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?

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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. I have two 1 ton bags of what I hope to be compost made from last years shredded hedge clippings – mainly privet but also other clippings such as laurel. It looks like compost!

    My shredder is not perfect and thus I have in the compost large amounts of small twigs.

    Ideally I need a trommel or sieve to separate out the small twigs.
    But I do not have the skills and time to make a trommel and sieving would be hard work and time consuming.

    I wanted to use this compost on my lawn/grass and my question is :

    Can I get the nutrients (the nitrogen for instance) from the compost by steeping it in water for a few days and then watering the grass with this steeped water ?

    Or do I need to spread it, twigs and all, over the grass, wait for a while whilst the rain and time does its job (perhaps over winter?) and then pick up the twigs when I mow?

    What is your advice please ?

    Reply
    • Stepping removes some nutrients, but most are still tied up in large molecules. Use this on a garden instead.

      Reply
  2. Hello Robert, I am writing about gardening without any actual gardening equipment – ie if you are not getting out of the house. I have been wondering what you could feed your plants if you had no actual plant food, and apart from urine – which seems like a pretty good option,at least short-term – I was wondering if compost tea could work I am talking about for people gardening in pots indoors. I wondered if a type of compost tea made from your ‘kitchen caddy’ where you put your veg peelings etc, would make a useful liquid plant food? What do you think? kate

    Reply
  3. I do not think that a product is meanwhile to look at from n-p-k values when growing vegetables. I personally never look amendments that aren’t raw and natural such as comfrey, alfalfa, kelp, guano, and various types of compost, such as worm castings. When brewing compost teas, the goal is to multiply the bacteria, and microbes, in hopes of making soil alive, it’s especially good when working new soil. If you add raw materials to the soil, they will be broken down faster and available quicker. The microscopic life formed within a good quality brew is exponential through a 24 to 48 hour period depending on ingredients used and studies show that without any doubt. Now that I agree that the nutrients aren’t multiplied, adding ingredients to the compost feeds the bacteria and allows them to multiply at an incredible rate. They feed off the ingredients with the help of natural sugars. Those bacteria can then be applied to the soil, increasing the bio availability of the nutrients already present in the soil, or added in the tea. Adding raw kelp for example would take a while for the plant to absorb.

    Reply
  4. Hello Robert! It must be weird to still be getting comments on this post years after making it.
    I’m new to all of this and am definitely no scientist!

    I make my ‘tea’ the lazy way too, just throw some weeds in a bucket and let it sit for a couple of weeks.
    Isn’t the benefit of making a ‘tea’ that it breaks down faster than solid compost? Like the plants breaking down by themselves on the ground would take a lot longer to break down and rot, than when they are rotting in the water? That’s what I thought, correct me if I’m wrong. I tend to see the ‘tea’ as a nice quick drink of nutrients that the plants can drink up that same day rather than waiting for layers to break down on top.
    Take care

    Reply
    • Maybe. But how much faster? Is the time saved worth it?

      They do not break down a lot faster, and the tea is mostly water.

      Reply
  5. Interesting article! I think you bring up some really interesting ideas, and it sounds like what we need are some controlled experiments. The one thing I’d say is that one of the reasons I’m making tea (and I compost a lot as well), is that I have comfrey plants and its easy for me to throw them in a bin and let them break down and just water with the liquid. Some of the areas that I have that are densely planted are a PAIN to try and spread out my compost between and if I’m being honest I”m just not always going to do it, so for me this is a way to use the nutrients and add them in an easier way. I usually add my compost to new plantings or when preparing new beds or when I pull everything out to switch seasonal gardens. Anything that makes gardening easier for me while still utilizing what I have is worth it. I make lazy tea too- throw in a bucket with water and I want to see if it seems to make much of a difference for me. Like you said, adding more nutrients can’t hurt, right? (hopefully)

    Reply
  6. Robert you always refer back to scientific evidence. And my question to you would be why do you need scientific evidence. The evidence of our plants is enough evidence to prove you wrong. And for your information scientist are wrong Most of the time so if I were you I would definitely not put my trust in scientists. I would put my trust in the evidence of my plants in how they grow. I understand you want everybody to think you are so right and that you have all the information when in fact you do not. And you have so much pride that you can’t even admit when you are wrong that is very sad

    Reply
    • Your statement,”scientist are wrong Most of the time” is both insulting and completely wrong.

      If you don’t believe in science that is your business. I hope you are not driving a car, or using a cell phone – both results of good science work.

