Living SoilWorm castings have a reputation that borders on magic. Sprinkle a little, the story goes, and your plants will thrive. Is that true? Or is it the kind of claim that sells bags?
The answer sits in the middle, and knowing where the line is will save you money and, occasionally, a seedling.
What worm castings actually do
Worm castings, also called vermicompost, are what worms leave behind after digesting organic matter. The research describes them as a fully stabilized soil amendment that is much more microbially active than the material the worms ate. They improve soil porosity and water holding, and they supply nutrients in forms plants take up readily.
Here is the part the marketing skips. Digestion concentrates the nutrients already in the food, but it does not create nutrients that were not there in the first place. One extension page claims castings may contain 4 percent more nitrogen than ordinary compost, but gives no study for that figure, so treat it with care. Castings release their nutrients at a medium pace, not overnight.
How much to use
This is where people go wrong, because "more is better" feels natural with something so gentle sounding. One research guideline is to add castings to potting mix for indoor or container plants at no more than a quarter of the mix by volume.
Why the limit? Pure worm castings can be high in soluble salts. Use them sparingly, and keep them away from direct contact with the roots of seedlings, which are the most sensitive to salt.
Making your own: the worm bin
Castings are easy to make at home, and worms will eat a surprising amount of kitchen waste.
- The right worms. Use red wigglers, also called redworms. Common garden worms and nightcrawlers quickly die off in a bin.
- Bedding. About six inches of bedding in the bottom of the bin, wetted until it feels like a wrung out sponge.
- Temperature. Somewhere that stays between 55 and 80 degrees Fahrenheit, out of direct sunlight.
- Food. Kitchen scraps, but never meats, eggs, fats, oils, dairy products or citrus peels.
How much do worms eat? Estimates vary widely. One reference says redworms can eat their own weight each day in raw organic matter. Another says a pound of red wigglers in an established bin eats about a half pound to one pound of scraps each week. Start small and watch what disappears.
Patience matters here. It takes 6 to 8 weeks for worms to produce a noticeable amount of vermicompost.
Worried about worms escaping into the garden?
Red wigglers are a different worm from common garden worms. If you would rather not introduce them outdoors, freeze the vermicompost for one to four weeks before use. That kills any egg cases and tiny young worms.
A worm bin is not a compost pile
Do not add worms to a hot compost pile. A working pile is too hot for them. The two methods are different processes that make different products, and both are good. For the hot method, see how to make compost at home.