July 25, 2005 | General


BioCycle July 2005, Vol. 46, No. 7, p. 61
A great potential exists for compost tea science – requiring cooperation between scientists, industry and observant users of all stripes.
Lynne Carpenter-Boggs

ANY solution – from, of, or based on compost – can be loosely referred to as compost tea. Most in the industry define compost tea as a brewed or steeped solution of approximately 1:1000 parts compost:water that is used as a foliar spray or soil drench. Teas are generally weak fertilizers, having only the nutrients extracted from a small amount of compost and/or other additives, but are thought to gain efficacy through the action of living microorganisms and/or microbial metabolites that inhibit plant pathogens and/or stimulate plant growth. The true mechanisms of compost teas’ activity are not known and are likely multifold. Although the chemistry and microbiology of compost tea is enormously complicated, enough can be understood to make and use compost teas beneficially.
A small industry is developing around compost tea, estimated to be growing at 25 percent per year. Currently about 10 companies sell brewers and ingredients to homeowners, nurseries, and ag producers. One company – SoilSoup Inc.- has introduced “kitchens” that continually produce fresh aerated tea. A few companies, and many more market gardeners, sell fresh or off-the-shelf teas locally. Purchased finished teas have the disadvantage of losing their freshness and aeration, and possibly changing their microbial community, by the time they are applied at home or on farm. When brewing stops, the microbial community changes, as some groups die off or are consumed by higher trophic groups. Teas are usually applied to plants or soil within 24 hours of the end of brewing to minimize chemical or biological changes. The fresh option is brewing your own, but batch-to-batch variability can be high. Being consistent in terms of inoculant compost, water source, additives, brew time, and conditions like temperature and aeration, means that more consistent tea is produced.
Many purchasers and brewers of compost tea rave about faster seedling growth, bigger plants, brighter flowers, prolific fruit, and reduced disease in their gardens and farms. Not everyone is raving, though, and the growing controversies around compost tea are reaching into universities. Right now the industry faces a challenge by critics who point out the scarcity of convincing studies and the potential for microbiological hazards. Both are legitimate concerns that can only be addressed by more education and research under strict guidelines. Several studies have successfully controlled plant diseases and increased plant growth with compost teas. Some published works also show no effect, less plant growth, or worsened disease on plants treated with teas. Much work is needed to reach clarity about which diseases may be controlled or worsened on which plants in specific conditions.
The issue of microbiological safety must be taken seriously, and should be addressed in a fashion similar to potentially hazardous food preparation activities like canning and krauting. University extension services have greatly improved home food safety by recognizing the value and danger of these activities and prescribing safe methods. In making compost tea, a high quality finished compost or inoculant is absolutely necessary. If unfinished, poor quality, or contaminated compost is used, one could indeed culture dangerous microorganisms. With tens of thousands of home brewing machines already sold in the U.S., compost tea is not going away anytime soon, and should be addressed seriously by agricultural institutions and agencies. The responsible answer is to study and recommend recipes and methods that preserve human safety but also allow freedom for specialized products.
Compost teas and liquid extracts have been used for decades, have gained significant interest and popularity in the last five years, and are supported by uncounted anecdotes. Conversely, microbiological and chemical research regarding their efficacy and mechanisms are indeed sparse. One tea-related research project is currently funded by the Washington Technology Center, which couples Washington state businesses with university and nonprofit agency researchers. I bring to the partnership a scientific background in composting and other soil and agricultural microbiology. The industry partner is SoilSoup Inc., a Seattle based company that develops and markets equipment to brew aerated compost tea.
Appropriate biotechnology research is being used to identify the beneficial components in SoilSoup compost teas. The first step is to develop appropriate positive and negative controls for testing tea efficacy. Next we have started to develop an understanding of the microbial and chemical workings of compost teas, defining which characteristics or fractions provide their plant growth-enhancing and pathogen-resistance properties. When the beneficial constituents are better understood, the next phase of research will reformulate and test new products. Many years or careers will be needed to fully understand and engineer these brews.
In 2003, a small core of researchers and consultants founded the Compost Tea Education and Research Foundation (CTERF). It provides a forum for open exchange of ideas and collaboration for research, development, and outreach about compost teas. We see a need for a “base theory” about the microbial communities in compost tea – which ingredients and brew conditions lead to what type of community, which organisms contribute to the various beneficial effects. The current work with SoilSoup is a step toward this base theory. Classifying an entire microbial community is easier said than done, but with a combination of classical and molecular tools we can get closer than ever before.
Compost tea science is wide open. Besides the microbial communities and chemistry of the teas themselves, there may be different effects of the same teas on different plants, diseases, in different climates and soils. Where does one begin? One place to start is listening to current users of the teas. By borrowing the observations of those most experienced with teas, we can gain substantial insight to their workings.
As a veteran of embattled research topics including biodynamic agriculture and microbial signaling, diving into a controversial study area is becoming a familiar process. Like biodynamic agriculture, compost teas are intriguing both scientifically and sociologically. Scientifically, they involve dynamic microbial communities and their effects on agricultural soils and plants. Sociologically, a subculture has arisen in support and belief of its use, individuals not dissuaded by criticism from academics.
Like many advances, these tools sprung from intuition and have been developed by observant practitioners, but require strict analysis for further improvement. And although the scientific method is slow to uncover mechanisms, the positive results seen by millions of customers does give some assurance that beneficial biochemistry is at work.
There is great potential for developments in compost tea science that will benefit both the industry and its clientele. In the future we should have compost teas for general use, organic fertilization, growth enhancement, inducing disease resistance, and fighting specific diseases. It will take cooperation of scientists, industry, and observant users of all stripes to get there.
Dr. Lynne Carpenter-Boggs is in the Department of Crop and Soil Sciences at Washington State University in Pullman, Washington. She can be contacted via e-mail at

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