Composting FAQ

lettuceWhy compost? Organic material makes up approximately one-third of all material sent to landfills. Composting reduces the amount of garbage produced and creates a useful, garden-enhancing product from kitchen and garden waste.

Compost is beneficial for lawns, gardens, trees, and potted plants. It adds organic matter to the soil, enhances the ability of the soil to hold moisture, increases soil porosity, reduces erosion, improves plant growth, and aids the development of healthy root structures. Composting also reduces greenhouse gas production, helps to recover valuable materials, and reduces water use.

Compost is easy to produce and apply to the garden. Just choose a method that fits your time and space requirements, and you can create a more environmentally friendly home and garden in a relatively short time.

For those who are just getting started with composting, here are the answers to a number of commonly asked questions, covering composting methods, materials, applications, and problem-solving.

How do I create compost?

There are a number of ways to compost, including bins, tumblers, trenches, piles, and vermicomposting.

Bins and tumblers

Regular compost bins can be built or purchased (many municipalities offer inexpensive bins and composting workshops). There are also tumbler systems commercially available. These are more expensive, but they produce good compost rapidly (in as little as 3 weeks) because the drum has a handle that enables daily turning of the compost. This aerates the mix, facilitating swift decomposition.

Trenches or piles

Many people don’t bother with a compost bin. Instead, they maintain a compost pile in a corner of the garden or bury kitchen and garden waste in an 8-inch trench and leave it to rot for several months, after which they plant above it. Trench or pile composting is easy, though it can draw pests to the garden.

Worm bin composting

A vermicomposting system can be kept outdoors or inside because it is completely enclosed. This type of system requires special worms that are suited to living in decomposing organic materials rather than garden soil (such as the redworm, also known as the red wiggler or manure worm). A vermicomposting system requires a layer of carbon-rich material that acts as bedding (shredded cardboard, paper, or dried leaves) to which kitchen wastes can be added. The worms consume and aerate the materials as they move through them and eventually convert them to worm castings, which make excellent compost.

What can I put in my compost pile (and what items should not be included)?

There are two types of compost material: green and brown. Green items are nitrogen-rich materials such as kitchen scraps, fresh-cut grass, feathers, hair, and manure, whereas brown materials are dry, carbon-rich items such as shredded paper and cardboard, dried leaves and grass, peat moss, sawdust, cornstalks and cobs, and straw. Alternating layers of greens and browns will increase the likelihood of developing successful compost in a relatively short time.

Ideally, you should start with a base layer (6-10 cm) of brown materials to facilitate air circulation. Collecting and storing fall leaves is a good way to ensure a year-round supply of carbon-rich material that can be layered into the compost as needed.

Compostable Materials

The following household and garden materials are compostable:

    • Coffee grounds and filters
    • Dryer link (if your clothes are made from natural fibers)
    • Egg shells (rinsed and crushed)
    • Feathers and hair (don’t add any that have been coloured using chemicals)
    • Fireplace ashes (don’t use ashes from chemically treated wood)
    • Grass
    • Leaves
    • Old potting soil
    • Plants (don’t use weeds that have gone to seed, as you may end up adding viable weed seeds to your garden with the finished compost)
    • Sawdust (don’t use sawdust from chemically treated wood)
    • Shredded paper and cardboard (use only uncoated paper that is free of toxic inks and grease)
    • Straw
    • Tea bags
    • Vegetable and fruit trimmings (and rotten produce)

Chop or shred compostable materials to small pieces and layer wet and dry items. Keep the layers varied rather than adding thick layers of any one ingredient.

The microorganisms that will convert your garbage into compost require oxygen. This is why compacted materials in landfills take ages to decompose, and why they produce methane (a greenhouse gas) in the process. To avoid these problems, leave a little space for air in the compost bin and turn the materials over once a week (or at least every 2 weeks) to mix and aerate them.

