Compost Tea & Liquid Extracts

Water-based extracts of mature compost, brewed to carry compost's biology and chemistry onto leaves, roots, and soil. A practical primer on what teas are — and are not.

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Definitions and distinctions

"Compost tea" is a loose term. Three very different products travel under it, and one impostor (compost leachate) is often sold as tea — it should not be:

Product	Process	Character	Typical use
Aerated compost tea (ACT)	Compost steeped in water with continuous aeration for 12–36 h	High microbial numbers and diversity; complex flavor of biology	Foliar and soil applications; the "biological" product
Non-aerated compost tea / extraction (CTE)	Compost steeped in water, passively or briefly stirred, 1–7 days	Fewer bacteria, more soluble chemistry; variable biology	Soil drenches, leach-mineral extraction, cost-sensitive operations
Compost water extract (humic-rich)	Long, controlled steep (days–weeks) aimed at humic/fulvic fractions	Chemistry-focused, low live biology	Seed treatment, potting-mix amendments, biostimulant claims
Compost leachate ⚠	Drainage water from an active composting pile or pad	Dark, high-BOD, high salts, often anaerobic, can carry pathogens	None for food crops. Manage as wastewater (see Environmental Impact)
Plant teas (comfrey, nettle, etc.)	Fermentation of green plant material	Nutrient-rich, microbe-poor, strong odor	Foliar feeding; a fertility product, not a biological one

Leachate is not tea Some operations sell "compost tea" made from pile drainage or from soaking compost in a closed barrel for days. Drainage leachate is unprocessed wastewater — high in salts, BOD, and potentially pathogens — and should never be sprayed on edible crops or treated as a biological inoculant process.

What's actually in the tea

Microbes: a well-brewed aerated tea typically carries 10⁵–10⁸ bacteria per mL plus fungi, protozoa, and small numbers of nematodes, drawn from the compost itself. Microbial counts and diversity reflect the compost far more than the brewer.
Soluble chemistry: small amounts of NH₄⁺, NO₃⁻, K, and micronutrients, plus organic acids, amino acids, and plant hormones left over from decomposition. Nutrient concentrations are generally an order of magnitude below fertilizer-strength — tea is not a nutrient delivery system.
Humic/fulvic fractions: soluble humic substances, relevant for their chelation and root-stimulation effects, are more concentrated in long, un-aerated extractions than in short ACT brews.

Mental model Aerated tea is mainly microbe delivery; long extraction is mainly chemistry delivery. Decide which you want before you choose a brewer.

What teas are for — and not for

Reasonable uses

Inoculating leaves with a living microbial community (foliar)
Soil and compost-pile drenches to distribute biology at planting
Seed and transplant drenches to help establishment
Delivering humic chemistry and small nutrient doses (extraction)
Including compost-derived biology into no-till and direct-seeded systems

Unreasonable expectations

As fertilizer: nutrient load is small; budget fertility separately
As pesticide: teas are not registered pesticides; health claims based on disease "control" are legally risky in several jurisdictions (e.g., pesticide-registration regimes) and scientifically shaky
As rescue for bad soil — it is a microbial seed packet, not compost's replacement
As a sanitation tool — extraction never "cleans" bad compost; see food safety below

Brewing: design & parameters

Parameter	ACT (aerated) typical range	Why it matters
Compost:water ratio	1:5 to 1:10 by volume	Extraction concentration; dilute for composts with high salt
Aeration	Continuous, vigorous (bubbler or venturi); keep O₂ > 6 mg/L	Aerobic microbes dominate; anaerobes and pathogen regrowth appear when oxygen lapses
Brew time	12–36 h; rarely > 48 h	Growth plateaus; longer brews go anaerobic and smell
Temperature	15–25 °C	Mesophilic window; too cold stalls, too hot shifts community
Feedstock activation (molasses etc.)	Small amounts — ~0.1–1% by volume (e.g., 5–10 mL/L unsulfured molasses)	Supports bacterial bloom; food-based additives also feed any pathogens present — use only with tested compost
Filtering	Coarse (250–1000 µm) or none for soil drenching; fine for sprayers	Fine filters remove fungal hyphae and the largest protozoa
Storage	Use within 4–8 h; or keep aerated up to 24 h	Decomposition resumes immediately; stale tea becomes anaerobic

Brewer scale: a 20–100 L bucket/barrel with an aquarium pump or venturi is enough for a small farm; 200–1,000 L tanks with blowers and bag filters serve larger operations; commercial brewers (sometimes 5,000 L+) exist but the biology does not scale linearly — batch size mostly just repeats the same brew.

Section view of an aerated compost tea brewer: a tank of dechlorinated water at 15 to 25 degrees Celsius with a coarse mesh bag of mature compost, an air pump feeding diffusers on the floor, and a chart of dissolved oxygen holding above 6 milligrams per litre through a 12 to 36 hour brew before falling away in stale tea. — **Oxygen is the design.** Everything in an aerated brewer exists to keep dissolved oxygen above the aerobic floor: a bag that lets water through without clogging, diffusers that move the whole column rather than one corner, and a brew time short enough to stay ahead of the oxygen curve. Note the shape of the chart — the microbial bloom plateaus long before the oxygen does, so brewing longer buys nothing and risks the anaerobic tail.

