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
Hey interesting article! Several valid arguments up in here on this I like it! Heres my two pieces. (Side note I am an indoor organic grower) if you need citation to specific articles on a specific point of interest in my reply please message I will provide.
I guess I’ll start with my view on the adding nutrients situation.
There is Truth in the argument of not “adding nutrients” to your compost tea by brewing it. The NPK isnt going to change from original ingredients. However from my understanding, and from what you stated the nutrient Availability of some of the organic matter is changed from the microbial life in the tea as you brew it to an immediately available form. (As opposed to letting this happen through the horizons of your medium over time, which also happens, just a little quick boost in the aerated tea. think of eating an orange, or eating a vitamin C suppliment)
As for the microbes. They multiply at an extremely high rate in aerated oxygenated water. When you brew it you are accelerating the rate of the aerobic “good bacteria” and they will devour and suffocate the “bad bacteria” out which is trying to survive in a non-hospitable environment.
In my opinion its a surefire way to ensure a sustainable amount of the beneficial microbial life needed in my pots. Worms eat bacteria, protozoa eat bacteria, the bacteria eat themselves. ( there are also many product lines of beneficial bacteria to add to your soils I use *great white*)
If anyone has any doubt on microbial and fungal life being incredibly beneficial to your organic garden ask yourself this: who runs out and puts fox farm nutrients on the Forrest? Lol. No one. The plants secrete sugars and carbs from their roots, fungus and bacteria come and start eating that. worms come and eat them. all that dies around the plant decomposes, all that is consumed is pooped out. The cycle continues. Plants cant move they bring life to them. We have to mimic that life in the rhizosphere.
Side note: aerated water is awesome alone!!!!
Roots love them some oxygen!!!
My brew points: fresh compost, lots of air, 12-24 hours.
Pests: the more benificial life you have in your soil the less chance of hostile takeover.
To the living soil grower dude up there, that soil build you just described is a “living soil” You can keep ammending that over time and reuse it indefinitely. Its essentially a nutrient factory. you can cut your plant down around the base and leave the roots in there. The soil life will compost it. Replant directly into it. red wiggler worms are always a good edition to living soil for extra oxygen pathways and worm poops 😀 springtail mites are like bodyguards for your soil. Nom nom If you are not using a cover crop I would suggest a nitrogen fixing plant such as clover,(local preferred!! Pick some from your yard!!)
Or frass. brew for a shorter time maybe(12-24hours) You also have like 12 different sources of nitrogen im not suprised your tips are yellow lol if your soil is dialed in the bacteria and fungus will adjust your ph for you. Keep that vapor pressure deficit in check buddy lol den stomata are fickle
Love you all happy growing.
*brew it, dont brew it; just do it*
Bacteria lowers your ph when they work
Fungus raises your ph when its doing what it does.
Find the balance and use the force you must.
If your composting at home a wet greener hotter compost pile is going to be bacterial dominant, a dryer slower leafyer pile is going to be fungally dominant. Hope this helps.
Peace
The trick to it is what other ingredients you are using .. I use 4 other ingredients.. which billion the blooms.. I’ve done enough of my own research over the years at home to know where I went wrong and now have perfected it to my liking .. so with variants it does enhance the bloom
The other ingredients might enhance the bloom – not the tea.
Robert. I use living amended soil.
I’m growing white widow canabbis
Every second watering I add EM1 some sort of korean microbes in molasses
Stepwell soil inc.
-bone meal
-blood meal
-kelp meal
-fish bone meal
-alfalfa meal
-basalt rock dust
-gypsum
-yucca
-domite lime
-humic acid
-perlite
The soils got everything in it. But Im a new grower this is my first grow. And my plants almost died while i was at work because my wife doesnt understand heat stress and cannabis.
Long story short my plants were wilted. Yellow leaves. Curled leaves. Droopy. Literally almost dead.
To fix it I got my grow tent environment all dialed in. Then started making some teas WITH top dress in the tea. They are now flourishing. Green and healthy.
