Compost Teas and The Fifth Element
How bacteria and fungi in a compost tea might just be the fifth element, here to save the world...
I’m looking down a microscope at a drop of Aerated Compost Tea. It’s a whole other world. Below me, untold numbers of bacteria are going about their busy lives ambivalent or unaware of my presence. Eating, procreating, hunting, being hunted… I am both irrelevant and central. It’s a vertiginous feeling and also awe inspiring. I’ve brewed the tea as part of an experiment to see if we can significantly impact crop yields in the market garden using aerated compost teas, and if there are optimal times to apply it.
As I watch the communities of bacteria and protozoa that have been brewing in my bucket universe for a couple of days I also think it’s like watching a driving scene in Fifth Element. Multiple levels of hover craft cars flying around, busy and industrious, a million lives existing and the world just waiting to be saved from disaster… Can we hope agroecology offers such a positive conclusion to the story?
At its core, the Fifth Element is a simple story of Good vs. Evil and as I watch a ciliate protozoa rotate its way across the slide I think more about the parallels between this delightfully ludicrous 90s sci-fi romp and Compost Teas, farming and food security.
A malevolent presence is heading for Earth on a mission to wipe out all life in the universe. A vast anti-life force, a sort of black hole of death
A lot like how the current geopolitical state and polycrises facing us feel…
Standing in its way are four stones, harnessing the power of Earth, Water, Wind, and Fire, and the eponymous ‘fifth element’, a supreme being called Leeloo, with the ability to emit the “Light of Creation“ - the life force itself which can animate the inanimate
The natural world and our bacterial and fungal friends ready to help us?
Military-industrial types unsurprisingly want to employ Leeloo for their own ends
BigAg and Tech worlds industrialisation of synthetic fertilisers, biostimulants as “products”, seed monopolies and chemical agriculture
She needs to work with a team to save the world
Nothing great happens without community.
I look back through the microscope at the bustling drop of tea and imagine a thousand Korbens’ and Leeloo’s zooming about down there ready to help us save the world.
So what is an Aerated Compost Tea?
Aerated Compost Teas (ACT) are a kind of compost tea that specifically focuses on increasing the beneficial bacteria and fungi in your soil, to help plants thrive. It involves adding good quality compost to water, oxygenating the water and adding a food source to help microbial communities grow. When we brew an ACT what we’re really doing is throwing a microscopic party for all the beneficial microbes living in the compost by waking them up, feeding them and increasing their numbers.
Here’s a sub-2 minute video of how to set up a brew:
Time to grab a microscope! (the science of teas)
So your tea is brewing away, but what then? How do you know whether your microbial party is taking off? Using microscopes can be an amazing way to get a real appreciation and sense of what’s happening in your brew and enable you to determine if it’s growing in the way you want and if it’s ready to apply.
This video by the amazing Mary of Life to Lab, gives an overview of the types of organism you might find brewing in your Compost Tea. She’s looking at a sample of 5 day old tea through the microscope and gives great insight into what communities are being brewed and how you might choose to feed your bucket differently depending on what your goals are.
It’s 10 minutes long, but I learnt so much from her and this I think it’s worth it :)
Using your nose (the art of teas)
What if you don’t have a microscope though? (or time to use one). Having the confidence to also trust our intuition, as well as the science, is a critical tool in the science and art of compost teas.
So what other ways could we know how our tea is doing? There seem to be a couple of good proxy indicators of microbial diversity and a quality living tea that you can keep an eye/nose out for if you don’t have a microscope.
This 4 minute video shows a compost tea brew changing over a week and looks at how it’s smelling and looking, alongside what’s happening under the microscope.
Each time we brew a batch of tea there are so many variables that change; from the compost or amendments you add to the bucket, your water source, the temperature and weather and so forth. Whilst it’s fun to try and be rigorous in the science of brewing it’s also about building confidence in trusting our senses and intuition.
Key indicators of your tea getting to the right stage of bustling microbial diversity:
a nice smell, sweet but no longer like molasses
appearance of large persisting surface bubbles
This point can be anything from around day 2-5 depending on temperature etc
Why do we care?
We want to grow more nutrient dense, healthy vegetable crops reliably in intensive systems. A market garden is actually incredibly intensive both in terms of labour and when you consider the type of production you want to produce on a small acreage. If we’re aiming for multiple crops a season from any single bed over many seasons, we too run the risk of exploiting and extracting too much from the soils that sustain us.
