How to Make Actively Aerated Compost Tea to Fertilize Your Garden

When people ask what we feed our plants, more often than not, my answer is an exuberant: “Compost tea!” Actively aerated compost tea, or AACT, is a biologically-active, nutrient-rich, mild-but-strong natural fertilizer. It can be made with worm castings or other high-quality compost. We try to feed our plants with aerated compost tea at least once every month or two. I guess the the proof is in the pudding… because our plants are as happy and healthy as can be!

Do you want to learn how to make aerated compost tea? Read along and I will teach you how! We’ll go over the benefits of using compost tea, the brewing supplies needed, the step-by-step process of how to make aerated compost tea, and the various ways you can use it in your garden.

There is a tutorial video that shows the entire process at the end of this post! It’s really quite simple, but there is certainly some insight I hope to provide you. Including, what proper compost tea is, and what is not.

WHAT IS COMPOST TEA?

Compost tea is a natural liquid fertilizer that is made by steeping compost in water, with or without the addition of air. The purpose of brewing compost tea is to extract beneficial microbes and soluble nutrients, and then provide them to plants in a form that they can readily uptake and utilize.

What are beneficial microbes? They include bacteria, fungi, protozoa and nematodes – who all have an important role in soil health! Using aerated compost tea in your garden is a great way to enhance the soil food web. We talked a lot about the soil food web in our “Building the Perfect Organic Soil” article, if you’d like to learn more.

“Chemical-based pesticides, fumigants, herbicides and some synthetic fertilizers kill a range of the beneficial micro-organisms that encourage plant growth.  On the other hand, compost teas improve the life in the soil and on plant surfaces. High quality compost tea will treat the leaf surface and soil with beneficial micro-organisms instead of destroying them.”

Soil Food Web Institute

Historically, gardeners and farmers have made a passive or non-aerated compost tea by soaking a sack of compost in water for an extended period of time, often up to two weeks. This passive brewing of non-aerated compost tea (NCT) has been occurring for centuries! In more recent years, modern agriculturalists have began to brew super-charged compost tea in a much shorter duration of time, by introducing oxygen, food, and additional nutrients (ACT).  This is what we do!

Why Aerate Compost Tea?

If you want to make the most of your compost and create the best tea possible, brewing actively aerated compost tea is the way to go! Why not take a stellar product, like homemade compost or worm castings, and make it even better? By introducing air and a food source for the beneficial microbes, their populations within the tea increases by the thousands.

The process of making actively aerated compost tea significantly enhances the strength and effectiveness of your starting compost. However, the process will only amplify the inhabitants and nutrients that are already in the raw compost used to brew the tea. Therefore, it is important to start with high-quality, well-aged, properly composted material. Quality in equals quality out! We’ll talk more about the types of compost that can be used in the supplies section to follow.

“Aerobic organisms are the most beneficial as they promote the processes that a plant needs in order to grow without stress and with a greater resistance to disease. To enhance this community of beneficials, the compost tea must remain aerobic. Anaerobic conditions during brewing can result in the growth of some quite detrimental microbes and also produce some very detrimental metabolites.  It is best to avoid extremely low oxygen concentrations during brewing.”

Soil Food Web Institute

The reference to “detrimental microbes” above includes the potential development of human disease-causing organisms. It is only in anaerobic or low-oxygen conditions that harmful human pathogens can outcompete beneficial microbes and flourish.

In summary, aerating compost tea encourages the best microbes possible, both the type and quantity, while reducing the risk of pathogens.

What are the Benefits of Using Compost Tea?

• Compost tea enhances the soils ability to retain nutrients. The nutrients in the soil will runoff and be depleted less quickly. Therefore, there is less need to use other fertilizers.
• An enriched population of beneficial microbes, introduced via compost tea, can increase the bioavailability of nutrients to plants. They break down organic matter and free up minerals. This means the plants can uptake nutrients from the soil more readily.
• A healthy soil food web can buffer soil and plants against pollution. For example, compost-rich soil is excellent at reducing the impacts, uptake, and concentration of pathogens, contaminants, chemicals, and heavy metals that may be introduced or present in soil.
• Compost tea can help improve the soils moisture retention properties. This prevents stress to plants by maintaining a more evenly moist soil, and reduces the need for more frequent watering.
• Plants fed compost tea are reported to not only grow stronger, but also have a boosted immune system and improved ability to resist disease.
• Likewise, it increases a plants ability to tolerate and rebound from stress such as drought or pests.

Compost Tea versus Worm Bin Leachate

Before we get into the instructions on how to make aerated compost tea, I want to point out a common point of confusion for some people. Many gardeners, ourselves included, make aerated compost tea using finished worm castings from a worm compost bin. The process (and the instructions I am providing you today) results in an aerobic compost tea solution. This is NOT the same thing as the liquid or runoff that can be collected from the bottom of a worm bin. That is leachate, and it is anaerobic.

