Composting and the Carbon Nitrogen Ratio

Thomas J. Koske  |  4/16/2005 2:08:17 AM

Composting is a science, but it also 'just happens'.

What does the carbon-to-nitrogen ration tell us about materials we want to compost? Scientists tell us that the best blend for composting is about a 25 or 30 to 1 ratio between the carbon and the nitrogen contents. Or, for every 25 parts of carbon, you need to have about one part nitrogen for great compost.

Most materials available for composting don't have this exact ratio, but grass and clover-hay are very close. You can blend organic materials to approach this ideal blend and make the compost work most efficiently.

If you look at the table included, you will see the carbon and nitrogen ratio of commonly available organic materials you can use for composting. Sawdust has as high as 500:1 carbon to nitrogen (C:N) ratio. Because of this high ratio, sawdust decomposes slowly; it needs more N to work better. There is a severe protein-N shortage and the microorganisms in the sawdust reproduce slowly.

When you are creating your own compost pile, you want to make sure you blend organic materials so that the overall ratio of carbon to nitrogen is approximately 25 or 30 to 1 (25-30:1). This might mean that you take high protein waste, such as green vegetation, fresh grass clipping and manure, and mix it with materials that have a higher carbon content such as brown leaves, twigs and sawdust so the microbes have a more ideal and balanced environment to reproduce. When they die, their bodies are then transformed into the humus, vitamins and mineral foods for your plants and soils to use.

Carbon vs Nitrogen Ratios* of Common Materials

  • Activated sewage sludge 6:1
  • Humus, fresh chicken litter 10:1
  • Alfalfa hay, cattle manure 12:1
  • Food wastes 15:1
  • Green clover 16:1
  • Sewage sludge; digested 16:1
  • Grass clippings, weeds 19:1
  • Rotted manure 20:1
  • Leaves 40:1 - 80:1
  • Straw 80:1 - 99:1
  • Paper 170:1
  • Sawdust 500:1


*average ratios

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