It’s easy to think of a composting system as something that needs to be mixed, turned, flipped, forked, aerated, and generally fussed with on a regular basis.
In many cases, that’s true enough. Traditional backyard composting systems often benefit from some degree of mixing, especially when we are trying to speed things up or prevent soggy, smelly zones from developing. Worm composting bins also tend to benefit from a more active form of ‘TLC’ in order to operate at peak efficiency.
A Johnson-Su bioreactor is definitely a different beast.
Rather than relying on regular turning, this system is designed to stay aerobic while remaining completely static. In other words, once the bioreactor is filled properly, the material is mostly left alone to decompose slowly over an extended period.
- No turning.
- No tumbling.
- No new deposits.
The key is in the design of the system itself.
Johnson-Su 101
Before we go any further, it’s important to make sure we're all on the same page about what exactly a “Johnson-Su bioreactor” is.
Developed by Dr. David Johnson and his wife Hui-Chun Su, the Johnson-Su bioreactor is a passive composting system designed to produce a highly-mature, biologically-rich compost. Their work focused on developing a simple, low-maintenance composting approach that could support diverse microbial communities and help improve soil health.
Unlike many traditional composting methods, the system relies on a combination of passive airflow, moisture management, and time to facilitate effective, aerobic decomposition. It also involves the use of composting worms - something we will circle back to further along.
The bioreactor is typically built as a tall cylinder using wire mesh that is then covered with landscape fabric or some other permeable material. Vertical PVC pipes are inserted during set-up and then removed after the system is filled, leaving behind air channels that help oxygen move through the composting mass.
The system is then filled with a very diverse mix of organic materials, moistened thoroughly, and left to decompose for many months. It's important to mention, however, that actual “recipes” are a key part of the Johnson-Su process. In other words - for best results, you shouldn’t just be tossing in whatever organic matter you happen to have on hand.
While exact timelines can vary, a nine to twelve month maturation period is often recommended.
Compared to more traditional composting methods: Johnson-Su systems also usually contain a higher proportion of "browns" - materials such as wood chips, straw, fall leaves and other carbon-rich feedstocks. This not only helps maintain a porous structure within the bioreactor (helping with passive aeration), but these wastes also help support the development of rich fungal communities during the long maturation process.
Why Slow Composting Down?
Many composters are focused on speed. We shred materials, turn piles, monitor temperatures, and look for ways to produce finished ‘black gold’ as quickly as we can. After all, faster production means more compost available when we need it, right?
While there’s certainly nothing wrong with that approach - the Johnson-Su slow-and-steady philosophy does offer some perks. Rather than focusing primarily on how quickly organic materials break down, the emphasis is placed on creating conditions that support the long-term development of diverse microbial communities.
The goal is to cultivate beneficial biology.
Which helps to explain why Johnson-Su systems are often left to mature for up to a year. The additional time allows microbial communities to continue developing long after the most active phase of the composting process has ended.
What Makes the Finished Product Different?
Finished Johnson-Su compost (often referred to simply as “Johnson-Su”) is known for being rich in beneficial microorganisms - especially fungi.
This is one of the reasons the approach has attracted so much interest among growers and soil health enthusiasts. Rather than viewing compost primarily as a source of nutrients, the system encourages us to think more in terms of producing a living, biological amendment.
For this reason (and similar to worm castings), Johnson-Su is often used in much smaller quantities - more as a microbial inoculant than as a bulk compost.
Compost extracts are also an important part of the overall approach, once again with emphasis on the mantra, “a little goes a long way”! It is widely reported that as little as 1-2 lb of Johnson-Su per acre (applied in extract form) can be used to effectively introduce beneficial microorganisms into the soil.
How Do Worms Fit In?
A Johnson-Su bioreactor isn’t usually thought of as a vermicomposting system, but composting worms do in fact play a very important role in the process. They help to fragment, mix and aerate the organic matter while also potentially improving the overall quality of the end product.
