Compost tea…again

My not-fan Justin has emailed me again with some more substantial comments of my criticisms of compost tea.  I’ve posted his email here, along with my responses in a point-counterpoint format:

1.  “Compost teas do vary from batch to batch, the same way galaxies vary.  Without the complexity and biodiversity present in the tea, you might as well just be using water.”

Yes, they do vary, and this is why it is so difficult to conduct replicated and repeatable studies on the efficacy of compost tea.  The comparison to variability in galaxies is really not relevant, nor is it conducive to experimentation.

2.  “Generally speaking though, this can be overcome by the purchase of virtually any microscope capable of achieving 400x field of vision or greater. By looking at what is present in the tea and a little bit of background knowledge, one can make an educated decision as to whether or not it will improve conditions on one’s plants and soil.”

Purchasing a microscope does not overcome variability.  Furthermore, microbial species can’t be reliably identified simply by looking at them under a microscope.  The “little bit of background knowledge” is vague.  What, exactly, will help in making the “educated decision” in whether it will do any good to use it?

3.  “I assume that these steps were not taken in these experiments, because of the generally lacking method in what has been come to be labeled (tobacco science).”

The steps referred to (I assume in point 2) are not useful in assessing efficacy of a product – in other words, demonstrating an effect not seen in the control treatment.  What would be the control?  Not looking under a microscope?  Not having background knowledge?  An experiment requires experimental variables.  I hadn’t heard of  “tobacco science” and had to look it up.  Apparently it’s “science that is skewed or biased, especially toward a particular industry.”  The only industry I see in this discussion is the compost tea industry – and yes, it’s an industry.

4.  “First of all you are trying to disprove compost tea as a foliar pesticide only. You do not do a relatively new science justice by not looking at the wholeness. Any and all foliar applied pesticides are palliative in nature, and symptoms will recur if you do not deal with the source problem. Compost tea (aerated) is to be used in the rhizosphere first, foliage second, and surrounding environment third. If you are not talking about this mode of application, you are not talking about compost tea.”

Compost tea is not a new science.  It is a product.  To demonstrate efficacy of a product requires conducting a controlled experiment in which there are one or a few variables.  It’s not possible for science to look at the “wholeness” of compost tea – it has to be looked at systematically.  Neither is compost tea defined by its mode of application.

I do agree with Justin, however, that symptoms (of disease or whatever) will recur if the underlying problem isn’t addressed.  There are scientifically testable, consistently reliable methods for improving soil health and plant health. At this point, compost tea is not one of them.

5.  “In order to disprove compost tea, you must first explain to the reader how balances of microbial life (bacteria, fungi, protozoa, nematodes, microarthropods, earthworms) are different in various stages of ecological succession. You must describe how the OVERALL HEALTH of any plant depends on how it has evolved to live in the soil conditions in which it is planted. You must describe how human activity effects soil food webs and how soil disturbed or treated with substances toxic to microbial life will move the soil backwards in succession. This will create a soil that favors weeds over crops by reverting the soil to bacterial dominance.”

Disproving any hypothesis (e.g. “compost tea prevents foliar disease) relies upon scientific evidence.  What Justin is asking for is not experimental but explanatory.  (There are several inaccuracies in what he outlines above, but in the interest of sticking to one topic I’m ignoring them.)

6.  “If you are going to debate compost tea you must disprove its ability to create a more fungal soil and inoculate the rhizoshpere with arbuscular mycorrhizae, improving soil born nitrogen. Excuse me, soil born proteins in the form of microbial biomass that are released as ammonium or nitrates in detritus, when consumed by predatory microbes, that are generally found to be lacking in human disturbed soils.”

The first point is incorrect, and is one of the hallmarks of pseudoscience – reversed burden of proof.  It is up to proponents of compost tea – or any other product or practice – to demonstrate efficacy.  (Wikipedia has an excellent overview of the characteristics of pseudoscience.)

7.  “You must prove: that most garden or Ag soils have a stable food web, the food web is not necessary, or that compost tea does not create a more complete food web. You must create a fair experiment (not paid for by cargill) that tests foliar applications on crops planted into a healthy rhizosphere with a complete food web.”

No.  Compost tea proponents must demonstrate that compost tea has an effect.  Period.  (It’s also important to understand that science doesn’t “prove” anything.  It either supports a hypothesis or disproves it.  That’s why scientific inquiry is a dynamic process – you never know when new evidence will lead to a paradigm shift.)

8.  “If you cannot present your findings in this way, you are misleading your readers knowingly. I was raised to classify that as a lie.”

I don’t conduct these compost tea experiments (though I do conduct research in other areas).  Part of my job as an extension educator is to read the scientific literature and translate it for use by nonscientists.  I have posted an extensive bibliography of the compost tea literature on my web site.  If I were either deliberately or accidentally misleading anyone, I would be in serious trouble with my university.  Given that I started my criticism of compost tea on my web site over 10 years ago, it’s likely that the information is not misleading.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

26 thoughts on “Compost tea…again”

  1. I can’t really figure out what Justin is trying to say but I do have a question about compost tea. My worm bin generates a regular supply of “tea” and I have always assumed that it is sort of a weak erratic form of miracle grow. I spread it around and my plants seem to grow fine (no real control or research). I thought that the effect was chemical not biological. Has any research been done on the NPK content of compost tea or worm bin run off? As an ecologist (I study aquatic food webs) frankly the whole assumption that adding compost tea is going to alter the food web in your soil is a bit baffling. Why wouldn’t you just add the compost? Surely you are drowning some of the necessary organisms?

