Earthworms improve the soil by burrowing

"How do worms make the soil good?" asked a curious student at Libertyville Recreation Department's Kinder Korner Preschool program.

The earthworms you see when you dig in your yard are pretty short, but the story they tell is a long one with some twists and turns.

The first chapter has an abrupt ending. Life for all earthworms in North America's northern regions ended about 11,000 years ago when glacial ice sheets shaved off topsoil and completely wiped out native earthworm populations.

But the story wasn't over. Like much of the animal and plant life that surrounds us today, these silent, soil-bound invertebrates are a nonnative species. Spoiler alert - earthworms made a big comeback. Starting a few hundred years ago, they stowed away in plants brought by immigrants to the New World and began doing what they do best - burrowing under the soil.

These couple-inch-long worms felt very welcome in their new turf and quickly spread across North America.

"Most studies document between 20,000 and 1 million per acre," said Penn State University Associate Professor Sjoerd Duiker, a specialist in soil management and physics.

As they burrow into soil, they bring needed oxygen to plant roots. Water chases them through their tunnels and nourishes plants. Burrows actually help minimize erosion by increasing soil stability. Earthworm castings (a fancy word for poop) are made of digested plant material, which turns out to be a terrific food source for plants.

Earthworms are categorized by the three underground levels they inhabit: litter dwellers that live in the debris of crops or on forest floors; topsoil dwellers that burrow in the top two-to-three inches of soil; and subsoil dwellers with worm holes well below ground, as deep as six feet.

Earthworms are efficient producers, as Duiker explains, "Earthworms have been shown to process as much as 36 tons of casts per acre - this is soil that has passed through the gut of the worms where it was mixed with organic matter and subsequently pooped out."

They are also efficient at replicating, and sometimes can produce worm babies starting with only one worm acting as both male and female. The only thing that does stop them is dry soil. Earthworms need a moist environment to survive.

In any good story, there's some drama. Nonnative plants, animals and insects have a tendency to create an ecosystem upset, and the litter dweller earthworms are no exception. In the Chippewa forests of Minnesota, these machine-like composters are depleting important forest-floor litter and worming their way into tree root systems. In the Smokey Mountains, the Asian jumper earthworm is gorging on the forest floor. As a result, the native species of beetles and insects at the lowest end of the food chain have limited food supplies, creating an imbalance in the entire food web.

There are ways everyone can chip in to turn the page on the bad earthworms. Commonly used as fishing bait, earthworms find a path to forest floors through rivers. Great Lakes Worm Watch makes a call for fishermen to throw unused bait earthworms in the trash, not on the ground or in the water. Because of damage these worms have caused in the Smokey Mountains, the National Park Service prohibits using live bait for fishing.

Check it out

Cook Memorial Library in Libertyville suggests these titles on earthworms:

• "Garden Wigglers: Earthworms in Your Backyard," by Nancy Loewen

• "Inside the Worm's Hole," by Meish Goldish

• "It's a Good Thing There Are Earthworms," by Jodie Shepherd

• "Let's Look at Earthworms," by Suzanne Paul Dell'Oro

• "The Life Cycle of an Earthworm," by Bobbie Kalman

• "A Slimy Story," by Michelle Knudsen

• "Toil in the Soil," by Michelle Myers Lackner

• "Yucky Worms," by Vivian French