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Tweaking the aeration time, handling, and changing additives can create diverse compost teas—even though they were made with the same compost. Washington State University researchers saw differences in the microbial organisms, nutrients, pH levels, and salt concentrations of compost teas that all started from the same compost and water.

CeCe Crosby, who’s working on her soil science doctorate degree at WSU under Dr. Lynne Carpenter-Boggs, research leader of the Biologically Intensive Agriculture and Organic Farming program, worked with compost teas for her master’s thesis. That research evaluated the potential mechanisms of biological control activity against the pathogen that causes black rot in cabbage.

Anecdotal evidence from growers has claimed that compost teas have anti-pathogenic properties, but most of the research has not been replicated, making it impossible to draw scientific conclusions. The research literature on compost teas for plant disease control shows that results have been mixed—sometimes they do suppress disease, but at other times they worsen it, or there is no effect.

Crosby is quick to point out that compost teas have not been approved by the U.S. Environmental Protection Agency as pesticides, therefore, using them to suppress or control diseases without an experimental pesticide use permit is ­illegal.

“There’s been very little scientific proof that compost teas provide disease control,” Crosby said during a phone interview with Good Fruit Grower. But compost teas may have value as fertility supplements. Researchers are just now beginning to understand microorganism communities, an area that’s been hard to study and measure, she adds. Scientists are working to get a handle on what effect compost teas have on soil and plant microorganisms and how they interact with nutrient pools and plant ­hormones to affect plant growth.

Soil-applied vs. teas

Soil-applied compost does lots of good things, like adding soil organic matter and improving soil aeration, soil water holding capacity, and soil structure, but these benefits require large volumes of compost. And, while compost does add nutrients, most are bound within the organic material or adsorb to the soil and are only slowly released. Nutrients in most composts take a long time to break down and become available to plants.

Compost teas are generally applied as a foliar spray or to the root area of the soil and may be a faster way to deliver the available nutrients for plant fertility.

“The idea behind compost tea is that it’s essentially extracting micronutrients out of your compost and enhancing the properties of your compost,” Crosby explains. The extractable nutrients are immediately available, and if microorganism growth is stimulated during compost brewing, the researchers hypothesize that the microbes can pull nutrients out of the compost to make it more plant available, putting the nutrients into ­biologically available forms.

More research needed

Dr. David Granatstein, Washington State University sustainable agriculture specialist, in further expanding on the use of compost teas for plant nutrition, notes that nutrient levels of compost teas are low, dilute, and a single application would not be enough to sustain plant nutrition for commercial crops. “If you have a response, it tends to be in stressed plants,” he said. “But when everything is healthy and great, compost teas often don’t show a benefit.”

Granatstein explains that with compost teas, you’re working with a live microbe. There is inconsistency from both the lack of a standardized process or product and also because much can go wrong in working with live microbes, including brewing human pathogens in the teas, a problem with compost teas in the past. Can the microbes survive once they are taken out of the brewing environment and placed in the real world of the soil and leaves? Can they compete against soil microbes already present? Is there better chance of survival if they’re applied at certain times of the day? Can they survive going through a sprayer?

“We know the mechanisms of microbes and compost teas are real and there is potential to make the live microbes work for us, but we don’t know how to exploit that,” Granatstein said.

For organic growers with few nutrient options, experimentation with teas may make sense, he said, but adds that much more research is needed on specific combinations of crops, pathogens, type of compost tea, and ­application procedures.

Diverse teas

Crosby found that she could change the microbial community, salt concentration (electrical conductivity), pH, and nutrient levels in compost teas by varying the aeration, temperature, time, and amounts of compost and additives, even though the same compost was used. Four compost tea recipes and brewing methods, if carefully followed, reliably produced four different teas with predictable qualities. The electrical conductivity was highest in the compost tea made with seaweed powder, humic acids, and Azomite rock dust, and lowest in one that used straw (boiled in water) and red potato. Total inorganic nitrogen was highest in the compost tea with whey powder and chopped straw.

As scientists learn more about compost teas, specific uses may become associated with specific recipes and growers may be able to select the microbial community or enhance specific nutrient levels by following a certain recipe. But much more research is needed.

“By changing the recipe and how you handle it, you can change the properties of your compost teas,” Crosby said. “Tracking the pH levels and salts in your compost tea batches will help indicate if your teas are stable or inconsistent.”

There are all sorts of conditions—climate, soil, plant status—that affect the success or failure of compost teas for any particular purpose. “Simple teas of compost and water can work great as a nutrient supplement, but when you try to target disease pathogens, each that has a ­specific life cycle, that’s when you run into trouble.”