From a conceptual standpoint, there has been considerable debate in recent decades over how to best maintain agricultural productivity while also achieving systems that can maintain long-term productivity through resilience to environmental stress. These conversations often pivot around the idea of replacing industrial input-intensive agricultural practices with ecologically-based, knowledge-intensive systems. These ecologically-based systems are typically depicted as relying on on-farm biological diversity as a mechanism for increasing crops’ resilience to environmental conditions, whereas industrial systems are maintained with off-farm inputs. Even as biological diversification enters the agricultural ethos, there continues to be a pull towards achieving these biological outcomes through off-farm inputs. We typically think of chemicals and energy as the off-farm additions to conventional systems; however, products that mimic the biological effects of diversification practices can similarly be introduced from external sources rather than fostered on the farm. While AMF inoculation has indeed shown some benefit in more industrially managed systems, in the present study we observe that in a more diversified system, augmenting a field’s endogenous AMF community does not improve plant outcomes. Rather than replacing one external input with another , flood and drain hydroponics we find that farmers who already practice diversified management will likely have better luck pairing local climatic conditions with locally-adapted microbial communities.
More broadly, the full fungal community in dry farm, irrigated, and non-cultivated soils were distinct, indicating different selective pressures in each soil condition. Irrigation seems to be a filter on agricultural soils, resulting in a smaller community that overlaps substantially with dry farm soils. Given that in this study only tomatoes were present in dry farm soils, while crops on irrigated soils varied from field to field, we likely overestimate the diversity of irrigated soils relative to dry farm, making this community shrinkage in irrigated soils even more pronounced. While fungal community responses to drought vary widely in the literature, there is precedent for deficit irrigation shifting bacterial communities in processing tomato fields, and natural experiments with drought conditions have led to increased fungal diversity in cotton rotations. This lower fungal diversity in irrigated systems may be driven by lower soil temperatures that are less conducive to fungal growth, or directly linked to changes in fungal competition induced by water stress that enhance diversity in dry farm systems. On the other hand, agricultural soils and non-cultivated soils seem to be distinct communities with roughly equal magnitudes of taxa numbers despite high levels of disturbance that might act as a narrowing selective pressure. Dry farm fungal diversity may be caused by external inputs that introduce non-endogenous taxa to cultivated soils. Dry farm soils were not only distinct from the other soil locations, but consistently enriched in taxa in the class Sordariomycetes.
These indicator taxa formed a dry farm “signature” that was not only present in dry farm soils, but increased in magnitude in soils that had gone multiple years without external water inputs. This signature showed positive associations with fruit quality outcomes, which is of particular importance to farmers in this quality-driven system. Sordariomycetes were also associated with an increased likelihood that a plot would not have any marketable tomatoes on a given harvest day; however, as this was a rare occurrence that happened almost exclusively in the first/last weeks of harvest when yields were low for all plots, we do not expect that farmers will see an association between Sordariomycetes and yield declines. If anything, farmers may notice a slight truncation of harvest season duration in fields that have been dry farmed for several years. Sordariomycetes themselves may not be causing these outcomes, but rather point to the fact that soil microbial communities–possibly including bacteria and other microorganisms in addition to fungi–are consistently adapting to dry farm management. Sordariomycetes enrichment may indicate other community shifts that are ultimately the cause for enhanced fruit quality. Endophytes in the Hypocreales class, which was enriched in dry farm fields, are known to increase drought resistance and decrease pest pressure in their hosts, though none of the specific species known to exhibit this behavior were enriched in dry farm soils. On the other hand, Nectriaceae, the family that contains the Fusarium genus, was found to be enriched, though similarly no known pathogenic species were enriched in dry farm soils.
