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
This maybe a stupid question on my part. If I were growing hydro/aeroponics and using “synthetic” nutrients would a compost tea help? If so that would be useful putting it in a filter bag and mixing for a few hours so that nothing would get in the system. Then the question remains would it even help?
If you are already adding the required nutrients it will do nothing except add more of the same nutrients.
Adding Biology to Soil and Hydroponics.
By Dr. Elaine
Teaming with Microbes‘series’. Three separate Books
By Jeff Lowensfels
These hv the real information
no they don’t.
Both promote ideas that are not accepted by science https://www.gardenmyths.com/teaming-with-microbes-close-look-part-one/
Wouldn’t one advantage of using compost tea be to prevent the spread of weed seeds? No matter how hard you try, it’s difficult to get compost completely free of weed seeds. I also think it’s cheaper and easier to spray a large garden with tea than try to spread that much compost. You can make a lot of tea with a little compost. A 50’ raised bed 30” wide would require .7716 yards of compost at 2” depth. You could spray significantly more area using tea from far less compost than that. I’m not pro tea or anti tea. Just my thoughts.
Are seeds killed in the brewing process? Never heard that claim for compost tea.
” it’s cheaper and easier to spray a large garden with tea than try to spread that much compost.” – Just because you dilute a small amount of compost nutrients with a lot of water, does not mean you are spreading the same amount of nutrients. This is the same illogic used for homeopathy. It does not work.
Usually the compost is put in a fine mesh bag when brewing compost tea. You feed the tea and multiply the microbes and then apply the tea. I would think that would pretty much eliminate the weed seeds. I don’t have a microscope so I have no idea if it multiplies microbes or not. Like I said, I’m not pro or anti tea. Just thought it might be merited in some situations. Do you know for a fact that microbes are not increased?
Hello from Italy. I just found you and signed up for your Facebook page. While I do have serious doubts about aerated compost teas, primarily because of the extremely low fungal/bacterial ratios, I do find the idea of compost extracts to be interesting for most people who cannot get hands on enough compost. I think this study was flawed because it looked at trees. Forests often have soil with 100:1 to 1000:1 fungal to bacterial ratios and this tea was 0.03:1, so it could not have any positive impact on a tree. I doubt it could even help vegetables too much since they like on average a 0.7:1 fungal to bacterial ratio. A compost extract would seem more appropriate without this brewing of excess bacteria which should have a more balanced fungal to bacterial ratio. Thanks for helping me think.
It is true that forests have higher fungal ratios than say grasslands, but I have not seen any evidence that changing the ratio that exists in soil makes any difference.
Do you have some research links to support that idea?
My point is that it is a strange study. Seems like giving cat food to a bird and wondering why it isn’t growing. If forests have high fungal to bacteria ratios it is because trees like fungi more than bacteria, so you shouldn’t see any boost from giving them more bacteria. Do you see any value to compost extracts if one doesn’t have enough compost? Shouldn’t the fungal to bacterial ratios be similar to compost (and more to the liking of my vegetables)? Then maybe one could mulch with other more available materials? Thanks again.
“If forests have high fungal to bacteria ratios it is because trees like fungi more than bacteria”
It is much more likely that fungi like trees and there grow better in such areas.
Compost teas can have great results if made properly from great compost. You will never know if the compost and compost tea contain a high amount of beneficial microbes and lacks pathogens unless you use a microscope. I’m studying microherding under Dr. Elaine Ingham. She can teach anyone to easily prevent and to recognize anearobic conditions definitively with a microscope (anaerobic material contains pathogens, high ciliate #’s are an indication) versus aerobic conditions (lack ciliates, contain beneficial organisms like Basiomycetes – easy to recognize due to clap connections on hyphae). Properly made compost has reached temps above 131F for long enough to kill pathogens and seeds – again, must check with microscope every time! If a tea is brewed properly from properly made compost it is constantly aerated with about 80 psi moving through the compost, so pathogens cannot grow there and beneficial organisms can multiply. Clients of Dr. Ingham’s Soil Food Web, Inc. have had amazing results, 100-200% yield increases, elimination of disease and pest cycles, steep slope stabilization, compaction layers and infiltration problems solved – because they use microscopes and know exactly what they are applying to plants and soil. Everyone else is shooting in the dark!
You can’t tell the difference between pathogens and anaerobic microbes with a microscope. You may be able to identify some in each group, but remember scientists have only identified about 30% of the species. They don’t even know what most of them look like.
You are being fed nonsense by Dr. Ingham’s promotions for selling her courses.
https://www.gardenmyths.com/soil-bacteria-myth-identification-managment/
In agreement there about IDing bacterial species via direct microscopy. It cannot be done. PCR/DNA needed. I have had to reteach so many of Ingham’s students. Some believe ciliates are anaerobic organisms. Most believe that only pigmented fungi are good and hyaline are bad doo doo, don’t know testate and naked amoebae are 2 different microbes…on and on. The benefits of aerated compost tea are
1/ one can use less compost per square foot per time parameter for roughly equivalent results to topdressed compost
2/ one can (with microscopy) fine tune the microbial population prior to application; e.g. flagellates combined with bacteria to influence immediate nutrient cycling; high volume of active fungal hyphae, etc.
