Composting 101

Why Compost

Composting as Waste Reduction

Compost is good for the garden and helps put back what we take from the soil. In addition, it diverts waste from going into our landfills.

Canada produces more garbage per capita than any other country in the world. This should be cause for great concern. We cannot continue to build new landfills and dump our garbage without consequence. Landfills can be sources of pollution, leaking fluid wastes into the soil and groundwater and emitting noxious gases into the air. Therefore, it is essential we try to reduce the amount of waste that goes into our landfills. Recycling is one way. Composting is another highly effective method of processing waste. Approximately 40% of household waste in Ontario consists of yard material or food waste that is compostable. Therefore, backyard or indoor composting can make a significant difference in the amount of waste we send to landfill sites.

Compost Builds Healthy Soil

  • Compost prevents erosion. Compost absorbs and holds water well, making the soil more resistant to erosion by heavy rainfall.
  • Compost prevents plant dehydration. Compost stores water as a film on small soil crumbs called aggregates. During prolonged rainless periods, plants can survive on the water stored in this way.
  • Compost improves soil structure. Compost will loosen heavy clay soils so that they absorb water and circulate air more easily. Sandy soils are bound by compost particles, increasing moisture and nutrient content.
  • Compost provides nutrients for plants. Compost is rich in a wide range of nutrients, unlike commercial fertilizers, which usually only provide a few isolated elements. The nutrients are released from compost slowly, consistent with the rate of nutrient uptake by plants. Commercial fertilizers usually deliver their nutrients all at once, which means that plants may only absorb a small amount before the excess gets washed away.
  • Compost stores minerals. Positively charged elements such as potassium, sodium, calcium magnesium, iron and copper are held by negatively charged particles of compost called colloids. Minerals are easily dissolved and washed away in soils that are low in compost.
  • Compost neutralizes toxins in the soil. The organic acids in compost form stable complexes with elements such as aluminum and lead, leaving the toxins unavailable for plant uptake.
  • Compost can extend the growing season. Compost makes soil darker, therefore allowing it to absorb more heat from the sun.

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 waste. 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.

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 ideal 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 may 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 concern, 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

Indoor Composting

Indoor (Worm) Composting

Indoor composting is done with worms, and is also called vermicomposting. It is an easy and efficient way to compost organic wastes in an apartment, house, office or school classroom. Vermicomposting takes place in an enclosed container. To build your own, the container should be perforated to allow air in. Drainage holes are required in the bottom and air vents in the sides to maintain the right moisture and air levels. The dimensions of the container will depend on the amount of waste you need to process. Surface area is more important than depth. You will need approximately 30 cm (one square foot) of surface area for every pound of food waste to be composted per week.

The Worms

The worms used for vermicomposting are called red wigglers. These worms are not found outdoors as they live in temperatures between 15 degrees Celcius and 25 degrees Celcius. They are efficient processors of food waste and other organic materials, producing dark and fertile compost. The worms reproduce quickly. If you know someone with an active vermicomposter, they will probably have enough worms to give you a “starter” amount. Otherwise the worms can be ordered with the vermicomposter unit or sold separately.

What do the Worms Eat?

You can compost all of the kitchen wastes listed in the DO COMPOST list in your vermicomposter. The worms will also efficiently process small amount of meat or fish waste. Yard materials, because of their volume, are unsuitable for this composting method.

Getting your Vermicomposter Started

  • The ideal bedding for the worms is shredded newspaper. There should be enough newspaper to fill 2/3 of the container. Soak the bedding in water and place it in the vermicomposter.
  • Add the worms. Leave the lid off the composter for about an hour. The worms are sensitive to light and will burrow into the bedding when the lid is off.
  • You can now add kitchen wastes to the bin. Dig a small hole in the bedding, add the waste, and cover the hole. You may want to mark the spot with a twig, or a popsicle stick or other marker so that you can add the next day’s waste in a different spot.
  • The most important part of vermicomposter maintenance is keeping moisture levels the same as a wet sponge. Add water or wet wastes if the material becomes dry. Add more shredded newspaper if the bedding is too wet.

Harvesting from the Vermicomposter

It will take between 3 and 6 months for the bedding and waste in the composter to turn into finished compost. When the compost is ready, it will be a dark, uniform material with an earthy smell. You can harvest the compost in two ways:

  • The slow way to harvest the compost is to move the contents in the bin to one side and add fresh bedding (and waste) to the other. After about one month, the worms will occupy only the fresh bedding and you can then remove the compost.
  • The quick way to harvest the compost is to open the bin under a bright light. The worms will retract into the compost, so that you can remove the upper layer of material. Continue until only a thin layer with the worms in the bin remains. You can then add fresh bedding and start over.