Items That Should Not Go Into the Compost Bin or Pile

There are a number of kitchen items that will cause the compost to smell foul and attract rodents and other pests. Such materials, which are also very slow to break down in the composter, include:

    • Baked goods (breads, cakes, muffins, etc.)
    • Dairy products
    • Grease/oil/fat (and paper towels contaminated with oily substances)
    • Meats/poultry/fish/bones
    • Pasta
    • Rice
    • Sauces and salad dressings that contain fats, oils, dairy, or meat

Items that shouldn’t be composted due to their toxic content include:

    • Ash from coal or barbecues
    • Chemically treated wood products
    • Glossy coated paper
    • Medications
    • Most types of kitty litter
    • Metallic wrapping paper

Other items that can be harmful if composted include seeds of aggressive weeds, which will later grow in the garden after the compost is used, and diseased plants, as there is a risk of spreading disease back to new plants when the compost is used.

How long does composting take?

Composting usually takes anywhere from a couple of months to a couple of years, depending on the effort you put into it and the materials used. Chopping or shredding materials and turning the pile regularly speeds the process significantly. If properly tended, composting should take 2-3 months, though letting the compost age for another couple of months is beneficial. If you don’t chop up the materials, it may take 4-8 months or more. Compost tumblers can reduce the processing time to a few weeks.

There are some materials that may take longer to break down in home composters, such as compostable plastics. Compostable plastics are made from organic materials (corn, potatoes, soy, etc.) that can decompose back into water, biomass, and carbon dioxide after composting. However, most home composting systems don’t produce enough heat to break these plastics down quickly, so the process may take a couple of months longer than with other compostable materials, and as long as 2 years for some compostable plastics. Commercial composters, which grind materials and generate more heat, can decompose these bioplastics more quickly.

Composting is complete when the material is crumbly and dark, with an earthy smell. You can remove completed compost from materials that are not yet broken down by sifting the material through wire mesh and then returning the partially decomposed items to the pile.

How do I add compost to my garden?

To extract the finished compost, remove the bottom layer of decomposed matter. Commercial bins have a door at the bottom for this purpose. If using a compost pile, you’ll have to remove the layers on top to get at the finished compost underneath. If you have sufficient space, you can have two compost bins or piles going at the same time so that you can draw from the finished pile while the newer pile carries on decomposing.

There are several ways to use compost in your garden, including adding a layer on top, applying as a tea, or making your own rich potting soil.

Use as a top dressing

Add a layer of compost on top of the soil and mix it in using a shovel or rake. You can add up to 6 inches for a new vegetable garden and 4 for an established garden (lawns require only about half an inch).

Make compost tea

To make compost tea, add compost to a cloth or burlap bag and submerge it in a bucket or barrel of water. The ratio should be approximately 5 parts water to 1 part compost. Steep the tea for a week, stirring a few times. Then use the compost tea to water your plants. The compost in the bag can be spread over the garden or added back to the composter.

Make potting soil

You can make your own rich potting soil by mixing 1 part compost, 1 part vermiculite, and 1 part soil or peat moss. Adding some organic fertilizer can help to improve growth, as compost often does not provide sufficient nutrients on its own.

How can I keep pests away from my compost pile?

Unfortunately, compost tends to attract various types of pests, including animals and flies, but there are ways to prevent or at least reduce this problem.

Meat scraps, fatty or oily foods, bread, cake, pasta, and rice will attract pests, so avoid adding these materials to your compost. Use a lidded container or wire mesh around the base of an open compost pile to discourage wild animals. Many pests can also be discouraged by adding pet hair to the compost. In addition, turning the pile regularly to release trapped heat can reduce the pile’s attractiveness as a nesting site for rodents, which may burrow in seeking warmth during the colder months.

If flies are a problem, burying kitchen wastes under a layer of dry materials can reduce the compost’s attractiveness to insects. If you’re collecting material in the kitchen, keeping it in a covered pail in the freezer or refrigerator is recommended.

