Compost Chemistry


The role of oxygen and pH levels, carbon-to-nitrogen ratios and other chemical relationships at work in your compost bin or heap.

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Carbon/Nitrogen Ratio

Every article, book and website about how to compost contains the information that a carbon/nitrogen ratio of approximately 25:1 or 30:1 is optimal for compost piles. This ratio is one of the key conditions for a successful hot pile. It’s so crucial that most instructions about how to build compost piles have as much to do with getting this ratio right as with anything else.

Like so many things, this ratio is a bit of an oversimplification. Microbes can no more make use of pure carbon than you can, and most cannot make use of pure, atmospheric nitrogen. Swallowing diamonds will not meet your carbon needs; putting them in the compost bin will not help the pile heat up. Microbes require carbon-containing compounds and nitrogen-containing compounds. The C/N ratio is a convenient short-hand for this.

Carbon is one of the basic building blocks of life. In chemistry, “organic” means “carbon-containing.” Nitrogen, too, is a part of every living cell. It is essential for building most proteins as well as DNA and RNA. The microbes that do most of the composting work are like every other living thing in that they require both of these elements. (Some of them consist of almost 50% carbon.) Generally, there’s plenty of dead leaves, wood chips and other carbon-rich brown material around. It’s usually the nitrogen supply, the so-called green material, that limits composting speed and heat.

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Given plenty of carbon but only a limited supply of nitrogen, the microbial population won’t be able to reproduce freely. It will increase to the point allowed by the supply of nitrogen and then stay approximately constant. This is what’s meant by the statement that nitrogen is tied up.

However, though the nitrogen is being used, it does not get used up. When the microbes using the available nitrogen die, that nitrogen becomes available again for another generation of microbes. This is why composting can proceed in a pile with limited nitrogen but cannot take off.

Much of the carbon in a compost pile is given off in the form of carbon dioxide (CO2), one of the major by-products of composting. Since the carbon content of the pile actually falls while the nitrogen content does not, the carbon/nitrogen ratio falls during the composting process, usually stabilizing at about 10:1.


Oxygen, like nitrogen and carbon, is needed by all living things and is a limiting factor for aerobic decomposition. While many of the chemicals in compost contain oxygen, most decomposing microbes require atmospheric oxygen.

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If a pile becomes too moist or too dense, air gets driven out and the aerobic microbes will be replaced by anaerobic ones which do not make use of atmospheric oxygen. If the working microbes use up all the available oxygen, the pile will again go anaerobic unless new oxygen is introduced.


The early stages of composting produce significant amounts of organic acids which can lower the pH to about 5.5. These acidic conditions kill off some pathogens — viruses, bacteria and fungi — which can harm both plants and people. They also encourage some fungi and actinomycetes that break down particularly tough materials such as lignins and cellulose.

As composting proceeds, the pH rises, sometimes to above neutral (7), but it will usually fall again without any interference. The pH of finished compost is usually between 6.5 and 7.5.

Related Questions

  • What products to compost


    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
    Peat Moss
    Saw dust
    Stems & twigs

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

    ​Things to Avoid
    ​Diseased plant material
    Colored paper
    Cat/dog waste
    Manures from carnivorous animals
    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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