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Compost Tea

KOM-post TEE
🐛 Worms & Vermicomposting By the Fed 'n Happy Team · Updated October 18, 2026

Definition

Compost tea is a liquid extract made by steeping worm castings or compost in aerated water. The process extracts and multiplies the beneficial microorganisms from the solid castings into a liquid that can be applied by foliar spray or soil drench.

Aerated compost tea brewer with a mesh bag of compost and air pump, microbial food being added, microbe close-ups and the finished tea sprayed on plants
Compost tea is brewed with active aeration to multiply the microbial population beyond what's naturally present in the source castings or compost. — Source: Fed 'n Happy (original illustration)
Quick Facts
General published figures for actively aerated compost tea (AACT); actual counts vary by brewing conditions and source material.
24–36 hrs
Typical active brewing time
~10⁷ per mL
Peak bacteria counts reported in aerated tea (one study)

Sources: University of Connecticut Plant & Soil Health Center, NC State University Plants for Human Health Institute, Plant Pathology Journal (2015)

Why It Matters

Compost tea allows the biological benefits of castings to be spread across a much larger area quickly and economically. A small amount of castings can produce hundreds of gallons of biologically active liquid.

How It Works

Castings are placed in a mesh bag and submerged in dechlorinated water. An air pump provides oxygen that fuels rapid microbial reproduction. After 24–36 hours of brewing, the liquid holds a larger microbial population than the source compost, mostly bacteria.

Benefits

  • Extends the reach of a limited supply of castings
  • Fast foliar application for stressed plants
  • Boosts microbial diversity in soil
  • Can be applied through irrigation systems

Sources: USDA Agricultural Research Service, Compost Science & Utilization (2011), via University of Hawaii CTAHR, Plant Pathology Journal (2015), Washington State University (Chalker-Scott)

How to Brew Compost Tea

Timing and aeration are the two variables that matter most.

Use Dechlorinated Water

Chlorine kills the beneficial microbes you're trying to grow. Let tap water sit out uncovered for 24 hours, or use rainwater instead.

Aerate Continuously

An air pump running the whole brew keeps oxygen levels high enough for aerobic microbes to multiply. Interrupted aeration risks the mix turning anaerobic.

Brew for 24–36 Hours

This window balances population growth against the risk of the tea peaking and starting to decline.

Use It Promptly

Apply within a few hours of brewing. Once aeration stops, the dense population starts consuming available oxygen and can crash quickly.

Sources: NC State University Plants for Human Health Institute, UC Cooperative Extension Master Gardeners, NC State University (Rhonda Sherman), via WSU Extension

Compost Tea vs. Compost Extract vs. Worm Tea

All three start from the same source material — brewing time and method are what set them apart.

Compost Tea Compost Extract Worm Tea
Process Actively aerated and fed for 24–36 hours Steeped without aeration, ready in a few hours Informal umbrella term — can be either method
Microbial population Multiplied well beyond the source material Roughly matches the source material Depends on which method was actually used
Shelf life Just a few hours once aeration stops Up to about two weeks Depends on the method used to make it
Equipment needed Air pump, mesh bag, brewing container A container and water, no aeration required Varies by method
Best for Maximizing microbial density for foliar application Simple, low-equipment soil drench Whatever the specific batch was made to do

Sources: University of Connecticut Plant & Soil Health Center, NC State University Plants for Human Health Institute, UC Cooperative Extension Master Gardeners, University of Arizona Cooperative Extension

Common Mistakes to Avoid

Compost tea rewards careful timing more than most liquid biology products.

Brewing past 36 hours
The tea peaks and then declines. Brewing longer doesn't add more biology and can actually work against you as the population outstrips available oxygen.
Using chlorinated tap water untreated
Chlorine is designed to kill microorganisms, including the beneficial ones you're trying to multiply. Let water sit out 24 hours or use rainwater instead.
Letting the tea sit too long after brewing
Once aeration stops, the dense microbial population starts declining within hours. Apply promptly rather than storing finished tea.
Starting with low-quality compost or castings
Compost tea multiplies what's already in the source material, it doesn't create biology from nothing. Poor-quality source material limits the final result.

Sources: UC Cooperative Extension Master Gardeners, NC State University Plants for Human Health Institute, University of Connecticut Plant & Soil Health Center

Frequently Asked Questions

What is compost tea? +
Compost tea is a liquid extract made by steeping worm castings or compost in aerated water. The process extracts and multiplies the beneficial microorganisms from the solid castings into a liquid that can be applied by foliar spray or soil drench.
How long does compost tea take to brew? +
Actively aerated compost tea is typically brewed for 24-36 hours. Brewing longer than that risks the microbial population crashing or the mix turning anaerobic, so timing matters.
How many microorganisms does compost tea actually contain? +
Estimates commonly cited in compost tea literature put actively aerated compost tea at around 4 billion bacteria per teaspoon, compared to roughly 1 billion per teaspoon in the compost or castings it was brewed from, since aeration and feeding actively multiply the population beyond what's in the source material.
How is compost tea different from compost extract? +
Compost extract is made without aeration or added microbe food, so it's ready in a few hours and roughly matches the microbial population of the source material. Compost tea is actively aerated and fed for 24-36 hours specifically to multiply that population beyond what's naturally present.
How soon do I need to use compost tea after brewing? +
As soon as possible, ideally within a few hours. Once aeration stops, the dense microbial population starts using up available oxygen and can crash or turn anaerobic quickly.
What can I add to compost tea while brewing? +
Common additions include unsulphured molasses or other simple sugars as a microbe food source, along with kelp, humic acids, or fish hydrolysate, though even a basic compost-and-water brew produces a biologically active tea.
Can compost tea replace regular fertilizer? +
Compost tea is primarily a biological input, delivering microbes rather than concentrated nutrients. It works well alongside a fertility plan, but isn't generally meant to fully replace fertilizer for crops with heavy nutrient demand.
Fed 'n Happy Insight

The quality of compost tea depends entirely on the quality of the castings used — brewing multiplies what's already there, it doesn't create biology from nothing.