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Compost Tea – Does it Work?

Compost tea is all the rage but does it really work? Research studies so far have produced mixed results. A 2007 meta data review on compost tea by Dr. Linda Chalker-Scott concluded that “Clearly, the science is not strong for aerated tea use on crop plants, much less on lawns, shrubs, and trees”.

There seems to be a lot of work done in this area but much of it is not published in peer reviewed journals and most of it has been done in labs and greenhouses, not in the field. Until the work is repeated in the field we can’t conclude it works in gardens.

The other problem is that controls have been poorly selected. For example, in one study (ref 3) they compared ACT compost tea to water, using lettuce that had been under-fertilized. Guess what, compost tea improved growth. This only proves that adding nutrients, when they are deficient, will improve growth. The study never compared compost tea to adding nutrients in other ways.

In this post I will review one study that compared the use of compost to compost tea, in field conditions.

Effects of compost tea on the growth of trees, by Garden Myths (data from reference 2)

Effects of compost tea on the growth of trees, by Garden Myths (data from reference 2)

What is Compost Tea?

I have discussed this in detail on another post; Compost Tea. In this post I will only look at ACT, also called AACT – Actively Aerated Compost Tea. This tea is made by adding compost to water, and bubbling air through it for several days.

Overview of the Research Project

The research paper is called “Wood Chips and Compost Improve Soil Quality and Increase Growth of Acer rubrum and Betula nigra in Compacted Urban Soil”, by Bryant C. Scharenbroch and Gary W. Watson.

Testing took place in an urban-like setting, designed to mimic a new development. Top soil was removed, the soil was compacted with construction-type equipment and 3 cm of top soil was replaced. Trees and grass, were planted to mimic a normal backyard. The trees were treated in a variety of ways; only water, compost tea, commercial bacterial concoction, wood chips, compost or fertilizer.

The purpose of the study was to look at the effect of each treatment on soil and on tree growth.

Each tree received the same amount of water, either as part of the treatment, eg compost tea, or as a separate watering. Analysis of soil samples were done by independent labs. Half of the 60 trees were removed after 4 years, and the remaining ones after 6 years. Average results are reported for 5 years.

During the test period, the liquid additives were applied on a regular basis, and compost and wood chip thicknesses were renewed yearly.

ACT compost tea was made using equipment designed for the purpose; Geotea-250 made by Greater Earth Organics. The compost used was obtained from Purple Cow Organics and the standard procedures of NOP (National Organic Program) were used to make the tea.

Soil Foodweb Inc, associated with Dr. Ingham, was used to test ACT as well as the compost and wood chips used in this study. Dr. Ingham is a strong proponent of compost tea, and by using her testing procedures we can be sure that it was done without bias against compost tea.

The Importance of Microbes

Proponents of compost tea will tell you that it is the microbes in the tea that make all the difference. Dr. Ingham has made these claims; ACT will increase nutrient availability and retention via microbial mineralization and immobilization, build soil structure and decrease the effects of compaction, detoxify soil and water, and suppress disease by inducing competition among disease (anaerobic) and beneficial (aerobic) organisms.

In this post we will look at ACTs ability to increase microbial populations, decrease compaction and influence plant growth.

Measuring Microbes

The main philosophy behind ACT is that by taking compost and brewing it in the presence of air, you can increase the microbe population. The resulting high levels of microbes will benefit the soil and the plants. How many microbes are in ACT?

This study used Soil Foodweb Inc to measure the number of microbes. The table below shows normalized results.

Microbe populations in compost and compost tea, by Garden Myths (based on data from reference 2)

Microbe populations in compost and compost tea, by Garden Myths (based on data from reference 2)

The brewing process reduced the bacterial population, on a weight basis, by 68% and the fungi population by 99%. This does not mean that brewing reduced the number of microbes. 8 L of compost was added to 840 L of water to make tea. Assuming a bulb density of 650 Kg/cubic meter, bacteria increased by a multiple of 52 and fungi doubled.

Microbes in ACT, Compost and Wood Chips

How does the microbe population compare between the main treatments used in this study? The following table shows the data. Note that the compost used to make ACT was not the same compost that was used as mulch.

