Many plants have also evolved to promote the activity of ants on their tissues

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, 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, large pot with drainage 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 fine tuning 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. Interestingly, prior studies in the region found that organic fertilizer use in the early organic movement was potentially more widespread. For example, early organic farmers in Yolo County who were interviewed by Guthman et al. in the early 1990s used high nitrogen-based organic fertilizers such as pelleted chicken manure, seabird guano, and Chilean nitrate to supply fertility to soil in their organic production; based on interviews here, several decades later, farmers appear to have significantly cut back on the use of such high nitrogen-based organic fertilizer products. Several of these farmers have explicitly realized that “more is not better” when it comes to organic fertilizers; as discussed above, the majority of farmers interviewed here have shifted towards implementing a synergy of management practices that promotes good soil structure, increased soil microbial activity and soil organic matter, and adequate soil moisture rather than using high nitrogen-based organic fertilizers. Third, drainage collection pot these organic farmers unanimously agreed that soil test results could be more useful to them if the numerical results were also provided with meaningful interpretation, ideally in the form of a direct conversation—and that importantly, moved beyond prescriptive recommendations for nutrient additions and organic fertilizer application. Farmers interviewed used a variety of rich metaphors to elaborate on this point, such as likening soil test results to the fuel gauge in a car; both provide little insight into the actual mechanics of how well the system, be it an engine or a soil ecosystem, is actually functioning. This key takeaway from farmers in this study suggests that available soil indicators do not fully account for the complexity of their ecological farming systems, and that farmers see the interpretation of soil test results as an essential part of addressing the underlying complexity, and holistic soil function in their broader agricultural ecosystem. Our study provides an initial window into farmer knowledge of soil function in relation to soil fertility; however, as PetrescuMag et al. emphasize, deeper research on this particular gap in farmer knowledge of soil function is essential to determine the specific content of interpretations accompanying soil test results that would be practical and informative to farmers. Another potential way to bridge this gap in applicability for farmers would be to incorporate descriptive indicators for soil fertility in conjunction with available quantitative soil indicators. As Romig et al. suggested several decades ago, descriptive indicators can integrate well with existing soil metrics, and therefore provide mutually acceptable alternatives to discuss soil health and fertility among farmers and scientists alike. Finding a common language through which to engage is at the heart of this current gap in soil health research . Indicators for soil fertility measured here provided limited effectiveness in differentiating between fields deemed by farmers as “most challenging” and “least challenging” , which suggests that current scientifically developed metrics for measuring soil fertility do not align well with farmer developed benchmarks for soil fertility.

This outcome additionally suggests that nutrient availability was not the driving factor for farmer perceptions of soil performance, at least in terms of soil fertility. Of the eight indicators for soil fertility measured in this study, total soil nitrogen was the only indicator that was able to detect differences in soil fertility ; however, fields selected by farmers as “most challenging” showed on average higher values of total soil nitrogen than fields selected by farmers as “least challenging.” Because higher total soil nitrogen values are generally equated with higher soil fertility in the soil health literature, we hypothesized that the “least challenging” fields would show on average higher values of total soil nitrogen . This alternative outcome here suggests that while this soil chemical property shows sensitivity to differences perceived by farmers in their selected fields, this commonly used indicator does not adequately capture the direction of farmer knowledge of soil fertility between their selected fields. On the one hand, it is not surprising that total soil nitrogen was the only soil indicator able to detect differences between farmer-selected “most challenging” and “least challenging” fields, especially given that after nearly a century of research total soil nitrogen remains one of the most predictive measures of soil fertility status . However, the contradictory direction of our results for total soil nitrogen between farmer-selected “most challenging” and “least challenging” fields emphasizes that current scientific application of this soil indicator does not readily transfer for use on-farm. One potential reason for this inconsistency may be because as a soil indicator, total soil nitrogen reflects both the amount of chemically stable organic matter and more active organic matter fractions, and therefore gives a rough indication of nitrogen supplying power in the soil. However, in practice it is possible that fields deemed by farmers as “least challenging” have depleted their nitrogen supplying power due to more frequent crop plantings, for example— compared to fields that are “most challenging” and therefore may be less frequently planted with crops throughout the year. This finding underscores the current lack of interpretation of soil test results in community with both agricultural researchers and farmers present together; the current gap in interpretation of soil testing results was repeatedly emphasized by farmers during interviews, and suggests that— moving forward, contextualizing and interpreting soil test results in local farming contexts is key to disentangling potential mismatches between farmer knowledge systems and agricultural researcher knowledge systems. To move toward this outcome requires deep listening and relationship building on the part of agricultural researchers not currently widely applied . Whereas another similar study found that active carbon was the singular most sensitive, repeatable, and consistent soil health indicator able to differentiate between fields in their study on organic farms in Canada , we highlight that one potential reason for this difference in our results might be as a result of differences in management in each study. While our study consisted of farms along a gradient of organic management , the prior study focused on three organic farms with similar management. This divergence in results highlights the importance of accounting for a gradient in management when evaluating the efficacy of soil health indicators on working farms. Much remains to be learned about how inherent soil properties and dynamic soil processes interact with complex management systems on working farms .