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
You seem to frequently apply what most would label common sense or logic. The issue with “common” sense is its 100% dependent on an individuals experience and knowledge. So unless that is”common” which it rarely is common sense is equally as rare.
Is not one of the major push button issues of commercial farming use of pesticides herbicides the loss of microbe diversity in our soils?
You can see the same example in westerns gut/colon bacteria bio diversity. Its over 10 fold less. There is now shown a direct corrilasion between gut diversity and general health. Same with our crop soils. Plants get there microbes from the soil thus if its low so are those on the plants. Given thus just as in fecal transplants and intake of probiotics etc it applies to the soil and plants.
You throw out that the soul and plants already have health balanced good poluations. You state this as a cornerstone fact to your position. Issue us this is not always even rarely us the case. Most peoples yards have had yrs of herbicides pesticides with next to know plant diversify. I think it safe to adding diversity to the plant surface and soil is a positive.
The argument of adding pathogenic bacteria is also disengenious as you well know there is likely not a soil around that does not contain e coli Most human guts contain it. Its the balance and diversity that keeps it in check.
As far as nutrients being added to soil from liquid compost versus throwing some on top is also slanted You know all too well there isca reason liquid spray fertilizers are used unless the billions spent on crops and University agro all have it wrong. No more fish emulsions just throw it on the soil and water it in…. Oh wait I can do that in one step with a liquid or emulsion.
I agree anerobic vs aerobic for our plants and their soil is correct in general terms.
Do not forget vast majority of our gut flora comes from what we ingest.
One thing industrial farming has shown us is you can produce beautiful healthy looking produce that may even taste good but is but a fraction of the phytonutrients and soil born bacteria that are all beneficial to our bodies. I am more than happy to provide countless research references from NIH pubmed researchgate springer or google scholar..
I would go as far to say anything we can do to add biodiversity to the bacteria of the soil the better given its state over the last 50 yrs.
Care should be taken running a site that is focused on disproving or weakening ideas or practices in general. It creates a unhealthy bias. You tend to disregard the data points that support it or discount their importance yet magnify the opposing or critical ones You tend to stay shallow on the support not looking for a deeper more complex reason but rather dif to the bottom to find and support the opposing view. It is clearly displayed in this article. I am willing to go point by point thru this if you wish as I one did so cursorly above.
I think all of your comments hing on the fact that adding bacteria from compost tea result in an increase of those bacteria in the soil.
Do you have some scientific proof of this? I could not find any.
this is all very interesting indeed.regarding the fertilizer/compost tea- i understand roberts point and agree that putting the compost on the soil will give the same amount of nutrients so why bother with the tea?
however,surely for hanging baskets etc-you cannot add layers of manure ,compost during the growing season without disturbing roots and/or soil overflowing so surely pouring in a liquid boost in the form of the tea is a good idea?isnt this the same principle as buying a liquid fertilizer from the shop?
if not -would be delighted to know why not?
thanks!
It may be the same as buying a liquid fertilizer because some of these are very weak dilutions.
Compost tea is unlikely to provide enough nutrients for containers, but if you use enough compost they might.
“Why not just use compost itself rather than tea?” Because of ease of application. Aerobic compost tea is easy to make and in a liquid form. It’s not a substitute for compost, but a different means of application—and it’s cheaper than fertilizer and an extension of what I am already generating in my compost pile. Especially when amending soil is inconvenient or impractical. I think of it as a booster shot.
Is it easier to apply? Try to take a yard of manure and make tea from it? On a commercial scale you can do this, but as a gardener you need a lot of buckets to make the tea. It is much easier to spread the solid material.
I am very surprised that PH is not mentioned in your article. For me – this is one of the most important reasons to make compost tea, at least in my case.
In searching for better ways to garden indoors organically, I’ve found that it can be difficult to consistently provide liquid feed for plants that is within the ideal PH window for that plant. Compost tea has helped me greatly with this.
My tapwater comes out of the tap with a PH of around 8.3 where I live. I have run into great problems using this water as-is with nutrient uptake – and boy can nutrient lock-out and other issues make you scratch your head. For example when a plant is showing signs of a calcium deficiency because it cannot uptake calcium due to a PH imbalance, the most common move is to feed more calcium. Ugh.
