story to find out more.
In an earlier composting study, microbiologist
Patricia Millner and research assistant David Ingram examine bacterial colonies
on nutrient media to detect and count various pathogens in manure samples
before composting. Similar examinations are later conducted on finished compost
to ensure that pathogens have been killed. Click the image for more
information about it.
Recommendations for a Safer Compost Tea
September 21, 2006
Compost tea is a brew favored by
many organic growers. It's made by adding small amounts of mature compost to
unheated water and leaving it to sit, or steep. The finished tea is
then applied as a foliar spray or soil drench to promote plant growth and
Now new recommendations for making compost tea are being offered, thanks, in
part, to research conducted by Agricultural Research Service (ARS) microbiologists
Millner. Their studies had shown that additives sold for making compost
teasuch as soluble kelp, fish hydrolysates, humic acid, rock dust and
proprietary nutrient solutionscan spur the growth of bacteria.
Generally, composting generates enough heat to reduce potentially harmful
bacteria. But Ingram and Millner, with the ARS
Microbial Safety Laboratory in Beltsville, Md., found that ingredients
commonly added to compost tea may promote growth of a variety of microbes,
including pathogens that can cause illness in humans.
Experiments showed that when compost with very low numbers of
Salmonella and Escherichia coli was used to make compost tea
(fewer than two cells per milliliter of tea), the pathogens multiplied when
additives were included in the initial water mixture. However, they remained
undetectable in all the compost teas made without commercial additives.
According to Ingram, this work counters the view among some compost
tea-producers that the aerobic bacteria in compost will inhibit growth of human
pathogenic bacteria when aerobic conditions and nutrient additives are present.
Compost tea supplements can give even a few pathogenic bacteria a boost, so
testing of the final tea before application may be necessary to ensure the
absence of human pathogens.
Recommendations and guidelines for safe production and use of compost tea
have been developed by the Compost Tea Task Force, formed by the
National Organic Standards Board.
more about the research in the September 2006 issue of Agricultural
ARS is the U.S. Department of
Agricultures chief scientific research agency.