Compost Tea

There’s been a lot of excitement in the last 15 years about the benefits of compost tea and nearly as much confusion about what it is.

The benefits are all the same as those things microbes do in the garden that were listed in the soil food web lesson, with insect and disease control and plant health being the main reasons people use it.

Cup of TeaNice with biscuits, but it won’t do much for your garden.

The confusion happens because of the name.

Gardeners have been making a form of compost tea for centuries by putting a small amount of compost in a pail of water, sometimes inside a burlap sack, stirring it once in a while for a few days, and then applying that water to the soil.

To me, that sounds like tea because it’s a lot like making a cup of tea — put the tea bag in the cup, add water, stir and drink. That’s tea.

Nowadays, this method might be called non-aerated compost tea, or simply the bucket method, which is also confusing because we use a bucket for the new method, too.

This older method can extract some nutrients and aerobic (oxygen-breathing) microbes from the compost if it remains aerobic, but the microbes generally won’t be very active and won’t stick to the leaf surfaces when applied.

Even worse, some funky teas can result from the old method that may harm plants.

You can add foods to the tea to wake up the microbes, but in doing this then they use up more oxygen and the tea can quickly go anaerobic (no oxygen).

YouTube video

Immature compost may also make an anaerobic tea. Anaerobic tea may have its special uses, but it will be mainly composed of anaerobic bacteria and yeasts, not particularly diverse, and lacking aerobic microbes like fungi, protists, and many bacteria.

We want aerobic tea to restore more of the beneficial microbes.

So non-aerated compost tea can be helpful or harmful, and it’s generally not optimal. There’s also traditional manure tea, which isn’t aerated.

The potential to extract harmful microbes that exist in animal manure is greater, along with antibiotics, hormones and other things we feed animals that aren’t organically raised.

Then there’s compost leachate, which is basically just the water that comes out of a compost pile that is too wet. Some people call that compost tea, but it’s very often an anaerobic, potentially toxic stew.

There’s a newer game in town that involves bubbling air through the water with some kind of air pump and adding specific foods to feed and multiply the microbes, and that is now called compost tea.

It’s more accurately called aerated compost tea, but it’s mostly just shortened to compost tea. I understand the confusion, but on this page, we’re looking at this new version of compost tea.

Aerated Compost teaCompost tea making at Granton Vineyard by Stefano Lubiana is licensed under CC0 2.0

What’s so great about it?

The new system can extract and multiply an astonishing number and diversity of beneficial, active, aerobic microbes. We mostly want the aerobes (aerobic organisms), because they’re generally the beneficial ones. Really good compost tea can contain as many as 100 trillion bacteria per 0.03 oz. (1 ml).

One of the reasons we do this is to inoculate our soil with microbes when we don’t have enough good compost available.

The other main benefit is that we can actually inoculate the leaves of our plants, something we can’t do with compost. With more experience, we can also get fancy and brew specific teas for specific circumstances. We may brew a tea to combat powdery mildew on grapes, or a tea to alter the microbial population in a soil in order to allow us to establish an orchard.

There has been some good research on compost tea, mostly on a larger scale.

Vineyards have achieved good control of mildews and even been able to harvest their grapes several weeks early, giving them a head start on winemaking.

Food growers have controlled diseases and documented many other benefits for plant health. Golf courses and parks have reduced pesticide, chemical fertilizer, and water use, substantially lowering costs while creating healthier turf.

When I started using it in my gardening business in 2006, I didn’t personally know any other gardeners or landscapers using it. It’s catching on now, but there are still very few professionals taking advantage of these great benefits of compost tea.

Even Harvard University has been using compost tea for 10 years.

They did a one-acre test of mostly turf in 2008 using compost, compost tea, mycorrhizal fungi, humic acids, liquid kelp, and an organic fertilizer. Compared to the control, root growth was two times greater and nitrogen produced was three to four times greater.

Yet they were able to mow 50% less, presumably because there wasn’t such a big hit of nitrogen and other chemical fertilizers all at once which often produces green grass and fast growth, but eventually causes problems.

They cut water use by 30% and that was expected to increase to 50% for a savings of two million gallons a year. Apparently, all 16 acres at Harvard are now organic.

