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.

Plant 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. I don’t know if anyone has mentioned this bit of information, as there are too many comments for my attention span to allow the reading of, at the moment. But, I believe that the point of making compost tea instead of simply top dressing is all in the speed of nutrient delivery. It is my understanding that making the tea, will supply a larger concentration of soluble, readily available nutrients to the plants roots, immediately, compared to the slow release via top dressing. I am referring to dried meals like alfalfa, kelp, bone etc. And not of actual compost that’s already been broken down by the microbes in a compost bin or tub or pile. As for the not knowing which microbes are going to be present in the tea, this is true, however; adding specific bacterial and fungal cultures to the tea before brewing certainly raises the chances of having beneficial microbes in the mix. There are a few I buy and use for this purpose but I didn’t read any rules and don’t know if I’m allowed to use the product names and I’m too lazy to go read the bottle for the specific cultures, but it includes bacillus strains, mycorrhizae and if I’m feeling like I can afford it, a certain product containing phosphorous solubilizing bacterium that has a nifty side benefit of being one of the only microbes I’ve heard of that can digest the phytotoxin produced by walnuts(because squirrels continuously bury them in my garden beds). Add the cultures before you brew, and I’d be willing to bet, that they’ll likely be the dominant species in the mix. Another thing a good tea can contain is the various phytohormones that were produced by and contained in the plants being brewed into fertilizer. Mainly cytokinin from the kelp, auxin from aloe vera, and if you add some polysorbate 20, a tiny bit of triacontanol from alfalfa(alternatively, and more effectively for triacontanol, which is basically insoluble in water; one could extract triacontanol from alfalfa into alcohol and dilute to a safe level for foliar feed (going to admit here though that since you have no idea how much triacontanol will be in it, just to buy powdered triacontanol and follow the instructions)).
    So to sum up, the point of compost tea is:

    1. Allow microbes to solubilize the nutrients from previously un-composted organic matter quickly, thus becoming an instant release liquid fertilizer from something that if top dressed, would take weeks to be fully available to plants(then top dress with the leftovers of your tea if you don’t mind attracting flies to the plant material and possums, mice and raccoons to bone meal)

    2. Assuming you add specific cultures to your tea before brewing, you will be adding a large amount of beneficial microbes to your soil and plants

    3. Providing exogenous phytohormones to your plant that can act as growth stimulants and growth regulators, albeit at unknown concentrations. However, since most phytohormones provided to roots or leaves tend to work better at low concentrations, simply start slow and work your way up in dosage.

    Just my $0.02, your mileage may vary. But it works well for me on everything, but be careful foliar feeding anything. With corn, the spray tends to collect at the base of the leaves along the stalk and sits there for a while and can cause some weird growth to happen.

    Reply
    • “It is my understanding that making the tea, will supply a larger concentration of soluble, readily available nutrients to the plants roots,” – it might speed this up a very small amount, especially for potassium which is not bound in large molecules, but most of the organic matter is still in the bottom of the pail after making tea – so it is not decomposed. Which means the nutrients are still tied up in large molecules.

      Have you seen NPK values for teas? They are low. In fact many people use use tea say it is not about the nutrients because they are so low.

      Reply
      • Those compost tea NPK values are NOT low. they only look low because you’re comparing them to dried NPK values which are different. For instance, let’s compare compost tea with fish emulsion. Fish emulsion is 5-1-1 but you don’t just put fish emulsion directly on your soil, right!! No. You dilute it at a ratio of 256 parts water to 1 part fish emulsion. so that 5-1-1 after being diluted into water is actually 5/256 – 1/256 – 1/256 so the nitrogen percentage is actually about .02% and compost tea is aobut .05% to .07%. So compost tea (which already has water added) actually has more nitrogren than fish emulsion, after the water is added!

        Reply
        • A lot of people also dilute the compost tea which dilutes it even further.

          You also need to account for the amount being added and the area to which is is added. A quick Google search gave me “25 gallons/acre” That is very little nutrients per acre.

          Reply
  2. Your post reminded me of something I heard Dr. Elaine Ingham say in one of her videos. She recommended using a wide variety of compost inputs. Her reasoning was truly bizarre: she claimed that each compost input has different microbes on its surface (probably true), and that this would somehow create a more microbially diverse compost (crazy talk).

    There is no need to concern oneself with which microbes are active in a compost pile. The most efficient bacteria and fungi will dominate at any given point in time, and this will continue to evolve throughout the composting process. You can’t control this, and why would you want to? And when the compost is added to soil, different microbes will dominate that environment, as the microbes that dominated during the composting may not be competitive. We need not concern ourselves with any of this. People are overcomplicating compost.

    Reply
  3. In my mind there is no argument that compost tea used on a soil less media has incredible benifits, however I agree with you have a diverse soil it’s not absolutley nessary. BUT in my experoence and testing, foilar feeding compost tea has a direct increase on the brix scale, which is undoubtedly a huge benifit. But yeah these microbe guys can take it a little to far.

    Reply
  4. I find your posts in general to often be contradictory or contain incomplete knowledge. For example, you state that there is no real benefit to compost tea. But then when you’re backed into a corner with facts showing it CAN provide real benefits, then you say something like “well it can provide benefits but not more than just using compost”

    Whether compost tea has extra benefits that regular compost doesn’t have is debatable. Also most gardeners that use compost tea do not assume the NPK ratio of the tea is higher than the original compost – that would indeed be a retarded assumption. However, anytime your plants get fed via compost it is through the water passing through the compost and allowing your plants to absorb it – so even with compost it won’t really be any more “concentrated” in terms of the water that they consume (if anything, probably less since most of the water drains quickly in soil and doesn’t steep for several hours prior to application). And it isn’t like you have to waste the compost that you used to make tea – one can spread it over their garden so that any nutrients which didn’t enter the tea can slow release into the soil.

    At the end of the day, the point “compost tea works but might not be better than just using compost” is not the same as the point “compost tea does not work at all”. This is what I mean when I say your points are often contradictory.

    I have had great success using jiffy pellets (and I remove the plastic mesh when transplanting into solo cups 2 weeks later) and disagree with your conclusions in that article. I have never used compost tea but plan to use it for my first time this year. I read this article to see if it really could prove compost tea wrong, but it doesn’t. There’s a big difference between “I haven’t tried this and believe it doesn’t work because of stuff i read on the internet” versus actually doing experimentation for yourself to see if it works. There are studies that will support virtually any opinion on anything, and i will be doing my own experimentation to see if it works. I’ll also be adding some fish emulsion and liquid kelp, so while not much NPK will enter via compost, the other additives should help. I’ll also be using roots organic nutrients, and for my cannabis plants I’ll also try some fox farm nutrients. I also apply plenty of organic composted manure from a dairy farm that I am friends with.

    Reply
    • “it CAN provide real benefits, then you say something like “well it can provide benefits but not more than just using compost”” – if it does not provide more benefits than making tea then it does NOT provide benefits.

      Reply
      • Except it’s way easier to apply on a large scale to feed in an in line inductor then it is to hand apply every bit of compost that you feed the plants, so it IS providing benefits in that crucial difference alone. Unless you like breaking your back every week. But yeah I’d never direct feed anaerobic tea, that just sounds stupid. Also there are an insane amount of amendments that can be added to the tea that would be a pain in the ass to go to every plant and do directly, so maybe think about it logically.

        Reply
        • I don’t agree. It is easier to apply a liquid on a large area than a solid – that is true. But the liquid is just very dilute compost. To apply the equivalent amount of organic matter as solid compost, you would need to apply huge amounts of liquid, repeating the process many times.

          Reply

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