Does compost tea work against plant disease?
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Only the simple steeped version has support. A review in Compost Science & Utilization found non-aerated compost tea suppressed a range of foliar and soil-borne pathogens, while very few controlled studies existed for aerated tea. University trials tabulated by Washington State University Extension then found aerated compost tea ineffective in 16 of 18 crop and disease combinations tested.
What do people believe about compost tea?
Compost tea, especially actively aerated with a pump and molasses, suppresses plant disease and inoculates soil with beneficial microbes.
What does the evidence show?
The distinction between non-aerated and aerated tea is the whole story, and it usually goes unmentioned.
The foundational review in Compost Science & Utilization distinguishes the two and finds a range of foliar and soil-borne pathogens suppressed by non-aerated compost tea, while noting that very few controlled studies existed for aerated tea.
Washington State University Extension then tabulated university trials from 2003 to 2005 - Cornell, Iowa State, Ohio State, Michigan State, Penn State, Rutgers and Minnesota - and found aerated compost tea ineffective in 16 of 18 crop and disease combinations tested.
So the version with some support is the simple steeped one, and the version that attracts equipment, additives and enthusiasm is the one that failed.
What is the safety issue with compost tea?
Two points from the WSU review deserve prominence. Aerated compost tea is not registered as a pesticide, so disease-control claims for it are not permissible. And some batches harbor E. coli - brewing with molasses at ambient temperature is an effective way to multiply whatever was in the compost, including human pathogens. On food crops, that is not a theoretical concern.
What works instead?
Use the compost. Applying compost directly delivers the organic matter, nutrients and microbial diversity that tea is supposed to extract, without the pathogen risk or the equipment. Where a specific biological control is wanted, use a characterized registered product with a known organism and a known count.
Key takeaways
- Non-aerated compost tea suppressed a range of foliar and soil-borne pathogens in the foundational review.
- Aerated compost tea was ineffective in 16 of 18 crop and disease combinations in university trials from 2003 to 2005.
- Aerated compost tea is not registered as a pesticide, so disease-control claims for it are not permissible.
- Brewing with molasses at ambient temperature can multiply E. coli and other human pathogens present in the compost.
- Applying compost directly delivers the organic matter, nutrients and microbes without the pathogen risk.
References
Frequently asked questions
Is aerated compost tea better?
No, the evidence points the other way. Non-aerated tea has support for suppressing foliar and soil-borne pathogens, while aerated tea was ineffective in 16 of 18 crop and disease combinations tested in university trials.
Is compost tea safe on edible crops?
It carries a real food safety risk. Some batches harbor E. coli, and brewing with molasses at ambient temperature is an effective way to multiply whatever was in the compost, including human pathogens. On food crops, that is not a theoretical concern.
Is compost tea registered as a pesticide?
Aerated compost tea is not registered as a pesticide, so disease-control claims for it are not permissible. Where a specific biological control is wanted, use a characterized registered product with a known organism and a known count.
What should you use instead of compost tea?
Apply the compost itself. It delivers the organic matter, nutrients and microbial diversity that tea is supposed to extract, without the pathogen risk or the brewing equipment.
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