Human interaction with earthworms extends beyond the direct benefits of providing fishing bait or a gross-out tool for little boys' use in impressing little girls. Worms complete critical, valuable tasks in enhancing soil quality.
"Earthworms are beneficial because they increase pores in the soil, whereby (we get) deeper and faster penetration of water and oxygen," said Ward Upham, Kansas State University Research and Extension Horticulture Response Center director. "They really do a good job of opening up soil."
The night crawler, Lumbricus terrestis, remains the largest species at four to eight inches and prefers soils high in organic matter. The common field worm, Allolobophora caliginosa, becomes more common when soil quality decreases.
"The night crawler belongs to a group of earthworms known as deep-burrowers," said Upham. "The deep-burrowers build large, vertical, permanent burrows that may reach as deep as five or six feet. Night crawlers pull plant material down into their burrows."
Both earthworm species feed on organic debris taken in from the surface. The holes aerate the soil and their waste, called casts, improves soil friability. The earthworm casts also contain concentrated nitrate, phosphorous, exchangeable magnesium, potassium and calcium.
"The burrows open up channels for water and air to penetrate," Upham said. "Roots also like these channels due to the ease of root penetration and nutrients found in the casting material lining the burrow."
Earthworm tunnels help plants by allowing oxygen into the soil, as well as improving conditions for beneficial soil bacteria. They also provide access to deeper soil levels for the numerous smaller organisms which contribute to soil health.
Worms ingest the soil and digest any organic matter as they tunnel through the soil, Upham said. Although many can be found within six inches of the surface, earthworms also work in the subsoil.
Earthworms make other contributions, such as adding calcium carbonate, a compound which helps moderate soil pH. Each day, they produce 60% of their body weight in urine, which contains high levels of nitrogen--also beneficial for plants.
Activity is seasonal and most noticeable in the spring and fall when soil conditions are moist. Populations peak in the spring and decrease in the dry summer months. Greater numbers of young worms are often found in the wetter, cooler fall.
When adding new worms to a lawn or garden, gardeners should distribute them evenly with only a few per square foot, Upham said.
To grow an earthworm crop, feed the soil with grass clippings, corn stalks or green leaves.
Artificial fertilizers do not provide food for earthworms. Some fertilizers may repel them. As the fertilizers become soluble, they may leach into the soil and force the worms to seek refuge elsewhere.
"Some gardeners want to protect the nightcrawlers due to their positive effect on soils," Upham said. "Pesticides used for other pests vary widely in their toxic effects on earthworms."
Chemicals with no effect on earthworms include diazinon, Dylox (Proxol) and Oftanol. Dursban and malathion may be slightly toxic.
Sevin, Benomyl, copper sulfate and the arsenicals (MSMA, DSMA) are extremely toxic. Upham said to avoid using the latter group while nightcrawlers are active.