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compost ratio question

Posted by ken1 az (My Page) on
Sun, Feb 12, 12 at 1:47

Question here.
I looked at a compost ratio calculator (klickitat county)
and put in 5 cu ft grass and 5 cu ft leaves , which gave me a ratio of 24.66. optimum ratio is 25 to 30, so that looks pretty good. Now if you add laying hen manure to the mix, your ratio drops to 13.72. Why would adding chicken manure screw up the ratio so bad? I've been doing exactly this ratio for several years

Follow-Up Postings:

RE: compost ratio question

That's a good question. You've been on GardenWeb a long time, so you obviously know manure has a lot of nitrogen in it, but maybe not so much to lower the C:N ratio so drastically to 13 to 1, as the calculator says.

Organic Gardening for Dummies lists old manure at 20:1 and lists N-P-K for chicken manure as 3-1-1 (rabbit's even higher at 5 - 3 - 2 -- got lotsa rabbits here).

The Organic Lawn Care Manual lists chicken manure a little lower in Nitrogen than the Dummies book:
2 - 1.5 - 0.5

Again, your question is very good. Not sure why your calculator drops the C:N ratio so much to 13:1 when Dummies lists general manure at 20:1.

There's variation in the books because Let It Rot! list general manure at 15:1 and manure with bedding at 23:1.

Let It Rot! lists fresh hen manure N-P-K at 1.1 - 0.9 - 0.5 and dried hen manure with litter at 2.8 - 2.8 - 1.5 .

Hen manure is nothing like bat guano at 10 - 4.5 - 2

Maybe the calculator is confusing hen manure with bat guano??

At what web site did you find that calculator on?

There's a calculator on the web site at
and if you select poultry manure and enter 1 pound, it gives a C:N of 10:1. So maybe that's why. Maybe hen manure and bat guano are those special manures with lots and lots of nitrogen.

RE: compost ratio question

Which ratio are you using? Carbon to Nitrogen, C:N, or the amount of manure to vegetative waste?
The C:N ratio is 30:1, Carbon to Nitrogen. To know that you need to know what the makeup of each material is.
If you use the rougher 3 parts vegetative waste to 1 part manure that is much easier and is very close to that optimal 30:1 C:N ratio. If you are not using animal manures as you N source it gets a bit more complicated, but if you build your pile and the ratio of C to N is pretty close and there is just enough moisture along with air the material should, in 3 or 4 days, heat up, sometimes sooner. If the pile does not heat up in that time frame something is not right.

RE: compost ratio question

  • Posted by pt03 2b Southern Manitob (My Page) on
    Sun, Feb 12, 12 at 8:48

You are adding a strong N component (manure) to a mix that is already on the N side. It is like adding hot water to warm water, it increases the temperature of the entire mix.


RE: compost ratio question

Ken, in your post, you mentioned you entered into your calculator equal volumes of grass and leaves, and you liked the 24:1 ratio that resulted. But when adding hen manure to that mix, the calculator then gave you a low ratio of 13:1.

At your calculator's web site, see what ratio it gives you if you remove the grass component, and just use the leaves with the hen manure.

RE: compost ratio question

Thanks for all the input. I guess I just never really thought about messing up the balance with what I was doing. I thought I was simply improving the quality of the compost with the manure.
ZoysiaSod, I get a totally different ratio with that calculator. Don't think that it matters much, as I do understand why it's wrong, better.
I like Lloyds answer with the hot water analogy.
Thanks again

RE: compost ratio question

Lots of unanswered questions.

What kind of leaves? Shredded, if so how fine. Leaves dry or wet, or in between?

Grass is said to have a C:N ratio or 19:1 or 20:1, but how fresh is it? I use it the day its cut.

Measurements based on ingredients by weight probably are a lot more accurate than those calculated by volume.

And that is just for a start, if your interest is in getting a high temp to speed decomposition. Then the next question would be, is the bin large enough to retain the heat generated by your materials.

RE: compost ratio question

My experience with composting is that it is a very forgiving and flexible process. Just because the mix isn't optimal doesn't mean it won't compost. It just may not compost as quickly, etc. Remember that 30:1 C:N ratio is just a guideline. I've done 10:1 and gotten along ok. The down side of a low C:N is that you can lose more N to ammonia volatilization, rather than retaining it in the finished compost. Hope that helps.

RE: compost ratio question

The leaves are shredded in a bearcat shredder fairly fine. The grass is added the day it is cut about an equal amount in volume but less in weight due to being fresh cut, and the leaves being shredded are much denser. I add chicken manure along with water to dampen the mix. I get heat just fine. My question was just about the calculator showing I'm basically wrong to add the manure as I am messing up the ratio. I just wanted to hear how the manure could possibly be bad. I thought adding the manure would just make the compost richer in N, guess I'm mistaken.

RE: compost ratio question

Many people think they can speed up the composting process by adding more Nitrogen and then wonder why their compost pile smells like ammonia as the excess Nitrogen is gased off to the atmosphere. If anything I would prefer to have more Carbon in the mix then Nitrogen.
You need enough N to feed the bacteria that are digesting the Carbon, but not too much for them to use.

RE: compost ratio question

ken1, keep in mind too that the calculator is just using average data for your ingredients. The actual C:N ratio of each ingredient could easily vary 10-20% from the values the calculator uses. The cumulative effect could be a calculated final C:N that is significantly different than your actual C:N. The only way to know for sure would be to have the ingredients tested by a competent lab. But its probably not worth the cost.

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