How to Make Compost Tea That Actually Helps Your Plants

The Short Version

To make compost tea, hang a porous bag of finished, hot-composted compost in a clean bucket of drinking-quality water, bubble air through it with an aquarium pump for 24 to 48 hours, and pour it on the soil promptly. Add nothing else. University extension services advise against the molasses that many recipes call for, because sugar can feed harmful bacteria as readily as helpful ones. Expect modest results. Research supports compost tea as a mild liquid feed more than as a disease cure, and solid compost spread on the bed does more for the soil.

What Compost Tea Is

Compost tea is water in which compost has been steeped so that soluble nutrients and microbes move into the liquid. University of Arizona Cooperative Extension compares it to a tea bag in hot water. It is different from leachate: University of Vermont Extension points out that compost tea is not the dark solution that leaks out of the bottom of a compost pile.

There are two ways to make it. The original method is passive. A bag of compost sits in a bucket of still water, which Washington State University Extension notes produces a nonaerated tea whose main components are anaerobic microbes and nutrients. The newer method adds an aerator. Oregon State University Extension explains that aeration raises the dissolved oxygen in the brew and shifts the microbial community toward bacteria and fungi that tolerate oxygen.

Time is the practical difference. Vermont gives at least five to eight days of fermentation for non-aerated tea and 24 to 48 hours for aerated tea. Aeration shortens the wait but adds equipment and complexity.

What the Research Says It Does

Sales claims for compost tea run well ahead of the evidence. Washington State University Extension reviewed the peer-reviewed studies as of 2007 and found 34 papers on nonaerated tea and disease control. Good results were common in the laboratory and more variable in the field, with a trend toward fewer foliar disease infections but no reduction in their severity.

Aerated tea had much less behind it. The same review found seven papers. Results were less consistent in the greenhouse than in the lab, and two field studies reported that aerated teas failed to prevent foliar disease and made it worse on apples and potatoes. The review’s conclusion was that the science is not strong for aerated tea on crop plants, and weaker still for lawns, shrubs and trees.

Vermont’s summary is more neutral: research sometimes shows that compost tea suppresses disease and sometimes does not. It also describes a multi-year study in which tea gave more measurable benefit in stimulating crop growth, yield and quality than in suppressing disease. One reason findings vary is that the product varies. Washington State notes that compost teas differ widely in microbial and nutrient content from batch to batch.

The fair reading is that compost tea is a dilute liquid fertilizer with living passengers. It can give container plants and transplants a gentle feed. It is not a dependable fungicide.

The Food Safety Rules That Shape the Recipe

Compost can carry bacteria that make people sick, including E. coli, Salmonella and Listeria. University of Arizona Cooperative Extension reports that each has been found in compost tea, and that adding nutrient additives and water can cause pathogens to proliferate. Arizona’s publication is careful to add that links between compost tea and foodborne illness have not been established in peer-reviewed medical literature. The potential exists, though, and a few rules keep it small.

• Start with properly made compost. Vermont specifies compost that has held at least 131°F for 3 days and has been mixed so the whole pile heated. Our comparison of hot and cold composting explains how a pile gets there.
• Leave out additives. Vermont says to avoid them when fermenting compost tea, and names simple sugars such as molasses in particular. Rutgers Cooperative Extension explains why: an additive may itself be contaminated, or it may stimulate the growth of pathogens present at low levels.
• Use potable water for brewing and for diluting, and sanitize the equipment.
• On food crops, put the tea on the soil. Arizona advises applying it to the soil above the roots instead of directly to the foliage.

Both Vermont and Arizona also refer to a waiting period drawn from the organic rules for raw manure. Arizona describes it as 120 days before harvest for crops whose edible part touches the soil and 90 days for other edible crops. Vermont applies the 90 to 120 days to tea made with additives and not tested, or tea that fails water quality guidelines. Arizona’s simplest advice is to use compost tea only on non-edible plants or the non-edible parts of plants, which removes the question.

How to Brew Aerated Compost Tea, Step by Step

Arizona calls the bucket-bubbler the most common technique among home growers. It needs a bucket, an aquarium pump, one or two air hoses with aquarium-style air stones, and a porous bag for the compost. Arizona’s diagram uses a nylon stocking.

1. Choose the compost. It should be finished and come from a pile that held at least 131°F for 3 days.
2. Sanitize the bucket, tubing and stones.
3. Fill the bucket with potable water and start the pump so the water is moving.
4. Fill the bag with compost, tie it shut and hang it in the water. Add no molasses, kelp or other feed.
5. Aerate continuously. Vermont considers 24 to 48 hours sufficient, and Arizona’s version of the method runs for 3 days.
6. Lift out the bag and use the tea promptly. Return the spent compost to the garden.

None of the extension publications above gives a home-scale quantity of compost per bucket, and the review from Washington State explains why a precise recipe would promise more than it can deliver: the result varies from batch to batch regardless. A bag that leaves room for water to circulate around it is the working rule.

How and Where to Apply It

Pour the tea on the soil over the root zone with a watering can. For vegetables and herbs, that placement follows Arizona’s guidance and keeps the liquid off the parts you eat. Ornamentals, lawns and container flowers carry no harvest concern, so they are the easiest place to use it.

Apply it the day it is finished. Vermont says brewed tea should be applied promptly, and Washington State notes that an aerated tea becomes a nonaerated one once aeration stops. If it needs thinning to go further, dilute it with potable water. Wash your hands and clothing afterward, which is one of Arizona’s four guidelines.

When Solid Compost Is the Better Tool

Tea carries what dissolves or floats out of compost, and the bulk material stays in the bag. Washington State’s review asks why a gardener would leach materials out of compost when the intact compost could go on the bed as part of an organic mulch layer. It lists the documented benefits of organic mulch, among them better soil moisture, less erosion and compaction, improved soil nutrition and fewer weeds. For an established bed, spreading the compost puts all of it to work and needs no pump. Our guide on how much compost to add to a garden bed gives the rates, and the best mulch for a vegetable garden compares compost with straw and leaves as a surface layer.

Tea makes the most sense where solid compost is awkward: pots that are already full, seedlings in cell trays, and a new transplant that will take a liquid feed. It does not replace a feeding plan for pots, which is covered in fertilizing container plants. Used that way, on the soil, with clean water and no sugar, it is a low-risk extra for the price of an aquarium pump.