In our “Vermicomposting 101: How to Create & Maintain a Simple Worm Compost Bin” article, you’ll see that we don’t add drainage holes to the bottom of our tote-style worm bins. The moisture is kept at an ideal level, the consistency of a damp but wrung-out sponge, through care and upkeep of the bin! This includes not giving it too much wet food, and adding new bedding or brown material when it is fed. When the worm bin is too dry, we give it a small shower of water.

When worm bins are maintained too wet, they leak. The runoff is often collected in a catchment system below, depending on the type of worm bin. Some gardeners do use this runoff to water their plants, calling it “worm tea” or “worm wee” even. Unfortunately, that isn’t the good stuff. But wouldn’t that be nice and easy?! This leachate liquid may be okay to use, but it is also more likely to contain negative bacteria and pathogens due to its anaerobic state.

So let’s learn how to make the good stuff, shall we?

7 SUPPLIES NEEDED TO MAKE AERATED COMPOST TEA

1) Compost

As we already discussed, the quality of your starting compost directly dictates the quality of your aerated tea! Whatever is in that compost is going to be amplified. A variety of compost types can be used for making aerated compost tea, though they may provide a slightly different end product.

“Research suggests that carbon-rich feedstocks (e.g. dry leaves, sawdust, wood chips, shredded newspaper), produce a compost with a higher fungal content. Nitrogen-rich feedstock (hay weeds, coffee grounds, herbaceous material and manures) produce compost with higher bacterial content. Vermicompost is used as an ingredient in many compost tea recipes. This compost is typically the highest in available nutrients.”

San Francisco Department of Environment

No matter what you choose to use, ensure that it is well-aged, balanced, and properly decomposed. For example, do not use fresh animal manures, or compost that is anaerobic and stinks like a landfill.

Most often, we use vermicompost from our worm bin to make AACT. If you need help getting a super simple worm bin started, see this post. Or, check out “Composting 101: What, Why, & How to Compost At Home” to learn about 6 different ways to compost.

If you do not have your own homemade compost, you can usually buy finished worm castings at your local garden center or buy some online. Another options is to use convenient compost tea bags from Malibu’s Compost – our favorite organic biodynamic compost company. The compost is already “bagged” and ready to steep! They offer various types of tea bags, including ones for trees, veggies, and even houseplants.

2) Brewing Vessel

This one is super simple. For an average home garden, a basic 5-gallon bucket or two is adequate for making aerated compost tea. Larger gardens, grow operations, or farms may choose to utilize bigger tanks instead. If it is important to you, there are BPA-free, food-grade 5-gallon plastic buckets available too.

We started brewing aerated compost tea using one 5-gallon bucket. Over the years, we have added more and more 5-gallon buckets to our brew day routine. Now, with the help of a multi-port air pump (described in #4 below), we can brew up to six 5-gallon buckets at a time! Most times, we make three buckets.

3) “Tea Bag”

It is called “compost tea” for a reason! The compost needs to be contained and steeped within a little sack, just like tea would be. You can get pretty creative here. The idea is to create a sack that is breathable to allow the exchange of microbes and nutrients between the compost and water,  but won’t let too many larger particles through.

We have made sacks from burlap in the past, and still use one of them. Recently, we have been using nylon paint strainer sacks. They work perfectly and are easily available. Cheesecloth could work, if it is layered several times, but may be more difficult to reuse. There are also some really nice quality, uber-durable compost tea bags on the market too – ready to cinch close and hang!

4) Air Source

As you’d expect by its name, aerated compost tea needs some air! An air pump is used to introduce oxygen into your compost tea brew. In the past, we used a basic aquarium pump. It worked okay… But to be honest? Not nearly as well as the little commercial air pump we use now!

This air pump that we love and use not only creates a ton of bubble action, but also has 6 ports so you can brew several buckets at once! The ports are adjustable, so you can turn off the ones you aren’t currently using, or dial the ones you are using up and down for more or less air flow.

5) Air Stone or Bubbler, & Tubing

The air pump is what generates oxygen flow, but you’ll also need a tool to get the air from the pump and down into your brewing vessel. Air stones are often used to make batches of aerated compost tea. When we first starting brewing AACT, we used a basic air stone like this. They do okay, but can get clogged easily and therefore need to be scrubbed after each use. Similar to our air pump, we have since upgraded to something that we find works much, much better!

To aerate our compost tea, we have been using these bubbler snakes by TeaLab for the last few years. We love these things! They produce some serious bubble activity through the perforated holes in the bottom of the “snake”. The bubblers fit perfectly in a 5-gallon bucket, and have a little loop where you tie and suspend your tea sack from. It is uber convenient, effective, and also keeps the snake weighed down in the bucket. Fun fact: The bubble snakes are made in Humboldt County, California!