Should I be concerned about slugs and snails in my compost pile?

Many people believe that slugs and snails in the compost are a problem, but they’re actually useful in helping to break materials down. In addition, the smorgasbord of favourite slug and snail foods in the compost bin helps to keep them away from garden plants they’d normally attack.

Are black walnut leaves and hulls toxic in compost?

Black walnut leaves, bark, and hulls contain a substance called juglone that is toxic to some plants. However, this toxin can be broken down through exposure to bacteria, water, and air. For black walnut leaves, at least 2-4 weeks of composting is required to get rid of the toxin (or a couple of months sitting in regular soil). Sawdust or chips from black walnut tree prunings and black walnut hulls should be composted for at least 6 months (and preferably longer) to ensure safety, so you may wish to set up a separate system for walnut-related materials. If you’re not sure that the toxin has been eliminated, plant tomato seedlings in the compost to see if they’ll grow.

Is it safe to compost pet waste?

Although manure from vegetarian pets (cows, sheep, horses, gerbils, guinea pigs, rabbits, etc.) is considered safe for composting, manure from meat-eating pets such as cats and dogs can be problematic due to the risk of spreading zoonotic diseases (illnesses that can be transmitted from animals to people).

Some individuals and organizations do promote composting cat or dog waste, and there are even pet waste composters commercially available. However, if composting pet waste, set up a separate system for it (away from all food production gardens and water sources such as wells), and don’t apply the finished product near produce gardens or drinking water sources.

Why does my compost stink?

When compost contains a proper balance of materials and receives sufficient aeration, it does not produce strong, unpleasant smells. When there is an imbalance or lack of air circulation, compost may reek of rotten eggs or ammonia, depending on the problem.

A rotten-egg smell indicates that the compost is not getting sufficient air. Stir or turn the pile to aerate it and break clumps apart. Then add a thin layer of soil on top. Adding some dry, carbon-rich material (such as straw or dried leaves or grass) can help to keep the pile drier and more aerated.

An ammonia smell occurs when compost has too much nitrogen-rich or “green” material (kitchen scraps, wet grass, etc.) and not enough carbon-rich material. In this case the pile will likely be soggy and muddy. Add some carbon-rich material such as dry leaves, turn the pile over, and add a thin layer of soil on top.

Why is my compost dry and cold (or soggy and muddy)?

Compost should get smaller and generate heat. It should be moist (like a damp sponge), though not muddy and waterlogged. Add water if it’s too dry and mix in more nitrogen-rich material (kitchen scraps, fresh-cut grass, etc.). If using an open compost pile, covering it to prevent moisture loss or relocating it to a less sunny spot can be helpful, particularly if you live in a hot, dry region.

If the pile is too small, it may have a warm, moist center but the outer portion will be cold and dry. Add more material and mix it in well to increase the size of the pile.

If the compost is cold and sweet-smelling, add some fruit or vegetable scraps, organic fertilizer, or fresh-cut grass to add nitrogen. Turn the pile to mix in the new ingredients, and add a thin layer of soil on top.

If the compost is soggy, turning it more regularly to aerate it, keeping it covered during the rainy season, and adding more brown material (dry grass or leaves, straw, peat moss, etc.) will often fix the problem.

For more gardening articles, visit the main Gardening page.

Information for this Composting FAQ series was derived from the following sources:

    • Compost Council of Canada. (2010). “The How Tos of Composting” and “Using Compost.”
    • David Suzuki Foundation. (n.d.). “Composting Dos and Don’ts.”
    • Metro Vancouver. (n.d.). “Here’s the Dirt: Backyard Composting.”
    • New Brunswick, Canada, Department of Environment. (n.d.). “Composting.” GNB.CA.
    • Southern Idaho Solid Waste. (n.d.). “Compost FAQ.”
    • (2009). “Composting Guide – Composting Fundamentals.”
    • World Centric. (2011). “Compostable Plastics.”

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