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.
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
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
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
I agree that compost tea and compost are no different in terms of total nutrients. I have never used it before. But could compost tea be a faster acting/faster release of nutrients since more of the organic matter area is covered by moisture and more of these can fill in the gaps in the soil? Since the compost on its own tend to have less exposed area, decomposition of organic matter is slower. In other words, compost tea is fast release and compost being slow release.
Compost in or out of water will have the same surface area. Moist compose will decompose faster than dry, but being in water will not speed up the decomposition. The brewing process will probably release a bit more nutrients in the short term if air is added, but keep in mind that compost takes at least 5 years to decompose. Speeding it up by a day or two is not making much difference.
Just my thoughts: I know this is an old thread….
1) Most people use compost tea in conjunction with compost, in a traditional garden setting, to gain whatever benefits they can from both.
2) The argument that keeps getting raised is “use commercial fertilizer, if you want that result”. In my experience, gardeners using compost tea, generally try to stay away from chemicals… which is why the exploration of this option.
3) Isn’t there room for new advances, and the mistakes and breakthroughs that come along with the research involved in getting there, in gardening?
You can’t use “most people” when talking about compost tea. Everyone suggests different practices. Most in fact believe in tea so strongly they don’t use compost. Quite honestly – there would be no point in using both. If you plan to use compost – why use tea?
The nutrients in tea and in compost are no different from the chemicals in commercial fertilizer.
There is nothing wrong with trying things. However, there is a big problem in making claims which can’t be substantiated.
I know of many gardeners who use both compost tea and compost(its not a either or thing for me) compost tea is a organic gardening practice in my experience organic gardeners return all unused organic matter to there gardens you statement is inaccurate.
Which of my statements is inaccurate?
I strongly disagree with this author. Can only compare the article to my own results though.
Blimey! Came across this as looking into worm composting on the balcony just for fun (I am a plant analphabet) and couldn’t understand the need of the tea step. Until clearly proven otherwise i will just pass on the tea. but your fencing and good responses to each point raised are priceless!! thanks.
Well I didn’t read many of the replies but I had a few cents to add, sorry if this repeated. First as far as the beneficial microbes goes you have some great points, lack of control of what kinds of microbes you are growing is an issue. First you should always grow your tea in an aerobic environment, 99 % of what the tea is for is for the introduction of be beneficial “fungi” which for the most part prefer fresh air. what fungi do is digest nutrients in the soil and trade them to the plant for other nutrients in a symbiotic relationship much the same way we rely on the micro flora in our stomachs to digest our food some bacteria can be beneficial and some bacteria can be grown in an an aerobic environment and fungi can be grown in an anaerobic environment but for the most part an aerobic environments grow the best fungi and bacterial colonies for these purposes. So by growing in the aerobic environment you are setting up the environment to favor the microbes you desire. Your second point was different compost make different tea and that you would a full lab to really control the tea and this is on the nose accurate. Thankfully are a bunch of companies that supply these beneficial microbes such as “ancient forest” giving you known strains of these beneficial microbes. For $10 – $15 you can buy several of these packets and it’s enough to brew tea for two or three years. as a foliar spray I have no idea of its efficacy and that’s why I was looking this up in the first place.
Most discussion of brewing aerobic tea focus on the value of bacteria – not fungi. Bacteria or fungi – does not really matter. The soil has lots of both of these and does not need more to be inoculated. Feed the ones in the soil and they will grow.
Amazon says that “Ancient Forest” contains “more than 35,000 species of bacteria and over 5000 species of fungi”. You say that Ancient Forest ‘knows’ the strains they are providing. Do you have any idea what it would take to identify this number of microbes?? What if ,in a new batch, a few were different from a previous batch – would they stop selling the product? If a strain is missing in a new batch would they add it back in? The claim that any company knows the strains in soil is ridiculous. Most soil microbes have not even been identified, so no company is doing this.
But it is good to know that “Ancient Forest is a natural product consisting of 100-percent pure forest humus” – if anyone wants some all they need to do is go to a forest and scoop some up into a bag – costs nothing.
By the way – the company does not even know what humus is. Pure forest humus would not contain any microbes!! They are using the term ‘humus’ incorrectly. See https://www.gardenmyths.com/what-is-humus/
You can throw all the science you think you know at this but all I know is proof is in the results. You have no idea what species of bacteria /fungi are in there but guess what. ..they sure do work. I bring back plants from the dead with a simple tea so something good is going on. Don’t care about the rest. That’s for you to try to figure out. I go off of strictly results and they are solid as a rock. Funny thing is it works every single time too. Next day is like an explosion when off, truly amazing. So I’m gonna keep mixing this useless/toxic brew and hopefully some day someone wicked smart can tell me what’s in it!
A claim like “I bring back plants from the dead” is a sure sign that your results can’t be believed. Not physically possible.
If your results are “solid as a rock” then report on them. Lets get the details. What controls did you use? How many replicates did you use? etc. I really doubt your results are solid as a rock. But if you believe – then continue doing what you are doing.
