Abstract
This review was conducted to synthesize current knowledge, learn producer and Extension specialist perspectives, and identify gaps in understanding of the role of soil health in sustaining production in high tunnel (HT) systems. This synthesis includes findings from scholarly resources related to soil health in HTs, including research and Extension-based literature, perspectives from experienced HT producers and technical assistance providers, and the direct observations of a broad network of university research and Extension personnel working with HTs. Findings are intended to identify knowledge gaps and additional research and Extension resource needs of greatest priority to the HT producer community and technical assistance providers that support them at the time of publication. A review of 68 research articles and 58 Extension resources was conducted. Focus group interviews were conducted with small groups of experienced HT farmers in four regions of the eastern half of the United States, with in-depth farm case studies conducted in individual farmers in three of these regions. Growers across regions identified soil fertility management, soilborne diseases, soil compaction, and lack of consistency of soil analyses specific to HTs as the greatest soil-related challenges to HT production. Research and resources for technical assistance providers on mitigation strategies to remediate yield-limiting HT soil conditions, such as excessive soil salinity and high pathogen populations, were also lacking. As such, process-based research on techniques such as leaching, soil steaming, solarization, and anaerobic soil disinfestation in tunnels that consider short- and long-term costs, benefits, and effects on soil and plant productivity should be prioritized in the future when considering the impact of HT production on soil health. Interviews also indicated a need for networking opportunities for technical assistance providers across agencies (e.g., Natural Resources Conservation Service, Extension, nongovernmental organizations). Despite a high and increasing rate of adoption, there is currently a lack of information about maintaining HT systems. Given that HTs play a critical and growing economic role for specialty crop growers throughout the eastern United States, comprehensive intervention across the research–Extension spectrum to sustain productivity in HT systems is recommended.
High tunnels (HTs) are passively heated and ventilated greenhouse structures typically used to extend the season for in-ground crop production (Kadir et al. 2006; Lamont 2005). These protected environments exclude rainfall and moderate weather conditions but typically lack active heating and cooling systems, thereby providing some of the benefits of a greenhouse at a much lower cost (Janke et al. 2017; Lamont 2009; O’Connell et al. 2012). Benefits include increased marketable yield (Blomgren et al. 2007; Ernst 2020; O’Connell et al. 2012; Waterer 2003; Yao and Rosen 2011), postharvest quality (Batziakas et al. 2020; Bruce et al. 2019), and production capacity on limited land area, with reduced pest and disease pressure (Ernst 2020; Frey et al. 2020a) and weather-related management risks such as low temperatures, precipitation, and wind (Belasco et al. 2013; Blomgren et al. 2007; Ernst 2020). High tunnel adoption is increasing because of the production benefits described previously, and the US Department of Agriculture (USDA) Natural Resource Conservation Service cost-share programs (including the EQIP High Tunnel System Initiative, https://www.nrcs.usda.gov/programs-initiatives/eqip-high-tunnel-initiative), which have invested more than $170 million in HT production from 2010 to 2020 (data from Natural Resource Conservation Service 2021, Fig. 1). However, this number is only a fraction of the scope of HT production because it does not account for HTs constructed before 2010 or self-funded by producers or other cost-share programs.
Number of high tunnels funded through the US Department of Agriculture Natural Resource Conservation Service programs 2010 to 2020 by state. Regional geographies for the purposes of this study are broadly depicted by colored shading. Total number of producers interviewed in region indicated in pin buttons.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05460-24
With more growers investing in HTs, it becomes more important to ensure that growers will achieve long-term benefit from the tunnel. All previously discussed benefits may be reduced or disappear if soil degradation resulting in reduced yields is not effectively managed. Previous studies have identified long-term soil fertility maintenance and increased input costs as important challenges that affect HT profitability (DiGiacomo et al. 2023; Fitzgerald and Hutton 2012; Knewtson et al. 2012; Rudisill et al. 2015).
There is a growing concern that the intensive production practices typically used in unique HT microclimates may deplete the soil resource and jeopardize long-term system sustainability. Frequent tillage and great nutrient demands in HT systems may result in soil use intensification (Wildung and Johnson 2012), decreasing long-term soil fertility and quality. In addition to frequent tillage, warmer soil temperatures, and optimal soil moisture levels (i.e., within the wetted zone of the irrigation system) have been shown to lead to increased nitrogen (N) and soil organic matter mineralization rates (Shrestha et al. 2021). Furthermore, due to higher fertilizer application rates and lack of leaching rainfall, HT soils are susceptible to increased salinity levels (Sanchez and Ford 2020).
