What is compost tea (and how do you make it)?

Brad Rowland
Thursday, 23rd June 2011

Compost tea brewing - one of those great dark arts of the organic or permaculture garden. What's the best approach for making compost tea - aerobic or anaerobic? How do you use it? Is it safe? Permacultural entrepreneur Brad Rowland reveals all!

As with everything, there is controversy around how to make compost tea.

The two basic camps are anaerobic, or aerobic, and there is 'scientific research' to back up both sides that you can easily find on Google. The aerobic method 'brews' beneficial bacteria with added oxygen, introduced by something like an aquarium pump, creating an environment that favours bacteria that require oxygen. Anaerobic is the opposite - you put the lid on, and keep the air out.

After reading way too much on this topic, we invested in some supplies to start making the aerobic kind of compost tea. Two books have really started to reframe my mental approach to yardening, Masanobu Fukuoka's classic The One Straw Revolution, and the more recently released Teaming with Microbes.

I'm now beginning to understand why people are making such a fuss about till versus no-till, and the negative impact to your soil life even from something as simple as a basic chemical fertiliser. As an aside, this guy gets into my Hall of Fame for doing no-till in an old refrigerator!

Aerated Compost Tea

Brewing compost tea aerobically increases the number of beneficial bacteria and other microbes, which results in naturally healthy plants with good yield. Most importantly it creates or strengthens a soil web of life that controls disease and creates its own fertiliser for the plants.

Some people swear by it, some people say it doesn't work at all, and one guy said it killed some of his plants! My advice is to test some on your plants first just in case. We are having positive results already and have not seen any plant mortality. The most positive results so far have been controlling a powdery mildew (or something that looks like it) and some big plant growth.

My actively aerated compost tea brewing kit cost under $20, but you can probably do it for less. I couldn't find an aquarium pump at my local pet store or garage sale so I bought a new one from Amazon because I had a credit there. I bought a two outlet air pump, 10 feet of plastic tube and two 'air stones'. I also got a five gallon plastic bucket from the garden area and a big rock to hold the air stone down. Here are the pics of the Tea Pot, and some white plant disease we are trying to kill.

Editor's note: The capacity of the air pump is important. 1/2 to 1 x the air, in relation to the capacity of the container (fluid). 

Becoming a compost tea brewer

You will also need:

  • A couple of cups of compost – I use worm castings from my giant worm bin.
  • Water with no chlorine. Let your bucket of water sit out in the sun for a couple days and you should be fine. I use a 5 gallon bucket which needs 4 gallons of water.
  • A source of sugar. Most sites recommend molasses because of the additional nutrients available to the microbes. I'm using white table sugar, which works just dandy for other bacteria like Kombucha and seems to work fine for this. I use a tablespoon per gallon.

Making compost tea

Take your bucket of water and turn on the air pump. Mix the compost and the sugar and add it to the water. I let mine run for about two days. It should have a sweet soil smell and it will produce foam and bubbles. When it's done you can use it as a soil drench or a foliar (leaf) spray. I do both. I also water it down a lot but I've also put it on the plants straight.

Every article and book I read reports different ways to do brew compost tea. Recipes can be for bacterial dominant tea or fungal dominant tea. Didn't know this would be so darned complicated, eh? Here are some articles to give you a well rounded approach to investigating and experimenting:

Don't run the aeration pump too long or the bacteria run out of food and start to go dormant. Don't do it without oxygen or you grow bad bacteria (apparently). If you add other stuff, like fish emulsion, then something else happens but I don't remember what!

I'm trying to keep it simple: sugar and oxygen feed good bacteria that promote balanced healthy soil. Give it a go - and report back to me!

Editor's note: The bacteria need a nutrient source. Liquid seaweed is normal, but nettle/comfrey juice works too. If the brew becomes anaerobic, there are compounds produced that may be harmful to plants. Composts that derive from animal manures may be an e coli/salmonella risk if the aeration is not good enough. Important if you are spraying salads, or selling produce. As far as I am aware, they are both facultive anaerobes.

This post originally appeared on Brad's brilliant permaculture blog site, Highly Uncivilized

mark |
Thu, 23/06/2011 - 16:31
I reckon you could have aerated the tea using a solar-powered pond pump - make it that extra bit more eco-friendly! Great idea though - always stayed away from compost tea because it was meant to smell so bad...
mik |
Sun, 02/03/2014 - 20:41
"Editor's note: The capacity of the air pump is important. 1/2 to 1 x the air, in relation to the capacity of the container (fluid)." I'm sorry, I haven't a clue what that means! But I've never used an air pump before. Could someone please explain it to me?