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 is the first time I’ve found Mr Pavlis to be misinformed (or rather under-informed). It’s clear he hasn’t followed the work of Dr Elaine Ingham, Dr David Johnson etc etc.
The reason to apply compost tea-extract is to add beneficial bacteria/fungi which then attract protazoa and nematodes who then provide plant-available nutrients after consuming the bacteria and fungi. It’s abundantly clear that applying extract/teas has significant benefits for plant health/nutrition. It’s not simply old-wives tales or wishful hippy-dippy thinking.
1) I follow both of these people. Dr. Ingham says a lot of things that are NOT science based, especially about compost tea.
2) “It’s abundantly clear that applying extract/teas has significant benefits for plant health/nutrition” The science does not support the idea that adding tea increases the microbe level in soil.
3) Where is you scientific references to support your claim?
In Dr Elaine Ingham’s masterclass on how to build great soil, she said her first preference will “always be to spread compost” but she added “but then if you have 10,000 acres of range…” then she recommended compost tea but made in a very particular way. So the writer of this article may well have mistakenly, misunderstood her, that is, he seems to think that she is/others like her are advocating compost tea in preference to compost itself for every gardener. No, she actually repeated that if you can afford “that much compost, then use compost” for the entire place, as that is by far, better & she also added that spreading compost is more beneficial than the more laborious strategy of brewing compost tea. That’s why as a novice gardener with a small plot, I don’t make compost tea since I have bought tons of high quality compost albeit only as my initial outlay. I shall have to make compost in future (not compost tea as an organic commercial farmer or someone with a massive plot would).
People make compost tea because they think it will be more effective and somehow cover more ground effectively.
If you can’t afford enough to cover the whole garden – then just cover part of it – making tea won’t work.
As a container grower I don’t have the space to continually top dress. So for me watering with the tea makes perfect sense.
I don’t have access to fresh compost, I use dry bagged and I add soluble seaweed to my tea that has a K value of 16 for flowering. You can add nutrients to the tea.
I have read a few of these garden myth things and it seems to me you are intentionally misleading ppl? While most of your points actually hold up you seem to choose and pick them carefully. For instance, compost tea isn’t supposed to increase npk, it’s a microbial booster (which you acknowledge) and I’m sure you know boosting certain microbes has been shown to increse available amounts of macro and micro nutrients to the rhizome?
You make very specific points with only one or two studies but dont mention the multiple studies that have shown that innoculations of certain microbes do reduce and fight diseases. I have read a few of them and I’m pretty sure you have too.
Most of your points are based on unrealistic conditions like perfect soil with thriving microbial life and yes compost tea probably won’t do anything very noticeable to perfect, fully amended soil. In that situation it probably helps to maintain the soil health though. In soil lacking microbial life what are the effects compost tea???
look a little deeper and not just for information to prove your own bias (confirmation bias is very real)
Besides all that…..
There are ppl who grow some of the most impressive and healthy plants ive ever seen this way.
I think ill listen to the all the ppl growing amazing plants not the 1 guy telling them they are wrong haha
“For instance, compost tea isn’t supposed to increase npk, it’s a microbial booster (which you acknowledge) and I’m sure you know boosting certain microbes has been shown to increse available amounts of macro and micro nutrients to the rhizome?”
1) Lots of people do claim that compost tea increases NPK
2) “it’s a microbial booster” – no it is not – I also debunk this claim. https://www.gardenmyths.com/buying-soil-probiotics/
3) “certain microbes has been shown to increse available amounts of macro and micro nutrients to the rhizome” – microbes in soil can do this – but there is no evidence that adding microbes to soil will do this. But since you are so certain – provide links to the studies to confirm your position.
It is truly amazing that comments such as these make all kinds of claims and yet the author provides no proof of their claim. And yet the post they are commenting about is full of links to studies and facts. And I am the one misleading people !!!
Thank you for this article, a very interesting read, I know the article is regarding compost tea made with pre rotted compost, but do you know how this compares with teas made from green materials such as a nettle tea? These are green materials rotted down within the water, not added after rotting, does this significantly change the benefits of the tea? Or does this process face similar issues?
https://youtu.be/tB7cxfzPFQc
I was looking for a scholarly article to try to clarify some endless issues for anyone to understand compost teas. Unfortunately these basic items are still a mystery, even after this “scholarly” article.
