Aerated Compost Tea: Does it Work?

There’s a lot aerated compost tea brewers on the interwebs!

I’ve been asked by Urban Farm Magazine to write a short piece on the pros and cons of aerated compost tea (ACT for short). I’ve been sifting through the peer reviewed literature on the subject. Most of the studies show, at best, mixed results. And, honestly, my bias is against gardening techniques that require gadgets or novel techniques with no analog in nature. I’ve also tried it myself and found that a thick mulch of plain compost seems to work better.

That being said, I want to present a balanced story. I’m interested in hearing from readers about their ACT experiences. Have you tried it? Do you think it works? Or are you skeptical? Leave a comment or send me an email with your name, where you live and whether ACT did or did not work for you. I’d like to gather some anecdotal reports for the story and your help is greatly appreciated.

For those of you not familiar with ACT, here’s a good explanation with some resources via Permaculture Magazine: What is compost tea (and how do you make it)?

On the con side of ACT, horticulturalist Linda Chalker-Scott has a set of pdfs as well as a long list of ACT studies on her gardening myths page.

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15 Comments

  1. Great idea Erik. I’ve often wondered about this. On the same general topic of validating or debunking gardening myths, has anyone ever done a definitive study on the application of rock dust (Azomite etc.)? Rock dust gets a lot of play on John Kohler’s widely followed website, “Growingyourgreens.com”, but I have yet to read any independent research on the subject.

    • A good question and something I’d like to look into. I’m always wary of the “notions and potions” school of gardening. I would suggest doing a soil report to see what’s lacking and work from there in terms of adding anything. There’s a very technical literature review on rock dust here that is above my head, sadly. From the conclusion it seems that the results were mixed and depended heavily on per-existing conditions.

  2. While I’ve not gone through the formalized use of compost tea, have grabbed the drippings off my barrel composter for the garden and really not noticed much from it. Do better with a nice batch of fish fertilizer, or, as you say, a mulch of compost. Don’t think we can create enough of a concentrate from our compost to make an effective tea, in general. My problems in the garden this summer exemplify this; it was so hot, that the plants took up the water so fast from the pots, that it wasn’t there long enough to absorb nutrients from the soil and blossom end rot was the result in just about everything. Realize these tea makers let the water sit with the compost longer, and there is the additional question as to what is in your compost. Know Prince Charles swears by Comfrey tea and grows a particular variety for just that purpose!.

  3. I think this technique has its roots in the central dogma of biology.

    I used to feel that sourdough starter with a special story was worth seeking out and getting: my parents’ starter traces its roots back to the Gold Rush, and I’ve had friends sneak me starter from great bakeries in my city.

    What really matters, though, is what conditions the starter is kept in. There’s enough diversity in un-sterilized flour to make a healthy starter, and its taste will be a predictable function of temperature and feeding schedule, anywhere in the world.

    This goes double, perhaps ten times over, for soil organic matter. If the conditions were right for these bacteria, they’d be there already; if the conditions are not, they’ll soon be dead.

    Making compost tea seems like a good way to produce highly soluble fertilizer, with very low carbon content. Bacteria are good at chelating minerals, so I’d expect this method to beat industrial liquid fertilizer at its own game…I’m just not eager to play such a game. I could imagine making a calcium or an iron solution to address some sort of emergency deficiency, like the blossom end rot that Morgaine mentions.

    A related practice, less gadgety and with analogues in nature, is something called anaerobic soil disinfestation. It seems like a way to cycle nightshades and other disease-sensitive plants more frequently, without resorting to chlorinated fumigants. The basic idea is to do anaerobic composting in the topsoil, then bring the soil back to aerobic conditions once spores etc. have mostly been eaten/scorched. Healthy soil microbes seem to bounce back a lot quicker than parasites (which typically rely on their hosts for more stable conditions). I imagine this also results in a lot of soluble nutrients shortly before planting time.

