A book from the 1950s by J.I. Rodale called the Encyclopedia of Organic Gardening refers to ‘compost water.’ “It is no trouble to make,” writes Rodale. “All you have to do is fill a sprinkling can half with finished compost and half with water.” Rodale points out that some of the valuable nutrients in compost can quickly dissolve in water, and in solution these nutrients are readily available for plant uptake.
For decades farmers and gardeners referred to this material as compost tea, and it has been used for watering house plants or spraying acres of field crops. In recent years almost everything about it has become sophisticated: How to make it, how to use it, its benefits, and, recently, its risks. And, of course, now that USDA has control of organic certification, regulations are suggested when compost tea is used on commercial organic farms. Perhaps the most interesting development is the discovery that compost teas can suppress plant diseases.
Types of Compost Teas
A good place to start the discussion is to present some new terminology that distinguishes among materials that were once all called, simply, compost tea. First let’s distinguish a very different material called compost leachate. This is the dark solution that leaches from the bottom of the compost pile. It is not compost tea. Most likely it is rich in nutrients, but it will not offer the many other benefits associated with true compost tea. Also, compost leachate may contain high levels of plant and human pathogens that may have been in the feedstock.
Traditionally tea was made from a porous bag of compost suspended in a barrel of water for about 7 to 14 days. New, widely accepted terminology distinguishes two materials, based on preparation methods, that used to be lumped together as compost tea. Both start with well made and finished compost. I will adopt the terminology used by Scheuerell (2002) and refer to them as nonaerated compost tea (NCT) versus aerated compost tea (ACT). The extraction method for NCT has become sophisticated by the work of German researcher Heinrich Weltzein. Many variations of NCT exist, but a basic description from an ATTRA bulletin (Diver 2002) suffices: “Compost teas were obtained by covering compost with tap water at a ratio between 1:5 to 1:8 (volume/volume). They were stirred once and allowed to ferment outdoors between 15° and 20° C (59-68° F). After a soaking period referred to as “extraction time” the solution was strained through cheesecloth and then applied with ordinary sprayers. Extraction periods ranged from 2 to 21 days, although most were between 3 to 7 days.”
The ACT method, described in the same ATTRA bulletin, first gained favor among a number of farmers and advisors on the West Coast. This “aerobic method” can be accomplished in several ways. Amigo Cantisano described this method at the November 1995 Acres, U.S.A. Conference in St. Louis, Missouri: “Compost teas are prepared with a heavy emphasis on aeration. A 12-inch-wide PVC pipe is cut in half lengthwise, laid on its side, and mounted several (at least 4 feet) above a tank that will hold the compost tea leachate. Next, numerous holes are drilled into the bottom of the PVC pipe to allow for drainage. Burlap bags containing compost are placed inside the trough created by the PVC pipe. A water line is run horizontally along the top of the trough. As the water collects and then runs through the burlap bags containing the compost, a leachate is created which then drops 4 feet through the air into the tank below. A sump pump in the bottom of the tank collects the leachate and distributes it back through the water line at the top of the trough, and so on. Through this process, which lasts about 7 days, the compost tea is recirculated, bubbled, and aerated.” Many homemade systems accomplish this aeration, and many kinds of machines are marketed now to produce ACT. (See the ATTRA bulletin by Diver, 2002, for a list of compost tea brewing equipment suppliers.)
Disease Control Benefits of Compost Tea
Compost teas are gaining increased attention among gardeners and large scale organic crop growers as a tool to control plant diseases. Weltzein (1990), Brinton (1995) and Scheuerell (2002) have summarized the benefits of compost tea. Benefits seem to relate to biological control, primarily, accomplished by the wide array of microorganisms – bacteria, yeasts and other fungi – living in the tea. The tea inoculates the plant surface with these organisms. Many species are involved, and studying the mode of their activity is a young science, but some species are assumed to directly antagonize pathogenic organisms, while others simply outcompete pathogens for space or food on the plant surface. In addition, research shows an induced plant defense in response to tea sprays, such as lignification in cucumber leaves. Another study demonstrated a nonbiological control as well, because tea that was heated or filtered to inactivate biological mechanisms still reduced diseases.
Disease suppression was not absolute in many of these studies. In many cases particular diseases were not controlled, or particular teas were no more suppressive than the control. Also, very little research compares the disease suppression of ACT with that of NCT; research that does compare the two is often conflicting. In fact, Scheuerell (2002) demonstrated that the source of compost had a greater effect on activity of the tea than aeration did. He and Diver (2002) both report that animal manure-based compost had better results than compost made from only plant material.
