BioCycle Magazine

Understanding Compost Tea

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Courtesy of BioCycle Magazine

 Understanding Compost Tea

COMPOST TEA describes many different preparations made using compost as a starting material and producing a liquid extract or in some cases a “liquid version” of the original compost. There are many home-designed pieces of equipment and some commercially available equipment made to produce compost tea. New ideas abound on how to fabricate the better tea-maker and different designs are tried each year with efforts to improve efficiency and decrease costs.

Additionally, there are probably as many recipes for compost tea as there are for chili in Texas, with more ideas on improvements and a focus on its use for more specific applications. For instance, when making compost tea to combat plant pathogens, the trend is to have as much microbial diversity as possible. When making a tea to supplement plant nutrients, many producers are fortifying the tea with supplements either during production or as a postproduction addition.

Optimism about compost tea is high, but understanding its limitations and having realistic expectations are necessary. With all of the variations in compost tea production, there are some applicable basic guidelines independent of recipe or equipment differences.


Compost tea is a readily available form of compost that will impact the plant more quickly than compost mixed into the soil. Compost quality issues, including maturity and microorganism content, become very important for making effective compost tea. The transformation of compost into compost tea cannot improve on the original quality of the compost.

A good compost has the potential to make a good compost tea if made properly; a poor compost will always make a poor compost tea. Many imperfections in the starting compost such as high salt concentrations, high levels of anaerobic microorganisms and the presence of pathogens may actually be amplified in the final compost tea. It is critical, therefore, to use only the highest quality compost available. Save the lesser quality compost for soil applications and use only the best for compost tea production.


Compost that is rich in microbial numbers and diversity can result in compost tea with these same qualities. Even so, the typical representation of microorganisms in compost tea differs from the original compost. Some types of microorganisms like to live attached to particulate matter and a compost tea made using a fine mesh strainer popular for tea destined for drip irrigation, doesn’t let a sufficient amount of particulate matter through to support these microorganisms. The beneficial fungi and actinomycetes prominent in a good compost may be poorly represented in the compost tea simply due to the necessity of straining out the material to which they would attach due to the demands of the irrigation system equipment. Keep in mind that compost tea microbiology is most impacted by oxygen availability, nutrient availability and the initial microbiology of the compost used to make the tea.

Compost tea is analyzed for the same microbiological parameters as compost. This includes beneficial microorganisms including aerobic and anaerobic bacteria, fungi, actinomycetes, pseudomonads and nitrogen fixing bacteria, as well as pathogens such as E. coli and Salmonella. Keep in mind that compost tea microbiology is most influenced by oxygen availability, nutrient availability, and the initial microbiology of the compost used to make the tea.


Unlike wine, compost tea does not typically improve with time. For best results, it should be used as soon as possible and should be stored in a shaded area with agitation and ventilation to the tank. Long storage times will negatively impact the diversity of microorganisms as well as the nutrients carried by the tea for plant use.

Compost teas are applied either to the soil or to the plant foliage. Those applied to the soil will move into the root zone and affect the rhizosphere of the plant. Nutrients carried in the tea will be used by the plant as well as the microorganisms in the soil. The microbes in the compost tea may have a lot of competition with other soil microorganisms, but have the opportunity to become a part of the soil and rhizosphere microbial ecology.

Alternatively, compost teas applied to the plant foliage will immediately impact the plant and there is very little room for forgiveness from the plant if a tea with toxic qualities is used. A good quality compost will provide beneficial microorganisms and nutrients to the surface of the plant to assist the plant in disease suppression and nutrient availability. A poor quality compost tea may be supplying the plant surface with unwanted components such as salts and problem microorganisms. Compost tea destined for foliar applications in particular should only be made with the highest quality of compost to avoid problems such as salt burn and the distribution of pathogens in critical areas of the plant.

Compost tea like compost itself has the potential to be a powerful tool for agriculture. In addition to being a stand alone product, growers are discovering its potential to be an efficient carrier for other agricultural components. Our understanding of the science behind this complex product continues to grow and help us to understand some of its possibilities and limitations

By Vicki H. Bess

Time for (compost) Tea in the Northwest

DURING last year’s damp, cool summer, farmers on Bainbridge Island (west of Seattle, Washington) battled gray mold on bean crops. Several lost their entire crop. But Art Biggert, who operates a community supported agriculture farm that serves 55 families with a season’s share of his crop, was an exception. Biggert’s beans — treated with tea spray from vermicomposted cow manure — survived and thrived, allowing him to provide all his customers with beans and still have 60 pounds a week left for market.