    Thanks, Wendy

  2. Reading Point 5 of Justin’s, I come close to seeing a hint of a believer in vitalism or “biodynamism”.

  3. Wendy, I’ve always thought that compost leachate (the water leaching through your bin) should be used for watering plants. Its nutrient and microbe content will be variable, but I’d rather see it used than wasted as runoff. The compost tea I object to is that which requires pump aeration 24/7. What an incredible waste of energy. As you suggest and I strongly encourage, the use of compost itself provides the nutrients as a “slow food”. It’s certainly more natural, and when it rains you get compost tea.

  4. Curmudgeon G., I agree. It’s another example when proponents of a belief system want the patina of science but not the rigor.

  5. What is essentially in issue here as far as I’m concerned, as Linda mentioned, is the burden of proof. Proponents of compost teas and the like tout such products as miracle additives, some even going to the extent of describing them in etherial overtones with the odd ‘sciency’ word thrown in to make it sound legit (“The scienciness will set you free,” Linda). As yet no research has supported proponents’ claims that such products improve plant performance. There seems a growing trend amongst the laymen to attack science itself when it does not support their often half-baked postulating. That worries me, and that’s why I like this blog.
    We could talk about food webs and biodiversity until the cows come home, and in the end we’ll all likely agree that a greater diversity of species at all levels of an ecosystem is a desired outcome. As far as modern forms of agriculture such as broad acre farming and monocultures are concerned, such systems can be sensitively managed to maintain soil health, even improve it – and this includes the use of chemical fertilisers in such systems.
    When one is abreast of the science on soil health in regards to plant systems there is one conclusion that is difficult to reach, and that is that nothing is ever as simple as an “artificial fertiliser=bad, organic-fertiliser=good” dichotomy.
    I hope my comments don’t open up another can of worm juice!

  6. This whole arguement seems so silly. Gardeners, beekeepers, farmers, scientists, and environmentalists are all extremely opinionated. We all have our tried and true methods. We can argue scientific proof vs. feel-good and “it works for me” trials forever. In the end we are all trying to do the best thing. As long as we are not creating any harm, let’s listen and consider the different thoughts and techniques and move on.

  7. Nancy, I disagree. How do you know whether or not there is any harm, without the science done to test.

    Sheet mulching, a practice that I thought was tried and true, and used, is actually harmful, I recently learned here. In the Compost Tea discussions earlier, I learned the potential risks about legionairres disease. I had read about and had in the past bought red mulch for my tomatoes until the Profs here basically said the improvements were very modest, at best. So a “Let’s listen and consider, why can’t we all just get along approach” is not welcome, from my perspective.

  8. 1. Compost tea is oxygenated for 24 hours, not 24/7.

    2. Compost tea is the stuff of life. Microbial life dictated plant performance for centuries before the advent of “conventional” ag.

    3. The scientific method….like i said has come to an endgame in which it can be used to distort reality.

    4. DOES YOUR SCHOOL GET MONEY FROM BIG AG OR NOT?? simple question that I have asked you 3 times with no answer.

    5. How much scientific literature has there been saying organic foods have no added benefit or that cellphones and cigarettes do not cause cancer? I hope to god that your readers do not put as much faith in hollow science as diabetic americans with lung and brain cancer.

    Compost will save the world. Not Cargill, and not scientific literature.

    I took classes under John Jeavons Scott Pittman and Elane Ingham. They are far finer scientists and debaters than you or I.


    the truth is here.

  9. I have conducted many experiments. All you need is a 400x microscope and you can see and count fungi. You can look at soil, compost, or tea.

    You haven’t done any of that. You just read (yank squirt) scientific literature.

    Get a microscope.

    P.S. this isn’t a debate if I have to use the reader comments box.

  10. GP Chalker-Smith.

    Thank you for putting up with this kind of behavior for the sake of helping people to become informed gardeners rather than the disciples of ideology.

  11. The Justin, there is nothing wrong with comments section for debate. It is obvious you’re passionate, but I’d recommend refraining from guilt-by-distant-association as a debating technique.

    Attacking the scientific method because it makes your ideas look bad doesn’t mean the scientific method is suddenly outdated. But attacking the scientific method DOES make your ideas look questionable. If the scientific method makes your claims look bad then you need to claim something that can be tested (reproducibly by others) as accurate.

  12. So now it’s a war on the scientific method, is it? What’s the point in debating when that’s your starting point, Justin? Nothing any of us can write will convince you otherwise, that much is clear. As my farther told me when I was a whipper-snapper, the problem of people with an axe to grind is that they’re seldom very good at swinging it….

