Soil Health Management Drives Soil Organic Matter More Than Edaphic Properties Across Working Organic Farms

Abstract. Rebuilding soil organic carbon (SOC) on working lands plays a pivotal role in mitigating climate change and improving soil function, yet its accumulation is constrained by both management decisions and inherent soil properties. Scientists and farm advisors recommend that farmers plant cover crops, reduce tillage, and add organic amendments to increase SOC, yet the effectiveness of practices intended to improve soil health may be limited by underlying edaphic controls such as mineralogy, texture, and pH. Given that SOC consists of two distinct fractions—particulate organic matter (POM) and mineral-associated organic matter (MAOM)—which differ in their stability and response to management, a critical question emerges: How much do inherent soil properties limit the effectiveness of recommended soil health practices in increasing SOC? Despite extensive research in controlled field settings, real-world farming contexts remain less understood, limiting our ability to predict SOC gains across diverse soil conditions. Here, we evaluate how in-season and recent (<5 yr) implementation of soil health management systems on working farms affects SOC fractions and stocks across 28 organic fields growing leafy greens in the Central Coast of California. We find that continuous living cover (e.g., through cover cropping) increases three of our measured carbon pools – free POM, MAOM, and surface soil total carbon stocks – while reduced disturbance (i.e., less tillage) increases two – free POM and MAOM. Crop diversity enhances both free and occluded POM fractions. Surprisingly, organic matter amendments do not show any relationship with any of the measured carbon pools. On average, management variables explain 3.7 times more variance than edaphic variables across carbon fractions, whereas, for carbon stocks, the opposite is true: edaphic variables explain ~2.1 times the variance compared to management. Our findings highlight that soil health practices, and in particular continuous cover, can significantly increase soil carbon levels, including both particulate and mineral-associated organic matter fractions, across diverse soil conditions.