Most commercial potting soils contain no soil at all. Instead, they are composed of peat moss, bark and perlite, which is puffed volcanic glass. It is then steam sterilized. While potting soils provide a loose, well-drained planting medium, they provide no minerals without the addition of fertilizer. Earthworms are sensitive to small changes in soil pH and are vulnerable to chemical fertilizers, weed killers and other compounds used in many gardens. Chances are, if your worms are dying off, the potting soil you added to your worm bin might be the culprit.
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Though we joke about becoming worm food, earthworms do not eat dead flesh. Earthworms eat leaves, grain, fruit and vegetable peelings and other organic plant-based material. Worms turn all these ingredients into castings, which are rich in nitrogen and other minerals. Two pounds of worms can create a wheelbarrow full of rich, black soil within six months, just by eating your kitchen scraps and yard waste.
Building a Better Bin
Worms can be harmed by more than just the chemicals in potting soil. Because it is intended to drain quickly, potting soil is a poor choice for a worm bin. Worms must have some moisture, but not enough to pool in the bottom of the bin. Worms not kept moist have trouble burrowing and often fail to reproduce. Shredded paper, cardboard, tree prunings, grass clippings, tea leaves and coffee grounds stay just moist enough. You will need to drill holes in the top and bottom of the bin to allow excess water to drip away and provide the oxygen needed to keep the bin from becoming moldy.
Potting soil can be expensive, especially when compared with other sources of organic matter. Ranging in price from $2 to $4 in Tuscon, Arizona in 1999, to over $10 in 2009, not only is potting soil not practical for use in worm bins, it is too expensive to use in the garden when other alternatives are available for just the effort it takes to open the worm bin and toss in some kitchen scraps. When your worm castings are ready for harvest, pour them around the base of each plant in your garden for instant fertilizer.
Despite the fact that commercial potting soil is not good for worms, they still need a little yard soil or sand to provide grit. Grit is what enables worms to grind woody stems, paper pulp and kitchen waste into castings to make the rich, black soil prized by every organic gardener. Sprinkle a handful or so of soil from your garden on top of the organic matter in your worm bin.
Worm castings can be mixed with peat moss and perlite and sold as potting soil. To make enough soil to meet demand, you will have to build additional worm bins. Get worms for the new bin from an older one, once your worms begin to multiply. This prevents worm bins from becoming overcrowded. Worms sell at $1 a dozen during fishing season. They can be placed in plastic containers with holes in the top for air.