Currently, there is a scarcity of research on soil processes and properties within HT systems and a lack of HT-specific fertility and soil management recommendations to guide producers. Furthermore, the efficacy and economic feasibility of management practices to mitigate or avoid problems of declining soil quality in HTs are unknown. This review was undertaken to summarize existing HT soil research and resources for technical assistance providers and identify priority research and outreach activities needed to sustain soil health and productivity in HTs. Interviews with experienced HT producers and industry stakeholders supplemented the review of the research and Extension literature related to HT soil properties and management. The result of these syntheses is intended to assist HT growers, technical assistance providers, and applied researchers to better understand the most common soil health issues in HTs in the humid eastern regions of the United States. Furthermore, this review identifies prospects for future research to address these soil-related challenges in HTs.
Materials and methods
A literature review was conducted in Fall 2020 with the aim of identifying and synthesizing peer-reviewed primary literature and Extension resources related to HT soil quality and productivity. Because soil-related parameters may be discussed in the context of crop production or other management activities, all literature related to the broad search terms of “high tunnel,” and in combination with “soil,” “soil quality,” and “soil health,” were reviewed. Searches were applied to Web of Science, ResearchGate, and Google Scholar engines for research sources, and an additional search using Google was used to identify Extension publications. There were no restrictions on publication date. Sources were reviewed if they included discussion of soil properties or suggested soil management practices. A total of 68 academic publications (articles and conference abstracts) and 58 extension resources were reviewed. Findings were organized into key themes, including soil fertility, soil organic matter management, salinity, soilborne pathogens, and irrigation/water management.
Focus group and individual farmer interviews were conducted with experienced HT producers in Spring and early Summer 2021. The objectives of the interviews included 1) improving understanding of farmer challenges with HT soil management, 2) how these challenges change over time and how farmers adapt, and 3) identifying the specific obstacles and technical assistance needs they identified to inform future research. Producer participants were recruited to solicit representation from four regions in the Eastern United States (Upper South, Great Lakes, Deep South, and Northeast) (Fig. 1). Producers were recruited based on the authors’ existing primary or secondary Extension contacts. Criteria for focus group selection included at least 3 years of HT production experience. A total of 27 producers participated in the focus groups, with an average of seven producers per group. Selection criteria for individual farmer interviews included at least 5 years of production experience and self-identification as managing farms in which HTs were of economic significance to their overall farm operations. Focus group interviews were intended to generate dialogue between growers, and allow for input from slightly less experienced growers, while case study interviews allowed for lengthier conversation with highly experienced, individual producers to explore issues at depth. Summary information on farm characteristics, production systems, HT production experience, and the economic importance of HT systems to the farms are presented in Fig. 2. Additionally, three producers were selected for individual interviews, from Kentucky (Upper South region), Georgia (Deep South region), and Minnesota (Great Lakes region). In total, these more experienced HT producers managed 28 HTs, and represented more than 50 years of HT production experience.
Summary data from focus group participants. Data were self-reported and collected during meeting registration, before focus group participation. Production system responses provided for “Other” category included descriptions of noncertified organic production, use of natural and organic methods, sustainable no-till and hand cultivation, and similarly themed descriptions.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05460-24
All individual farmer and focus group interviews were facilitated by a primary interviewer and a team with at least one plant or soil scientist, one agricultural economist, and one additional note taker from the research team. Protocols and interview instruments were reviewed and approved by the Institutional Review Boards at each interviewer’s home institution and/or in a reliance agreement with the University of Kentucky. The interview protocols consisted of open-ended questions to allow the producers to describe soil and irrigation management challenges in their HTs, organized into management-related themes. Specifically, questions were designed to solicit information on HT production challenges related to soil fertility, pathogen and pest management, soil water/irrigation management, the economic impacts of these challenges and issues, as well as sources of information that inform farmer decision-making and learning. A qualitative research approach was used to analyze information from case study and individual interviews. Notes from each interview were reviewed by all interviewers and coded and analyzed. The coding process consisted of three readings: an initial reading identified general themes, a second reading identified important themes common across all farms, and a third reading reclassified themes into general themes and subthemes common across all farms.
Findings from interviews and the literature review were compared, then synthesized into key themes related to soil management that pose challenges in sustaining crop productivity in HT systems (Fig. 3). These are presented in the next section, with each theme described from the perspectives of stakeholder input, contextualized via summary of research and Extension resources, and key knowledge and outreach gaps identified. These findings are then summarized into recommendations for future applied research and Extension programming.
Conceptual model of soil health concerns in high tunnels, by biological, chemical and physical parameters, and their driving management, climatic, and edaphic factors.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05460-24
Results and discussion
Crop fertility, soil nutrient, and organic matter management
Foundational work on HT soils has indicated that growers have long considered changes in soil quality and productivity to be of concern (Grubinger 2012; Knewtson et al. 2010; Rudisill et al. 2015). This was echoed in our producer interviews, where soil salinity and fertility imbalances were described as widespread issues in HTs. Nearly all HT growers with more than 5 years of experience that were interviewed had experienced yield declines due to disease and fertility issues in crops of economic significance for their farms. Growers indicated that these problems differed from those they had experienced in open field systems. Soil salinity and nutrient imbalances contributing to these issues are likely linked both to the unique edaphic factors and management practices producers are using in HT systems (Fig. 3).