Farther down the coast at the Oregon research farm of Territorial Seeds, a regional wholesale and retail seed company, tomato plants treated with a foliar spray of compost tea over the growing season were lush and productive. Unlike adjacent untreated plants, the sprayed tomatoes set blossoms from June till frost, and produced multiple, large tasty fruits without blight.

The preliminary, anecdotal results of compost tea use appear positive and are spurring worldwide interest. Compost feedstocks for tea are being analyzed so the organisms in compost can match the micronutrient needs of the plants. Recipes are developed to provide jump-start food for the microbes present in the compost extract. Research is being done to assess its effect on plant growth and disease suppression for various growing conditions, soils and plant types. Two Northwest companies are steaming into the compost tea market with brewers designed to quickly and easily produce large volumes of tea for a wide range of applications.


“Applying technology to sustainable agriculture one drop at a time,” Growing Solutions, Inc. (GSI) in Eugene, Oregon offers three sizes of its Microb-Brewer (500, 50 and 12 gallon models). The brewers use a patented nozzle design adapted from automobile carburetion systems to “entrain” oxygen into the liquid compost solution. According to company president Michael Alms, compared to “bugs in a jug” fertilizer that contains five or six species of microbes, tea can add thousands of specific species to the soil to enhance soil health and tilth.

The noncomputerized Microb-Brewers are fully assembled and ready to go. An on/off system uses the same pump to mix and discharge the liquid. “The future holds a lot of changes to these machines as there is a lot of technology that can be used to enhance them,” says Alms. “An amazing diversity of growers are trying out this product.” Tea is being tested on potatoes, wheat, calla lilies, cherries, pears, delphiniums, papayas, sugar beets and turf in a number of states.
 A second company in the Seattle area, Soilsoup, Inc., offers a Bio-Blender that arrived on the market last spring. Operating with a small electric motor clamped onto the user’s mixing container, the Bio-Blender differs from the Microb-Brewer in design and aeration technology, but introduces air into continuously circulating liquid to create an oxygen rich environment. The company offers a complete system with 12 or 30 gallon capacity targeted for use by smaller farmers with biointensive operations.

In addition to package setups that include vermicompost as a basic feedstock, Soilsoup provides research recommendations for others, including cattle manure and mushroom composts that have been said to be effective in trials with specific plant pathogen infestations. Jerry Erickson, a partner in the compnay, has also developed a Bio-Blender operated off a 12 volt battery and solar panel that the company would ultimately like to see used for small plots in developing countries.

Though simple in nature, teas must match the plants to which they are applied: Trees and shrubs require a fungal dominated soil to thrive; vegetables, turf, and row crops do best in bacterially dominated soils; and perennials take a balance of the two. Hence the nutrient rich teas must be mixed by recipe, taking the compost feedstock into account as well, to produce the type of brew appropriate to either the plants’ nutritional needs or state of disease. Both Soilsoup and GSI offer premeasured, packaged nutrients to be added before the brewing process begins.

Both companies recommend a dilution ratio of 10:1 to provide the best results. In other words, five gallons of tea concentrate will yield 50 gallons of foliar spray. There is some variability with dilution as GSI’s estimate is based on the vineyard industry, where there is a moderate density in the canopy of grapes mid season. Other situations may need more or less and users are encouraged to experiment with dilution ratios and application rates as well as compost feedstocks. Erickson recommends that cropland, depending on conditions, receive about ten to 20 applications per year.

Growing Solutions collaborates with Elaine Ingham of Soil Foodweb, Inc. in Eugene, Oregon, whose laboratory has been working on a variety of projects and field trials to accurately document tea results. Soil Foodweb tests soils and composts to determine what bacterial or fungal organisms are present and in what ratio.


Bainbridge Island grower Biggert and another organic farmer have used compost tea successfully for two years to combat gray mold and tomato blight. The farmers tailor made tea made from vermicomposting about 20 pounds a day of both crop residues and dairy manure for use on a variety of food crops, either as a foliar spray or a soil drench using a roto-tiller drawn sprayer.

With his farm on tight clay soil, Biggert faces major problems with buttercup infestation. “Applying compost tea monthly has increased the tilth and significantly decreased the buttercup presence in my fields,” says Biggert. Using a tank sprayer with both a boom and wand spray, he applies tea year-round, gauging the frequency with the crop, its level of maturity, and presence or absence of disease. At season’s end, he drenches his soil and also inoculates his cover crop for his one and one-half acre operation. “Fresh, finished compost requires the least amount of time in the blender to produce great tea,” he notes.