Heres a brief list.
1. Compost is already bacterial digested matter. To use it for further bacterial digestion (in compost tea) would seem a poor food choice.
2. Adding any bacteria to improve poor soil would seem fruitless, since the food required by bacteria would separately need to be added to the soil.
3. Your assertion that only nutrients existing in the compost will end up in the Tea would seem incorrect. I presume the nitrogen, primarily (or at least additionally), comes from the air.
4. Nitrogen fixing: Im not sure i that is the correct term, but some vegetative materials, when added to soil (maybe compost tea starter stuff too) consume nitrogen (lock it up / make it unavailable to the plant) when added to soil eg leaf litter. How does this affect the choice of starter food?
5. Cow manure particularly is loaded with antibiotics. I have had several unsuccessful teas due to NO microbial activity when using Cow Manure, I suspect for this reason.
I am currently trying Fish meal (ground fish products) and bone meal for starter food. I was hoping to clarify whether this will make satisfactory tea as it is not pre-composted, and I’m not sure if such “raw” products would lock up nitrogen away from plants..
” Compost is already bacterial digested matter.” – that is not true. Compost is “partially” digested material. Most of the nutrients have not yet been converted to plant available forms.
“only nutrients existing in the compost will end up in the Tea would seem incorrect.” – not true even for nitrogen. True some nitrogen might be fixed in the water, but this would be a very minor amount. All the other nutrients with the possible exception of sulfur must come from the compost.
Compost tea is more about cultivating beneficial microbes than stretching existing nutrients from compost. Mycorrhizal fungi will secrete acids that break down rocks and supply those nutrients to your plants in exchange for sugars and carbon. It can form connections between plant and tree species allowing them to share and nurture each other. Bacteria can also provide nutrients that the plant would otherwise not be able to get on its on. some bacteria are literally squeezed by the roots to release nutrients, others are eaten and excreted or die on their own releasing small doses of nutrients to plants. The more diverse and robust soil food webs create more available nutrients which will help your plants stay healthy and fight off diseases and produce more healthy growth. The microbes living in the soil are what will feed your plants, not the compost.
There is a study on how just putting an earthworm in a petri dish with salmonella will kill off the harmful bacteria. Salmonella will die when it passes through a worms gut, but it will also die when just touching the outside of an earthworm. It should also die within 48 hours in an aerobic environment, which is one reason why you add oxygen to the tea. When you create a good compost, it is essentially pasteurizing the soil by keeping it between 135-170 degrees f. This should kill off e-coli present from manures. Adding mycorrhizal fungi to a compost or when brewing tea will ensure at least some of it is present. If you aren’t looking at your tea or compost through a microscope it is still a guessing game, but the quality of compost gives you a decent idea of whether your putting in good microbes or not, if thats not enough, you can buy a microscope for children and just mix soil with water or tea and put it on a slide if you want to really know whats in there. You don’t need a fully equipped soil testing lab, just a cheap microscope.
There are varying degrees of quality. Many people don’t know what their doing and have poor results. Or they buy local “compost” that hasn’t been thermophmillicly pasteurized or has antibiotics or dewormer or pesticides and chemicals in it. I assure you, when you add good compost tea the results are obvious and dramatic. I have grown crops side by side and seen almost double the growth and yield. It is almost like looking at 2 different varieties. I have had some plants that didn’t bear fruit, and ones right next to them that got a 1 time compost tea spray that were covered in fruits. Don’t buy “compost” from the city and expect that to work wonders, or buy something in a bag where all the microbes used up their O2 four hours after brewing and then died and then the tea sat on a shelf for weeks in the sun before you apply it. Compost it yourself, brew it yourself, take the time to learn how to do it right and you will be notice the results.
