What is Compost Tea anyway?

I get asked this question a lot. So much in fact, that I hesitate to even use the term 'compost tea' because everyone's understanding of what this is seems to be different. Often times nowadays I'll talk about 'compost extract' of 'actively aerated compost extract,' just to avoid the confusion that comes whenever I say 'compost tea.' In my experience, people have told me that compost tea is anything from a burlap sack filled with horse-poo steeped in water for three days in a closed container, to the smelly liquid that seeps out the bottom of your compost or worm bins. These definitions make me cringe. It feels to me like someone saying that making bread is no more than some flour and water. Similarly, the reductionist understanding of compost tea often leads people who might be expecting a 'loaf of bread' to be disappointed by the tasteless lump of dough they pulled out of the oven.

A slight dated photo of me next to my compost tea brewer

In my search for how to amend this, I went to the source (or at least the most popular source of relevant information in today's modern age), Wikipedia. According to the general masses:


"Compost tea is a liquid solution or suspension made by steeping compost in water. It is used as both a fertilizer and in attempts to prevent plant diseases.^[15] The liquid is applied as a spray to non-edible plant parts, or as a soil-drench (root dip), such as seedlings, or as a surface spray to reduce incidence of harmful phytopathogenic fungi in the phyllosphere.^{[16] "}


While this definition certainly seems to reflect the general attitude, it is a big source of frustration for me. The very most basic premise is spot-on. Compost tea is a combination of compost and water, and yet it is really so much more.

Compost Tea Equipment from my company BioLogic Systems

What irks me about this definition is the lack of recognition that compost is much more than a fertilizer. In so many ways, we are still approaching farming and gardening from a very conventional NPK perspective. I will often see compost branded as a fertilizer, and while it is true that compost does have a nutritive chemical component, that chemistry is all a result of, and dependent on, the microbiology that really makes compost what it is.

My own definition of compost is: "Organic matter, and the microorganisms that eat it." I find this to be a simple and accurate description of what compost really is, and why it is so important. As an example, one teaspoon of healthy compost will contain around 33,000 different species of bacteria and fungi. That isn't even counting how many of each species there are... Without the microbes, nothing would compost. The process of composting is almost entirely a biological process, not a chemical one, and though there are certainly chemical reactions happening within a compost pile, most of those take place in the guts of microorganisms.

Therefore, compost tea is much less a fertilizer, and much more a biological inoculant. I recently spoke with a agricultural extension agent who told me that the farmers using biological inoculants are a fringe group within organic farmers. I asked him how many farmers use compost, to which he replied that most, if not all of them do. I didn't dare correct him in the moment, but in my mind I was shouting "That means everyone uses bio-inoculants!"

A photo (through a microscope) of my own Compost Tea

So, in my efforts to dispel the myth that compost is an inert fertilizer, similar in some way to the synthetically produced fertilizers we all know and love, here is my definition of compost tea:

"Compost tea is a liquid suspension of compost-dwelling microorganisms, the organic matter on which they feed, sometimes with the addition of specific ingredients to further feed those microorganisms. The liquid is actively aerated (brewed) until the organisms have reached a peak in their population levels at which time the liquid is applied foliarly and as a root drench for plants."

While it may be true that my definition is not necessarily better than wikipedia's, it feels to me to be more accurate. It is one of my deeply held beliefs that if we can start to recognize the living nature of our compost, compost teas, and soils, we would change the way we manage land, grow food, and treat our soil.

More information on this topic in a recent post of mine about resources!