Learn all about using earthworms to improve soil in your garden.
Charles Darwin, who was fascinated with earthworms, wrote, “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.”
Worms have made the soil of many nations fertile, and they will do the same for your soil. Having thousands of earthworms in your garden is like having a thousand tiny tillers running day and night, and each tiller is pulling a compost spreader behind it.
Earthworms mix the soil as they eat their way through it, and their tunnels help loosen compacted clay or silt, allowing water to seep through. This also helps increase aeration of the soil, sometimes up to 75 percent, which in turn allows the tiny organisms around your plants’ roots to thrive. (However, earthworms are not good for some northern gardens; more on that in a minute.) Earthworms recycle nutrients and disperse them through their castings, making them more available to plants. Earthworms will do this work for you all day and even while you sleep. What’s not to like?
Black Gold
Earthworm castings are as good as gold to a plant. As worms eat their way through the soil, they devour organic material and soil, and inside the worm’s crop and stomachs, the organic material is broken down into nutrients while the soil is ground into extremely fine particles. This “soil soup” in the worm’s gut mixes with beneficial microorganisms from inside the worm, and nutrients that would otherwise be difficult for plants to absorb are made more readily available when excreted.
USDA tests show us that castings compared to the soil from which they were made have about five times the available nitrogen, seven times the phosphorus, three times the magnesium, eleven times the potassium, and one and a half times the calcium. Even the slime on the earthworm is high in nitrogen. That nitrogen is the reason when you dig up a plant, the roots have wrapped around the earthworm burrows. The burrows also help catch fertilizer runoff and bind it to the soil to keep within reach of plant roots.
The worms in an acre of land can produce up to 700 pounds of castings in a day. Imagine how much 700 pounds of fertilizer would cost. What’s more, according to Dennis Linden, a USDA Agricultural Research Service scientist in St. Paul, Minnesota, worms can create roughly 250 miles of tunnels per acre every week, allowing for better aeration and better rain absorption.
How to Increase Earthworms in Soil
If the worms in your soil are healthy, then you know that the soil biomass is also healthy, and this in turn keeps your plants strong and vigorous. Here’s how to encourage those worms into your garden without a lot of heartache and hassle.
Earthworms eat organic material, including compost, rotted manure, and decaying plants. Once the earthworm population has increased, you’ll need to continue adding organic materials to keep them fed. Cow manure, grass clippings, green clover leaves and other high-nitrogen materials are great for worms.
The kinds of organic matter you can provide worms are endless. Worms love all kinds of kitchen waste: fish bones, fruit and vegetable peels, leftovers. If you don’t have a compost pile or you want to cut out the middleman, incorporate it directly into the ground. According to Dr. Linden, earthworms are creatures of habit. If they find food once, they’ll keep returning for more. If you till your organic matter into the soil, the field worms will dig along horizontally. If you leave it on top of the soil, the worms will dig up through the soil, and these upward-facing tunnels will allow more rain and air into the soil than horizontal burrows.
Mulching Madness
Avoid acidic mulches like large quantities of oak leaves or pine needles. An acidic soil, one with a pH of 5.4 or less, will kill worms. If you have acidic soil, try amending it with lime to bring the pH up, and test the soil annually to be sure it remains neutral. However, if you are trying to grow acid-loving plants, such as blueberries, magnolias or azaleas, don’t lime the soil.
Sandy soils generally have fewer worms than silt or clay soils do. Applying more organic material will help build the soil and attract more worms, though the process will take a while.
Worms can die in extreme summer temperatures or during sudden autumn freezes if they’re not protected. To help protect them, cover your garden with a thick, organic mulch. A thick layer of newspapers, about ten sheets covered with grass clippings or leaves, will provide ample protection. This will keep the weeds down, keep the soil cool, hold moisture in, and will rot away in winter, adding nutrients to the soil. Continue adding mulch as it decays.
When soil temperatures rise and the soil dries out, field worms will dig deep into the soil, curl up into a little ball to conserve moisture, and go dormant for a while. Mulching will keep the soil cool and moist, and field worms will continue to be active.
