Composting Ratios (C:N)

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.

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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 (yes, there are 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.

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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.”

Estimated Carbon-to-Nitrogen Ratios

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!” Learn more about building a compost pile here.

Related Questions

  • What products to compost

    Hello,

    You will be able to source all of the essential elements in order to build a great compost pile without having to look too far! As long as your carbon to nitrogen ratio is optimal (25-30:1) your compost pile will be breaking down properly. Here are some lists of acceptable additions:

    Carbon Rich Material "Browns"
    Cardboard (free of dyes)
    Corn stalks
    Fruit waste
    Leaves
    Newspaper
    Peat Moss
    Saw dust
    Stems & twigs
    Straw

    Nitrogen Rich Material "Greens"
    Alfalfa/Clover/Hay
    Algae
    Coffee grounds
    Kitchen food waste
    Garden waste
    Grass clippings
    Hedge clippings
    Manures
    Vegetable scraps
    Weeds (that have NOT gone to seed)

    ​Things to Avoid
    Meats
    Bones
    Fats/oils/grease
    ​Diseased plant material
    Colored paper
    Coal/charcoal
    Cat/dog waste
    Manures from carnivorous animals
    Onions
    Garlic
    Citrus peels

    As for the rhododendron and holly leaves, you can definitely put them in your compost pile. However, it is a good idea to really chop or shred them up, as they take much longer to break down due to their fibrous and waxy make up. It really depends on how quickly you are trying to create usable compost. It might be a good idea to have a separate pile going that you incorporate those leaves into and another pile that you do not. That way you can have a pile you know will rapidly break down into garden goodness and have yet another ready to use later on. Good luck!

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