Compost Tea: Science Says It Doesn't Work & May Be Risky

Dr. Linda Chalker-Scott (WSU) reviews the evidence: compost tea lacks scientific support for disease control and poses pathogen risks.

Two Types, Weak Evidence

• Non-aerated (NCT): Passive steep. 34 studies on foliar disease—lab success, field variability. Reduces incidence, not severity.
• Aerated (ACT): Actively bubbled. Only 7 studies. Lab fungal control (1), greenhouse inconsistent (3), field studies worsened disease on apples & potatoes.

Why So Little Research?

High batch-to-batch microbial variability → high data variance → inconclusive results often unpublished. Rigorous trials require 20+ field replicates × 3 repetitions = impractical for most.

Safety Concerns

• ACTs "enhanced" with molasses/kelp documented to harbor E. coli and Salmonella.
• No compost tea products EPA-registered as pesticides.
• WSU Master Gardeners cannot recommend unproven, unregistered products.

Better Alternative: Organic Mulch

Intact compost as mulch component provides proven benefits: moisture retention, erosion control, temperature moderation, nutrition, disease suppression (biological, chemical, physical), weed reduction, less pesticide need—all backed by solid science.