Do I need a 30:1 ratio of carbon to nitrogen
Thursday, 6 February 2014 | HOTBIN Composting
Is a 30:1 Carbon to Nitrogen Ratio Needed in HOTBIN Composting?
In the HOTBIN, you do not need to analyse materials and mix them to achieve a strict 30:1 carbon to nitrogen ratio. It is easier and more important to balance 'easy to digest and 'slow to digest' types of waste (ie green and browns) as they affect the temperature and aeration and hence speed of composting.
Where does the 30:1 carbon to nitrogen come from?
Bacteria use carbon as a source of energy and nitrogen to build proteins needed for to operate and make new cells. Overall they use 30 carbon for every 1 nitrogen atom – hence the optimal ratio of carbon to nitrogen for bacteria growth is 30:1. If you wish to analyse and achieve 30:1 then there are online calculators to help. In our experience, this level of detail is not needed to get achieve hot composting.
What happens if I do not have 30:1 ratio?
Decomposition occurs in wastes when the ratio is considerably away from the optimum. Wood decays (200:1 ratio), grass decays (10:1). It is more accurate to say: if the nitrogen content is exceptionally low (eg wood), composting can be 'limited' and when nitrogen is in excess (eg grass), the excess nitrogen can be lost as ammonia gas. Losing nitrogen is generally to be avoided, as we want to retain the nitrogen nutrient in the final compost for the plants.
What are typical high and low nitrogen materials?
Are the high nitrogen materials fast and easy to digest?
Yes in most cases there is a close relationship between nitrogen rich (green) materials and those items that are easy for bacteria to digest. There is also a close relationship between hard to digest and woody (high carbon materials). To some extent, this partly explains why the 30:1 ratio works – you end up mixing “greens and browns” and “easy and hard”.
You can test and prove it is not the ratio that is affecting how fast your compost process works by doing these tests:
If a 30:1 Carbon:Nitrogen is not essential, what is?
b) Structure and Aeration:
Balancing carbon and nitrogen going into the HOTBIN by using a mix of cooked food, shredded paper and sawdust versus cooked food, shredded paper and wood chip.
The first option will quickly go anaerobic and form a solid stinking mush. The later however will compost well without odour. Both had easy to digest food at 30:1 C/N, both had high carbon wood (200:1) that did not compost. But only one mix worked effectively because wood chip supports aeration and sawdust compacts and blocks aeration.
In summary worry less about carbon to nitrogen and focus on adding easy to digest materials that will generate heat. Add some hard particles (e.g wood chip) that will maintain aeration structure.
Note: Updated April 2015