Actively-Aerated Compost Tea
Why It ROCKS, and How to Make It

I used to think that compost tea was nothing more than a bucket of water with a bag of compost suspended in it for a few days to extract the nutrients. But I now make it in a completely different way, a way that dramatically boosts the populations of the beneficial bacteria and fungi that are so necessary for making nutrients available to plant roots. By using a special recipe and bubbling air through the tea, I can create a very special juice called actively-aerated compost tea, or "AACT".

AACT explodes the populations of beneficial bacteria and fungi in your garden soil, which can dramatically increase the nutritional quality of the vegetables. These microbial critters are what make the nutrients in the soil available to the plants, and when the plants are better nourished they are more resistant to disease - and more nutritious for us. Now this new kind of tea still contains nutrients like the old kind did, but it is also packed with helpful microbes.

Start with Good Finished Compost or Worm Castings

The highest quality compost is made from many diverse ingredients, and you really need to make your own to get the best. See How to Make Compost for instructions. Compost isn't hard to make, and you don't have to have a lot of space.

Most storebought compost is made from waste products like wood chips and feedlot manure, and won't make good AACT. Good compost made from diverse ingredients will contain nitrogen and carbon compounds, some phosphorus, potassium, magnesium, sulfur, calcium and micronutrients, all needed by plants. It will have lignins and be rich in humic and fulvic acids, which are necessary for plant health, soil structure, and water and nutrient-holding capacity in the soil. And it is important that the compost be completely broken down. When you look closely you should not be able to tell what the ingredients used to be.

Good, completely finished compost is filled with billions of beneficial bacteria, fungi, nematodes, and actinomycetes, and it is these microbes that we want to increase by making AACT.

While these critters might not sound so appealing to you and I, to plants, they are like an army of waiters delivering yummy dishes for the plants to dine on. They actually participate in the transfer of nutrients into the roots of plants, and many of them also eat or outcompete many pathogenic organisms that can cause plant diseases.

You can also use fresh worm castings if you have a worm bin. This is my favorite way to use my worm castings, because I feel I am making them go farther. When I make them into compost tea, I can spray the whole garden, instead of just pampering a few lucky plants.

How to Make Actively Aerated Compost Tea

There are two ways to make actively aerated compost tea.

  • use a professional compost tea brewer
  • make a homemade brewer

As a diehard DIYer I spent my first summer making AACT in a homemade brewer, using a good aquarium pump and a big 5" airstone. No matter how careful I was, it always ended smelling a bit off, but I used it anyway, thinking "Well it doesn't smell THAT bad... I think it's okay".

But I'm now convinced that it wasn't okay. To really aerate five gallons of compost tea sufficiently to keep anaerobic bacteria (like Ecoli) from getting a foothold, and to really produce big numbers of the good guys, you need a LOT of aeration.

I have modified the DIY brewer instructions below to double the output, but I'm now using the Keep It Simple commercially-made brewer, because I know it produces a great and safe finished product.

I highly recommend the Keep It Simple Compost Tea Brewer, because it has lab numbers to back up its claims that it produces extremely high fungi and good bacteria populations.

Here is the Compost Tea Recipe:

Four days before actually beginning the tea, prep your compost to enhance fungal growth in the tea. (See instructions next paragraph).

  • 4-6 cups excellent, finished compost (prepped beforehand, see below)
  • baby oatmeal or quick rolled oats (3-4 Tbsp per cup of compost)

After fungi have developed, add it to:

  • 4 ½ gallons of chlorine-free water (let the bucket of water sit overnight, or just run the bubbler in it for a couple of hours before adding the compost. This will allow the chlorine gas to dissipate.)
  • 2 Tablespoons of unsulfured molasses, honey, or fruit juice
  • 2 Tablespoons liquid kelp
  • 2 Tablespoons fish emulsion
  • some rock phosphate or kelp meal to provide substrate for fungi to grow on

"Feed" the Compost Four Days Before You Start the Tea

Fungi take longer to grow than bacteria, so about four days before you start your compost tea batch, add the baby oatmeal to the compost you're going to use. By feeding your starter compost a few days before you use it in the tea, you give the fungi a chance to multiply too.

Keep it out of the sun, and keep it about as moist as a wrung-out sponge. Lowenfels and Lewis, in their excellent book Teaming with Microbes, also recommend adding a bit rock phosphate or kelp meal at this point to help the fungi grow and to give them something to attach to when they get mixed in the water.

If you are using a commercial brewer, follow the instructions that came with it.

Making AACT in a DIY Brewer

(I cannot guarantee that the results you get will be satisfactory, but if you have problems, my suggestion would be to add more aeration.)

Here are the supplies you will need:

  • two 5-gallon plastic buckets (one for brewing, one for straining into)
  • two large aquarium air pumps
  • an old, 3-hole brick
  • two large aquarium airstones (“Top Fin” brand, 5-inch round airstone)
  • about 8 feet of ¼ inch air hose (2  4-foot long pieces)
  • a cup of oat baby cereal or finely ground oatmeal
  • an old colander that will fit into a 5-gallon bucket without falling in (thrift store)
  • an old piece of coarse fabric, cheesecloth, huge pair of pantyhose, old Reemay, or other filter for straining

To make your compost tea brewer, start by attaching the air hoses to the two large aquarium bubblers. I use one of those 3-hole bricks on the bottom of the bucket, set on its side, and I run the air hoses through the holes before attaching the airstones. This keeps the airstones from floating up or moving, without squashing the air hoses. The Top Fin brand 5" airstones are pretty heavy though, and I've never had a problem with them staying put.

Run the hoses up and out the top, and down to the air pumps. Put the bucket in a location that does not get direct sunlight (UV light kills the microbes) and is between about 60 and 80 degrees. I make mine in the garage.

Start your brew in the late afternoon or early evening, so that when it is ready in about 24 hours, you can spray it right away. This is because the microbes you have just grown are killed by strong ultraviolet light, and should be sprayed on the plants when the sun is low, so they have time to soak into the soil, and be taken up by the plant leaves before getting hit with sun in the morning.

Fill the bucket about ¾ full, and then run it without compost for 2 hours to dissipate any chlorine in the water. Take the compost you fed the baby cereal to four days ago, and gently crumble it between your hands down into the bucket.

Add the molasses or honey to feed the bacteria, add the kelp and fish emulsion to add micronutrients and protein for the bacteria, plug in the air pumps, and let the brew bubble for about 24 hours. 

When the tea is ready, take the other 5-gallon bucket, set the colander on top, line it with the strainer fabric or pantyhose, and pour the AACT through the strainer. 

There are several ways to spray it, but make sure that your spray is gentle. The fungal strands (‘hyphae”) are delicate living things, and getting shot out of a high pressure hose will break them up and kill them. I use a 1 1/2 gallon handpump sprayer that I got from the nursery if I'm spraying plant leaves, and a big watering can with a "rose" on the end for applying it to the soil.

The AACT should not smell bad. If it does, it means there was not enough oxygenation and the batch has gone anaerobic. Toss it out, and use more aeration in the next batch.

For more information about actively aerated compost tea, read Teaming with Microbes by Loewenfels and Lewis. It makes a wonderful wintertime read when you’re inside dreaming of next year’s vegetable garden.

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