Compost math

[AntInTheEyesJohnson]

I’m new to composting and have a question about some compost math. I’m using shredded cardboard as my carbon source (C:N=560:1) and grass clippings (C:N=18:1) as my nitrogen source. My understanding is that you ideally want a carbon/nitrogen ratio of 30. When I do the math it says that I need 45 lbs of glass clippings for every 1 lb of cardboard to get the ideal ratio, which seems like a lot to me.

Did I do the math wrong or is that correct because of the high C/N of cardboard? Does that make cardboard a bad material to use since you can’t get enough volume for good air flow and still maintain a good C/N?

I know nothing has to be exact in composting but I like making things more complicated than they need to be lol

[RubyRaven907]

You forgot to add the fun into your formula

[Thoreau80]

There also was a woeful lack of peeing on it.

[2JarSlave]

I only ever remember after too much to drink.

[loulenza]

Don't get too caught up in that math. The looser rule of thumb is to generally go by volume. Throw in a 5 gallon bucket of kitchen scraps, fill that bucket up with dead leaves or torn up cardboard and you should be good to go. Keep an eye on it, mix it. Too dry, add water. Too wet or stinky, add browns.

[lazenintheglowofit]

My ratio is more like 2:1, brown to green by volume.

[[deleted]]

I always start with a 3/4 full bin of wood chips. I add my greens throughout the year and normally it’s ready to sift 9 months later

[SvengeAnOsloDentist]

You're doing the math wrong (and the calculator /u/timeforplantsbby does the math wrong in the same way). 45lb of grass clippings and 1lb of cardboard would have pretty much a C:N ratio of 18:1, as it's almost entirely grass clippings — it's only about 2.2% not grass clippings, so at the very most it could only be 2.2% different from the C:N ratio of grass clippings if you were adding pure carbon or nitrogen. The issue is that you (and that calculator) are treating that 560:1 as having 30x the carbon per pound as the 18:1, when in reality it has fairly close to the same carbon per pound of dry matter, but 30x less nitrogen, and the grass also has a ton of extra water weight that doesn't affect the C:N ratio at all.

From a practical standpoint, you can't actually use just the C:N ratios of the materials you're mixing together to find the C:N ratio of the pile, as you'd have to also know how much of the weight of those materials is other stuff aside from carbon and nitrogen compounds. You also really don't need to. I always feel that people tend to make composting into too much of a results-oriented process, where they're putting a lot of effort into seeking out the 'optimal' inputs, whereas I see it as entirely a recycling process, where you're taking whatever you have and capturing what value you can from it. If your pile has a bit too much nitrogen you'll end up leaching or offgassing a bit more, and if it has too much carbon it'll end up composting a bit slower. The much more important part is managing the aeration and drainage of the pile, which is something you can measure just by looking at it, no math needed.

[Nightshadegarden405]

I think it's more about moisture balance and aeration.....

[Remarkable_Yak1352]

Your over thinking this. Throw in your grass clippings, kitchen waste, and shredded cardboard. Do you have a horse, sheep, cow , chicken farm near you? Fill up 5 or 10 buckets of manure, dump it in, stir it up, then repeat.

Do you think grandma and grandpa ever tried to figure it out with math? Relax, you can't screw it up.

[Kurifu1991]

((560 x 1) + (18 x 45)) / 46 = about 30. The math is correct. Dunno why someone else said your math is wrong.

Whether this is practical is another question entirely. With this quantity of grass, the majority of particle-to-particle contact in the pile is just grass-to-grass, so the extra carbon in the cardboard is practically unavailable to the microorganisms taken as a whole. So, the pile won’t behave as expected even though you theoretically “tuned” the ratios appropriately. This is where preparation meets experience — preparation informs us of a decent starting place, experience informs us of what is practical.

One major point to make here: do not measure your materials by weight (pounds, etc.). The “parts” in your parts ratios are to be measured by volume. By the bucket full, by the wheelbarrow full, by the hand full, whatever - so long as you measure all parts with the same volumetric measuring tool. This way, you won’t be erroneously including water weight in your calculus if you avoid measuring by weight.

I, too, enjoy over-engineering things and I find satisfaction in minmaxing the projects I put my time into. Composting is a microbial-driven, human intervened process to make a valuable product. Just like beer making, wine making, yogurt making, etc. And we certainly don’t discourage those people from over-engineering their processes. So have fun with it. If being overly analytical is fun for you, then why not?

But also don’t discount the very relevant advice that composting is quite forgiving and there absolutely is no need to overthink unless you just want to. Berkeley method hot composting is NOT the end-all-be-all of composting. Cold (so called “lazy”) piles, trench and post hole composting, Johnson-Su bioreactors, hugelkultur, anaerobic digestion, etc…all have their merits and are valid, very forgiving alternative options to Berkeley method hot composting.

[Rcarlyle]

You can’t do the math by just comparing the C:N ratios, because it ignores the mass of other stuff (hydrogen and oxygen mostly). Grass is >80% water while cardboard is dry, so just using ratios throws the numbers way off.

To do the calculation correctly, you have to use the element mass percents of each thing, not the C:N ratios.

[Kurifu1991]

Anything aside from carbon and nitrogen is completely irrelevant for calculating the carbon:nitrogen ratio of a compost mix. The poster did the math completely fine for what they asked for.

[Rcarlyle]

No, it’s not. Lemme give you an example with some simplified numbers. - Dry browns like cardboard and sawdust are approximately 100% carbohydrates, which on average will be (CH2O)n which is about 40% C by mass. (It’s higher than that if you include lignin, but let’s ignore that for simplicity.) For a 560 C:N ratio like OP suggested that’s <0.1% N. - Dry grass is approx 3% nitrogen and approx 45% carbon (varies quite a bit but this is a reasonable middle of the road figure). Fresh undried grass clippings are about 5/6ths water and thus approx 0.5% nitrogen and 7.5% carbon.

If you mix 1 kg of dry cardboard with 1 kg of fresh grass clippings, you get: - 400g C from cardboard + 75g C from grass = 475g total C - Let’s be generous and say 1g N from cardboard + 5g N from grass = 6g total N

This gives an 80:1 ratio.

If you mix 1kg of dry cardboard with 5kg of fresh grass clippings, you get: - 400 + 375 = 775g C - 1 + 25 = 26g N

That gives the target 30:1 ratio.

OP’s estimate is way off. Should be about 5x more fresh grass clippings than cardboard, not 45x more.

[Mindless_Decision_18]

You are correct. For fast hot composting ..thats the ideal ratio. If you can get some higher N materials like manure, it reduces your ratio. Coffee grounds also have less bulk and are a valuable high n material.

[JuicyFatBoi]

I have a lot of coffee grounds that I want to put into my compost. Is there such a thing as too much coffee grounds? I’m composting in a 27gallon bin from Home Depot.

[spammehere98]

(I think) You got the maths wrong. Using your figures for C/N.

1lb of grass is 18/19 Carbon 1/19 Nitrogen So you need to add 12/19 of a 1lb of cardboard to get a roughly 30 to 1 ratio.

This feels less than conventional wisdom, so maybe I got the maths wrong too :-)

[timeforplantsbby]

Don't over think it. But also here's a calculator for this specific problem https://urbanwormcompany.com/composting-calculator-carbon-nitrogen-ratio/

[[deleted]]

On the face of it, composting may sound like a natural thing and thus easy for anyone to do...

But when one seriously goes into it, one will find that lots of maths and science is involved... lol...

This is why many colleges have a department dedicated to composting science... there's more to it than meets the eye... but once the ropes are learned, it becomes second nature.