Soil Health Monitoring and Compost Trial Evaluation
In 2018, Sonoma Mountain Institute partnered with the Sonoma Resource Conservation District to launch a multi-year Healthy Soils Demonstration Project, funded through CDFA’s Climate Smart Agriculture program. Over the next three years, we applied compost to 53 acres of treatment plots and established 22 acres of control plots without amendments. These efforts were paired with adaptive rotational grazing and systematic ecological monitoring.
From 2018 to 2021, initial sampling showed promising increases in soil carbon. To evaluate whether those improvements would persist, we returned to all original sampling locations in 2025–four years after compost application had ceased–and re-sampled using the same laboratory, methods, and personnel to ensure continuity and scientific rigor.
2025 Results
● Treatment Plots: On average, soil carbon increased by 22% relative to 2018 levels.
● Control Plots: Even without compost, control plots showed a 5.35% increase in soil carbon.
● However, when we exclude Treatment Area 1—which showed poor results in both treated and control areas— even stronger outcomes are apparent:
○ Treatment plots improved by 26%
○ Control plots improved by 12%
● Increased carbon leads to significantly increased water retention for the soil. ○ At a 26% relative carbon increase, an additional ~4.4 million gallons of water is held in 400 acres of soil, and nearly 72.1 million gallons across 6,500 acres. ○ At a 12% relative carbon increase, approximately 2.05 million additional gallons of water are held across 400 acres.
These trends suggest that the landscape as a whole is improving, and the treatments may have catalyzed broader soil-building processes, which could be amplified by grazing management and improved plant community function. Our re-monitoring effort this year demonstrates the long-lasting effects of early regenerative interventions. Even after halting compost applications, the system appears to be continuing on a positive trajectory. Significantly, the use of consistent sampling protocols enhances our confidence that these are genuine gains, rather than simply artifacts of measurement.
While there is more to be parsed from the dataset, including variations between subplots and site-specific responses, the overall findings are encouraging. They suggest that regenerative practices can build lasting soil health resilience, reduce the risk of carbon loss, and potentially amplify gains over time, even in control areas.