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Using Compost to Create Compost Tea
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what is compost tea?  
compost teas vs. compost extracts    
liquid organic extracts vs. compost teas   
how to brew compost tea 

ALSO SEE:
outdoor composting
indoor composting
other ways to recycle food scraps
composting equipment

*Text excerpted and edited from the National Sustainable Agriculture Information Service.


illustration: fungusWhat Is Compost Tea?

Compost tea is a liquid that helps feed your plants the nutrients they need when used as a liquid drench that also helps to organically prevent pests and disease when applied topically to your plants.

Compost tea is produced through a brewing process that extracts microorganisms from compost, and encourages microbial growth and multiplication. This includes beneficial bacteria, fungi, protozoa, and nematodes.

When compost teas are sprayed onto a plant, these beneficial organisms occupy spatial niches on the leaf surface and gobble up leaf exudates that pathogenic organisms (bacteria or fungi that can cause disease) would otherwise feed on to prosper; other microbes directly antagonize and interfere with pathogenic organisms.

Ideally, compost teas contain both an abundance (immense total number) and a diversity (vast variety) of beneficial microorganisms which perform different functions. Pathogenic organisms that land on the leaf surface simply cannot compete with the beneficial organisms and therefore have a greatly reduced chance to initiate disease.

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illustration: nematodesCompost Teas Vs. Compost Extracts

How do compost teas differ from compost extracts or compost leachates?

Compost leachate—the dark-colored solution that leaches out of the bottom of the compost pile—will most likely be rich in soluble nutrients; but in the early stage of composting it may also contain pathogens. It would be viewed as a pollution source if allowed to run off-site. Compost leachate needs further bioremediation and is not suitable or recommended as a foliar spray.

Compost extract is made from compost suspended in a barrel of water for 7 to 14 days, usually soaking in a burlap sack. This centuries-old technique yields a liquid fertilizer with soluble nutrients.

Compost tea, in modern terminology, is a compost extract brewed and aerated with the addition of a microbial food source. (Examples of microbial food sources: molasses, kelp powder, and fish powder. Examples of microbial catalysts: humic acid, yucca extract, and rock dust.) The compost-tea brewing technique extracts and grows populations of beneficial microorganisms.

Compost teas are distinguished from compost extracts both in method of production and in the way they are used. Teas are actively brewed with microbial food and catalyst sources added to the solution, and a sump pump bubbles and aerates the solution, supplying plenty of much-needed oxygen. The aim of the brewing process is to extract beneficial microbes from the compost itself, followed by growing these populations of microbes during the 24- to 36-hour brew period. The compost provides the source of microbes, and the microbial food and catalyst amendments promote the growth and multiplication of microbes in the tea.

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Liquid Organic Extracts Vs. Compost Teas

Compost teas and herbal teas enhance plant fertility and inoculate the phyllosphere and rhizosphere with soluble nutrients, beneficial microbes, and the beneficial metabolites of microbes. 

Here are some other common organic extracts used as a liquid drench or foliar spray:

Herbal Tea
Plant-based extracts, usually from stinging nettle, horse tail, comfrey, or clover. A common method is to stuff a barrel about three-quarters full of fresh green plant material, then top off the barrel with tepid water. The tea is allowed to ferment at ambient temperatures for 3 to 10 days. The finished product is strained, then diluted in portions of 1:10 or 1:5 and used as a foliar spray or soil drench. Herbal teas provide a supply of soluble nutrients as well as bioactive plant compounds.

Manure Tea
A manure-based extract is a soluble nutrient source made from raw animal manure soaked in water. For all practical purposes, manure tea is prepared in the same way as the compost extracts described above: manure is placed in a burlap sack and suspended in a barrel of water for 7 to 14 days, yielding a liquid fertilizer with soluble nutrients. SEE CAUTION.

Liquid Manures
Mixtures of plant and animal byproducts seeped as an extract—stinging nettle, comfrey, seaweed, fish wastes, or fish meal. Liquid manures are a blend of marine products (local fish wastes, seaweed extract, or kelp meal) and locally harvested herbs, soaked and fermented at ambient temperatures for 3 to 10 days. Liquid manures are prepared similarly to herbal tea: the material is fully immersed in the barrel during the fermenting period, then strained and diluted for use as a foliar spray or soil drench. Liquid manures supply soluble nutrients and bioactive compounds. SEE CAUTION

CAUTION: Do NOT try this at home. Because of concerns over new pathogenic strains of E. coli, growers are advised to avoid manure teas and/or to work with a microbial lab to ensure a safe, worthwhile product. Caution is urged: Manure teas are NOT the same thing as compost teas or compost extracts.

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How To Brew Compost Tea

Bucket-Fermentation Method
Compost extract is prepared by immersing a burlap sack filled with compost into a bucket or tank for 7 to 10 days, stirring occasionally. This method dates back hundreds of years in Europe, and yields a watery compost extract rather than a brewed and aerated compost tea.

Bucket-Bubbler Method
The equipment setup and scale of production are similar to the bucket method, with the addition of using an aquarium-size pump and air bubbler, plus 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. Make sure to use chlorine-free water so you do not kill the microbes.  Allowing the water to sit in the bucket for a day before you add the compost will allow the chlorine to off-gas.

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 the solution. Single-strand mesh materials such as nylon stockings, laundry bags, and paint bags may be 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 several feet above a tank. 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. The solution runs through the burlap bags containing the compost, and the leachate drips through the holes, dropping several feet through the air back into the open tank below, where the sump pump recirculates it back to the top of the trough, and so on. The compost tea is recirculated, bubbled, and aerated for about 7 days. The purpose of the microbial food source is to grow a large population of beneficial microorganisms.

Commercial Tea Brewers
Research has shown that commercial compost tea brewers produce the greatest numbers and diversity of beneficial microorganisms. Usually there is a compost sack or a compost leachate basket with drainage holes. This container is filled with compost and 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.

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