Local and regional food systems, soil phosphorus, and resilience in a northeastern regional farmers’ market network

Objectives 1:

a) Interviews indicated that there is a significant degree of variation in soil fertility management among participants, but participating farms could be grouped into three general soil fertility paradigms, as defined below:

• Biomass Fertility (n = 8) – The application of organic matter to soil to meet plant nutrient needs. Inputs compost (farm-made and purchased), manure (raw, composted, and pelletized), soy meal, fish and kelp emulsions, humates, and cover crops grown as fertility amendments (green manure).
• Synthetic Fertility  (n = 3) – The application of chemical compounds derived from industrial processes to produce highly soluble and concentrated nutrient fertilizers.
• Hybrid Fertility  (n = 8) – The use of soil amendments that correspond to both general categories to meet plant nutrient needs.

Initial classification included a fourth management group, rock mineral fertility, which included farmers that used crushed rock mineral inputs that are used by many certified organic and organic-minded farmers. While these types of inputs are commonly used by farmers in this group, most of these farmers also use cover crops and compost (though not necessarily every year), thus there were very few farms and farm fields that fell squarely in this group. Given these circumstances, the rock fertility group was combined with the hybrid fertility group.

There are also three participating farms on the Black Dirt soils of the Walkill River Valley of Upstate New York that do not fall into any management group. These farms, as is typical of vegetable producers in the Black Dirt region, do not actively manage soil fertility, and rely instead on the in situ fertility that is charactaristic of drained histosols. These farms were not considered a separate management paradigm since their practices are a result of unique soil resources and therefore not applicable to farms outside the region. Rather these farms are used a type of idea case for comparison to other management types.

The presence of a significant number of participants that use hybrid fertility management strategies suggests that these categories might be better thought of as a spectrum of approaches. Most of the hybrid fertility participants use a combination of rock minerals and biomass-based amendments, which is in keeping with certified organic practices (four participant are certified organic and three are certified naturally grown, which has similar restrictions). However, two participants in the hybrid category also use synthetic fertility amendments, which makes it difficult to provide a more precise definition to the category.

b) Nutrient network mapping, seen in the maps below, produce complex webs of phosphorus supply. While there is some variation in local suppliers among the three sub regions, there is also significant overlap. Figures 2 and 3 depicts this first link in the phosphorus network that flows from local feed and input distributors to each sub-region. 

As these maps indicate, with the exception of one supplier in norther Florida, the majority of farmers source their phosphorus nutrients from distributors in the northeastern U.S. The majority of the overlap in networks occurs in south-central Pennsylvania, where several important distributors of organic-approved soil amendments are based.

The primary distributors of grain and soil amendments depicted in figures 2 and 3 are, with few exceptions, not the locations of origin for the the phosphorus nutrients used by participants. Each sub-region has farms that uses municipal leaves as a key component of compost amendments, and these inputs supply phosphorus extracted by trees from local soils. Additionally, one farm in the Lower Delaware sub-region uses composted manure from their small herd of dairy cattle that are entirely grass-fed. Beyond these exceptions, all other sources of phosphorus used by participating farmers eventually link back to mined mineral phosphate extracted in Florida, North Carolina, and Morocco, as depicted in figure 4. In the case of synthetic and rock mineral suppliers, this connection was directly documented through network tracing. Biomass-based inputs were only traceable to either the animals that produced manure, or the feed that was fed to such animals. Since tracing beyond that point was not possible, subsequent links are implied. However, all such sources found in this study document links to conventionally grown grain, which national statistics indicate are predominantly grown with synthetic phosphorus fertilizers. Thus the phosphorus contained in these biomass-based inputs originated from the same mined mineral sources (Macdonald et al. 2009).

This line of analysis provides two important findings for the sustainability of local food system initiatives such as the Greenmarkets. First, while the farms are located within a clearly defined region, the network of nutrient supply, at least for phosphorus, is certainly not a local network. In terms of conventional fertilizers this is not surprising as there are no mineral phosphate deposits in the northeast. However, this analysis also indicates that even fertility based on locally produced biomass is largely reliant on the same sources of phosphorus, albeit through longer supply chains. Furthermore, all farms, with the exception of the three growing on Black Dirt and one farm in the Catskills, import phosphorus from these sources on an annual basis, and their fertility management does not reflect in situ soil phosphorus resources. These findings indicate that building truly sustainable and agroecological soil fertility requires not only switching from synthetic to boimass-based fertility, but changing the paradigms under which these fertilizers are produced and applied.

c. The results of soil analysis indicate that there are significant differences between the three management groups, and the Black Dirt group differs from all three management groups (Figure 5).

For the SOM, POX C, CO₂ Burst, and Ptase assays, higher numbers are better as they indicate soil building, microbial community size and vigor, and in situ nutrient cycling. For each assay, the management groups broke down as Biomass>Hybrid>Synthetic, with the differences between each group statistically significant at the α<0.05 level.