Compost tea has shown some value as an anti-pathogen, especially non-aerated. Look to Allison Jack – Cornell and European Institutes (can’t recall) ~ Tim Wilson
Re: “one can use less compost per square foot per time parameter for roughly equivalent results to topdressed compost” – That would only be true for anti-pathogen effects. It can’t be true for providing nutrients.
I’m currently doing a major literature review of peer reviewed articles regarding effectiveness of compost tea as a nutrient source when foliar and drench applied. I’m kind of disappointed in your assertion that there are no peer reviewed articles that support compost tea as a tool for disease suppression and/or a valuable source of nutrients. I could easily list 30 studies that contradict this and this is only after a week or two of reading papers. There certainly is a need for more research to clarify the nutritional and phytopathogenic effects, however there is much support for use of compost tea as viable and valuable tool in addition to compost. Teas provide chelated forms of plant nutrients throughout the growing season (as opposed to the more stable and less readily available nutrients in compost that is applied only once in the beginning of the season) to improve both yields as well as nutritional quality. I think you may need to do a bit more homework on this one. There really is a ton of research supporting what you are denouncing.
Here are just a few studies I’ve come across…
Zaccardelli M, Pane C, Villecco D,
Maria Palese A, Celano G. Compost tea
spraying increases yield performance of
pepper (Capsicum annuum L.) grown
in greenhouse under organic farming
system. Italian Journal of Agronomy.
2018;13(3):229-234
Taha SS, Seoudi OA, Abdelaliem YF,
Tolba MS, El Sayed SSF. Influence of
bio-spent mushroom compost tea and
potassium humate as a sustainable
partial alternate source to mineral-N
influence of bio-spent mushroom
compost tea and potassium humate as
a sustainable partial alternate source
to mineral-N fertigation on. Egyptian
Journal of Basic and Applied Sciences.
2018;33(1):103-122
Omar AEDK, Belal EB,
El-Abd AENA. Effects of foliar
application with compost tea and
filtrate biogas slurry liquid on yield
and fruit quality of Washington navel
orange (Citrus sinenesis Osbeck) trees.
Journal of the Air & Waste Management
Association. 2012;62(7):767-772
Pane C, Palese AM, Spaccini R,
Piccolo A, Celano G, Zaccardelli M.
Enhancing sustainability of a processing
tomato cultivation system by using
bioactive compost teas. Scientia
Horticulturae [Internet]. 2016;202:117-
124.
Santiago-López G, Preciado-Rangel
P, Sánchez-Chavez E, Esparza-Rivera
JR, Fortis-Hernández M, Moreno-
Reséndez A. Organic nutrient solutions
in production and antioxidant
capacity of cucumber fruits. Emirates
Journal of Food and Agriculture.
2016;28(7):518-521
Scheuerell S, Mahaffee W. Compost
tea: Principles and prospects for plant
disease control. Compost Science &
Utilization. 2002;10:313-338.
Radin AM, Warman PR. Assessment
of productivity and plant nutrition
of Brussels sprouts using municipal
solid waste compost and compost tea
as fertility amendments. International
Journal of Vegetable Science.
2010;16(4):374-391
Hargreaves JC, Adl MS, Warman
PR. Are compost teas an effective
nutrient amendment in the cultivation
of strawberries? Soil and plant tissue
effects. Journal of the Science of Food
and Agriculture. 2009;89(3):390-397
Hargreaves J, Adl MS, Warman PR,
Rupasinghe HPV. The effects of organic
amendments on mineral element uptake
and fruit quality of raspberries. Plant
and Soil. 2008;308(1-2):213-226
Your first link is a greenhouse study – not a field study. As I have said a dozen times in the comments – almost all of these compare compost tea to water.
Give me a link to one study that shows compost tea grows better plants than compost. Than we can discuss it.
Wow. So much information around here. Thank yall for your input! In the end, there is only one way to know! Do it!
I had astonishing results just after 24hours of spraying my aerated worm tea on my plants.
Leaves became thicker, greener, and theres just something magical going on in there that i cannot explain.
Try it yall!
Let us know your experience 🙂
Leaves do not get thicker in 24 hours!
I could see compost teas being potentially beneficial for plants grown in containers as the microbiome may be significantly barren compared to landscape soil, especially if paired with other organic fertilizers that need to be broken down as well. I could also see compost teas being beneficial at scale when distributing compost is more labor intensive than fertigating with tea.
That being said, I am consistently floored at the average gardener’s logic that somehow soil organic matter isn’t available to plants unless we step in to orchestrate. There are obviously good arguments for the use of liquid feed fertilizers in different situations, but if you use your compost to build soil health year to year you shouldn’t be needing to use this type of fertilizer anyways since all of your nutritional needs are already met by the soil. The same bacteria are doing the same process to the same material, and there is less waste when you just put it in the ground; there is plenty of that compost left at the end of the tea making process after all. If the argument is that the NPK doesn’t matter, why wouldn’t you want as much of the carbon to be able to be fed to the microbes like would be available on the ground over time?
