A different farmer echoed this sentiment, saying that “I’m not going to magically get rid of issues that soil tests show… I can only slightly move the needle, no matter what I do.” Most farmers recognized that soil tests produced inconsistent results because of differences in timing and location of sampling. As one farmer noted, “You can take the same sample a couple months apart from the same field and get very different results.” Likewise, another farmer shared that, “I still struggle with the fact that I can send in two different soil tests and get two very different results. To me that seems like the science is not there.” Farmers also emphasized that each of their “fields are all so different” with “a lot of irregularity in [their] soil.” According to several farmers, soil tests did not account for variations in soil texture and soil structure, despite their observations of the influence of both edaphic characteristics on soil test results. For example, one farmer pointed out that fields that were plowed or were previously furrow irrigated created marked differences in soil test results. Similarly, another farmer shared that if a sample for soil testing was taken from an irregular patch in a field with heavier clay, differences in soil texture across samples skewed soil test results. If a systematic sampling approach was not considered, several farmers emphasized that results of soil tests might be “misleading.” Another source of inconsistency that farmers voiced stemmed from variation in protocols used across different labs that processed soil samples. One farmer stated that in their experience, “soil tests are not really accurate, botanicare rolling benches because if I use a different lab, a different person [ie, consultant] doing the soil test, it’s all different.”
Several farmers also raised issues related to how well soil tests were calibrated to their type of farm. For example, one farmer pointed out that they do not use soluble forms of nitrogen, and instead relied on their animal rotations and cover crops to supply nutrients as part of their fertility program; this farmer emphasized that, “I think we need to get to a place with soil testing where it would be more applicable or be more accurately useful for a farm like mine. This farmer questioned if available soil tests were calibrated to their type of farm, given that soil tests were designed for conventional agriculture . Relatedly, farmers expressed that soil tests often did not match up with their own observations of their soil and fields. One farmer plainly stated, “I’ve had soil tests that I felt were wrong; they often do not match up with what I’ve observed and gathered.” So instead, this farmer created a work around, “I usually just rent a backhoe every year and dig up one of my fields.” Another farmer also discussed this gap in soil tests, and stated the reason for this misalignment in farmer knowledge of soil and soil test results occurred because soil tests only provided “snapshots” and that observation was “just more practical in the end” because of the historical, iterative knowledge-making farmers engage in. To this farmer, these snapshots were a “another tool” but not as powerful as direct observation; as a result, soil test results did not inform decision-making on this farm. These sentiments were often directly related to the issue of sampling discussed above. By far, the largest limitation of soil tests that nearly all farmers discussed related to the lack of analysis and interpretation of results provided by most commonly available tests.
Farmers used a variety of metaphors to get at this general point. For example, one farmer likened using soil tests as a fuel gauge. This farmer stated that “the soil test tells me my tank is half empty, but it doesn’t tell me how far you’re going to be able to go… I think what’s lacking from soil tests, if someone with experience [could] help me interpret the results.” Another farmer wished they could ask “someone who has a lot of experience with doing soil tests—what do the results mean to you? Then I would incorporate my thoughts into the results… but there is not expertise and no dialogue.” This lack of dialogue was echoed by several farmers that saw the usefulness of soil tests in the collaborative interpretation of the 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, ebb and flow tray 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.