The high level of organic matter in the Black Dirt soils contained significant tannins that dyed the Ptase extractant even after filtering, and interfered with colorimetric analysis. Therefore the Black Dirt soils are not included in the phosphatase enzyme comparison. In all other assays, however, the Black Dirt soils were significantly higher than all three management groups. The differentiation between Black Dirt and other management groups was most pronounced for SOM and POX C, with the difference in CO₂ Burst statistically significant, but less pronounced. There is also more significant variation in the CO₂ Burst assay, with numerous fields in both the biomass and hybrid groups scoring at least as high as the Black Dirt fields. Hurisso et al. 2016 argue that this indicates the dominance of C mineralization rather than C storage in non-Black Dirt fields, which is likely due in par to much lower levels of organic matter.

Mixed effects modeling was also conducted to account for clustering issues related to the significant variation among farms. While this approach did reveal significant clustering among farms, management type remained a significant predictor at the α<0.05 level for each assay. These models explained at least 85% of the variation for each assay. These models also revealed that Ptase more significantly correlated with CO₂ Burst than POX C, which supports the relationship between CO₂ Burst and short-term nutrient mineralization put forth by Hurisso et al. 2016.

These models also indicated close relationships between SOM, organic C, and enzyme activity that was difficult to tease apart with this analysis. Future research utilizing structural equation models may better reveal the nuances of these relationships.

Mehilch 3 extractable phosphorus was also compared across groups with the following relationships: Black Dirt≥Synthetic>Hybrid≥Biomass. In this case, Black dirt M3P levels where slightly higher on average than Synthetic fields, but the difference was not statistically significant. The same relationship holds between Hybrid and Biomass fields. Both Black Dirt and Synthetic fields are significantly higher than Hybrid and Biomass fields at the α<0.05 level. Unlike the other assays discussed here, higher levels for Mehlich 3 P indicate excessive levels of soil phosphorus that offer no agronomic benefit beyond a certain threshold, and threaten connected ecosystems with nutrient loading and eutrophication. While Hybrid and Biomass fields did have lower M3P levels, all groups had mean extractable P levels of at least 134 ppm, with 85% of all fields containing excessive levels of M3P. For reference, in most northeastern soils, M3P levels above 50 ppm are considered excessive. So while management practices that rely more significantly on biomass do have lower soil test P levels, none of the management groups (as they are employed by participants in this study) seem able to manage soil phosphorus bellow levels that threaten other critical ecosystem services.

While very few of the fields in the non-Black Dirt farms have assay scores that attain the levels of this ideal case, these results clearly indicate that farm management that relies more heavily on biomass-based inputs have greater potential for in situ nutrient supply that can reduce reliance on external inputs and nutrient loading in ground water and connected ecosystems. However, this analysis also suggests that this potential is not being fully realized by participating farms.

Objective 2:

Interview analysis indicates that the primary influence on soil fertility management strategy is a participant’s formative experiences as farmers. Nearly all participating farmers manage soil fertility (and many other environmental aspects of farming) in a manner that is consistent with the way they learned to farm – whether that be through apprenticeship, experiences as in an intergenerational family farm, or through reading and self-education. Geographic location is also an important factor that not only controls soil type (and thus native fertility), but also access to resources – especially raw materials for biomass amendments. Geographic location is particularly important when considered in combination with the total cultivated acres. Participating farms cultivating more tan 100 acres of vegetables are more likely to purchase fertility - either synthetic or biomass - due to cost considerations and the large volume of biomass fertilizers needed for that size operation.

Interview analysis reveals a complex relationship between farmer’s market participation and soil fertility management. Direct interactions with consumers in the context of the market do not seem to have a direct influence on soil management, however this is likely due in part to the fact that very few consumers ask questions about soil management. There is frequent producer-consumer dialogue about other environmental aspects of farm management, such as the use of pesticides, and interview analysis suggests that over time, farmers do begin to adopt different approaches to pest and disease management that are more environmentally friendly. In terms of soil, the farms that use synthetic fertility are beginning to change, or rethink, their approaches to soil fertility, and this does seem to be indirectly related to participating in the Greenmarket farmer’s markets. However, it is also very common for producer-consumer interactions to reinforce current practice rather than induce changes. This is because farmers are often able to effectively explain their practices and assuage the concerns of potential customers. And the establishment of long-term relationships with repeat customers only seems to bolster this effect. In other words, there is currently no economic pressure in the Greenmarkets for farmers to change practices. And while the relations of care and trust that are central to many farmers' markerts (Hinrichs 2000) are important, farmers are able to establish and maintain those relations without changing practices.

It is is interesting to note, though, that over relatively long periods of participation, some participating farmers did change management practices in ways that might be considered more environmentally benign and sustainable. One farmer adopted integrated pest management (IPM) rather than using chemicals, another farmer shifted soil management from synthetic to compost based fertility, and two others are considering altering soil management to practices that build organic matter. In each of these cases, the farmer has at least 18 years of Greenmarket tenure, and changes did not manifest until at least ten years of participation had passed. Interviews present these cases as a result of personal transformation on the part of the participating farmer, thus drawing a direct connection to the farmers' market participation is tenuous. However, the language used by farmers to describe these changes frequently express care and concern for customers, and for their land. This suggests that the relations of care and trust that are important to the functioning of farmers' markets may lead to changes in farming practices over time, though the exact out comes of these changes are heterogeneous.

