Compost tea is a liquid extract brewed from finished compost that delivers beneficial microorganisms and soluble nutrients directly to plant roots and leaves. When brewed correctly, compost tea can suppress foliar diseases, improve nutrient uptake, and boost soil biology. This guide covers everything from choosing your compost to brewing, applying, and troubleshooting compost tea.
What Is Compost Tea?
Compost tea is not simply water that compost has soaked in. Properly brewed compost tea is a living biological product teeming with bacteria, fungi, protozoa, and nematodes extracted and multiplied from high-quality compost. These organisms colonize leaf surfaces and soil, outcompeting disease-causing pathogens for resources and space while making nutrients more available to plants.
Aerated vs Non-Aerated Compost Tea
Actively Aerated Compost Tea (AACT)
AACT is brewed with continuous air supply from an aquarium pump or dedicated brewer. Oxygenation encourages aerobic bacteria and fungi to multiply rapidly, reaching populations in the billions per milliliter within 24 to 36 hours. This is the preferred method for most applications because aerobic organisms are the most beneficial for plant health and disease suppression.
Non-Aerated (Passive) Compost Tea
Passive tea is made by steeping compost in water for several days without aeration. While simpler, this method favors anaerobic organisms that can produce compounds harmful to plants if the brew goes too long. Passive tea is best used within one to two days and is primarily a nutrient extract rather than a biological inoculant.
How to Brew Aerated Compost Tea
Start with a five-gallon bucket of non-chlorinated water. If you use municipal water, let it sit uncovered for 24 hours or use an aquarium dechlorinator to remove chlorine and chloramine, which kill beneficial microbes. Place one to two cups of high-quality finished compost in a mesh bag or old pillowcase and suspend it in the water.
Add microbial food sources to boost organism reproduction. Use one tablespoon of unsulfured molasses to feed bacteria, and one tablespoon of liquid kelp or humic acid to feed fungi. Attach an aquarium pump with an airstone to the bottom of the bucket and turn it on. The water should bubble vigorously. Brew for 24 to 36 hours at temperatures between 65 and 80 degrees Fahrenheit.
How to Apply Compost Tea
Use compost tea within four to six hours of completing the brew, as organisms begin dying once aeration stops. Apply as a soil drench at full strength, pouring one to two gallons per 100 square feet around the base of plants. For foliar application, strain through a fine mesh and spray early morning or late evening when leaf stomata are open and UV light is low. A backpack sprayer or watering can with a fine rose both work well.
When to Apply Compost Tea
Apply compost tea every two to four weeks during the growing season. Key application times include transplanting seedlings, at flowering, during fruit set, and whenever plants show signs of stress. For disease prevention, apply as a foliar spray before problems appear rather than as a cure for existing infections. Spring applications help establish beneficial organisms in the soil as the growing season begins.
Signs of Good vs Bad Compost Tea
Well-brewed compost tea smells earthy and pleasant, similar to forest floor soil. It should be brown and slightly frothy from the brewing process. If your tea smells sour, putrid, or like ammonia, something went wrong. Common causes include using immature compost, brewing too long without adequate aeration, or contamination with anaerobic organisms. When in doubt, dump a bad batch on your compost pile and start over.
DIY Compost Tea Brewer
Build a simple brewer from a five-gallon bucket, a dual-outlet aquarium air pump rated for at least 30 gallons, two airstones connected with airline tubing, a mesh bag for compost, and a stick or dowel to suspend the bag. Total cost is under twenty dollars. For larger gardens, upgrade to a 20 to 50 gallon brewer using a pond aerator or regenerative blower for more powerful aeration.
Frequently Asked Questions
Can I store compost tea?
No. Compost tea is a living product that begins losing viability within hours of brewing. Always use it the same day you finish brewing for maximum benefit. Stored tea quickly becomes anaerobic and potentially harmful to plants.
Does compost quality matter?
Enormously. Your tea is only as good as the compost it comes from. Use well-aged, fully finished compost that smells earthy and has no recognizable food or plant material. Compost made from diverse inputs produces the richest microbial diversity in tea.
Can compost tea replace fertilizer?
Compost tea supplements fertilizer but does not replace it for heavy-feeding crops. It primarily delivers biological benefits: disease suppression, improved nutrient cycling, and enhanced soil biology. Continue your regular fertility program alongside compost tea applications.