All About Greens and Browns

All About Greens and Browns

Do I Need a 30:1 Carbon to Nitrogen Ratio in the HOTBIN?

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.

What Are Greens and Browns?

Compost pile works best with a ratio of greens to browns – not achieving this balance can lead to an anaerobic heap or one that will not produce any heat and take ages to break down the contents.

What is a Green?

These are nitrogen-rich materials, often with a high moisture content that are easy to digest and produce heat quickly.

What is a Brown?

These are materials which are rich is carbon (woody based material) but are often harder to digest and therefore slower to break down in a compost pile. They often used as a bulking agent to promote airflow.

What Are Typically Nitrogen and Carbon Rich Materials?

Nitrogen Rich
Green Waste

Carbon Rich
Browns Waste

Grass, garden waste,  manure (especially chicken), tea leaves, coffee grinds, 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)

What Else Is Important to Consider in the HOTBIN?

1. Temperature:

The rate of decomposition doubles for every 10°c increase in temperature (up to 70C). You achieve fast efficient composting with heat which is produced naturally by bacteria digesting food. It is a virtuous circle, the higher the temperature goes, 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.

2. 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

How Does the HOTBIN Require the Use of Browns?

Nitrogen-rich waste has a high water content, which when it breaks down can create a wet mush that bocks airflow, which then results in anaerobic conditions. To avoid this, the HOTBIN uses two techniques involving browns:

  1. Woodchip - added to maintain airflow
  2. Shredded paper/corrugated cardboard – these materials absorb excess moisture

Consider This Scenario

Balancing carbon and nitrogen going into the HOTBIN by:

  1. Using a mix of cooked food, shredded paper and sawdust
  2. Using a mix of 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 woodchip supports aeration and sawdust compacts and blocks aeration.
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