Compost teas have been promised by some companies to be a wonder-all product, protecting plants from disease, increasing growth, breaking down toxins in the soil, enhancing taste of fruits and vegetables, and more. Though few of the claims are supported by science, there may be something to learn from compost teas in the area of beneficial microorganisms and biochemistry.

At its simplest, compost tea is an infusion of compost into water. Good compost tea must start with good compost. Recipes are diverse and can use 1 part compost in up to 100 parts water. Additives can be used to boost plant nutrients, change pH, and other properties, and teas can be aerated or static (fermented).

Aerated teas can be made using a simple aeration system, such as a fish tank aeration pump, and be made in five- or ten-gallon buckets or under more sophisticated setups. Aerated teas are generally aerated for short periods, up to 24 hours, and don’t have a lot of unpleasant odors, said CeCe Crosby, soil science graduate student at Washington State University.

Static or fermented teas often take a week or longer to reach maturity, Crosby said. “They take more planning and control, and there’s more time for things to go wrong. They tend to get more fungi. And, fermented teas tend to be incredibly smelly.”

Compost teas can be applied with boom sprayers or as fertigation in drip or microsprinkler irrigation lines, but she cautioned against using high-pressure sprayers if viable microorganisms are wanted because the high pressure can damage the microorganisms. Filtering is needed to remove any chunks of compost that could clog ­application equipment.

The source of compost is important. The compost used should be high quality, pathogen free, and made from a consistent source. Growers should make sure the compost has been tested for pathogens. If the crop is being grown organically, the compost must fit the organic compost standards as well, or be handled and applied as though it is raw manure.

There is potential for pathogens to grow in compost teas, particularly if a sugar source like molasses is added. “If your compost is not perfectly clean [of pathogens], the compost tea can grow plant pathogens or even human ones, like Escherichia (E. coli) or Salmonella,” Crosby warned. If anything besides compost and water is added to the compost tea, the tea is considered by the National Organic Standards Board as raw manure and a harvest interval must be followed. For fruit crops, the harvest interval is 90 days.

As scientists continue to study compost teas, they may learn if compost teas have benefit in the areas of micronutrients, microbial inoculation, plant immunity stimulation, plant hormones, and microbial signaling.

Microbial community

Dr. Lynne Carpenter-Boggs, soil scientist and research leader for WSU’s Biologically Intensive Agriculture and Organic Farming program, stated during a session at the Washington State Horticultural Association annual meeting that there is potential for compost teas to change the plant or soil microbial community. Although repeated application and/or habitat enhancement is usually necessary to change a microbial community over the long term, it may be possible to culture and develop microbial communities if scientists know the needs of beneficial organisms.

New studies in the world of soil microbiology may offer clues in how microorganisms communicate with each other through volatile or diffusible molecules, releasing hormones or other signals. These signals may induce a change in the activity of other microorganisms, increase growth rates or reproductive rates of microbes in the same species, or signal antibiotic molecules.

“The chemistry and complexity of microbial signaling is a new frontier in microbiology,” Carpenter-Boggs stated.

More research is needed to learn if compost teas have a role in this new frontier.