How to Brew Aerated Compost Tea

Aerated compost tea (ACT) is brewed by aerating a mixture of finished compost and water for 24--36 hours, allowing the microbial population in the compost to multiply dramatically. The resulting liquid is applied as a soil drench or foliar spray with the goal of introducing beneficial.

—- title: "How to Brew Aerated Compost Tea" slug: how-to-make-compost-tea hub: care category: "Soil" description: "Aerated compost tea amplifies the microbial population of finished compost into a liquid form for soil drench or foliar spray. This guide covers the equipment, process, timing, and the honest evidence on what it actually does." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Aerated compost tea (ACT) is brewed by aerating a mixture of finished compost and water for 24—36 hours, allowing the microbial population in the compost to multiply dramatically. The resulting liquid is applied as a soil drench or foliar spray with the goal of introducing beneficial microorganisms to the soil or plant surface.

The claims surrounding compost tea range from credible to overblown. The credible ones — that ACT delivers concentrated beneficial microorganisms and soluble nutrients to the soil — have reasonable support. The overblown ones — that it cures plant diseases, eliminates the need for fertilizer, or dramatically outperforms regular compost application — do not hold up in controlled trials. Per Cornell Cooperative Extension, the honest assessment is that ACT is a useful supplemental tool, not a replacement for the foundational practice of incorporating solid compost into soil.

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How Aerated Compost Tea Differs from Compost Extract

These two terms are often confused.

Compost extract (non-aerated): Compost steeped in water for 24—48 hours without aeration. Per Penn State Extension, this leaches soluble nutrients and some microorganisms into solution but does not significantly increase microbial populations because the organisms are not receiving enough oxygen to reproduce rapidly.

Aerated compost tea (ACT): Water oxygenated continuously with an aquarium pump while compost brews in it. The high dissolved oxygen content supports aerobic microbial reproduction. Per NC State Extension, microbial populations in properly brewed ACT can be 10—100 times higher than in non-aerated extract.

Which to use: ACT is preferred for introducing microbial inoculum. Compost extract is adequate for delivering soluble nutrients. Most serious growers use ACT.

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Equipment

Per Cornell Cooperative Extension, a functional home ACT brewer requires:

1. A bucket or reservoir: 5-gallon bucket is standard for home use; 20—55 gallon barrels for larger operations
2. Aquarium air pump: Sized to deliver 1 cubic foot per minute per 5 gallons of water. For a 5-gallon bucket, a pump rated at 1 cfm is adequate. Per NC State Extension, under-aerated tea produces anaerobic conditions that kill aerobic beneficial organisms and can produce compounds toxic to plants
3. Air stones or diffusers: Multiple stones distributed throughout the bucket produce finer bubbles and better oxygenation than a single stone
4. A mesh bag or sock: For containing the compost and allowing easy removal without straining
5. Dechlorinated water: Per Penn State Extension, chlorine in tap water kills the microorganisms you're trying to cultivate. Let tap water sit uncovered for 24 hours before brewing, or use well water or rainwater. Do not use water that has been treated with chloramine — chloramine does not off-gas. If your municipal water uses chloramine, use a carbon filter or add a dechlorination agent.

Total equipment cost: Per these specifications, a functional 5-gallon brewer can be assembled for $20—40: a 5-gallon bucket ($4—6), aquarium air pump rated for 20—40 gallons ($10—15), two air stones ($3—5), and airline tubing ($2—4).

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The Compost: Quality Is Everything

Per Cornell Cooperative Extension, the quality of the finished compost used in ACT determines the quality of the tea. ACT amplifies whatever microorganisms are present in the compost — if the compost is poor (partially composted, slimy, anaerobic) the tea will also be poor and may contain pathogens.

Use compost that is:

• Fully finished (dark brown, crumbly, earthy smell — no recognizable original materials)
• From a well-managed hot compost pile that reached 131—160°F during active composting, per USDA, which kills most pathogens
• Not derived primarily from animal manures unless the hot composting process is verified — per NC State Extension, manure-based compost that hasn't reached proper temperatures can harbor E. coli O157:H7 and Salmonella, which would be amplified in ACT and could contaminate food crops

Do not use: Fresh manure, partially composted material, compost from unknown sources, or compost with a sewage smell.

