Compost Tea

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Robert Pavlis

Compost Tea has become a very popular topic. The following is a quote from Fine Gardening (ref 2):

Gardeners all know compost is terrific stuff. But there’s something even better than plain old compost, and that’s compost tea. As the name implies, compost tea is made by steeping compost in water. It’s used as either a foliar spray or a soil drench, depending on where your plant has problems.

Why go to the extra trouble of brewing, straining, and spraying a tea rather than just working compost into the soil? There are several reasons. First, compost tea makes the benefits of compost go farther. What’s more, when sprayed on the leaves, compost tea helps suppress foliar diseases, increases the amount of nutrients available to the plant, and speeds the breakdown of toxins. Using compost tea has even been shown to increase the nutritional quality and improve the flavor of vegetables. If you’ve been applying compost to your soil only in the traditional way, you’re missing out on a whole host of benefits.

Let’s look at the facts.

Compost Tea
Aerated Compost Tea

What is Compost Tea?

This seems like a simple question, but it’s not. There is no clear definition of compost. Compost can be made from a large variety of materials, and each compost is different. If you make tea from two different types of compost you will get two different types of tea.

The nutrient content of each type of compost tea will be different.

One of the reported benefits of compost tea are the ‘microbes’. If we assume this to be true then is it not important to know which microbes are in the tea? It certainly is. The problem is that unless you have a fairly sophisticated lab you won’t know this. Home gardeners have no way to know which microbes are in their tea.

Compost Science for Gardeners by Robert Pavlis

The microbe content of each type of tea will be different.

Tea can be made in two very different ways; aerobically and anaerobically. The term aerobic means that the tea is made in the presence of oxygen; you usually bubble air through the tea as it is brewing (see picture above). When tea is made anaerobically, it is made without added oxygen. You simply let the smelly sludge sit in a pail. The method used to make the tea is very important because microbes tend to favor one or other of these living conditions. They either like living with oxygen present or they prefer less oxygen. So the method you use to create the tea is very important to determine the type of microbes in the tea.

Aerobic soil bacteria inhabit soils that contain a lot of air; the light fluffy type of soil we all know to be good for plants. Anaerobic soil bacteria tend to live in wet, compacted clay type soils where there is little oxygen present – not the kind of soils we want. So why is it that many recipes for compost tea use the anaerobic method? That makes no sense and I can’t explain it.

There is also something called manure tea which is the same as compost tea except it is made from manure.

Bokashi composting is something completely different and is described in detail in Bokashi Composting Myths.

What Are the Benefits of Compost Tea?

Proponents of compost tea ascribe a wide range of benefits – see the above quote from Fine Gardening.

One thing is clear to me. If a product or gardening technique does everything under the sun, it is always too good to be true. When it sounds like snake oil, it probably is snake oil! Run for the hills.

There are a few main benefits that would be worth discussing. Compost tea is claimed to provide:

  • An increase in nutrients
  • A decrease in diseases
  • Additional microbes for the soil

A recent study compared AACT compost tea to using just compost and is described in Compost Tea – Does it Work?

Does Compost Tea Increase Nutrients?

To clarify the question it should be stated more clearly as; Does compost tea add more nutrients than compost alone? There is no doubt that compost tea adds nutrients. But does the process of making tea increase the level of nutrients compared to just using compost without brewing? If they both add the same amount of nutrients–why bother making tea?

If you think about it for 2 seconds you will realize that this is a silly notion. Think about what you are doing in making tea. You take a handful of compost and you put it in a bucket of water. Microbes take over and start digesting the compost.

Your original handful of compost had a certain amount of nitrogen, phosphorus and potassium. No matter what process you use, you will never increase the amount of these nutrients in a plastic bucket (except for some minor organics falling in an uncovered bucket). The microbes might breed and grow and digest things, but the total amount of nutrients remains the same. In fact it might actually be less since some of the nitrogen might be converted to ammonia which evaporates into the air.

