Why Are Earthworms Good For Gardens?

Earthworm Body Parts Cultivating a healthy garden is an enlightening and fulfilling experience, but as veteran gardeners know, such an achievement requires not only aesthetic creativity but also a connection to and an understanding of the soil in which plants are grown. There are many techniques that gardeners employ to create ideal soil conditions, such as gentle tilling and scheduled watering for aeration and moisture.

However, there is a biotic community, which is less commonly considered, that does a lot of the soil-conditioning work for us. Made up of many different kinds of organisms, this underground society supplements the gardener’s efforts and ultimately mediates a soil’s ability to provide a desirable plant habitat. One of these key players is the earthworm.

Are earthworms good for your garden? Many gardeners believe that earthworms are a benefit while those who garden in a very heavy clay soil would be quick to disagree. To find out more, read Are Earthworms Really Good For Your Garden?"

Test your knowledge: try the Earthworm Quiz

What Are Earthworms?

The term Earthworm refers to a specific group of invertebrates within the taxonomic phylum Annelida. Earthworms belong to the Oligochaetes-which means 'few bristles.' Worldwide there are over 3,000 species described. Earthworms are, as their name suggests, terrestrial, their skin is permeable and they need a moist environment so they don't dry out. The different species of earthworms have individual requirements just as a dog has from a cat, some earthworm species live in compost, some live in permanent burrows deep down in the soil, others are content with the middle ground and make complex networks of tunnels as they explore the earth.

"The plow is one of the most ancient and most valuable of man's inventions; but long before he existed, the land was in fact regularly plowed and still continues to be thus plowed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.    Charles Darwin

How Do Earthworms Benefit The Garden?

Soil Formation

Earthworm Soil Formation We all know that a healthy compost heap is writhing with worms, working their way through the kitchen and garden waste to help create organic matter to feed our soil—these are mostly brandling worms, recognized by their red, stripy appearance—but I’m talking about the worms which, if your soil is good, you’ll find in large numbers throughout your vegetable plot. How useful are they?

Well, first, they’ll be munching on leaf fragments and other plant debris that accumulate on the soil surface. Research indicates that, having passed through worms, these bits and pieces are broken down more easily by microbes. Result? Humus! That black gold beloved of all organic gardeners is produced more quickly with the help of worms than by microbes alone.

Nutrient Cycling

With worms having little to do but eat, they shift a surprising amount of material. One study on forests came to the conclusion that worms buried over 90% of the plant debris left on the soil surface. Another indicated that an impressive forty tonnes of soil per hectare were brought to the surface in worm casts in the course of a year.

This is great news as worm casts tend to be very fertile, holding more calcium, iron and phosphorous than the soil around them. They also contain significant amounts of nitrogen and help to stabilize carbon, reducing the release of carbon dioxide from the soil. The small particle size in the casts means they can even hold more water.

Water Absorption

Of course, worms also burrow, and not all in the same direction. Epigeic species burrow in the litter on the soil surface; anecic species live in vertical burrows that can descend several feet. Endogeic species burrow below the surface in horizontal or random lines. Aristotle described worms as "the intestines of the soil", which is a good way to regard the passages they create. Burrows open up the soil to air, bringing oxygen to plant roots, and create pathways for water and roots. The presence of earthworms has been shown to have a notable effect on the amount of water a soil can hold and on how fast it sinks in.

Earthworm Structure

An earthworm consists of a digestive tube housed within a thick cylindrical muscular tube that forms the body. The body is divided into segments, and furrows on the surface of the body mark the division between each segment.

The first segment encloses the mouth, and has a fleshy, muscular lobe on the top. This lobe can be pulled in to seal the mouth, or extended forward to probe the immediate surroundings. All segments, except the first, have eight retractable bristles which help the earthworm to grip surfaces as it moves.

Earthworm Structure

Earthworm Reproduction

Earthworm Reproduction Earthworms are hermaphrodites, meaning they have both male and female reproductive organs.

