Compost Tea: Is It Beneficial? - November 2, 2011
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Last weekend, I was fortunate enough to hear Washington State University Horticulturist, Linda Chalker-Scott, speak about garden myths. Linda is known as a myth buster and one she spoke about was rgarding the benefits of compost tea. What follows is just one example of how a gardening practice can explode on the gardening scene fueled by purported benefits beyond belief – often in the absence of sound science and research.

The use of compost as a soil amendment has been used for centuries to return essential plant nutrients to cultivated soils (i.e. fertilization). Liquid compost extracts have also been used in this manner. Usually, the compost extract or is made by collecting runoff from a compost pile or placing compost in water and allowing it to soak for several days. Today, we refer to the resulting mixture as non-aerated compost tea or “NCT”.

In recent years, aerated compost tea (ACT) has come into popular use. It requires some equipment to maintain an aerobic (oxygenated) environment. While they are marketed as fertilizers, they have more recently been marketed as antimicrobial agents that displace pathogens on the surface of the plant that could cause foliar and fruit disease.

Initially, the popular press and trade journals were quick to sing the praises of ACT using statements like “beautiful blooms”, “delicious fruits and vegetables” and “thick green turf”. The claims also included: “keep garden plants, turf, and crops free of disease” and “provide beneficial organisms”. These claims seemed to have some substance, but are all claims supported by scientific research. In the case ACT, is has shown positive results in the laboratory, but field studies gave less consistent results.

In reviewing the available literature, Dr. Chalker-Scott found only four controlled studies using ACT. One study stated that NCT was more effective at preventing disease than ACT – the anaerobic teas worked better than aerobic teas contrary to claims made in non-scientific literature. Another study looked at the fungal disease called apple scab and found that ACT was not effective at preventing it, and in some cases, appeared to enhance apple scab. A third paper reported mixed results in controlling “damping off” in cucumbers. The fourth study reported the occurrence of human pathogens where ACTs were used. This last finding is particularly worrisome as human pathogens in food supplies continue to be a major public health concern. These findings do little to support claims that ACT prevents plant disease and raise other larger questions.

Another “fly in the ointment” related to compost teas are findings that report excessive nutrients released from compost tea application (NCT and ACT) could cause water pollution. This was the case in six studies reviewed by Dr. Chalker-Scott. Compost applied as a soil amendment and to the soil surface as mulch provides a slow release of nutrients. Conversely, increased levels of nutrients in solution from ACTs and NCTs can contribute to groundwater and/or watershed pollution.

To summarize, plain compost, when used as an amendment or mulch, is most effective at disease suppression and NCTs are also somewhat effective. The use of ACT for disease suppression is not supported by published scientific work. In addition, ACT may contribute to environmental contamination and increase risk of exposure to human pathogens. Given the state of the available science, people should not purchase “compost tea brewers” or ready-made “compost tea”. Instead, they should purchase or build a suitable composting system that allows them to recycle kitchen and garden waste into compost. Making compost is an art, but the process is also very forgiving.

I also hope this article cultivates critical thinking skills among gardeners – as with most things, if it sounds too good to be true, it probably is! Products labeled as pesticides (showing active ingredient percentages, EPA registration and establishment numbers, signal words, safety information, etc.) have been subjected to peer-reviewed science and should perform as noted on the label when used correctly. Other products may make unsubstantiated claims and the buyer must evaluate their merits. Ask product vendors or companies to provide references to peer-reviewed research that support their product’s claims. You may also ask your local Cooperative Extension office about product claims. Below are links to Dr. Chalker-Scott’s other myth busting activities.

Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: November 21, 2011
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