I’ve got some veg top dress from kootney biological soils (boogie brew canada)
NPK 6-3-3.5
-feather meal
-bat guano
-fish meal
-seabird guanos
-oyster shell
-alfalfa meal
-volcanic ash
-blood meal
-crab meal
-rock phosphate
-fish bone meal
-glacial rock dust
-sulfate of potash
-langbeinite
-kelp meal
But instead of top dressing I make a tea of my own concoction for my 4 plants in 7 gallon pots
-4 litres of RO water
-4 tablespoons of top dress
-4 tablespoons of worm castings
-1 tablespoon kelp meal
-1 tablespoon unsulphured molasses
I let it aerate with an air stone for 15-30 hours depending on when I remember to make it.
I give each plant 1 litre of straight tea and another half litre to wash it down.
Tea to water ratio being 2:1
My plants love it. They have the smallest bit of nute burn on a few the tips.
So I was gonna cut back on the tea recipe to once every third watering (about every 9 days) instead of every second as I know less is more.
I know what I’m doing is probably overkill.
And I dont plan on reusing my soil. Might throw it in my garden after I’m done but I’ll always start with a fresh batch.
What can I cut out of the equation in between my soil, top dress and tea?
What’s your opinion of it all?
What’s your suggestions?
I love everything you’ve said. And the reason I’m on your page is because I am researching compost tea recipes and the benefit if any.
I never trust anybody’s word for the hell of it. I’m a seeker of truths.
Keep up the good work, I always love a good debate.
Thanks brother
-Mack
Robert, You’ve spent years trying to convince people in this comment section that compost tea is ineffective vs compost, I know because I get an email every time you post a new reply. After the last couple years of growing seasons I tent to agree that compost tea is a waste of time because of how much extra time and resources it takes vs just laying compost down under plants. And as a farmer that makes 100% of my living farming, I need to be as efficient as possible in all farming tasks that I need to complete. What I don’t understand is what is the big deal to you if someone wants to put compost tea on their garden. If it’s because you don’t want them to waste their time, then my question to you is how much time have you spent responding to comments on your article when you could have been outside enjoying the day or tending to your own plants. If it’s because you wish to convince people that you can’t believe everything they read on the internet, then I would say you should write a book about it.. hope you’re having a great day in the garden!
My goal in doing this is education. My other reason for doing it is that I learn a lot along the way.
I have written a book about gardening myths – two in fact. Garden Myths Boos and Book 2.
Anyone who doubts the effectiveness of compost tea should look at the guy from Alaska that holds a dozen or so Guinness Book World Records and a couple thousand awards for size and quality of vegetable growth. His videos are shocking. My garden THRIVES!
You are missing the point. Nobody says compost will not increase growth. But compost tea does not contain more nutrients than the original compost. To prove otherwise you need a proper comparison. One person growing giant veg is not proof unless they also do proper controls.
Hi, I like your article. You fairly gave tea its props while pointing out the snake oil salesman overkill sellers use to promote it. I do use worm casting tea for an additional line of defense against pests. I have a little over a hundred succulents (trying to grow my own nursery) and everytime I turn around there was a pest issue. I do everything in my power to avoid chemicals as I’ve read articles about pest becoming resistant to common pest products. As a nurse I know people become resistant to antibiotics so why can’t plants too. I spray with neem oil, Castille soap if I actively see pest and water with undiluted worm tea every 1 to 2 weeks. Does it work? Your guess is as good as mine. I haven’t seen a pest since but couldn’t that just mean there hasn’t been any find there way to my plant since? Idk if it helps with growth or soil but all my plants are healthy happy and thriving. That could be due to my awesome plant parenting too. Idk but I figure it isn’t hurting my plants, it’s possible helping them so I continue to use it until I’ve debunked its claims. If I do find out that it isn’t helping or that my plants grow 10 feet taller than normal from use, I would love to share! Thank you and I’m glad I just discovered this, I love when science meets world… Anything can happen. Take care
I have a question about compost tea.
