Using compost to make a foliar spray or soil drench to promote plant growth and suppress plant diseases has gained popularity in the past 10 years, especially among organic farmers. These “compost teas” are made by adding small amounts of finished, mature compost to unheated water and allowing the mixture to steep or brew.
Composting generally reduces pathogens. But Agricultural Research Service microbiologists David Ingram and Patricia Millner have found that ingredients commonly added to compost tea may promote growth of bacteria that can cause illness in humans.
Ingram and Millner, who are in the Environmental Microbial Safety Laboratory in Beltsville, Maryland, examined the potential for such bacteria to grow during both aerobic and anaerobic compost-tea production. They studied the effects of additives—such as soluble kelp, fish hydrolysates, humic acid, rock dust, and proprietary nutrient solutions—on growth of pathogenic bacteria as well as microbes that some farmers feel are beneficial and necessary to enhance soil and inhibit foliar pathogens.
Ingram found that, in general, when compost with low numbers of Salmonella and E. coli is used to make compost tea, the pathogens grew only when additives were included in the initial watery mixture; pathogens remained undetectable in all the compost teas made without commercial additives.
“Use of supplemental nutrients and other additives to produce compost tea gives even a few pathogenic bacteria a growth boost, so testing of the final tea before spraying may be necessary to ensure the absence of human pathogens,” says Ingram.
Recommendations and guidelines for safe production and use of compost tea have been provided by the Compost Tea Task Force, formed by the National Organic Standards Board. The report can be found at: www.ams.usda.gov/nosb
(Agricultural Research magazine, September 2006, www.ars.usda.gov/is/AR/)