What about the quoted statement above “compost tea makes the benefits of compost go farther “. The nutrient content (NPK fertilizer numbers) of say 500 ml of compost is 2.6 – 0.9 – 2 (average value for composted cattle manure; source Alberta Agriculture Department). If I now add this to a 5 gal pale (about 20 L), I still have the same ratio of nutrients, namely 2.6 – 0.9 – 24, but it is now diluted 40 times (500 ml to 20 L). The nutrient value of the tea is now 0.07 – 0.02 – 0.05. That is an extremely dilute fertilizer. For comparison human urine has a nutrient value of 11 – 1 – 2.5, that’s 160 times as much nitrogen as compost tea. Sure you can probably spread the tea over a larger area than a handful of compost, but if you do that the amount of nutrients added to the soil is  negligible – so why bother??

The fact is that making tea from compost does not increase the amount of nutrients. It does not make the compost ‘go further’. If you want to add nutrients to the garden just add the compost directly.

In the post, Compost Tea NPK Values, I have a closer look at the NPK values and what manufacturers of the tea and kits for making tea have to say about their products.

Will Compost Tea Decrease Diseases?

This topic has been evaluated extensively, in reference #3 (link no longer valid). There are limited studies about disease reduction by compost tea, and the results are inconclusive.

The concept here is that the tea has a high concentration of microbes. When these are sprayed onto leaves they populate the surface of the leaves to such an extent that invading pathogenic microbes can’t take a hold. The good tea microbes out compete the potentially bad ones.

For this to work, the sprayed on microbes would need to colonize the leaves (ie live and breed on the leaves). This requires that the new environment, ie the leaf surface, has enough food for them and the oxygen levels are right for them.

Clearly, the oxygen levels would be high and so you can expect that anaerobic microbes would die out quickly. Anaerobic tea just won’t work.

The native microbes on plant surfaces are not well understood. There are anywhere from 1 to 10 million microbes on each 1 square centimeter of plant. Nobody knows what happens when more microbes are sprayed onto the leaf. I can’t help wondering why the large number of naturally occurring microbes can’t out compete the potentially bad ones and yet the ones sprayed on in the tea will do this??

In summary, there is little scientific evidence to support the idea that compost tea solves disease problems.

Does Compost Tea Add Microbes to the Soil?

There is no doubt this is true. You have a pail full of slimy microbes and if you spread it around the garden you are certainly adding microbes to the garden.

There is a new gardening  trend of adding microbes to the soil under the assumption that the soil ‘needs microbes’. I’ve looked at this myth in more detail in the post Soil Microbes. In summary; the soil already has lots of microbes and adding a bit of tea is not going to make much of a difference.

If you are interested in identifying the microbes in tea you should read this before buying a microscope and taking Dr. Ingham’s course: Soil Bacteria – The Myth of Identification and Management.

The scientific study discussed in Compost Tea – Does it Work? clearly shows that adding microbes from AACT tea does not impact plant growth.

Can Compost Tea be Dangerous?

It is important to ask this question. Even if there are some minor benefits for using compost tea, they could be outweighed by risks.

Think about what you are doing when you make the tea. You are creating an incubator for microbes. You are providing the moisture, the food and the right oxygen levels to grow microbes. But which microbes are you growing? You have no idea know.

The reality is that along with the ‘good’ microbes you might also be growing ’harmful’ ones. You could be growing microbes that will make you or your plants sick. Tea that is aerated can contain Salmonella and E. coli both of which can prove to be deadly to humans. Remember the contaminated lettuce? That was E. coli contamination. You could also be growing microbes that are harmful to plants.

This study shows that adding molasses to increase the microbe populations can significantly increase the population of salmonella and E. Coli 0157.

The process for making compost tea is not selective – you grow whatever is in the pot.

I am confident that the risk is low. But why take the risk when the benefits of compost tea are at best, minimal?

Conclusion:

If you want to make some compost tea, go ahead. You will probably not harm anything and you just might have some fun doing it. But understand that there is currently no evidence that compost tea is any better than using just compost. Be a smart gardener and just spread the compost on the soil as a mulch. Nature will do the rest.