Scheuerell (2002) reviewed the disease control activity reported in research on ACT and NCT. He reported whether tea suppressed the disease more than the control . Table I gives some examples from Scheuerell relevant to crop production in Maine:
|Table I. Did compost tea suppress the disease more than the control did? (modified from Scheuerell, 2002)
|Early blight on tomato
|Gray mold on bean
|Gray mold on strawberry
|2 yes, 2 no, 1 mixed results
|Gray mold on tomato
|3 yes, 1 no
|Late blight on potato
|1 yes, 2 no
|3 yes, 2 no
|Early blight on tomato
|1 yes in spring and 1 no in summer
Risks of Compost Tea
Escherichia coli 0157:H7, Salmonella and Listeria are now common household terms that have raised the fears of many consumers. Foodborne outbreaks of diseases caused by these organisms have been associated with the consumption of raw fruits and vegetables and have been linked to the use of uncomposted manure during the growing, harvesting and washing of these commodities. Composting manure effectively reduces pathogens to nondetectable levels (Lung, 2001), but composting is not sterilizing. It is reasonable to assume that very low levels and resting stages of these pathogens may be present, even though they may not be a health risk. When compost is land applied, the soil environment will not foster regrowth, and the low levels of pathogens will quickly wane. In contrast, when compost is brewed in a tea, conditions may favor growth of populations of pathogenic organisms. This was the case when simple sugars were added to the brewing tea. Bess (2002) and Duffy et al. (2002) brewed ACT with molasses and other simple sugars and demonstrated that these microbial nutrient supplements not only enhance the growth of beneficial bacteria in the tea but also support and may select for the growth of E. coli and Salmonella. Both authors suggest that eliminating sugars can reduce or eliminate the pathogens.
Ingham (personal communication) disagrees with the results of these studies. She claims that the degree of aeration is important, and that no risk is present if the brewed tea is kept properly aerated, which she claims the two studies mentioned above failed to do.
Use of the label “organic” on agricultural products is regulated by the USDA following 7 CFR Part 205, National Organic Program; Final Rule, which is affectionately referred to by many as The Rule. The Rule regulates the use of manure in crop production in Section 205.203(c). The entire Rule can be seen at the NOP Web site, www.ams.usda.gov/nop. The crux of the section says that the producer must manage plant and animal materials in a way that does not contribute to contamination of crops, soil or water by plant nutrients, pathogenic organisms, heavy metals or residues of prohibited substances. It describes waiting periods between applying raw manure and harvesting crops. But it states that the producer is exempt from the waiting period if the manure is composted.
The Rule does not specifically mention compost tea. As you can imagine, some people think that tea is then permitted without meeting the manure waiting period, because it is simply a mixture of two permitted materials, compost and water. And some think that tea is not permitted unless it meets the manure waiting period, because it is not specifically listed as an exemption. They point to the preamble of the Rule, which says, “The final rule does not contain provisions for the materials commonly referred to as compost teas.” Of course, the preamble to the Rule does not have true regulatory power. The National Organic Program has not clarified this confusion, so confusion persists.
The National Organic Standards Board (an advisory board to the National Organic Program) has created a task force to study this issue and make a recommendation. This task is difficult because membership of the Task force was built to represent the split views about tea safety seen in the organic community, and developing a recommendation for compost tea regulation has become increasingly difficult with recent health scares centered around food.
Potential Research in Maine
Chris Rheberg-Horton (Maine’s sustainable agriculture specialist; 581-2942) and I are considering some farmer-driven research into the efficacy of compost tea. If you are interested in participating, please contact one of us.
Citations, References and Suggested Reading
Bess, V.H., M. Manes, B.S. Richter and J.L. Snodgrass, 2002. “E. coli Survival in Compost Tea Using Different Nutrient Substrates.” Proceedings International Symposium of Composting and Compost Utilization.
Brinton, William F., 1995. “The control of plant pathogenic fungi by use of compost teas.” Biodynamics. January-February. p. 12-15.
Diver, Steve 1998. “Compost Teas for Plant Disease Control.” Appropriated Technology Transfer for Rural Areas (ATTRA) publication. (ATTRA publications can be obtained at www.attra.ncat.org or by calling 1-800-346-9140.)
Diver, Steve 2002, “Notes on Compost Teas: A Supplement to the ATTRA Publication ‘Compost Teas for Plant Disease Control.’” Appropriate Technology Transfer for Rural Areas (ATTRA) publication. (See above.)
Duffy, B., C. Sarreal, R. Stevenson, S. Ravva and L. Stanker, 2002. “Regrowth of Pathogenic Bacteria in Compost Teas and Risk of Transmission to Strawberry Plants.” Proceedings International Symposium of Composting and Compost Utilization.
Elad, Y., and D. Shtienberg, 1994. “Effect of compost water extracts on gray mould (Botrytis cinerea).” Crop Protection, Vol. 13, No. 2. p. 109-114.
Ingham, Elaine, Soil Foodweb, Inc. www.soilfoodweb.com
Lung, A.J., C.M. Lin, J.M. Kim, M.R. Marshall, R. Nordstedt, N.P. Thompson, and C.I. Wei, 2001. “Destruction of Escherichia coli 0157:H7 and Salmonella enteritidis in Cow Manure Composting.” Journal of Food Protection 64:9:1309-1314
Scheuerell, Steve, 2002. “Compost Teas and Compost Amended Container Media for Plant Disease Control.” Doctoral Dissertation, Oregon State University
Sideman, Eric, 1996. “Enhancing Natural Disease Control with Water Compost Extracts,” The Maine Organic Farmer & Gardener, March-May, p. 30-31, 41.
Weltzein, Heinrich C., 1989, “Some effects of composted organic materials on plant health.” Agriculture, Ecosystems and Environment, 27:439-446
Weltzein, H.C., 1990, “The use of composted materials for leaf disease suppression in field crops,” p. 115-120. In: Crop Protection in Organic and Low-Input Agriculture, BCPC Monographs No. 45, British Crop Protection Council, Farham, Surrey, England.
About the author: Eric is MOFGA’s director of technical services. You can contact him with your questions about farming or gardening at [email protected] or by calling 568-4142.