Compost Tea: The Smart Way to Boost Soil Microbes

What Is Compost Tea? And How Is It Made?

People have many assumptions when they first hear the term “Compost Tea.”

Do you drink it? Is it like sweet tea? Is it made out of composted tea leaves?

People come up with all kinds of ideas, lots of them wrong.

This article will introduce the topic of compost tea, how it helps plants, and how compost tea compares to compost.

Compost tea being applied as a soil drench
Compost tea being applied as a soil drench

The easiest definition of compost tea is that it is a liquid form of compost. Compost is placed inside a mesh bag and agitated in water to extract the microorganisms from the surfaces of organic matter into the water, much like preparing an actual cup of tea.

Foods for microbes (like kelp meal or black strap molasses) are added to the water and it is oxygenated with an air pump for a period of time to get these microorganisms to eat and reproduce.

This explodes their populations one hundred, even one thousand fold.

(If you’re wondering…..yes, someone mistakenly drink my compost tea. This was an experienced farmer I thought would know better. I guess he could skip his kombucha that day!)

Microbes from Compost Tea: Soil’s Facilitators

Image from Garden Therapy

Many individuals believe that compost is a form of fertilizer supplying plants with the nutrients they need to grow and thrive.

While this is essentially true, it is actually the microorganisms present in that compost that are making those nutrients available to plants. Plants and microbes have maintained symbiotic relationships for hundreds of thousands of years, well before we came along and screwed it up.

Through the process of photosynthesis, plants use sunlight to create energy and compounds that they use for growth. Plants create excess carbohydrates, proteins, and sugars that they release through their roots to feed soil biology.

These foods are known as exudates and attract beneficial bacteria and fungi to the area around the root, also called a rhizosphere. When predators eat microbes or when the microbes simply di, the extra nutrients from the microbes are released in a plant-available form to be taken up by the roots of plants.

Therefore the plants manage their own nutritional needs so long as microbes are present.

So it’s up to growers to feed the microorganisms in the soil and let those microbes provide plants with the nutrients, micronutrients, and minerals that they need. Compost tea helps to work as a liquid inoculant to get all of these helpful microorganisms into soils and onto plant foliage.

Compost tea is brewed for a particular amount of time depending on the ambient temperature to increase microbial populations. The result is a concentrated liquid that can be diluted and applied to farms, landscapes, and lawns.

An image showing a crop treated with compost tea vs a control group. The control group has lower growth
Image from Ecosoil

Modern agricultural practices can drastically reduce the amount of biology in soils.

Plowing and tillage tear apart the tiny microscopic fungal hyphae which help plants access a greater variety of nutrients and minerals.

Chemical fertilizers, pesticides, and herbicides are also detrimental to all of this life in the soil. Heavy machinery also compacts soil, depriving soil of its ability to breathe while promoting harmful anaerobic bacteria and fungi.

Tea Application: Far More Economical Than Compost

When there is a major loss of life in the soil, the most natural way to replace that life is through compost and/or compost teas. 10 tons (or 20 cubic yards) per acre is a generally accepted application rate for solid compost. Market gardens may apply up to a 4-inch layer of compost mulch to achieve the same effect.

If compost costs $50-$60 per yard, that’s $1000-$1200 for one acre.

Compost tea is a much more cost effective way to get beneficial biology onto that same amount of land.

Compost tea made from 15 cups of compost can deliver similar biology to 10 tons of compost

Your soil needs organic matter, which compost can provide. But when it comes to delivering biology, compost tea can be applied at a rate of 10-20 gallons per acre and use a mere 15 cups of compost rather than 10 tons of it.

It’s not just the cost of the compost vs the compost tea that matters.

Tea is more efficient in terms of time and energy. Compost tea can be applied at seeding and/or transplanting, which means less driving on the soil and less compaction.

At larger scale, it is also more cost effective to transport a few hundred gallons of diluted compost tea than it is to ship buttloads of compost. The calculus works even more when you can produce the tea on site!

It also means less time spent on a tractor which saves fuel *and* time.

Production, transportation, and application are all more efficient.

That’s a “win-win-win” scenario.

The Basics of a Quality Liquid Compost Extract or Tea

Compost being turned by pitchfork. Excellent compost tea requires excellent compost!
Incredible compost tea starts with quality compost

A quality compost tea starts with high-grade, high-biology compost.

