What you should know about the earthworms in your soil

Wednesday, May 2, 2012

These prolific creatures – up to three million in every hectare – benefit plants and add to soil quality. But some, like the night crawler, can cause runoff and soil losses, and are not so good for our forests

by MIKE MULHERN

Worms! You walk over them all the time. You dig them up, plow them up, fish with them squirming on a hook. But, what do you really know about them and how much they do for you? Not you the fisherman, you the farmer!

There could be three million worms in every hectare of your ground. Only one in 25 is easily visible because most are the tiny ones that live their lives underground, always eating their way forward. The most visible ones are the night crawlers, the big ones that spend their entire lives in vertical burrows that can extend six or seven feet below ground.

If you have sandy soil, you have fewer worms because they prefer the dampness of clay, partly because they have no lungs and need moisture to breathe through their skin. If you no-till or ridge-till, you have more worms than if you till conventionally because plowing kills some worms, disrupts others and offers many more up to gulls. Worms prefer soybeans to corn because they like nitrogen-rich protein residues legumes leave. They absolutely love hay crops, partly for the soil stability they provide.

Adam Hayes, an OMAFRA soil management specialist for field crops stationed in Ridgetown, says earthworms "provide quite a significant benefit to the soil." Hayes notes that smaller earthworms "help with the residue breakdown and nutrient cycling, that sort of thing." His observations agree with others that reduced tillage allows worm populations to grow.

If you attended the Southwest Agricultural Conference in Ridgetown in January, you might have heard Odette Menard of the Quebec Department of Agriculture, Food and Fisheries, talking about worms and the good they do.

Menard is an agricultural engineer from St-Hyacinthe, Que. In 2005, she became the first female member of the Soil Conservation Council of Canada's Canadian Conservation Hall of Fame and the first member from Quebec. Menard has been studying soil structure and worms for 24 years and she had lots to say, not just about the worms but also about the other things going on in the soil that the presence of worms signifies and supports.

"Earthworms will tell us when we are on our way to getting better soil," is how she puts it. Their numbers imply a greater or smaller presence of beneficial fungus and bacteria.

The fungus is interesting because it grows in webs beneath the surface that reach out to plant roots. They exchange sugar from the root for nutrients pulled from the soil by the fungus.

Fungi live like a web," Menard says. "Every time you work the soil, you break up the web and stop them from doing their job."

Menard says there are three species of earthworms in our soil – a tiny one that works the top 10 centimetres of soil, a slightly larger one that burrows horizontally in the top 30 to 35 centimetres and the largest, the night crawler, which burrows vertically. It's the one that creates little piles of vegetation, called middens, in the field at the opening of its burrows.

Dr. Jill Clapperton, a freelance scientist living in Montana, used to work for Agriculture and Agri-Food Canada at the Lethbridge Research Centre. In a presentation to the Certified Crop Advisor Conference in Indianapolis, Ind., in 2006, Clapperton said there is more going on below ground than above.

"Living in the soil are plant roots, viruses, bacteria, fungi, algae, protozoa, mites, nematodes, worms, ants, maggots, other insects and insect larvae (grubs), and larger animals," she told the conference.

"Together with climate, these organisms are responsible for the decay of organic matter and cycling of both macro- and micro-nutrients back into forms that plants can use. Micro-organisms such as fungi and bacteria use the carbon, nitrogen and other nutrients in organic matter. Microscopic soil animals such as protozoa, amoebae, nematodes and mites feed on the organic matter, fungi, bacteria, and each other."

Clapperton has done years of research on worms. She favours the smaller ones that inhabit the top 30 to 35 centimetres of the soil and burrow horizontally.  

"If you've got earthworms working to 30 centimetres, and that's not unusual, that means you have a nutrient profile all the way through the rhiosphere," she says. "Nutrients are being made available to the plant the whole depth and that's really important." In that environment, Clapperton says, plants are healthier and more disease-resistant. There have even been some studies, she says, indicating that more worms lead to better yields.

Brassica plants are the ones worms like best, Clapperton says. "They absolutely love brassica plants – cabbages, turnips, radishes, canola. We don't know why. After canola, the next spring, the worms are fatter and there are lots of them. They just really like brassica plants for some reason."

Clapperton says the ideal worm population is anything above 200 per square metre (two million per hectare). A rough way to calculate worm populations, she says, is to put a spade in the ground early in the spring. You should do this when the ground is above five degrees Celsius but before the ground warms up above eight degrees. If you dig up a spade of earth and find five earthworms, you are in good shape. But if you find 10 or more, you are in "really good" shape, Clapperton says.

Although she says most worms are good, some worms are less favoured. The larger night crawler, she observes, is one such worm. They live in permanent burrows that often go down to just above groundwater level, enabling farm chemicals to mix with ground water. Because they eat crop residue and leave bald spots on the land, there is evidence that night crawlers enable soil erosion.

A study at the Ohio State University Agricultural Technical Institute in Wooster conducted in the late 1990s and published in 2002, concluded that high populations of earthworms, particularly night crawlers, could increase runoff volume and soil losses in the first few years of converting sod to row cropping. "However," the study found, "moderate earthworm populations, if strongly dominated by endogeic species (they burrow at 10 to 30 centimetres below the surface), can provide considerable protection from runoff and soil losses."

The night crawler, brought to North America from Europe, is also a problem in our forests. Ryan Hueffmeier of the Natural Resources Research Institute, University of Minnesota in Duluth, is the program co-ordinator for the Great Lakes Worm Watch. He says there are no native earthworm species in the Great Lakes region.

"All earthworms we are dealing with now were imported," he says. "They are exotic species. They are not native." The problem that presents for forests is that earthworms destroy the duff, the layer of organic matter that builds up above the mineral soil.

Historically, the duff was broken down slowly by fungi and bacteria. "The natural decomposing cycle happens very, very slowly," Hueffmeier says, "and in this duff layer, trees put roots out because this is where the nutrients are."

A forest floor without earthworms is rich in herbaceous plants, tree seedlings, and shrubs. With earthworms, most of that is replaced by grasses and sedges. When worms eat away the duff layer, plant growth slows, fewer new trees are seeded and the habitat for some ground birds is compromised.

Farmers, Hueffmeier says, can check their woodlots to see whether the duff is intact, allowing woodlot regrowth. If all or part of the forest floor is lush, he advises, farmers should try to preserve that by not driving equipment through or in any way introducing earthworms to that part of the woodlot. Earthworms are sometimes introduced when fishing worms or wormy compost is dumped in an area where there were no earthworms. Sometimes they are introduced by having earth transferred from fields in the treads of tractor tires.

Even if earthworms do exist in some areas of your wood lot, they will move very slowly to other areas unless they get some human help. BF