Objective 3:

Resilience is defined by ecologists as the ability of a system to maintain certain ecological processes or structures in the face of specific disturbances. While this framework has been widely applied, it has not been widely adopted by those working on food and agricultural systems. In this case study, we develop the concept of agrobiogeochemical resilience, which we define as the ability of a farm system to maintain their fertility needs through in situ nutrient cycling, while reducing (or eliminating) dependence on non-renewable nutrient sources and the transfer of nutrients to connected ecosystems. This study considers agrobiogeochemical resilience in regard to phosphorus, and focuses particular attention on the relationship between participation in local food systems and resilience.

As the results from objective 1 indicate, there is greater potential for in situ phosphorus provision with biomass-based fertility, yet farms utilizing these management strategies continue to import phosphorus to their fields on an annual basis, even though biological and chemical soil test results indicate that additional phosphorus application is not necessary in nearly every case.

This suggests two issues for soil management. First, the differences among these management groups is largely one of input substitution rather than a reliance on artificial versus ecological processes for fertility. Secondly, most farms using biomass-based fertility in this study are using annual compost amendments to supply all fertility rather than using annual cover crop mixtures to supply nitrogen and maintain other nutrient levels.

The compost-only approach has two consequences that can make it difficult to reduce excessive phosphorus levels. Firstly, because the nitrogen:phosphorus ratio in compost and manure is generally 4:1, management that seeks to meet nitrogen demand with compost generally over applies phosphorus by a factor of four (crop demand is typically 16:1). Secondly, since compost is a broad spectrum fertilizer, it is impossible to calibrate fertility inputs to accommodate in situ resources. This may explain why many participating farmers using biomass based fertility continue to import phosphorus when soil resources are sufficient to meet plant demand.

These results suggest that building agrobiogeochemical resilience in this local food system should focus on building capacities and infrastructure for the use of ecological processes for soil fertility that can assist farmers in moving away from input substitution.

The results of nutrient network mapping also indicate that all farms in this study, regardless of management type, are significantly dependent on the same non-renewable phosphorus resources. Biomass-based fertility recycles nutrients, and thus these links back to mined phosphate are longer, yet this type of recycling does not address the underlying problem of soil fertility paradigms that rely on non-renewable resources. These results indicate that building agrobiogeochemical resilience in this (and likely other) local food systems requires regional restructuring to create regional systems of grain and manure that can close fertility cycles and create regenerative recycling. This is a daunting structural challenge, but this analysis clearly indicates that this type of paradigm shift is necessary to reduce reliance on non-renewable fertility resources.

Finally, the results of Mehlich 3 extractable phosphorus indicate that much work remains to be done to reduce the potential for nutrient loading from agriculture on these participating farms. The potential fixes outlined above will address this challenge, but this effort will likely also require investment in regular soil testing on regional farms. Most of the farms participating in this study did not regularly test soil, particularly farms cultivating less than 100 acres. The primary barrier is time for collection, though financial considerations are also important in many cases. It can take many years of concerted effort to draw down excessive phosphorus levels, and a long term commitment to monitoring is essential.

Based on this analysis, the agrobiogeochemical resilience of this local food system initiative is relatively low, yet this study does offer a silver lining. Many of the farms participating in this study use at least some biomass for fertility, and results suggest that this approach has great potential (with modifications) to increase agrobiogeochemical resilience. While there is certainly some selection bias in the relatively high number of farmers using biomass-based fertility (both in terms of study participation and farmers' market participation), this study suggests that long-term farmers' market participation can nudge farmers toward soil fertility practices that build soil organic matter. This suggests that with directed effort, local food systems like the Greenmarket farmers' markets can build agrobiogeochemical resilience.

Achieving these goals will require coordination across sectors, but these efforts are attainable in the long term, and study participants indicated that there is interest in meeting these goals. Food system localization is not a panacea, but it does show some potential for building resilient and sustainable food systems.

Hinrichs, C.Clare. 2000. “Embeddedness and Local Food Systems: Notes on Two Types of Direct Agricultural Market.” Journal of Rural Studies 16 (3): 295–303.

Hurisso, T. T., Culman, S. W., Horwath, W. R., Wade, J., Cass, D., Beniston, J. W., ... & Lucas, S. T. 2016. Comparison of permanganate-oxidizable carbon and mineralizable carbon for assessment of organic matter stabilization and mineralization. Soil Science Society of America Journal, 80(5), 1352-1364.

MacDonald, James, Marc Ribaudo, Michael Livingston, Jayson Beckman, and Wen Huang. 2009. “Manure Use for Fertilizer and for Energy.” Report to Congress AP-037. USDA Economic Research Service. https://www.ers.usda.gov/publications/pub-details/?pubid=42740.