As we all know, PH is very important for nutrient uptake with plants in general. That sweet spot is very well provided by Mother Nature, given time, outdoors. For indoor gardeners who need to replicate this quickly, compost tea is a huge help. As bacteria, fungi, and other helpers break things down in the soil in nature, everything is fine, but how can we speed this up for indoor growing?
Here’s what I’ve found. I can take my tapwater (PH 8.2), some wood ashes (PH over 9), some castings from my own worm bin (PH neutral-ish), a calmag additive, some coffee grounds (PH 6.5 or so) and some organic molasses or maple syrup, and put the air to it. Initially, the PH is around 8.0 – certainly not ideal at all for nutrient uptake. Within three days, just like clockwork, the tea will go from 8.0+ to 6.5 – 6.8. Why exactly is this, I don’t know – I’ve read that these organisms, over time, work to create an environment in which they thrive the best.
So, unless you are arguing that PH does not affect nutrient uptake, compost tea for me is highly beneficial. Why? (1) Because I don’t have to wait for weeks/months for that to happen in my soil, (2) I can PH balance liquid nutrients from wood ashes and other sources that are otherwise at dangerous PH levels (money savings not buying nutrients), and (3) Oxygen supplied into my liquid nutrients is obviously also a huge benefit also.
I am certainly not a botanist, but have dealt with many nutrient uptake problems at this point and currently have compost tea solving these problems for me. As soon as I dialed in on the PH issues I started learning a great deal about what not only what macros & micros do in the medium, but the environment that the bennies create. I have a worm bin, and otherwise make endless free nutrients. Works extremely well for me!
The brewing proces extracts the nutrients from the organic matter followed by further processing by the biological population.
The benefits of Liquid (organic) fertilizers in horticulture and agronomic practices are well documented in literature, including compost water extracts.
Your position on foliar feeding is kind of outdatet. Foliar feeding is standard practice in dutch tomato cultivation.
Provide a link to one study that shows foliar feeding with compost is better than foliar feeding of just the nutrients in water.
We can agree that spraying plant leaves with nutrients will benefit the plant, at least short term. What the research needs to show is that the magical additives above and beyond what is in the compost, ie created during brewing, is also beneficial.
This also needs to be a field study – not a lab study.
It all depends on the rate of decomposing in the brew. The faster the rate, the more feedstock can be used in a given time, the more concentraded the brew will be in nutrients and bioactives
No magic here.
True, but nobody doubts the fact that the nutrients help plants.
What is in doubt is the brewing process.
To me it shows that with brewing a tea it is possible to concentrate nutriënt content beyond the content of nutrients in compost, wich is usualy to low to be considered as a fertilizer.
Further more, there seems to be a positive effect of compost tea on stomatal opening when foliar applied.
Compost tea, while having most of the same properties as compost, is more versatile in (faster acting) delivering methods of nutrients.
Brewing tea can not increase nutrients – it is physically impossible. The one possible exception is nitrogen. If the bacteria in the brew are able to fix nitrogen the nitrogen in the brew+compost could be higher than the original compost.
I have not seen any evidence that this actually happens nor have I seen anyone claim it happens.
It is true that brewing results in decomposing to speed up, and so nutrients are converted to plant accessible nutrients faster, but that may or may not be beneficial to plants.
Dabing Xu, Shujun Zhao, Yousheng Xiong, Chenglin Peng, Xiangyu Xu, Guohan Si, Jiafu Yuan & Qiwei Huang (2015) Biological, Physicochemical, and Spectral Properties of Aerated Compost Extracts: Influence of Aeration Quantity, Communications in Soil Science and Plant Analysis, 46:18, 2295-2310, DOI: 10.1080/00103624.2015.1081693
The goal of this experiment was to investigate the effect of aeration quantity (0, 11, 33, 55, and 77 L·min−1) on the growth of aerated compost extracts from a pig manure–straw compost. When the aeration quantity was 11 L·min−1, lettuce root growth enhancement of normalized compost extracts was at a maximum. As the aeration quantity increased, the total water-soluble organic carbon (TWSOC), total nitrogen (TN), total phosphorus (TP), humic carbon (humic C) content, and humification degree of compost extracts improved gradually. No differences in functional group structure were found among the aerated compost extracts. The positive root growth could be attributed to physicochemical and spectral characteristics, such as TN content, humic substances content, humification, aromaticity, and the low content of carboxyl groups. In conclusion, the aeration quantity of 11 L·min−1 was suitable for the production of aerated compost extracts, which obtained much greater promotion growth.