      Reply
    • Richard, how can you say that ‘the evidence of our plants is enough evidence to prove…..’ the point about evidence collected ‘anecdotally’ is that it can’t prove anything because it doesn’t ‘control’ for spurious variables. People used to believe that bad smells caused disease – not a bad guess, given the correlation they observed – but correlation is not causation and in the end it takes a properly constructed scientific study to tell the difference. There may well be some benefits to compost tea compared to nothing, Robert’s point is it has no advantages over using the compost itself – and it also seems that a lot of people are selling products based on the ‘hype’ about compost tea. Robert’s analysis will help to stop people from being duped into paying for an over-priced product when many other cheaper and probably better products are available.

      Reply
    • 1) This is not a field study. It seems they grew seedlings in pots and then analysed the larger seedlings.
      2) The soil is “coconut beat and peat moss” – not sure what coconut beat is, but peat moss has virtually no nutrients.
      3) This study makes the same mistake as so many compost tea studies. The control was given just water. So what this study shows is that if you don’t add nutrients to a nutrient poor soil, you get less growth. If you add various amounts of nutrients, via the tea, you get better growth.

      But we know that – adding nutrients causes more growth.

      What this study should have done, was compare adding the compost to the soil as the control, and then showing that using tea makes a difference. And it needs to be done in the field.

      Nobody doubts that compost contains nutrients.

      Reply
      • compost tea supplies bacteria, yeast and fungi to the soil.. it’s the metabolites and symbiosis with the plant that releases nutrients from the soil which then in turn benefit the plant.

        Reply
        • That is what Pro-compost tea people say. Now show me the science that compost tea adds microbes to the soil so that they survive long enough to do anything.

          Reply
          • empirical evidence… as following practices pertaining to permaculture yield soil GROWTH. That is why people are doing it… it’s proven to work… compost tea is but an ingredient to a diverse system of land management.

          • The following is a list of the benefits of compost tea claimed by Elaine Ingham. This comes from her site @ https://www.soilfoodweb.com.au/about-our-organisation/actively-aerated-compost-tea-information . As for testing… i know people constantly test their soils for composition and it would seem senseless if there wasn’t improvement noted.

            The benefits of using a compost tea that contains ALL the food web organisms are:

            Improved plant growth as a result of using beneficial organisms to protect the plant surfaces. The organisms occupy infection sites and can also prevent disease-causing organisms from finding the plant.
            The tea improves the nutrient retention of the soil thus stimulating plant growth. If your soil can retain its nutrients it helps minimise the need to use fertiliser. A healthy soil is less likely to leach its nutrients into ground and surface waters.
            Increasing the nutrients available to the root system leads to a stronger healthier plant. The predator-prey interactions increase the available nutrients required by the plant and enables it to absorb them in the correct dosage at the time the nutrients are required.
            Compost tea assists in reducing the negative impact that chemical-based pesticides, herbicides and fertilisers have on beneficial micro-organisms in the ecosystem.
            Improves the intake of nutrients by increasing foliar uptake. The beneficial micro-organisms increase the time the stomata stay open, while at the same time reducing evaporative loss from the leaf surface.
            Reduces water loss and improves the water retention of the soil thereby reducing the need for frequent watering.
            Improves tillage by building a better soil structure. Only the biological components in your soil can build its structure, and ALL the organism groups in the food web are required in order to have this occur. Thus your soil must contain not only bacteria but also fungi, protozoa, nematodes and micro arthropods. (Please be aware that the plate count method on its own does not supply you a complete overview of your existing soil foodweb.)

          • But compost also does all of those things – that is the point. Show me one study that compares compost tea to compost, and demonstrated value to making the tea.

          • >>>But compost also does all of those things – that is the point. Show me one study that compares compost tea to compost, and demonstrated value to making the tea.<<
            I don't think a study like that exists and the reason is that compost is never the same. Compost depends on it's ingredients and the environment is is developed in. Compost tea has the ability to add missing microbial life not only to the soil but also directly onto the plant and studies that suggest foliar application of compost tea is beneficial do exist but to me studies like this are less valuable than testimony given by honest people. Compost and Compost Tea are never the same so that needs to be taken into consideration..
            I could lay out a list of studies on compost tea and many of them are negative i suspect because of the intent of the study. But there are also very positive papers written in unbiased manner.

          • The study would be quite easy to make. Take a pile of compost. Split it into two. Make tea from one half. You now have equal amounts on nutrients in each pile. The only difference is the presents of bacteria, and maybe a faster decomposition taking place in the tea. Now treat two sets of plants equally.

            As long as research compares compost tea against water, they prove nothing.