 

Further Comments:

This post now has quite a few comments. Many of them are from people with feelings about this topic but without any scientific evidence that their feelings are correct. If you have some references to discuss – please continue posting comments. If you have no valid references to support your position – don’t bother commenting, because I will not approve the comment.

For more information and explanations about the myths promoted by Dr. Ingham, have a look at these posts:

Teaming With Microbes – In Depth Book Review

Soil Bacteria – The Myth of Identification and Management

Teaming with Microbes – A Close Look, Part 1

Teaming with Microbes – A Close Look, Part 2

Compost Tea – Does it Work?

 

References:

1) Application of Two Microbial Teas Did Not Affect Collard or Spinach Yield: http://hortsci.ashspublications.org/content/44/1/73.full

2) Brewing Compost Tea : http://www.finegardening.com/how-to/articles/brewing-compost-tea.aspx

3) Link no longer valid.

4) Photo source for Aerated Compost Tea: Lily Rhoads

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

376 thoughts on “Compost Tea”

  1. Hi Robert
    Great post [ as always, I’m a big fan ] but I’m looking for a organic, locally produced, low cost fertiliser for a football [ soccer ] pitch I work on. As a club we want to get away from using fertilisers produced non-sustainably by the chemical industry. The wider area has large amount of invasive Russian Comfrey that a local person is steeping to make liquid fertiliser [ he does not aerate so anaerobically I guess ] I have read your other articles and it’s clear you do not think there is large amount if NPK in fertiliser made like this and that compost applied to plants gives far far better growing improvements which I have no doubt of at all. But. We can’t use solids due to the nature of how pitch grass grows so a Comfrey based liquid seems a good compromise. Diluted urine would probably be best but I doubt I could get that agreed! 😀 Any thoughts appreciated. Cheers Glyn

    Reply
  2. You mention you don’t understand why would someone use fermented compost? It also has it’s uses. I don’t understand why people have the single minded associatiation of anaerobic conditions and bad stuff alone. In fact many of the most healthy things are anaerobic products such as, pickles, kombucha, etc. We also enjoy beer and wine. In fact our colon is populated overwhelmingly by anaerobes.
    Going back to soil, it’s also known that soil treated with anaerobic bacteria has a strong impact on root diseases of plants ( https://link.springer.com/article/10.1007/s00253-018-9119-x) its a link as an idea where this is also mentioned . In my own experience as a skeptic I have tried this applied to plants in my garden and have definitely seen great amount of root exudation as evidenced by very thick root dreadlocks in those in which the treatment was applied directly to the planting hole, compared with none, some of which actually had just clean roots.
    My point is, one has to be open to the possibilities and explore them and don’t be polarized on one thing vs another.

    Reply
    • This man’s argument was extremely flawed. I’ve read multiple people contradicting him now, some even providing evidence. All he does is skirt around it. Dr Cho Han Kyu has even provided scientific evidence that ferments and anaerobic “teas” are extremely beneficial since the bacteria actually helps keep the soil loam and fluffy. Helps the facilitation of nutrient breakdown and will help add trace nutrients and minerals into the soil. Teas aren’t really meant for adding nutrients to the soil, but adding bacterial and fungal life to the soil. He’s even shown studies that some of these teas and ferments will actually help break down hard soil and clay soil. As well, he doesn’t add the fact that some bacteria and nutrients can become water soluble when adding water, because of the covalent bond formed at the molecular level. Example would be a protein molecule is attached to a water molecule, that protein is now immediately available to the plant. As apposed to just adding it to the soil where it has a chance to be attached to another molecule that is facilitated for breaking down material. This man’s article is extremely polarized and flawed.

      Reply
  3. My only question/comment is this: I agree that you may not be *adding* nutrients to the soil, but the process of extraction through steeping in a tea increase the “bioavailability” of those nutrients by simply enriching the water more thoroughly, and therefore, the soil where the roots dwell?

    Reply
    • This may be true, but this change is insignificant. Remember that the brewing process is done in a couple of days, which means you only get a couple of days more decomposition, and while that is going on, the compost that was not used for making tea is also continuing to decompose. So the difference in nutrients is only the difference between the two processes, which is minimal.

