Compost tea is a water-based extract of finished compost that concentrates and proliferates the microbial populations present in the source material. When prepared correctly under aerobic conditions, it delivers billions of bacteria and fungi per millilitre to soil or foliage surfaces — a biological amendment that costs a fraction of commercial microbial inoculants and uses materials already on hand in any composting system.
The following covers equipment requirements, the brewing process, quality indicators, and application guidelines relevant to Canadian growers from Quebec to Alberta.
What Compost Tea Is — and Is Not
Aerated compost tea (ACT) is not compost leachate. Leachate is the runoff that drains from a compost pile, often containing soluble nutrients but also anaerobic metabolites that can be phytotoxic. ACT is brewed deliberately: finished compost is submerged in water and continuously aerated for 24 to 36 hours to promote aerobic microbial growth. The resulting liquid contains living organisms, not just dissolved compounds.
The distinction matters. Anaerobic extracts can harbour E. coli and Salmonella if the source compost was not fully thermophilic. Properly aerated tea from fully finished compost presents a much lower risk profile, and several Canadian provincial extension services describe ACT as generally safe for vegetable gardens when compost temperature records show adequate kill temperatures.
Equipment
- Brewer bucket: A 20-litre food-grade plastic bucket is sufficient for most residential gardens. Commercial brewers use 200-litre tanks with submersible pumps.
- Air pump: A pump rated for at least two to three litres of air per minute per litre of water. An aquarium pump rated at 20–30 L/min works for a 10-litre batch. Under-aeration is the single most common cause of poor tea quality.
- Air stone or diffuser: Placed at the bottom of the brewer to distribute bubbles finely. Coarse bubbling from an unobstructed tube provides inadequate oxygenation.
- Mesh bag or nylon stocking: Used to hold compost material suspended in the water, allowing easy removal after brewing without straining the entire batch.
- Water without chlorine or chloramines: Let tap water sit uncovered for 24 hours before use, or use rainwater collected from a roof surface that has not been treated with biocide coatings. Municipal water from most Canadian cities uses either free chlorine (which off-gasses readily) or chloramines (which do not off-gas and require a vitamin C tablet or commercial dechlorinator to neutralise).
The Brewing Process
Step 1 — Select and Measure Compost
Use fully finished compost that smells like forest floor — earthy and pleasant, not ammonia or sulphur. For a 10-litre batch, use 500 g to 1 kg of compost material. Higher concentrations do not linearly increase microbial output and can create oxygen deficits in the brew.
Step 2 — Add Microbial Food Sources (Optional)
Bacterial populations respond well to simple sugars. Adding 5 to 10 mL of unsulphured molasses per 10 litres of water provides an accessible carbon source. Fungal populations are better supported by kelp meal (1 to 2 g per 10 L) or a small amount of humic acid. Avoid fish hydrolysate in covered or indoor brewing setups — the odour is significant.
Step 3 — Fill and Aerate
Place the compost mesh bag in the bucket. Fill with dechlorinated water at 18–22 °C. Colder water — common in Canadian spring — significantly slows microbial multiplication; brewing at 12 °C may require 48 hours to reach the same population levels as 22 °C brew at 24 hours. Start the air pump. Confirm bubbling is vigorous and distributed across the water column.
Step 4 — Monitor and Extract
At 18 to 24 hours, the brew should have a pleasant, earthy, slightly sweet aroma. A sharp sour smell indicates the tea has gone anaerobic — oxygen supply was insufficient, or the batch ran too long. Discard anaerobic batches; do not apply them to food crops.
Remove the compost bag. The used compost can be added back to an active pile or applied directly as a surface amendment. Use the tea within four hours of stopping aeration. Microbial populations decline sharply without continued oxygen supply.
Application Methods and Timing
Soil Drench
Apply 1 to 2 litres of tea per square metre of bed surface, using a watering can without a fine rose (to avoid shearing organisms). Apply in the early morning or on an overcast day — UV exposure from direct midday sun reduces bacterial viability at the soil surface within 30 minutes of application. Water lightly before applying to ensure soil has adequate moisture for organism survival.
Foliar Application
Dilute 1:4 with dechlorinated water. Apply with a pump sprayer, covering both leaf surfaces. Foliar applications are most effective at establishing populations that compete with fungal pathogens such as powdery mildew. Apply in early morning so foliage dries before nightfall — sustained leaf wetness in cool Canadian nights can itself promote fungal disease.
Timing Within the Season
In Canadian growing conditions, the highest-value application windows are:
- Two weeks before transplanting, as a soil drench to establish microbial communities before plant roots arrive.
- At transplant, direct to the root zone.
- Every three to four weeks through the season for continuous support, particularly after any disturbance such as cultivation or heavy rain.
- After harvest, before seeding a cover crop — the biological activity supports rapid cover crop establishment.
Quality Indicators Without a Laboratory
A simple microscope at 400× magnification reveals whether a batch is biologically active. A teaspoon of tea should show actively moving rods and cocci (bacteria), along with visible fungal hyphae in good batches. If the field of view shows slow or non-moving particles only, the batch was under-aerated or used poor-quality source compost.
Smell remains the most accessible quality indicator. Earthy and pleasant equals aerobic and usable. Sour, sewage-like, or sharp equals anaerobic and unusable. No reliable visible test replaces basic olfactory assessment.
Source Compost Quality
The tea can only contain what the compost contains. Finished backyard compost that reached thermophilic temperatures (55–65 °C) for at least three consecutive turnings provides a broad biological spectrum. Vermicompost — worm castings — produces tea that is consistently high in bacterial diversity and protozoa. Municipal green bin compost, available across Ontario and British Columbia, is variable in quality but generally acceptable after visual and olfactory assessment.
Never use compost from piles that received diseased plant material unless the pile was confirmed to reach and hold 60 °C throughout. Incomplete composting risks re-inoculating garden beds with the very pathogens being managed.
Storage and Shelf Life
Fresh ACT cannot be stored. There is no safe or effective method for preserving living microbial populations in liquid form at ambient temperatures for more than a few hours. Commercial dried products and capsules use stabilised spores, not the range of vegetative organisms present in freshly brewed tea. Brew only what can be applied in a single session.
