How To Calculate C:N Ratio for a Compost Pile
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
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.
When we get the C:N ratio worked out, we need create a pile at least 3x3x3 feet or 1 cubic yard (1cubic m) or place it in a compost tumbler for easier turning
It is important to monitor compost temperature, all material should reach at least 131F (55C) for a minimum of 3 consecutive days to kill pathogens and weed seeds. The best way to monitor temperatures is with a Stainless Steel Compost Thermometer. If temperature begins to get close to 149F (65C) the pile needs turning or tumbling to stabilize temperature.
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.