What is a Johnson-Su Bioreactor?

Wesley Soule
Sep 2
6 min read

The Johnson-Su bioreactor is a static, aerobic composting system, developed by Dr. David Johnson and his wife Hui-Chun Su, designed to produce biologically rich, fungal-dominant compost over a period of 9–12 months- all without turning.

Unlike traditional compost piles or tumblers that require regular turning, the Johnson-Su system is hands-off, consistent, and produces compost that is rich in beneficial microbes, especially fungi. We can achieve this by using a static system which is more suitable for the fungi that suffer the most from soil disturbances. If you want to understand how to return them to the soil, read along to find out.

Or if you want to see a video explanation, scroll to the bottom of this page.

Fungal growth is visible with mushrooms popping up in the bioreactor.

See our Bio-reactors at Stark-Conde Wine Estate in Stellenbosch

What are the Benefits of Johnson-Su Compost?

Fungal-dominant and microbially rich: Fungi are crucial for building soil structure and supporting long-term carbon sequestration. From our soil testing experience, they are also the most ‘endangered’ in the conventional system due to their sensitivity to environmental pressures. Applications of Johnson-su compost provide the soil with an inoculation of the beneficial microbes necessary in promoting nutrient cycling and healthy plant-microbe interactions.
Minimal Labour: This system is designed to be static, where aeration comes from vertical perforated pipes instead of the regular turning that is required with other compost systems. This means once set up correctly, there are far fewer things that can go wrong, which they often do in hot composting due to the more intensive management practices required.
Cost-effective: This system is built with simple materials of wire mesh, weed barrier fabric, perforated pipes, and pallets or wood frames. Once you have the structure, then we can include the compostable material. A strong focus on high-carbon materials, which feed our fungal communities, is always best here. We always recommend looking at the farm’s waste streams to collect these materials. The cost-to-benefit ratio is greatly in your favour if you use waste materials from the farm.

Odourless and clean: The aerobic process prevents the anaerobic smells often associated with many composting systems. This material will end up smelling like a forest floor with a hummus-rich rich earthy odour.

Why use a Johnson-Su Bioreactor?

One of my favourite tools for building both soil health and microbial diversity is the Johnson-Su bioreactor. This low-maintenance, high-performance composting system is an easy-to-manage system which works in the background to deliver high-quality inoculation-grade material. Due to the high concentration of beneficial microbes, this material works best as a compost extract or tea, allowing a small amount of compost to go far as an inoculation. Setting up this system means adding a resource on-site that can provide a huge range of soil-enhancing benefits from a single input.

Fungal growth in the vineyards because of the Johnson-su compost. — Fungal growth in the vineyards

How do I build a Johnson-Su bioreactor?

Constructing a Johnson-Su bioreactor doesn’t require much, but how do you do it? Here’s a rough overview of how it works:

Frame setup: A cylindrical frame (usually ~1m in diameter) is created with wire mesh, supported by a pallet base. We use cable ties or wiring to connect the wire mesh, allowing it to open and close easily. You’ll thank us later when harvesting the finished material!
Aeration pipes: Six vertical perforated PVC pipes are placed evenly inside the cylinder to ensure oxygen flow. Drilling holes in your PVC downpipes is required to diffuse air into the bioreactor. I also suggest cutting holes in the top layer of the pallet, allowing the downpipes to sit snugly in. This makes for easy filling of the bioreactors, as the pipes tend to fall over if not secured.
Lining: The frame is lined with weed barrier fabric or landscape cloth to keep compost in and pests out. We want to use breathable materials here; avoiding anaerobic conditions is key.
Filling: The bin is filled with shredded, moist organic matter. Ideal inputs include leaves, manure, straw, food scraps, and wood chips. Contact us if you need help designing a recipe for this; we work with your ingredients.
Initial watering: Compost should reach 60–70% moisture (about as wet as a wrung-out sponge). Presoaking the materials is very useful.
Inoculation: By bringing in the beneficial microbes required for compost making, you can ensure the process runs smoothly. Finished compost is a great inoculum for new compost. If you need assistance here with sourcing material, feel free to contact us.
Monitoring: I suggest putting up some small irrigation systems over the reactors to allow moisture consistency. Allowing this to dry out is a key failure point. Drip lines work great here on a timer. The best practice is small doses of watering frequently, especially in the warmer months.
Worms: A great way to finish this compost is by adding worms into the mix. These composting worms finish and homogenise the compost in its final stages. Consider doing this from months 4-6. A small number of worms is perfect, they will breed.

