Gardeners know that earthworms in the garden are a sign of healthy soil. These earthy allies loosen the soil, create fertile soil clods or aggregates, provide pathways for plant roots, redistribute organic matter, and drain and aerate the soil. We routinely till in applications of organic matter to encourage earthworm activity, but now it appears that how we apply it matters more than we thought. Recent research presents some surprising evidence that deep tilling is not the best method.
Dennis Linden at USDA-ARS soil lab in St. Paul, Minnesota, has found that earthworms are creatures of habit. Once a worm stumbles onto its food source, it remembers where the food is located and keeps coming back for more. The pattern of these return trips depends on the location of the earthworm's main food source: organic matter. If the organic matter has been tilled into the soil and incorporated six inches or more, earthworm tunnels will tend to be horizontal as they patrol to find food. However, if the organic matter is left on the soil surface, the tunnels tend to be vertical as the earthworms dig upwards to their food source.
Vertical tunnels not only allow for better air and water penetration into the soil layers than horizontal tunnels, they also help with removing chemical toxins. The soil in the burrows created by vertical tunnels harbors more beneficial microbes, and these microbes can degrade pesticides and fertilizers.
So this fall, unless your soil quality is poor, till organic matter just a few inches into the soil, leaving a 3- to 4-inch-thick layer on the surface. Earthworms will do their good work through the winter as long as the ground isn't frozen.