HarvardHarvard lawns.

I’ve had great success with compost tea in my own gardens and the gardens of clients.

I’ve controlled diseases such as mildew and insects such as spider mites. It can’t be marketed as a pesticide and people have found themselves in legal trouble for doing so, but of course, it can help to control some plant predators because that’s what beneficial microbes do.

In some gardens, one application of compost tea has cleared out plant predators overnight and perked the plants up as if it was exactly what they needed. In other gardens, a few applications have been necessary, and in other gardens, not much change was noticed.

Sometimes, an inadequate soil food web is the problem, and sometimes it’s something else.

Making Compost Tea

To make a good tea, you need to get a lot of factors right — air pressure, water quantity, size of the air bubbles, amount and types of compost and microbes foods, and on and on.

That means you either need to purchase a quality brewer that has been thoroughly tested and has the data to support that, or build your own and test and tweak it until you get it right.

The first option may cost you at least a couple hundred dollars for a five-gallon brewer, but that’s actually cheaper than the testing that needs to be done to properly build your own.

I use excellent brewers made by Keep It Simple, and there are other great brewers out there, too. [Update: I’ve now started selling these brewers here.]

You can also buy a microscope and learn to test compost tea yourself — not cheap, but lots of fun. You can learn more about how to brew good tea from Dr. Elaine Ingham’s Compost Tea Brewing Manual.

There’s plenty of advice out there about how to build your own brewer for as little as $25. This can be a lot of fun and you might make a decent tea, but it generally works out that most homemade brewers don’t make a good tea until they’ve been tested and tweaked.

There are just too many variables. You probably won’t do any harm, though, if you use good compost and if the resulting tea smells good, so it’s a great way to start experimenting.

If you use a cheap aquarium pump, I recommend using only one gallon of water in the bucket in order to ensure you have sufficient oxygen.

Whether you buy a brewer or make your own, here’s how it works.

You start with a small amount of exceptionally good, aerobic, fully finished compost.

Finished compostFinished compost should look like rich, black soil, without any visible chunksof whatever went into it (except for things like egg shells which break down very slowly).

A mixture of two or three different composts is even better. Using different composts will bring more microbial diversity, and you can even throw in a small amount of healthy soil.

If you want to promote a fungal tea, perhaps in order to establish an orchard or shrub garden or strawberries, use compost that was made with a lot of woody material and still contains a small amount of woody material.

Even better, mix some oatmeal or oat bran into the compost a few days before you brew at three tablespoons per cup of compost. Keep this moist, dark and warm at 75F (25C) to promote fungal growth. For a bacterial tea, go for a less woody compost.

You can put this compost first into a mesh bag or directly into a bucket of clean, room temperature water.

By clean, I mean it can’t have chlorine in it. If you use city water, you need to let that bucket of water sit out for 24 hours for the chlorine to dissipate, or you can turn your air pump on for 20 minutes instead and that also does the trick.

If your city uses chloramine to disinfect the water, you need to tie it up by adding ascorbic acid (vitamin C) or humic acids. I use no more than half a tablespoon of my particular brand of humate in a five-gallon brew.

Your pump will blow air through tubes that are in the bottom of the bucket, the tubes attached with waterproof tape or weighed down somehow. The air goes through the water and compost, keeping the environment aerobic to favor the aerobic microbes, and physically pulling them off the compost.

In the old method, those microbes would mostly stay attached to the compost with the sticky substances they manufacture, and wouldn’t have enough air to multiply. The new method gives them the right amount of air, plus we add the foods they need to multiply.

Examples of good microbe foods include molasses, kelp, fish, humic acids and rock dust. Obviously, these products should not have preservatives in them, because preservatives are designed to kill microbes.

Molasses, other sugars, fruit juice and kelp promote more bacteria growth. Fish, seed meals, humic acids, yucca and rock dust promote more fungal growth.

Other than yucca, which is added at the end, these are all added at the beginning of the brewing process. Mycorrhizal fungi can be added at the end of the brewing process if you’re doing a soil application.

YuccaExtracts from the yucca plant can be used as a biostimulant and wetting agent.

