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Everyone seems to have an opinion on this, but is there any definitive answer, ideally backed up by link/s to reputable sources or research?

Ratio by weight or by volume.

  • Are you really going to weigh kitchen scraps and hedge trimmings? – Stephie Jun 22 '15 at 8:35
  • Sounds crazy, but yes - at least a sample to give a representative idea weight-to-volume. Funding for community growing scheme coming from climate change fund, and part of this is tied to showing that household waste is being diverted from landfill. - but you're right from the point of view of this question, only volume is relevant. – GC. Jun 22 '15 at 10:04
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The notions of "brown" and "green" material are only a proxy for Carbon and Nitrogen rich materials to make it easier to mix your materials. If you really want to measure to have optimal composting conditions, you should look into the Carbon and Nitrogen ratio. The ideal starting ratio for fast composting seems to be 25:1 to 30:1, though 20:1 through 40:1 are acceptable. There seems to be consensus on this, here are some example from the FAO, Cornell, Washington State University and Oregon State University.

To convert this into proportion of "green" and "brown" materials you need to find the C:N ratios of your main materials, then find the ratio of each to achieve 25:1 to 30:1. You can easily find C:N ratios for common materials on the web. The sources mentioned contain tables, but here are a few more from homecompostingmadeeasy.com and Planet Natural.

I would recommend doing back of the envelope calculations by weight, otherwise use an online calculator. Here is an easy to use one, otherwise Cornell's is more customisable (though you'll need moisture content for the latter).

As you mention a scheme about household landfill waste reduction here is an example. Most of your waste comes from food waste (20:1), garden waste (30:1) and newspapers (175:1) with the bulk of the volume being the first two. By mixing food and garden waste 1:1 you reach a 25:1 starting mixture. A few shredded newspapers added in bring you closer to 30:1 but well within the 40:1 upper boundary.

I started composting last year spent a while researching trying to get ideal proportions (mainly broiler litter, goat manure, grass clippings, leaves and kitchen waste). From my experience, you shouldn't sweat too much on this, as your material will compost even without perfect starting proportions. Meeting moisture and oxygen requirements, and breaking down materials to smaller sizes seem more important to me for fast composting. My litmus test for C:N ratio is smell. If the compost heap smells of ammonia (for me usually due to chicken manure), the initial mix was too rich in Nitrogen and i add brown material to the mix.

Hope this helps and have fun composting!

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Too much green (glass clippings) and you will get a smelling mess!

Too much carbon (wood chippings etc) and it will take a lot longer to break down.

In a normal garden, you will not get fast composting, as most people will not turn the heap often enough or have a large enough heap.

Therefore I will say it is ideal to have too much browns, as that way people are not put off by the smell and therefore they will keeping composting….

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