Compost Tea for Cannabis: Brewing, Application, and What the Science Actually Says

Compost tea is liquid biology. You take high-quality compost, brew it in aerated water with food sources to multiply the beneficial microorganisms, and apply the resulting liquid to your soil or leaves. The goal is to inoculate your growing medium with diverse populations of bacteria, fungi, protozoa, and other microbes that form the soil food web.

It's one of the most talked-about techniques in organic cannabis growing. It's also one of the most over-marketed. This guide covers how to do it right AND what the science actually supports.

What compost tea is (and isn't)

What it IS

• A liquid extract of compost, produced by steeping compost in aerated water for 24–48 hours
• A delivery vehicle for beneficial microorganisms (bacteria, fungi, protozoa)
• A way to rapidly increase microbial populations in soil
• A complement to a good soil-building program

What it ISN'T

• A replacement for properly amended soil
• A significant source of plant nutrients (NPK content is negligible)
• A guaranteed solution for soil problems
• A standalone fertilizer

The honest assessment: compost tea adds biology. If your soil already has good biology (well-amended, not sterilized, not overwatered), the marginal benefit of compost tea is modest. If your soil biology is depleted (reused, chloraminated water, synthetic-only feeding), compost tea provides real, measurable improvement.

Types of compost tea

Actively Aerated Compost Tea (AACT)

This is what cannabis growers mean by "compost tea":

• Compost is steeped in water with continuous aeration (air pump + air stone)
• Food sources (molasses, kelp, fish hydrolysate) are added to fuel microbial reproduction
• Brewing time: 24–36 hours
• The result is aerobic — dominated by beneficial aerobic bacteria and fungi

This is the only type recommended for cannabis. Aerobic conditions prevent pathogenic anaerobic bacteria from multiplying.

Non-aerated compost extracts

Soaking compost in still water without aeration. This goes anaerobic quickly and can harbor plant pathogens (Pythium, E. coli). NOT recommended.

Fermented plant extracts (not technically compost tea)

Korean Natural Farming (KNF) ferments like FPJ (fermented plant juice) and FFJ (fermented fruit juice) are sometimes called "teas" but are a different process entirely — they're lacto-fermented plant material. Effective but distinct from compost tea.

How to brew AACT

Equipment

Total cost: ~$40 for the complete setup. Reusable indefinitely (clean between brews).

Ingredients

Base compost (choose one or combine):

• High-quality earthworm castings (most common, most reliable) — ½ to 1 cup per 5 gallons
• Well-aged garden compost — 1 cup per 5 gallons
• Forest humus (leaf mold from a deciduous forest floor) — ½ cup per 5 gallons (excellent fungal inoculant)

Microbial food sources (choose 2–3):

• Unsulfured blackstrap molasses — 1 tablespoon per 5 gallons (feeds bacteria)
• Liquid kelp extract — 1 tablespoon per 5 gallons (feeds fungi, provides trace minerals)
• Fish hydrolysate — 1 teaspoon per 5 gallons (feeds both bacteria and fungi)
• Humic acid — 1 teaspoon per 5 gallons (fungal food, chelation agent)

Do NOT use: sulfured molasses (sulfur kills microbes), honey (antimicrobial), granulated sugar (feeds bacteria at the expense of fungi), anything with preservatives.

The brewing process

1. Fill the bucket with water. If using chlorinated/ chloraminated tap water, treat first — chloramine kills the microbes you're trying to grow. Use RO water, or treat with a Campden tablet 30 minutes before brewing
2. Add the compost in a mesh bag. Suspend it in the water so it's fully submerged
3. Add the food sources (molasses, kelp, fish) directly to the water. Stir to dissolve
4. Turn on the air pump. The air stone should produce a continuous rolling boil of bubbles. Vigorous aeration is essential — it keeps the brew aerobic
5. Brew for 24–36 hours. Stir occasionally. The liquid should smell earthy and slightly sweet — like forest soil
6. Check the smell. If the brew smells bad (rotten, sour, sulfurous), it has gone anaerobic. Dump it and start over. Good tea smells earthy and alive, not foul
7. Use immediately. Compost tea has a short shelf life — the microbes need oxygen and food. Apply within 4–6 hours of turning off the pump. After 8 hours, microbial populations decline rapidly

Temperature

• Ideal brew temperature: 65–80°F
• Below 60°F: microbial reproduction slows significantly. Brew longer (36–48 hours) or warm the water
• Above 85°F: some beneficial organisms die. Aerobic bacteria produce less diverse populations at high temperatures

WNY winter note: brewing in a 55°F basement in January produces a weak tea. Move the bucket to a warmer room or use an aquarium heater set to 72°F in the brew bucket.

Application

Soil drench (primary method)

• Dilute the finished tea 1:1 with dechlorinated water (or apply undiluted for maximum biology)
• Water the soil as you normally would, using the tea as your watering solution
• Apply during lights-off or low light — UV from grow lights kills microbes on the soil surface
• Water normally afterward — the microbes establish in the root zone

Foliar spray (secondary method)

• Strain the tea through a fine mesh to remove particles (they clog spray nozzles)
• Spray all leaf surfaces (top and bottom) during lights-off
• Foliar application inoculates the leaf surface with beneficial bacteria that can outcompete pathogens (powdery mildew, Botrytis)
• Stop foliar tea applications by week 2 of flower — same rule as all foliar sprays

Application frequency

• Soil drench: every 2–4 weeks during veg and early flower
• Foliar: every 2 weeks during veg only
• After transplanting: one application to inoculate the new root zone
• Don't over-apply — more frequent isn't better. The microbes need time to establish and find equilibrium

What the science says

Supported by evidence

• Compost tea introduces diverse microbial populations to the root zone
• The bacteria and fungi in tea CAN establish in soil and contribute to nutrient cycling
• Foliar applications of compost tea show disease suppression in some studies (particularly against powdery mildew)
• Tea-treated soils show increased microbial biomass

Partially supported

• Claims of significant nutrient delivery — tea provides minimal NPK. The biology it introduces helps make EXISTING soil nutrients more available, but tea itself is not a meaningful fertilizer
• Claims of pest suppression — some evidence in controlled studies, but results are inconsistent

Not supported

• "Compost tea replaces fertilizer" — it doesn't. Tea supplements biology; the soil still needs amendment-based nutrition
• "More tea = better results" — frequency studies show diminishing returns beyond monthly application
• "You can fix any soil problem with tea" — dead, depleted, or contaminated soil needs physical amendment, not just a microbial spray

Common mistakes

1. Brewing without aeration — still water goes anaerobic within hours. Anaerobic tea can harbor plant pathogens. The air pump is non-negotiable
2. Using chloraminated water — chloramine kills the microbes you're trying to multiply. Treat your water before brewing. See our water quality guide
3. Storing tea for later — compost tea has a 4–6 hour window after brewing. You can't bottle it and use it next week. Brew fresh each time
4. Over-feeding the brew — too much molasses causes a bacterial explosion that consumes all the oxygen, killing the aerobic organisms and crashing the brew. Use the recommended amounts
5. Expecting tea to replace soil building — compost tea adds biology. It doesn't add structure, minerals, or organic matter in meaningful amounts. Build your soil properly first, then use tea to enhance the biology

For the living soil approach, see our living soil guide. For organic amendments, see our amendments guide. For water treatment, see our water quality guide.