      Reply
  4. Good point. My only thought was that given the horse droppings never broke down that breaking them down would aid the composting process. The pile got to 165 degrees Fahrenheit but even after a year the droppings are still intact.

    Reply
  5. We have a barn where we keep 30 horses. I have tried to compost the bedding (shavings and horse droppings). It will get to temperature but it won’t break down the horse droppings and it has not composted in spite of having it in place for over a year. I got to looking at the compost tea article wondering if I did go through the steps of making compost tea and then pouring that over the bedding whether that would make a difference.

    If you have any thoughts, suggestions and ideas I would appreciate the feedback. I know I could add nitrogen or urea but in an ideal world I would use what I have.

    Thanks

    Reply
    • Absolutely none.

      I got a load of horse manure last year. Problem was it was mostly saw dust. It never heated up or composted. Not enough nitrogen for the high carbon content.

      Making compost tea does not increase the amount of nitrogen in the compost you started with.

      Reply
    • typically you add a mixture of brown and green material to generate a good compost. brown being the manures and green the leaves, grasses and so forth… about 50/50 to make a pile about 5 ft…high X 8 ft. diameter

      Reply
  6. I am reminded of Thomas Edison’s remark that “until man can make a blade of grass, Nature can laugh at all his so-called science”. Compost tea and the debates around it demonstrate that we humans almost all filter information, “facts”, through our emotional predispositions, not to mention our “scientific” and technological vanities. What does nature do!? Can we improve on it? Where are the “aerators “in nature for Pete’s sake!? In a bog or a swamp what do we see? We see plant matter decomposing in the presence of moisture, largely anaerobically, producing the most wonderfully fertile soil. All this idiotic hardware and brouhaha over air pumps and stainless pipes etc. is just comical. Spraying compost tea on plant leaves?! Where does that happen in nature people?! The microbiome of plant surfaces is hardly understood; you really think you’re going to somehow improve on it by spraying compost tea around? LOL. The idea is so utterly illogical only a combination of arrogance and ignorance could propose it. FOLLOW NATURE! We can perhaps speed up or accentuate natural processes, but we are not going to “improve” on them. If the GMO debacle doesn’t prove the cost of this kind of hubris, what does?! I make compost tea: I dump compostables in garbage cans full of water and let it rot. Periodically I give it a stir and ladle the stuff out ON THE SOIL around my plants; they LOVE it – why wouldn’t they? It’s a concentrated dose of nutrients that compostables rotting on the surface would typically leach into the soil from rainfall. Plant juices are assimilated by humans almost without digestion; it compost tea is like juice for your plants. It’s worked in nature for countless millenia, and incredibly enough it still works. By all means get the chlorine out if you can, by all means improve the water’s “structure” (Schauberger, Pollak etc.), but please, let’s not be silly.

    Reply
    • There is an extremely large dry lake bed in Africa called “The Bodélé”. It is known as the duties place on earth. The dust from there is carried by trade winds across the Atlantic Ocean and fertilizes the amazon jungle through foliarapllication. Nature IS doing this.

      Reply
    • I did a quick look at the first reference. I guess the term “in vitro” is new to you. It means “in a test tube”.

      Such a study does not prove that this is valid in the field.

      Reply
  7. You make some sound points, but I think many of your conclusions and assertions contradict what I understand from Dr. Ingham’s research. Your argument also brings to mind that the absence of evidence is not the evidence of absence, and that biology is never a simple process where one variable can be isolated with a single result. That’s not how life works, especially in soil where the interactions between microbes, plants, and the physical/chemical environment are so dynamic. I have been a professional gardener for 25years (I started a landscaping company at 12), and have seen the results of compost tea enough to use it at least once a year. If I thought it did not make a significant difference, or that spreading compost alone would be more efficacious, I would happily not do the extra work of making and spreading teas as soil drenches and sprays. I hope you do not shoot this post down due to my reliance on basic logic and widely accepted ecological/biological principles (I have also taught ecology professionally), and not just peer reviewed research that anyone can easily find themselves to confirm or disconfirm the basic information.

