Does Compost Tea Work?

When I first started looking into low cost organic gardening I went looking for a replacement to the traditional fertilizer products. Looking for an alternative was nothing more than trying to replace the practice of using fertilizers during the growing season. I did not question at the time the need for the method at all.

Upon some preliminary research the alternative that seemed to jump out was Compost tea. Compost tea was something I could make at home with water, compost and some relatively inexpensive equipment.

On this month’s installment of the testing garden assumptions series I am going to evaluate the two main claims made about compost teas.

Define Compost Tea vs Compost Extraction
For the purposes of today’s episode it is important identify that there are different definitions for compost tea. I will be using the definitions as outlined in the book Teaming with Microbes. They define two different types of compost tea, Actively Aerated Compost Tea (AACT) and Compost Extractions.

What is Actively Aerated Compost Tea?
AACT is a method that recommends the use of compost and a sugar source such as molasses (Unsulfured) or kelp with supplemental air provided by air stone or bubbler to create a tea that has a high concentration of bacteria and then to use it in the garden.

What is a Compost Extraction
Compost extractions are a different method where it is recommended to add compost to water and let it sit for a week or two allowing the nutrients in the compost to leach out into the water. Eventually the water should become an organic fertilizer.
Why are water soluble nutrients important in the garden?

Understanding why a fertilizer is of use in the garden is a little more intuitive. By providing a water soluble nutrient to the plant the roots can usually take them up right away. Bacteria however may require a little more explanation to see why they are important in the garden.

Why is Bacteria Important in the Garden?
As I have spoken about in the past Bacteria along with other organisms form the nutrient cycle in the soil. Bacteria break down more complex molecules and release nutrients plants can use where they can be accessed.

In other cases beneficial bacteria can fix atmospheric Nitrogen again releasing it into the soil in a form plants can use.
Compost Extractions and AACT are said to add more fertilizer and bacteria to the soil. Plants need both nutrients and bacteria so using compost tea in the garden should be beneficial right?

Let take a look to see if these compost tea methods live up to their two main claims.

Commercially Available Compost Teas
I want to be clear today we are not testing commercially available compost tea products. I will touch a little more on these products in a follow up video to this one.

Does AACT compost tea live up to its claims?
Today’s first hypothesis is that AACT increases the number of bacteria through the brewing process however is not necessarily an organic fertilizer.
The second hypothesis is that Compost extractions are an organic fertilizer.

In order to test today’s hypothesis I sent in a number of samples to Maxxam analytics for analysis including the component materials, rain water, molasses and compost I used to make the tea.

Compost Source
I used homemade compost that was made using a hot bacterial driven compost method followed by a slower fungal dominated one. This should ensure the compost is well broken down and should have rich concentrations of bacteria and fungi. (More detail can be found on
I started with homemade compost made using free and local resources including, autumn leaves, used coffee grounds, brewing waste, garden waste and comfrey among a variety of other materials. In order to ensure the compost was consistent I took a sample mixed it in a 5 gallon bucket until it was evenly mixed and homogenous.

I ran the compost for analysis to know what the base level of nutrients were contained in the compost. (The results today represent both the plant available and unavailable. )

Water Source
The water was collected form my rain barrel and was all from the same batch. No new rain was added during the course of this trial.
This was the same rain barrel that I ran the analysis on to see if rainwater was safe to use in the garden. [1] Sugar Source

In order to cultivate the bacteria found in the compost it is noted by advocates that a sugar source is required. Most advocates recommend unsulfured molasses or kelp. I used molasses as I have extreme reservations on the use of kelp in the garden. [2]

Finished Compost, Rain Water and a Molasses Blank
The first set of samples I sent in was to set a baseline for each of the individual components used to make the AACT.
I ran the finished compost, rain water and a Molasses Blank using the rain water and same amount of molasses as the AACT simply without adding the compost.
These results gives us a baseline to understand if there are nutrients where they come from.

Trial Batches
Trial Batches, Compost Extraction, Actively Aerated Compost Tea and AACT Trial Blank
I produced three batches of compost tea using slightly different methods. All batches were kept in the same area at room temperature.

Batch 1 How I made my Actively Aerated Compost Tea.
The basic recipe I followed to make the AACT was based on an amalgamation of the methods presented by advocates. I chose to focus on AACT that can easily be made at home.

To make AACT you need a bucket, air source, compost, water and a sugar source. I used 5 gallon pails from a big box store ensuring they were reasonably clean. The air source I used a high output, microbubble air stone and an aquarium pump that is sized for a 30 gallon tank (7.5 times more volume than the water being aerated.)