Whilst there’s plenty of research to suggest applying ACTs could help increase yields, I was interested to see if we could find any indications as to how much it might increase yields and if there are optimum times or ways to apply it? In a Market Garden time is precious. We have to be fairly ruthless about where we focus in order to meet our production goals and get out high quality produce to our community.
I’m interested in Compost Teas, and in Biostimulants more generally, as a piece of this puzzle and part of (re) discovering new (old) knowledge. It seems like a more accessible and sustainable way of building soil health and converting pasture land to productive vegetable beds than relying on buying vast amounts of compost produced elsewhere to amend our beds.
With increasingly precarious food security, the global impacts of war, famine, drought… a 24 hour news cycle pumping us with disaster, ineptitude and hopelessness. We need more lo-fi, accessible solutions that work. There are plenty of companies offering pricey off the shelf options, but brewing and making your own amendments is empowering, decentralises power, promotes resilience and probably creates better ‘products’.
We were fortunate enough to get funding from the Royal Society of Edinburgh through their “Healthy Planet, Healthy People: Catalyst Award” to help us explore these questions.
Enter the Glamorous assistant
Every show needs a glamorous assistant and (aside from the lovely market garden team) Maureen was mine. She’s a tasty, sweet and crunchy hearting little gem lettuce. She would be our experimental subject, and then be ready to head out in our CSA member veg shares once the experiment completed.
The plan was to investigate the effects of applying compost tea at different stages (seed, propagation, transplant, and foliar) on a dependent variable (crop yield).
The independent Variables (Factors) and Levels:
Seed Inoculated: (yes, no)
Propagation watering: (yes, no)
Root drench at transplant: (yes, no)
Foliar feed: (yes, no)
With these four variables there are 16 different permutations in factorial design; from never applying tea, applying it at two or three points (in various combinations) or at all stages. Ultimately we were interested in measuring the yield (in this case cut weight of final head lettuces). To get enough data for robust statistical analysis we sowed 40 Maureens for each test set and planted out 25 of the best for each test set.
We also brewed and maintained two types of tea and kept these test sets separate. We made one brew with compost and one brew with worm castings.
So overall on this adventure 1,280 Maureens were sown, 800 joined us at transplant and 568 made it to the finish line getting harvested and weighed in. They were delicious and went in our CSA veg shares to help nourish our community in North Edinburgh as well as contribute to our research project. Maureen, a pretty impressive lettuce.
1 minute video of growing some lettuce… no voiceover, just a feeling that Maureen also needed her moment in the spotlight.
The Results
Firstly there seems to be a strong overall pattern (before statistical analysis) with a clear monotonic trend: More treatments = higher weight.
Approximate mean weights by number of “yes” treatments:
Number of active treatments // Typical mean weight
0 (all no) // ~25–30 g
1 // ~40–60 g
2 // ~55–75 g
3 // ~75–100 g
4 (all yes) // ~85–110 g
This alone implies large, statistically meaningful effects.
Digging into the data more it seems we can show statistically significant positive effects
Root application seems to be One of the strongest single drivers with application almost always shifting weights upward by ~20–40 g.
Foliar application showed a strong positive effect and was especially effective when root or prop was also present. It’s interesting that foliar had a significant impact as it was only applied once. It may be that considerably more gains could be achieved through increasing the number of foliar applications.
Propagation watering showed a moderate-to-strong positive effect and though it seemed less powerful alone, it did seem to amplify root and foliar effects. It’s interesting, because it was definitely noticeable that seedlings receiving the watering treatment during propagation seemed visually larger and more robust this didn’t translate to similarly large gains at final harvest. Seemingly it shows how pivotal later stage treatments are root / foliar applications with legacy effects tailing off from propagation - makes sense!... they can’t capitalise forever from an early boost!
Treating the seed did not show a strong independent effect and is likely non-significant. It’s possible that it could act more as a background or enabling factor, but not a driver.
Interaction effects are where it gets interesting and strong interactions were detected. The data strongly supports synergistic effects, not just additive ones.
