carbon + nitrogen

Getting the carbon/nitrogen balance right

Most of us know we need a blend of carbon-rich and nitrogen-rich ingredients to make a good compost mix. The perfect compost ratio recommended by most compost sources is approx 30-parts carbon to one-part nitrogen.

But what is a carbon nitrogen ratio?

All living things have both carbon and nitrogen in them, just in different quantities.

Carbon is the basic building blocks for all living things. Anything that is, or has been, alive, has lots of carbon making up its structure, but there will also be nitrogen there too, often in the form of proteins. Things like meat scraps will have a very high nitrogen count, whereas wood has a very low nitrogen count, even though both are still made up primarily of carbon.

What is a carbon nitrogen ratio?

The perfect compost ratio recommended by most compost sources is approx 30-parts carbon to one-part nitrogen.

A carbon nitrogen ratio is a way of expressing this. Every ingredient that goes into our compost pile will have its own carbon nitrogen ratio. Straw has a C:N quoted ratio of approximately 80 parts carbon to one part nitrogen, or 80:1 and is considered a ‘carbon-rich’ ingredient. Fresh cut grass has a quoted C:N of around 15 parts carbon to one part nitrogen, or 15:1, what we call a ‘nitrogen-rich’ ingredient.

Great, but what does this mean in the real world of our backyards?

In practice we can’t know the exact carbon nitrogen ratio of any specific ingredient without sending it off to a lab for testing. This would be too costly, too slow and not very practical for us backyard composters.

Don’t despair, here are two handy tips to help you better understand the C:N ratio of your compost ingredients.

Firsty, try this little thought experiment: imagine a particular compost ingredient left in a bucket of water for a week in summer, then ask yourself, how stinky would it get? If you imagine it being very stinky (say food scraps) you have a pretty high nitrogen ingredient. If you think it would not smell too much (say wood chips) it is likely to be pretty high in carbon, with very little nitrogen. Most people’s intuition on this is pretty good and it is lots cheaper than sending everything off for lab testing!

Secondly, look up a list of common compost ingredients you use and their average C:N ratio to give yourself a ballpark figure. We have one on our YIMBY website, HERE or below It's just a guide of course but have a look.

When we are layering up our compost pile (remember those thin, lasagne style layers?) it is good to think about the approximate C:N of what we have just put down, then balance it with an opposite ingredient; carbon-rich this layer, nitrogen-rich the next.

Compost getting a bit wet and stinky? Lean into those carbon-rich ingredients. Pile getting a little dry and woody looking? Go a little harder on the nitrogen-rich ingredients.

Next week Mikaela Beckly will be exploring compost options for your next holiday.

AUTHOR: JOEL MEADOWS

Joel Meadows works with Yes In My Back Yard, (YIMBY), a community-scale composting initiative in Castlemaine and surrounds. Send questions or comments to hello@yimbycompost.com

This article first appeared in Midlands Express on 17 October 2023. View it here.