CBP is an unnamed, commercial biological product that provides microbes for soil enhancement. It also contained maltodextrin (48%), yeast extract (5%), soluble seaweed (13%), humic acids derived from leonardite (17%), precipitated silica (8%), leonardite extract (6%), and polyethylene glycol (3%).

Microbe populations in compost, compost tea, and wood chips, by Garden Myths (based on data from reference 2)

Microbe populations in compost, compost tea, and wood chips, by Garden Myths (based on data from reference 2)

This table shows the microbe populations in the material used to treat the trees. It does not take into account the actual amount added to each tree.

A big surprise for me was the microbe count on wood chips. They are not as high as ACT or compost, but they are significant. What is not clear is how easily these microbes get into the soil. Do they stay attached to the wood, or does rain wash them onto the soil?

compost-tea-microbes-add-to-soil

Amount of microbes used, in Kg/100 square meters/year, by Garden Myths (based on data from reference 2)

This table shows the amount of bacteria and fungi that were actually applied to the trees. The numbers are Kg/100 square meters/year. Other microbes such as ciliates are not expected to have an effect on soil or plants, so their number is not included.

It might surprise you to see that the amount of microbes used for ACT and CBP is so much lower than compost and wood chips. The amounts added follow standard application procedures for each product. It is common to see ACT recommendations that use very dilute solutions and the application rates used in this study are within the range recommended by Dr. Ingham.

Improvements in Soil Quality

The following soil parameters were measured; density, moisture, organic matter, respiration, pH, nitrogen, phosphorus and potassium. Density is a measure of the degree of compaction. A lower density indicates soil that is less compacted and of better quality.

Respiration is a measure of the amount of CO2 produced. A higher level indicates that the microbe population is higher and more active in decomposing organic matter – the soil is healthier.

compost-tea-benefits-to-soil

Effect of compost tea, compost and wood ships on soil, by Garden Myths (based on data from reference 2)

ACT and CBP did improve the soil density but not nearly as much as compost or wood chips. As far as the other soil parameters go, ACT and CBP had limited effect.

It is not surprising that ACT does not add a lot of nutrients – it is mostly water. For more on this see my post, Compost Tea NPK Values.

The amount of bacteria added sounds like a lot, but it isn’t when compared to the organic matter in soil, which explains why ACT did not increase organic matter.

What should be surprising to the proponents of compost tea is that respiration did not go up. This is a major claim for compost tea. ACT adds missing microbes to the soil and increases the level of both bacteria and fungi. The results of this study show that neither ACT nor CBP resulted in an increase in microbe activity.

Compost added twice as many bacteria as wood chips, but had the same respiration rates. Adding bacteria to soil does not seem to make a big difference.

Both compost and wood chips had significant effects on all of these parameters. I have discussed this in my previous post; Mulch – How Does It Affect Soil?

The numeric values can be seen in figure 2, reference 2.

Improvements in Plant Growth

Enough of this science data – did ACT and CBP improve plant growth? That is really the important question.

To measure tree growth, the total mass of the tree, including roots, was weighed.

After five years the total tree mass under wood chips was 170% greater than the control trees which received just water. The mass of trees receiving compost were 82% higher. Neither ACT nor CBP had any effect on tree growth.

Effects of compost tea on the growth of trees, by Garden Myths (data from reference 2)

Effects of compost tea on the growth of trees, by Garden Myths (data from reference 2)

Adding Microbes to Soil

One of the main claimed benefits of compost tea, and ACT in particular, is that the soil needs more microbes. Adding microbes makes the soil come alive, making it healthier.

In this study the addition of microbes does not correlate with changes in soil or plant growth. ACT and CBP added microbes but had limited effect on soil and none on plant growth. Compost added the highest level of bacteria, but produced less growth than wood chips.

The problem with compost tea is that it adds very little organic matter. Without the organic matter, you can add all the microbes you want, they won’t do much because they lack food.

Garden soil already has lots of microbes and I bet that if the compost was sterilized before being used, it would show the same results.

Value of Commercial Microbes

CBP is a commercial biological product that not only includes microbes, but several other ingredients that have been claimed to improve soils and make plants grow. In particular it included yeast extract (5%), soluble seaweed (13%), and humic acids. None of these improved plant growth.

