Garden Myth Busters! Compost Tea

By Rachel Vecchio, Extension Master Gardener & Kirsten Conrad, VCE Extension Agent

THE MYTH: Compost tea suppresses pathogens and enriches garden soil.

THE REALITY: In recent years, compost tea has been touted as the be-all-to-end-all for healthy soil, capable of increasing plant health and yield, and suppressing plant disease. Is it possible that something that is so easy to make can be that fabulous? 

Compost tea is made from compost that has been steeped, or brewed, in water. A long-held theory is that while the compost is brewing in the water, beneficial bacteria and fungi will steep the liquid, and those benefits will pass into the garden soil when the tea is applied. There are two types of compost tea: non-aerated compost tea (NCT) and aerated compost tea (ACT). NCT is simply compost that has been put in a mesh bag and suspended in a barrel full of water. It usually takes one to three weeks for this tea to finish. ACT is brewed in the same way as NCT; however, additives such as sugars (molasses), yeast extracts or kelp are added to the barrel while the water is constantly oxygenated. The additives are thought to catalyze and increase microbial growth and the oxygenation provides the needed aerobic environment for growth. ACT generally takes 24 hours to brew before it is ready for use as a foliar spray or soil amendment.

Little scientific evidence exists to show that NCT and ACT are effective at plant pathogen suppression. In fact, the opposite has been observed. ACT has been shown to contain harmful pathogens, such as E. coli and Salmonella, as a result of the additives and the aerobic brewing environment, which allows these pathogens to grow. The Organic Materials Review Institute classifies compost tea in the same category as “raw, uncomposted manure”and states “compost tea used as a fertilizer or soil amendment is subject to the same restrictions as raw, uncomposted manure. It may only be (i) applied to land used for a crop not intended for human consumption; (ii) incorporated into the soil not less than 120 days prior to the harvest of a product whose edible portion has direct contact with the soil surface or soil particles; or (iii) incorporated into the soil not less than 90 days prior to the harvest of a product whose edible portion does not have direct contact with the soil surface or soil particles.”

While compost tea is marginally effective at pathogen suppression, there is evidence that it improves plant health and quality, and may increase yields by giving plants a quick boost of nutrients. However, these results have been inconsistent.  If you want to include compost tea in your garden toolbox, non-aerated compost tea is recommended over aerated compost tea, and restrict its use to landscape plants, not edibles.  For best results with soil amendments, avoid compost tea and stick with the tried and true use of compost as a mulch or as a bed-building soil amendment. .

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