Choosing the compost

Mature and stable: raw or immature compost extracts poorly and can bring phytotoxic salts and ammonia into the tea (see maturity). Use Solvita-tested or equivalent stable product.
Pathogen-screened: for any edible crop, brew only from compost with documented pathogen testing (fecal coliform < 1,000 MPN/g and/or Salmonella not detected). See pathogen standards.
Feedstock history: no biosolids or sewage-derived compost for food crops; be alert to persistent herbicide and PFAS liabilities in untested feedstocks.
Diverse source: a mixed-manure-and-yard compost carries a more diverse community than a single-feedstock product.

Application methods & rates

Route	Typical use rate	Timing notes
Foliar	Dilute 1:4 to 1:20; apply 100–500 L/ha	Early morning or late evening; not in hot sun; re-apply after rain
Soil drench / at planting	Undiluted to 1:5; 10,000–30,000 L/ha (or per transplant ~0.5 L)	Band near seed row or drip line; incorporate promptly
Seed soak / dressing	1:5–1:10, 5–15 min soak	Rinse after; use only tested compost for food crops
Compost pile activation	1:5, sprinkle into layers at build	Complements but never replaces recipe management

Tea has essentially no residual value — treat it as fresh produce: brew, apply, repeat on a schedule rather than storing.
Sprayer hygiene: rinse lines and nozzles immediately; dried tea is a biofilm hazard in irrigation systems.
Keep the same brew moving in one session where possible; split batches for multiple sites rather than dragging one batch all day.

Three recipes

1) 20 L bucket brew (small farm / trial)

2 kg mature, pathogen-tested compost in a mesh bag
18 L dechlorinated water (let tap water sit 24 h or use rain water)
2–5 mL unsulfured molasses (only if starting from tested compost; omit for food crops)
Aerate continuously with an aquarium air pump (two stones) for 24 h at 18–24 °C
Use within 4–8 h; dilute 1:5 for foliar work

2) 200 L farm brew

20–30 kg compost in a 500 µm filter sock; suspend in a 200 L drum
Fill with dechlorinated water; aerate with a small blower and two diffuser stones
Brew 24–36 h; keep temperature below ~28 °C (shade the drum in summer)
Strain coarse; apply to soil at 1:3–1:10 with a sprayer or injector

3) Passive humic-rich extraction (no pump)

10 kg mature compost in a barrel; fill with water; cover loosely
Steep 3–7 days in shade, stirring twice daily
Strain; expect a chemistry-rich, biology-poor liquid in excellent condition
Use as seed soak or soil drench at 1:10; do not spray on food crops without testing (long anaerobic passage)

Safety & food-safety limits

The food-safety bottom line Brewing does not sanitize. Oregon State University research (and later work) showed that food-based additives can allow E. coli O157 and other pathogens to regrow in tea within 24 h even when the starting compost had low counts. For crops eaten raw: use only tested, pathogen-negative compost, skip molasses-type additives, brew aerobically for ≤24 h, and use the tea promptly.

Labeling/claims: in many regions (e.g., Oregon, Canada, EU member states), advertised disease-suppression claims can classify a tea as a pesticide requiring registration. Market it as a soil- and plant-conditioning inoculant, not a control product.
Organic production: teas brewed from NOP-compliant compost are generally allowed under NOP, but verify with your certifier; leachate and non-composted-manure teas are not compliant feedstocks.
Operator exposure: stirring, filling, and spraying release bioaerosols (see bioaerosols); wear gloves and a fitted respirator when handling concentrated brews, and avoid overhead spraying on windy days.
Water quality: start from potable or clean rainwater; never from manure-contaminated sources.

What the research shows

Biology: multiple studies confirm ACT carries viable bacteria and fungi in the 10⁵–10⁸/mL range, with diversity tracking the source compost. Community composition is highly variable between batches.
Disease suppression: the seminal review (Scheuerell & Mahaffee, 2002) found teas reduce some foliar diseases in some trials (e.g., Botrytis, powdery mildews) but effects are inconsistent, method-dependent, and far weaker than managed compost amendments in soil. Few field-scale confirmations exist.
Plant response: positive responses are frequently reported for seedling establishment and early growth; whether this is hormonal (humic/auxin-like), nutritional, or microbial is unresolved — treat claims of yield gains skeptically unless strip-tested.
Pathogen risk: the OSU regrowth finding is robust; food safety must be engineered, not hoped for.
Bottom line: teas are low-cost, low-risk biological conditioning with plausible but modest benefits — an addition to, never a replacement for, good compost and agronomy.

Tea troubleshooting

Symptom	Cause	Fix
Strong sour/rotten smell by 24 h	Oxygen lapsed or brew too long	Increase aeration, shorten brew to 12–18 h, use fresh compost
Thick foam overflowing	Over-fed with molasses or very N-rich compost	Cut additive to ~0.5% or omit; dilute next batch 1:2
Clear, no cloudiness	Old/dead compost or over-filtered	Use freshly finished, mature compost; filter coarsely or not at all for soil
Microbial counts low (if testing)	Chlorinated water, cold temperatures, stale compost	Dechlorinate; brew at 18–25 °C; brew from a blended, rich compost
Phytotoxicity on sprayed leaves	Salts or ammonia from immature compost; too-concentrated	Dilute 1:10+; switch to tested mature compost

Key takeaway Compost tea delivers microbes and humic chemistry, not fertility and not pest control. Brew aerobically from tested, mature compost, use it fast, and let strip trials — not marketing — tell you whether it earns its place on your farm.