Carbon-to-Nitrogen Ratios
What to add and how much is key to reaching the perfect balance of brown and green materials in your compost bin or pile. Here’s the numbers.
All organic matter is made up of substantial amounts of carbon (C) combined with lesser amounts of nitrogen (N). The balance of these two elements in an organism is called the carbon-to-nitrogen ratio (C:N ratio).
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. 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.
Below are the average C:N ratios for some common organic materials found in the compost bin. For our purposes, the materials containing high amounts of carbon are considered “browns,” and materials containing high amounts of nitrogen are considered “greens.”
| Browns = High Carbon | C:N |
| Ashes, wood | 25:1 |
| Cardboard, shredded | 350:1 |
| Corn stalks | 75:1 |
| Fruit waste | 35:1 |
| Leaves | 60:1 |
| Newspaper, shredded | 175:1 |
| Peanut shells | 35:1 |
| Pine needles | 80:1 |
| Sawdust | 325:1 |
| Straw | 75:1 |
| Wood chips | 400:1 |
| Greens = High Nitrogen | C:N |
| Alfalfa | 12:1 |
| Clover | 23:1 |
| Coffee grounds | 20:1 |
| Food waste | 20:1 |
| Garden waste | 30:1 |
| Grass clippings | 20:1 |
| Hay | 25:1 |
| Manures | 15:1 |
| Seaweed | 19:1 |
| Vegetable scraps | 25:1 |
| Weeds | 30:1 |
Note: Many ingredients used for composting do not have the ideal ratio of 25-30:1. As a result, most must be mixed to create “the perfect compost recipe.” High C:N ratios may be lowered by adding grass clippings or manures. Low C:N ratios may be raised by adding paper, dry leaves or wood chips.
Many home gardeners prefer to put up with a slight odor and keep some excess nitrogen in the pile, just to make sure there is always enough around to keep the pile “cooking!”
CITY FARMER COMPOSTING RECIPE
STEP ONE
In a rodent resistant bin, create a base of 3″ – 4″ or woody, brushy material to promote aeration (do not mix into pile).
STEP TWO
Alternate layers of green and brown materials; keep the layers 2″ – 4″ deep. Common green (nitrogen) materials are grass, food scraps (uncooked fruit and vegetables, coffee grounds, filters, tea bags and egg shells) (wet), garden trimmings. Common brown (carbon) materials are, fall leaves, straw and newspaper strips (dry). Chop up larger materials for faster decomposition.
STEP THREE
Whenever you add a food scrap layer, make sure you sprinkle it with soil and then cap off with a brown layer to prevent smells and flies.
STEP FOUR
Mix bin contents often (minimum once every two weeks). This introduces air and gets bin heating up again. Mix older materials with newer materials for faster decomposition.
STEP FIVE
Moisture content of bin should be like a wrung out dish rag. Only add water if pile is very dry after mixing.
STEP SIX
Pile will shrink. Continue to add and mix until bin is almost full. Place carpet on surface of pile to retain heat and moisture.
STEP SEVEN
Compost is generally ready to use when it looks like humus (after about two to three months). However, aging the compost for another 1 to 2 months is recommended.