Balancing Carbon and Nitrogen (Bedding and Food)

To maximize microbe production in the worm bin, there needs to be a balance of carbon and nitrogen in the bin. We call that the C:N ratio. Carbon is used by microbes as food energy, while nitrogen is used for protein production so the microbes can reproduce. In conventional composting, you would balance the carbon and the nitrogen when you build the pile, but that causes a microbe bloom and can heat up. The heat can kill your worms. So in worm composting, we start with a lot of carbon and add nitrogen gradually. So that we can agree on our terms, we call the high carbon materials bedding, and we call the high nitrogen materials food. We sometimes refer to them as browns and greens, even though not all bedding is brown, and not all food is green. The worms live in both and eat both.

Bedding tends to be dry, bulky and absorbent. Food tends to release moisture as it breaks down. Excess food often means excess moisture, which can cause a bin to go anaerobic.

There are a number of C:N ratio calculators on the web that can be used to estimate the approximate C:N ratio of mixtures of different beddings and foods. However, these can be hard to apply to a worm bin for a number of reasons. One is that C:N ratios are calculated by the mass of the material, and we tend to build compost piles by volume. Some materials will vary in the amount of carbon and nitrogen if the material is compacted or ground up so that the same mass of material takes up less volume. Some materials will vary depending on their makeup, like food scraps, which can be made up of different types of foods. And some materials, like grass clippings, will vary depending on their age and moisture content. Freshly mown grass clippings are food. But grass clippings lose their nitrogen as they dry out. Completely dry grass clippings are bedding.

The ideal C:N ratio is about 25:1. A C:N ratio table can help you determine how much bedding or food you may need to add to your bin. For example, horse manure has a C:N ratio of about 25:1. If you are using horse manure as bedding, you would not want to add food. Shredded cardboard has a C:N ratio of 350:1. You can add a lot of food to a bin if you use shredded cardboard as bedding. Coconut coir is closer to 100:1, so you would not add as much food if you are using coconut coir as bedding. Fall leaves can range from 30:1 to 80:1, so you would want even less food if you are using leaves as bedding.

Any rule of thumb is just that, a rule of thumb, and may or may not be an accurate guideline. We do know that if the C:N ratio is just right, the bin can heat up. If it leans heavily toward nitrogen, the bin can become a mucky mess. If it leans heavily toward carbon, the organic matter in the bin breaks down slowly.

Feeding is as much an art as it is a science. As you gain more experience, you will get a feel for how much of what types of materials you can add to the bin. As the bin gets more established, food will be eaten faster. Some foods create a mucky mess. Others break down slowly. Others are eaten very quickly.

A rule of thumb I like to go by is to feed an amount of food about equal to the total weight of the worms. So if you are starting out with a pound of worms, give them about a pound of food. Don’t feed again until the first pound is mostly eaten.

I have read that a mix of about 50/50 bedding to food is about right, but it depends on what you are using for bedding and food. I would lean more heavily toward bedding until you get feel for what works in your bin. Keep an eye on the bin. Check to see if the worms appear happy and if there are any bad smells, or noxious critters attracted to the bin.

If the balance looks about right, try to maintain that balance. If the bin is becoming a mucky mess, add more bedding. If the bedding is breaking down very slowly, add more food. Over time, you will get a feel for how much food and bedding your bin needs.