I added 2 cups of compost, 1 tablespoon of unsulfured molasses to 4 gallons of rain water. (473 ml of compost, 14 ml of molasses to 15L of rain water)
Once mixed in the pail I added the air stone and ran it for 48 hours as recommended by leading advocates. The addition of sugar and oxygen should allow the bacteria concentrations to increase.

Batch 2 Compost Extraction
The second batch I made was a simple extraction. I placed the same volume of compost as the AACT batch in the same volume of rain water and left it for 7 days with a firm lid sealing the container. This method should allow the maximum amount of nutrients to transfer from the compost to the water.

Batch 3 Actively Aerated Compost Tea Blank
The third batch I made was an AACT blank was the set up the same way as the AACT however not allowed to brew for 48 hours rather the sample was taken right away. This batch should allow use to figure out if there is an increase in the nutrients and bacterial numbers in the compost tea.

Overall Nutrient Results.
It is now time to test the nutrient content of the compost teas to see if they have value as an organic fertilizer. I am only going to touch on the elements that have proven or suspected to be essential or beneficial for plant growth.

Initial Sources of Nutrients of the water, molasses and compost
The final AACT could get nutrients from all of the ingredients. The water and molasses blank had minute traces of some elements (Calcium, Magnesium, Potassium, Sodium and Sulfur). It is not a surprise that the compost used had the majority of the nutrients with 14/15 commonly tested for elements. These results represent both the available and unavailable nutrients.

RPD analysis comparing the AACT, Extraction and Blank
The following results represent the available or extractable nutrients. Changing between water and soil as a sample means we can’t compare directly however within what’s called the matix (Water or Soil) we can compare.

Comparing AACT to Compost Tea Blank
I used the Real Percent Difference (RPD) statistical tool to determine if there is a difference between the control to the AACT.

The results did show that there is a statistical increase in the concentrations of 7/15 commonly tested for nutrients (Boron, Calcium, Copper, Magnesium, Nickel, Phosphorus, Potassium) in the AACT when compared to the control.

Comparing AACT to Compost Extraction
I then compared the compost extraction to the AACT. If the compost extraction really does allow more nutrients to leach into the water it should be higher than the AACT that had only spent 48 hours brewing.

That same RPD analysis showed no statistically significant difference in the nutrient content between both methods.

Discussion: Fertilizer potential of Compost Tea.
This is not surprising as the same volume of compost was used and it is reasonable to assume that there is only a certain amount of leachable nutrients in that compost. If you increased the volume of compost you might see a little higher nutrient concentration in the water. There however will be a point where the water hits an equilibrium or balance point and no matter how much more compost you add the water will not pick up any more nutrients.

This casts some doubt on the claims made about compost extractions even though they extraction had nearly a week to break down the compost. With the lack of oxygen those bacteria that could survive are simply less effective at breaking down complex molecules in a reasonable timeframe.

Is there Nutrient Value in AACT or Compost Extractions?
There are some nutrients in the AACT and extraction however without context it is hard to know if it has enough to be considered a useful source. In order to set a baseline I had a 20-20-20 synthetic fertilizer tested that was already diluted in water to the recommended rate. (I am not advocating the use of)
I used a synthetic fertilizer as the baseline because the companies making it are not going to add anything in to high or low concentrations. They are going to do just enough that when people use their products they see defensible results. It is reasonable to use this rational to justify the synthetic fertilizer as a fair baseline to determine if the compost tea batches have enough fertilizer to be valuable in the garden.

When comparing the synthetic fertilizer to both AACT and Compost Extraction all three had equal concentrations of three elements (Boron, Calcium, Magnesium) but for the most part the synthetic fertilizer had significantly more. (Copper, Iron, Manganese, Molybdenum Phosphorus, Potassium, Sodium Sulphur and Zinc)
These results indicate there is limited value in the fertilizer potential for AACT and Compost Extractions made under these common methods.

This is not really a surprise as most homemade compost teas are not made with the intent of having significant concentrations of organic nutrients rather they are made to increase the bacterial concentration.

The protocols are different when you sample soil vs water making it hard to directly compare them. Anecdotally though both AACT and the Compost Extraction did not have significant enough concentrations to be relevant.

Does making AACT increase the concentrations of beneficial bacteria?

Lets move onto the claim that AACT increases the concentration of bacteria.

