2005: The debate surrounding compost tea used
to linger solely around its effectiveness in controlling
plant fungal diseases and other pathogenic systems --
that was until reports of compost teas contaminated
with E. coli starting emerging. Focus quickly
shifted from efficacy to safety. Concerns that tea usage
could lead to bacterial contamination of food crops
caused such an outcry within the organic community that
the National Organic Standards Board (NOSB) created
a Compost Tea Task Force and a slew of research experiments
were set up to investigate the situation.
In April of 2004, the Compost Tea Task Force released
its recommendations—which included using potable
water, sanitizing equipment and brewing areas and following
additional guidelines when using additives such as molasses—without
much available science to go on. Now newly published
research is emerging and at least initial studies are
backing the committee’s common sense approach.
One such study, conducted at the Woods End Research
Laboratory in Mt. Vernon, Maine, and titled, “Compost
Teas: Microbial Hygiene and Quality in Relation to Method
of Preparation,” was published in the summer 2004
edition of Biodynamics. Dr. William Brinton and team
set out to determine the potential health risks of compost
tea by recording the bacterial levels of several preparation
methods. The methods studied were commercial compost
tea kits, commercial kits in which farm-made compost
was substituted for purchased materials and a European
method in which homemade compost is steeped in water
and lightly stirred for several days. The results showed
that after 24 hours of aeration and 36 additional hours
of steeping none of the preparation methods tested positive
for E. coli provided the ingredients were not
contaminated prior to mixing. The researchers found
that small amounts of E. coli in the tea will
actually decrease to the point of extinction in 72-120
The results of the study lead researchers to conclude
that hygiene is the greatest threat facing compost teas.
When teas were produced with bacteria-free water and
compost in a laboratory setting, E. coli was
virtually undetectable. On the other hand, when E.
coli was present in the compost or water at the
time of brewing, the process served as an incubator
and bacteria levels grew rapidly. In order to reduce
the risk of bacterial contamination researchers stressed
the importance of clean handling techniques including
washing tools and storing brews in areas protected from
outside contamination and using composts that have been
tested for E. coli.
Although supplemental tea materials were not the focus
of the study, researchers also found support for the
cautious approach taken by the task force with regard
to the use of additives, especially molasses, in compost
teas. Based on the findings that showed the European-style
teas to actually inhibit the growth of E. coli
over time, researchers hypothesized that the lack of
available carbon (provided by the molasses) and the
absence of active aeration may have created an inhospitable
environment for E. coli and hindered replication.
Researchers recommended avoiding teas made with a brew
time of 24 hours or less and those using molasses additives,
determining that these popular brews may in fact be
among the least safe and—according to previous
research*—least effective methods of producing
In October, the NOSB voted to accept the recommendations
of the Compost Tea Task Force and is now working to
merge the April report with its previously established
guidelines on Compost Tea.
*Weitzien, H.C. 1963. Untersuchengen über die Uraschen
der keimhemmung von Pilzsporen im Boden. Zbl. Bakt.
Parasit. Infekt. Hyg. 116:131-170.