Calculating C:N ratios

Calculating C:N ratios – hitting the sweet spot when blending multiple feedstocks

Cooking may allow the chef to add a pinch of this and a daub of that to create an incredibly edible meal, but more exact measurements and methodologies are required for successful baking.  That’s because cooking is mostly about building and enhancing flavors.  Baking is about understanding and exploiting science.

Composting is about science, too.  Just as curdled custard and flat cookies are indicative of science gone awry, a sluggish composting process and offensive odors are signs someone failed to adhere to the science.

One of the common culprits of composting-science-gone-bad is getting it wrong when calculating C:N ratios.

C means Carbon.  N stands for Nitrogen.  Together, they influence microbial feeding activities.  C provides energy and microbial cell structure while N is linked to proteins, enzymes and other substances integral to cell growth and other biological functions.

The trick, of course, is to hit the composting “sweet spot” with a carbon-to-nitrogen ratio of about 30:1 for the initial feedstock blend.

Forget about “brown” and “green” for calculating C:N ratios

The ubiquitous brown:green rule of thumb for composting — brown materials are carbon, green are nitrogenous — is often promoted to help the backyard composter recognize differences in materials.  But one cannot categorize feedstocks based on color alone.

Coffee grounds may be brown, but they are rich in nitrogen and can actually serve as a substitute for manure (which is also a brown).  Therefore, coffee grounds are “green.”  Peanut shells, also brown, have a near-perfect C:N ratio for composting of 35:1 without combining with any other carbon or nitrogen.

Adding to the brown-green confusion is a mistaken belief that the ratio relates to volumes of brown material and volumes of green, as in 30 buckets of sawdust to one bucket of cut grass.

It does not.

C and N values are derived from the actual carbon and nitrogen content of the individual blend ingredients, not by feedstock volumes, and those numbers are derived from materials testing or generic charted values based on someone else’s tests.

At a micro-scale, the margin of error associated with color confusion may be small enough to make little difference in odor generation or degradation rate for a home compost pile.  But at industrial-scale, calculating C:N ratios needs to be more precise.

The best method of determining carbon and nitrogen content is feedstock testing.  Then, calculations can be made to determine the mix.  When testing isn’t possible or practical, assumptions can be made based on charts like this one.  Washington State University also makes a Compost Mixture Calculator available online that does the math.

C:N impacts on the composting process

C:N is an indicator of the nutrient content of any given material.  Sometimes, a C:N ratio may be expressed as a single number — i.e., 45, 30, 13, etc. This means 45:1 or 30:1 or 13:1.

A lower “C” number indicates higher nitrogen.  Higher Cs indicate more carbon.

For a composting blend, anywhere in the 25-35:1 range is considered good.

A higher carbon content will slow the biodegradation process.  But if carbon content is too low, odors and anaerobic conditions can become management issues.  Too much nitrogen can also raise pH, killing off desirable microorganisms.

Carbon content will naturally drop during composting as microbes use it for energy.  Carbon is also released as CO2.  Nitrogen, however, gets recycled, so the amount at the end of processing is the same as it was in the beginning.

C:N impacts on compost use

The 30:1 ratio is the Goldilocks Zone for composting, but a typical finished compost might have a ratio of 20-25.  (Soil microbes prefer a C:N ratio of around 24:1.)

When added to the soil, composts with higher numbers can encourage microbes to lock up any available nitrogen for their own use, leaving less for plants. Lower ratios reduce the likelihood for food (nitrogen) competition between plants and microbes, since the feeding microbes will still leave plenty of N for plant growth.  Even at a 10-20:1 ratio, there will still enough carbon and nitrogen to allow plants and microbes to successfully share.

However, when the C:N ratio drops below 10, degradation rate for organic matter becomes very high, negating some of the benefit of compost use.

Soil tests, product tests and a chat with the local agricultural or horticultural extension agent should tell growers what they need to know to maximize yields based on the C:N ratio of finished compost.

LEARN MORE:  Carbon to Nitrogen Ratios in Cropping Systems