Johnson-Su compost, what are the results?

Look inside a bioreactor to see all the life which can be brought into your soils.

I’ve found the system incredibly manageable, even for teams with limited time or composting experience. We've implemented the Johnson-Su bioreactors on various farms, including Stark-Conde, Joostenberg and Lammershoek, among others. I am impressed by the quality and microbial diversity of the finished compost and the feedback I get from these clients is positive in terms of the ease of management.

Results Highlights:

Graphs of the balance of beneficial microbes present in a sample from Lammershoek a bioreactor that is still developing. — Have a look at the balance of beneficial microbes present in a sample from Lammershoek a bioreactor that is still developing.

Diversity of beneficial Testate Amoeba found in Johnson-su compost

Beneficial Fungal Hyphae (dark hair-like structure) growing on carbon-rich material from Johnson-su Compost.

Bacterial-feeding nematodes are key to maintaining a balance of bacteria to fungi. We are aiming for a fungally dominant material.

What can go wrong when making Johnson-su Compost?

While this system is forgiving, a few key missteps can hinder its effectiveness:

Not maintaining proper moisture: The first 3-4 weeks are crucial for ensuring the proper moisture levels. If the compost starts off too dry, microbial activity won’t kick in properly. Letting it dry out at any point is to be avoided.
Using too coarse or too fine material: Extremely coarse material may not decompose well, while fine material can compact and block airflow. Using a mix of both is best. Diversity of materials here is key.
Removing the pipes too early: Aeration pipes should be left in place until the structure is fully settled; some even leave them in permanently.
Expecting quick results: This is a slow, cold process. The reward is in the richness of biology, not the speed.
Weed seeds: Seeds are not killed in the composting cycle, as this is cold compost. For this reason, we focus on making liquid extractions from it to ensure weed seed is not spread.

Think of it like a sourdough starter:

Just like a sourdough starter, where you cultivate beneficial microbes and use them to inoculate new batches of dough/compost, Johnson-Su compost becomes that starter culture.

Rather than being used in bulk like traditional compost, Johnson-Su compost is applied in smaller amounts because it's not just about adding organic matter; it’s about inoculating the soil (or other compost piles) with a concentrated community of beneficial microbes.

This compost is biologically dense, rich in fungi, bacteria, protozoa, and other vital microorganisms that help restore soil structure, enhance nutrient cycling, and improve plant health. Because of its microbial potency, it’s used more like a starter culture than a bulk soil amendment.

When we refer to “inoculating” a pile or soil with Johnson-Su compost, we’re essentially populating that system with the kind of biology that leads to long-term fertility, structure, and resilience.

Microbial sample taken from Johnson-su compost — In that single sample, there are millions of diverse microorganisms, perfect for the soil application.

How do I use Johnson-su compost?

We recommend using this as a compost tea or extract starter culture. Placing this in a tea system will easily allow you to extract the microorganisms from the compost into a liquid solution. From there, it is all about applying them to the soil through irrigation or sprayers. You can cover a large area with a small volume of compost due to its high concentration of beneficial microbes.

A very general application rate would go as follows: 40 Litres of compost in a compost tea tank with 2000 L of water and microbial foods can be bubbled for 24 hours to produce an inoculum that can cover up to 10 hectares. This varies on a number of conditions, but gives you a guideline to work with.

Compost tea bubbling to support aerobic microbial development as we extract them into an easy to apply form.

If you're interested in setting up your own Johnson-Su bioreactor or learning more about composting for soil health, contact us- we’d love to support you in this process. Below is a video tour to show you what we have been working on at Stark-Conde.

Watch the full video tour of the Johnson-su bioreactors.

Have you made Johnson-Su Compost before?