Often ignored by yardeners, earthworms are Nature's own tillers and soil conditioners. Their favorite food is the dried leaves, grass clippings, organic mulch and other organic materials yardeners want to incorporate into the soil throughout their property. Worms drag these materials down into their burrows and break them down. If you have a healthy population of earthworms in your yard, that organic mulch laid over the surface of the soil in beds and under trees and shrubs in the spring will be gone by fall without your having to do anything but spread the stuff in the first place; the worms take care of the rest of the job. In turn, earthworms also work in the sub soil, bringing mineral-rich soil from below up to the soil’s surface. This adds to the plant's nutrient store. Research shows that earthworms in 100 square feet of lawn soil will bring to the surface between 4 to 8 pounds of dirt each year.
Besides integrating organic material into your soil, earthworms manufacture great fertilizer. Every day, they produce nitrogen, phosphorus, potassium, and many micronutrients in a form all plants can use. Worms make other very significant contributions to your yard’s soil. They secrete calcium carbonate, a compound that helps to moderate soil pH. Earthworms can help change acid or alkaline soils toward a neutral pH over time. The rearrangement and loosening of the soil by earthworms enhances aeration, which allows more oxygen to penetrate the soil. This not only helps the plant directly, but also improves conditions for certain beneficial soil bacteria. Finally, by their tunneling, earthworms create access to deeper soil levels for countless smaller organisms that contribute to the health of the soil.
You can add earthworms to your soil to increase the worm population. Be sure you get the variety (Lumbricus terrestris) that will survive in garden soil. Many worms sold commercially are designed to live in the heat of a compost pile and will die when left in the garden soil. When you add new worms, spread them around the yard so that there are only a few introduced in every square foot.