Worm Tea, Compost Tea & Leachate

Did you know that you can steep your finished compost or vermicompost in water to create a nutrient-rich liquid called “compost tea” that enriches soil and reduces plant pests and disease?

Worm tea, a type of compost tea made from vermicompost, has the same benefits as worm castings but in liquid form. Castings are produced when worms break down the organic matter in soil during the vermicomposting process. Castings are also called “worm manure” or “worm humus” and are present in worm beds. When you run water through these castings, nutrients such as nitrogen, calcium, and magnesium are picked up.

What is Compost Tea & Leachate?

Compost tea, which includes vermicompost tea, is a nutrient-rich liquid created by steeping finished compost or vermicompost in water. It is sometimes sprayed on foliage to reduce pests and disease or used as a soil drench around plants to enrich soil in the root zone. Leachate is produced when water drains over compost. It is the liquid that is leached out of a compost pile or worm bin. Since there is a possibility liquid traveled through uncomposted matter, leachate is not recommended for foliar, or leaf, application.

Why Make Tea or Use Leachate?

When created and used properly, many growers claim that compost tea and leachate provide benefits for themselves and their plants, including:

  • Improved plant growth and overall health
  • Disease suppression
  • Pest resistance
  • Additional nutrients for plants and soil
  • Less toxic chemicals and pesticides used

Given the high variability of compost materials used, production processes, application methods, and differing environmental conditions for each batch of compost created, it is hard to obtain scientific proof of the benefits claimed when using compost tea. Many of the benefits listed are based on grower testimonials and should not be taken as evidence that compost tea will improve your garden.

Regardless of whether a gardener chooses to use compost tea or leachate, the ultimate goal is to have healthy soil that produces healthy plants. A gardener’s first priority should be to build and sustain healthy soils by amending with quality compost.

How to Make Worm Tea
To make worm tea, you will need a large bin or worm compost bin. The bottom tray should have a drainage spout and holes for aeration.

First, soak a handful of worm castings in at least five liters of warm water. Allow the castings to soak for a couple of days. If desired, add a teaspoon of molasses (optional). Molasses will promote the growth of micro-organisms. If you feed your worms a balanced diet such as fruits and vegetables (no meat or dairy), they will produce the best castings and worm tea. The water must be chlorine free because chlorine will destroy “good” bacteria, or bacteria that performs essential functions for soil and plants.

Looking for environmentally friendly sources of non chlorinated water? Try rainwater. Using rainwater saves energy, since it does not have to be treated at a plant or transported. Rainwater can be collected via rain barrel, which many cities subsidize. Please visit solanacenter.org/rain-barrels/ for more info on discounted rain barrels and rebates in your city.

Pete Ash, an experienced gardener, long time master composter, and organic farming and gardening teacher, crafts a tea bag of compost and vermicompost to soak in water. He suggests using an aquarium pump to keep the water aerated to stimulate micro-organism growth. Pete says, “The idea is to wash the microbes out of the compost into the water; add a simple starch or sugar to the brew to feed the bacteria that are breeding. Use the wash water from rice rather than washing it down the drain.”

How to Use Worm Tea
Worm tea is potent and works best when diluted. Pete dilutes his worm tea with 4 to 6 parts water (or more) for foliar spray applications. He recommends using worm tea within a couple of days; if it smells bad you should not use it.

Pete harvests his castings regularly because mucus can build up along with bacteria and can actually become toxic for the worms. As Pete says, “No one likes to live in their own feces.”

Benefits of Worm Tea
Worm tea and compost is excellent for a garden. Pete uses worm tea as a foliar spray and compost tea as a field spray. Here are a few ways to use worm tea to grow healthy fruits and vegetables:

  • Use worm tea as an inoculant for potting soil. The nutrients in worm tea help seedlings grow strong. It is suggested that inoculation should be done two weeks before you plant your seedling.
  • Worm tea also helps recover polluted soil. If you add worm tea repeatedly, microbes in the soil will convert and metabolize organic and inorganic chemicals. The worm tea will help sequester the heavy metals found in chemicals.
  • Sometimes lawns become sterile due to chemical treatment. Worm tea will repopulate the soil with microbes, enrich the roots, and break down the thatch, turning it into food for grass.
  • During hot summer days, worm tea can help soil retain water.
  • If you decide to use worm tea as a foliar spray, it will help your plants produce more foliage and larger stems. This greatly helps plants that are lacking enough sun.
  • You may also add worm tea to a compost pile to accelerate decomposition.

By using worm tea, you can help the environment by reducing and even eliminating the use of chemical fertilizers that cause water pollution. Studies show an average American family produces a ton of waste each year. 1/3 to ½ of household waste is estimated to be organic matter (kitchen waste). By vermicomposting, you reduce the amount of organic matter that ends up in landfills, help mitigate global warming, and make nitrogen-rich organic fertilizer and worm tea for yourself. Vermicomposting is nature’s way of completing the recycling loop.

Things to Consider:

Recent research brings up concerns about the presence of pathogens in compost tea. When compost tea is used as a foliar (leaf) spray on food crops, pathogens may be transmitted to humans. Follow these guidelines to avoid pathogens:

When using leachate or compost tea:

  • Use potable water when brewing or diluting.
  • Avoid additives (molasses, kelp, fish byproducts, etc.) when making leachate or compost tea, as they can promote the growth of harmful organisms.

When using leachate: 

  • Use only as a soil drench. Given that some leachate is produced from unfinished compost or vermicompost, it is not recommended for use in foliar, or leaf, application.

When using compost tea: 

  • The safest use of compost tea is as a soil drench.
  • If compost tea is being used as a foliar application on plants that are to be ingested, it is recommended that it be used 3-4 months prior to harvesting.
  • After the process is complete, sanitize equipment used.

To Aerate or Not to Aerate:

Aerating a tea means introducing oxygen into the mix by using a bubbler or method of showering compost over a suspended tank. Some claim that aeration can speed up the time it takes to produce finished tea and produces a more nutrient-rich product. However, aeration does not eliminate the concern of pathogen presence, and results are inconclusive on whether aerated or non-aerated teas are better for your plants and/or soil. Both methods require potable water, incubation time, and filtration of the end product if it is to be used as a foliar application. Most users recommend aerating tea for 24-36 hours, and letting non-aerated tea ferment for 5-8 days. More research is needed to confirm the benefits and concerns related to using aerated or non-aerated teas. 

Resources for Compost Tea information: