Southwest Environment

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Department of Soil, Water and Environmental Science
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Got Worms? Vermicompost provides plant nutrients, improves soil

By Melanie Martin

Maggie Whiteside scoops up some soil containing vermicompost.
Photo by Melanie Martin.
Maggie Whiteside scoops up some soil containing vermicompost.
Shadows dappled the ground early in the morning in late October, with the air just beginning to take on the brisk quality of autumn. Maggie Whiteside knelt down and scooped up a pile of soil enriched with worm castings.

It was several shades darker in color than the surrounding sandy soil on the Marana Heritage Farm, and much lighter in weight. Whiteside, farm coordinator, noted how the plant growth had improved after she switched from cow manure to worm castings.

“I think it did a way better job than the cow manure.  I think it made a much better, healthier potting soil,” Whiteside said of the farm, located about 25 miles northwest of Tucson. The farm is owned by the Town of Marana and operated by the Community Food Bank of Southern Arizona.

Composting with worms, or vermicomposting, is simple, inexpensive, and can be done in your own home. When worms consume food scraps, their feces – termed castings – are mixed in with the soil.  The resulting enriched soil is called vermicompost, which has many benefits for plants and soil.

Plants tend to grow better when nourished with vermicompost.Photo by Melanie Martin.
Plants tend to grow better when nourished with vermicompost.

Benefits for Plant Growth

Incorporating castings into potting soil results in faster germination and plant growth, and increases crop yield, researchers have found. This is possibly due to the presence of plant growth hormones in worm castings, said Rhonda Sherman, a Cooperative Extension specialist with North Carolina State University who organizes the world’s only annual conference on mid- to large- scale vermicomposting. These hormones, called auxins, are a byproduct of worm digestion.


“The plant growth hormones make the plants germinate more quickly,” said Sherman. “The plants grow bigger and usually have a higher yield.”

Worm castings also contain humic acid, formed during digestion of plant matter, according to Sherman. Humic acid frees up nutrients present in the soil, making them more available for uptake by plants. In a 2002 study by Rola Atiyeh of Ohio State University and others, adding humic acid from vermicompost resulted in increased growth in tomato and cucumber seedlings.

“By just adding a little bit of vermicompost to soil, no more than 20 percent by volume, you’re likely to increase your yields, so you’re going to produce more food,” Sherman explained.

Maggie Whiteside scoops up some soil containing vermicompost
Photo by Melanie Martin.
A worm bin at the Marana Heritage Farm
Adding vermicompost to soil also helps repel pest insects, Sherman explained. A 2004 study in BioCycle by Clive Edwards and Norman Arancon showed that applications of vermicompost to tomatoes, peppers, and cabbage resulted in a decrease of pests such as aphids and mites.

“The castings do all sorts of great things for the soil,” Sherman said, noting that castings can improve soil texture and structure, reduce the risk of erosion, and increase a soil’s ability to store water.

Composting with worms is easy

Making a worm bin is simple and inexpensive.

“The biggest cost is the worms,” explained Sherman.  In the U.S., worms can cost anywhere from $25 to $50 a pound, and can be purchased over the internet. Luckily for Tucsonans, there are several local worm suppliers, such as Acme Worm Farm in northwest Tucson.

Photo by Melanie Martin.
Vermicopmost is typically darker than the surrounding soil.
A worm bin can be as simple as a large plastic bin, the kind people often use to store holiday decorations. Whiteside mentioned that on the Marana farm, the first worm bins were constructed from old apple crates.

Earthworms need bedding, such as shredded newspaper, mixed with some soil to introduce beneficial micro-organisms that help the worms digest food scraps. Worm bins do not produce any foul odors because they are kept covered, and contain only food scraps such as fruits, vegetables, and egg shells. Meat and dairy products are not appropriate food for worms, as they attract rodents and flies and make for a smelly bin.

She also explained that turning vermicompost is not necessary like it is with regular compost, since the worms digest the food scraps so quickly and aerate the soil as they move around. Whiteside said vermicompost is easier to use than cow manure.

“We were using composted cow manure, but there were big chunks in it that you had to break apart, and sometimes they’re so big that you can’t break them apart,” Whitesaid said. “So if I have worm castings I’d rather use those because they’ve already been sifted so there’s not chunks, it’s just really light and fluffy.”

People from 83 countries have contacted Sherman about vermicomposting, she said, noting that many Americans don’t realize how prevalent vermicomposting is worldwide.With the growing popularity of Sherman’s annual conference, in addition to the many workshops and lectures she gives each year, vermicomposting could become more accessible to people here in the U.S..

“It’s a pretty simple technique,” explained Sherman, who has had worm bins in her home for years. “I have friends who made a lovely coffee table that was a worm bin.”

Melanie Martin is a senior studying environmental science in the Department of Soil, Water and Environmental Science at the University of Arizona.  She is interested in conservation of wildlife and natural resources, and hopes to work for the Natural Resources Conservation Service after graduation. This story was submitted for publication on December 13, 2012.

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