C:N ratios

Share this post

Share on facebook
Share on linkedin
Share on twitter
Share on email

When fresh organic material undergoes decomposition in soil, both the rate of decomposition and the amount of humus formed are related to the C:N ratio of the residue. When other conditions are equal, the rate of decomposition increases as the C:N ratio narrows.

Four conditions that are constant for all residue decomposition.

  1. A maximum of 35% of the carbon in fresh organic material will be converted into soil humus IF there is sufficient nitrogen present.
  2. A minimum of 65% of the carbon in fresh organic material will be given off to the atmosphere as carbon dioxide due to microbial respiration.
  3. The humus formed from the decomposition of fresh organic material will contain approximately 50% carbon and 5% nitrogen. In other words, the C:N ratio of the humus is 10:1.
  4. Most fresh plant material contains 40% carbon. The C:N ratio varies because of differences in nitrogen content, not carbon content.

Below are the average C:N ratios for some common organic materials found in the compost bin. For our purposes, the materials containing high amounts of carbon are considered “browns,” and materials containing high amounts of nitrogen are considered “greens.”

Estimated Carbon-to-Nitrogen Ratios for some common composting materials:

Browns = High Carbon C:N
Ashes, wood 25:1
Cardboard, shredded 350:1
Corn stalks 75:1
Fruit waste 35:1
Leaves 60:1
Newspaper, shredded 175:1
Peanut shells 35:1
Pine needles 80:1
Sawdust 325:1
Straw 75:1
Wood chips 400:1
Greens = High Nitrogen C:N
Alfalfa 12:1
Clover 23:1
Coffee grounds 20:1
Food waste 20:1
Garden waste 30:1
Grass clippings 20:1
Hay 25:1
Manures 15:1
Seaweed 19:1
Vegetable scraps 25:1
Weeds 30:1

 

Further reading

100 years later… Farming Today

Some of the principles that Howard wrote about 100 years ago are just being talked about in this broadcast of Farming Today. From the Groundswell

Support Our Projects

Whilst Good Gardeners International is lucky enough to generate some income through our own products, we still rely heavily on invaluable donations from people such as yourself. If you can support us and the work we do, there are four options below to get involved.