Claims checked › Claim
Compost tea feeds the soil and suppresses disease
“Spraying aerated compost tea feeds the soil food web, suppresses plant disease and replaces fungicide.”
Five hundred and sixteen case studies say the effect is real sometimes and unpredictable always. The confident version of the claim comes from brewers and consultancies, aeration turns out to matter less than the compost used, and molasses can grow pathogens in the tank.
Where it comes from
Compost tea brewers, "soil food web" consultancies and microbial-inoculant vendors, whose material dominates what a farmer finds online. The idea has a real research literature behind it, which the marketing quotes selectively.
Who benefits
The people selling brewers, additives and courses. Compost tea is cheap to make and expensive to be taught.
What the evidence says
The most comprehensive synthesis to date, 39 peer-reviewed studies and 516 case studies from 2010 to 2025, concludes that compost tea has "biological potential" but "substantial variability that limits its reliable deployment"; that the field "cannot consistently forecast when a given compost tea will achieve commercially relevant control versus marginal suppression"; that aeration status is "secondary to feedstock quality"; and that bacterial and viral diseases are entirely unstudied and perennial crops "grossly underrepresented" (Applied Soil Ecology, 2026). Field trials in the reviews show the pattern: no significant reduction of mummy berry in blueberry; mixed results on pecan scab, generally below fungicide performance (WSU compilation of the literature). One study in that compilation found that molasses added to aerated tea regrew E. coli O157:H7 and Salmonella, a food-safety hazard on crops eaten raw.
Compost tea also has no drought mechanism. It adds no organic matter, does not change infiltration or water-holding capacity, and cannot do what the bulk compost it is made from does.
What to say instead
"Use the compost; skip the tea." Bulk compost has 21 years of European trial data and the only farm-account win on this site; the tea has an unpredictable disease effect and a pathogen risk. If you want disease suppression, the compost itself, worked into the soil, has the better record. The compost page grades the two separately.