Just to add in another question to yours..... if you are getting the sawdust from a cabinet shop or the likes, it will most likely have a lot of plywood sawdust... which I was told years ago would not be the best to add to you garden due to the glues used in it's manufacturing. I hope that's changed more recently.... but just wondering if that's still the case.

So no more formaldehyde in the glues? I have a big old dust collector bag full of mixed stuff and would like to put it to use if I could too.

I'd use the sawdust as a "brown" to mix with grass clippings, kitchen scraps or other high moisture ingredients.

Yes, me too, lazy_gardens.

Greens are needed, not more browns. Coffee grounds must be browns, if they have a C:N ratio of roughly 20:1

Also, what is this love affair that people are having with coffee grounds? Do they have easy access to them and therefore want to use them?

UCGs should be at the top of every composters favourite thing to use. A pox on those single cup coffee makers. Boo, hiss.

BTW, UCGs are a 'green' in the context of composting.

Providing further information, which I'll edit and add to the original post, I have researched both online and with our coffee pot to compare dry grounds to wet grounds. Most of this research points to wet, used coffee grounds weighing 2.5X more than dry coffee grounds. With all of this, if my math is correct, it seems like I would need approximately 75 lbs of wet, used coffee grounds for each 1 lb of wood chips to meet a 30:1 ratio, but I could easily be wrong. Any thoughts on this concept? Thanks!

To tete_a_tete, as Lloyd stated, in the context of composting, used coffee grounds (UCG) are considered a "green" resource, even though they are brown in color. Scientists have found (unless I'm mistaken) that an ideal composting ratio is around 30:1. Being 20:1, UCG are actually considered to be high in nitrogen. Refer to this link to see some differences of ratios between "brown" and "green" composting items: https://www.planetnatural.com/composting-101/c-n-ratio/

Formaldehyde is volatile anyway, so if there was any in the original wood product, it will offgas (as well as biodegrade). At those tiny levels it is not a risk to the garden anyway as plants won't absorb it and it does not accumulate in plants or on vegetables. The issue with formaldehyde is that it can offgas from building materials into the indoor air, where it is an inhalation risk. Outdoors this problem literally goes away on the breeze. If you are inhaling your tomatoes you have bigger problems than this anyway. :-D

Thanks yonley.

I must say that I thought that the C:N ratio of greens was going to mean that the N was higher than the C.

Off topic a little, I've read here on GW that manure is considered a green. Goodie, I thought, and went collecting horse manure again. (Any excuse will do.) However, when playing with my compost I notice that wherever there is a splosh of wet lucerne (alfalfa) hay, there is heat. And yet, all around there is horse manure. That same horse manure has not performed as a green in the way that the alfalfa hay did. So now I have my doubts about whether manure really truly-ruly is a green.

ETA: Furthermore, sometimes (if I can ever get the mower started again) I make holes in my huge and ever-growing compost pile and place grass clippings inside. It heats it up in those places like nothing else can (except maybe a heater turned on full blast).

"With all of this, if my math is correct, it seems like I would need approximately 75 lbs of wet, used coffee grounds for each 1 lb of wood chips to meet a 30:1 ratio, but I could easily be wrong. Any thoughts on this concept?"

My main thought on this is that one shouldn't focus excessively on the specific ratio - or put differently, what are you trying to achieve by getting that precise ratio? Do you want the pile to heat up? Compost as quickly as possible? That 30:1 'ideal' ratio is just a rule of thumb, and non-ideal piles compost eventually, too.

As tete-a-tete has noticed, different compost items - independent of their number ratios - have a different tendency to heat up. In my experience, sawdust does not heat up as readily, and because it is high in lignin (which is quite resistant to decomposition) will compost relatively slowly.

Extra greens or nitrogen will help but it will still be relatively slow - and quite possible that some of the lignin will not decompose much and will just become a 'stable' part of the end-product (I've read that humus in soil contains a fair amount of lignin). It can become compacted and so turning is good - fortunately since it is in sawdust form the particles will get plenty of air, as long as not too compacted.

In other words, give it time, air, some water, and occasional turns, and more time. As many 'greens' as you can add will be good, but I wouldn't stress about the precise ratio, or trying to get 75X as much coffee as sawdust. It will need some nitrogen/greens, but the fungi that will do a lot of the composting are quite resourceful.

Continuing the manure semi-tangent, Horse manure usually consists of bedding (straw, sawdust, etc), #1 which is soaked up by the bedding, #2 which is digested grain, straw, pasture, whatever. Of all this, the #1 is the main "green". The overall C:N ratio depends on what is used as bedding and how much bedding is in the overall mix.

The stable I visit uses sawdust bedding and some feed hay is always dropped and mucked out. The resulting pile is smoking hot and composts quickly-even if I mix an equal volume of leaves.

That is a good point. To 'goose' the nitrogen level it would be easier - in terms of volume - to add higher-nitrogen additives than coffee grounds. #1 is pretty concentrated.

My favorite gooser is urea. At $20 for 50lb bag and 46%N, it's cheap and a little goes a long way.

Aside from it being mixed with browns already in some cases, another thing about manure is, how long has it been sitting? A poop apple begins to compost as soon as it hits the ground, and it doesn't take long for it to give up its super-green ability to heat up a pile. If you get a truckload from the farm, it's very possible to have stuff from days to weeks or even months old.

rayzone, the people at your stable must be making a superior manure pile to where I go.

