Carbon to Nitrogen Ratio

The carbon to nitrogen ratio (C:N) describes relative proportions of carbon and nitrogen in a substance. That substance can be soil, compost, or plants.

Carbon is used as energy and a building material, much the way carbohydrates and sugars are used by us. Nitrogen is needed by to generate proteins, amino acids, and enzymes, making it the steak and salad portion of a garden diet. The energy contained in carbon can only be used if there is enough good health provided by the nitrogen.

With the proper C:N ratio, plants can thrive, soil can support plant growth, and compost can decompose quickly. Improper C:N ratios can interfere with the soil microorganisms that make everything else in the garden possible.

Soil microorganisms

Soil microorganisms are responsible for the decomposition of dead plant and animal matter. They also break down minerals and chemicals found in the soil, turning them into plant food. Soil microorganisms prefer a C:N ratio of 24:1. This means they perform best in an environment that is 24 parts carbon and 1 part nitrogen. Since some carbons break down more slowly than others, the Golden Rule of C:N ratios is 30:1.

Microorganisms have a C:N ratio of 8:1. When they consume carbon and nitrogen, 16 parts of the carbon is burned off as energy, while 8 parts are used for maintenance. If higher levels of carbon are available, such as right after applying straw as a mulch, the microorganisms will pull nitrogen from the soil to maintain the balance they need. This means there will be less nitrogen in the soil for your plants. This tying up of nitrogen is called immobilization because the nitrogen is unavailable until the microorganisms die and decompose. The latter half of this cycle is called mineralization, because minerals are returned to the soil. Materials added to the soil with a C:N ratio of less than 24:1 means there will be an abundance of nitrogen left over for your plants. 

What’s your soil’s C:N ratio?

Complex lab tests are used to calculate C:N ratios for commercial agriculture. Since nitrogen doesn’t stick around for very long, you don’t need to go to the trouble or expense for an actual test. Instead, you can manipulate the C:N ratio with cover crops, crop rotation, mulch, and composting, If you have the correct C:N ratio, your plants will be able to eat and your compost will break down quickly, If the C:N ratio is out of whack, things won't be running as smoothly.

Cover crops and C:N ratios

Many cover crops, such as fava beans and other legumes, are grown to both protect the soil from erosion and compaction, and to add nutrients to the soil. You need to find a balance point between allowing the cover crop to grow, when to cut it, and what to replace it with as it decomposes in place. Cover crops are an excellent way to protect and feed the soil between regular crops. Depending on the plants used as a cover crop, you can ensure that your soil microorganisms and your plants have the nutrients they need.

C:N ratios and crop rotation

If you grow crops with low C:N ratios, such as legumes (peas and beans) or brassicas (cabbages and broccoli), you will want to rotate with crops that have higher C:N ratios, such as corn, sunflowers, sorghum, millet, or wheat. After crops with higher C:N ratios, you will want to follow with, you guessed it, crops with lower C:N ratios.

Mulching for C:N ratios

There are many good reasons to mulch. Mulch stabilizes soil temperatures, holds moisture, reduces weeds, and, eventually, adds nutrients to the soil. Different mulches have different effects on the carbon to nitrogen ratio. Different parts of the same plants can also have very different C:N ratios. Stems and flowers (anthesis) contain more carbon, while leaves contain more nitrogen. For example, oat leaves have a C:N ratio of 12:1, while oat stems have a C:N ratio of 78:1.

Infographic of common C:N ratios (USDA)

Composting for a better C:N ratio

As you add materials to your compost pile, keep in mind the ideal C:N ratio of 30:1. Now, not all materials break down at the same rate. Temperature, oxygen levels, and moisture content also play a role. This is not an exact science. To reach a point that is Perfect Enough, simply strive for 50% green and 50% brown. The ‘greens’ will all be higher in nitrogen, while the ‘browns’ will be higher in carbon. Also, stems contain more carbon, while leaves contain more nitrogen. For example, oat leaves have a C:N ratio of 12:1, while oat stems have a C:N ratio of 78:1. Below are some common materials and their C:N ratios:

High carbon
dried leaves         30:1 to 80:1
straw                     40:1 to 100:1
sawdust              100:1 to 500:1
bark                     120:1
paper                   175:1
newspaper        560:1
cardboard         560:1

High nitrogen
kitchen scraps        15:1
coffee grounds      20:1
grass clippings      20:1
manure                     5:1 to 25:1

If your compost has a C:N radio below 20:1, all of the carbon will be consumed, leaving nitrogen behind. This excess nitrogen is then converted into ammonia, which means it stinks and is lost to the atmosphere. If your compost pile smells bad, flip it, to add oxygen, and stir in more carbon.

Keep your garden healthy by maintaining a good ratio of carbon to nitrogen. The proper C:N ratio improves decomposition rates and nutrient cycling within the garden. Strive for a C:N ratio of 30:1 in your garden. Your soil microorganisms will thank you!