Root × Foliar (very strong interaction)
Root alone: moderate increase
Foliar alone: moderate increase
Root + foliar together: disproportionately large increase
Prop × Root
Prop boosts the effectiveness of root treatments
Root treatments without prop are consistently lower
Three-way synergy: Prop + Root + Foliar
This combination consistently produces some of the highest weights
Often comparable to or exceeding “all treatments yes”
Any interaction involving seed alone had weak or negligible interactions
The Bottom line
Root and foliar treatments drive weight.
Propagation watering amplifies them.
Seed treatment has minimal independent impact.
The biggest gains come from combining root + prop + foliar
Cautionary note: Not all Teas are created Equal.
Worm Tea also had a strong overall pattern.
More treatments = lower weight 😱
It seems that we managed to inhibit growth using tea made from worm castings…
Given what is widely known about the benefits of worm castings I suspect this reflects our production technique rather than what could be achieved positively from brewing teas with worm castings.
Some ideas about why our worm cast tea inhibited rather than helped with yields
The inhibition/lower weights observed may be partly explained by nitrogen immobilisation, along with the high C:N ratio and high fungal:bacterial ratio of the vermicompost used in the brew.
High C:N ratio suggests that the vermicompost was still relatively carbon-rich and was ‘unfinished’.
While lab results showed sufficient total nitrogen, plant-available nitrogen forms (ammonium and nitrate) were very low, indicating that much of the nitrogen was likely still immobilised in organic matter or microbial biomass. This would make it less suitable as a fertiliser for immediate plant cultivation, particularly for nitrogen-hungry crops such as lettuce.
Biologically (under the microscope), the vermicompost was incredibly diverse and well-balanced, with all key functional groups present. It also appeared to have a relatively high fungal-to-bacterial ratio, likely reflecting the carbon-rich feedstocks originally added to the bin.
Lettuces and fast-growing N-loving annuals tend to prefer more bacterial dominated environments.
By applying the tea at multiple stages, it is possible that the nitrogen immobilisation issue was amplified, with each treatment introducing more microbial activity that increased competition for available nitrogen needed by the lettuce.
Adding molasses may also have exacerbated this effect. Molasses introduces a spike of readily available simple sugars, which microbes respond to very quickly in a liquid brew.
If the starting material was already low in plant-available nitrogen, that molasses carbon input may have further stimulated microbes to scavenge any available nitrogen. As a result, the final tea may have contained even less plant-available nitrogen for the crop... and they would have continued to scavenge post application!
Leaving some solid matter in the brew batch could also have been a contributing factor as old organic matter in a brewer may create conditions that favour anaerobic microbes which can produce toxins; acids, alcohols etc. which can inhibit plants…
Exploring and improving our worm casting teas are definitely on the menu for the next growing season.
Brew It Yourself
The basic set up for brewing your own teas is quite cheap and accessible. To get started you will need:
25L bucket (~£25, for 3)
Aquarium pump + stones (~£20)
Mesh Bag (~£7, for 6)
Molasses (~£23 for about 5L which would keep you going for years. You could buy less. Try to ensure you buy an organic one without sulpur
Rainwater
Good quality homemade compost
For application in propagation we used standard watering cans. Root drench used standard trug buckets. For foliar application we used a Stihl Mistblower SR450. This is an expensive item of kit and allows us to scale up for field applications. In a smaller context you could apply foliar sprays with a backpack sprayer or hand misters which are considerably cheaper.
What next?
I’ll definitely be brewing and applying more teas where we can at key plant stages with a focus on root drenching transplants and foliar applications for in field crops. The project indicateds that watering at propagation would also give us solid gains, but the reality of time and energy in the market garden makes this feel like a real stretch.. something I’d like to work towards getting a system in place to make it more feasible.
It would be interesting to explore the impact of the number of foliar applications (this is the easiest intervention to plan for, manage and apply at scale for our context).
I’m also keen to work on ‘recipe development’ and learning from other tea and amendment bodies of knowledge (IMO, EM, natural farming, biodynamics, Fermented Plant Juice etc) and explore what else we might add to teas to boost certain aspects of their impact and what protocols we might be able to develop for applying a different range of teas over crops lifecycles e.g. teas to promote flowering, fruiting, vegetative growth or seeding
It feels like we’ve forgotten so much in only a handful of generations but I am hopeful we can rediscover ways of being good stewards of the land and creating generative systems that help communities of all species flourish. Brewing Compost Teas seems like a fun, accessible and impactful tool for us on this journey.
Wow, that's fascinating and impressive work!