The Low-Till Method
Another way to protect your worm population is by not tilling or tilling less. Tilling through the year tears up worm burrows and kills field worms. Maintain several areas in your garden that won’t be tilled (easy enough if you have a strawberry or asparagus bed), or simply have a no-till garden. If you prefer tilling, fall tilling is best. Tilling helps aerate the soil, which insulates the earthworms against winter’s extreme temperatures. Also, there are lots of young worms in the fall, so any extra organic material added to the soil would provide some winter food for them. After tilling, mulch the soil heavily with chopped leaves. The worms will remain active through the winter under the mulch, and you’ll find worm castings under the leaves even in February. How’s that for service?
Subdivision Soil
Sometimes, you’ll have a soil that has been completely wrecked. Some people have subdivision soil, which is that churned-up clay leftover from house construction; some people have fragipan; some have desert asphalt. If your soil is completely dead with no worms, build a raised bed on a thick layer of newspapers, and add worms, compost and other organic material. This process can take between three and five years, but you will eventually bring the soil back to life. Keep adding the good stuff, and eventually the worms will return.
You can even “seed” your garden with earthworms. Find a pasture that is rich with worms, cut out a large block of soil, “plant” it into your garden, and surround it with organic material. Keep adding mulch and digging organic material into the dead soil. You can also pick up earthworms after a rain and put them in your garden, or buy them at a bait shop. Avoid red wigglers, which do best in compost only. Go for the nightcrawlers instead.
Where Worms Can’t Go
Not all places and soils are welcoming to earthworms. In Minnesota and around the Great Lakes, non-native earthworms are actually considered an invasive species. For thousands of years, the northern glaciated forests grew without worms present to help breakdown decomposing leaves, trees and branches. These forest floors are filled with a loose layer of decomposing leaves and plants called “duff.” Fungi and bacteria have been the principal means of duff decomposition since the last ice age glaciers receded thousands of years ago, as the earthworms were killed off by the ice and cold.
Lately, however, earthworms have been moving into northern soils. Sometimes they hitch a ride in the soil of plants being transplanted from other states. Some worms are brought in as fishing bait to northern lakes and then discarded. At any rate, earthworms are slowly moving into the northern forests, where they break down the duff too quickly for the vegetal forest dwellers to respond. Many of the native northern plants and some small animals are adversely affected by the rapid duff removal. A number of plants can’t survive in the duff-less soil and they die off. Earthworms also dine on the helpful fungi that used to do the work of decomposition.
In northern gardens, it’s best to not introduce earthworms, but seek out soil-building methods that work for those particular types of soil. For the rest of the nation, where earthworms have long been a part of the soil biota, they are wonderfully helpful little animals to have on your side.
A Field Guide to Earthworms
The common earthworm (Lumbricus terrestris) is known by a variety of names, but nightcrawler is most often used in North America. Nightcrawlers form deep burrows and are surface feeders, unlike other worms that get their food as they burrow through the soil. Since they feed and mate on the soil surface, they’re seen more often than other earthworms. They are also great for fishing bait.
• Aporrectodea caliginosa and rosea, known as angle worms, are common in fields and gardens, and they do a great job of mixing the upper part of the soil. If you want a nice thick layer of black soil in your garden, better get these guys.
• Dendrobaena octaedra is a tiny earthworm living in the leaves on top of the soil, feeding mostly on bacteria and fungi. These are commonly found in forests.
• Leaf worms (Lumbricus rubellus) live in the top leaf layer and the first few inches into the soil.
• Red wrigglers (Eisenia fetida), also called tiger worms due to their stripes, are surface dwellers only and make great worms for vermicomposting purposes.
Melinda R. Cordell, a former horticulturist, lives in northwest Missouri with her family and two red hens. She is the author of the book Courageous Women of the Civil War: Soldiers, Spies, Medics, and More, Â from Chicago Review Press.