  4. Hey all, I have had great luck with compost tea. It is simple, effective, and easy to make. You can make it with almost all found/salvaged items except for maybe an aquarium pump. Here is how I first did it … http://autonomyacres.wordpress.com/2011/07/10/diy-compost-tea/ … Since then I have scaled up my production to about 35 gallon batches (still using the same aquarium pump). While that sounds like a lot, it is pretty easy to use that all up quickly. All my fruit trees get sprayed, along with most vegetables except for leafy greens (lettuce, spinach, arugula. I try to spray atleast twice a season, but would prefer to 3-4 applications. See Michael Phillips book, The Holistic Orchard, for more details on compost tea …. Andy

  5. Thanks for taking this one on, Erik. Disclosure: I worked for Elaine Ingham at Soil Foodweb Inc. for 3.5 years, so I learned about tea from the master. I was even part of the back-n-forth over E. coli in compost tea. So many variables (compost, water, aeration, time, nutrients, pump style and more) that comparing studies is at best frustrating. We all need to agree on the definitions. There are lots of old stories and new observations to explore regarding anaerobic teas, and herb teas like the comfrey mentioned above. Lots of work to do there.
    As far as personal experience, I’ve used carefully brewed, aerated 12-hour teas for years without any negative results. At least, I know I’m inoculating poor soil with organisms. The most dramatic experience I can offer is spraying peach trees that had evidence of leaf curl with two sprays a week apart and the leaf curl stopped progressing. Did that two years in a row. A fruit-growing friend said leaf curl didn’t just stop like that on its own.
    ACTs and rock dusts (mentioned above) seem to be tools for healing soils. There’s no substitute for a living healthy soil, and we know hardly anything about what makes that up. I would use them to build up soil or super-charge compost, then design things so the soil builds itself.
    Thanks for your work and your site!

    • Erik – I haven’t tried it yet. I have all the components to make the brewer (purchased them at Walmart) and plan on getting into compost tea production this coming Spring. I hope to do my own “field test” by foliar feeding and watering the same variety of tomato with the tea, and one without (same soil and light conditions). I thought the narrator on the video link I posted made a pretty compelling case. We shall see.

  6. Hi, my friend brews the tea in a hundred gallon compost tea machine, she sells it for 5 dollars a gallon to people with more money than me. She uses compost and a fish emulsion. She says she can see the difference, but im all for her making money on this. I make some worm tea in a trough i have under the gutter down spout, i use a paint strainer on the vermicompost and add some soft rock phosp, and bat guano,then add a bit of unsulferated molasses to the water and i have a fish tank bubbler in the bucket. I let it go 24 hrs. I feel good about it, its fun to be a little chemist. At the end I empty the paint strainer on a plant that looks like it could use a boost and parse out the liquid on the rest of my vegetables

  7. I backed up the pfeiffer center’s barn raising kickstarter (they’re a biodynamic education center in NY) and my reward was a sandwich bag of barrel compost that is I imagine similar to compost tea. It’s mixed into a bucket of water and stirred for quite awhile. While I’m not too keen about the more spiritual woo woo parts of biodynamics, I do know that a soil scientist had visited them and mentioned that she suspects that their sprayed preps encourage and nurture microorganisms in the soil (bacteria, fungi), in addition to adding many trace elements that people tend to overlook as critical to plant health. I imagine adding something is better than nothing. :)

  8. I have been working with non aerated, aerated compost for more than two years in a big garden in Israel.
    When I started, the concept improving soil & plant health through biological means was attractive. Yet I had my dose of skepticism.
    I decided to try the concept on several flower beds. Success did not come immediately. I have learned that the quality of the compost matters a lot.
    In fact, a good compost would usually give similar if not better results than compost tea alone made from the same compost.
    Simply because microbial diversity in the mother compost would be higher than in the compost tea. You will get in the compost tea only those microorganisms that will grow well on the ingredients added to the compost tea.
    Furthermore, the benefits tend to be incremental, with small bits of improvement at a time. I have noticed that the life span of my flowers get prolonged with the use of compost tea, and that the general health of plants is visibly improved therefrom. When you have a closer look at compost teas, you realize that the ingredients added to them during brewing (i.e., rock dust, humates, fish hydrolysate, seaweeds, deep seawater, molasses, ect.) have been reported in the literature to have, on their own, properties that are beneficial to plants. I have used several (i.e., rock dust and fish hydrolysate, seaweeds) with visible improvement for the flowers.

  9. Thanks, Erik. As a fellow follower of the Garden Professors blog, I’m glad you’ve linked Linda Chalker-Scott’s articles on the subject, which are as always, top-notch. I think we both know how strongly they feel about Compost Tea over at the GP blog. :)

    I myself have been making and using both aerated compost teas and anaerobic compost teas for a few years and I’m convinced that it is fairly effective as a liquid fertilizer but I have seen no benefits from foliar feeding. Many compost tea advocates seem to attribute numerous benefits to the tea that seem to be a little far-fetched but I’m totally open to its possibilities, I just haven’t seen the evidence yet.

    Personally, I like the ability to quickly make and add free liquid fertilizer to my garden, but won’t ever pretend that it will replace a high-quality homemade compost.

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