Be a Better Farmer: Understand
the Living Soil
Part 4 - Those Wonderful Worms
Originally Printed in Country Guide, Spring 2001
Table of Contents
- Earthworm Species and Habitat
- Influencing Earthworm Populations
- Three Facts From the World of Worms
- Related Links
Earthworms don't just improve soil texture. They also tell you a lot about the quality of your soil management.
Earthworms are the most easily recognized soil organism. Farmers
and gardeners have long viewed them as a sign of soil health. New
no-tillers eagerly document the increase in worm numbers as a sign
of improving soil health -- and they are right to make that assumption.
In fact, worms are often studied as an "indicator species"
for monitoring changes in soil quality.
As farmers, we can learn a lot about our soils by monitoring worm populations. There is also a soil management angle. Many activities that affect earthworms also affect other soil organisms. Because of their size, however, worms are easy to study and monitor. Let's take a closer look at the mighty earthworm.
There are thousands of earthworm species in the world but only about 20 in Canada. Those we find across Canada (except in a small area on Vancouver Island) are not native. The original earthworms were wiped out by glacial ice sheets that covered Canada until about 15,000 years ago, Canadian earthworms are immigrants, carried here from Europe by the early settlers on root stocks and in the earthen ballast of ships.
Earthworms are most numerous in fine and medium textured soils (clays and loams). They are less common in sands, gravels and acidic soils. Earthworms breathe through their skin and need to keep it moist to stay alive. Soils that are dry for prolonged periods tend to desiccate worms.
Two types of worm are found in agricultural soils in Canada. First is the deep burrower, Lumbricus terrestris. Commonly called nightcrawlers or dew worms, these are the worms most commonly sold for fish bait. Adults are generally 10 to 30 cm long. They create large vertical, permanent burrows up to 2 meters deep in the soil profile. They pull surface plant residues, and in some cases living plant material, down into the mouth of the burrow to soften and be eaten.
In fields with enough surface residue, you will see the middens (a mixture of plant residues, worm droppings, small stones) that these worms build over the mouths of their burrows. These little piles protect the burrow and also serve as a food reserve.
Nightcrawler populations are heavily influenced by tillage and rotations that reduce surface residue levels. Tillage can disturb eggs laid on the soil surface, damage burrows and injure the worms. If you want nightcrawlers, leave some crop residue. It's necessary for both food and protection.
Shallow-dwelling worms also inhabit cropland. Many of these smaller species live in the top 10 to 30 cm of soil. Adults vary in length from less than 20 mm to greater than 15 cm. These earthworms do not build permanent burrows, but travel randomly through soil. They are also affected by tillage in that any reduction in residue reduces populations. Growing crops and crop residues provide food and protect the soil from dramatic moisture and temperature changes that reduce worm populations.
Together with other organisms that cycle soil nutrients and stabilize soil structure, earthworms contribute to soil health. Examining the response of worm populations to farm practices can tell us how these practices influence other soil life.
Worms mix soil and crop residues, promoting the breakdown of organic matter and the cycling of nutrients. They leave nutrient-rich casts behind, coating the walls of their burrows. Plant roots often grow along or through worm burrows, taking advantage of the better soil structure, higher nutrient levels, and ease of root expansion along the channel.
Burrowing increases soil porosity, promoting movement of air and water. Macropores formed by nightcrawler channels enhance drainage. When improved drainage is seen early in the adoption of no-till, these vertical worm channels are responsible.
Earthworms have a lot of predators - birds, raccoons, other small animals, and a variety of insect pests including the cluster fly. A common pest in old farm houses, cluster flies reproduce by laying their eggs on a particular species of earthworm.
Human activities, however, have the greatest impact on worm populations. Earthmoving for building houses and large-scale soil disruption has the greatest effect, followed closely by intensive tillage practices and poor crop rotations. Pesticides application does not usually affect worm populations.
It pays to encourage large and varied worm populations in your soil. Some people talk about seeding fields with earthworm eggs but improved cultural practices work better. If the food source and habitat aren't there, worms will not grow and reproduce.
To boost the worm population on your farm, reduce tillage or go to no-till; use a varied crop rotation that includes forages and cereal grains; apply manure or maintain surface crop residues; and plant cover crops.
- Worm picking for the U.S. bait market is big business. Most fields with worm populations high enough to interest pickers (no-till fields, hay, pasture, turf) can sustain limited picking with little or no ill effect. Be careful when making a contract with pickers. For more information on worm picking, check out the Ag Canada web site http://res2.agr.gc.ca/london/faq/earth-terre_e.htm.
- Seagulls are often blamed for high worm losses after plowing. But it's the plow, not bird feeding that does the most damage. Seagulls eat worms and other disturbed soil life that the plow has injured or exposed.
- Application of anhydrous ammonia and soil fumigants does kill worms in the sterilized soil band. Usually the impact is short-lived as populations are replenished from surrounding soil. Caution: repopulation may not happen if rotation and tillage practices do not support worms.
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