When two earthworms are ready to mate they adopt a head-to-tail position, cover themselves in a layer of mucus, and exchange sperm. The saddle produces a mucous tube which detaches and moves forward along the body, collecting on the way the earthworm’s own eggs and the sperm received from its partner.

Fertilization occurs in the mucous tube which is shed from the front end of the earthworm. This dries in the soil to become an egg capsule, from which one or more young earthworms will eventually hatch. Many species can reproduce several times a year.

There Are 3 Groups Of Earthworms

There are thousands of earthworm species worldwide. These species are placed into one of three groups based on their feeding and burrowing habits. Each group contributes to soil health in different ways, but they all have a couple of things in common.

Epigeic Group

Earthworm types Epigeic, Endogeic and Anecic graphic This group is composed of species that primarily consume plant litter and organic matter at the soil surface and make nutrients more rapidly available for plant uptake. Epigeics are small worms with short life spans (three to four years) and high reproductive rates. During winter, they bury themselves under the litter layer to survive, but if the top foot of soil freezes, they die.

Endogeic Group

Members of this group create continuous horizontal burrows within the soil profile and feed on subsurface soil enriched with organic matter. These earthworms help aerate the soil as they burrow. Endogeics have lower reproductive rates than epigeics, are larger, and have longer life spans (up to six years). In winter, they burrow deeply in the ground and fall into a dormant state.

Anecic Group

This group of earthworms, which feed on surface litter and create deep vertical burrows, incorporates organic material into the mineral portion of the soil (below the surface) by pulling surface litter into their burrows. The vertical burrows also improve water flow through the soil. Anecics have the lowest reproductive rate but are the largest earthworms with the longest life span (four to ten years). They retreat deeper into the soil and go dormant when cold temperatures arrive.

Did You Know?

  There are over 100 known native earthworm species in the United States. They primarily reside in undisturbed native habitats (forests and grasslands) that were not glaciated.
  Exotic earthworm species were accidentally brought over on colonial ship ballasts and in potted plants during the 1700s through the 1940s. They thrive in and dominate large stretches of land that have been disturbed (agricultural and urban areas).
  Earthworms can process around 0.08 to 0.3 grams of soil in one day, depending on the species.
  Earthworms are hermaphrodites (have both male and female reproductive organs). When two earthworms mate, both transfer sperm to one another and grow egg cocoons in the clitellum (the engorged region close to the middle of a mature earthworm).
  Earthworm eggs hatch in eight to wenty weeks under favorable soil conditions.
  Earthworms can live from three to ten years, depending on the species and soil conditions.

Looking After Worms

Earthworm Cartoon Given their activities, it’s a good idea to encourage as many as possible to hang out in your soil, so my first suggestion is to dump the rotovator. Agricultural research has shown that the more soil is tilled, the fewer the worms are in it and, despite rumours to the contrary, worms cut in half do not grow into two worms. It’s true that, in some circumstances, one half might survive, but on the whole you can assume that a bisected worm is a dead worm.

Digging also presents a clear and present danger to them, though on nothing like the scale of a rotovator, and worm safety is one of the reasons I’m a great advocate of No-Dig beds which leave soil undisturbed. If you must dig, using a garden fork rather than a spade is less likely to damage worms.

Bear in mind, too, that worms eat organic matter and, if supplies drop off, they will vote with their contractile tissue and move elsewhere. They also need moisture, which is why in dry weather you’re likely to find them in the damp soil under stones or wound into a tight ball. It’s their way of protecting themselves from heat and dryness until conditions improve.

The best answer to keeping them happy is to look after your soil. Ensure that it’s well fed with compost and spread with mulch, and don’t dig unless you have to. Not only will it be good for your plants, but the organic matter and improved water retention will make the worms want to call your garden home.

Read More:

 Take The Earthworm Quiz!
 Are Earthworms Truly Good?
 Fertilizer Basics: A Tutorial
 Soil Biology
 Native Plant Root Systems