The ancestors of modern earthworms survived the brutal conditions of the ice age. Since that time, 3000 species of earthworms have evolved to inhabit almost every ecosystem on earth. As many as 500,000 earthworms may work to improve the soil of a single acre of cultivated land. They play a vital role in maintaining healthy soil, so be mindful of them when you work your garden.
An earthworm uses its head to push through loose soil, in essence, plowing it. In hard and compacted soils, earthworms eat the soil, creating a series of interconnected burrows up to a depth of several feet below the surface. Soil eaten by earthworms is excreted as castings, and they are a source of rich nutrients, such as nitrogen, which is otherwise unavailable naturally to plants. The actions of earthworms break up compacted soil, allow air and water to penetrate it, and thus improve conditions for roots.
The body of an earthworm is 72% protein, and as nitrogen is the foundation of proteins, earthworms require large quantities of nitrogen. Compost that is rich in nitrogen will attract and benefit earthworms, but they will avoid synthetic nitrogen fertilizers. Earthworms, having a permeable skin that makes them sensitive to physical and chemical conditions in soil, avoid the salty conditions created by synthetic fertilizers.
If you are transitioning from conventional to organic gardening, or are otherwise concerned that your yard or garden has few earthworms, you can raise your own indoors. Use a garbage can, washtub, or wooden box to create your own worm bin. Conditions must be kept moist but well-drained, so use a rigid divider to create a drainage area in the bottom of the bin separate from living space for the earthworms. Keep flies away and light out with a loose cover. Use 2 parts commercially available steer manure, 2 parts sawdust and 1 part shredded leaves to fill the bin. Mix this well and add enough water to dampen it thoroughly. This mixture will begin to compost and generate heat, so avoid adding earthworms for the first few days. Store it in a cool, dark place as a compost bin for kitchen scraps and a home for your own colony of earthworms.
If you intend to introduce your earthworms to your garden, excavate a few from your garden to begin the colony. Store bought earthworms typically are species that survive in only very rich soil and may not survive in your garden.
Add well-chopped vegetables and water to your bin to feed your earthworms. Give them only soft foods the first few days, such as oatmeal, toast, fruit and vegetable trimmings, and coffee grounds, all put through a blender. If the food is not completely consumed within 24 hours, reduce the amount you are feeding them. Your colony should double in size within a month and your bin should be composted within 60 days.
To separate the earthworms from the compost, place the compost outside on a sheet of heavy plastic or fabric. After about an hour, the earthworms will have clustered together to avoid the heating surface of the compost and to maintain their moisture. Dig through your compost to find the cluster. Save some earthworms to start a second colony in a fresh bin and put the remainder to work improving the soil of your garden.
All-New Encyclopedia of Organic Gardening: The Indispensable Resource for Every Gardner, Fern Marshall Bradley and Barbara W Ellis, Editors