January 7, 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 hours.
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.