This is an interesting article. I stumbled across it looking to see if I could improve my vermicompost tea (CT). I’ve read some of the articles you have linked to as well. I’m no scientist nor do I have a science background. What you and the linked articles state makes sense and I would be inclined to believe it, but my anecdotal backyard gardening evidence the past four years leads me to disagree. I’m not trying to disprove your or the others findings but just only point out I’ve only seen positive effect from CT. I’ve done many side by side same species comparison with different types of plants and veggies in my garden. As expected, my vermicompost (VC) plants out perform my store bought soil plants. More shocking for me is that after discovering CT, my CT and VC plants performed better than VC plants. Again, the science behind whatever is going on is above my head & my pay grade but there is an old saying that’s fitting here “if it ain’t broke don’t fix it”.
Firstly, congrats that you got something right to have consistently positive results. I’m still on the way figuring out mine.
Along my learning path, I found helpful articles and presentations (particularly by Dr. David Johnson) that support and scientifically interpret the WHY behind the results. One example is the F:B ratio, as it increases, increased the absorption of Carbon at the roots & shoots; thus, resulted better photosynthesis, more sugar, more cellulose for plant and hence more growth. Another experiment by Ashish A. Malik and colleagues in 2016 showed higher F:B results more SOC, etc. My point here is, under the hood, microbes do a lot more in producing and converting the chemistry in soil & plants, given the same amount of other inputs on test environments. Per Dr. David Johnson, he said we should start with biology side of it rather than the conventional way comparing NPK, etc. So, I suppose you’ve done something right biologically to “add” positive effects using CT while it may seem diluting the inputs. This takes me to the second point, which is more about the article.
While the author provides opinion about why CT not beneficial and not worth for taking extra steps b/c the nutrients would be diluted, there’s no proofs of preventing diseases, etc. I more or less do agree to those points although it’s still, just a logical opinion. Not everything is as it appears; especially when it comes to micro biology. From what I’ve learned so far, what makes CT work is supposedly (1) the ability to cultivate microbes, specifically more on fungi side, that then do powerful works below the surface (back to the paragraph above!) While it’s also true this whole F:B science is still a giant black box that demands for much more in-depth studies, we can’t refute the proofs that many have successfully done over decades. And the second benefit (2), in my opinion, is the ability to input cultivated microbes more efficiently when needed (crop field, grasslands). I don’t think I need to prove this point.
Back to my failed experiment with CT… I sprayed CT three (3) times without much dilution each time, so technically, the CT test bed should have at least the same amount of nutrients as my Compost bed. So why doesn’t it work? I just got a lab assay with F:B at 1:1. Per Dr. David Johnson, that’s just a normal environment without significant success (for crops) until F:B > 3:1. If I can raise F:B to 3.5:1 early enough for the plants to grow and thrive, the outcome may be different then. Another possible explanation is probably my fungi groups are not the right ones for the plants. There’s so much in the black box to be able to pinpoint the exact cause of failure and even success, but I think there’s enough proofs and reasonable interpretations of experiments to support CT’s positive results. And because there are still so much unknowns, anyone can easily make unsuccessful CT without realizing so, yet keeps thinking the science behind it is totally false.
What about products like jimz save a tree which comes in gallon jugs and has organic nutrients. Is this like a compost tea or just a natural fertilizer.
No idea what ” jimz ” is?
I’m a big fan of your posts and knowledge. But I feel like I need to point out a big fact you seem to have left out: When people make compost tea, they generally ADD in sugar (molasses) for the microbes to consume. Would this change any of your conclusions?
Good question.
There are two claims for compost tea; add nutrients and add microbes.
Adding a sugar source to compost tea will not change nutrient levels, except maybe some sulfur from molasses. Sugar is just carbon, oxygen and hydrogen. So sugar does not change the nutrient level.
The sugar is added to increase the number of microbes. Sugar is an energy source for them and adding it will cause an explosion in growth – at least until the sugar is used up. So compost tea with sugar may have more microbes than compost tea without sugar- depending on when it is harvested.
Do the microbes in compost tea affect the soil? The answer is no. https://www.gardenmyths.com/buying-soil-probiotics/
If the number of microbes in the compost tea were important you could also just double the amount of compost you add to the water, or reduce the amount of water. Either one will increase the relative amount of microbes in the tea. In fact that would work better than adding sugar. Sugar without a nitrogen source does not work very well. Compost adds both sugar (organic matter) and nitrogen.
