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What is Compost Tea?

As the name implies, compost tea is made by steeping compost in water. It's used as either a foliar spray or a soil drench, depending on where your plant has problems.

Bucket-Fermentation Method
“Passive” compost tea is prepared by immersing a burlap sack filled with compost into a bucket or tank, stirring occassionally. Usually the brew time is longer, from 7 to 10 days. This is the method that dates back hundreds of years in Europe, and is more akin to a compost watery extract than a “brewed” and aerated compost tea.

Bucket-Bubbler Method
The equipment setup and scale of production are similar to the bucket method, except that an aquarium-size pump and air bubbler are used in association with microbial food and catalyst sources added to the solution as an amendment. Since aeration is critical, as many as three sump pumps may be used in a bucket simultaneously.
With homemade compost tea brewing, a compost “sock” is commonly used as a filter-strainer. Ideally, the mesh size will strain compost particulate matter but still allow beneficial microbes—including fungal hyphae and nematodes—to migrate into solution. Single-strand mesh materials such as nylon stockings, laundry bags, and paint bags are some of the materials being used; fungal hyphae tend to get caught in polywoven fabrics. If burlap is used, it should be “aged” burlap.

Trough Method
Large-scale production of compost teas employs homemade tanks and pumps. An 8- or 12-inch-diameter PVC pipe is cut in half, drilled full of holes, and lined with burlap. Compost is placed in this makeshift trough. The PVC trough is supported above the tank, several feet in the air. The tank is filled with water, and microbial food sources are added as an amendment. A sump pump sucks the solution from the bottom of the tank and distributes the solution to a trickle line running horizontally along the top of the PVC trough filled with compost. As the solution runs through the burlap bags containing the compost, a leachate is created which then drops several feet through the air back into the open tank below. A sump pump in the bottom of the tank collects this “tea” and distributes it back through the water line at the top of the trough, and so on. Through this process, which lasts about seven days, the compost tea is recirculated, bubbled, and aerated. The purpose of the microbial food source is to grow a large population of beneficial microorganisms.

Commercial Tea Brewers

Commercial equipment is available for the production of brewed compost teas (see a list of suppliers below). Usually there is a compost sack or a compost leachate basket with drainage holes, either of which are used to hold a certain volume of compost. The compost-filled container is placed in a specially designed tank filled with chlorine-free water. Microbial food sources are added to the solution. A pump supplies oxygen to a specially-designed aeration device which bubbles and aerates the compost tea brewing in the tank.

Depending on your scale of production and the level of financial resources available to purchase commercial brewing equipment vs. making some kind of homemade brewer, there are several methods to choose from. Research at Soil Foodweb, Inc. in Corvallis, Oregon has shown that differences exist in the beneficial attributes of compost teas, with commercial tea brewers producing the greatest numbers and diversity of beneficial microorganisms.

By Steve Diver
NCAT Agriculture Specialist
February 2002