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What Are Worm Castings and Why Are They Beneficial?
by George Hahn

Note: George Hahn is president of California Vermiculture, LLC, (P.O. Box 95, Cardiff by the Sea, CA 92007, www.wormgold.com, 760-942-6086). His company manufactures Wormgold® pure worm castings and other worm casting products. This is the first of four articles that appeared in the "Fruit Gardener", journal for the California Rare Fruit Growers, Inc., Fullerton Arboretum - CSUF, P.O. Box 6850, Fullerton, CA 92834-6850, www.crfg.org, 714-840-7694. The original article has been edited for space considerations.

Most people involved in gardening and commercial agriculture have a generally positive feeling about worm castings. Over 100 years ago, Charles Darwin was the first person to carefully observe the action of the earthworms and the benefit of the castings they produced. Several universities have researched the benefits of worm castings, but specific knowledge by the general public and even trained agriculture scientists is still very limited.

A goal for this series of articles is to provide education about worm castings and show that that they can provide benefits for all plants.

Worm castings are not actually compost. The vermiculture process is a cool temperature digestion that allows a wide array of beneficial biology to survive in the finished product. Composting worms eat animal manures, newspaper, green waste, household scraps, bio-solids (human sewage), etc.-anything that has previously been living. The digestive process adds a complete array of biological organisms to the castings. These organisms include beneficial bacteria, fungi, actinomycetes, pseudomonads, plant growth regulators, yeasts, and molds. As many as 10,000 different biological species are added! These organisms provide conversion mechanisms so plants can access needed nutrients.

Worm castings is the polite name for earthworm feces.

Worm Castings Production

Many individuals operate private worm beds [compost bins] to convert food waste and landscape trimmings from their own home into worm castings for personal use. A few commercial-sized facilities are operated to convert various materials into worm castings on a larger volume scale. Worm beds come in various forms, from home-sized above-ground boxes and on-ground piles to commercial-sized above-ground, continuous-flow reactors and windrow farms.

The facility which produces Wormgold® castings, is a 50-acre windrow farm. The windrow beds are 12 feet wide and 600 feet long. At the date of this article [August 2000], the windrows extend for 3 miles, giving a production capacity of 50,000 tons per year.

The worms used are predominantly Eisenia foetida. The concentration of worms is approximately 1 pound per square foot of surface. There are about 1,000 E. foetida earthworms per pound. E. foetida eat from 50-75 percent of their body weight every day. So 2,000 pounds of worms will eat from 1,000 to 1,500 pounds of feed per day!

Earthworm beds can be fed a wide array of feed. The Wormgold® facility feeds only landscape trimmings that have been allowed to cure [compost] for at least 120 days. The curing process allows a partial breakdown of the landscape trimmings as well as destroys any weed seeds or grasses. Each worm bed is fed 4 inches of feed every week. The thickness of the feed layer will shrink from 4 inches down to less than -inch in one week. When the beds have grown to over 3 feet tall, the top 10 inches are removed to start new beds, and the remainder is sold as castings. A 10-inch layer is removed because the worms and egg capsules are contained in the top 6 inches. By saving the top 10 inches, all of the worms are kept on the farm and not sold with the castings.

The castings layer is removed from the bed area and allowed to air dry from about 80 percent moisture to under 30 percent. This allows the castings to be screened. The castings are then screened to 1/8-inch as the final product.

Many products on the market contain some worm castings mixed with compost and other elements. The amount of worm castings in most products is 1-3 percent. Wormgold® is castings from the 1/8-inch screen, and is not mixed with any other product. This is defined as pure worm castings.


The vermiculture process provides a pathogen-free end product without the necessity of high temperatures. The cool process allows the rich biology introduced by the earthworm digestive process to thrive. High temperatures are necessary in thermal composting to eliminate the human pathogens. High temperatures are not required to eliminate human pathogens in castings because digestion in the worm's gut kills them.


Chemical fertilizers, fungicides, pesticides, and herbicides all require special handling to deal with the high level of toxicity. Worm castings are non-toxic and do not require any special care and handling. There is no fear of any danger to yourself, your pets, or your children.