Not all composts are created equally.

There are varying degrees of quality when it comes to compost and one cannot tell how much life is in a compost without checking that compost with a microscope. Some composts are more of a mulch than a compost as they barely contain any biology.

This is where vermicompost – which also has varying degrees of quality – plays a key role.

Start Making Vermicompost Today!

Due to the nature and actions of worms as they work through material, vermicompost will almost always have greater populations of beneficial biology than regular compost. This makes it a superb choice when looking for ingredients to make a compost tea.

Quality also depends upon the plants intended to be grown in that soil.

A quality tea for one crop may be less than optimal for another. In general terms, plants like shrubs and trees that are higher on the plant succession scale will need a more fungal compost and compost tea.

Annuals, on the other hand, are lower on the plant succession order and will benefit from a more bacterial compost or tea.

How to Make Compost Tea

Compost tea isn’t complicated.

You can make a small compost tea brewer for $25-60 (USD) depending on what you have on hand already.

All you need is a five gallon bucket, clean water, compost or vermicompost, foods for microbes, and a small air pump with a hose.

The brewing process should take place in the same ambient temperature as the plants that will receive the application of compost tea. So place the brewer in a location with similar ambient temperatures.

Water must be clean!

It can be tap water, spring water, or rain water. If using tap water it is best to allow the water to off-gas any chlorine it may contain. 

Any clean 5-gallon bucket works!

Brewer Components

Bucket

Any clean five gallon bucket will work. There should be no chemical residue. Check with local bakeries for free buckets, preferably with a lid!

I personally get these buckets for containers to collect food scraps in my kitchen. They are large enough that they hold several days worth of scraps, and small enough that the scraps don’t get stinky and nasty. 

Tea Brewing Bags

Compost tea brewing bag available on Amazon. Perfect for a 5-gallon bucket.

You’ve got two good options for brew bags. An inexpensive elastic-top paint strainer bag works for individuals who don’t plan on doing much brewing, or are just dipping their toes in the water before spending more money.

These bags cost $2.50-$3 (USD) at your local paint store, or on Amazon. They are specifically made to fit tightly around the rim of a 5-gallon bucket. The elastic keeps debris and compost out of the water. 

If you plan on brewing compost tea on a more regular basis I recommend purchasing a higher quality brew bag. I tested a few brands of brew bags a long time ago and have stuck with the bags that can be found at Earthfort for $21.95 (USD) plus shipping. Similar bags can also be purchased on Amazon. These bags stand up to the beating of the brewing process better.

They should last a lifetime if kept clean.

Air Pump

The other main expense is a small air pump. You can find these at your local pet supply, aquarium, hydroponic stores, or Amazon. I recommend an air pump with an output of 100 gallons per minutes for a 5-gallon brew.

Soluble Humic Powder from The Seed Supply

A small air pump with an output of 100 GPH should cost between $15-20.

Food for Microbes

The foods for microorganisms that I like to use are humic acid, fish hydrolysate, and kelp meal. When choosing any type of item like this I look for OMRI-approved items to ensure quality. One quality source is from The Seed Supply, which offers a soluble humic acid powder.

The Seed Supply also offers a soluble kelp powder. Kelp not only helps to feed microbes it also offers minerals and micronutrients that are beneficial to plants. A little goes a long way when it comes to soluble kelp. It especially doesn’t take much for compost tea.

Soluble Kelp Powder from The Seed Supply

The last food source I use for microorganisms is fish hydrolysate. Most of the fish products found in garden stores are fish emulsions. Fish hydrolysate is basically the entire fish made into a liquified version, using the fats and oils extracted from the fish emulsion-making process. We want those fats and oils as foods for microorganisms so be sure to watch out for the word hydrolysate.

The best fish hydrolysate that I have found so far is Brown’s Fish Hydrolysate. This stuff is the real deal! I’ve had a cat steal my brew bag and drag it away due to the fish smell. Potent!

Neptune’s Harvest offers an OMRI-approved fish and kelp combo. The brand’s popularityy makes it easy to find.