          • I don’t understand your idea of comparing the effects of compost tea to compost because they are not the same thing? They have different purposes in life though while some overlap, and that being microorganisms. Compost proves protection from the elements and helps keep the soil moist and cool while also proving carbon, a home and a food source for microorganism and which it is designed to have, or should have. And on top of that, it does have bio-available micro nutrients in it. Compost tea on the other hand is largely just a brew of microorganisms which can be brought into the plant’s environment for a number of reasons, one of which is to bring back balance to a depleted soil. It has a much more concentrated effect in that regard than compost has alone.

          • It is the only way to prove that making tea works. It is basic science and how all research is done.

          • but what you’re suggesting doesn’t seem to me like science… please explain more completely your idea because i don’t understand the purpose of it.

      • Hi guys, researcher here. I’ve been exploring this thematic quite a lot, and while many aspects and comments here point out some valid point, I would recommend you guys to open up to a few different visions.

        The thing is, actually we have no evidence that some worm tea is more efficient than any real compost, its efficiency will depend on how it’s made and there’s a lot of room for amelioration. But note that it is not always meant to just replace compost.

        For exemple aqua/hydroponics systems are taking over and should fairly developp in the decade to come. While most of the nutrients and the nitrogen comes from the fish, it is usually necessary to add in nutrients. At this point compost tea is really usefull, you can control it easily and add it without putting dirt in your system.

        The truth is, we still need some serious assays on its different properties (composition, microrganism profile, pest-reppelant effects, etc) but it sure is a cheap way of producing some rich fertilizers.

        Also it could have COMBINED properties, because of the microorganisms grown that wouldn’t be in others conditions. The idea isn’t always to REPLACE, sometimes you use it for something else, or you combine it with other technologies. Another nice thing is the idea that you can actually dose it, which is interesting. It is also a good alternative for green plants in appartments and stuff.

        Also, our big-scale agricultural fields are heresies without any life and any worms. And well it’s too labour expensive to just grow your compost and put in everywhere in the fields (also knowing that the machines will do the work of the worms and also kill them together with the pesticides…). Compost tea once again is a cheap solution to enrich your soils and its life slitghly without using the typical potassium rich fertilizers coming from mines that we might have entirely used in the next 60-80 years… Compost tea is easier to get in the field and can be added with other molecules (pesticides, etc)

        That’s all I had to say. Different products = different applications. You need to compare the methods and chose the good one. But Saying that one is pure trash usually means you can’t think beyond your nose.

        Reply
  7. You clearly don’t understand what compost tea is supposed to be doing. It has nothing to do with NPK levels. It’s about nutrient cycling. You grow mostly bacteria and flagellates and the flagellates consume the bacteria and release the excess nutrients in a plant available form. That is how plants grow, not from NPK. Just look at vermicompost, you can’t deny its benefits for the soil and it has almost zero of the “nutrients” you think plants need to grow.

    Reply
    • Actually plants do grow from the NPK – that is basic biology.
      Nutrient cycling happens without adding compost tea. That is one of the main points – there is no science to support the idea that nutrient cycling increases by adding compost tea.

      Reply
  8. I’m new to composting and am waiting for my pile to be usable. I’d like to use potassium now to stimulate root growth so I’m thinking of blending up banana skin and adding it to the soil around my plants. In this scenario I know what my tea is made of and hope to be adding potassium and a little phosphorous to the soil. Is that thinking on the right track? Also I’ve seen some people who promote this say that they wait a week or two, letting the banana soak. If I blend it and increase surface area will that not release nutrients immediately?

    Thanks!

    Reply
    • 1) potassium does not stimulate root growth
      2) a banana is not going to add much potasium.

      A banana smoothy will release most of the potassium immediately, because potassium is not tied up in large molecules. There is no point making tea.

      Reply
  9. Hi! Thanks for the information. With springtime there are dandelions popping up throughout my yard. I’ve read up on their nutrient analysis (.4% potassium, .07% phosphorus, .036% magnesium, .187% calcium, .03% iron, etc.). I’ve been wanting to use them to give my plants all the trace nutrients this plant is packed with, but don’t know the most effective way to apply. Should I compost, or just blend them and turn them into the top layers of the soil, etc.? From your post I’ve decided making a tea out of them may not be beneficial, but would it be beneficial to crush them and soak them in water, then using the water on my plants? Thanks for your advice!

    Reply
      • Maybe more efficient to just eat the dandelions. Leaves, buds,and blossoms are quite tasty and nutritious. Mix with a bit of vinegar and oil, other salad greens if desired, dried or fresh fruit, a few nuts and you have a banquet. (( Oh, yeah, maybe a crumble of cheese, too,)

        Reply

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