Worm Castings: Actually a fertilizer?

Castings are considered an extremely valuable fertilizer in gardening because plants respond so well to them. A simple top-dressing application of a small amount of castings - even a tablespoon! - is enough to restore the vigour of most plants, along with improved flower/food production and better soil conditions within the area. When combined with a seed starting mix, castings can set the stage for ideal plant development, improved germination rates, more robust seedling health, and quicker growth.

Keep in mind, though, that the actual fertile content of worm castings is negligible. Most scientific studies place the N-P-K values at just 1-0-0. So how do castings work their magic?

Castings are not so much food for the plants, but rather food for the microbes that are the most valuable to plants. Microbes multiply when food and conditions are right for them; so more microbe food means more microbes converting other fertile substances into molecules the plants readily absorb through their roots. Fresh worm castings (unpasteurized) also contain a well-balanced population of beneficial microbes to kickstart a sterile or inhospitable soil environment. Therefore, castings are valuable because they are the best microbial feed and inoculant we can add to help plants thrive.

Using castings in the garden, tips and advice

It is always best to use fresh, homemade castings rather than those bought at a gardening store, to get the full strength of the inoculant.

Seed starting is one of the most finicky and important parts of gardening, Worm castings included in your seed starting mix (20% ratio is ideal) will have significant positive effects on germination rates, seedling health and growth speed. 

Use other organic fertilizers like compost in conjunction with castings. Beefing up the nutritional content in the mix will have many benefits. In addition to castings and compost, consider any mineral amendments that may be necessary for your soil. Your microbial inoculant will ensure your plants make the most of these amendments.

Beware any additives in the water you use in your garden. Most municipalities use chlorine or some other anti-microbial substance to sterilize the water we drink. While the levels of these additives present in the water do not significantly affect our health as humans, chlorinated water used in the garden will certainly have a negative impact on the microbial life present in your soil, and therefore plant life as well. There's no point adding an inoculant like castings, then watering it in with sterilized water. Fortunately, you can dechlorinate municipal water: leave it standing in the open for several hours (overnight to be certain) and the chlorine will evaporate.

A properly-run worm system shouldn't have drainage (called leachate), but as it is, do not use any system drainage liquid immediately in your garden, especially if growing edibles (and especially leafy greens). Leachate contains harmful microbes (eg. e. coli, salmonella) in addition to the beneficials. Leachate must be brewed into vermicompost tea before it can be added to the garden.

Large-scale application of worm castings

It's kind of amazing how little compost you get after the worms are done with the system material. Worms are the ultimate trash compactor, and for those gardening on a large scale or farming, such a small yield can seem impractical. Some organic farmers brew vermicompost "tea" to inoculate many acres of soil, and for which a small amount of castings will suffice. The brewing process essentially encourages beneficial organisms present in the castings to multiply in liquid, under carefully oxidized conditions, to stretch the castings’ microbial yield farther. There are many videos available on Youtube which detail the DIY process for making worm tea safely.