Steps To Brew

  • Fill the bucket with water 12-24 hrs in advance to allow it to get to ambient temperature and off-gas chlorine or chloramine.
  • After allowing the water to sit, add the foods. 
    • 1 tablespoon humic acid
    • 1 tablespoon soluble kelp
    • 1 tablespoon fish hydrolysate

(*If using the Neptune’s Harvest fish/ kelp combo use two tablespoons)

  • Slide some type of weight (small metal nuts can work) on the ends of the pump’s air hose. Do not use air stones as they can breed harmful bacteria in the crevices.
  • Place the ends of the air hose on the bottom of the bucket and turn it on. We want the air to travel through the entire column of water.
  • Add 2 cups of vermicompost to the brew bag and place the brew bag in/on the bucket. Use your hand to break up clumps and aggregates in the vermicompost. Massage the bag for a couple of minutes to work the material. You are officially brewing!
  • Let brew for 24 hours.
  • Remove the brew bag from the bucket and allow it to strain.
  • Pour the compost tea into applicator.
  • Clean brew bag and bucket immediately after your brew.
  •  Apply the compost tea.

Application

Compost tea can be applied by water can, handheld/ backpack sprayer, or any other means by which you might apply liquid fertilizers. Make sure to filter the compost tea after brewing if you will be using a sprayer that might get clogged. 

You can see results in as little as five gallons per acre, but I generally use at least 10-20 gallons per acre. Compost tea can be applied with or without diluting.

Obviously it is difficult to apply only five gallons to one acre of land, so dilution is necessary. 

You can dilute compost tea however much you need. If it normally takes you 10 watering cans to water your raised bed gardens, you can split the compost tea up evenly over those 10 cans to apply it all more evenly. 

How to Use the Spent Castings

Castings from tea brewing can be added back into your bin!

This is the cool part. These “spent” castings retained their value.

The brewing process simply separated much of the biology from the surface of the vermicompost, but the rich, well-fed castings can feed the microbes in your worm bin yet again!

Depending on your castings, you may find that adding these castings back to your bin can boost your fungal population. Score!

Results

You may or may not see some results for a period of time. I’ve noticed reactions from plants 24 hours later and sometimes have not noticed differences until the following spring. 

While working at Rodale Institute, I was making, applying, and researching with compost tea on my own time. It was late July and the tomato plants were about four feet tall with small yellow flowers. The yellow flowers were all drooping down, facing the ground.There were several rows of identical varieties of tomatoes. I sprayed one row with compost tea and left the rest. 

Exactly 24 hours later, and noticed a change in the row that I sprayed. Almost all of the yellow flowers had turned up towards the sun, like tiny satellites soaking in the sun’s rays. The same flowers on the rows that had not received compost tea were still drooping and face-down towards the ground.

Color me convinced!

Wrapping It Up

Compost tea is not a fancy new hipster alternative to $6 cold brew coffee or another form of sweet tea.

Rather, it’s a beneficial brew full of life that aids in the health of plants and soils.

This article has taught you how these soil microbes in compost tea work with plants to provide the nutrients and minerals that they need to grow. Brewing and using compost tea is also an excellent way to save time and money as opposed to using solid compost.

In a future article, I’ll be going into how to make a compost tea full of life that will work in concert with your plants! The nerding out about soil, vermicompost, and (of course) tea is just beginning!

This article was co-written by the Urban Worm Soil Biologist Troy Hinke, a Soil Food Web trained expert in soil microbes. Check out his What’s Brewing Podcast on Apple Podcasts and Spotify.

If you want to know if your soil is appropriate for your crop, or if you need help our help with anything else, engage our services today!

33 thoughts on “Compost Tea: The Smart Way to Boost Soil Microbes

  1. Can we please get the recipe for the tea, how much molasses and time to brew, how much airflow? Thank you Pamela

      1. Thank you very much. I am very thankful I found your company. Your instructions and ideas are easy to follow.
        Thank you again.
        Pamela

      2. What do you do with the compost after it’s been brewed? Can it go back into the worm bin? Also do I need to use all of the food types or could I just use one, like the kelp meal?
        Thank you so much for writing this!

        1. Hey Deborah,
          We added an update to the article addressing this. Bottom line: Putting them back into your bin is great!

