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Compost Tea, Worm Tea, & Leachate

This is a compilation of information from the USDA's 2004 Compost Tea Task Force.

Compost tea, which includes vermicompost tea/worm tea, is a nutrient-rich liquid created by steeping finished compost or vermicompost in water. The compost infuses the water with beneficial nutrients and is sometimes sprayed on foliage to reduce pests and disease or used as a soil drench around plants to enrich the soil in the root zone. 

Leachate is produced when water drains over materials that are in the process of decomposing.  This liquid may drain from a traditional compost bin or a worm bin, but is easier to collect from a worm bin. Since the liquid is traveling through partially decomposed matter (as opposed to finished compost), leachate is not recommended for foliar application, but is great to dilute and use as a soil drench.

Why Make Tea or Use Leachate?

When created and used properly, many growers claim that compost tea and leachate provide benefits for them and their plants.  These benefits include:

  • 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 the different environmental conditions for each batch of compost created, it is hard to obtain scientific proof of the benefits claimed when using compost tea.  Basically, no two batches of compost are alike, so results vary.  For any scientific experiment, controls, replicated treatments, and repeated trials must be done to obtain verifiable results.  Therefore, many of the benefits are based on grower testimonials rather than scientific evidence given the tendency for inconclusive scientific results.  Regardless of whether a gardener chooses to use compost tea or leachate, the ultimate goal is to have healthy soils that produce healthy plants.  A gardener’s first priority is to build and sustain healthy soils by amending with quality compost.

How do I make Compost Tea or Worm Tea?

While you will find a large number of resources and recipes for compost tea and worm tea, one of the most simple is as follows:

  1. Fill a 5-gallon bucket (or other container) with water.
  2. Take a scoop of finished compost or worm castings and place the matter in a nylon sock, or another breathable material.
  3. Soak the sock full of compost in the bucket of water overnight.
  4. In the morning, you will have a bucket full of liquid compost to water your plants with! This is a great option for those who don’t want to spend lots of time incorporating compost into the soil, or for those who want to stretch their finished product the furthest.

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. This article from the USDA has more information. The following are some guidelines to follow when creating or using leachate and/or compost tea in your garden to promote safe practices:

When using leachate on or around edibles:

  • Use potable water when diluting.
  • Use only as a soil drench.  Given leachate may have been produced from unfinished compost or vermicompost, it is not recommend for use in foliar application.
  • Do not include any additives (molasses, kelp, fish byproducts, etc). These have been found to increase the likelihood of pathogens.
  • Allow 4 months before harvest without application of leachate (even as a soil drench) for most edible crops

When using compost tea:

  • Use potable water when brewing or diluting.
  • The safest use of compost tea is as a soil drench, but Task Force guidance allows unrestricted us of compost tea that does not include additives.
  • Avoid additives (molasses, kelp, fish byproducts, etc.) when making compost tea, as these can promote the growth of harmful organisms.
  • 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 as some have previously believed.  Results are inconclusive on whether or not 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.