That said, it’s very important not to add them right away since the system can heat up considerably during the early thermophilic stage (well above what would be considered “worm-friendly”). Once the material cools down sufficiently - ideally, below 27° C - the environment should be ready for their arrival.
Vermicomposting enthusiasts may notice some familiar themes here:
- Importance of moisture management
- Importance of habitat quality
- Mesophilic temperature range (for most of the process)
- “A little goes a long way” (end product)
But a typical Johnson-Su bioreactor makes even the most “neglectful” worm bin maintenance look like micro-management! 😂
This is great for anyone who really appreciates a “set-it-and-forget-it” composting strategy, but it does raise the question…
Is the Johnson-Su Method Practical for Everyone?
Definitely not!
A full-sized Johnson-Su bioreactor can require quite a lot of material, space, water, and time compared to your average home composting system - not to mention upfront planning and labour!
And, for all our mentions of just how passive this type of system is, it’s important to note that it still needs to be set up, monitored and managed properly if you want to achieve optimal performance. Your moisture-management practices, alone, can make or break the process.
Bottom-line, setting up Johnson-Su bioreactors probably isn’t the best starting place for someone just looking for a way to convert their food scraps or yard wastes into compost, or even necessarily for many of the more experienced backyard warriors.
This approach may, however, be a good fit for:
- Serious composters with a keen interest in soil biology.
- Small farms and market gardeners.
- Community composting projects.
All with a critical underlying willingness to be very patient! In other words, unless you are prepared to wait at least 9-12 months to reap the rewards of your composting efforts, we recommend you opt for one (or more) of our “Big 3 Methods”.
Our Own Johnson-Su Adventure
Back in the Spring of 2025 we set up our first four Johnson-Su bioreactors, each with its own distinct recipe of starting materials. Our primary aims were: 1) to gain a better understanding of the set-up and management requirements for these systems, 2) to evaluate the end product, and compare it to other PC composts, based on Soil Food Web School (and other) compost quality criteria, and 3) to end up with a large quantity of highly-valuable Johnson-Su inoculum that could be used extensively on the farm, and made available to as many PC customers as possible!
Some things that stood out from our experience:
- This is an approach that requires a lot of upfront planning, time and effort.
- Once up and running, these systems can be fairly “easy” (assuming you don’t cut corners with #1), but they definitely still require monitoring.
Moisture-management in particular is something you need to take very seriously (it should be automated for best results). - The end product is indeed quite a bit different (in a “good way”) than the more typical composts we produce on the farm.
From a (beneficial) microbiological standpoint, our Johnson-Su amendment has the highest fungal biomass and F:B ratio of anything we’ve produced, and it contains many if not all of the plant-beneficial groups of organisms that we expect to see in a “biologically complete” material!
Final Thoughts
Can slower composting produce better results? As with many things in composting, the answer depends on what you're trying to achieve.
If your primary goal is producing large amounts of finished compost as quickly as possible, there are certainly faster approaches.
If your goal is producing a highly mature compost with an emphasis on microbial diversity and biological activity, the Johnson-Su system offers a fascinating alternative worth exploring.
Ultimately, one of the most interesting aspects of the Johnson-Su approach may be the reminder that composting is not just about breaking down organic matter. It is also about creating the conditions that allow beneficial biological communities to thrive!
Have any Johnson-Su questions, or experiences to share? We’d love to hear from you!
Recommended Johnson-Su Resources
Best Management Practices: Johnson-Su Bioreactors
Compost for Soil Regeneration: Johnson-Su Bioreactors
Helpful Related PC Resources
What is “Biologically Complete” Compost?
How to Use Worm Castings & Liquid Extracts
Setting Up Your GeoBin For Worm Composting Success
What is “Living Material”?
Can I Add Red Wigglers to My Garden?
Outdoor Vermicomposting 101
Recommended Products
Johnson-Su Compost
Living Material
pH Buffer/Grit
BioChar