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Worm Composting: Converting Food Waste to Soil - January 2007

By: Kristi Mahy

What is worm composting and why would you want to do it?

Worm composting refers to the practice of keeping red wiggler worms in a container, feeding them food scraps and using their waste or 'castings' as a fertilizer for gardens, potted plants, etc. Worm composting is also called 'vermicomposting' or sometimes even 'vermiculture' and can range in scale from a unit suitable for one person to large enough to handle the food waste of an entire apartment building. For the purposes of this article, we'll stick to discussing the small scale type

Setting up your bin: what you'll need

First you will need to find an appropriate container. Assuming your bin will be used to handle the food scraps for a single house, a container that is about 40-50 centimetres cubed should do fine. The bin can be rectangular or cylindrical and these are just estimated dimensions. Plastic or wood tend to be the best bin materials. There must be aeration holes (which can be made with nails in the sides and bottom of the bin) and you may need a tray under the bin to collect excess moisture.

Next, you will want to get yourself some worms. One pound should be enough for a household of four people. Make sure you get red wiggler or brandling worms, as other worm types don't process the food as quickly. A couple of people I have spoken with have tried to use earthworms without success. You may be able to find a friend who will give you some worms from their own compost bin. If not, see the resources section of this article for suggestions about where to get your worms.

Prepare the bin for the worms by shredding up some news paper or cardboard. A paper shredder will work very well, or you can tear the paper by hand. This material will act as bedding for the worms (a place for them to live and reproduce) and should be kept moist, like a wrung out sponge. Spread the bedding on the bottom of the bin to a depth of about ten centimetres, give or take a little. The idea is to have enough bedding to bury all food about three centimeters deep. Sprinkle a handful of soil from outside over the bedding to introduce beneficial soil organisms to the system. Next you can add your worms. Let them get used to their new environment for a few days before you begin to feed them.

Feeding the worms

Worms tend to like fruit and vegetable scraps and peels. A bit of mould won't hurt! However, you shouldn't put meat, dairy or grain products in the compost. Experiment a bit and see what your worms like best. Mine really seem to enjoy banana peels and citrus rinds, but they won't touch onion skins or carrot peels. Other people find their worms are fine with some of these foods. It is important to add crushed eggshells to the bin to provide grit for worm digestion, to reduce acidity and to provide the worms with enough calcium to reproduce. If you don't eat eggs, ask your friends to save you their shells.

Make sure the worm food is chopped up finely. The smaller the pieces, the more surface area the worms have access to for eating. Start slow with the feeding and build up. Eventually the worms will be able to eat half of their body weight in food each day! Just bury the food scraps at least three centimetres deep in the bedding. Wait until one batch of food has been processed into castings before putting more food in the bin. Place the food in a different section of the bin each time so the worms aren't forced to hang out in their own waste.

The finished product: the richest compost of all

Possibly the most exciting part of this process is harvesting the finished compost. Contrary to what you might expect, worm castings have a pleasant, earthy smell and are moist and crumbly.

There are many ways to separate the worms from their castings. One method involves using one end of the worm bin for food and bedding and the other for storing castings. Each time you feed, push the finished castings toward the storage end of the bin. Then put down new bedding in the small, cleared section. Eventually there will be a good amount of compost at one end of the bin for you to harvest. Although you will still need to go through the compost carefully to make sure you aren't removing any worms, most of them will stay near the food and out of the castings. For more information on different harvesting techniques, you can consult the resource list below.

This compost is a very precious resource. Use it to fertilize potted plants by sprinkling it on the surface of the soil. Mix it in with soil when planting any seeds: a little goes a long way. I used it to plant lettuce in pots to grow by my window. This is the beauty of vermicomposting: it allows you to close the energy loop by turning your food waste into soil and then growing your own food in it. Imagine how much energy this saves in terms of offsetting food transportation and the manufacture of artificial fertilizers.

Solving issues with the worm bin

Fruit flies can become an annoyance if food is improperly buried. Make sure all food is covered with bedding. Another potential problem is mite infestation. Mites are tiny white bugs which may appear if you are over-feeding the worms. They like to eat food that has putrefied. If you notice mites, remove all food scraps, wait a couple of days and start feeding smaller amounts of food than before. Always wait until one batch of food has been eaten before feeding again.

Finally, please don't dump your worms outdoors if you find you no longer want to compost this way. Pass them on to a friend! Enjoy your new compost bin; I'm sure you'll develop quite a special relationship with your worms. After all, they are doing you a big favour!

Kristi Mahy is the University of Guelph's Composting Coordinator. Visit Sustainability at U of G for more info on sustainablity.


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