Here’s a recipe I have adapted and evolved for a five-gallon homemade brewer.

This takes one to five days to make. I don’t really know when it’s done if I’m not testing it, but two to three days is a good time frame to start.

The mix is: 2 cups finished compost, 1 Tablespoon unsulfured blackstrap molasses, 1 Tablespoon liquid kelp, and 1 teaspoon liquid fish to 5 gallons of clean water. Brew for 2-3 days.

If you use a purchased brewer, they often use less compost. You may be able to buy excellent compost from the brewer manufacturer and a mixture of the microbe foods too.

As I said, this generally doesn’t produce a tea that gives much protection from disease or a big boost in nutrition, unless you get into using a better air pump and doing some testing to make sure you get all of the variables right.

The main ingredient variables are water quality and temperature, compost quality and microbe diversity, and the mix of extra microbe foods you use.

The main brewer variables are the oxygen level that is maintained in the water, the speed, size and placement of the bubbles, and the buildup of anaerobic pockets. Then there’s the question of how long the brewer should be left on.

It’s of critical importance to clean the brewer and all of the tubing from the air bubbler very thoroughly right after a brew is done. You don’t want any biofilm residues left over, as they promote anaerobic conditions.

Compost tea needs to be used as soon as possible after you make it because not long after that pump goes off, the oxygen in the water drops rapidly. It can be used with eight hours, but the sooner the better.

When possible, I use mine within one hour of its being ready. If used in farming or for other commercial purposes, you’ll want to do some testing to make sure you’re making a good tea. Use it in the morning or the evening because many of the microbes don’t like UV rays.

My method of applying it is to strain it through nylon or cheesecloth into a quality backpack sprayer, such as those made by Solo. This allows me to spray a mist at 60 psi and thoroughly coat both sides of the leaves of all of my plants. A watering can will suffice for soil applications, but some kind of spray is best.

A hose-end sprayer would work, but that water coming through the hose often has chlorine in it and it’s very cold. I’d rather not shock my microbes like that.

Hose-end sprayerHose end sprayer.

I add EM and biostimulants in with the tea, even if I already added a small amount to the brew. Commercial growers and wineries might spray weekly during disease season. Home gardeners might spray anywhere from once a month to once or twice a year.

Using a spray is best because research by Elaine Ingham says that we want our leaves at least 60-70% covered by microbes in order to prevent and even cure some diseases.

For foliar applications, which is the main method of application you’ll do, you’ll generally spray compost tea undiluted at 1 pint per 1,000 square feet, which means a good quality five-gallon brew can do about an acre.

It doesn’t hurt to do more than that, though. You could spray your five gallons on a few thousand square feet and that’s just fine.

Of course, if you have tall trees, you’ll need more.

The rule is 1 pint per 1,000 square feet for each six feet of tree height you have. If your trees are 12 feet tall, you need one quart.

We don’t dilute the foliar sprays because we want the maximum number of microbes on the leaf surface as possible.

PondA wider, shallower pond would also need only a tiny amount.

It can also be applied to the soil at 1-4 pints per 1,000 square feet. This can actually be diluted with clean, dechlorinated water in order to provide sufficient coverage. When planting, you can drench each new seedling with one to two pints of compost tea.

There are other uses, too.

Hydroponic systems use compost tea at 1 gallon per 40 gallons of water. It’s applied to ponds at as little as 1 gallon per acre-foot. That means if you have a pond that is an acre big and one foot deep, use 1 gallon of compost tea.

In a small backyard pond the size of a king-sized bed and four feet deep, that would be 1 tablespoon of compost tea. You also need to apply sufficient aeration throughout the pond so the aerobes can actually live there.


  • Compost tea is a great way to revive the soil food web when there’s not enough compost to go around and to establish beneficial aerobic microbes on the leaves of our plants, as well.
  • It’s made by pumping air through water that contains a small amount of compost along with foods for aerobic microbes to multiply. Here’s an example brewer.
  • It’s usually applied through a sprayer of some kind, to the leaves of plants undiluted at 1 pint per 1,000 square feet, and to soil at one to four pints per 1,000 square feet, potentially diluted.