  2. Great info! I made some Kelp Tea for this yrs tomato plants. I sprayed all but one flat. Within 3 days there was a significant difference in the size. I look forward to giving this Compost Cocktail a go!
    Thanks Brother🌴

    1. Right away is generally best. Within hours after brewing the beneficial biology will start to die off. They need to be given more food, either by going into the soil where plant exudates are waiting, or fed more molasses/kelp.

      1. Thank you. Molasses? The recipe to make the tea didn’t include molasses. How would I use it if I couldn’t use it right away?

    2. Compost tea should be used as soon as possible. But will last for 4 or 5 days if kept in a a container that doesn’t allow light in. However the micro organisms you have created will dissipate in quantity

    3. Hi,
      My Worm Tea is fungi dominant by about 4:1 F:B. What can I add to get a Bacteria Dominant or /and Balanced result. Really appreciate your information. Keeps me experimenting.
      Cheers,
      Don

      1. Hey Don

        To get more bacteria reproducing in the compost tea you could use a teaspoon of unsulphured blackstrap molasses. If your compost tea is truly fungal dominant it is mainly the result of the compost your are using. A compost that has higher populations of bacteria would be better to use.

    4. Hey Beverly. You will want to use the compost tea within 24 hours in the hot summer months and within 48 hours when it is cooler in the spring and fall. The brew can start to go anaerobic as the microorganisms use up all of the food and oxygen. I do not advise adding any more foods to the mix after brewing.

  3. how do you use the spend worm castings after brewing. Thanks so much for the step by step. I’m guessing it is too cold. Does it matter if the plants have started their spring growing or not?

    1. Hey Jani,
      We added an update to the article addressing this. Bottom line: Putting them back into your bin is great!

        1. Hi Adam

          Compost tea CAN be applied to any type of plant. The soil chemistry is dictated by the biology. Forest soils, like where you would find blueberries, contain more fungi. Fungi secrete acids and enzymes in the soil which lower the pH.

  4. Can you add too much compost tea? How critical is the application rate. I have 4 raised beds do I call this down to a cup or two of brew or just over apply and not worry about it?

  5. what do you do with the spent worm castings? Is it best to wait till later in the spring when the soil is warmer to apply the tea?

  6. Such a great article! I learned a lot, and have a few questions:
    Is there a recipe you can share with a molasses option?
    With the given recipe, how detrimental is it if we can get all the listed additions? Would you change how much of the other ingredients you add?
    Looking forward to learning more!

    1. Hey Alessandra

      It’s not that big of a deal of you aren’t able to get one of the microbial foods. I would just omit that ingredient and use the same amount of other microbial foods as the article states.

  7. Hello Troy and Steve,
    Thanks for the concise article with ONE all-purpose recommendation: one can get really bogged down wading through the internet annals trying to find “the best” Compost Tea recipe since individual opinions are so divergent (and abundant).

    I’m wondering if you recommend this recipe as a soil drench or as a foliar spray, or both? Would you brew different recipes for soil application and foliar application, or is this recipe offer good performance in both applications.

    Thanks!
    Keep up the excellent education UWC team!

    1. Hi David
      This recipe for compost tea can be used as both a foliar application and soil soak. Thank you for the kind words!

  8. On the topic of dilution, do I need to let tap water off gas before mixing it with the brewed compost tea before applying it?

    1. Hey Joe

      Good question. Yes it is best to allow water from public municipalities to off gas chlorine over 12-24 hours and get the water to ambient temps. You can also use humic acid to bind up the ions of chlorine or chloramine. Humic acid is one of the foods for microbes used in the compost tea recipe.

  9. Using your recipe for compost tea how do I know if it is fungal or bacteria compost tea? For my garden I think I need bacterial tea.

    1. The recipe given will make a quality all around compost tea, whether or not it is has a high fungal content will depend on the compost. You don’t have to be concerned about there getting to be too much fungi in the soil. It won’t happen that easily.

  10. I have been researching Vermi-Compost Tea articles all week. This is by FAR the most informative and concise item that I have come across. You hit all of the highlights in one article! I look forward to reviewing more of your items. Well Done and Thank You.

    1. Thank you for the compliment Mark. If you are a podcast listener you may want to check out Troy’s ten episode series titled “What’s Brewing? A Compost Podcast”. The first and only podcast all about compost tea.

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