Gardeners have used compost for years to “brew” compost teas, liquids produced when compost mixes with water. Compost teas can provide soils and plants with some micronutrients and nutrients, like nitrogen and phosphorous. Research also suggests that beneficial microorganisms occurring in some kinds of compost may transfer from the compost tea onto plants, which could improve plant disease resistance.
Compost teas have long been made by filling a burlap sack or cheesecloth with finished compost and soaking in a bucket or tub of water. According to an article from University of Illinois Extension, the gardener should allow the water to saturate the compost, stir the sack around, and then let the mixture sit for a few days. Remove the sack and transfer the compost tea to a watering can or sprayer; compost tea can be sprayed over plants. Gardeners have often used compost teas to water seedlings and young plants, which seem to respond well to the liquid fertilizer.
Gardeners and organic farmers have more recently brewed compost teas using aeration, called actively aerated compost tea. In this method, a specific amount of compost (usually with a known chemical property profile, like you would receive in a soil test) is mixed with a certain amount of non-chlorinated water. Oxygen is then bubbled through the mixture over a set period of time. The idea behind the bubbling is that keeping plenty of oxygen provides an environment to increase beneficial, oxygen-loving bacteria coming from the compost. Some scientists think adding these bacteria by spraying compost tea on the plant, or watering it into the soil, improve soil and plant health.
There is still a lot unknown about exactly how different compost teas work. While compost teas can help treat some plant diseases, the effect of the compost tea depends greatly on the quality of the compost used and the brewing process. For that reason, compost tea cannot be labeled as a disease preventer. Another caution for using compost tea, especially for fruit and vegetable gardeners: Naturally-occurring organisms which can cause illness in humans could be present, and even increase, in some compost tea brewing methods.
The benefits of compost tea, properly made and applied, have made it a popular addition in many gardens. If you choose to spray or water your plants with compost tea, consider these suggestions given in a 2013 University of New Hampshire Extension publication:
Start with good-quality compost that has maintained a temperature of at least 131 degrees F for three days
Apply compost tea quickly after brewing; this ensures a maximum population of aerobic bacteria
Avoid applying compost teas on plants that will be used for food within 90 days of harvest