Johnson–Su: compost without turning, with microbial life in focus
The Johnson–Su method combines passive aeration, consistent moisture and long curing to produce microbially rich compost for biological soil inoculation.
David Johnson and Hui-Chun Su developed a static system in which material remains unturned for about a year. Its design emphasises air access and conditions supporting microbial development.
How the bioreactor works
A cylindrical frame lined with permeable fabric stands on a raised base. Pipes create vertical channels during filling and are removed about 24 hours later. The remaining channels circulate air, avoiding routine turning.
In a University of California Cooperative Extension implementation, the pile initially heated and then cooled. Red composting worms were to be added after cooling to continue processing.
Moisture needs continuous attention
Less turning reduces labour but does not eliminate monitoring. The revised 2024 manual calls for about 70% moisture, frost protection and worm introduction after the thermophilic phase. Irrigation is recommended for uniform moisture.
In a SARE farm project, outdoor bioreactors could not be kept sufficiently and uniformly moist despite watering during dry periods. Decomposition suffered. Water supply and location are central design decisions.
Why wait about a year?
Extended processing and curing allow community development in a material intended as an inoculant. Pat DeSimio, a collaborator of the method’s developers, notes that the original reactor is filled in one operation, requiring enough prepared material. He has also presented adaptations for smaller household quantities.
What field trials show
A SARE-funded trial at Pata Viva farm in New Mexico examined Johnson–Su applications alongside cover crops and reduced fertilisation. In 2022, compost application had no statistically significant effect on yield per plant. Heat, insect damage and establishment problems complicated assessment.
This illustrates the need for comparisons under each farm’s conditions. Producing biologically rich material and obtaining a crop response are distinct stages requiring monitoring.
Our field work
Johnson–Su could be compared with thermophilic composting and vermicomposting at the Center. Recording materials, temperature, moisture, labour and microscopy would help assess final biology and suitability.
Sources and further reading
- Pat DeSimio, Adapting Johnson-Su Composting Bioreactors for Home Use
- UC Cooperative Extension, Master Gardeners Build Compost Bioreactor
- Johnson-Su Composting Bioreactor Technical Manual, Revised July 2024
- SARE, Final report FNC21-1262
- SARE, On-Field Trial of Cover Cropping and Johnson-Su Microbial Inoculations