Composting
Carbon Nitrogen Ratio
Is It Really That Important?

The role that the composting carbon nitrogen ratio (C:N) plays in the composting process is a topic that frequently comes up when discussing how to compost. But is it really as important as people make it out to be?

It Sure Is!!!

As we so often discuss on this site, the composting process is almost entirely dependent on the health and "happiness" of the micro-organisms within your compost pile, or windrows. And aside from adequate oxygen and moisture levels, your microbes need a couple more VERY important things to thrive - they need the right balance of compost ingredients.

Why Your Microbes Need Carbon & Nitrogen

On a very basic level, the dietary requirements of the micro-organisms in your compost are not very different than those of the typical human being. That is, just like us, your microbes use carbon for energy and growth, and they use nitrogen for protein and reproduction. If you remove either of these components, or overfeed your microbes too much carbon or nitrogen, you can kiss your chances of making high-quality compost good-bye.

The Magic Ratio

Why do we call this the magic ratio? Because when your composting carbon nitrogen ratio falls within this range, that's where the magic happens. So what is this ratio?

As we mentioned on our ingredients page, the ideal composting carbon nitrogen ratio is between 20:1 --> 40:1. Now before you start breaking out your calculators, please understand that if your ratios are not within this magical range, your pile will still breakdown; it may just happen more slowly.

Please note, if you're a commercial compost producer, you really want your ratios between 25:1 and 30:1. This will ensure your compost breaks down as quickly as possible and you able to process your materials in the most efficient manner.

Refer to the chart below, or use our compost ingredients page to determine the composting C:N ratios for the ingredients you have locally.

Are Your Starting Ingredients Wet?

If so, throw the C:N ratio talk out the window for the time being. Instead, focus your attention on the combined moisture levels of all of your ingredients.

Failing to properly compensate for wet materials can lead to a lot more headaches than trying to find the ideal C:N ratio. If your materials are overly moist, you run the risk of your pile quickly going anaerobic, increased potential for odors, delayed heating, and unnecessary seepage. If your operation is located anywhere near other businesses or towns, and you fail to compensate for wet materials, you'll have the "authorities" breathing down your neck in no time.

Once you've got your moisture levels balanced, then reconsider the composting carbon nitrogen ratio for your starting recipe. This same warning does not apply to overly dry materials. If that's the case, you can easily add water.

Not All Carbon Is The Same

Just because we pin a number to the "C" component of all composting ingredients does NOT necessarily mean they're all equally comparable.

For instance, let's imagine three piles of carbon-containing materials: each pile is either entirely wood chips (100-400:1), straw (75:1), or high-sugared fruit wastes (15-25:1). Say we had worked out the volume of each of these ingredients so we had roughly the same number of pounds of carbon per pile. From a visualization stand-point, the size of our wood chip pile would be much smaller than the size of our straw pile, which would be smaller than our pile of fruit wastes.

compost wood chips

Now even though each of these piles contains an equal number of pounds of carbon, our microbes will not process these ingredients at the same rate.

The rate at which your microbes process your carbon sources is dependent on two main criteria: the surface area of your ingredients, and a factor referred to as lignification.

The first factor is quite easy to understand. The larger the particle size of the ingredients in your compost, the slower they will decompose. This is why almost all commercial compost producers use a compost grinder to pre-digest their materials.

The second factor - lignification - refers to the complex cellular structure of woody substances. Lignin is very resistant to decomposition, so the more lignin an ingredient contains, the slower it will break down. That is why compost made with fruit scraps will break down more quickly than compost made with straw, and compost made with straw will breakdown more quickly than compost made with wood chips (even IF the particle sizes have been greatly reduced).

Note - Although it may take more time to breakdown, a critical ingredient in the creation of fungal-dominated compost is woody material. If done properly, this compost is one of the most beneficial soil amendments you can produce, and/or use.

Common Carbon Nitrogen Ratios

Below is a list of the composting carbon nitrogen ratios for several common composting ingredients.

Ingredient C:N Ratio
Poultry Manure (no litter) 10:1
Cattle Manure 20:1
Horse Manure 25:1
Pig Manure 6:1
Paper Sludge 105:1
Peat 70:1
Mushroom Compost 40:1
Garden Wastes 40:1
Rock Powders N/A

Note - If the ingredient you are searching for is not in the above table, please check the tables on our composting ingredients page. We tried to avoid duplications between these pages.

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