Compost Teas Production Practices
Dr. Steven Scheuerell
Oregon State University
Compost tea requires compost, water and time but additional options, including added nutrients, aeration, filtration, dilution, spreaders and stickers and tank mixes, may be used. The primary goal of brewing compost tea is to obtain nutrients and cultivate the beneficial microorganisms found in compost for application to nursery and farm crops. Anecdotal evidence and some industry data indicate that compost teas can have an impact on yield; however, as compost teas are relatively low in nutrients, mechanisms such as changes in soil biology or increased nutrient efficacy probably play a role. Laboratory and replicated field studies indicate that compost teas can induce disease suppression but the effectiveness varies with production method, amendment, application, environment and pathogen. Additional research is needed to better evaluate how the stability of the compost, brewing techniques, and application timing and methods affect the benefits of compost teas.
Discussion:
Brian McSpadden-Gardner (OSU) asked if the benefits for foliar diseases depend the number of microbes on the plant surface. He also questioned whether the potential for pathogens in the compost tea or metabolites being produced in situ are a concern for food safety. Scheuerell noted that for Botrytis and Pythium, research has indicated that the number of compost tea organisms colonizing a plant has been positively correlated to disease control. For concerns about human pathogens, if they were present in compost tea then they could be applied to plants, however, survival of pathogens in this situation has not been investigated under agronomic conditions. Currently, there is no data indicating whether metabolites are created by the compost tea organisms on plant surfaces.
Harry Hoitink (OSU) asked if, for humid climates, the rain splashing on manure applied to fields acts as a "natural" tea. Such practices are not usually associated with contamination of crops. Scheuerell noted that the issue is complex and more data is needed that identifies the organisms present on the plant surface and the survival rate for varying conditions.
Fred Michel asked if non-aerated compost teas are anaerobic. Scheuerell indicated that the dissolved oxygen content in non-aerated systems depends on the stability of the compost, amount of water and the addition of fermentation nutrients. If the compost is relatively stable and the compost to water ratio is at least 1:10, the dissolved oxygen content is usually above 4 mg/kg (ppm).
Hoitink noted that the literature indicates that if compost is stable, coliform bacteria will not be a problem even with fermentation, but if soluble nutrients that support coliform growth are added, there may be a problem. Also, yard waste can support salmonella populations, so manure does not pose the only risk of contamination. Scheuerell noted that the literature does address regrowth of pathogens in composted biosolids in relation to compost stability. The relationship between stability of the compost used to make compost tea and the potential to support multiplication of human pathogenic bacteria in compost tea has not been evaluated. While commercial compost tea systems usually test clean, there is still a need for data to assess the risk across a range of compost tea production practices.
Sally Miller (OSU) asked for recommendations for compost teas, especially in nurseries with mini vegetables where damping-off is problem when watering is not done properly. Scheuerell responded that there are not specific recommendations and suggested discussing the pros and cons with the producer. Although there are no known contamination issues nor restrictions for conventional growers, they should be aware that there is a potential risk and that there could be liability issues. All available data indicate that if no nutrients are added to the tea, there is not a problem with increasing coliforms. For organic growers, the use of compost teas has yet to be clarified in the regulations, so the matter should be discussed with the certifier.
Miller asked whether damping off control is possible with no added nutrients. Scheuerell noted that it has not been demonstrated. He suggested that the best potential method to obtain sufficient quantities ofmicrobes, without adding nutrients that could support human pathogens, is to use high quality compost, a high ratio of compost to water, agitate extensively for about an hour, filter, and spray. Another compost tea production method that should be investigated for avoiding the growth of coliforms, yet obtaining high populations of beneficial organisms, is the use of cool water and relatively short culture times. A consultant in California has been successful in
producing compost tea using a process that includes 6-8 hours of active aeration, 50-60 degrees F water, and additions of molasses and kelp. In addition, in the west, many producers focus on applying compost tea to the rhizosphere using surface or buried drip irrigation, so there is little risk of surface contamination.
Matt Kleinhenz (OSU) asked whether potato growers had been assertive in adopting the compost tea application practices used by one supplier of compost tea in Idaho that resulted in significant increased crop yields. Scheuerell noted that the data he had shown is not widely available but is available to him because of a close working relationship with the company. Other Idaho growers trying compost teas have had logistical problems with the fledgling compost tea industry that have resulted in some negative attitudes toward the use of compost teas. For this reason, once growers decide that compost tea can provide them measurable benefits, they usually chose to make their own compost tea to reduce costs and be in control of quality and timing.
Notes by Mary Wicks
The OCAMM (Ohio Composting and Manure Management) Program is funded by OARDC/OSU.