The C:N Ratio

This is a page written for compost nerds.  If you love to crunch numbers this is right up your alley.  This is how you get it just right, every time.

If you prefer a more laid back, let nature do it approach, read no further.  Also, you need to know that I no longer compost in a bin, and therefore I don’t worry about getting the ratios just right.  But for those of you who want to produce high quality compost in a reasonable amount of time in a bin or a traditional compost pile, you really need this information.

The Perfect Ratio
A 30:1 ratio of carbon to nitrogen is the optimum balance for a perfect compost pile. It is not necessary to have optimum balance to produce compost.  Organic materials will decompose and produce a useable compost in their own time.  However, the closer you get to it, the faster your pile will turn from raw organic materials to finished compost.  Your finished product will also be of higher quality.  Not all compost is created equal.

Commercial Bin Claims
The manufacturers of some rotating compost bins claim that you can “produce compost in fourteen days”.  These claims are based on the perfect ratio, perfect timing, and perfect moisture content to support the aerobic, thermogenic bacteria necessary to process the materials in this short window of time.

Calculating the C:N ratio
For perfect compost every time you must know the Carbon and Nitrogen content of each material you will add to the bin or pile.  Of course, it is still an approximation, because we will be relying on published average percentage, but it is as close as you will get without testing each batch.  You will learn to tweak the combinations of materials as you gain experience and get to know the nature of your locally available materials.

Step One: Weigh each material – parts are measured by weight, not volume

Step Two:  Calculate your pile’s Total Carbon Value
Multiply the percent carbon of each ingredient by the number of parts (by weight) of that ingredient. Use the chart below for estimated carbon percentages.  Add up the carbon totals for all the ingredients.

Step Three: Calculate your pile’s Total Nitrogen Value
Multiply the percent nitrogen of each ingredient by the number of parts (by weight) of that ingredient. Use the chart below for estimated nitrogen percentages.  Add up the nitrogen totals for all the ingredients.

Step Four:  Find the C:N Ratio
Divide the carbon total by the nitrogen total to get the C:N ratio.  If the result is between 25:1 and 35:1, your pile should compost quickly.  If the ratio is higher or lower than that, adjust the proportions of the ingredients to bring it into the range of 25 to 35 parts carbon for each one part nitrogen.

Our ingredients are 50 pounds of non-legume hay, 10 pounds of kitchen scraps, and 2 pounds of coffee grounds

Carbon Value
50 lbs hay x 40% C = 20 lbs C
10 lbs kitchen scraps x 10% C = 1 lbs C
2 lbs coffee grounds x 25 % C = 0.5 lbs C
20 + 1 + 0.5 = 21.5 lbs C

Nitrogen Value
50 lbs hay x 1% N = 0.5 lbs N
10 lbs kitchen scraps x 1% N = 0.1 lbs N
2 lbs coffee grounds x 1 % N = 0.02 lbs N
0.5 + 0.1 + 0.02 = 0.62 lbs N

Therefore, 21.5 / 0.62 = 34.7 parts carbon to 1 part nitrogen

Carbon and Nitrogen Content of Common Compost Ingredients:

Material % Carbon % Nitrogen
Alfalfa pellets 40.5 2.7
Blood meal 43 13
Cottonseed meal 42 6
Soybean meal 42 6
Legume hay, dry 40 2.0 to 2.5
Non-legume hay, dry 40 1.0 to 1.5
Fresh manure, cow 12 to 20 0.6 to 1.0
Fresh manure, horse 20 to 35 0.5 to 1.0
Fresh manure, laying hens 10.5 to 20 1.5 to 3.0
Fresh manure, broiler chickens 20 to 32.5 1.3 to 2.0
Wheat or oat straw, dry 48 0.5
Grass clippings, fresh 10 to 15 1 to 2
Fallen leaves 20 to 35 0.4 to 1.0
Newspaper or cardboard, dry 40 0.1
Wood chips or sawdust 25 to 50 0.1
Coffee grounds 25 1
Vegetable wastes, fresh, leafy 10 1
Vegetable wastes, starchy 15 1
Kitchen scraps 10 to 20 1 to 2
Fruit wastes 8 0.5
Seaweed, fresh 10 1
Weeds, fresh 10 to 20 1 to 4