Kefang Nie, LaZaria McWright, Joanna G. Rothwell, Mariel Mondragon-Becerra, Halima Malal, Cristina Lazcano, Daniel Geisseler, Layla Scott, J. Elliott Campbell, Jorge Gomez-Ortega, Hannah Waterhouse, Alexie Leauthaud, Shrijana Duwadi, Rebecca Ryals, Alda F. A. Pires
Organic soil amendments, particularly compost, are widely used in organic agriculture because of their benefits to soil health and environmental sustainability. Commercial compost producers are generally required to comply with pathogen reduction standards and are under regulatory oversight. This study assessed the microbial quality of finished soil amendments collected from 35 organic waste processing sites across California, including both CalRecycle-registered facilities and alternative organic waste processing units. A total of 184 finished soil amendment samples were analyzed. The mean fecal coliform concentration was 3.10 log Most Probable Number (MPN) per gram dry weight (gdw). Samples from registered facilities, 75 of 108 (69.4%), met the fecal coliform regulatory limit of ≤1,000 MPN/gdw, and no foodborne pathogens (i.e., Shiga toxin-producing Escherichia coli, Salmonella, or Listeria monocytogenes) were detected. Compared with registered facilities, alternative organic waste processing units had significantly higher fecal coliform concentrations (β = 1.37, 95% CI: 0.30-2.44) and were the only source of pathogens. These differences may reflect reduced feedstock preparation, monitoring, and process control practices in alternative systems. Higher moisture content (β = 0.06, 95% CI: 0.04-0.08) and longer intervals between sample collection and microbiological analysis (β = 1.83, 95% CI: 0.68-2.98) were also associated with increased fecal coliform concentrations. The findings emphasize the need for targeted training and resources to improve microbial quality in finished soil amendments, particularly among alternative organic waste processors. Efforts focused on feedstock preparation, process control, and postproduction management may reduce microbial contamination risks. Strengthening the microbial safety of finished soil amendments can support the wider adoption of composting as a sustainable waste management strategy and a safe use for food production.