The only benefit this product showed is a slight decrease in compaction. It is not worth buying.

Is there Value in Compost Tea?

This study shows no benefits for compost tea. Given the extra work to make it, there seems to be little reason for using it.

There is still the claim that compost tea can suppress diseases when used as a foliar spray. The science for this claim is weak, but it warrants more investigation.

This is just one study and it only looked at the growth of trees, but other studies on compost tea show similar results. Just use compost or wood chips to mulch your garden.

Tea is for drinking on a cold winter night with a little rum in it.

References:

  1. Compost tea: Examining the science behind the claims; https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/compost-tea-4.pdf
  2. Wood Chips and Compost Improve Soil Quality and Increase Growth of Acer rubrum and Betula nigra in Compacted Urban Soil; joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=3337&Type=2

 

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

29 Responses to 'Compost Tea – Does it Work?'

  1. Calliandra says:

    What nobody seems to have seen, is that the AACT they used was totally bacterial, and used to inoculate plants that prefer fungal dominance.
    A set up bound to fail, necessarily!

    It’s not that AACT “doesn’t work”, but just that the basic principle of succession and what that means for the make-up of the microbial herd was not understood, leading to nonsense results and nonsense conclusions.

    • The concept of adding the correct ratios of bacteria and fungi to soil, for specific plants is not generally accepted.

      But lets say you are right – do you have a similar reference that looks that the value of compost tea in field conditions?

  2. Is DR. Ingham the same one that was responsible for the Klebsiella nonsense?

  3. Mat says:

    So you’re trying to say compost tea doesn’t work off the chelation of ions like every other bottled organic or synthetic fertilizer? Once your dry inputs/castings are steeped in water the nutrients become immediately available to the plant just like any other bottled nutrient. Comparing mulching compost to using tea is like comparing top-dressing dry ferts to feeding bottled nutes. Sorry to burst your bubble, but you’re very wrong. Maybe you should just put it into practice and see for yourself ?

    • Re: “So you’re trying to say compost tea doesn’t work off the chelation of ions like every other bottled organic or synthetic fertilizer?” – never said that – not quite sure what that even means.

      Re:”Once your dry inputs/castings are steeped in water the nutrients become immediately available to the plant just like any other bottled nutrient.” – this statement is clearly false. Much of the organic matter has not yet been decomposed so the nutrients are still tied up in large molecules. For example the nitrogen in a protein molecule is not released by adding the protein to water.

      Since the proponents of compost tea claim it is better than just using compost – comparing tea to compost is precisely the test that needs to be done. How else are you going to prove one is better than the other?

  4. Howard Davis says:

    Very interesting article. In our garden we have been using multiple of these methods (wood chip mulch, compost mulch, and compost tea). The accurate information you have provided on how well each of these work in isolation is terrific. I do have a question about the ingredients in your compost tea. Could you let us know what was in the compost tea? I ask about this because the compost tea we make has a mix of ingredients, some of which are actually fertilizer, and so it is a foliar fertilizer in addition to having bacteria. We use liquid kelp and liquid fish emulsion, in addition to molasses. I assume that you used molasses as most compost tea recipes that I have seen use that as the ingredient to promote bacteria growth. It would be great if you could list what you used. We did see some very visual results of our compost tea helping a few plants that were obviously struggling as they would perk up for a few days after each spraying and eventually returned to full health. However now I am wondering how much of that may have been the foliar fertilizer component vs. the bacteria in the compost tea.

  5. Ari says:

    So it is food for microbes that matters, not microbes and since much of the matter that made up compost was already predigested, the result is that microbes now have less food to work with

    • Most of the organic matter in finished compost is NOT predigested. It will take another 5 years and maybe longer to digest it.

      • JC Farmer says:

        I do believe, that you sincerely believe, that the knowledge you have and have gained in your life has profound and logical meaning. Unfortunately, those of us who have dedicated our entire lives to studying and experiencing nature understand that anyone one, these days, can find some study, or analysis that was conducted by any number of so called “experts” and funded by un-named sources for unknown agendas, to fit or support personal or corporate myths, theories, products or beliefs. The supportive study becomes the theory of structure on which later beliefs are predicated. The more socially promoted the claim (media, advertising, Universities, Government agencies, etc.) the more it begins to erroneously be accepted as truth.