Many chemical products, fertilizers, fungicides, pesticides, and herbicides, are toxic to worm castings. Some concentrations of chemical products can destroy the beneficial biology in the worm castings and eliminate their benefits.

Organic Use Approval

Understanding the vermiculture process makes it clear why worm castings are approved for use by all Organic Certification organizations.


Worm castings test low on a normal soils report. The nitrogen level of Wormgold® tests at less than 1 percent. Phosphorous tests around 1 percent. Potassium tests slightly higher than 1 percent. These levels appear to be low but all of these nutrients are in a form that is readily available for use by the plants.

Benefits of Worm Castings

Improved Plant Growth

Extensive university plant growth research has been performed by Ohio State, Cornell University, U.C. Davis, the Australia SIRO, and other agriculture organizations to test the benefits of worm castings. These tests have shown improved plant size, bloom quantity, quality, and color for flowers. Fruit and vegetable tests have shown significant yield increases as well as improvements in taste and appearance. Ohio State research showed that the optimum ratio of worm castings to native soil is 10-20 percent.

Soil Softening

Soils can become hardened with the reduction of beneficial soil biology. This limits fertility. Adding worm castings provides the biology needed to return the soil to a softened state. Within a few weeks, the hard soil is workable allowing ease of plant growth.

Biological Decomposition

The decomposition rate of many biological materials is dramatically increased with the addition of worm castings. Testing has shown the decomposition of prawn and crab shells is doubled when worm castings are added to the soil medium. These shells cannot become valuable in the soil until they are fully decomposed.

Beneficial Nematodes

Some nematodes are harmful and others beneficial. The harmful nematodes are root eaters that can destroy plants, and switcher nematodes that eat fungus then switch to consuming roots. Beneficial nematodes are fungus eaters. Research by Dr. Elaine Ingham at the Soil Foodweb, Inc. lab in Corvallis, OR has shown that worm castings have only the beneficial fungus-eating nematodes.

Fungus Control

Four published university research projects have shown that the complete soil food biology found in worm castings can quickly control fungus problems on a long term basis. Fungus-eating protozoa and fungus-eating nematodes are found in the worm castings. Fungus is an important part of healthy soil but causes nitrogen lock-up and other problems when it gets out of control. Using worm castings as a part of the soil mix will control fusarium, rhyzoctonia, phytophthora, and sclerotinia fungi, and will provide improved plant growth with the release of excess nitrogen as the soil fungi are brought into balance.

Odor Elimination

Worm castings can very quickly and effectively eliminate odors. Mix 10 percent worm castings with compost or composted dairy manure, horse manure, or chicken manure and the objectionable odors are eliminated within a few hours. With the addition of worm castings, animal manures can be used as effective fertilizers without the offensive odors. Worm castings have been tested for odor absorption compared to activated charcoal and found to be more efficient and much less expensive than activated charcoal.

Insect Repellency

Testing has shown that several microorganisms found in worm castings work as effective repellants for a large array of insects. A key element for insect repellency is the level of chitinase-producing organisms. Chitinase is an enzyme that will dissolve chitin. The exoskeleton of bugs is made of chitin [so obviously they don't want to hang out where their skeletons will get dissolved!]. Bugs have various detection mechanisms to determine the level of chitinase in plants and soil. Once the chitinase level is high enough, they will leave the area. Worm castings test with a very high level of chitinase-producing organisms.

Also, some elements in worm castings are able to activate the chitinase-producing organisms found inside plants. Once triggered, these organisms multiply to a level that can be detected by the insects. The repellency effectiveness has been tested and confirmed for white fly, aphids, spider mites and various other bugs. Twenty various plants have been tested showing effectiveness. The rate of repellency is in direct relation to the size of the plant. Spider mites will leave houseplants in about 2 weeks, aphids leave roses in about 6 weeks, and white flies will leave full sized hibiscus in about 3 months.

In November 2002, California Vermiculture was granted a U.S. Patent for the use of worm castings as an insect repellant, and they in the process of obtaining EPA approval to label their worm castings as an insect repellant. Currently, Wormgold® castings are being tested for use against the pine bark beetle.

Wormgold is a registered trademark of California Vermiculture, LLC.

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