I have a ton of weeds. I don’t want to add them to my compost because many of them spread by rhizome.
I have heard that if you put weeds in a big bucket of water and let it ferment for a month, the water soluble nutrients will soak into the water, making a free fertilizer.
This is not the compost tea mentioned in the article but I could not find any information on whether this is effective.
Weeds are plants, therefore they contain nutrients. As these plants decompose the nutrients are made available to other plants. This is true of composting or of anaerobic decomposition taking place in water.
There is no additional benefit to decomposing in water.
i grow in coco coir and adding compost to my coco is not ideal. it holds the perfect air/water ratio and that is thrown off if i add compost. for that reason i use teas. i also can’t foliar feed solid worm castings or compost.
Absolutely nothing wrong with using compost tea. It becomes and issue when people ascribe magical powers to it.
Good day Robert,
I have no references but a certain point of view, so perhaps you’d like to add this comment – or not – doesn’t matter much.
In organic gardening or for field crops I’m inclined to agree with most if not all of your points. In a healthy soil you don’t need additional microbes and in a very bad soil no compost tea will ever be enough to restore the soil to good health. I think that’s pretty obvious, nonetheless there will always be people who fight for what feels right to them even in the complete absence of evidence or logic reason for their doing.
Thanks for putting up with all the nonsense and lack of understanding for all those years!
My point is, and I thought that would be the main use for compost or worm casting teas, that they can in fact be very beneficial in soilless culture and hydroponics.
I think most evidence in this field of application is still anecdotal but in contrast to it’s use in the field or in organic farming the concept how the benefits come to be are quite logic and reasonable.
I’ve seen some early studies regarding this matter and they were mostly positive and on the first glance proper research.
I thought that soilless and hydroponics were the main field of application for compost teas.
Would’ve never thought the use of it is that widespread in organic farming, it seems so pointless.
Anyway, perhaps you could write something about the use of composts, worm castings and their teas or microbes and fungi in soilless/hydroponic culture in contrast to soil culture. Perhaps that would make some points more clear for those who believe in the absence of evidence.
What is the difference between using compost tea or compost in hydroponics? It seems to me that in both cases you would add compost to water and then grow plants in it. Would they not be exactly the same?
I know this is a very old article, but from reading the responses, I can see how more research needs to be done. After reading all the responses and people’s scientific theories, all that can be said is more research is needed.
Agreed. I don’t think the science has changed much in the last few years.
I think science is pointing in the direction that indicates all plants and animals have a primordial relationship with yeast, fungi, bacteria and virus that it is just beginning to get insight into. Compost tea not only adds active bacteria that work to whatever degree improving aeration and water holding capacities but as these critters die they leave behind carbon that feeds other soil critters.
True, but the amount of bacteria in compost tea is minuscule compared to what is already in the soil. Why not add the compost directly and feed the bacteria already in soil?
yes, comopost in some way is almost essential however the advantage of compost tea is similar to the concept behind orthomolecular medicine and that is to provide what has the ability to heal itself all the resource in abundance. Once soil health is established and thriving compost tea is not necessary but can benefit plant health through foliar feeding or direct soil applications.
“Orthomolecular medicine, a form of alternative medicine, aims to maintain human health through nutritional supplementation. The concept builds on the idea of an optimum nutritional environment in the body and suggests that diseases reflect deficiencies in this environment.”
But…. compost tea does not have more nutrients than the compost. I agree compost is good for soil.
did you know they’re considering classifying the human microbiome as another human organ? No.. the idea behind orthomolecular medicine is to supply the body with what it NEEDS in excess. when compost tea is applied it supports, and if not colonizes, what is already there.
bruh r u guys serious … the point of compost tea is not ‘aeration and water retention’ its a massive amount of pre fixed nitrogen…… its about improving nutrient value and accessibility
No it is not! Making tea can not increase the nutrient level over and above what was in the compost. And I have never seen anyone claim it uses nitrogen fixing bacteria. The rhizobiums that fix nitrogen for legumes are anaerobic, so they would die in aerated tea.