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?

Nitrogen rich (green) wastes Carbon rich (browns) wastes
Grass, most manure (especially chicken), meat, eggs, and fish Wood (shavings/sawdust/chips), garden pruning, newspaper & cardboard, white paper, moss, straw (dried wheat/oat stems) and hay (dried grass stems)

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:

  • Add sugar to your heap (zero nitrogen) - does it decay fast? Yes, faster than anything! (it will be used in the absence of any nitrogen for a simple reason not all food is used to reproduce and create new cells - some energy is need to respire (breath / live).
  • Add 1 Kg wood chip to 70Kg grass (to give a 30:1 ratio), does the wood decay? Nope. The grass does and the wood remains. In fact you are more likely to get excess nitrogen released as ammonia from this mix despite being balanced 30:1- see aeration below!
  • Add 1Kg of shredded paper to 70Kg grass - do both disappear (compost down)? Yes. Did it compost any faster - no the rate at 60C is the same. Do you smell ammonia? No. The bacteria have consumed both the carbon and nitrogen. The ratio of carbon to nitrogen did not determine the speed or composting. Everything got used up quickly and the bacteria population would have grown rapidly.

If a 30:1 Carbon:Nitrogen is not essential, what is?

a) Temperature:
The rate of decomposition doubles for ever 10C increase in temperature (up to 70C). You achieve fast efficient composting with heat. Heat is released from bacteria digesting food. It is a virtuous circle the higher the temperature the faster they work and the more heat is released. The key to this is the knowledge that bacteria can digest some waste faster than others: in an analogy to the human gut - we digest glucose/sugars faster than carbohydrates (fibres) and we are unable to digest cellulose or lignin (wood). Bacteria digest sugars faster than >proteins > cellulose > lignin.

b) Structure and Aeration:
Bacteria need oxygen hence air needs to diffuse through the heap to each piece of waste. A lot of "browns" (high carbon materials) are woody and do not break down into mush - they stay as pieces and create aeration pathways

Try this: balance you carbon nitrogen going into the HotBin first by using a mix of cooked food, shredded paper and sawdust Now try cooked food, shredded paper and wood chip. The first will quickly go anaerobic and form a solid stinking mush. The later 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. 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