By Asha Kreiling
Compost tea is defined as a nutrient-rich water extract by brewing or steeping compost. Compost tea can be differentiated into three types:
- Manure tea: a water extract of animal manure, including earthworm castings and bat guano, that can contain soluble nutrients such as nitrogen, phosphorus, and potassium but it also contains high levels of bacteria, root-feeding nematodes, and often human and animal pathogens.
- Compost extract: tea produced by draining water through a compost pile that contains soluble nutrients nitrogen, phosphorus, and potassium but very few beneficial microorganisms.
- Compost leachate: this is produced when water drains from over-saturated compost. Typically, leachate contains only soluble nutrients and few organisms. This leachate is often anaerobic which allows for the growth of certain pathogens. 1 Applying leachate from raw manure during the growing season is not recommended.9
Compost tea is used as either a foliar spray or a soil drench to promote plant growth and control foliar and root diseases. Using good quality compost will provide beneficial microorganisms and nutrients to the surface of the plant.2 When sprayed on the leaves, compost tea will immediately impact the plant and can help suppress foliar diseases, increase the amount of nutrients available to the plant, and speed the breakdown of toxins. Compost tea can be applied as a soil drench to develop the biological barrier around roots, improve plant growth, and improve soil biology in general.1 Using compost tea has been shown to increase the nutritional quality and improve the flavor of vegetables.
The University of Hawaii conducted laboratory work to study common assumptions of the use of compost tea and found that the application of tea to the root zone can increase plant yield and root growth significantly when applying a water:compost ratio of 10:1 -100:1. Soil biological activity increases with the use of compost tea. A study done by BioCycle at Ohio State University showed that the use of vermicompost tea decreased the amount of damage by spider mites, suppressed aphid populations significantly, and reduced root knot galls on tomato roots. Plants treated with regular compost tea had less substantial results.11 Aerated vermicompost tea has been shown to induce significant plant growth responses compared to nonaerated teas.10
However, a poor quality compost tea may be supplying the plant surface with unwanted components such as salts and problem microorganisms, and the presence of human pathogens is a potential hazard when consuming fresh produce.3 If composted improperly, compost can contain E. coli, salmonella, and other pathogens. If human pathogens are present even in small concentrations in the compost used to prepare compost tea and the conditions of compost tea production allow pathogenic growth, then the likelihood of contaminants being retained on foliage increases substantially when the tea is used as a spray. Therefore, control of foodborne pathogen growth during compost tea production is essential for reducing potential hazards of compost tea application. Implementing certain measures can reduce the potential for pathogens to enter compost tea.4
The use of compost tea supplemental additives, particularly molasses, stimulates plant-beneficial microbial populations, but also favors growth of human pathogenic bacteria. Studies have shown that regrowth of Salmonella and E. coli is positively correlated with molasses concentration in compost tea. 5
Aerating compost tea can breed a larger population of beneficial aerobic bacteria and fungi. Organisms consume the majority of the oxygen during aerobic metabolism, so using a bubbler to replenish the oxygen at a faster rate than what is consumed by the bacteria, the tea will remain aerobic.1 Aerated teas are not free of potential pathogens though, because E. coli is readily cultured under aerobic conditions. According to a study done at the Woods End Research Laboratory, if a small amount of E. coli is introduced into a tea, its population levels will decrease to the point of extinction in 72-120 hours. With adequate aeration maintenance, E. coli will not survive in the tea because of competition and predation by other organisms. These results were independent of the presence or absence of aeration.6
The Agricultural Marketing Service of the USDA advises that compost tea made without tea additives can be applied without restriction, whereas compost tea with additives should be pre-tested to produce compost tea that meets the EPA recommended recreational water quality bacteria guidelines of fecal contamination. Raw manure extracts and compost leachates may be applied to the soil with a 90-120 day pre-harvest restriction, and foliar applications are prohibited. 7
Compost tea made from aerating worm castings is best made from mature, high quality vermicompost. The material should be a fairly odorless, dark earthy compost that resembles coffee grounds. Potable water should be used when making compost tea, and all equipment used should be sanitized. As discussed earlier, compost teas brewed with additives should be used cautiously. Application to the root zone reduces risk of microbial contamination of leafy greens.High quality vermicompost tea can help promote germination growth, flowering, plant yields, and root growth.Plant diseases, parasitic nematodes, and arthropod pests such as mealy bugs, spider mites, and aphids can all be suppressed using vermicompost tea. 8
In conclusion, the quality of the compost used to make compost tea is extremely important. Using good mature aerobic compost without animal manure reduces risk of spreading contaminants into the tea brew. Application to the root zone is preferable to spraying leaf surfaces in order to maximize efficacy and reduce risk of microbial contamination of leafy greens.8 Aerated tea made from vermicompost has shown to produce the most beneficial effects on plant growth while reducing the chance of spreading disease compared to other compost teas.
