How Much is Enough?
Worm farms generally sell castings by the pound, rather than area (for example, 5 lbs instead of X cubic feet).
Part of the reason is that castings aren't readily comparable to traditional compost or fertilizers.
But the inevitable question (whether it's because the material is sold by weight, or because gardeners aren't really familiar with the product) is "how much does that cover?"
A fair question, and so it seemed appropriate to try to answer it here. Of course this is a generalization, because if you have a worm bin, your castings won't be the same as someone else's who feeds their worms differently. There really are almost limitless variables when working with a living organism, which in many ways is the most technically accurate way to describe vermicast, or castings.
While there are most definitely nutrients in there, the power is not in the chemical makeup, but rather in the biological. Castings are "alive" and teeming with beneficial microbes — bacteria, fungi, nematodes, etc. that are vital to plant health and vigorous growth. The microbes provide the plant roots what they need, and science has found that the microbes communicate in a way with the plants: They recognize when the root zone is low in a particular mineral or nutrient, and go out and find that in the surrounding soil, and return it to the roots. With the boost of nutrition, the plant's roots release sugars that the microbes feed on, known as exudates.
It's a symbiotic relationship, and the healthier the ecosystem between plant roots and microbes, the less chance there is for the "bad guys" (harmful bacteria that stress plants, stunt growth and lead to disease) to get a foothold.
So back to how much do you need for your garden. Our vermicomposting friend Steve Churchill at Urban Worm Company did some figuring (which was great, since we're not so strong in the math department).
Boiling things down to the most basic terms, Steve calculated the ideal amount of castings to garden bed is 1/10 (one-tenth) of a pound per square foot of bed.
For a 4-foot by 8-foot raised bed (32 square feet), you'd want to use 3.2 pounds. If you have 100-square feet of garden beds, you'd use 10 pounds.
We sell a 5-pound bag because we've found the 4-ft x 8-ft bed is fairly common, and the additional 1.8 pounds (approximately — we always provide more than 5 lbs to allow for any possible moisture loss that might lessen the weight) can be used for starting seedlings, or creating a worm tea brew to provide a supplemental boost as your plants start growing.