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What are Worm Castings?

Niki Foster
Updated May 16, 2024
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Worm castings, also called worm manure, or worm humus, are the end product of the breakdown of organic matter by certain earthworm species called composting worms. They are used as a form of organic fertilizer to improve soil and plant growth. The process of harvesting this material is called vermicomposting. It is practiced both on a large scale for large farms, landscaping, or commercial sale, and on a small scale for home gardens.

Worm species suitable for vermicomposting include Red Wigglers and European nightcrawlers. They may be purchased from a bait store or from commercial vermicomposting operations. Large-scale harvesting of worm castings uses either a windrow, a row of mowed hay or other grain crop, or a raised bed. Since Red Wigglers are top-dwelling worms, and the food source is placed on the top of the bed, a raised bed allows the collection of castings from below without disturbing the worms.

Vermicomposting at home is similar to the raised bed method. A large bin with air holes, and a drainage spout, and a collection tray on the bottom, is used to house the worms. Some experts recommend leaving the bottom open so that the worms are not trapped, allowing them to escape in the case of adverse conditions. Worm castings can be harvested when there is little to no visible food or bedding left in the bin.

Composting bins should ideally be made of recycled or semi-recycled plastic. Though they do not need as much drainage as plastic bins, bins made of wood will decay over time and must be replaced, and some woods contain oils that are harmful to worms. Metal bins are not recommended as they are prone to rust and can release heavy metals into the compost. Styrofoam bins may also produce a toxic environment.

Compost worms can be fed with a variety of organic material. Commercial vermicomposters use grass clippings and wood chips, cow and pig manure, sewage, food processing and grocery waste, and agricultural waste. Home vermicomposters use kitchen and garden trash including fruit and vegetable rinds and peels, coffee grounds and filters, tea bags, moldy bread, and leaves and grass. It is not recommended to use dairy products or meat because they can putrefy and attract pests.

Worm castings have 10 to 20 times as much microbial activity as the soil and food the worms ingest. They contain worm mucus, which helps the soil hold nutrients and retain moisture. They also attract other earthworms that continue to improve the soil. Vermicomposting also reduces biowaste and greenhouse gas emissions.

HomeQuestionsAnswered is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Niki Foster
By Niki Foster , Writer

In addition to her role as a HomeQuestionsAnswered editor, Niki enjoys educating herself about interesting and unusual topics in order to get ideas for her own articles. She is a graduate of UCLA, where she majored in Linguistics and Anthropology.

Discussion Comments

By anon991020 — On May 21, 2015

I live in southern Arkansas. I raked my leaves and grass clippings into a corner of my back yard and add all my coffee and tea grounds and food scraps to the pile,I also shred my newspaper's and add them. The pile of leaves ( Compost pile ) are full of night crawler worms. They are fast at work making castings that I will add to my garden. The worms found the compost pile on their own and went to work .

By anon926279 — On Jan 17, 2014

What are the advantages of using castings?

By honeybees — On May 22, 2012

I really need some help with my soil and this sounds like it would be a great thing to try.

When I first started gardening I just thought I could plant some seeds in the soil, and in a few months, I would have this great harvest of vegetables.

I have found out that one of the secrets to a good crop, is good soil. I am working on having an organic garden and it sounds like using worm castings is my next logical step.

It's too bad I can't use all the earthworms I see outside after every rain. If I could use those, I wouldn't have to ever worry about finding a source to buy them from.

By sunshined — On May 22, 2012

I have had a compost bin in my back yard for a few months now. It makes so much sense to put scraps of food here that I would otherwise just throw away or put down the garbage disposal.

This is one way I plan on improving my soil and getting a better crop of vegetables. I am also very interested in using worm castings.

It sounds like the food scraps I am already putting in my compost bin would be perfect for feeding the worms. I just need to build it differently, get some worms, and I should be ready to harvest my own worm castings.

By myharley — On May 21, 2012

I have used organic worm castings in my vegetable garden for a few years now. I feel much better about using something like this than some of the fertilizers you buy at the store.

The worm castings also don't burn the plants like some of the fertilizers will do.

Once I started using them, and realized how much more yield my garden produced, I won't use anything else.

The soil is so much more moist and pleasant to work with, and the roots of the plants seem much stronger.

By LisaLou — On May 20, 2012

I always wondered what the specific benefit was of using worm castings. I have seen these advertised in some of the flower and gardening magazines I receive.

When I read that worm castings can have up to 20 times the amount of microbial activity, I finally realized why they can be so helpful.

If you don't know much about it, it can sound kind of gross, but this would make the ideal fertilizer.

You are also recycling because otherwise the castings would just decompose on their own. They would still enrich the soil, but it would be much better if that soil was in a garden.

By JimmyT — On May 19, 2012

@Emilski - Those people are right for wanting the worm casting compost for their tomatoes. My grandma taught me as a kid to always make sure you used worm compost in your garden. She always had the best plants, too.

The problem I have now, though, is that I can't find anyone around me who sells worm castings, so I just have to go with regular fertilizer. I have found a couple of places online where you can buy the castings, but it's a little too expensive considering the shipping costs.

After reading this article, I am thinking maybe it wouldn't be too hard for me just to set up my own worm farm and use the results for my garden. Has anyone ever tried this before? How much work would be involved, and how much would the start-up price be for the worms and plastic bins?

By Emilski — On May 18, 2012

@Izzy78 - I had a friend who used to have a worm farm where he sold the worms for fishing. The way he had it set up, though, he could also harvest the castings to sell to people. I don't recall which species of the worms he used, though. They weren't the normal night crawlers you find in your yard. They were a little thinner and yellowish.

Anyway, you won't get rich trying to get people to buy worm castings, but it was a nice little side business. That's especially true when you consider that he had to do very little extra work besides throwing in some food scraps.

There were some people that swore by using worm castings to grow their tomatoes. They came back every year wanting to buy the stuff. I know he sold it in one-pound bags, but I don't know for how much.

By Izzy78 — On May 18, 2012

@matthewc23 - I'm with you. I didn't realize there was a whole industry around this. I don't think I would ever get involved with vermiculture, since I think worms are pretty gross, but I am interested in it.

If you were to try to do this on your own at home, how long would it take from the time you put in the first batch of food scraps until you had sufficient worm castings to use in a garden or something? Also, would you be able to produce enough to have your worm castings for sale? How much would they sell for compared to something like potting soil or other types of fertilizers?

By matthewc23 — On May 17, 2012

@shell4life - Yes, worms are excellent for the soil. Obviously, you got the hint from this article that earthworm castings are good for the soil, but worms have a lot of other benefits, too. As they move through the soil, they leave small tunnels. The tunnels act like pipes for water and air to get to the roots of plants, which is very important. That is the reason you should till before planting and why you don't want the soil to be compacted. Worms get all of their nutrients from the soil itself and should never adversely affect plants.

That being said, I never realized that there was an actually business of generating worm castings. Is this done in any particular part of the country, because I have never heard of it until now?

By shell4life — On May 17, 2012

I had no idea that worm castings serve as fertilizer! Actually, I never knew that worms could do any good to the plants that they share the soil with, and I actually thought that they should be removed so that they couldn't eat the plants.

I grow a lot of flowers in my garden, and every time that I till the soil, I find tons of worms. I suppose this means that I have lots of natural worm castings in my soil, though I have no idea how to spot them.

I'm glad to learn that the worms are good for the soil. Now I don't have to worry about plucking them out, and I may not even need additional fertilizer.

Niki Foster

Niki Foster


In addition to her role as a HomeQuestionsAnswered editor, Niki enjoys educating herself about interesting and...

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