Healthy soils are the basis for resilient agricultural systems. Yet, on working organic farms in the United States, disentangling the features of soil management that support agricultural resilience remains a key challenge. Importantly, available soil indicators have facilitated efforts to measure expression of soil health on-farm. Despite the widespread focus on quantitative soil indicators, farmers—who are closest to the land, and soil—have been largely omitted from research on soil health. However, if available soil indicators are to be considered effective by farmers, they must be grounded in farmers’ realities and local soil contexts. Engaging with farmer knowledge of soil—and farmers’ approaches to soil management in practice—therefore represents an important and underutilized opportunity to widen our research frame around how assessment of and management of soil on farms is realized. In this dissertation, I take a case study approach, focusing on 13 unique organic farms in Yolo County, California.

Across three chapters, I investigate three main questions at the intersection of farmer knowledge, soil nutrient dynamics related to soil health expression, and on-farm management—First, how do organic farmers who are engaged in alternative agriculture acquire, translate, and apply knowledge of their soil? Second, how can we better pinpoint soil nutrient dynamics, in particular nitrogen availability to crops, on working organic farms, and also consider the role of management and soil edaphic characteristics in influencing these belowground soil nitrogen processes? Third, how can farmer knowledge enhance current understanding of soil fertility and nutrient management, especially in relation to available indicators for on-farm soil fertility?

In the first chapter, I use in-depth interviews with farmers to present ways in which farmers in this location are thinking about their soil and soil management; in the discussion, I propose a framework for understanding the substance of farmer knowledge and farmer knowledge formation, and I then apply this framework to this modest group of farmers and document farmer knowledge of soil management in the region. In the second chapter, across the same farming community (and a single research station), I create farm typologies based on indicators for soil quality to understand nitrogen cycling and crop nitrogen availability—and the role of management and soil texture in explaining differences in soil quality. Overall, I found significant differentiation among farms based on soil organic matter quality, strongly driven by both recent management and soil edaphic factors; I also found that soil texture may play a more significant role in determining soil organic matter levels, especially compared to management. Finally in the third chapter, I assess the utility of available indicators for soil fertility in informing on-farm management across this same farming community; overall, results underscored the current overemphasis of crop nutrient availability in building on-farm soil fertility, and the importance of calibrating indicators for soil fertility within local soil contexts by working in collaboration with local farmers.