The Carbon:Nitrogen Ratio (C:N) in Composting

Did you know that all organic matter has a ratio of Carbon to Nitrogen (C:N) in their tissues? For microorganisms, carbon is the basic building block of life and is a source of energy, but nitrogen is also necessary for such things as proteins, genetic material, and cell structure.

Balance of C:N is Key
Decomposition of organic materials in your compost pile is greatly increased when you create the proper balance between the carbonaceous materials (called BROWN because they are dry) and the nitrogen-rich materials (called GREEN because they are more fresh and moist).

In compost lingo, this balance is referred to as the Carbon-Nitrogen ratio, and shown as C:N.

Now, it is true that most people simply don't give a hoot about this scientific hocus-pocus stuff. Waste is waste! And when you just want to throw the stuff away, you're not inclined to stop a moment to ask, "Gosh, is this Carbon or Nitrogen?"

But magic is afoot out there in nature. And much of the sleight of hand of composting, whether you are aware of it or not, has to do with the organic materials' content of Carbon and Nitrogen. Blow this stuff off and you might get a surprise when you open the lid to your bin: it may reek to holy hell, like rotten eggs or ammonia, or it may just be sitting there doing absolutely nothing! Which is to say your pile has become a cold couch potato, and it ain't going nowhere fast!

So, back to this necessary balance between the Carbon content of your waste material and the Nitrogen content. For best performance, the compost pile, or more to the point the composting microorganisms, require the correct proportion of Carbon for energy and Nitrogen for protein production. Compost scientists have determined that the fastest way to produce fertile, sweet-smelling compost is to maintain a C:N ratio somewhere around 25 to 30 parts Carbon to 1 part Nitrogen, or 25-30:1. If the C:N ratio is too high (excess Carbon), decomposition slows down. If the C:N ratio is too low (excess nitrogen) you will end up with a stinky pile.

A Little More Science Behind the C:N Ratio
Microorganisms that digest compost need about 30 parts of carbon for every part of nitrogen they consume. That's a balanced diet for them. If there's too much nitrogen, the microorganisms can't use it all and the excess is lost in the form of smelly ammonia gas. Nitrogen loss due to excess nitrogen in the pile (a low C:N ratio) can be over 60%. At a C:N ratio of 30 or 35 to 1, only one half of one percent of the nitrogen will be lost. That's why you don't want too much nitrogen (fresh manure, for example) in your compost: the nitrogen will be lost in the air in the form of ammonia gas, and nitrogen is too valuable for plants to allow it to escape into the atmosphere.

Scientists have determined four conditions that are constant for all residue decomposition:

1.A maximum of 35% of the carbon in fresh organic material will be converted into soil humus IF there is sufficient nitrogen present.

2.A minimum of 65% of the carbon in fresh organic material will be given off to the atmosphere as carbon dioxide due to microbial respiration. (Uh-oh! An argument could be made that composting contributes to greenhouse gases and warming of the Earth's atmosphere. However, consider this, nature is always decomposing everywhere; so, what you are doing in your little compost bin is a mere iota of carbon release compared to nature's vast compost bin in forests, rangeland, etc.)

3.The humus formed from the decomposition of fresh organic material will contain approximately 50% carbon and 5% nitrogen. In other words, the C:N ratio of the humus is 10:1.

4.Most fresh plant material contains 40% carbon. The C:N ratio varies because of differences in nitrogen content, not carbon content. (Note: Dry materials are generally in the range of 40 to 50 percent carbon, and sloppy, wet materials are generally 10 to 20 percent carbon. Therefore, the most important factor in estimating the carbon-to-nitrogen ratio of plant or food waste is how much water is present).

HOW TO USE THE C:N RATIO

Principle #1: The Ideal 30:1 Ratio

A hot, fast pile (with temperatures up to 140°F/60°C) is obtained when the C:N ratio of all the materials you add averages 30:1 (50:1 is adequate for most slower, lower-temperature piles). You can be sure, then, that the little microbes are stuffing themselves. Of course, this is ideal: you may not necessarily obtain this!

Don't Make This Mistake: This ratio describes the chemical composition of a material and does not mean that you need a volume of brown materials that is thirty times greater than the amount of green matter! Don't make this mistake!

Principle #2: 2 Parts Green to 1 Part Brown
(The best stragey to mix your compostable materials)

Generally speaking, you can get C:N ratios of 30:1 to 50:1 by adding two parts of a GREEN material to one part of a BROWN material to your bin. A "part" can be defined as a certain quantity of the material, such as two 5-gallon buckets of GREEN and 1 packed bucket of BROWN.

Play with the chart below. For example, food scraps, grass clippings and leaves come close to an average of 30:1. How? Add-up the Carbon side of the ratio for all three materials, i.e. 15, 17, 60, and divide by the number of materials, i.e. three. 92/3 = about 31:1.

