Compost’s Carbon: Nitrogen Ratios

In order to understand compost’s carbon:nitrogen ratio, know that everything organic has a carbon:nitrogen ratio of carbon to nitrogen (C:N) in its tissues. See below for a list of C:N ratios of common organic wastes. It is the combination of materials that creates the ideal climate for compost microbes – a C:N ratio of 30:1. This combination, along with moisture, volume and surface area, is what makes a fast, hot pile. Some composters like to keep things simple and use the terms brown (carbon) and green (nitrogen), and follow the general rule of 1 part brown for every 2 parts green.

Other composters like to build their piles using a variety of materials and using the following formula to determine the C:N ratio of their compost pile. This balance is difficult to achieve exactly, but if you are interested, you can come close by combining materials and calculating the resulting (C:N) ratio. For example, mixing two parts green grass clippings (which have a C:N ratio of 20:1) with one part dry leaves will give you a C:N ratio of 33:1, very close to the ideal. The process is this: add 20/1 + 20/1 (the 2 parts green) with 60/1 (the leaves). The total is 100/3 or 33/1.

Let’s look at this another way:

60/1 + 20/1 + 20/1 = (100/3)/3 = 33/1

Using the C:N (C/N is another way to write it) ratios on this worksheet, see if you can work out the following problems. We’ll do another one for you. Keep in mind that all volumes are equal – each pile of material in the following problems is approximately the same size.

• Making a pile of 2 parts green grass, 1 part pine needles and 1 part fresh manure yeilds a C:N ratio of _____?
• 20/1 + 20/1 + 110/1 + 15/1 = 4 165/4 = 41/1
• Your pile is a mixture of 1 part straw, 1 part dry leaves, and 3 parts green grass. What is the C:N ratio? _____.
• You have all the waste from your yard and garden and you have some sawdust to add to the pile. The ingredients are: 1 part sawdust, 2 parts green grass and 1 part fresh weeds.
  The C:N ratio of this pile is _____.
• If you make your pile with 1 part manure, 1 part straw and 2 parts green grass. what is the C:N ratio? _____.

See? Wasn’t that easy?

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