Just adding a handful of earthworms to poor soil will probably not do much to improve it. Don’t add them to a pile of sand or clay and expect to get rich loam. You must add organic matter such as compost, peat, manure, and/or ground up decaying plant material and keeping it moist with a near-neutral pH. Avoid or minimize the use of pesticides because of their harmful effects on earthworms.
As earthworms tunnel through the soil, they ingest the soil and digest any organic matter in it. Organic matter is dragged into their burrows and is broken down. Although they are the most numerous in the top 6 inches, they also work in the subsoil, bringing mineral rich soil from below to the surface. This adds to the supply of nutrients available to the plants. Research shows that in 100 sq ft of garden soil, earthworms may bring from 4-8 lbs of dirt to the soil surface each year.
Besides incorporating organic matter to your soil, earthworms are good manufacturers of fertilizer. Castings have a nutrient level and organic matter level much higher than that of the surrounding soil. Each day they produce nitrogen, phosphorous, potassium and many micro nutrients in a form that all plants can use. For example, a 200 sq ft garden with a worm population of only 5 worms/cubic foot will be provided with over 35 lbs (about 1/3 lb per worm) of top-grade fertilizer by the worms, each garden year.
Not only do they produce this fertilizer but spread it thoroughly within the top 12 inches of soil. They may also incorporate it as far down as 6 ft. A soil that is well managed, rich in humus may easily support 25 worms per cubic foot, which translates into at least 175 lbs of fertilizer per year for the same 200 sq ft garden.
This means that your garden or lawn can be supplied with far more fertilizer and of superior quality than a dry or granular fast-acting chemical fertilizer of 10-20 lbs. In fact, these fertilizers may even repel the earthworms that are present. As the fertilizers become soluble, they may leach down into the soil and force the earthworms to seek refuge elsewhere.
Worms make other contributions, such as adding calcium carbonate, a compound which helps moderate soil pH. Overtime earthworms can help change acid or alkaline soils toward a more neutral pH.
Earthworm tunnels help to aerate and loosen the soil. This allows more oxygen in, which not only helps the plant directly, but also improves conditions for certain beneficial soil bacteria. Finally, the tunnelling of the earthworms provide an access to deeper soil levels for the numerous smaller organisms that contribute to the health of the soil.
In summary, earthworm activity in your soil is beneficial and should be encouraged. They help incorporate organic matter, improve the soil structure, improve water movement through the soil, improve plant root growth and minimize thatch build up in lawns.
To increase the earthworm population you may want to add earthworms to your soil. When adding new worms, distribute them around the garden so that there are only a few per sq ft. Dried leaves and other organic materials are their favorite food and need only to be spread on the surface in the spring. They do the rest!