Carbon (browns) and Nitrogen (greens) are both critical elements of a healthy compost. Carbon is an abundant, naturally occurring element that forms the basis of most living organisms. Nitrogen is a gaseous element that exists in plant and animal tissues and is released in decomposition. It’s important that balance between carbon and nitrogen be maintained. Why? It’s all about the bugs!
The microorganisms living in our compost and doing the work of decomposition to create soil need a proper balance of carbon, nitrogen, air and water to function optimally. The microorganisms consume carbon for energy, nitrogen for protein for cell structure and water facilitates the biological metabolic functions that allow them to consume food and reproduce. If there is too much carbon, decomposition slows down as available nitrogen gets used up and the microorganism population drops. Some of the organisms use stored nitrogen to form new cell material burning more carbon in the process. In the soil, using organic matter with excess carbon can create problems. To complete the nitrogen cycle and continue decomposition, the microbial cells will draw any available soil nitrogen in the proper proportion to make use of available carbon. This is known as “robbing” the soil of nitrogen, and delays availability of nitrogen as a fertilizer for growing plants until some later season when it is no longer being used in the life-cycles of soil bacteria.
When there is not enough carbon needed to fuel the conversion of nitrogen into protein, organisms make full use of what carbon they can get and lose excess nitrogen as ammonia. The release of ammonia into the atmosphere means it’s not being stored in the soil and can’t be used later by plants. Loss of nitrogen should be kept to minimum if we want to use this compost to feed our gardens.
For the purposes of our hot compost system the suggested chemical ratio is 25 to 30 parts carbon to 1 part nitrogen. In real-world terms, this should be about a 2 to 1 ratio of browns to greens added to the compost bin.
Organic waste designated for the compost can be separated into two distinct categories based on the elemental properties they bring to the compost. Carbon-rich organic waste materials like newspaper, dead leaves and egg shells are referred to as “browns”. Organic vegetable waste, grass clippings and other material high in nitrogen are referred to as “greens”. Together with proper aeration and moisture, browns and greens in the right ratios create a well-balanced environment to speed up decomposition and transform organic waste into nutrient-rich soil humus.
How Does Carbon and Nitrogen Work in Compost?
- A maximum of 35% of the carbon in fresh organic material will be converted into soil humus IF there is sufficient nitrogen present.
- 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.
- 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.
- 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).
While carbon and nitrogen exist in both browns and greens, in different ratios, we generally assume that a brown to green ratio of 2:1 will achieve the 30:1 carbon:nitrogen ratio we’re looking for.
Browns are generally the dry, carbon-rich material added to compost. Here is a short list of typical browns we’ll use in our compost bin:
- Dried Leaves
- Straw, Hay
- Shredded Newspaper
- Wood Chips
Greens are generally the moist, nitrogen-rich organic material added to compost. Here is a short list of typical greens we’ll use in our compost bin:
- Organic Food Waste
- Grass Clippings
- Coffee Grounds
- Vegetable Scraps and plant clippings
Carbon: Nitrogen Ratios in Common Materials
|GREEN (Nitrogen)||BROWN (Carbon)|
|Aged Chicken Manure 7:1
Fresh manures are way to hot and can burn your plants and roots!
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