COMPOST TEA
MICROBIOLOGICAL INTERPRETIVE GUIDE
What is compost tea? Compost tea is typically made as an aqueous aerobic extraction of compost and contains both the soluble nutrients from the compost and the extracted portion of the microorganisms from the compost. Many times compost tea also contains nutrient supplements to help the microorganisms to grow after they are extracted.
Why is compost tea used? Compost tea is used as a nutrient component for the plant and a microbial inoculant. Some compost teas contain organisms that specifically assist in activities such as stimulating plant growth and suppressing plant disease. Other compost teas may provide a general microbial population that serves to out-compete some plant pathogens for nutrients and other resources rather than to specifically inhibit the growth of a pathogen.
How is compost tea applied? Compost tea is used both as a soil drench and as a foliar spray. It may be used full strength, although it is often diluted prior to use.
How does microbiological quality impact compost tea? Since most compost tea users are depending on compost tea to serve as a microbiological inoculant of either soil or foliage, the concentration and variety of microorganisms present have a very significant impact on the compost tea performance.
Microbial Concentrations
FUNCTIONAL GROUP
INTERPRETATION OF COMPOST TEA BIOASSAY
Heterotrophic Bacteria (Aerobic)
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[/FONT]>10 million (107) Colony Forming Units/milliliter (CFU/ml)
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[/FONT]Insufficient numbers of these organisms may leave the tea without enough microbial coverage for good foliar pathogen suppression.
Anaerobic Bacteria
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[/FONT]Ratio of Aerobes to Anaerobes in the compost tea should be at least 5:1 or greater
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[/FONT]Too many anaerobes in the tea are an indication of insufficient aeration, too much nutrient supplement, or a combination of both.
Fungi (filamentous)
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[/FONT]>1000 (103) CFU/ml
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[/FONT]These organisms, while important, prefer to grow on a solid surface in their filamentous form and not in a turbulent liquid medium. Therefore, concentrations of filamentous fungi are usually significantly lower than would be expected from the starting compost. Yeasts, which are non-filamentous fungi, grow readily in compost tea.
Actinomycetes
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[/FONT]>100 (102) CFU/ml
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[/FONT]Actinomycetes, an important part of compost, do not extract and grow well in most compost teas.
Pseudomonads
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[/FONT]>1 million (106) CFU/ml
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[/FONT]Pseudomonads are important in nutrient cycling, plant growth promotion (rhizosphere), and the biological control of plant pathogens. They extract and grow well in compost tea.
Nitrogen-Fixing Bacteria
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[/FONT]>100,000 (105) CFU/ml
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[/FONT]Populations of these free-living nitrogen-fixing bacteria will increase as the available nitrogen in the compost tea decreases.