use it on pathways to keep out weeds.  Sawdust can be used for composting although some advise not using on gardens.  Sawdust is great for paths and areas where you want weed suppression.
Sawdust can be used for composting   Sawdust is great for paths and areas where you want weed suppression. It is probably best to avoid it near gardens.   