In Harmony Landscape Services of Seattle offers clients an organic approach to tree, shrub, and lawn care. “Our focus is reducing or eliminating as many pesticides as possible while teaching our clients to be more tolerant of some weeds,” asserts partner Ladd Smith. The company serves a thousand clients in the greater Puget Sound area, two-thirds of whom desire regular care and maintenance of lush green lawns. The landscaper jumped on the idea of using compost tea as soon as he learned about it.

“I brew 50 gallons a day of bacterially dominated tea. Then using chlorine free water and a 10:1 dilution rate, I divide the 500 gallons among my service vehicles,” says Smith. “We have 12 to 15 hours after brewing to apply it with hose sprayers, so it’s a pretty good window for applications.” In this mild maritime climate, In Harmony crews may do lawn maintenance year round, using compost tea spray five to seven times annually for clients. He is presently using compost from a small Washington fruit orchard, two cubic yards of which keeps the operation in tea for about six months. Smith is interested in finding other compost sources that will test well for either bacterial or fungal life. “Right now I’m tending to think that smaller, more niche market composters will probably have the quality feedstocks I need for tea,” he speculates.

Hendrikus Schraven, president of Schraven Landscape Construction and Design in Issaquah, Washington, is sold on the advantages of compost tea. Using a 100-gallon spray pump and a reel hose, Schraven’s crew has used tea for two years to inoculate leaf, stem and soil on all new installations once planting and initial fertilization is completed. For maintenance clients, his crew sprays a bacterially dominated tea in late spring after plants have leafed out to help jump-start the growing season, and again in late fall, after most leaf drop has occurred, with a more fungally dominated tea to promote biodegradation on the ground.

Schraven has used compost tea in a variety of applications, including sliding hillsides. In one extreme project, his company had to contend with inaccessibility, exposure from weather off Lake Washington, heavy rainfall, and a cool planting season. In September, 1997, his crew was suspended with climbing gear down a 55 to 80 percent slope whose topsoil was sliding toward a waterfront house. After containing the 35 gallons a minute coming from multiple dispersed springs in the hill, the crew went to work to control erosion and recreate healthy habitat.

Using a three-way nozzle of his own design, Schraven’s crew sprayed the 210 linear feet of bare, uneven hillside one to two feet deep with his own soil mixture, water, and compost tea. The tea was a strong 1:3 dilution. The crew then “planted” worms in the new soil, and in mid-September hydroseeded the slope with a native grass seed mix. Beginning in October, unusually heavy rain fell for nearly two months; however, no erosion occurred on the hillside. In mid-December, after three cool, wet months, Schraven tested grass growth and found root mass four to five inches deep.


The effectiveness of compost tea in many anecdotal situations has been substantial enough to pique the interest of local governments and schools. The city of Eugene, Oregon uses tea for its rhododendron garden, and the Agricultural Research Service at Oregon State University purchased four Microb-Brewers last season and is starting trials now. In Washington, Snohomish County, the Port of Seattle, and the city of Seattle have all purchased units for use in their parks and grounds departments. According to Edward Hook of the Seattle Office of Environmental Management, the opportunities are wide open to test the tea.

Hook plans to take the tea out for trial studies on city golf courses, the city rose garden, and in greenhouse propagation. There is great interest in its efficacy to suppress both fungal turf diseases and rose ailments such as mildew and black spot. “I anticipate using compost tea to inoculate root balls prior to planting and in back filled soil,” he says. Following staff training and program setup, the first applications occurred in June 2000. “Compost tea isn’t a silver bullet,” cautions Hook. Good, regular care and maintenance procedures are equally critical to healthy plant growth. Further, he believes anyone using tea shouldn’t overlook the value of soil testing on a somewhat regular basis.

Northward in Snohomish County, planner Sego Jackson has stimulated the interest of the parks department, whose staff wants to try tea as a replacement for some biocide and fertilizers on an experimental basis. With an eye toward cutting fertilizer discharge in local waterways, Jackson plans to make one of the tea brewers available to Master Composter volunteers to use personally and demonstrate publicly. The volunteers will be asked to track use and results, with comments about ease of use and other parameters.

Finally, in a significant move, the USDA’s Western Sustainable Agriculture, Research and Education division awarded funding for a three-year study of the efficacy of tea in combating Botrytis on Oregon wine grapes. Field trials scheduled to begin early next year should keep interest in compost tea high and help provide the hard research needed to better assess this new age brew.

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