California Department of Resources Recycling and Recovery (CalRecycle)

Organic Materials Management

Making Compost Tea

To produce a highly beneficial compost tea, start with quality ingredients (feedstocks). There are several production methods you can choose--based on your needs.

Feedstocks

The basic feedstocks for compost tea are compost, water, and several optional additives:

Quality Compost
Start with high quality, stable compost. Compost products will vary in quality based on the original feedstock and the conditions under which it is decomposed. For small-scale composting, feedstocks include yard trimmings, grass clippings, wood chips, dry leaves, fruit and vegetable scraps, paper products, and untreated wood chips and sawdust.

To learn about making quality compost on a small scale, including information on bins and piles, ingredients, techniques, and troubleshooting, please see our home composting page. You might also be interested in more detailed information about characteristics of different types of compost for different situations.

Pure Water
It is important to use water that is as pure and as uncontaminated as possible. Water containing high levels of salts, heavy metals, nitrates, pesticides, chlorine or pathogens should not be used. These will affect the survival and reproduction of beneficial organisms from the compost and may also adversely affect the plants on which the compost tea is applied.

Additives
Certain stimulatory additives can be included during brewing to improve the final quality of the compost tea. These are materials that are added in the process of making compost tea (as distinct from spray adjuvants that are tank mixed immediately prior to application) and that are presumed to sustain and enrich microbial growth. Examples include molasses, yeast extract, fish-based products, kelp, and green plant tissue.

The information below provides more information on additives.

Other Considerations

In addition to high quality feedstocks, you should also consider the following factors when choosing a production method and making compost tea.

Characteristics of the "Tea Bag"
The mesh size of the "tea bag" determines which components of the compost are extracted into the water. With a fine mesh bag, only the tiny, soluble components will enter the water. This is important if the compost tea will be applied with a sprayer or in irrigation systems. Farmers and researchers have found that old nylon stockings make fine tea bags, though fine-weave cotton and silk will also work. Nylon window screening, plastic feed bags, and burlap can also be used. Use clean material for the tea bag that has not been treated with any preservatives or other chemicals.

Aeration and Recirculation
Choose an aeration system that provides the proper amount of water agitation. When compost tea is not adequately agitated, oxygen can become depleted thereby reducing aerobic microbial growth, creating anaerobic conditions, and resulting in poor extraction of materials from the compost.

Ratio of Compost to Water
If there is too much water for the amount of compost, the tea will be diluted and not provide maximum benefits. However, if there is too much compost, it is possible that there will be an excess of nutrients for bacteria, which can lead to oxygen depletion and anaerobic conditions. Experiment with different quantities in your system to achieve the best ratio.

Brew Time
The longer the compost remains suspended in the water, the greater the amount of soluble materials that will be extracted from the compost. These include both living organisms and the nutrients that feed them. Compost tea that is well-aerated and recirculated requires a shorter brewing time than tea made without adequate agitation. Using a sophisticated micro-brewing system, it is possible to produce good quality compost tea in 18 to 24 hours. Under more basic conditions, it may be necessary to allow the compost to steep for a few days to a few weeks.

Environmental Conditions
Temperature, humidity, and evaporation can affect the quality of the compost tea. If water is too cold, extraction will be reduced and microbe growth slowed, but if it is too warm, microorganisms may be inhibited or excessive evaporation may occur. It is hard to change the ambient weather, but a cover over the container in hot weather should help control evaporation.

Production Methods

Bucket Fermentation Method
"Passive" compost tea is prepared by immersing a burlap sack filled with compost into a bucket or tank and stirring occasionally. Usually the brew time is from 7 to 10 days. This is the method that dates back hundreds of years 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 materials being used; fungal hyphae tend to get caught in poly-woven 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. 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.

Note: Research by 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.

Frequently Asked Questions

Q. My tea stinks, is that bad? What should I do about it?

  • A. Don't make compost tea without good aeration equipment. If the tea is not aerated constantly, the organisms in it will quickly use up the oxygen, and the tea will become anaerobic. That is what causes the smell. Never use a smelly compost tea on your plants as it can harm them. Healthy, adequately oxygenated compost and compost tea should smell sweet and earthy. If your compost tea smells bad, add a second pump with more bubblers and stir it more often. Aerate it until the smell goes away. Likewise, if your compost pile smells bad, turn it more frequently.

Q. I've made my tea and it looks and smells good, but what do I do with the compost I used to make the tea with?

  • A. The compost will still have plenty of good bacterial and fungal foods left in it, as well as humus. Just put the compost solids back into your compost pile or add it into your garden soil.

Compost Tea Home

Last updated: April 3, 2012
Organic Materials Management http://www.calrecycle.ca.gov/Organics/
Contacts: http://www.calrecycle.ca.gov/Organics/Contacts.htm