When you watch the demonstration video at the end of this post, you’ll see what a significant difference there is between the snake bubblers and the air stone bubble activity. And they are all hooked to the same air pump! We’ve been meaning to buy another bubble snake soon, to eliminate the use of the air stone completely.

Last but not least, slender silicone airline tubing is used to connect the air stone, snake, or bubbler to the air pump. The TeaLab bubbler tubing is 1/4″ and the standard air pump tubing is 3/16″, so we use these adapter pieces to connect the two hoses together.

6) Microbe Food Source

In addition to the compost itself, other nutrients are often added to aerated compost tea brews. The purpose is to feed the microorganisms in the tea, and thus increase their activity and quantity.

Common additions include kelp, fish hydrolysate, molasses, and humic acid. Most often, we use a little organic molasses, seaweed extract, and/or kelp meal. We were out of kelp meal at the time we made this example brew for you all, so that is why you don’t see it!

7) Dechlorinated water

As much as possible, the water used for brewing aerated compost tea should be free of disinfecting agents such as chlorine or chloramines. Those are meant to kill microorganisms, so using chlorinated water sort of defeats our purpose here! We use captured rainwater from our rain tanks for brewing compost tea.

If you are on municipal tap water that uses liquid or gaseous chlorine as a disinfectant, you can simply fill your buckets a day or two in advance, let them sit out in the sun, and most of the chlorine should dissipate. Unfortunately, chloramines do not “burn off” the same way chlorine does. Another solution to this is to use filtered water. These carbon filters that attach to your hose will help do the trick! We use them extensively in our garden.

Now that we have our supplies covered, let’s brew!

Directions: How to Make Actively Aerated Compost Tea

Prepare your Brewing Vessels

Add dechlorinated water to your brewing vessels. If needed, let your water sit out for a day or two to let any chlorine burn off. If you are making compost tea in 5-gallon buckets like we do, fill them up most of the way. We generally leave a few inches of room on the top to allow for bubbling and possible foaming.

Prepare Your Compost Tea Sacks

Using your compost of choice, fill your tea sacks with several cups. There are varying recommendations out there for exactly how much compost to water should be used. We generally use anywhere from 2 to 5 cups of compost per sack, per 5-gallon bucket, depending on how much available compost we have at the time. If you need some tips for how to harvest finished worm castings from a compost worm bin, check out the link to see how we harvest and screen ours!

If you would like to add kelp meal as your microbe food source, add a quarter cup per tea sack at this time.

Tie the sack closed on top with a string, hemp tie, or similar. Leave a little length to the string so you can suspend the teabag in the bucket.

Steep & Feed

Dunk your ready compost tea sack in the brewing vessel. Just as you would with a tea bag, lift and lower the bag in the water several times to moisten, agitate, and encourage infusion. Tie the extra length of string to the handle of the bucket. Or, if you’re using a TeaLab snake bubbler like ours, tie it to the designed steeping loop at the top of the snake.

At this time, add an additional food source for the microbes – unless you already added kelp meal in the previous step. We typically add 1/3 cup organic molasses to each 5-gallon bucket during every brew. Sometimes, we also add a slug of seaweed extract in place of the kelp meal.

Aerate

If you haven’t already, insert your air delivery device (air stone, snake bubbler, etc) into the brewing vessel. Ideally, it should rest on the bottom of the bucket, with the tea bag suspended above it. This prevents the bag from sitting directly on the bubbler and blocking the air flow.

Kick on the air pump! Allow the compost tea to bubble for 12-48 hours. This is the ideal timeframe for optimal microbial activity and nutrient extraction.

Aerated compost tea should ideally be protected from extreme temperatures while it is brewing. Excessive heat and sunlight or freezing cold temperatures can impact the microbial activity. We don’t stress about this too much. However, during the winter, we brew our tea in the garage to keep it a tad warmer. In the summer, we keep the brewing vessels out of the hot sun.  

Use Your Compost Tea Right Away!

At the end of the designated brewing period, be prepared to use your finished tea in the garden immediately. Actively aerated compost tea becomes anaerobic very quickly, and its benefits and strength quickly degrade. Therefore, we recommend that you make use of your AACT within an hour or two after removing the air source. The quicker, the better! We’ll talk about the many ways to use compost tea in the garden below.

But, what do I do with the “spent” compost in the tea bags?

There are a few different options for utilizing the compost within the tea bags! Which option you choose depends on how you want to use the compost tea.

If you are going to pour the tea into garden beds, containers, or around other plants straight from the bucket, the worm castings or compost can be incorporated into the tea itself. For example, we most often scoop out helpings of finished tea with a large liquid measuring cup. Once the tea has finished brewing, we simply open up the tea bags and empty the contents back into the bucket. Then, as we give the plants compost tea, we stir the bucket frequently to prevent settling of the castings and ensure even distribution.