The next test we ran was a simple bacteria plate count on samples of all three batches as well as the rain water used to make them. If the process of brewing AACT actually increases the numbers of bacteria we will see higher concentrations of bacteria in the AACT when compared to the AACT blank.
There are limitations to the plate count method. The first is not all bacteria can be cultured however it those that can represent a wide variety and should be representative enough to see a comparative increase.

The second is this method does not actually tell us which bacteria are present just a total number of culturable bacteria.

Does AACT actually increase the number of bacteria in the soil?
These results were quite surprising. In all cases the samples had more than 12,000 coliform form units per milliliter.

Maxxam analytics even tried to raise the detection limit to see if they could differentiate between the samples and even that failed to do so.

These findings do not support the hypothesis that AACT actually increases the bacterial concentration in the water when using this method. I was unable to support nor refute the claim as all of the samples already had such a high concentration of bacteria already.

This null finding though does refute the core need to culture bacteria in the first place. If rain water and compost already have so many bacteria why would you go to the effort to try to culture more?

What is the Fertilizer Potential of AACT, Compost Extractions and Molasses
It would appear as we suspected that none of the samples including compost tea, molasses blank and compost extortion had any significant concentrations of nutrients.

For AACT that was expected however there are claims out there that compost extractions and molasses can be used as organic fertilizers.

Compost extractions if you were to add much more compost you might be able to extract more nutrients into the water however there is a limit to how much even if available can be extracted. At some point for all nutrients they will hit an chemical equilibrium meaning it is balanced between the compost and the water. no further exchange will happen until something is taken out on either side.

Most commercial organic products are concentrations meaning they increase the concentration by removing water while mechanically trying to force the nutrient source to give up more. In a home situation it is unlikely people would go to that extent.

In the case of the molasses batch this is just incorrect. even with higher concentrations of molasses being added there is simply not enough in the molasses itself. Infant sugar has been known to attract distractive pests.

Bacterial Content of AACT, Compost Extractions and Rain Water

Todays results after a little thought are not surprising at all. The rain barrel has tons of bacteria in there and my compost and subsequent garden soil are absolutely filled with bacteria. In fact a healthy soil has more than 10,000 bacteria species per gram.
That is a whole lot of bacteria and if your garden soil is healthy or even if you collect and use rain water you likely already have more than enough bacteria in the soil.

Today I set out to evaluate compost tea as a fertilizer in compost extractions and the ability of AACT method to culture bacteria.

It would seem that the fertilizer potential of a compost extraction is very low and you likely do not need to culture more bacteria through AACT as the compost and rain water you are already using have so many.

Normally at this time in my videos if the evidence is not supporting the method being tested I like to propose an alternative method to gain the same benefits. Usually the method requires less time allowing you to enjoy the garden more. But as I feel I have only half addressed this question I am going to delay that recommendation until I have researched some of the other claims made about compost teas.

There are plenty of claims made about compost tea products, application techniques and resulting benefits of its use in the garden. I would love to find out which claims you are interested in so that when I release the follow up video I can address them.

In two weeks I am going to address the remainder of the claims and make a determination if I will still recommend its use in the garden or abandon it. If I abandon the practice I will also propose a replacement strategy.

Related Videos:
Is Epsom Salt useful in Organic Gardening?

Part 2 Is Compost Tea a Valuable Method in Organic Gardens? Testing Garden Assumptions Series Playlist:

Lab Results:
20 – 20 – 20 Synthetic Fertilizer Dry and Wet
Compost Tea Lab Results Including AACT, and Compost Extraction

Supporting Material:
[1] Is Rain Water Safe to Use in the Garden?

[2] Kelp fertiliser and amendments in Organic Gardening

[3] Dr. Linda Chalker-Scott’s resource paper on foliar application
[4] Dr. Linda Chalker-Scott’s resource paper on Compost Tea’s Pesticide use
[5] Dr. Linda Chalker-Scott’s resource papers

Horticultural Myths

[6] Bacteria 1 Photo Free to Use Google search March 3rd 2016
[7] Petri Dish 1 Photo Free to Use Google search March 3rd 2016
[8] Bacteria 2 Photo Free to Use Google Search March 3rd 2016
[9] Petri Dish 2 Photo Free to Use Google Search March 3rd 2016

About Stephen

The Alberta Urban Garden Channel hopes to promote organic gardening that is simple, sustainable and does not have to cost a lot. We do this by investigating the Science behind gardening, methods, practices and products to make sure that you will have the best chance of successfully growing your own food at home.