Figure 1 summarizes the spatial distribution of all farms based on PCA results with PC 1 as the x-axis and PC 2 as the y-axis. As shown in Figure 3, the results of the nearest neighbor analysis order each farm from 1 to 13, and provide a basis for visualization of the gradient in management. Therefore, this gradient in management, strongly driven by the amount of external N-based fertilizer applied on-farm, served as the basis for further visual comparison of Fields A and FieldsB across all farms . As shown in Figure 2a, the difference in soil ammonium concentration between fields was low among farms on the low end of the gradient. At the middle and high end of the gradient, farms showed greater soil ammonium concentrations in Field B compared to Field A—with the exception of two farms. Farm by farm, net N mineralization rates followed trends identical to soil ammonium concentrations. Soil nitrate concentrations varied widely among farms and did not produce any consistent trends ; however, a majority of farms showed greater soil nitrate concentrations in Field B compared to Field A regardless of the management gradient. Like net N mineralization rates, net N nitrification rates followed trends analogous to nitrate concentrations farm by farm. For both mineralization and nitrification rates, a majority of farms showed greater rates in Field B compared to Field A, regardless of the gradient in management.
Differences between Field A and Field B for total N, total C, and POXC followed identical trends farm by farm . Among farms on the high end of the gradient, cannabis grow setup the difference in total C between fields was consistently low . Similarly, the difference between fields in soil protein values were also consistently low at the high end of the gradient . Radar plots provided further comparison of Field A and Field B across all eight indicators for soil fertility along the gradient in management developed above . As mentioned, because the level of N-based fertilizer input was a strong driver of the management gradient, radar plots were divided to reflect low, medium, and high N-based fertilizer inputs. Shown in Figure 3L is the high overlap in soil indicators, with the exception of net N mineralization and nitrification rates, between Field A and B. However, among farms with medium N-based fertilizer input , the overlap of soil indicators between fields is minimal; Field B tended to show higher concentrations of soil ammonium and soil nitrate than Field A, while Field A tends to show higher values for total N, total C, POXC, and soil protein among these farms. Among high input farms , differences between fields were less evident in terms of soil ammonium concentration, total N, total C, POXC, and soil protein, though soil nitrate concentrations and net N mineralization and nitrification rates did show noticeable differences in values between the two fields.
The results presented above are reflective of the perspectives, observations, and experiences of a sample of organic farmers in Yolo County, California, USA, and offer an enhanced understanding of soil health and fertility from this particular node of the organic movement . Here, we focus less, as prior studies have commonly done, on a comparative analysis that quantitatively compares farmers perception of soil health to results of soil laboratory analyses ; instead, we lead the discussion with farmer knowledge of soil health and fertility, and explore emergent synergies with ongoing soil health research and soil indicator results. Establishing definitions of soil health among farmers in this study was important to gauge as a starting point to discuss soil fertility, and also for selecting fields used for soil testing. Among farmers in this case study, there was general consensus on defining soil health, with strong overlap in the particular language used by farmers. Because farmers who participated in this study were geographically located within a significant node of the organic movement in California and many of the farmers interviewed participated directly or indirectly in the growth of this movement , the similarity in responses to define soil health suggests that—on the one hand, these farmers continue to draw their understanding of soil health from the culture and guiding principles of the organic movement to this day . Indeed, maintaining healthy soils was a central component of the organic movement, as stewardship of soil represented a direct connection to the land and a form of environmental protection . At the same time, the aspects of soil health that farmers touched on here were also similar to findings by other previous studies , which suggests that—on the other hand, more recent codification of the five soil health principles by the US Department of Agriculture Natural Resources Conservation Service has led to widespread integration of a national soil health lexicon, as put forth by federal policy .
