How to Make Compost Tea to Fertilize Your Garden (Aerated)

Want your plants to grow bigger and better than ever before? Come learn how to make your own compost tea in this step-by-step tutorial. This guide covers the benefits of using aerated compost tea (as opposed to non-aerated), the required supplies, and various ways to use it in your garden. Video included!

When people ask how our garden looks so healthy, my usual reply is: “compost tea!” We feed our plants and fruit trees with aerated compost tea several times per year. Actively aerated compost tea, also known as AACT, is a biologically-active, nutrient-rich, mild-but-strong natural fertilizer.

RELATED: Want to learn more about composting at home? Learn how to set up a simple DIY worm compost bin here, or how to build a 3-bay compost bin here. I also show you how to harvest worm castings from a worm bin in this guide.

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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. You can make compost tea with worm castings or other high-quality compost. The purpose is to extract beneficial microbes and soluble nutrients, and then provide them to plants in a form that they can readily uptake and utilize.

Compost tea is full of beneficial microbes including bacteria, fungi, protozoa and nematodes, all of which have an important role in soil health. Using aerated compost tea in your garden is a great way to enhance the soil food web – the best way to grow big healthy plants!

“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

Why Aerate Compost Tea?

Aerating compost tea makes it exceptionally rich in nutrients and beneficial microbes, while reducing the risk of potentially harmful pathogens.

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). By introducing air and a food source for the beneficial microbes, their populations within the tea increases by the thousands.

“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.

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

Many folks confuse worm bin runoff or “worm juice” with compost tea, but it’s important to note that they’re quite different! The liquid or runoff that can be collected from the bottom of an overly wet worm compost bin is leachate, and it’s anaerobic. It isn’t nearly as rich in beneficial microbes, and may even contain negative bacteria and pathogens.

As you’ll see in our tutorial on How to Make Worm Compost Bin, we don’t add drainage holes to the bottom of our tote-style worm bins. Proper feeding and maintenance keeps the bin at the ideal moisture level, eliminating both smell and runoff.

Supplies Needed to Make Aerated Compost Tea

1) Compost

The quality and nutrient content of your starting compost directly dictates the quality of your aerated tea. Whatever is in that compost is going to be amplified, so it’s important to start with good stuff! A variety of compost types can be used to make 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

We usually make compost tea with homemade worm castings. Or, you can buy finished worm castings at your local garden center or buy some online. 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.

2) Brewing Vessel

For an average home garden, a basic 5-gallon bucket or two is adequate to make 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!

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 does. 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, are reusable, and easily available. Several layers of cheesecloth could work, 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) An Air Source

Next we need an air pump to introduce oxygen into the compost tea brew. In the past, we used a basic aquarium pump. It worked okay, but not nearly as well as the little commercial air pump we use now.

This air pump 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 (and 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.

Now we use these awesome bubbler snakes by TeaLab to make compost tea. They’re designed to fit perfectly in the bottom of a 5-gallon bucket, and produce some serious bubble activity through the perforated holes in the bottom of the “snake”. There is also have a little loop to tie and suspend the tea sack from, which also keeps the snake weighed down in the bucket. Fun fact: The bubble snakes are made in Humboldt County, California!

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.

7) Dechlorinated water

As much as possible, the water used to make 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 water from our rainwater collection system to make aerated 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 the water 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

1) Prepare 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.

2) Fill Tea Bags

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 into each 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.

3) Feed and Steep

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.

4) 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.  

5) Use 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.

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. In that case, we simply open up the tea bags and empty the contents back into the bucket after removing the pump and bubbler. 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.

Ways to Use Compost Tea in Your Garden

Using Compost Tea Soil Drench

Our preferred method for using compost tea in the garden is applying it as soil drench. A soil drench is just another way of saying “watering with it”. It is quick, easy, and effective! Using a measuring cup with a handle, we simply scoop finished compost tea from the bucket to pour around the base of each plant.

The amount applied varies 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.

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! So if needed, you can dilute a smaller batch of tea to create more volume and feed more plants.

Alternatively, we sometimes add compost 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. Using a large funnel, we ease the finished compost tea into a watering can after removing the tea bag.

When and how often?

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.

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 (instead of a foliar spray) 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.

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.

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

Demonstration Video

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!

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