Compost tea is a liquid biological amendment made by extracting beneficial microorganisms and soluble nutrients from finished compost into water. Applied to plants and soil, this liquid introduces a diverse microbial population beneficial for nutrient cycling and overall plant health. Using compost tea improves the soil food web, enhancing a plant’s ability to absorb available nutrients and strengthening its defenses against disease. The goal of brewing is to multiply the numbers of helpful bacteria, fungi, and protozoa found in the starting compost material.
Required Materials and Starter Ingredients
Aerated compost tea requires equipment designed to maintain high oxygen levels. The main vessel is typically a clean, food-grade 5-gallon bucket paired with an air pump that delivers sufficient oxygen. Air stones or diffusers attached to the pump tubing are placed at the bottom of the bucket for vigorous aeration. A mesh bag, such as a paint strainer or fine cheesecloth, holds the solid compost, preventing particles from clogging the air stones or sprayers during application.
Biological inputs are important, starting with high-quality, fully finished compost or worm castings, which source the beneficial microbes. This starting material should ideally have been hot-composted to eliminate potential pathogens and should not contain fresh manure. To encourage the rapid reproduction of the extracted microorganisms, a microbial food source is added to the water. Common activators include simple sugars like unsulfured molasses, which preferentially feed bacteria, or complex sources like fish hydrolysate or humic acid, which support a broader range of fungi and protozoa.
Step-by-Step Aerated Brewing Method
The actively aerated brewing process begins with preparing chlorine-free water, as disinfectants like chlorine or chloramine kill beneficial microbes. Tap water should sit uncovered for 24 hours to off-gas chlorine, or a dechlorination agent like humic acid or Vitamin C powder can neutralize it quickly. Next, one to two cups of high-quality compost per five gallons of water are placed inside the fine mesh bag and suspended within the bucket.
Once the compost bag is submerged, the microbial food source is added directly to the water, offering an immediate energy supply for the organisms to multiply. The air stones are placed into the bucket, and the air pump is turned on, initiating continuous aeration. The aeration must be strong enough to create a vigorous, rolling churn on the water’s surface, ensuring the dissolved oxygen levels remain high throughout the process.
The optimal brewing temperature generally falls between 65°F and 75°F for the most successful microbial growth. The tea is typically brewed for a period between 18 and 36 hours; brewing past this timeframe risks the microorganisms consuming all available food and oxygen, causing the tea to transition into an anaerobic state. A successful batch of aerated compost tea should have a pleasant, earthy aroma, indicating a healthy, aerobic environment.
Understanding Non-Aerated (Passive) Tea
Non-aerated or passive compost tea, sometimes called compost extract, is made by simply soaking compost in water without an air pump or bubbler. This simpler method requires only a bucket, compost, and water, and steeping lasts longer, often one to seven days. The lack of continuous forced oxygen, however, is a major differentiating factor that carries significant risk.
Without active aeration, the microorganisms quickly deplete the limited dissolved oxygen in the water, causing the mixture to become anaerobic. This low-oxygen environment favors the growth of anaerobic bacteria, which can include potentially harmful human pathogens such as E. coli or Salmonella, especially if the source compost contained animal manure. The resulting liquid often develops a foul, putrid odor, which is a clear indicator of the unhealthy, anaerobic conditions. Due to the potential for cultivating detrimental organisms, the actively aerated method is preferred as a safer alternative for gardeners.
Proper Application and Usage Guidelines
The finished compost tea must be strained immediately after brewing, using a fine mesh to remove solid particles. This straining step is especially important if the tea will be applied using a sprayer, as residual compost sediment can easily clog nozzles. The resulting liquid concentrate is then ready for application, but it must be used quickly to ensure the viability of the live microbial populations.
The beneficial microbes rapidly consume oxygen once aeration stops, so the tea should ideally be applied within four to six hours of brewing completion. The tea can be applied as a soil drench, pouring it directly onto the soil around the base of plants to benefit root health and enhance the soil food web. Alternatively, it can be used as a foliar spray, coating the leaf surfaces to create a protective biological film that can help suppress plant diseases.
While the tea can sometimes be applied undiluted, it is common practice to dilute it with dechlorinated water at a ratio of 1 part tea to 4–10 parts water, which helps to spread the microbial inoculum over a larger area. For foliar applications, the tea should be applied in the early morning or late evening, as the ultraviolet rays from the midday sun can be detrimental to the live microorganisms. Compost tea does not store well, and any unused portion should be discarded after 24 hours, as the microbial community will have largely died off or turned anaerobic.