On the other hand, if you’d like to apply the compost tea with a watering can or sprayer, you want to keep the spent compost separate. It will clog the holes in a can or sprayer! Some folks even further strain their compost tea if they’re going to put it in a sprayer. In that case, pull up the tea bag, give it a good squeeze to ring out as much liquid into the bucket as possible, and then make use of the spent compost elsewhere in your garden instead! We often empty and spread the contents of the tea bags directly into a garden bed, or around the base of fruit trees.

How to Use Aerated Compost Tea in Your Garden

Using Compost Tea As A Soil Drench

Our preferred method for using compost tea in the garden is applying it as soil drench. A “soil drench” is essentially just another way of saying “watering with it”. It is quick, easy, and effective!

All we do is scoop out a helping to pour around the base of each plant, which may vary from one-half cup to several cups each, depending on the size of the plant. Experts recommend to apply as much volume of compost tea as necessary to saturate a plants root zone. That means that smaller plants such as seedlings will need less – because they have such small roots at that point. Larger plants, like established tomato plants or even fruit trees, will appreciate more volume!

As an alternative to scooping out finished compost tea with a measuring cup, we sometimes add the tea to a watering can. This is particularly helpful when we want to evenly distribute compost tea across an entire bed of small plants, such as with carrots or radishes. Again, this is with the tea alone – not with the spent compost too! Using a large funnel, we ease the finished compost tea into a watering can after removing the tea bag.

It is best to apply compost tea to soil soon after a routine watering, when the soil is still damp. Damp soil more readily accepts more moisture than dry soil. Meaning, it will more easily absorb and less will run off. Additionally, you probably won’t need to water for a few days following, which gives the tea some time to do its work before getting diluted.

We like to spoil our plants with an application of AACT once every month or two, but especially for newly transplanted seedlings! Another treat for transplants or stressed plants is an aloe vera soil drench.

Aerated compost tea does not need to be diluted before application. It is mild and cannot “burn” your plants like many other fertilizers can! Use as much as you’d like, but also keep in mind, a little goes a long way! If needed, you totally can dilute a smaller batch of tea to create more volume and feed more plants.

Benefits of a Compost Tea Soil Drench

Using compost tea as a soil drench is the most bang-for-your-buck, especially since we usually add the spent compost into the tea solution as well. Additional filtering, such as what may be required for use in a sprayer, creates an extra step. It also removes suspended particles that may contain nutrients & microbes. A soil drench is full-strength aerated compost tea, which delivers all those stellar benefits we previously discussed – straight to your plants root systems.

Another benefit of using compost tea as a soil drench is that there is minimal concern for potential pathogens. The soil and root system of the plant act as a buffer to filter out harmful pathogens that could be present in the brew. Because we use captured rainwater, we aren’t extremely comfortable with the idea of spraying something like leafy greens with our tea. I’m sure the good microbes are kicking butt and warding off pathogens in our brew as they should, but why risk it?

Using Compost Tea As A Foliar Spray

Rather than watering the soil and root system, you can apply compost tea directly to plant leaves! Plant foliage and their vascular system are extremely effective at readily absorbing and using nutrients from their surface. Many gardeners use this practice and swear by it.

To create a compost tea foliar spray, you may find the need to filter it further. This largely depends on the tea bags you use, and how fine or porous they are. Either way, do not empty your tea bag into your bucket if you plan to do a foliar application.

Add finished compost tea to a pump sprayer immediately after brewing, and apply to plants leaves until the point of runoff. Drench them! Like all foliage applications, it is best to wet leaves either early in the morning or in the evening hours. Direct sunlight on wet leaves can cause sunburn effects, and will also kill beneficial microbes present in the compost tea.

To be honest, this isn’t something we do very regularly. Mostly for the reasons above: it is an extra step, there are concerns of the sprayer clogging, and the slight risk of pathogens. I will say though, when we do make a foliar spray, I feel 100% comfortable using it on anything we aren’t going to consume directly! For example, on the foliage of tomato, squash, pepper, other veggies, cannabis, or non-edible plants where we aren’t consuming the raw greens. It’s just the fresh leafy greens that make me most nervous.

So, what do you say?! Are you ready to get brewing with us?

Check out this tutorial video that walks you through the whole process!

In summary, you can’t go wrong with actively aerated compost tea! It is easy, and your plants and soil will love it! Sure, you may need a few supplies upfront… But that is a small, one-time investment that can in turn provide you with an otherwise endless supply of free, killer, organic, homemade fertilizer for your garden ~ for years to come! To me, it’s a no-brainer.

I hope you found this helpful. Please feel free to ask questions, and spread the love by sharing this article with friends!