From a disease perspective, compost has repeatedly been shown to decrease diseases when simply applied to the ground, and this is just simpler and verifiably beneficial. Even if the core concept of adding good/inert bacteria to leaves proves true, shouldn’t we be trying to culture good bacteria from the leaves of healthy plants instead of random ones that prefer growing in compost as they are actually adapted to grow on the surface?
How many people I’ve heard espouse the “evils” of woodchip mulch and praise things like compost tea will never make sense to me. The reason wood chips can tie up nutrients in a landscape is by overpopulating microbes that breakdown organic matter, so why are we now trying to supercharge our microbiome instead of just letting nature take its course? People want a magic bullet when all they need is a soil test and consistent water; great thing about healthy plants is they don’t get sick.
I have read your article and a fair amount of the conversations that have followed. I have to say, it seems like you’re being a bit contrary just for the sake of it. It’s now 2021…I wonder if your perspective has changed. Seems to be universally accepted now that compost tea is beneficial… worm tea specifically is really great, even transforming the hydroponics landscape… what’s your take on vermiponics
“Seems to be universally accepted now that compost tea is beneficial” – not in the scientific community.
Since you are convinced that compost tea is beneficial – why not post a link to a scientific study that proves it works better than compost.
Read the comments first and see what is required to prove this case. Comparing compost tea to water in hydroponics is not a valid comparison.
The post has been up for several years and not one link has been provided to show compost tea works.
There actually are a ton of peer reviewed research studies that support increased yields, decreased damping off diseases and other fungal pathogens, and increased fruit and vegetable quality. I think you might have to get yourself on google scholar and have a look (authors such as Hargreaves, Khalid, Radin, Scheuerell, Pane, Mohd Din, Hegazi, Eudoxie, Riahi are only a few). I’m a research agronomist and I’ve just spent the last few weeks preparing a literature review of both foliar and drench feeding methods in preparation for a grant. There is certainly more work to be done, however, to make a blanket statement that there are no supporting studies is simply false, and is pretty irresponsible on your part to say the least. I’d love to waste my time listing articles, but I have a feeling you will either delete my post, or dismiss them with a few words as you have in all the other threads. If you really are interested in the science, I encourage you to continue to learn.
“There actually are a ton of peer reviewed research studies that support increased yields” – and yet you were not able to find even one???
Show me one study that proves an increased yield over using just compost.
You keep arguing that nobody can provide you evidence of compost tea being more effective than compost. How about you post evidence of compost alone being more effective than using compost and compost tea? The entire article you wrote is all your own speculation with no science to back, yet you keep tipping your hat to “the scientific community” as if its an entity that has given you direct validation of your theory.
1) I never made the claim that “compost alone is more effective than using compost and compost tea” – nor has anyone else made that claim. Why would I try to prove something I don’t believe and where simple logic concludes that your statement is probably not true.
2) There are several links in the article showing you data from studies, that support my position. Maybe you did not look at them?
3) It is not my theory, its actually what the science has shown to date and what others have said about compost tea. But I do believe it because in 10 years I have yet to see any evidence that the position is wrong.
4) Instead of arguing that my approach is wrong – why not provide the evidence showing my position is wrong?
Gardening in Canadian soil is quite different to the poor soils of the southern US. The blazing hot sun kills microbes in minutes when it’s 100 F and therefore your compost is toasted. Immediate penetration is necessary. I’m confident through personal experience that compost tea sprayed on and around the plant keeps my 25 acres healthy.
Unfortunately, beliefs don’t equate to scientific knowledge.
Even in hot weather soil is full of microbes. Consider this, a compost pile gets hotter than 100 F and yet it is full of microbes – or else you would not be able to make compost.
I have a hotbin and draw “tea” from it weekly. It’s definitely aerobically produced (odourless), so won’t be harmful. But is it beneficial added to water in can?
It’s pretty simple even if Npk values are low in the tea by mixing it with water your making them soluble and immediately available for the plant to uptake. If you amend the soil with straight compost the nutrients take a long time available to said plant. Just because the compost has a certain npk ratio does not mean the plants can take that ratio in everytime you water only very small amounts are absorbed that’s why it takes a month or longer before you have to fertilize again. When you water plants your essentially making your soil into a compost tea except the water doesn’t stay in the soil long enough to break the nutrients down enough to be completely available. When the nutrients are in water they are instantly available and when they are in the soil they take a long time to be used. You’ve overcomplicated this whole argument without thinking about how the plants take in nutrients most of your macro nutrients are what are considered movable meaning the plant pulls them in and move them through the plant it does this with water so yes a small amount of nutrients in a water solution will go a lot farther then amending with compost that’s why they invented salt fertilizers that you mix with water because they are instantly available.
“by mixing it with water your making them soluble and immediately available for the plant to uptake”
1) Adding water does not make them soluble. They are either soluble in the tea or they are not.
2) They are only plant available if they are in a form that plants can use and are soluble.
Over time the total nutrients in compost will be the same as in the tea and solids in the bottom of the pail – basic chemistry.
“when they are in the soil they take a long time to be used” – not true.