The carbon-to-nitrogen (C:N) ratio is often considered to be of utmost importance in composting, particularly hot composting. If the C:N ratio is too high, the compost will break down extremely slowly. If the ratio is too low, the pile can produce a displeasing smell as excess nitrogen escapes into the atmosphere in the form of ammonia.

While many gardeners probably obsess over C:N more than is strictly necessary (and those who use cold composting methods typically do not need to worry about it at all), attention to the C:N ratio of your compost pile can keep it working smoothly and quickly. And it may be useful for troubleshooting!

Experts disagree on the optimal C:N ratio, but most scientific literature typically recommends something between 25:1 and 30:1. Higher ratios are fine if a slow composting process is acceptable.

The C:N Ratio List

Here’s a list of the average C:N ratios of common compost ingredients, pulled from a variety of sources:

• Swine manure: 6:1.
• Aged chicken manure: 7:1.
• Hairy vetch: 11:1.
• Fresh-cut alfalfa: 12:1.
• Table/kitchen scraps: 15:1.
• Used poultry bedding: 15:1.
• Fresh cattle manure: 15:1.
• Sheep manure: 15:1.
• Legume hay: 17:1.
• Fresh grass clippings: 20:1.
• Coffee grounds: 20:1.
• Clover: 23:1.
• Horse manure: 25:1.
• Vegetable scraps: 25:1.
• Mature alfalfa hay: 25:1.
• Wood ashes: 25:1.
• Rye cover crop in a vegetative state: 26:1.
• Freshly pulled weeds: 30:1.
• Garden waste: 30:1.
• Used horse bedding: 45:1.
• Peat moss: 60:1.
• Leaves: 60:1.
• Fresh corn stalks: 60:1.
• Oat straw: 70:1.
• Wheat straw: 80:1.
• Pine needles: 80:1.
• Rye straw: 82:1.
• Shredded newspaper: 175:1.
• Hardwood bark: 223:1.
• Sawdust: 325:1.
• Shredded cardboard: 350:1.
• Wood chips: 400:1.
• Softwood bark: 496:1.

Using C:N Ratios

Gardeners often simplify matters by thinking in terms of color—materials with a C:N ratio higher than 30:1 are browns, and materials with a ratio lower than 30:1 are greens. (Note that high-nitrogen materials can actually be brown in color and vice versa.) However, a compost pile that has a ratio of 30 parts brown material to 1 part green material actually has a disproportionately high amount of bulky carbon. If you are using browns:greens instead of C:N, you will want to use the ratio 1:1 or even 1:2, both of which take bulk into consideration.

Do the math to see why 1:2 works. Let’s say we’ve chosen to use one part leaves for our brown and two parts fresh grass clippings for greens:

1. Add the ratios of each part (60:1 + 20:1 + 20:1 = 100:3).
2. Reduce the fraction to find the C:N ratio of the mixture (100:3 = 33:1).

This C:N ratio is slightly on the high side, but with patience should come out just fine. The mathematics will work on any other combinations of ingredients we choose to evaluate.

Also of interest is how the C:N ratio applies to plant residues left on the surface of the ground to protect the soil. The same 25:1–30:1 rule applies. If the ratio is lower, soil microbes will eat up all of the available carbon too quickly and leave the soil bare. If the ratio is higher, the microbes take a long time to eat up the high-carbon materials, leaving a great deal of chunky debris in the soil. Furthermore, the microbes will need to absorb more nitrogen to balance their diet, and this will have to come from the soil—leaving less nitrogen available for growing plants.

The practical implications? Often the best cover crop is a blend of high-carbon grains and high-nitrogen legumes. This mix, highly favored among organic gardeners, works because it keeps the C:N ratio close to optimal.

The C:N ratio will also affect the mulch you use. Wood mulches are attractive in ornamental gardens, but they pull a great deal of nitrogen out of the soil to balance out their high carbon levels—not good for growing sweet corn. For a vegetable garden, something a little closer to the optimal C:N ratio will foster healthier plants.

Helpful Resource

Composting Quick Start
Includes more essential information on balancing nutrients in a compost pile.