At 'my place' horse owners muck out the stables and yards daily but the manure is placed in a spread out fashion, creating long manure heaps.

I avoid picking up the sawdust but make a bee line for discarded hay. I am able to collect manure that is free from bedding.

However, recently the agistment owners tamed the manure heap. It had been scraped up, and in some places I found lovely looking stuff that, although was clearly sawdusty, appealed to me. So I collected that. I'm using it for the base of my newest compost heap.

toxcrusadr, I must experiment with fresh manure to test your theory. Regarding manure starting to compost when it hits the ground, I have to disagree. It can sit there for months in the long extended version of a manure heap, and if dry, does pretty much nothing, 'compost'-wise.

I will defer to your experience on that. It was more of a general comment that you can't necessarily depend on piled up manure/bedding to heat up a compost pile.

ABSOLUTELY NOT!

I would recommend mixing two parts coffee grounds by volume to one part raw sawdust. You may be able to mix up to one for one (no guarantees).

#1. Coffee grounds are green because 20:1 C:N ratio is on the nitrogen rich side of 25:1 to 30:1 that is ideal for composting.

#2. You can't average ratios; they are NOT percentages. and C+N does not make up the 100% of the item. Imagine two buckets: [CCCNxxxx] and [CNNNNNxx]. The C:N ratios are 3:1 and 0.2:1. The total is not 3+0.2 : 1, nor half that, 1.6:1. These are not addable because nitrogen amounts are different. The total in this case is CCCC NNN NNN = 4:6 = 2:3. You could average percentages of the whole, but not ratios.

Now, your solution, according to Humanure book:

Coffee grounds: 46% Carbon 2.3% Nitrogen ==> 20:1

Raw sawdust: 56% Carbon 0.11% Nitrogen ==> 510:1 (nitrogen is practically 0, it wouldn't matter if this ratio was 1,000,000:1, it shouldn't change final ratios to mix!)

I would recommend:

2x Coffee grounds 1x sawdust BY VOLUME

Hoping it makes:

2x Coffee grounds 0.5x sawdust BY WEIGHT

==> (92+28)/250 Carbon (4.6+0.05)/250 Nitrogen ==> 120:4.65 = 25.8:1

If you weight coffee grounds and sawdust, you can go with their weight ratio:

Weight ratio: 100:25 (Volume ratio: 2:1 ? Less?) ==> C:N Ratio: 26:1

Weight ratio: 100:40 (Volume ratio: 1:1 ? More?) ==> C:N Ratio: 30:1

Actually, you can also just assume coffee grounds to be close to kitchen scraps and follow recommendations for kitchen scraps. If you were mixing with leaves, which are lighter, recommendation would be 1 bucket kitchen scraps to 3 buckets of dried leaves. You can experiment and see if mixing a bucket of coffee grounds with a bucket of sawdust by volume shoots past 30:1 C:N ratio and doesn't heat up sufficiently...

I have some comments about this part:

You can't average ratios; they are NOT percentages. and C+N does not make up the 100% of the item. Imagine two buckets: [CCCNxxxx] and [CNNNNNxx]. The C:N ratios are 3:1 and 0.2:1. The total is not 3+0.2 : 1, nor half that, 1.6:1. These are not addable because nitrogen amounts are different. The total in this case is CCCC NNN NNN = 4:6 = 2:3. You could average percentages of the whole, but not ratios.

I've always assumed they WERE addable, but let's work this out. I would have always said that equal amounts of these two would add up to 3.2:2 or 1.6:1. But what you're saying is that the ratio of C to N in a material is independent of the amount of each (i.e., percentage). So a material with 10% C and 1% N is 10:1, but so is a material with 1% C and 0.1% N.

Yikes.

Take two materials:

Material A: CN 10:1

Material B: CN 100:1

The way I would have always calculated this, equal amounts of each gives 110 parts C and 2 parts N, for a final ratio of 55:1

This isn't right. Well, it's right only rarely, like a broken watch is right twice a day. Here's why.

Let's do two scenarios where A and B still maintain the above ratios 10:1 and 100:1, but have different percentages of C and N in the material. Then mix equal amounts of A and B. If ratios were additive, then the final C:N of the mix should be the same for both scenarios.

Scenario 1

A is 10% C, 1% N (CN=10:1)

B is 10% C, 0.1% N (CN 100:1)

Mix 10 lb of each. I made a little table of the contributions (in lb) [Edit: it didn't appear as a table so I added commas for clarity].

A, B, Total

C: 1, 1, 2

N: 0.1 0.01, 0.11

Resulting C:N = 2:0.11 or 18:1

Scenario 2

A is 30% C, 3% N, CN= 10:1

B is 1% C, 0.01% N CN=100:1

A, B, Total

C: 3, .1 , 3.1

N: 0.3, 0.001, 0.301

Resulting CN = 3.1:0.301 or 10:1

If ratios were additive, the final CN of the mixtures should be the same, but they're not.

As an env. chemist, I don't know how I missed this, so I'm having a cow pattie right now.

The above advice about 'observe and adjust' is the way to approach it. That and having a 'feel' for when the ratio is off or is going to be.

Sawdust is generally slow to break down, so even if you mix in a lot of high N material, it can lose some N to the air because the sawdust can't decompose fast enough to use it at the rate it's being liberated.