The Jury is Still Out on Compost Tea

Compost tea is currently hot in the gardening world, but will it also move beyond fad status?

Fine Gardening - Issue 107

compost tea in a bucket

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.

Compost is the best way to go

Rather than rely on compost tea, I continually enrich my garden soil with plenty of compost and other organic materials, an approach I have been using for 30 years.

I rely on time and temperature to do their job on my compost. I monitor progress and quality with my compost thermometer (not using my compost until it has cooled down from its typical 150°F), my nose (bad smells usually indicate poor aeration or too much nitrogen), and my eyes (finished compost is brown with few of the original materials recognizable). The regular addition of compost or other organic materials to soils maintains and improves them by keeping beneficial microorganisms thriving and multiplying, so I have no need for compost tea.

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.

You can also make compost tea from fresh vermicompost and worm castings. Watch  Brewing Your Own Vermicompost Tea  to see how it’s done.
View Comments


  1. ecompost 04/05/2016

    I think you are missing the point of compost tea here. Compost tea simply put allows me to provide value to the land over huge areas, where i would need literally tonnes of compost and any number of trucks and tractors to apply it and this is not economic nor is it very organic if we take the whole picture.
    Gardens in Korea have been using ACT for hundreds of years and now the whole Cho movement has thousands of follwers globally, farmers that have saved the land they farm because of its practice.
    You appear to have a limtied vision of how compost tea can help the gardener as another tool to help grow plants, or encourage good soil conditions and related biomes of life that work to support both plant growth and health.
    Many a garden has been spoiled by poor quality compost which may contain the wrong biology depending on your crop choice.
    Sourcing good quality compost is not as easy as you are making it sound here and buying it would cost money that perhaps you have but others not so much.
    If i live on a typical European estate, then I will have limited access to space, turning any of this over to composting is far from ideal. Even if you could collect enough matter in such a restricted environment its just not practical.
    Persons of limited physical ability can more readily apply an ACT than they might shovel heaps of compost everywhere.
    How is making a 20L bag of compost last 3 years not extending the value of the product?
    Perhaps you should put aside your opinion in favour of a more rounded debate on the many virtues of ACT since you have failed to provide science also to support not using it. The ability to extend the money spent on your space alone is worth investigation imo.. To start a discussion we a headline the jury is out, and then to make such a sweepingly confident statement as dont bother, seems a little disingenuous if you ask me.

    The cost or space and time required to make compost does not tally with peoples busy lives in my opinion.
    Pressure on peoples budgets another reason why brewing a good quality ACT will always out compete buying in tonnes of compost or trying to make it yourself.

    Perhaps you make compost and are worried you might sell less?

    1. pharmerdavid 06/14/2017

      Excellent comment - thanks! I have a book "Teaming With Microbes" (organic gardners guide to the soil food web), and I read that inoculating with mycorrhizae lowers the PH, which is helpful for growing cannabis. Powdery mildew is a problem here in Oregon, so I'm hoping the compost tea sprayed on the leaves will help prevent it - I would never spray it on the buds to prevent botrytis though - you never want to spray anything on the flowers.

      I'm about to make my first batch of compost tea of the year, and thanks to the advice here I'm also adding seaweed, molasses, kelp, humid acid, and maybe alfalfa and some other stuff. I have a metal stirrer from the hardware store, which is used for mixing paint manually, and it works well for stirring the compost tea in a 5-gallon bucket. I'll get a bubbler eventually, but right now I can't afford anything. Stirring periodically for three days should work almost as well.

      Compost tea is a no-brainer, and I too was wondering why this article doubted its effectiveness. That said, I'm sure you can grow healthy plants using solid compost too - the microorganisms grow in the soil like they do in the bucket I suppose. But compost tea gets more microorganisms to more area of the soil faster than adding compost on the top layer can, as other commenters have noted. Growing cannabis you don't want any nutrients left at the end of the grow, so adding compost or guano in the soil mix can have negative effects at the end if the plant's roots find pockets of it at that time, so compost tea is a better way to feed the girls.

  2. Grateful_Child 07/12/2016

    the jury may be out for you, but for me, I see dramatic results using compost tea, ...especially on my tomato plants. The plants are huge, ...and they usually are, but the significant difference is the amount of flowers and ultimately tomatoes it produces. For example, my Sun Sugar and Sun Gold cherry tomatoes usually have from 10 to 15 tomatoes on each bract. Now using compost tea, ...spraying the leaves and drenching the soil once a week, each bract can contain 50 or more tomatoes. I've been doing this for three years now. Peppers and eggplant also see extreme fruition.

    1. asafmazar 05/29/2017

      Amazing results. please share details of your recipe and application protocol. I am currently experimenting with tea prepared from home made vermicompost +kelp extract (sea magic) brewed for about 12 hours in a tea lab kit 5 gal bucket. I am still a bit cautious to spray on most of my plants leaves. I pour the filtered liquid into a fertilizer tank and distribute via drip irrigation.

      1. Grateful_Child 05/29/2017

        Hi Asaf..., I think you may not be brewing it long enough. I'm assuming by brewing, you mean you are using a fish tank aerator. That is necessary to provide oxygen to the beneficial bacteria you are feeding. It takes 24 - 36 hours before you see a lot of foam forming on top, ...which is the bacteria. That is the right time to filter it and spray it on the leaves. I cut it 50/50 with water, and then apply the rest of the uncut remainder to the roots. I use both hands and add two generous scoops of compost into a five gallon bucket nearly filled with water, some seaweed extract, and most important about 6 ounces of unsulfured molasses. That is to feed sugar to the bacteria. Good luck with it Asaf...

  3. user-7008242 09/03/2016

    I am a fan of compost teas.. a case like this when the only ingredient is Compost...the teas Will be of minimal value, as the ingredients are the similar so the outcome will be likewise.

    If one uses compost And other ingredients such as alfalfa, kelp. insect frass, humus, etc..the results are amazing...another key ingredient is the air pump...time of brew and application rates...good luck it works for me!

  4. WVlandscaping 03/30/2021

    Good Read. Thanks for sharing.

  5. WVlandscaping 03/30/2021

    Good Read. Thanks for sharing.

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  6. hedgesbc 07/07/2021

    I was really interested in your article on Compost Tea. Right now is the perfect time to try it out! Many people are getting great results with compost tea and many others want to see more research before committing to anything. hedge trimmer

  7. PeakLawn 08/17/2021

    Really enjoyed reading your post on compost tea. I'm not sure if you are a gardener or just interested in gardening but it's always exciting to see the newest trends and technologies that arise in this industry. :)

  8. r1ckwlb3rg 06/14/2022

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