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This is not the kind of tea you want in a warm mug to drink. Compost tea is a nutrient-rich fertilizer that you can make by brewing compost in water.
“Compost tea is a liquid byproduct of compost. You take the compost and you steep it, or you process it to get the liquid,” said Mark Hutchinson, extension professor at the University of Maine Cooperative Extension. “It’s kind of like taking a tea bag and putting it in a mug and drinking the tea.”
Some gardeners swear by the powers of compost tea to promote the growth, blooms and yield of flowering plants, vegetables, houseplants and crops alike. However, murkiness surrounding the scientific evidence supporting the efficacy of compost tea have lead to some skepticism about its virtues.
Why make compost tea
Compost tea apparently enhances the benefits of compost and allows for greater flexibility in its application.
While compost should only be added around plant roots in the soil, compost tea can be sprayed on the foliage as well. When sprayed on the leaves, research shows that compost tea helps protect the plants against foliar diseases.
When prepared correctly, compost tea promotes the growth of aerobic microbes, which will add nutrients to the soil and serve as active members of the microbiome of your garden. Proponents of compost tea swear, largely anecdotally, by the benefits of the added nutrient boost to their gardens.
“I’m a big fan of using compost and compost tea together,” Mark King, composting and organics management specialist at the Maine Department of Environmental Protection. “You’re going to have a better effect if you do both.”
The evidence against compost tea
The scientific evidence about the effectiveness of compost tea, however, is largely inconclusive or nonexistent. Research conducted at Cornell University, Michigan State University and Penn State, among other institutions, have shown aerated compost tea was ineffective in reducing specific diseases in many crops, including powdery mildew on tomatoes and apple scab on apple trees.
“There’s really a lack of a lot of scientific evidence that shows that it has any nutrient value or any yield efficacy for plant growth,” Hutchinson said.
Hutchinson said that not only is compost tea likely no more effective than compost itself, but the liquid extract also is missing some of the key elements that make compost so effective.
“If you add the regular compost you also have all the organic matter that goes along with it,” Hutchinson said. “You’re definitely missing out on the organic fraction of the compost, the solids are to me the beneficial part of the compost that improves soil structure and feeds soil microbes.”
Ultimately, given the lack of research, Hutchinson believes that compost tea is not worth the effort of steeping and aerating.
“I’d like to see more research done on it, or have research that supports it before I support the concept for somebody’s time energy and money,” Hutchinson said. “There’s been some limited [success], but it’s limited scale and very limited results and not repeatable results.”
Still, compost tea has its defenders. King points to the fact that compost tea has been shown to be rich in trichoderma, an antiviral fungi that can help fight diseases.
“You’re waking up the plants’ genetic ability to fight disease,” King said. “When you get a vaccination, you’re giving a tiny bit of the disease to strengthen the immune system. When you give compost and compost tea to a plant, you make it more vigorous.”
Even so, some compost tea skeptics encourage gardeners to experiment with the technique if they are curious about its effects.
“I wouldn’t count on necessarily improving my plant health that way, but if you want to experiment, by all means,” said Dawn Pettinelli, Assistant Extension Educator Department of Plant Science & Landscape Architecture UConn Soil Nutrient Analysis Lab. “It’s fun to experiment, but there has not been a ton of evidence that has found that [compost tea] makes a vast difference.”
Pettinelli recommended experimenting with perennial plants or disease-susceptible fruiting trees rather than edibles.
“Unless you have it tested for E. coli, I’m not positive myself if I would put it on an edible crop like lettuce or cucumber,” she laughed. “You don’t want to apply it to food crop and eat it right away. It’s sort of like manure.”
How to make compost tea
If you have seen the evidence and decided that you want to try compost tea anyway, start by making your own DIY compost tea brewer, a simple apparatus by which compost tea can be prepared.
“I would tell people if they want to experiment, they should try a really inexpensive base model: an aquarium pump and a five gallon bucket,” Pettinelli said.
You want to use a well-aged compost that is free of any dangerous pathogens (these can mainly occur in composts that use animal feces, so if your compost pile makes use of manure, make sure your pile is covered and there are no suspicious molds or fungi). You also want compost that is rich in “green” materials such as kitchen scraps and coffee grounds, which contain sugars that promote bacterial growth.
To create your compost tea “bag,” remove any worms or insects from the compost and place the compost into the water-permeable bag, using a piece of twine to tie the bag closed.
Fill the 5-gallon bucket with water, leaving an inch or two of space at the top. To promote the “stickiness” of the compost tea to the plant materials (otherwise, proponents say, it can easily be washed away), add molasses to the bucket, too, and stir well.
Put the bucket near an outdoor electrical outlet, place the aquarium tubing in the bucket and plug in your aquarium pump to provide the oxygen the microorganisms need to reproduce.
“If you oxygenate the water, you’re promoting the growth of the microbes,” Pettinelli said. “If you add those microbes to the plants or soil, theoretically, it makes sense.”
“Brew” the tea by letting the pump run for 24 to 48 hours. Once it is done, it needs to be used within 24 hours. Do not let it sit for longer or disconnect the pump, as the compost tea will become toxic without the oxygen supply.
When the tea is done brewing, unplug the pump, remove the tubing, and take out the bag of compost. Squeeze the tea into a watering can or pump sprayer and apply both around the base of your plants and directly to the leaves.
How often you should use compost tea
Compost tea proponents assert that proper timing is essential to the success of the technique.
“In order to be used legitimately, it needs to be spread with the stomata of the plants are open, usually around dusk when the plants are respiring,” King said. “As the sun goes down, it’s the best chance for material to soak in.”
Consistency also matters, according to King. Because the compost tea is easily washed away, at minimum weekly application is recommended, as well as re-application after rainy days.
Ultimately, King believes your success with compost tea is going to depend on your dedication to the experimentation.
“You don’t want it deemed as the next great elixir. That’s not the case,” King said. “I see compost tea used a lot with organic farmers. They swear by it, and they don’t care if they have to reapply it five times a week because it rains every day. It’s just being fastidious to a methodology.”