Outdoor Composting

Choosing a Composter: Outdoor and Indoor Units

The first choice to make when you consider getting a composter is whether you want an indoor or an outdoor composting unit. Indoor composting uses worms to process kitchen waste in a closed container. The smaller size of indoor units makes them unsuitable for processing yard wastes. Indoor units are active throughout the year. Outdoor or backyard composting units are larger, and can be used to compost leaves, grass clippings and other yard materials as well as kitchen wastes. In outdoor units, while the composting process will slow significantly during the winter, composting of kitchen organics can continue year round.

 Outdoor (Backyard) Composters

An outdoor composting unit is called a holding unit because it basically stores kitchen and yard waste in an organized way until these materials break down. Other holding units include perforated garbage cans, wood or wire bins and cinder block bins. Building plans for all these composters are available from TREA.

Holding units are slow producers of compost, but they are also the easiest way to compost. The finished compost is harvested from the bottom of the unit, while new materials are added at the top on a continuous basis. The alternative is to use a turning unit, in which compost is mixed on a regular basis to aerate the organic material. This method is more labour intensive, but reduces the duration of the composting process to only two months or less.

Compost Pile Construction

  • Start with a 10 cm to 15 cm (4″ to 6″) layer of carbon-rich material such as leaves or shredded newspaper.
  • Then add a layer of nitrogen-rich material of equal weight. For example, kitchen scraps or grass clippings. You may want to cover them with an additional thin layer of soil, sawdust, dried leaves or straw to absorb odours and discourage fruit flies. If you use grass-clippings, which are very high in nitrogen, the layer should be no thicker than 10 cm.
  • If the layers are very dense, you should add some dry, bulky materials (ie. shredded, fallen leaves) to increase the air flow inside the pile.
  • Mix the pile materials.

Your compost pile is now active and you can continue to add kitchen and yard wastes in the appropriate brown:green ratio or carbon:nitrogen ratio. Adding green and brown materials in equal amounts by weight will ensure roughly the right proportion.

Getting the Backyard Composter Started

Where to Compost

When you have built or purchased your backyard composter, the next step is to find an appropriate location. The composter should be sheltered from high winds that may cool the pile. Direct sunlight is all right as long as you add water to the pile when it gets dry. Good drainage is very important. Finally the compost pile should not be located against trees or wooden buildings, because wood in contact with compost may decay.

What to Compost – Carbon to Nitrogen Ratios

Compostable materials are usually divided into two groups. Carbon-rich materials or “browns” include dried leaves, straw, shredded newspaper, and wood chips. Nitrogen-rich materials, also called “greens”, include grass clippings and most kitchen wastes. Compost piles require the right combination of these two groups of waste. The relative carbon and nitrogen content of a particular material is indicated by the carbon-to-nitrogen (C:N) ratio. An idea C:N ratio for most soil microorganisms ranges from 15:1 to 30:1. The best way to learn to balance the C:N ratio is to be specific about it for a while and then relax into an intuitive assessment of what the pile needs. A useful guideline at first is that using equal amounts of carbon-rich and nitrogen-rich material by weight will give you approximately the right ratio.

Maintaining the Backyard Compost Pile

You need to observe the conditions of the compost pile in order to compost efficiently:

  • Make sure that the compost is always as damp as a wrung-out sponge. You can add water if the pile is dry, and add dry materials, such as dead leaves or shredded newspaper, if the pile is too wet.
  • The pile should be loose and porous in order to provide the bacteria inside with the oxygen they need. Do not let materials clump together. You can aerate by mixing the pile with a shovel, pitchfork, or special aerating tool.

    Preparing the Material

    Compostable materials will decompose faster when their surface area to volume ratio is increased. This means that you should try to compost only small scraps of material. Shred leaves before you add them to the pile (using a lawnmower works well). Cut kitchen waste into small pieces. Shred wood and newspaper before adding them to the compost. Smaller particles also allow more air to circulate in the pile.

    Do Compost Do Not Compost
    Greens Browns
    Fruit or fruit peels
    Nut shells
    Vegetable scraps
    Fresh grass clippings (small amounts)
    Coffee grounds/Filters
    Green leaves
    Garden wastes
    Aged manure
    Tea bags
    Crushed egg shells
    Grass clippings (dried)
    Paper (shredded)
    Corn stalks
    Leaves (dried)
    Hardwood ash
    Wood chips (untreated)
    Dairy products
    Diseased plants
    Pet wastes
    Woody yard waste
    Fish scraps
    Meat, bones or fat
    Weed heads gone to seed
    Crab grass

    Why Some Organic Materials Should Not Be Composted

    The materials in the DO NOT COMPOST list are organic and will eventually break down just like the materials recommmended for composting. However, meat and bones will attract animals to the compost pile. Oils putrefy and have a similar effect. Cat or dog manure can contain harmful pathogens that survive the composting process. Finally, certain plants such as crab grass or weeds that have gone to seed can survive the composting process and overtake other plants when you use the finished compost in the garden.

    Composting Herbicide-Treated Materials

    You should use caution when composting grass clippings that have been sprayed with herbicides, particularly if you intend to use the finished compost in your vegetable garden. The following table shows the persistence of some common herbicides in soil. You should allow these toxins to break down by composting sprayed grass clippings for at least a year before using the finished compost. You can avoid this cancern, preserve your local environment and protect your health by not using chemical lawn sprays in the first place.

    Balan, Balfin
    Betasan, Prefar
    Roundup, Kleenup
    many formulations
    many formulations
    less than 1
    Source: Rosen, et. al., 1988