Compost Tea – Does it Work?

Robert Pavlis

Compost tea is all the rage but does it really work? Research studies so far have produced mixed results. The science to support the use of aerated tea on crop plants, lawns, shrubs, or trees is very weak, at best.

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.

Microbe Science for Gardeners Book, by Robert Pavlis
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 bulk 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; https://www.researchgate.net/publication/288097189_Wood_Chips_and_Compost_Improve_Soil_Quality_and_Increase_Growth_of_Acer_rubrum_and_Betula_nigra_in_Compacted_Urban_Soil
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Robert Pavlis

I have been gardening my whole life and have a science background. 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!

53 thoughts on “Compost Tea – Does it Work?”

  1. A good author to read and reference in your writings would also be Jeff Lowenfels. He writes extensively on bacteria and fungi as well as other nutrients. With a science background he breaks down why, what, et. Fascinating.

    Reply
  2. The surprising thing to me is the effectiveness of wood chips, even though their nutritional value is less than compost. Is there a theory to explain this?

    Reply
    • My guess is that wood chips feed beneficial fungi in the soil to make them more abundant in the soil. Trees do really well in fungally-dominant soils since beneficial fungi help them get food from the soil in the form they like best, helps provide a slightly more acidic pH which they prefer & lots more good stuff (as discussed in books like Teaming with Microbes and Teaming with Fungi).

      Compost usually feeds fungi too but more so feeds bacteria. Trees benefit more from beneficial fungi than from beneficial bacteria. Hence the fungi-supporting wood chips should do better for trees than compost.

      But for another plant, like a vegetable, compost would likely do better than wood chips since vegetables prefer beneficial bacteria that give them more alkaline growing conditions and nitrogen in nitrate form which they like best. Just my guess though 🙂

      Reply

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