Due to high planting densities, vigorous growth, and increased annual biomass production of crops in HTs, increased fertilizer rates per unit area are often required in HT systems compared with open field systems (Guan 2020). However, there is a lack of HT-specific fertilizer recommendations that are based on HT fertilizer research studies, save a few notable exceptions (Fitzgerald 2013; Guan 2020; Langenhoven 2019; Sanchez and Ford 2020; Sideman et al. 2019). As such, producers are adapting fertilization regimes for their HT rotations with guidance based on research conducted in open field production. High fertilization rates, as well as increased compost applications (discussed further subsequently) were documented in our grower interviews and were echoed more broadly by the Extension specialists on our research team. Studies show that overfertilization in HTs can result in decreased nutrient use efficiency (Thompson et al. 2007), reduced plant health, and increased vulnerability to plant diseases and insect damage (Magdoff and van Es 2000).
High fertilizer application rates combined with the lack of leaching rainfall can result in the excessive accumulation of minerals in HT soils (Blomgren et al. 2007) and may lead to saline soil conditions that inhibit plant growth (Gluck and Hanson 2013; Little and Ristvey 2020). The producers interviewed noted the fertility imbalances and accumulation of salts, with experienced growers noting they had experienced loss of marketable crops (primarily tomatoes) due to fertility imbalances and their negative effect on fruit ripening and other physiological disorders. Producers relied on a variety of strategies to overcome fertility imbalances, including more frequent soil and plant tissue testing in HT crops and using crop consultants to guide custom fertility recommendations. Once physiological disorders and elevated salt levels were observed, producers modified their HT management practices, including opting for fertilizers with low salt indexes and limiting the use of compost with high salt levels. Some producers incorporated various strategies to leach salts between crop cycles, such as flushing a fallow tunnel with irrigation water, leaving tunnels uncovered seasonally when replacement of plastic is scheduled, and incorporating the use of movable tunnels. Producers noted the lack of specific guidance for salt and fertility imbalance mitigation strategies and the need for Extension resources specific to their soil types and climates.
The majority of producers (∼60%) interviewed discussed using compost as their primary method of soil organic matter management and acknowledged the potential for compost use to compound salinity and nutrient imbalance (e.g., accumulation of salts and increases in soil P levels) issues. These included both organic and non-organic growers. They noted compost applications had resulted in high soil P and K, increased soil pH, and salt accumulation in HT soils after repeated and/or large applications. However, most producers using compost perceived a lack of economically viable options to replace compost additions for soil organic matter management. Further, they noted a lack of research and technical guidance on efficacious soil organic matter management strategies, such as cover cropping or other strategies.
Literature on HT nutrient management has shown that application of large quantities of organic amendments (e.g., animal manures) can result in salt accumulation (Knewtson et al. 2012; Rudisill et al. 2015), P accumulation (Edmeades 2003; Reeve and Drost 2012), and increasing soil pH (Zhang 1998). These trends may be most pronounced at high application rates, particularly when compost is used to meet fertility requirements (rather than for organic matter management) for crops in HTs (Gheshm and Brown 2018; Knewtson et al. 2010). Compost applied at rates to meet N demand will exceed crop P needs and comes with the risk of carrying high cation (salt) levels (Montri and Biernbaum 2009). Further, although compost has been shown to be a source of mineral N to plants in HTs, N availability is driven by compost decomposition (Altamimi et al. 2016; Marshall et al. 2016), and significant quantities of compost derived-N may be largely exhausted after the first year (Marshall et al. 2016). The correlation between compost application and increased soil pH levels was less clear in the literature, although research in this area is limited. On-farm research by Sanchez and Ford (2020) investigating soil properties in HTs found the majority of HT soil samples used in the study had soil pH levels above the optimal range. However, they did not find a clear linkage between compost use in HTs and soil levels above pH 7.0 and note high soil pH levels were observed broadly in HTs, irrespective of compost application.
In open field systems, cover crops are commonly used to protect and increase soil organic matter and nutrients as part of integrated nutrient management systems (Snapp et al. 2005). Despite research and management guidance recommending the use of cover crops in HTs (e.g., Natural Resource Conservation Service 2021; Nennich and Wold-Burkness 2012), there is limited research on the effects of cover crops on soil processes in HTs, and the economic costs and benefits of cover crop use in HT rotations. Currently, Extension resources on cover crop use in HTs provide general considerations for cover crop management (Coolong et al. 2020; Perkus et al. 2018) and species selection (Coolong et al. 2021a, 2021b).