        With that stated, it would be wise to accept that humans are a miniscule, but essential part of an enormous living global entity, like being a single cell in the human body. Although the egos that many people acquire via “University Inc.” seem to be larger than their true capacity to learn life science after leaving their Alma Mater and falsely promulgate the “socially accepted myth” that we are smarter than the rest of the world,s flora and fauna. That would be their un-educated opinion. Opinions are a dime a dozen and for a few extra pennies one could acquire a study which supports their opinion. Opinions are simply that and they rarely confirm any understanding of the natural world. Compost Teas work if all the right components are present. Epsom Salts work and some times they may not work so well because there are a multitude of other complex factors involved. To say something is good or bad, that some things work or don’t work are simply opinions based on common teachings or individual experiences and do not take into play the complexity and depth of the perfect cycle that exists in nature. It’s a un-hindered system of checks and balances, of action and reaction which will always impart a correction to any threat. I have performed hundreds of soil and water tests on our family farm over the last 50 years and on our farm we have simply applied the principles of nature. We farm with a “non-intervention approach. We have come to fully understand nature as it exists on our 300 acres. In fact most of what we do here seems to be what many opinion sites profess NOT to do. We rarely weed, we let garden debris remain after harvest and we plant our seeds directly in the two acre plot designated as our manure/plant compost pile which has been added to for the last 100 years. Most of our summer crops come from the fallen fruits from the previous year which are left and covered by the added plant matter at the end of the season.. Heck, I cannot remember the last time I planted tomatoes, peppers, greens or cucurbits. Every year, since 1996, I sample some of the produce and send it out to test it for nutrient values and now Brix. All I can say is that the values are consistently high and DELICIOUS”. We won’t win any Better Homes and Garden photo contests but we also won’t be found sick in some profit center hospital either.
        I realize that people need to feel important or to sell products and some even feel the need to be told what to do by anyone who appears to present themselves as experts in a field where to this date we really, as a species, do not appear to come close to comprehending…Unless you live with nature every day(as any animal does) and try not to control it, nature will always win on her own time schedule and if you allow it to do what it wants you will always win…from an ecosystem point of view. So my point is, if you find yourself seeking answers to natural problems then maybe you should seek natural help. A man is only as good as his teacher, wisdom comes from a life of failure not a life of success. Every plant and every garden is like a fingerprint…It’s all unique. Best of luck.
        A Locally Raw Farmer

  6. jo says:

    Wondering about the merits of using compost tea in pots…as there would most likely not be many bacteria or fungi in this closed environment.

    • Potted soil contains quite a few bacteria and fungi.

      • Aurelien B says:

        Commercial potted soil are sterilized and thus contains very little if not any bacteria and fungi.

        • Ray Roberts says:

          Bacteria and fungi are ubiquitous in the environment. Even if a potting soil is sterilized, it won’t be for long when exposed to the air and water. Hopefully it won’t be a pathogenic species that takes up residence first.
          I’ve never seen a commercial potting soil that claims to be sterilized although some seed starting mixes claim to be, but again they won’t be for long when put to use.

  7. Aurelien B says:

    Growth is by no means a marker of the health, balance, or nutritional quality of a plant, and therefore is not a marker of value.

  8. Joe Mota says:

    We are totally organic vegetable gardeners. Through out the season we mulch between the vegetable rows with chemical free grass clippings. Would it be wise to add wood chips also?
    Thank you . You’re research is excellent!!
    Joe

    • You may think you are chemical free, but that grass is nothing but chemicals. If you have other options, don’t use wood chips in a vegetable garden. Vegetable gardens get more soil disturbance than landscape gardens and if you dig in too many wood chips, the soil it might have low nitrogen levels. If you do use them, just rack them aside for planting.

  9. Bonny says:

    Thanks as always for your time and care in your research. Have you shared your results with Dr Ingham? I would like to know her response.

  10. Tolga says:

    Excellent

  11. Dan oconnell says:

    Tea with rum? Is that an Ontario thing?

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