Gardening, like health, has its fads. Raised beds and whole grains, once considered trendy, are now recognized as important components of good gardening and good health. Compost tea is currently hot in the gardening world, but will it also move beyond fad status?
Compost tea is water in which compost has been steeped. Leached into that liquid are some of the compost’s nutrients, microorganisms, and a witch’s brew of poorly defined compounds called humates. Humates help plants better use nutrients already in the soil and offer a host of other benefits. Compost tea has long been used as a weak fertilizer, but in recent years, devotees of compost tea have shifted the focus away from the liquid’s ability to provide a small amount of nutrients and onto the microorganisms it contains. Microorganisms in healthy soils and composts provide protection against diseases, especially root diseases; improve soil structure with associated benefits of aeration and water retention; and improve nutrient uptake. Promoters of compost tea claim their microorganism-laden brew provides the same benefits. Those microorganisms sprayed on leaves, they say, will fight off garden diseases. To encourage microorganisms, tea making has turned high tech: Commercially available brewing machines provide constant, vigorous aeration, and added materials such as kelp, rock powders, and molasses further stimulate microbial growth. “Tea centers” have sprouted up, mostly on the West Coast, where you can purchase fresh brew for your plants, and you can find laboratories to quantify the microorganisms in and rate the quality of your tea.
Another benefit of considering microorganisms the workhorses of compost tea is that you can lighten up on the application. You no longer need to drench the soil with the prodigious quantities required when the teas are used as fertilizers; 15 to 20 gallons of tea can inoculate a whole acre versus the thousands of gallons required to feed it.
But scientific support for these claims is thin. Only a few studies have yielded positive results, and those results are trivial or meaningless to the backyard gardener.