Experiment (this isn't a perfect world!) to find your own style. Many people have very good success with equal parts. Just don't have too much brown or too much green!

QUESTION: "Can I use 1-part GREEN and 1-part BROWN?"
ANSWER: Yes you can. The best combination would be a mixture of GREEN sources, as shown on the left of the chart below and a BROWN source such as leaves (notice that leaves have a fairly low C:N ratio compared to other carbonaceous materials shown on the right of the chart. Leaves are ideal for composting!). Researchers have determined that effective compost can be made with equal parts GREEN and BROWN, or with 2 parts GREEN and 1 part BROWN.

QUESTION: I am confused. Half the articles I read suggest using 2-parts BROWN to 1-part GREEN. The other half say to use 2-parts GREEN to 1-part BROWN. WHAT IS CORRECT?!
ANSWER: When authors/researchers call for 2-parts BROWN, they are considering the fact that common BROWNS such as leaves or hay, etc. have a lot of fluff or air in a particualr volume as compared to the more matted volume of GREEN. So, they might say use two shovels full of BROWN and one shovel full of GREEN. We don't dispute this advice if you use unshredded leaves or hay. However, when shredded, a large volume of leaves/hay is cut to about 1/4 its mass. Therefore, 2-parts of such shredded BROWN would be too much carbon for the 1-part volume of GREEN. Experiment to see what happens.

Our caution to using the 2-parts BROWN to 1-part GREEN approach is that one can easily overestimate the volume of BROWNS whereas in fact the GREENS are vital for getting the pile heated (but not too many or you get smelly material). Too much BROWN and the pile is too dry and will not heat up. We attempt to use the scientifically known make-up of Carbon and Nitrogen in materials and use the the C:N Ratio formula for computing the balanced ratio. This is why the 2-parts GREEN to 1-part BROWN has been promoted. Our best advice is to try both formulas for GREENS and BROWNS. You may be surprised at what you find: probably good compost!


Use this Rule of Thumb when viewing the chart below

•Any organic matter that has a C:N ratio generally smaller than 30:1 is considered a GREEN.
•Any organic matter that has a C:N ratio generally larger than 30:1 is considered a BROWN.

Common Home Compostable Materials & C:N Ratios
(Example: Food Scraps has a Carbon:Nitrogen Ratio of 17:1,
meaning 17 parts Carbon to 1 part Nitrogen)

GREEN (Nitrogen)	BROWN (Carbon)
Aged Chicken Manure 7:1 Fresh manures are way to hot and can burn your plants and roots!	Leaves 60-80:1 One of the most important ingredients for composting, especially shredded or broken down (leaf mulch).
Food Scraps 17:1 Vegetable Scraps 25:1	Straw, Hay 90:1 The best way to use is to shred for faster breakdown.
Coffee Grounds 25:1	Sawdust 500:1 Commercially produced compost is high in sawdust or shredded bark chips. Use very sparingly!
Grass Clippings - Fresh 17:1 Dry clippings would be higher in Carbon. Therefore, use as carbon source if necessary.	Woody chips & twigs 700:1 Be sparing. Best use is small material at bottom of bin or pile.
Fresh Weeds 20:1 Make sure you don't compost weeds with seeds, unless you insure that your pile gets hot - over 140°F/60°C.	Shredded Newspaper 175:1 Has no nutrient content. Best used in vermicomposting. Always shred and soak in water for fast breakdown.
Fruit Wastes 25-40:1	Nut shells 35:1
Rotted Manure 20:1 Horse manure should not be used because it contains undigested seeds that can sprout in the bin.	Pine Needles 80:1 Use sparingly. Very acidic and waxy; breaks down slowly.
Humus (soil) 10:1 This is nature's natural ratio. Use sparingly in pile. Best used to "seal" the pile by putting a 1-2 inch layer on top.	Corn Stalks 60:1 Shred or cut up in small pieces for fast break down.
Seaweed 19:1	Peat Moss 58:1 Has no nutrient value. In the bin it is mostly filler.
General Garden Waste 30:1 Clippings from plants, stalks, dead flowers, etc. Excellent mix with leaves	NOTE The C:N Ratios given in this chart are average and may slightly vary according to source, researcher or scientist!

TIPS TO REMEMBER

• The 2-to-1 ratio of Greens to Browns is your best bet when creating a Batch Pile. This will aid you in creating about a 30:1 C/N Ratio. Adequate enough to get a hot pile.
  
  
• A 1-to-1 Ratio works well with the Add as You Go Pile as well as the Batch Pile. This will aid you in creating about a 50:1 C/N Ratio, adequate enough to get a warm pile.
  
  
• Stockpiling of leaves and food scraps or yard waste is perhaps the best composting strategy to make a heap large enough for the microorganisms to get hot and bothered.