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Compost Tea

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.

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

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.

Will Compost Tea Decrease Diseases?

This topic has been evaluated extensively by Linda Chalker-Scott, Ph.D in reference #3. 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. The picture below shows an electron microscope image of a leaf surface showing the microbes present. There are anywhere from 1 t0 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.

Compost tea and disease control

Microbes on the surface of a leaf

 Photo Source: Gerd Innerebner and Roger Wepf/ETH Zurich

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 aare 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.

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.

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.

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) Compost Tea and Disease Reduction: http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/magazine%20pdfs/CompostTea.pdf

4) Photo source for Aerated Compost Tea: Lily Rhoads

I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writting 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

16 Responses to 'Compost Tea'

  1. Tristan says:

    I am sorry to say this is a very lazy article. How can you use common sense and basic rhetoric to refute scientific knowledge? Compost tea is best for indoor or potted because it focuses on creating and sustaining the highest quality soil as opposed to blindly, and inanely, adding nutrients that your plants won’t be able to directly consume. The point of compost tea is to keep the microbes alive and healthy and feeding the plant. Yes beneficial bacteria and fungi(mycorrhizae) exist in nature but the more you have the better your plants will produce. You should go and read the book “Teeming with Microbes” and inform your opinions before you attempt to inform others…

    • Several references were given to back up the points I made – the comments are not just based on common sense.

      You say “Compost tea is best for indoor or potted” – not according to most of the people who advocate compost tea. Most use it in the garden. Please provide some references for using it in pots.

      You say “it focuses on creating and sustaining the highest quality soil as opposed to blindly, and inanely, adding nutrients that your plants won’t be able to directly consume”. So this must mean that you do a lab test on your compost tea? Otherwise you would have no way of knowing which nutrients your tea is providing, and in that case you are adding nutrients just as blindly as someone who uses fertilizer – no, that is not correct. The person using fertilizer actually knows what they are adding.

      You say “but the more you have (microbes) the better your plants will produce”. This is not necessarily true. Even if you add lots of microbes they will only live if you also provide organic matter for them to eat. Compost tea adds less organic matter than just adding the compost to the soil. Besides that – plants growing in pots don’t need microbes at all. Regular fertilization replaces them quite nicely. The ultimate potted plant is grown hydroponically – they grow just fine.

  2. Joerg says:

    We will make a scientific test of compost and compost tea here at the university. I can keep you updated in our results.

    • Please do. both topics are of great interest to me and the readers. Especially compost tea is a hot topic these days.

      I’ve looked at a lot of data about compost teas. What I find lacking is a good comparison between using the tea and the compost. Most work looks at benefits of the tea alone. What one really needs to demonstrate is that the effort of brewing tea is worth the effort ie it is better than just using the compost directly. In fact I would compare three things: the compost, the tea, and the sludge left over from the tea making process.

  3. Nick says:

    Incomplete understanding of soil biology. Also slimy anaerobic “compost tea” is just failed compost tea. To deny that providing the CORRECT soil microorganism increases the efficiency of the whole biological system is….well you fill in the gap. Compost tea is not meant to “add nutrients”. The whole gist of your article is plainly inaccurate.

    • I never said that it was not important to provide the CORRECT soil microorganisms–what I said is that nature does this for you. You just have to feed what is already there–straight compost does that. How do you know that the compost tea you are brewing contains the RIGHT soil microorgansims? Nobody is testing this stuff to see what is growing in it. In fact the FDA will soon release a cautionary statement that brewing compost tea can be a health risk due to the possible brewing of disease organisms.

      Just search the internet and you quickly find that there are two ways to make compost tea–anaerobic and aerobic. The one is NOT “failed compost tea”.

      You might not believe compost tea adds nutrients, but that is a benefit believed by many–just see the quote at the beginning of the post!

  4. David says:

    Well,
    The point is that microbes in the tea die in the soil and feed the plants nitrogen. Also one should be using worm compost as a means for growing the bacteria. It has all the correct soil bacteria in it. I agree with you compost is one of the best ways to add organic mater but also bacteria to the soil. But compost tea when done right really spreads out that compost you have made. I would also like to say that mulching is my all time favorite.
    In regards to compost tea, it does not necessarily grow giant vegetables, only if you use seeds that have the genes to grow that big, but I have seen that crops really benefit from the extra boost in my own garden over the years. To compare it with compost I would say it does give you more than the same amount of compost would do. This is just my experience.

    • It is true that microbes die, and the nitrogen and other nutrients help to feed the plants. But where did the microbes get the nitrogen? They got it from the compost, manure or worm compost. No matter what kind of tea you brew, you don’t increase the amount of nitrogen, unless the bacteria in the tea use nitrogen from the air, and there is no indication that they do this.

      Microbes don’t increase the nitrogen levels in the original compost. They just convert it from one type of compound to another. The statement “compost tea when done right really spreads out that compost you have made” is a complete myth, that is unsubstantiated by any scientific evidence I have seen. However, if you have some scientific studies that support your opinion, I would love to see them.

