How To Calculate C:N Ratio for a Compost Pile

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I’ve had frequent conversations about composting over the years. I’d read all the stuff about how a pile should have a 30:1 carbin to nitrogen ratio, I’d read the add 2 parts brown to one green (I never had good results this way). I’d even seen somewhat complicated compost calculators on the internet. It all seemed a bit complicated or didn’t really get me results. I kept wondering if there was an easier way. Finally I stumbled across a now forgotten article about how to calculate C:N ratios.

I’ll  share with you a way to calculate C:N ratios fairly quickly with a pen and paper and some remedial math. This way isn’t perfect but it’ll get you close enough to make some minor adjustments and get good results.

The easiest way to show you is with a couple of examples.

with 2 waste products we will use leaves and chicken manure. Looking up the C:N ratio of each I found the average for autumn leaves is about 80:1 for chicken manure 7:1.

Start with one part of each and add them.

80 +7 = 87 now divide by total number of parts (in this case 2. So 87/2 = 43.5 since this number is higher than our desired 30:1 we need more nitrogen (manure). Let’s try 1 part leaves to 2 parts manure 80 + 7 + 7 = 94 now we have 3 parts total in the pile so we divide 94 by 3 and we get 31.3

A pile of 1 part leaves and 2 parts chicken manure is pretty close to the 30:1 we want. We could build this pile and be pretty close to the ideal and would expect decent temperatures and results.

An example with 3 waste products lets select:
Pig manure 6:1
Sawdust 325:1
Coffee grounds 20:1

Start 1 part of each 6 + 325 + 20 = 351 divide by total parts (3) 351/3= 117 it’s above 30 so more manure and/or coffee needed try 1 more manure and one more coffee (total now will be 5 parts 117 + 6 +20 = 143/5 = 28.6 now we can build the pile with 2 parts manure 1 part sawdust and 2 parts coffee and just add a couple of extra handfuls (depending on the size of your “parts”) of sawdust. This will be pretty close to our 30:1 ideal ratio.

Since learning this method myself my piles maintain temperature better, decompose faster and more completely. This has allowed me to create better piles more quickly and create better compost. Give the method a shot or if you’ve done this method tell me your thoughts.

4 Comments

  • gud_hart says:

    Hi, I recently started getting distilled grain from a distillery in town, 100-200 gal at a time. The grain is very high in nitrogen, and very warm in the compost bin I made exclusively for it (it is too hot for my red wigglers). Very new at this, I covered it with straw, sawdust, and dirt, and mixed it in. It seems that the first batch thday i got about a month ago has turned to soil, and now I am covering/ mixing this soil with the second batch, thinking it has a start on the necessary microbes to help it along. Do you have any suggestions about howI might improve on this process, and when the mixture has cooled down, should i give it to the worms to finish? How do i measure how much nitrogen is in the grain? Thank you for you valuable and helpful information, Gudrun

    • Larry Shier says:

      Hi: thanks for reading and for your question. Doing a Google search for “C:N spent brewery grains” the results are between 12:1 and 17:1. What I’d likely do is calculate my pile using 15:1. Grains can be difficult to mix in a way to allow good airflow so an assortment of sizes of balancing C materials allows more oxygen into the pile. If you monitor temps then turn thenpile whenever it approaches 65 C (150F) or after 3 days between 57C and 65 C (131F and 150F)

      • gud_hart says:

        Thank you, Larry, very much for your helpful reply! The C:N calculator and the temperature guide are going to help take the guesswork out of the distilled grain composting. I will experiment with adding different high-carbon products. My shoveling muscles are getting pretty strong, too! Cheers, Gudrun

    • Larry Shier says:

      Your choices when temps begin dropping are to cure the compost in piles, spread it on gardens well in advance of planting, or feed it to worms during the curing process.

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