    First of all, Dr. Ingham explicitly states many times in her talks and books that synthetic fertilizers destroy the soil food web, though she also points out organic fertilizers can also be harmful in excessive concentration. This conclusion was even reached by the biochemist Fritz Haber, who invented many of the original NPK fertilizers, and he ultimately warned against their use. Synthetic fertilizers are kind of like the soil economy’s version of Amazon delivered “x” “y” “z” and “anything else you could possibly want in one place”. They may fill that immediate basic need in the plant, just like your new gadget delivered within a day may solve an immediate problem for you. However, in getting that synthetic fertilizer to the plant roots in chelated concentration, it often poisons soil life, and even if it doesn’t it will deny the preexisting microbes that are symbiotic plants the interaction/trading that keeps them alive and reproducing, making the plant dependent on ever more fertilizer that largely runs off underutilized by an ever decreasing soil life web. This is much like how Amazon destroys local businesses (often through price dumping), and all the problems that occur downstream from the increasing economic monoculture they’ve created. In the soil, the void of the biodiversity and biomass that once was sustained by symbiotic interactions with the plants (trading water or nutrients for sugars from the plant), ultimately collapses, reducing soil air and water retention, further increasing runoff and downstream over-nutrification and flooding.

    In contrast, actively aerating compost tea can breed a diverse and abundant array of microbes that can make nutrients available (chelation) that are already present in the soil but which were previously not available to the plants. Moreover, with AACT you can feed them with additional organic fertilizers in on top of the compost (used mostly for its biodiversity for inoculation) to deliver both chelated nutrients much more efficiently and less destructively (less runoff than just spreading manure or synthetics). We can also use diverse wild forest or prairie soil to inoculate our teas, which can then inoculate more compost or soil and plants.

    Finally, is this the study you deemed invalid?https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4564151/

    It is unfortunate they could not do a control with actual soil, but saying it is invalid due to its use of what amounts to a “soilless mix”, is hyperbolic. Dr. Ingham’s analysis of synthetically farmed soils, as well as urban and contaminated soils, shows an utter lack of biodiversity (sometimes less than 10 species!). This is likely similar to that of the soilless mix used, and I think the results show the value of at least an initial inoculation of a site with compost tea where adequate volumes of compost cannot be found or made. If inoculated with diverse wild soil/duff, (look up Korean Natural Farming methods), compost teas can undoubtedly increase microbial diversity.

    Ultimately, I wish you good fortune, but will I take your advice as much as I would take any advice from someone giving gardening advice wearing a tux.

    Reply
    • 1) You talk about “Dr. Ingham’s research” – Dr. Ingham has never done any research on compost tea – if she has, please post the links.
      2) “Dr. Ingham explicitly states many times in her talks and books that synthetic fertilizers destroy the soil food web” – yes she does – but that does not give any support to the idea. Dr. Ingham says a number of this that are not supported by science.
      3) If you understand basic composting and the decomposition of organic matter you already know that the nutrients provided by synthetic fertilizer is identical to that supplied by organic fertilizer, therefore if one harms micros – so does other other, but neither do.
      4) I doubt that I said your referenced study was “invalid”. If I did, present my actual quote. I may have said it does not prove what you think it proves – that is something completely different.

      Reply
  8. This post and the two years of comments I have read fail to maintain some fundamental distinctions needed for clarity.

    1. First is the definition of compost tea. There is actively aerated compost tea in which beneficial microbes in compost are substantially multiplied in an environment that maintans at least 6ppm dissolved oxygen in the water. This is quite different from organic matter, whether compost, comfrey, bannana, dandelion or anything else, that is soaked or steeped in water, i.e. extracts. Also, compost tea is not nutrition; all or almost all of the nutrients added at the start is consumed by microbes during the brew.

    2. Second is the purpose intended for the tea.
    a. As a soil drench to increase microbe counts;
    b. As a foliar spray to increase disease resistance:
    c. As a foliar spray to fight off existing disease infections.
    The tea is not nutrition, but liquid nutrients are sometimes added after
    tea is brewed so nutrition is distributed simultaneously with the tea.