Statements like “The positive root growth could be attributed to physicochemical and spectral characteristics, such as TN content, humic substances content, humification, aromaticity, and the low content of carboxyl groups.” clearly indicate the cause is not known and could also be anything else.
What this shows me is that a certain amount of aeration while making the tea produced the largest amount of decomposition of the manure. A higher rate of decomposition resulted in more nutrients on the tea and higher growth rate.
It does not show that the tea was any better than just using the compost.
I’ll also bet that this was not a field experiment. There are many such experiments, but they all fail to compare the compost to the tea in field experiments. It should be no surprise that adding more nutrients to a plant will produce more growth.
“I’ll also bet that this was not a field experiment. There are many such experiments, but they all fail to compare the compost to the tea in field experiments.”
Robert, I bet you would claim a field experiment is invalid because it wasn’t performed in a laboratory. I can’t believe you’re still harping on this and defending yourself. You just conceded that “A higher rate of decomposition resulted in **more nutrients** on the tea and higher growth rate.” due to the compost being mixed into water and aerated. Isn’t that your entire argument in the article?
You miss-read what I said. A higher rate of decomposition speeds up the release of nutrients – so in a sort period of time there might be more, but decomposition does not create more nutrients.
Why would anyone “claim a field experiment is invalid because it wasn’t performed in a laboratory”? That is nonsense.
Current knowledge suggests that compost teas work through a combination of chemical and biological mechanisms, which have not been fully unravelled. A ready supply of macro- and chelated micronutrients becomes more available to plants through hormonal action of humic acids and other phytohormones that act both on the roots and leaves. The variability that exists across compost teas’ chemical and biological constituents compounded by edaphic and crop factors challenges precise determination of mechanistic effects.
There are several organic fertilisers and nutrient sources available but with few liquid options. Compost tea presents the best alternative liquid organic nutrient source for horticultural and agricultural use. Its origin in compost ensures that the product is sanitary and contains soluble constituents of the compost. By definition, being associated with mature compost also minimises the potential for phytotoxic compounds and effects on crop health and soil quality. The term compost tea must be differentiated from other extracts and from other organic sources as these may have potential negative or non-stimulatory effects. Regardless of the nature of the composting system, composting feedstock and brewing conditions, compost tea has been reported to enhance soil quality through increased microbial diversity and nutrient availability and increase crop growth and importantly yield. The latter is especially so when compost tea is combined with mineral or organic fertilisers. Several mechanisms have been posited for the altered effects associated with compost tea use including increased availability and uptake of nutrients especially when applied as a foliar treatment. Secondary mechanisms include increased soil organic matter and nutrients turnover through microbial activity. Stimulatory effects occur on plants through PGRs, humic and other biostimulatory compounds present in compost teas. Further benefit is derived through the suppression of plant pathogens which provides the best opportunity for maximum growth. As an amendment its versatility betters even its source material. Compost tea has shown potential for being an ideal beneficial product in any cropping system.
https://www.intechopen.com/books/organic-fertilizers-history-production-and-applications/compost-tea-quality-and-fertility
I would disagree with the statement “Current knowledge suggests that compost teas work through a combination of chemical and biological mechanisms,”. At present it seems the only benefit is from nutrients. Studies do not show any benefits over and above the nutrients added.
The reference you list seems to be a book and not a scientific study. Do you have a study that supports your position?
Becky…
Thats fermented fruit fertilizer I believe.
You can put ya fruits in a container, add an equal amount of sugar and wrap a paper towel or newspaper over the top. It will separate. the juice formed at the bottom after a couple/few weeks is goody good suck it out with a syringe or make a DIY separatory funnel out of your container. Google fermented fruit fertilizer. Should help you out.