The Dirt on Earthworms
It is spring! We dig in our gardens and find wriggly earthworm in the soil. But depending on whether you are in a field or forest earthworms can be a benefit or hurt.
When the glaciers of the last ice age moved over North America, the ice scraped soil off the bedrock and wiped away native earthworms. Over the next 10,000 years our northern ecosystem and habitats developed without any earthworms in the soil. When European settlers arrived in North America, they brought European earthworms with them. These worms were in soil used as ballast in the ships and on farming tools. Anglers too have introduced earthworms by dumping unused bait on the ground. Currently there are 19 earthworm species in Vermont and none of them are native to the state.
How Earthworm Act on Soil
Earthworms are one of thousands of creatures which call soil home. There are many species of beetles, arthropods, nematodes, bacteria, fungi, and protozoa living together in our soil. But even earthworms alone have an incredible effect on soil. As earthworms crawl through the soil and carry on their lives they change the dirt. Their burrows create channels that allow air and water to move in and through the soil. Earthworms pull organic material from the surface down into the soil. They eat organic material and mix it into the soil. Castings, or “worm poop”, are rich fertilizer deposited on the soil surface.
Different species of earthworms have different habits: some live near the surface, some live in the top inches of soil, and others live in deep burrows and stay underground their entire lives. Each species of earthworm acts on the soil and organic material in a different way and all together the earthworms have a significant impact on soil heath and function.
Earthworms in Agriculture
In a farm field earthworms are very beneficial. They help move water and air into and through the soil which increases water infiltration and reduces soil compaction. They mix crop residues from the surface into the soil increasing soil organic matter. This organic material is food for other important soil-dwelling creatures. The presence of earthworms in a farm field is an indicator of good soil health. There can be up to 5 million earthworms per acre in a healthy dairy pasture. Today farmers utilize earthworms and other soil living species to naturally aerate the soil and reduce the need for tillage. This is a Natural Best Management Practice that protects both soil health and water quality.
Earthworms in Forests
In Vermont and northern forests, earthworms present a risk. Our native forest trees and plants, including sugar maples and spring ephemeral flowers like trout lily and trillium, evolved in soil without earthworms. The layer of duff and leaf litter on the forest floor is a thick mat which protects and shades the soil beneath. Many tree and shrub seeds and seedlings depend on this layer for the first stages of their growth. Some native plant seeds spend the winter on the forest floor, going through a freeze-thaw cycle before they can germinate. Other seeds depend on infection by a fungus to properly germinate. If earthworms mix up the soil layers, as they do so efficiently, the duff layer is broken up, seeds are brought below the surface too soon, and fungus communities are broken up before they can act on seeds.
What You Can Do
The best way to protect our forests from earthworm invasion is to control what we bring into the woods. Make sure compost materials and topsoil are not stored in forests and clean silviculture equipment between work sites. When fishing, take left over bait out of the forest and dump it into the trash.
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