I was reading about this ultimate tea for bioponics, that solve all the problems related to micronutrients, I am not a specialist but I knwo that Fe is a limitant element in aquaponics systems, and hidroponics I think so ! Well this famous or infamous tea really resolve this problem ? including the others micronutrients that the plant needs ?
What is your link for ‘ultimate tea’?
http://www.bioponica.net I made the question about the Fe and they answer that his tea provide enough to the plants
The phrase ‘ultimate tea’ is not found on the link you provided.
What makes this entire post invalid is his claim that compost tea does the same as a growing in Compost. The nutrient value is actually increased exponentially in tea rather than the slow turnover process of natural composting but that’s not my argument at all. What he fails to mention is that people are not using compost tea instead of growing in dirt. They are using it to INCREASE the nutrient value of their soil. Many people who grow in containers often use compost tea because they lack the ability to add more soil and compost .
…Your personal attacks on me have been removed – there is no place for them here!
Unfortunately you fail to understand basic chemistry and physics! For your statement “The nutrient value is actually increased exponentially in tea rather than the slow turnover process of natural composting but that’s not my argument at all” to be true, the most fundamental law of physics would need to be invalid – the conservation of mass. A handful of compost has a certain weight of carbon, nitrogen, phosphorus etc. No matter what you do to that compost, the weight will never change. So the ‘nutrient value’ can’t go up by brewing it.
Re: “Many people who grow in containers often use compost tea because they lack the ability to add more soil and compost . ” They might use compost tea, but not because they can’t add soil or compost. Compost can be added as a mulch and there is no point in adding more soil – the container already has that. what the plants need are nutrients and most people add them using fertilizer.
I think you have gone out of your way to turn a blind eye to the research that has been done. There are many Universities that have done research on compost tea including Harvard and Ohio State University. These are not new studies and there are many more that that tend to be ignored by people that wright these types of articles. If your looking to prove or disprove something way not start with actual studies?
I did precisely what you have suggested – I started with the studies. I did not find a single one that showed, in field trials, that compost tea performed better than just compost. There are lab studies, and there are field studies that show compost tea might have an effect – most in fact show no effect. But I have not found a single one that compares compost to compost tea in the field. They only compare compost tea to nothing or to water. That only proves that compost has nutrients – we already know that.
Since you know of such studies, please forward a couple of links for them. I would really like to see the evidence to prove compost tea works.
Erick, I am get lost, if you said that harvard and ohio u, have been involve in those investigations, what other information will be provide by the author if both are suppose to see the same reports.
Are you arguing that compost itself is bad? The only thing compost tea really does is multiply what is already in the compost by giving the microbiology the proper conditions for feeding and breeding. This seems like a jealous rant from someone who has never tried compost tea. There are several holes in your argument.
First, you spent much more time going over the improper ways to make compost tea. If you do a little research, you will find the most people recommend stirring your water as often as possible if you don’t have an air pump. Voila, oxygen!
Second, OF COURSE there are going to be different compositions of nutrients and microbes. That doesn’t mean that it’s not beneficial still. Plus, microbes do two things that you did not account for in this article: they form a hierarchy and play by the population rules that they all realize are in place. Microbes also “fix” nutrients in the soil and the tea, making them more immediately available. So there goes your “compost tea doesn’t add any additional nutrients other than what was already in the compost” argument
Third, When you say compost tea does “everything under the sun”. It makes me think that you don’t know how to garden at all. You obviously researched the topic enough to know exactly what compost tea is supposed to do and that plants need much more than microbial life and nitrogen to live and prosper.
Fourth, By the way, you don’t need a “sophisticated lab” to do testing on compost tea. All you need is a microscope that goes to at least x1000, which you can buy for about $100 these days. If you don’t have a microscope, you can use a laser pointed through a drop of tea pointed at a blank wall in a dark room. Works great as a microscope.
I never said compost is bad. It is very good for soil. I also never said compost tea is bad for soil. What I said is that compost tea is not better than compost.
Your statement “The only thing compost tea really does is multiply what is already in the compost” is not correct. Nutrients are not multiplied. The quantity of microbes in the tea may well be larger than the microbes in the compost – but nutrients are not increased.
Re: ” you spent much more time going over the improper ways to make compost tea” – I spent very little time going over the methods used for making the tea and there are many recipes on the internet – who’s to say which is the “proper” way? The fact is that the basic arguments I made don’t change with the method used to brew the tea.
Re: “Microbes also “fix” nutrients in the soil” – what does that mean? I Googled the expression and did not find one hit for it. If you are trying to say that microbes in soil make nutrients available in a form that plants can use – I agree. But this happens in soil anyways even if you don’t add compost tea. having more microbes in the tea will have limited effect once they are added to the soil. I have never seen a study that showed a marked increase in microbes in soil after adding compost tea.
You have said nothing that contradicts my point that “compost tea doesn’t add any additional nutrients other than what was already in the compost”. This is really simple – if the compost contains 100g of phosphorus for example – the tea cannot contain more than 100g of phosphorus. Brewing tea does not create new molecules.
A simple microscope is not going to allow you to identify the thousands of different bacteria species in tea or soil. Try and find a single reference where someone has identified all of the bacteria in a compost tea sample – you won’t find it because it is a huge undertaking. In fact most microbes existing in soil have not yet been identified – it is that big of a problem.
