Worm Farm Reviews

C:N Ratio: What is it and Why is it Important?

Home » Composting and Vermiculture » C:N Ratio: What is it and Why is it Important?
C:N Ratio

If you can remember back to high school chemistry, you might remember the chemical elements and how each of them has a letter as its symbol. H for hydrogen, O for oxygen…etc. When people use the term C:N ratio, they’re using the chemical symbols for Carbon and Nitrogen. So, the C:N ratio refers to the ratio of the amount of carbon to nitrogen within something. Most often it refers to the ratio within the soil. In the context of vermiculture, the C:N ratio is used to refer to the ratio within the bedding and what gets fed to the worms.

How Do Ratios Work?

Ratios work by having one number before another. The ‘:’ means ‘to’. So, a C:N ratio of 10:1 means ten parts of carbon to one part nitrogen, or, for every 10 units of carbon, there’s only 1 unit of nitrogen.

Carbon to nitrogen ratio in the soil can impact how processes within the soil function. Different levels of carbon and nitrogen on their own, and different ratios of the two, can change the way plants absorb nutrients. It can also affect the activities of the soil ecosystem breaking down organic matter. Different C:N ratios may also affect what can and can’t survive in the soil.  

RELATED: What Are Worm Castings? All You Need to Know About the “Black Gold”

What’s the Best C:N Ratio?

C:N Ratio

The C:N ratio you should be aiming for will depend on what you’re trying to achieve in the situation. For plant growth, a C:N ratio of 24:1 is the ideal condition for micro-organisms to break down organic matter that releases essential nutrients like nitrogen and phosphorus.

But this will also depend on the plants you’re growing.

Some crops require higher nitrogen levels and will require the application of fertilizers to reach these levels. For ornamental gardens, some plants cannot tolerate high nitrogen levels at all, so a lower ratio is needed. 

C:N Ratio for Composting

If you’re planning on building a compost heap, the ideal C:N ratio is a bit higher, at 30:1. Higher nitrogen allows microbes that prefer higher temperatures (that you find in compost piles) to thrive, which in turn speeds up decomposition, and pushes the temperature higher again. In a composting situation, you can increase your nitrogen levels through what you’re adding to the pile. 

High nitrogen wastes include green vegetable waste, ground coffee, and manures. But be careful, because there is such a thing as too much nitrogen, which will actually slow down decomposition. To balance this out, you’ll need to also add carbon-rich waste like sawdust, leaves, paper and cardboard. It’s a real balancing act adding the right ingredients to get the right ratio!


C:N Ratio for Vermicomposting

Vermicomposting is again different in the processes of breaking down organic matter, so needs a different ratio. In a worm farm, you don’t want the temperature to be too high. This means we want to avoid heat-based decomposition happening, so the C:N ratio needs to be above the 25-30:1 ratio. 

It’s generally accepted that worm farms work best with a C:N of more than 50:1. This means you want to be adding a lot more carbon than nitrogen to your worm farm.

RELATED: What Do Worms Eat? Feeding Your Worms For Ultimate Success

How to Fix Problems With the CN Ratio?

When carbon levels are too high, decomposition will slow and be very dry. When nitrogen is too high, the pile can begin to stink and get all slimy. The easiest way to fix your C:N ratio then is by adding more nitrogen or carbon to your soil to bring these levels back into the ideal range.

In a cropping situation, you can do this through the addition of fertilizers. These can be organic or synthetic and can be solid or liquid. You’ll just need to do some calculations to figure out how much you need to add to your soil to get you where you want.

To add C or N to a compost heap or worm farm, you’ll need to add materials that are high in one or the other. A very simple way of remembering what to add is that any scraps for vegetables or fruit that you eat, or waste that comes from an animal is usually high in nitrogen. Anything that comes from a tree, or plant material that isn’t edible to humans, is usually higher in carbon. 

RELATED: Composting With Worms: Common Problems and How to Fix Them

C:N Ratio

“Greens” versus “Browns”

If we expand on that just a little bit, we can say that vegetable and fruit scraps, cow manure, coffee grounds, and grass are high in nitrogen. These are often referred to as green materials.

Wood, tree branches, sawdust, shredded paper, and cardboard are materials high in carbon and are referred to as brown materials.

A good guide can be found here.

Generally speaking, your pile, or worm farm, should be made mostly of high-carbon materials. When you’re building or adding to your pile, work to the rule of 4:1 carbon to nitrogen materials. Going off mass or volume is also better than weight for this rule. Having a stash of old newspaper, dried leaves, shredded paper or cardboard is always a great idea. Add a generous amount to your worm farm before feeding them your food scraps. This should help keep the C:N ratio fairly consistent and improve the quality of your worm castings. Just remember though, that you don’t need to be super accurate. You can be, by all means, but sticking to these values generally will still do the trick.

In Summary

Having the correct carbon to nitrogen ratio is important, but exactly what ratio you need to have depends on your situation. Cropping situations will be different from composting, and composting will again be different to your worm farm. By adding the right organic materials to your compost pile you can adjust and correct your C:N ratio pretty quickly.

For worm farming, a C:N ratio of more than 50:1 is desirable. As a general rule though, add 4 times the amount of browns to greens when adding food for your worms. Newspaper, cardboard, and dried leaves are all great browns to add. Keep a stash handy so you’ll never be caught short.

Let us know what tips you have for keeping your worm bin’s C:N ratio optimal in the comments below. As always, happy worm farming!

The site for independent worm farm reviews, worm farming guides for beginners and all things vermiculture.

Leave a Comment

Your email address will not be published. Required fields are marked *