Compost tea is a liquid made by soaking compost in water and draining to save the murky liquid. This “tea” contains some nutrients and presumably some beneficial bacteria. You can strain the liquid through, say, a burlap bag as described in the above link, or you can buy it ready made at plant sales and some garden stores. You can also buy compost tea kits to make your own.
Does compost tea work?
Jeff Gillman, in his book The Truth about Garden Remedies, says that the theory is that the beneficial bacteria can kill various plant diseases, such as powdery mildew, bacterial spot on tomatoes and apple scab. Research, however, seems to show that the only effect actually reported is on powdery mildew, “which sometimes ...is controlled and sometimes made worse.”
He points out that every batch of this tea can be different because it depends on your compost and your technique. Further, you don’t know what microorganisms will be in your compost or already in your garden. And it could also include human pathogens, which might not be good at all. He concludes that the existing research seems to show that “the jury is still out."
Linda Chalker-Scott of Washington State University reviews the actual scientific evidence on her Horticultural Myths page. She discusses both aerated and non-aerated teas, but finds that the research is inconclusive, with some success in Petri dishes, but much less in the greenhouse and field. Clearly the #Science is not strong for aerated tea use on crop plants, much less on lawns, shrubs and trees. Her advice to Master Gardeners is not to recommend them because the science is so weak. She notes that it is illegal to sell unregistered substances as pesticides and that the danger of including human pathogens such as E. coli is significant.
Even the University of Connecticut Ag extension service is cautious about such teas, particularly cautioning about the dangers of making home compost from manure because the composting temperature may not be high enough to kill any human pathogens in the compost. Such pathogens may very well be responsible for a number of the recalls of organic vegetables around the world, where the manure is mishandled.
In other words, don’t waste your time (and money) on compost teas. #Health