The people with microscopes can positively determine an increase or decrease in aerobic vs aerobic life in a sample of water. Thus determining a proper way to make compost tea by experimentation. We can also determine fungal/bacterial ratios with a great degree of accuracy.
The one source you found that seems to imply that compost tea didn’t effect collard or spinach yield doesn’t go over how they made the tea (except for the fact that they use manure – which, in my experience, is a terrible idea), gives one of the most general definitions of compost tea that I’ve ever read, and it’s vague as to if they gave the compost group any nutrients at all. The way it’s written, they may have planted the compost group in the “previously sandy loam” soil without amending it or adding nutrients, in which case their experiment would be invalid for use in this case.
“Microbes don’t increase the nitrogen levels in the original compost. They just convert it from one type of compound to another.” Robert Pavlis, feb 19 2015
Exactly, they change it from one compound to another, the second compound being similar, but more immediately available to the plants. When you multiply the microbes in a tea by aerating it for 36-44 hours, they are doing all the work (to transform nutrients) in a short period of time compared to the longer period of time that it would take the soil microbes to break it down. And it also leaves food for the soil microbes to break down so they can continue living happily.
Your claim that your compost tea stunk, proves that it was not made correctly. My compost tea barely smells at all and if you get your nose right up to it, it smells like sweet tea.
My sources are all books and hands on experience.
“Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web” by Jeff Lowenfels
“Teaming with Nutrients: The Organic Gardener’s Guide to Optimizing Plant Nutrition” by Jeff Lowenfels
“The Contrary Farmer” by Gene Logsdon
Since you seem to like internet sources so much, give this one a read:
http://microbeorganics.com/
I agree with a microscope you can tell if you are increasing the number of microbes. But that has never been disputed. If you add water, and nutrients, and bacteria, you will grow more bacteria – everyone agrees on that point. Where I have a problem is that you don’t know which bacteria you are growing. Therefore you can’t say the ones you are growing are actually good for soil.
No one disputes that the bacteria in tea are decomposing the compost faster. So what! That does not mean you will get bigger yields especially if you dilute the heck out of it when you apply it to the garden. Even if this were the case, you could just use some commercial fertilizer and get a fast feeding – you would not need the tea.
“Teaming with Microbes” is a good read but not all of it is based on scientific fact. I would not use that as your source of information.
Re: “http://microbeorganics.com/”. Contains lots of information about compost tea. I did not see a reference to a published study that compared compost tea to compost, in a real field study. I admit I did not read the whole thing. If there is such a study I’d like a reference to it.
How about this if we are posting from reputable sites on this issue. Clearly not everyone in the educated community share your views. http://extension.oregonstate.edu/douglas/mg/dcmg/aerated-compost-tea
This is not a scientific paper – proves nothing. I know some people believe in compost tea.
All microbes are not the same. However, when a lab runs a biological assay on a compost product, all they do is look under a microscope with a gridded slide and count.
They measure fungal hyphae and rely on their experience, but with all due respect, they don’t truly know what they are looking at. Arguably, they can’t.
The true merit of biological products cannot be gleaned visually. It is unobservable. Think about it, can you see athletic or musical ability in people by looking at them?
As much as we like to tell ourselves that we “understand” how Nature works…we really don’t.
My understanding was that lacto fermented aerobic compost tea made the nutrients more easily accessible to the plants, perhaps in the same way that fermented foods are much easier for humans to digest? So for example a lacto fermented aerated comfrey tea would provide nutrients to the plant much faster then simply mulching with comfrey and waiting for the soil microbes to break it down. I haven’t researched this much yet so have no resources but am curious to hear your thoughts on this theory.
I think you are mixing up two types of composting. Bokashi is a lacto fermentation process. Compost tea is not usually.
Does a fermented comfrey leaf feed plants faster than a fresh leaf? maybe, but only slightly. To be fair, you would need to put the leaf on the ground at the same time you start making the tea. By the time the tea is ready, the leaf on the ground is no longer fresh and has started to decompose. Both leafs are mostly intact. both leaves still need a lot of composting. The decomposition process is a long one – years. Finished compost continues to break down for about 5 years – see my blog posts on compost.
Besides – why do you need faster composting? If you need fast nutrients for plants use commercial fertilizer. If you want a slow feed for plants compost.
Yes, to really get an idea of how the lacto-ferments are effecting the soil you’d have to compare an equal amount of plant matter decomposing into the soil, and thankfully research into this area is finally being done.
As for why you would need faster composting, and why not just use a commercial fertilizer…
lacto-fermented plant teas are free, can be harvested from the same land I’m growing on, and don’t require an industrialized system to bring them to me. I’m not making the assumption that the lacto fermented teas do indeed provide fast nutrients for the plants, but if they did prove to be as effective, or nearly as effective, as commercial fertilizers then there’s no chance I’d ever purchase a commercially made product again.
So far there is no evidence that teas either from compost or from Bokashi are any better than using the material before making tea. If you like lacto-fermented compost – go ahead an use it. It is also free.