Worm-bin construction made simple

By Kelly Grube
Originally posted on August 17, 2004

A variety of commercial vermicompost systems are available for purchase; but you can easily build your own for less.

The term vermicomposting simply refers to composting organic matter with worms. Vermicompost, the end product, is a high-nutrient form of compost containing nitrogen, phosphorous, potassium, calcium, sodium, magnesium, iron, zinc, manganese, copper, boron, and aluminum. These nutrients are often more abundant and more readily available to plants in vermicompost than in other types of compost. In addition, vermicompost can benefit your soil by improving its structure and porosity.

Preparing the bin
The vermicompost bin will consist of two, 10-gallon Rubbermaid (or similar) containers stacked on top of each other, with a third bin being added later on. Holes, one half-inch in diameter, will be drilled into the upper sides and bottom of the top bin (which will contain the bedding) to allow for aeration and drainage.

The bottom bin will be used to catch leachate drainage, materials falling out through the holes, and escaping worms. When the material in the initial bin has been sufficiently processed, a third bin with fresh bedding is added on top. The worms will migrate up through the holes to the fresh material, leaving behind vermicompost in the bottom bin to be harvested.

Preparing the bedding
Bedding is necessary for worms to burrow, bury food scraps, and also for moisture retention. Bedding material must be a non-toxic, fluffy material that holds moisture and allows air to circulate. Popular bedding materials include shredded newspaper, computer paper, decaying leaves, grass clippings, peat moss, or some mixture of the above.

Our own bedding consists of a mixture of peat moss (approximately 3.0-3.5 lbs dry) and one of the following: chopped wheat straw, shredded newspaper, or dried coffee bean hulls (approximately 0.65 lb dry). Dry bedding materials are mixed and slowly wetted with approximately 10 liters (2.6 gallons) of distilled water, mixing the materials as the water is added. Once worms are added to the bedding, the moisture level should remain approximately that of a wrung-out sponge.

The best composting worms are red wigglers (Eisenia foetida) which can be obtained from a local bait shop or through mail order. Generally, they cost about $20 a pound, not including any shipping fees. Regular garden and compost worms will not survive in worm bin conditions and therefore should not be used.

Red wigglers can typically consume about a half-pound of food scraps per one pound of worms in a 24-hour period (one pound of worms equals roughly 1,000 worms). You can adjust your bin size and worm population to accommodate your food-scrap load. Keep in mind that in about two weeks the worms will begin reproducing, increasing the population. One pound of worms should be a good starting population for the 10-gallon bin.

Once you have your worms, place them on top of the moistened bedding and they will quickly burrow in.

Worms eat all kinds of food including coffee grounds, tea bags, vegetable and fruit scraps, egg shells, and yard waste. Worms are known to particularly enjoy broccoli, watermelon, cantaloupe, and pumpkin.

It's best to avoid bones, dairy products, meats, garlic, onions, and spicy foods. Citrus foods should also be avoided as they can cause the bin environment to become too acidic (an over-acidic bin can be corrected by adding crushed eggshells). To speed the decomposition process, food should be chopped or shredded before being added to the bin.

To feed, pull back the bedding, put in the food scraps, and then re-cover with bedding. This will minimize mold, flies, and other pests.

Where to put the bin?
The main concern when deciding on a location for your worm bin is temperature. The ideal temperature for red wigglers to grow in is 55-75? F. Popular indoor spots include the kitchen, garage, laundry room, or basement. If you want to keep your worm bin outdoors, you must shade it in the summer and insulate it in the winter (with hay bales or other material) to maintain the proper temperature range. Also, if your bin is outdoors remember to protect it from flooding conditions as worms can easily drown inside the bin.

Harvesting vermicompost
You can harvest vermicompost after about three months. To do this, prepare your second bin of fresh moist bedding exactly as you did the first. Place the second bin on top of the first, making sure that the holes in the bottom of the second bin are flush with the surface of the bedding in the first bin (worms cannot crawl through air).

The worms will then begin to migrate up to the new bin, leaving behind vermicompost. From our experience, the process may take up to two weeks, and adding fresh food scraps to the new bin will encourage the migration.

Harvested vermicompost can be added to potting mix or garden soil to provide an excellent source of readily-available plant nutrients and organic matter.

As long as you maintain a healthy worm-bin environment (i.e. proper temperature, moisture, pH, and food), the worm population will stabilize itself and there will be no need to either purchase additional worms or remove surplus worms.

Happy vermicomposting!

Kelly Grube was a 2004 intern at The Rodale Institute.

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