Lets assume the sugar does have some effect. To make this comparison fair, you would also add the sugar when you use compost. You would apply compost tea with sugar and compare that to compost with added sugar. So no net effect of sugar.
Adding sugar to soil will increase the microbe population in soil, briefly, until the sugar source is used up. There is very little longer term benefit.
“ everything is everywhere and the milieu selects” – A common used phrase in evolutionary microbiology. You can put what you want in soil, but it will never outcompete the microbes that the local soil conditions have selected for. Like putting some domesticated cows in the African savannah. Nature at the microbiology level is also brutal and ruthless. Do you really think the microbes you culture in a completely different environment stand a chance against the native microbes that have spent countless generations evolving to the local environment?
I like the analogy “Like putting some domesticated cows in the African savannah.”.
Which is why one utilizes local compost – vermicompost when extrapolating microorganisms in aerated (or non-aerated) liquid. One is able, following guidelines or using a microscope to approximate a tuning of large numbers of bacteria/archaea and adequate numbers of flagellates and naked amoebae so that nutrients are immediately cycled to the root zone. (Rudimentarily explained) This occurs because as the protozoa consume the bacteria their nutrient/energy needs are fulfilled by an average 20% of that intaken and the excess 80% expelled is in the ionic form available to the roots. There can be added benefit of surviving and encysting microbes. If one has cultured fungal hyphae the benefit is further increased.
The economics of this is beneficial to the roots, as they do not need to expel their precious carbon into the soil solution to activate the ionic/nutrient exchange.
Of course the same thing could occur using the same volume of compost/vermicompost over an extended period. In my experience the use of ACT or LCE can bring about positive change to the soil/plants more rapidly than applying similar compost volumes; especially if experiencing pathogen pressure such as erwinia . Sometimes similar results are observed applying a microbial feedstock such as molasses.
” using a microscope to approximate” – You have clearly bought into the false claims of Dr. Ingham. You can’t ID soil microbes with a microscope.
https://www.gardenmyths.com/soil-bacteria-myth-identification-managment/
“In my experience the use of ACT or LCE can bring about positive change to the soil/plants more rapidly” – Science disagrees with you.
Sir, I have delved into this completely separate from Elaine Ingham. If you had read my material and research sent to you previously you would have a better grasp of what I am referencing. Of course one cannot ID bacteria/archaea by direct microscopy, however it is quite simple to discern flagellates, amoebae (naked & testate), ciliates and fungi. In addition one can observe volume/numbers of bacteria/archaea both motile and non-motile. This is not very different from noting the various types and sizes of birds in your back yard.
Based upon these observations one can make an educated estimation that nutrient cycling may occur.
The information regarding nutrient cycling is based upon research and papers by Vigdis Torsvik, Bonkowski, Bryan Griffiths, Marianne Clarholm and several other soil scientists. This is off the top of my head so apologies for spelling errors, etc. I’m sure a man of your talents can look them up.
As far as published efforts regarding successes utilizing CT and LCE please look to Betsy Ross [RIP] (Sustainable Texas – I’m guessing) and Steve Diver – Allison Jack.
If you send me a personal email address I’ll donate to you video I produced for ID of soil microorganisms purchased by universities and researchers globally and used by the Smithsonian in their soil science exhibit years back ~ Tim Wilson http://www.microbeorganics.com
“Based upon these observations one can make an educated estimation that nutrient cycling may occur.” – Show me some published studies that provide the method of doing this.
You may certainly read about nutrient cycling through the publications written by the people I mentioned. Do you really need to be spoon fed? Scientists use bioreactors in labs all over the world to grow out microorganisms. You did not even concede the ID of protozoa by direct microscopy. Are you just keeping your mind closed?
When I write a post I take the time to read the research, and give you my reader a link to it so you can read the same research.
If you want to refute the research I found – the least you can do is provide a link to your supporting evidence. It is not my job to prove the point you are trying to make.