1 Ingham, Elaine. (2000). Compost tea brewing manual. Corvallis, OR: Soil Foodweb, Inc. http://www.reboreda.es/Documentos/Manual,%20te%20de%20compos.%20Edition%203.pdf
2 Ingham, Elaine. “Brewing Compost Tea: Tap your compost pile to make a potion that is both fertilizer and disease prevention.” Kitchen Gardener. Oct/Nov 2000: 16-19. Print. (http://www.wnps.org/restoration/documents/GD/GD_Proj_Notes/GD_l5.pdf)
3 Bess , Vicki. “Understanding Compost Tea.” BioCycle Jounral of Composting & Organics Recycling (2000): 71. Web. 9 Aug 2011. <http://www.jgpress.com/BCArticles/2000/100071.html>.
4 Ingram, D.T., and P.D. Millner. “Factors Affecting Compost Tea as a Potential Source of Escherichia coli and Salmonella on Fresh Produce.” Jounral of Food Production 70.4 (2007): 828-834. Web. 9 Aug 2011. <http://ddr.nal.usda.gov/dspace/bitstream/10113/18297/1/IND43906748.pdf>.
5 Duffy, Brion, Chester Sarreal, Ravva Subbarao, and Larry Stanker. “Effects of molasses on regrowth of E. coli 0157:H7 and Salmonella in compost teas.” Compost science and utilization 12.1 (2004): 93-96. Web. 9 Aug 2011. <http://cat.inist.fr/?aModele=afficheN&cpsidt=15598691>.
6 Brinton, W., P. Storms, E. Evans, and J Hill. “Compost Teas: Microbial Hygiene and Quality In Relation to Method of Preparation.” Journal of Biodynamics 2. (2004): 36-45. Web. 9 Aug 2011. <http://www.woodsend.org/pdf-files/compost-tea-BD04R.pdf>.
7 National Organic Standards Board. “Compost Tea Task Force Report” (2004). < http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5058470>
8 Radovich, Ted, Archana Pant, Nguyen Hue, Jari Sugano, and Norman Arancon. “Promoting plant growth with Compost Teas.” Food Provider Mar-Apr-May 2011: 1-3. Web. 16 Aug 2011. <http://www.ctahr.hawaii.edu/sustainag/news/articles/V7-Radovich_etal-CompostTea.pdf>.
9 Nikolic, Jovan. “Overall study of compost water extract application though fertigation system in organic farming.” Jul 2003:Web. 16 Aug 2011. <http://orgprints.org/6661/1/JovanNikolic03.pdf>.
10 Arancon, Norman, Clive Edwards, Richard Dick, and Linda Dick. “Vermicompost Tea Production and Plant Growth Impacts.” Biocycle Nov 2007: 51-52. Web. 16 Aug 2011. <http://growingsolutions.com/shop/images/bc0711_51.pdf>.
11 Edwards, Clive, Norman Arancon, Eric Emerson, and Pulliam Ryan. “Suppressing Plat Parasitic Nematodes and Arthropod Pests with Vermicompost Teas.” Biocycle Dec 2007: 38-39. Web. 16 Aug 2011. <http://growingsolutions.com/shop/images/bc0712_38.pdf>.