This soil health lexicon, in combination with farmers’ deep cultural history with organic agriculture, likely unified definitions of soil health among farmers in this study. Interestingly, while nearly all farmers interviewed touched on the first four soil health principles in some capacity, even farmers who used integrated crop livestock systems did not explicitly mention the importance of livestock integration . This finding suggests that perhaps due to sensitivity around food safety concerns, farmers may not openly emphasize livestock integration in conversation, because although this practice may be considered beneficial to their soil, in reality, they face structural and policy limitations . Despite the emphasis on understanding nutrient cycling and nitrogen availability to crops in soil health research and fertility management , we found that for most farmers interviewed in this study, tracking nutrient levels was less important than other aspects of fertility management. Moreover, for these farmers, managing for soil fertility required a holistic approach that went beyond understanding nutrient levels. Farmers also underscored that measuring indicators for soil fertility was not particularly useful to maintaining soil fertility in practice, because assessment of soil indicators lacked integration with management practices. In most farmers’ experiences, assessing soil indicators was often associated with prescriptive rather than holistic solutions. In this sense, farmers stressed that the synergy of multiple management practices over space and time guided their approach to building and assessing soil fertility on-farm, rather than using soil nutrient levels as a guide—a key finding that is also emerging in recent literature . While farmers agreed that gauging soil nitrogen and other key soil nutrients was important to consider and be aware of generally, other aspects of soil management, such as promoting soil biological processes, maintaining adequate soil moisture and aeration, or planting cover crops in regular rotation, were more critical to adequately maintaining soil fertility on their farm. An analogous soil health study similarly found that among predominantly non-organic farmers in the midwestern part of the US, measuring nutrient levels in soil was generally not highlighted by farmers interviewed . When prompted to discuss key aspects of soil health, a majority of farmers in this past study completely omitted mention of the importance of gauging nutrient levels, vertical grow system or in their case “soil mineral fertility,” as an indicator for soil health. This prior finding in combination with our findings here suggests that measuring nutrient availability to crops may not be as important as initially hypothesized to organic and non-organic farmers alike. Importantly, Gruver and Weil posited that the lack of emphasis on soil mineral fertility among these midwestern farmers may have occurred because they perceived that their soil fertility was not currently limited by nutrient availability to crops. Our research with organic farmers in California corroborates this hypothesis, and we suggest further research in other farming contexts to see if this sentiment among farmers is more widespread. We learned that there were three related reasons for why organic farmers in our study expressed that measuring nutrient levels was not particularly relevant for gauging soil fertility on their farm operation. For one, as already mentioned, farmers emphasized that they relied on carefully orchestrated soil management practices—such as the application of cover crops and livestock rotations—rather than depending on organic nitrogen-based fertilizers—to supply nutrients to crops. Because a majority of farmers applied less than 25 kg-N/acre of additional fertilizer per growing season, farmers in this context emphasized that their soil chemical and biological processes related to soil fertility may potentially diverge from agriculture that was predominantly or exclusively fertilizer-based.
By creating internally regulated farming systems via diverse management practices, these farmers observed that in general nutrient availability to their crops was ensured over the growing season. This key finding shared by farmers overlapped strongly with hallmarks for resilient agriculture outlined by Peterson et al. , who summarized features of internally regulated farming systems and key management practices associated with these systems. Based on knowledge shared by farmers, we suggest that it is possible for farming systems that integrate multiple management practices rather than rely on external fertilizer inputs to create soil conditions that “buffer” soil nutrient levels. In these internally regulated systems, measuring nutrient availability to crops may be less practical or even achievable with available soil indicators, as certain nutrients only become available as needed by local soil processes, and strongly depend on plant root structure, associated mycorrhizal pathways, and microbial communities present . To this end, several farmers hypothesized that available soil indicators were not sensitive to alternative approaches to maintaining soil fertility, likely because these fertility management practices operated on different timescales of nutrient release compared to direct fertilizer application. These conclusions drawn by farmers on the limits of measuring nutrient availability to crops were not unlike broad thematic gaps in measuring bioavailable nitrogen to crops discussed by Grandy et al. and others previously . In particular, Grandy et al. discussed the importance of considering soil health gradients, especially on farms that are not “ecologically simplified” and do not rely extensively on fertilizer application; such farm systems, like the farms examined in this study, are not as dependent on soil inorganic N and instead rely on what Grandy et al. call “a highly networked supply of organic N.” In other words, as farmers in this study also pointed out in interviews, soil health and fertility depend on a variety of factors, such as plant root accessibility, the microbial communities present, and soil mineral properties . As hypothesized in recent soil health literature, available soil indicators may not fully capture the complex plant-microbe-soil interactions that regulate fertility, particularly on organic farms that use minimal organic fertilizer application—a sentiment supported by farmer knowledge in this region as well. Second, farmers in this study also questioned whether available indicators for soil nutrient levels were calibrated not only to alternative farming approaches but also to local soil conditions. Farmers emphasized that soil test metrics were not grounded in their farm operation and produced inconsistent results that were likely due to a combination of spatial and temporal variations in their land, and also due to differences in inherent soil characteristics. As most farmers also pointed out, soil indicators for fertility did not explicitly calibrate for inherent soil characteristics, such as soil structure and soil type, or soil management history. Yet, to farmers, local knowledge of prior and ongoing soil management were integral to making management decisions that improved, or at least maintained, soil fertility on their farm. Farmers in this region stressed that the synergy of management practices they applied were often calibrated to account for physical soil variability among fields, and therefore were closely informed by their local soil conditions and unique management histories. While the importance of considering soil aggregate stability, soil texture, and management history when assessing soil indicators is well-documented in the soil health literature , in practice there continues to be a gap in soil health indicators that are tailored to be site-specific and/or farming system relevant . Given that soil indicators can vary by region and soil type, farmer involvement to provide key knowledge of local soil necessary for calibration of soil indicators is one essential way forward toward closing this gap. Merging results of soil tests with farmer knowledge may also help to increase sensitivity and utility of soil indicators across varying local soil contexts.Relatedly, farmers agreed that finetuning management could alleviate challenges associated with inherent limitations due to physical soil characteristics . Local farmer knowledge from this study established that inherent limitations posed by their soil or poor prior management could not be overcome by adding more N-based fertilizers—even if soil indicators showed the contrary.