Our study explored dry farm management practices and their influence on soil nutrient and fungal community dynamics in 7 fields throughout the Central Coast region of California, allowing us to explore patterns across a wide range of management styles, soil types, and climatic conditions. Though we were able to sample from a large swath of contexts in which tomatoes are dry farmed, we are also aware that conditions will vary year to year, especially as climates change and farmers can no longer rely on “typical” weather conditions in the region. While we are confident in the patterns we observed and the recommendations below, we also encourage further study across multiple years to better understand the full scope of the decision space in which dry farm growers are acting.Given the scope of our current findings, we outline several management and policy implications for dry farmers and dry farming. Though we aim these implications towards the context of dry farm tomatoes in coastal California, we expect that they are likely to generalize to other dry farm crops grown in other regions with Mediterranean climates. First, given the expense and possibility that it is detrimental to fruit quality, we do not advise AMF inoculation for dry farm tomato growers. Second, we note the importance of nutrients below 60cm and the complexities of subsurface fertility management, and we recommend experimentation with organic amendments and deeply rooted cover crops that may be able to deliver nutrient sources that persist at depth, as well as planning several seasons in advance to build nutrients deeper in the soil profile. Finally, given our finding that dry farm soils develop a fungal signature that increases over time and its association with improved fruit quality, we encourage farmers to experiment with rotations that include only dry farm crops and even consider setting aside a field to be dry farmed in perpetuity. However, fully dry farmed rotations currently do not exist, likely due to a lack of commercially viable options for crops to include in a dry farm rotation. In order to experiment with potential dry farm rotations, as well as cover crops that can best scavenge excess nitrates and soil management regimes that can increase soil fertility at depth, farmers must be given both research support and a safety net for their own on-farm experimentation. Funding to mitigate the inherent risk in farmers’ management explorations will be key in further developing high-functioning dry farm management systems. Expanding land access to farmers who are committed to exploring dry farm management can additionally benefit these explorations. Dry farm tomato systems on the Central Coast point to key management principles that can both help current growers flourish and provide guidance for how irrigation can be dramatically decreased in a variety of contexts without harming farmer livelihoods. In these systems, managing nutrients at depth–at least below 30cm and ideally below 60cm–is necessary to influence outcomes in fields where surface soils dry down quickly after transplant. Fostering locally-adapted soil microbial communities that are primed for water scarcity can improve fruit quality. Farmers can otherwise manage nutrients to maximize either yields or quality, indoor vertical farming giving latitude to match local field conditions to desired markets. As water scarcity intensifies in California agriculture and around the globe, dry farm management systems are positioned to play an important role in water conservation. Understanding and implementing dry farm best management practices will not only benefit fields under strict dry farm management, but will provide an increasingly robust and adaptable example for how farms can continue to function and thrive while drastically reducing water inputs.Small, diversified farms on California’s Central Coast have been dry farming for decades, a practice that allows farmers to grow tomatoes and other vegetables with little to no irrigation in summers without rainfall, relying instead on water stored in soils from winter rains. Though dry farming was originally developed in this region to allow farmers to grow crops on land that had no water access, it has thrived from consumer demand for dry farmed tomatoes.
Superior flavors have enticed customers, allowing farmers to charge a premium for dry farm tomatoes and develop markets for this regional specialty. Tomato dry farming in the region has been notably devoid of involvement from academic researchers and extension agents; however, policy groups and the general public have shown growing interest in dry farming in recent years as water shortages in California force a reckoning with the precarity of the state’s agricultural water supply. Amidst growing urgency to develop low-water agricultural systems in the state, we interviewed ten Central Coast dry farmers, representing over half of the commercial dry farm operations in the region where the practice was developed, to collaboratively answer two central research questions: 1. What business and land stewardship practices characterize successful tomato dry farming on California’s Central Coast? 2. What is the potential for dry farming to expand beyond its current adoption while maintaining its identity as a diversified practice that benefits small-scale operations? We summarize farmers’ wisdom into nine themes about current dry farm practice, the potential for expansion, and future opportunities. We also synthesize farmer-stated environmental constraints on where dry farm management may be feasible into a map of areas suitable for dry farming in California. As we consider the process by which dry farming might expand to new areas and new operations, we highlight dry farming’s history as an agroecological alternative to industrial farming in the region and the need for careful policy planning to maintain that identity. Because policies that encourage dry farm expansion could change the economic landscape in which dry farming operates, we warn against the possibility that well-intentioned policies will edge small growers out of dry farm markets. At the same time, we emphasize the opportunity for dry farm tomato systems to model an agroecological transition towards water savings in California.Unlike other forms of dryland farming , in this region dry farm tomatoes are grown over a summer season where there is a near guarantee of no rainfall. Farmers plant tomatoes into moisture from winter rains, counting on soils to hold on to enough water to support the crops over the course of the entire dry summer and fall. While some farmers irrigate 1-3 times in the first month after transplant, severe water restriction is what gives the fruits their intense flavor, and farmers trade water cuts that lower yields for price premiums that consumers are more than willing to pay for higher quality fruits. Beyond Bay Area consumer’s enthusiasm for high-quality local produce, dry farm tomatoes also trace their origins to a richer food culture of justice-oriented and farmer-centric food distribution in the region . From the Black Panther Party’s Free Breakfast Program to strong community support for worker-owned and consumer food cooperatives , the Bay Area has become a hub of alternative values-based supply chains in a country largely dominated by an industrialized food system . Following this tradition, dry farm tomatoes originally found their footing in the United States in the Central Coast region 30 miles south of the Bay. In the 1970’s and 1980’s, innovative growers in small-scale cooperatives and teaching farms adapted an Italian and Spanish legacy of vegetable dry farming to the region’s Mediterranean climate, maritime influence, and high-clay soils . While these environmental features were necessary to grow tomatoes under dry farm management, the movement that sparked the reemergence of local farmer’s markets in the 1980’s also provided the access to direct to-consumer marketing that small farms needed to win consumer attention and loyalty, allowing farmers to both grow and sell this niche product. With their origins in local food distribution networks and local adaptations to a unique climate, dry farm tomatoes are now a signature of small, diversified, organic farms on the Central Coast and are a feature of many such operations’ business models. To this point, dry farming has largely followed its initial course and is only practiced at a small scale in the region, both in terms of geographic scope, and farm size.