MICROBIOLOGICAL QUALITY 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.
INTERPRETATION OF COMPOST TEA BIOASSAY
¾ >10 million (107) Colony Forming Units/milliliter (CFU/ml)
¾ Insufficient numbers of these organisms may leave the tea without enough
microbial coverage for good foliar pathogen suppression.
¾ Ratio of Aerobes to Anaerobes in the compost tea should be at least 5:1 or
¾ Too many anaerobes in the tea are an indication of insufficient aeration, too
much nutrient supplement, or a combination of both.
¾ >1000 (103) CFU/ml
¾ These organisms, while important, prefer to grow on a solid surface in their