Compost tea is an organic liquid soil amendment used by cannabis cultivators to enhance the natural microbial life in their growing medium. This brewed solution extracts and multiplies beneficial bacteria, fungi, and other microorganisms from high-quality compost. Applying the tea introduces these living organisms to the soil or plant surfaces, acting as a probiotic to foster a thriving ecosystem around the roots and leaves. This process supports the plant’s natural defenses and improves nutrient availability, offering a holistic alternative to synthetic fertilizers.
Essential Components and Setup
The foundation of a successful brew starts with selecting the right microbial source, typically high-quality, fully mature compost or vermicompost (worm castings). Vermicompost is often preferred because it naturally contains a high concentration of beneficial bacteria and enzymes needed for a robust biological tea. The compost should smell sweet and earthy, indicating aerobic conditions, and must not be sterilized, as the goal is to cultivate the living organisms within it.
The essential hardware for brewing actively aerated compost tea (ACT) consists of a five-gallon bucket, a small aquarium-grade air pump, an air stone, and connecting tubing. Continuous aeration is non-negotiable; it ensures the environment remains rich in oxygen, promoting the growth of beneficial aerobic microorganisms and preventing harmful anaerobic bacteria. The air pump should be powerful enough to provide an airflow of at least 0.4 to 0.6 liters per minute per liter of water to maintain sufficient oxygen saturation.
Water quality is a significant consideration, as beneficial microbes are sensitive to chemical treatments. Standard municipal tap water often contains chlorine or chloramine, which are added to eliminate microbial life. To protect the microorganisms, tap water must be dechlorinated by either letting it sit uncovered for 24 hours to off-gas the chlorine, or by adding humic acid or ascorbic acid to neutralize chloramine.
Once the water is prepared, microbial food sources are added to stimulate the rapid multiplication of the organisms extracted from the compost. These sources typically include simple sugars like unsulfured blackstrap molasses, which primarily feeds bacteria, providing a fast-acting energy boost. More complex foods, such as liquid kelp extract or fish hydrolysate, encourage a wider diversity of microbial life, including fungi and protozoa. The choice of food also influences the final tea composition; for instance, ingredients like kelp meal promote fungal growth, which is often favored during the cannabis flowering stage.
The Step-by-Step Brewing Process
The actual brewing begins once the water is dechlorinated and the equipment is assembled. The compost is placed inside a fine mesh bag or cheesecloth, creating a “tea bag” that allows microorganisms to be extracted while keeping solid material out of the final liquid. This prevents the solids from clogging sprayers or air stones, ensuring a clean and effective brew.
The mesh bag containing the compost is submerged into the water, and the microbial food sources are added and gently stirred to dissolve, particularly molasses or other thick liquids. The air stone, connected to the pump, is placed at the bottom of the bucket and turned on immediately, beginning the continuous aeration cycle. The constant bubbling action maintains high oxygen levels and physically agitates the mixture, knocking the microorganisms off the compost and into the water where they multiply.
Temperature is a primary factor influencing the speed of microbial reproduction and the required brewing time. The optimal range is generally between 65 and 75 degrees Fahrenheit (18–24°C). Within this range, the tea is typically ready in 24 to 36 hours, with warmer temperatures accelerating the process and cooler temperatures slowing it down.
Brewing for too long can deplete oxygen, causing the tea to become anaerobic and potentially promoting the growth of undesirable organisms, so monitoring is important. A healthy batch of actively aerated compost tea typically develops a light, earthy, and sometimes slightly sweet aroma, often accompanied by a layer of foam indicating high microbial activity. Once the required time has passed, the air pump is turned off, the mesh bag is removed, and the finished liquid should be strained one final time to ensure all solid particles are removed.
Application Techniques for Cannabis
The finished compost tea represents a concentrated population of beneficial microorganisms and is highly perishable, demanding immediate application. Microbial populations decline rapidly once aeration stops and oxygen is depleted, and the tea may turn anaerobic within a few hours. For this reason, the tea should be applied to the cannabis plants within four to six hours of brewing concluding to maximize the benefit of the living organisms.
The concentrated tea must often be diluted before application, though the precise ratio varies depending on the grower’s recipe and intent. A common dilution rate for soil drenching is one part compost tea to between five and ten parts non-chlorinated water. Some growers apply the tea at full strength directly to the soil, believing the risk of over-fertilization is low since the tea is primarily a microbial inoculant rather than a high-concentration nutrient solution.
There are two primary methods for applying the tea to cannabis plants: soil drench and foliar spray. A soil drench involves pouring the diluted tea directly onto the root zone, where the microbes immediately begin to improve nutrient cycling and soil structure. Foliar spraying involves applying the tea to the leaves, coating the plant surfaces with a microbial biofilm that helps fend off pathogenic organisms and enhances disease resistance.
Compost tea is generally applied during the vegetative stage to promote robust leaf and stem growth, and it can continue into the early flowering stage to support the development of larger buds. Regular applications every one to two weeks help maintain a strong microbial presence in the soil and on the leaves. Applying the foliar spray during the early morning or late afternoon is recommended, as this prevents the tea from evaporating too quickly and allows the plant to absorb the benefits.