The Jury is Still Out on Compost Tea

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.

Effect on diseases is questionable

The most promising reports about compost tea come from its use on leaves to suppress diseases, especially powdery mildew and gray mold (perhaps because the former is fairly superficial and the latter is a weak pathogen). Theoretically, “good bugs”—the microorganisms from the tea—sprayed on plant leaves could crowd out or antagonize “bad bugs”—the disease-causing microorganisms.

Before you start using compost tea by the gallon, be aware that most of the claims made concerning this liquid have been anecdotal and, even then, inconsistent. The positive results from scientific studies have been few and, again, lacking in results meaningful to backyard gardeners. A documented benefit created under sterile conditions, for example, does not translate to a benefit in your backyard, with its slew of natural microorganisms.

Soil receives little benefit

Benefits from compost teas are on even shakier ground when the tea is used to improve the soil. Compost has been shown to improve soil tilth, help soil retain nutrients, enhance nutrient availability, increase rooting depth, and suppress root diseases. Compost tea, by extension, is credited with providing these same benefits. Therein lies part of the great appeal of compost tea. Who wouldn’t rather improve an acre of soil with the recommended 15 to 20 gallons of compost tea rather than have to heave around 2 to 5 tons per acre of compost?

But compost and compost tea are not the same. They differ quantitatively and qualitatively in microbial makeup, and most dramatically, one is a relatively small volume of liquid and the other is a relatively large, mostly solid mass. Compost’s bulk comes mainly from carbon compounds, which are the major foods for beneficial microorganisms. Compost tea contains relatively little of these carbon foods. The beneficial microorganisms of composts and compost teas are already present in most soils and will multiply rapidly if food supplies permit. If your soil does not have beneficial microorganisms, it probably means the conditions aren’t hospitable to them. Unless you improve those conditions, any added microorganisms will die.  

Research into compost tea is ongoing. At the very least, the mixture does no harm, so if you feel that using compost tea isn’t merely carrying coals to Newcastle, give compost tea a try.