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Optional Additives (Microbial Foods)

Per Penn State Extension, adding a small amount of microbial food to the brew accelerates microbial reproduction:

• Unsulfured blackstrap molasses: 1 tablespoon per 5 gallons. Provides simple sugars for bacterial growth. This is the most widely used additive
• Kelp meal or kelp extract: 1 teaspoon per 5 gallons. Provides complex carbohydrates and minerals that favor fungal growth. Use when improving soil for tree and shrub plantings (fungally dominated systems)
• Fish hydrolysate: 1 teaspoon per 5 gallons. Amino acids favor bacterial growth

Per Cornell Cooperative Extension, do not over-add these materials — too much molasses or fish creates a bacterial overgrowth that depletes oxygen and crashes the aerobic population, producing anaerobic tea. 1 tablespoon of molasses per 5 gallons is sufficient.

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The Brew Process

Per Penn State Extension:

1. Fill the 5-gallon bucket with 4—4.5 gallons of dechlorinated water at room temperature (60—75°F)
2. Place 1—2 cups of finished compost in a mesh bag; suspend the bag in the water or clip it to the rim so it hangs mid-bucket
3. Add optional microbial food (1 tbsp molasses)
4. Insert air stones; connect to pump; start aeration
5. Brew for 24—36 hours at room temperature. Per NC State Extension, shorter brews (under 18 hours) have lower microbial populations; brews beyond 36 hours can deplete oxygen and shift toward anaerobic conditions
6. The finished tea should smell earthy or sweet — never sulfurous, sour, or putrid. Any unpleasant smell indicates anaerobic conditions; the batch should be discarded

Apply within 4 hours of completion. Per Cornell Cooperative Extension, ACT begins to lose aerobic microbial populations rapidly once aeration stops, as oxygen is depleted and organisms die or go dormant.

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Application

As a soil drench: Per Penn State Extension, apply 1 gallon of ACT per 10 square feet of garden bed as a soil drench. Apply to moist soil in early morning or evening to reduce UV-caused microbial death on the soil surface. Water in lightly after application.

As a foliar spray: Apply to both surfaces of leaves in early morning, allowing leaves to dry before midday. Per NC State Extension, the evidence for foliar ACT suppressing powdery mildew (Erysiphe spp.) is mixed — some university trials show suppression, others show no significant effect. Apply as a preventive, not a cure.

Frequency: Per Cornell Cooperative Extension, monthly applications through the growing season are commonly recommended. More frequent application is not harmful but is also not well-supported by evidence as more beneficial.

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What the Evidence Actually Shows

Per Cornell Cooperative Extension, peer-reviewed studies on compost tea produce mixed results:

• Several studies show significant improvements in soil microbial diversity after ACT application in depleted soils
• Studies in already-healthy, compost-amended soils show minimal additional benefit from ACT vs. solid compost alone
• Disease suppression results are inconsistent across trials
• No well-controlled study has shown ACT to replace the need for solid compost application

The practical conclusion: ACT is most valuable in soils that have had little organic matter management — degraded soils, new planting areas, container media that has been in use for a season. In well-managed beds already receiving annual compost additions, the marginal benefit of ACT is less clear.

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Safety Note

Per NC State Extension, do not apply ACT brewed from manure-based compost (unless pathogen-free status is verified) to edible crops within 90 days of harvest. The FDA's National Organic Program applies similar standards to manure application. When in doubt, use compost brewed from plant-based materials for food crops.

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Common ACT Problems

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FAQ

Can I use ACT instead of fertilizer? Per Penn State Extension, ACT is not a fertilizer replacement. It delivers microorganisms and trace amounts of soluble nutrients but not the quantities of N-P-K needed to support vegetable or flowering plant growth. Use it alongside, not instead of, an appropriate fertility program.

Does temperature affect brew quality? Yes. Per Cornell Cooperative Extension, brewing at 70—75°F produces the best microbial reproduction rates. Below 60°F, microbial activity slows significantly and brew times should be extended by 6—12 hours. Above 80°F, some beneficial organisms die off and pathogens reproduce faster.

Can I store leftover ACT? Per NC State Extension, ACT cannot be stored effectively. Once aeration stops, the aerobic microbial population declines rapidly. Apply immediately after brewing or resume aeration if there is a short delay. Do not store ACT for more than 4 hours after aeration ends.

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Sources

1. Cornell Cooperative Extension &mdash; <a href="https://cce.cornell.edu">Compost Tea</a>
2. Penn State Extension &mdash; <a href="https://extension.psu.edu/composting">Composting and Compost Use</a>
3. NC State Extension &mdash; <a href="https://plants.ces.ncsu.edu">Compost Tea Application</a>
4. USDA &mdash; <a href="https://www.usda.gov">Composting Standards</a>