  13. American consumers, if given the choice, will choose compost tea veggies over chemical veggies EVERY SINGLE TIME……Scientific literature that.

  14. Justin I don’t have a clue who funds research at WSU. In any case, I don’t get funding from “Big Ag,” nor is my work influenced by what other people do. I am very sensitive to this whole issue, however, as I was at the UW Center for Urban Horticulture in Seattle when it was firebombed by ecoterrorists in 2001. They readily admitted that what most of us did there – research on sustainability, plant conservation, restoration ecology – was not their target. We were merely “collateral damage.” So I really, really, really do not like being presumed guilty by association.

  15. Keep up the fine work, Linda. Although I am a faithful composter, I am POSITIVE that it won’t save the world. I hope that maybe, just maybe, science will.

  16. You are right about guilt by association. Just because you are saying something that people get paid to say, doesn’t mean you are getting paid to say it. I’m sorry….Elizabeth i find it sad that you hope science is gonna save the world. It’s not that science is bad. It’s the faith that science is gonna save us that led us to GE alfalfa, Lyme’s disease, kudzu invasions, killer bee invasions,and honeybee deaths.

    Science can’t WAKE YOU UP!

  17. All the buzz about compost tea bypasses the fundamental question of why compost tea would limit plant disease when sprayed on plant leaves? The theory goes that the good microorganisms colonize leaves to displace and/or fight off the bad guys.
    Compost tea contains some of the microorganisms from the compost that made the tea. These microorganisms are normally found in soils and, of course, composts. But why, evolutionarily speaking, would these microorganisms provide any benefit on plant leaves, for disease control or any other purpose? Furthermore, lacking the nutrients and environment in which they thrive (i.e. composts and soils), these organisms will not thrive, or even survive very long, on plant leaves. The same goes for soils: If the soil has the right environment for a particular set of microorganisms, they generally are there; apply microorganisms to a soil lacking the needed environment and those microorganisms cannot survive.
    Still, occasional research papers report on positive effects of compost tea applications with respect to plant diseases. In answer, I contend that if you spray just about anything on a plant leaf and measure closely enough, you’ll turn up some measurable response to the spray. That response might be very transitory and very small, but, with the right equipment or instrumentation, you’ll measure some effect. Whether that effect is of biological or practical significance is another story.
    With that, I suggest someone begin a series of experiments to see the effect on plant diseases of spraying — say — milk solutions on plant leaves. Wait! A google search just told me that milk sprays have been tested and are, in fact, effective in controlling plant viruses, powdery mildew, and other diseases. In contrast to compost tea, which provides microorganisms but little of the food they need to survive, milk provides a smorgasboard of nutrients to whatever microorganisms tag along for the ride.
    On the basis of the evidence, I’d go with milk rather than tea for my plants. And I’ll take my milk without tea.

  18. I’ve been trolling this site for the last few weeks, steadily scrolling backwards in time and gathering up loads useful information (Thanks!) and I’ve slowly come to realize that the “compost tea” I’ve been using in my garden and houseplants for so long should actually more correctly be called “fertilizer tea”. While I have taken to using an air stone, that’s only to keep the bacteria from going anaerobic in the mixture and getting putrid. (I didn’t spend anything on it, since the fish died years ago and I only make about 3-4 batches a year for seedlings, transplants and such.)
    This might explain why I’ve never heard such profound claims about compost tea being the end-all-be-all of organic gardening until now. I always assumed it was to be used as a supplement to compost and fertilizer at best. In fact, after years of trial and error I’ve found that it’s effectiveness is greatly determined by the amount of nitrogen I add to the mixture (eg: green manure, urine). Using it as a foliar spray seems hopeful at best, unless maybe your compost tea is fortified with milk. (Or nano-bots!)

  19. Pouring compost tea into the soil and expecting the microbes to thrive is like throwing a puppy out into the snow and leaving it to fend for itself. Geez. The soil does the work when you treat it right. Compost tea doesn’t even past the sniff test for making sense.

  20. Linda, your work is so valuable to many many people who are open to learning as much as they can in the field of gardening and biology! In my experience gardening can be like religion, people believe what they want to believe and nothing can sway their deeply held beliefs, whatever they may be. The problem is that you don’t need to be a critical thinker to be a gardener and biology is much more complicated than the average gardener can comprehend, leading to assumptions. There are many convincing products out there and doesn’t help that permaculture, for example, teaches the young and naive that all they have to do is pay to take a very short course and buy a few compost products and they will save the world from everyone else. Its very religion like for the ‘chosen ones’ who ‘hold the key’ and Justin seems to have swallowed his indoctrination wholeheartedly. I’d be willing to bet $ he’s a permie who has just started gardening as a way to give his life meaning. For everyone else who wants to hear a logical and evidence based discussion that is open to new evidence at all times, I thank you for your ongoing dedication.

    1. Thanks, Jodi! And yes, it’s hard to give up closely held beliefs. I gave up many along the way of transitioning from a marine biologist (my MS degree) and an urban horticulturist (my PhD). And I’m still learning!

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