  5. Lee Reich says:

    You’ve got to be careful criticizing compost tea — for advocates, it’s more religion than fact. I weighed in on compost tea in my blog also (http://www.leereich.com/2010/07/has-your-garden-had-its-tea-this.html) which you might also want to check out. As a fellow iconoclast, you might also be interested in my past comments on permaculture (http://www.leereich.com/2010/09/looking-around-at-my-fruit-trees-and.html). Keep up the good work.

    • Robert Pavlis says:

      Thanks for visiting and your comments. A religion–that makes sense–people do a lot of things for religion. I now have more recent data that shows the number of microbes in compost tea is in fact extremely low. All the hype would have you believe that the brew was full of them and that they add benefits. Turns out there are very few microbes in the tea–I guess there is just not enough nutrients for them to grow :) I’ll post more on this in the future.

      I have followed your writings for quite some time, both in books and magazines. They always made so much sense to me. I especially liked the series you did on soil in Fine Gardening. Thank you.

  6. Water says:

    You are dead wrong. I have personally witnessed the effects of compost tea on my wheat.
    It is a wonderful way of spreading compost. Microbes are the life of the soil. All around us is sterile soil because chemical fertilizers kill all microbes. You are dead wrong. Go do more research. All my Research that I have done is from books and experimentation. I did not just read some gardeners blog of the Internet. GO do 5 years of intensive study before you make the statements that you made above.

    • Robert Pavlis says:

      Chemical fertilizer, when used properly does not kill microbes. In most farm situation it is the cultivation and lack of organic matter that kills microbes.

      Microbes are important to the soil–that agrees with my statements. However, soil already has microbes and does not need more. What it needs is food for the existing microbes ie organics.

      Since you have performed experiments on compost tea, can you provide a link the the data you have collected? All of the research papers I have looked at where compost tea is tested in field conditions fail to show any benefits over and above that obtained from spreading just the compost. If you have such references I would very much like to see them.

  7. Kendra says:

    A lot of the things being said here make no sense to me…

    If compost tea could contain E.Coli or Salmonella, couldn’t the un-brewed compost contain this as well?

    Regular compost is going to differ, just as compost teas will. Why would I need to know the microbial content of a compost tea, but not of regular compost?

    If the nutrients in a tea would be overly diluted, couldn’t you just add less water? Like with frozen fruit juices.

    Lastly, while normal gardening soil might not need added microbes, indoor container gardens benefit from them, since the potting mixes used might not contain microbes. While normal compost would solve this issue, not everyone composts – commercially bought compost tea mixes can help.

    • Robert Pavlis says:

      Very good questions.

      Compost could also contain E.Coli or Salmonella. The process of making the tea tends to increase the population of these. People who feel that the tea has value feel that the more microbes they can add to soil the better–so they are trying to grow them. If the microbes in the tea are important, then you must also believe it is important to grow the right ones and to know which microbes are in the tea. People who promote compost tea don’t know which microbes they have, and each batch could be different.

      People who use compost directly are doing so because of the nutrients and organic matter in it. They don’t care which microbes are in it because they will all eventually die and turn into nutrients.

      The truth is that the microbes added to soil don’t matter. Microbes in soil are very important, but nature has already put the right ones in the soil. The ones we add will probably not grow well and just die.

      You could add less water to concentrate the nutrients, but the common recipes don’t suggest this. They add lots of water to a handful of compost and then expect it to contain a lot of nutrients. It does not work that way. People who believe in compost tea have been misinformed about the nutrients in tea.

      Potting soil, if new, will normally not contain microbes. Most people growing in pots will fertilize their plants and so the plants really do not need the microbes to provide the nutrients. In fact most people over fertilize and there are plenty of nutrients available to plant roots. If you are going to add fertilizer you can select from many types; synthetic, organic and even compost tea. In my post “What is organic Fertilizer” http://www.gardenmyths.com/what-is-organic-fertilizer/ I discuss the fact that all of these are the same–from the plants point of view.

      Let’s say you decide to add your fertilizer as compost tea. You can buy it in two basic forms; compost and already made tea. Buying it as compost does not make sense. Why not buy a finished fertilizer and save yourself the trouble of making the tea? Besides when you buy the compost as special “compost tea’ compost it is ridiculously expensive. See my post here about manure tea which is the same thing. http://www.gardenmyths.com/?s=tea&submit=Search

      Few companies sell ready made compost tea. I have not been able to find one source ( I have not looked real hard) that gives the nutrient value of the compost tea ie the NPK values. Without this you have no idea what you are buying. You are probably paying a lot for colored water because if the nutrient value was high enough to qualify as fertilizer it would be mentioned on the bottle–it would help sell the product.

      Potted plants are growing in a very artificial environment. Most of the cultural practices that are good for your garden, especially those for soil building, do not apply to potted plants.

  8. Robert Pavlis says:

    The added quote should end with phosphorus as per original reference. Unfortunately that advice is incorrect. It is true that you should not add phosphorus unless you know you have a known deficiency. But adding compost tea is no better since it adds no real nutrients and if it does add very low levels you don’t know what you are adding. Adding either makes no sense unless you know you are solving a specific deficiency problem.

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