Compost Tea vs. Compost Extract - who is who, and what do they do?

Every time I attend a workshop or presentation by an esteemed colleague “in the field”, I return to my desk full of buzz, bursting to share. So today I'd like to introduce you to Compost Extract, a low-tech, low-cost alternative to Compost Tea.

Way back when I went to gardening school, the science and practice of combining organic matter and water was in its infancy. Traditional methods involved steeping compost (or worse, manure!) in a barrel of water for days or weeks on end. Inevitably, the resulting anaerobic concoctions were unpleasant, ineffective, or outright dangerous, giving compost tea / extract a bad name — with good reason.

Over time, research and knowledge evolved.

Imagine my excitement when, in the early 2000s, I learned about the benefits of properly made, actively aerated, aerobic Compost Tea! A whole new world opened up, and for some fifteen-plus years, I have been selling brewers and kits, and educated countless tea-curious gardeners on how to use them.

In my enthusiasm, I overlooked Compost Tea’s older, lesser known cousin: Compost Extract.

Attending a presentation by Amy Luck-MacGregor, owner/operator of Spring Hill Soil Lab  in BC’s Cowichan Valley changed all that. So let me make up for lost time and explain the difference to you!

Compost Tea, as you probably know, is vigorously aerated. In addition, most recipes call for molasses and other ingredients to feed the microorganisms, prompting them to multiply exponentially. This favours some microbial species that can handle all that air and food. This tea is best suited for foliar / above ground spraying.

Compost Extract, on the other hand, is not aerated, and no microbe foods are added. As a result, the microbial density and diversity in the liquid closely resembles that of the compost used. This extract works best when applied into the soil and root zone.

So we can see that one is not better than the other, they just do different things.
Tea supports leaves. Extract supports roots.

Principles for Making Compost Extract

As with Compost Tea, only the best quality compost will produce the best quality extract. Use fresh rain water or clean well water, or dechlorinated tap water. (Humic acids neutralize chlorine / chloramine.)

In addition,
    1)    The compost microbes must be able to survive the extraction process.
    2)    The microbes must then be able to survive the application process.
    3)    And finally, the microbes must be able to survive in the soil.

What does this mean?

Extraction Methods: There are ways to utilize a hose-end spray nozzle, or a paint stirring paddle mounted on an electric drill, to agitate the compost-water slurry and dislodge the microorganisms into the water. Best results, however, can be had by simply massaging compost in a submerged mesh bag, using your hands.

There are no one-size-fits-all amounts and ratios of compost to water, but three cups of compost in one gallon of water is a good starting point. One of my brew kits contains approximately three cups of high quality, biologically complete compost.

It works like this; fill a small bucket with one gallon of water, and place one cup of compost in the mesh bag. Dip the bag into the water, holding on to its top with one hand while sticking the other hand inside, and gently swirl, spiral, dip, move the compost around in the bag, and the bag around in the bucket. Lift and squeeze, then repeat until the water running out of the bag becomes noticeably clearer. Dump the compost from the bag and add a fresh cup, repeating the process twice in that same gallon of water, until the three cups’ volume of compost is used up. Don’t rush — this can easily take 15+ minutes. The spent compost can go back into the garden, and the dark brown liquid extract is ready to use.

Watch Amy, from Spring Hill Soil Lab, hand massage for compost extraction, click here.

Application Methods: The easiest, and most microbe survival friendly way to apply Compost Extract, is by watering can. For larger areas, Amy can tell you all about in-line fertilizer injection systems that send microbes out the end of a garden hose with a gentle shower wand on the end (with a carbon filter installed if using tap water). Low pressure soil injection is another possibility.

Benefits are greatest if the liquid can soak at least 3” deep into the earth, and so the recommended application rate for Compost Extract is a lot higher than for Compost Tea sprayed on foliage. One gallon of concentrated extract covers 1,000 square feet. It can be diluted with dechlorinated carrier water as needed.

It makes sense to apply Compost Extract to loose soil that is already moist from a rain. Dry, hot, bare earth equals dead microbes! In compacted soil, the microbes will help to re-build structure so that roots can penetrate deeper. If you have compaction near the surface, it is best to break it up with a broadfork first. The aerobic organisms in your extract need some air in the soil to settle in.

Most important for microbe survival is that they need food. Specifically, they thrive on root exudates from a diverse mix of living plants. They also require a minimum of 3%, but preferably 5% organic matter in the soil. A mixed mulch layer is icing on the cake. No-till soil management, increasing soil organic matter content, mulching, adequate watering, and plant cover all make the beneficial effects of Compost Extract go further and last longer.

As with Compost Tea, the Extract has zero shelf life and must be used straight away. If it’s not used within four hours, aerate the liquid to keep the aerobes alive, but don’t offer food. When you’re done, be sure to thoroughly clean all equipment right away to prevent biofilm from forming.

Are you extract-curious now?

Go ahead and check out Amy’s excellent web site and blog!

And, my brew kits, plus humic acids, biochar, and mycorrhizal fungi are on sale until Nov 30, 2025! Even as plants prepare for winter dormancy, there still is lots of activity going on underground. That’s why October and November are good months for earth works, whether you plant trees or apply soil biology. Be sure to take advantage.

Speaking of extracts: Stay tuned for an upcoming blog series on making Plant Extracts this winter.