    It only makes sense to compare spreading compost with use of compost tea as a soil drench. Spreading compost is not an alternative when one’s goal is to increase the proportion of leaf surfaces that are covered with bacteria. in other words, criticism of the efficacy of foliar sprays of compost tea is not a logical argument for spreading compost.

    Neither man-made compost nor compost tea is needed for nature to grow healthy plants. Rather, they are both human schemes to improve what nature provides or decrease the time that nature takes to succeed.

    There are many benefits to spreading compost in most agricultural circumstances. One of those benefits is inoculation of soil with additional beneficial microbes.

    Examination of soil with a microscope can demonstrate that microbes, or certain microbes, are in short supply compared to an optimal quantity. Getting additional microbes to the root zone of plants can be done at far less cost with compost tea than by spreading compost. Further, inoculation of the root zone can be accomplished far faster with a soil drench than through compost on the soil surface. This can be critical for a crop with a short growing season. Subsequent examination with a microscope can demonstrate the increase in microbes.

    I am not advocating for compost tea. I am advocating against a poor piece of criticism that “concludes” it does not have significant value. Credible criticism follows substantial consideration of the available facts and expert opinions. What I see here is a conclusion based on a quick survey of information and a demand for contrary scientific proof before that opinion will be reconsidered in light of all available information. That is a disservice to me and other readers.

    Furthermore it is just too easy to poke at compost tea, especially since the topic has been exploited by so many flaky commercial claims that deserve a good poke. Soil science itself is a vast topic still in its infancy after a century of setback by agribusiness that lobbied for a chemical industry and suppressed competing science. Compost tea is a very new topic in the explosion of new soil science information during the last thirty years. Its failures deserve to be pointed out but it is far too early to dismiss the topic.

    Reply
  9. Your article seems to say that compost teas are useless and have no value. That you should just add compost. But that is not always an option. If you are growing in containers and their is no room to add any more compost. Then having it in liquid form is a great way to add it. Also if you have cover crops growing along side your plants you cant just always throw compost onto your garden.

    Reply
    • “Your article seems to say that compost teas are useless and have no value” – that is not what the article says. The article says that compost tea is nor more beneficial than compost.

      If you can’t add compost use synthetic fertilizer.

      Reply
      • Your response is to use synthetic fertilizer?! No way! It literally destroys the soil food web. The synthetics mean the plants no longer create the synergistic relationships with the microbes in the soil due to having “free” nutrients. With out those beneficial relationships the plants are at the mercy of “bad” microbes and you supplying constant nutrients. If you want healthy plants you must support a healthy soil food web! Read Teaming With Microbes by Jeff Lowenfels and Wayne Lewis….

        Reply
          • Why not post some of those studies that support compost tea or permaculture. There is very little research on permaculture.

          • >>Why not post some of those studies that support compost tea or permaculture. There is very little research on permaculture.
            -Effects of Permaculture Practices on Soil Physicochemical Properties and Organic Matter Distribution in Aggregates: A Case Study of the Bec-Hellouin Farm (France)
            https://www.frontiersin.org/articles/10.3389/fenvs.2018.00116/full

            permaculture is being studied widely by various groups, universities and also the empirical effects are observed…
            studies on compost tea have been published by Elaine Ingham and others and their are numerous studies to cite…
            https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4564151/

            The test I don’t believe available is one that shows nutritional differences between conventionally raised crops and those managed under permaculture… You can get a plant to grow and appear healthy when it is indeed lacking in nutrient and is actually so ill that it is unable to fend off predators, which is why conventional farmers have to use herbicides and pesticides…

          • This post is not about permaculture.

            Elaine Ingham has never published a study on compost tea.

            Re: Your study:

            1) it is not a field study so tells us nothing about what happens in real soil
            2) it compares using compost tea to water – this is a major fault of the study. We all know and accept that compost tea contains nutrients and that nutrients make plants grow better. If you want to show that compost tea is better than fertilizer you need to compare it to fertilizer – not water.

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