Carbon Nitrogen Ratio

mooserider(8)October 19, 2011

First, let me apologize, as I have no doubt this topic has been addressed many times here. I've read a ton on the web and in the forums on this to try and get a good grasp of its understanding, but I'm still missing something.

I understand that you want to keep the ratio low in the compost pile to make it head up faster and hotter (say ideally 30:1). I also understand that you want browns in there too for the bacteria, and that the browns typically have a higher C:N ratio. If you just have greens to keep the ratio low, you'll just end up with stuff rotting and smelling from what I gather... no food for the bacteria (or not enough carbon).

What I can't figure out is how much brown to add depending on the type of brown. For example, say I have rotted manure (20:1) and vegetable cuttings (25:1) in a 50/50 mix to make this easy (I pulled the ratios from the web). I'm looking at a 22.5:1 ratio at that point, but have no browns.

Browns can be like straw (80:1) or sawdust (500:1). So now I have to decide to either put a small amount of straw, or an even smaller amount of sawdust in order to keep that ratio around 30 or 40 to one. A little algebra could find how much exactly to add to keep it at that ratio, but it seems like there'd be way too little brown mixed in to sustain the bacteria.

This would make for a tiny amount of browns compared with the greens, yet I also see many articles saying that you should have (generally) twice as many browns as greens. How can you have twice the browns and yet keep the C:N ratio down low?

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scotty66(8 Hutto TX)

I'm still new to gardening and figuring out the "optimal" ratio for compost is tough...
I just add enough browns to keep the pile from getting stinky and slimy, everything eventually break down.
I put about 50/50 grass and leaves... my compost doesn't steam but it gets hot and breaks down quickly. I only turn my piles about every other week.
I have several bags of aged horse manure as well... but I'm saving that to mix in with the completed compost when I set up my raised beds in the spring. If it was fresh manure, I would add it to the compost now... to me, "aged" means it's sat around long enough that it has already composted (it won't burn plants). As I understand it, fresh manure would burn plants if tossed into a garden.

I do admit... I would like to actually see my pile get so hot it steams. closest I have gotten is when I left a pitchfork stabbed in my pile overnight and the next day the tines were "almost" too hot to hold.

    Bookmark   October 19, 2011 at 9:10PM
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Do not get too hung up on this question because many of us have found over the years that as long as what we pile up gets hot then the mixture must be close to the optimal 30:1 ratio. The bacteria that will digest the carbonaceous material need Nitrogen as a fuel as well as just enough and not too much moisture to work.
If you pile up some material and it does not start to heat up in about 3 days that might mean there is too much moisture or not enough Nitrogen. If that material smells of ammonia the mix has too much Nitrogen, while if it smells of sewage there is too much moisture in the mix.
Sir Albert Howard was not very concerned, probably not aware, about the C:N ratio and told us to layer 6 inches of vegetative waste, 2 inches of animal manure, and about 1/8 inch of good soil to a height of 4 feet because that is what the people in Indore, India had been doing for centuries and that worked quite well.

    Bookmark   October 20, 2011 at 7:31AM
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Just piling up half "browns" to half "greens" by volume usually works pretty well. You can always adjust it from there. It won't heat up? Add more greens. It gets smelly? Add some browns.
If you want some general ideals on the C:N ratio of different materials (though these are generalities and do vary), you can use a compost calculator.

    Bookmark   October 20, 2011 at 7:37AM
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Ya the subject of C:N confuses a lot of people. Think in terms of getting a warm bath. How much hot and cold water to add together to get warm depends on how 'hot' hot is and how 'cold' cold is and the volume of each. Sure it can be mathematically complicated but most of us figure it out pretty darn quick.

And don't get hung up on something just being called a green or a brown. Something at 25:1 is a green but so is something at 8:1. They are both greens but one is a much stronger green and would require more (volume wise) of a weak brown (40:1) than a strong brown (200:1) to get a targeted ratio.

I often use the terms "weak brown" or "weak green" when discussing composting, somehow people grasp that better than a black or white definition. But without exception everyone eventually understands "it" after a while.


    Bookmark   October 20, 2011 at 10:46AM
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toxcrusadr Clay Soil(Zone 6a - MO)

Good advice has already been given - 'observe and adjust' will do it.

The OP's concern about super-browns (like sawdust) not being able to 'sustain the bacteria' is not quite the way it works. The bacteria are going to be in there no matter what. They need N just like you need proteins, as well as C the same way you need carbs in your diet. Fresh grass has some C, just not a lot, and dry leaves have some N, just not a lot. But the critters will do their thing as best they can.

30:1 is optimal, but compost happens in a range on either side of that.

    Bookmark   October 20, 2011 at 11:23AM
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val_s(z5 central IL)

I agree with all above. When I first started composting I must have had an optimal mix 'cause it heated up nicely and made beautiful compost.

Now I don't worry about it, just throw everything in, turn a couple of times and I still get beautiful compost. Usually, I don't get a lot of heat or probably the truth is that I don't go check.


    Bookmark   October 21, 2011 at 7:54AM
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C:N ratio is one of the requirements for fast compost. Good particle size, proper moisture, and a bin large enough to retain heat are others.

If you really want to better understand the C:N ratio, Cornell has the best website. Their list of values of ingredients is the longest I've seen. The 'On-Hand Handbook' is the name of it last time I looked.

But many listings are a guess, a average, or one value among a number possible. Spring fresh-cut grass may be lower from fall fresh-cut grass. The value of your used coffee grounds may depend on how much water is present. Paper and wood vary quite a bit. Leaves, my main brown, may vary from 20:1 to 90:1, depending on the tree type and when leaves are collected.

    Bookmark   October 22, 2011 at 4:35PM
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Far too many people get far too hung up about the C:N ratios. While it is kind of nice to know that tree leaves, after they fall from the tree, have a C:N ratio of about 40:1 and fresh grass clippings have a C:N ratio of about 16:1 the particle size and the moisture level are of more importance then the C:N ratio. Too large particles will slow digestion a lot and too much, or not enough, moisture will keep the bacteria from digesting anything.
As a general rule of thumb about 3 parts vegetative waste (of the right particle size) to 1 part animal manure, with just enough moisture, will usually get a really good cooking compost pile in about 3 days.

    Bookmark   October 23, 2011 at 7:16AM
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I just pile up random greens and browns and make compost. I don't pay attention to ratios. That sort of thing will drive a person batty. I've composted nothing but freshly mowed grass clippings and made a fantastic compost. It had to be turned often to keep it from becoming tar but it was astoundingly rich.

Here is a link that might be useful: EZ grass compost

    Bookmark   October 29, 2011 at 11:34PM
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Yes, a good understanding of C:N ratios is not necessary for successful composting. But reading the manual for my tumbler; including the info on C:N ratios; was necessary before I got the 160F core temp the product promised. Now, years later, after futher reading I try to keep my hot mesh piles around 140F.

I thought the topic was interesting, after learning how to get the basics down. My comments to Mooserider were along the lines of further reading on the subject, which I thought he/she expressed an interest in.

    Bookmark   November 5, 2011 at 3:21PM
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lol never did good in algebra... at least when I was in high school, but my compost heats up :p
if you over think it it will drive you nuts. A little common sense goes a long way. the more varied the items the better.

    Bookmark   November 7, 2011 at 3:45PM
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I tried out the Compost Mix Calculator, linked below. I was simply amazed how wide a range of mixes of the materials they have data on will give acceptable composting C:N ratios.

Jim Marconnet

Here is a link that might be useful: Compost Mix Calculator

    Bookmark   November 13, 2011 at 6:51PM
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