Recommendations for a Safer Compost Tea

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 suppress microbes.

Now new recommendations for making compost tea are being offered, thanks, in part, to research conducted by Agricultural Research Service (ARS) microbiologists David Ingram and Patricia Millner. Their studies had shown that additives sold for making compost tea—such as soluble kelp, fish hydrolysates, humic acid, rock dust and proprietary nutrient solutions—can spur the growth of bacteria.

Generally, composting generates enough heat to reduce potentially harmful bacteria. But Ingram and Millner, with the ARS Environmental 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.

Read more about the research in the September 2006 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.