Research on cover crops in HTs has focused largely on the effects of cover cropping on soil, chemical, and physical properties when cover crops are included in HT crop rotations. Nitrogen fixation benefits from the use of legume cover crops in HTs has been consistently documented (Domagała-Świątkiewicz et al. 2019; Perkus et al. 2022) and the potential to reduce fertilizer inputs via cover crop nitrogen contributions (DiGiacomo et al. 2023). However, cover crop effects on soil nutrient levels has been variable and influenced by species composition, stand establishment, and timing (e.g., DiGiacomo et al. 2023; Perkus et al. 2022). Generally, findings indicate that cover crops may effectively increase soil N and microbial activity (Domagała-Świątkiewicz et al. 2019; Muchanga et al. 2020) and soil carbon levels (Hajime et al. 2009), although benefits to soil properties may take time to accrue (DiGiacomo et al. 2023; Domagała-Świątkiewicz et al. 2019) and may not translate into increased yield or crop quality (DiGiacomo et al. 2023; Muchanga et al. 2020; Zheng et al. 2019).
Although research is emerging, key knowledge gaps remain in evaluating the potential for cover crops to replace or reduce the use of composts for soil organic matter management. Further research is needed to determine best practices to prevent the over-accumulation of salts within stationary HTs and how to best flush tunnels that are experiencing this problem.
Soil compaction
Team members, as well as our focus group grower participants, noted soil compaction as one of the most challenging problems for HT producers. Participants explained that after a period of 5 to 10 years of HT use, soil compaction might become difficult to manage without tillage-based management. Several producers used deep tillage, and others were exploring cover crops in walkways as an alternative. Despite being described as one of the primary physical soil health–related parameters of concern in HTs, specific studies that address soil compaction problems and the effect on crop growth, nutrient cycling, and soil water dynamics in HTs were found to be lacking.
Water management
The exclusion of rainfall is a fundamental benefit of HT production and has been directly linked to reducing weather-related risk, lower incidence of foliar disease (Blomgren et al. 2007; Bomford et al. 2007; Carey et al. 2009; Conner et al. 2010; Demchak 2009; Knewtson et al. 2010) and increasing marketable yields and crop quality (Blomgren et al. 2007; Lamont 2009). However, in the absence of rainfall, soil water management is dependent on irrigation.
Universally, producers described a great need for water management and irrigation information tailored to HT systems. Producers noted a lack of information to guide water management for specific crops by growth stage and soil type. They noted that existing soil water monitoring tools, such as tensiometers, were not well adapted to the highly variable and sometimes harsh conditions (e.g., frozen soils and extreme temperatures) experienced in year-round production in HTs. Producers also noted the interaction between irrigation and fertility management and expressed a lack of confidence in understanding the interactions between nutrients, soil water status, and the overall effect of water management on soil microfauna and macrofauna in HT systems.
In our review, we found resources related to irrigation management; however, literature on the relationship between soil water and soil processes in HT systems were limited. Extension resources focused on irrigation strategies for HT crop production are currently based on open field systems. Although this may provide general guidance, crop water needs in open field systems differ greatly from HTs. HT-specific tools are a necessity due to complex interactions between greater crop growth rates, higher planting densities, general aridity of the soil environment, and lower evapotranspiration rates than the open field. For example, as noted by Montri and Biernbaum (2009), soil moisture content in HTs may vary at soil depths in ways that differ from open field systems. In contrast to open field systems where soil moisture content typically increases as soil depth increases, HTs on well-drained sites may experience drier conditions in the subsoil compared with the irrigated surface soil, resulting in conditions in which the rooting zone may become moisture limited in what appears to be well-irrigated soil on the surface. Producers interviewed noted the need to monitor soil moisture content at various soil depths for management of crop and soil health and the need for technical assistance in this area.
Drip irrigation has been the main strategy used to increase irrigation water use efficiency and leave the plant canopy dry in HT systems. Soluble fertilizer is often applied through the drip irrigation systems (Montri and Biernbaum 2009). Introduction of fertilizer salts through irrigation water may result in high soil salinity if the irrigation wetting front remains in the top few inches of soil as water is taken up by plants or evaporates before it can infiltrate any deeper (Gluck and Hanson 2013). Specialists conducting HT fertilizer trials in New England observed that when irrigation water was distributed unevenly over planting beds, even soluble fertilizers (e.g., potassium sulfate) would remain undissolved (thus unavailable) in soil in irrigation dry spots (Sideman et al. 2019).
Soil moisture is also a primary regulator of soil biological activity, of relevance to biological aspects of soil health in all HT systems and of particular interest in organic systems where soil microbial activity is a key driver of nutrient availability (Biernbaum 2013; Gaskell et al. 2000; Moore 2000). Studies have shown that abiotic plant stressors similar to those observed in HTs (such as dry conditions, high temperatures, and increased salinity) can affect soil microbes and other soil biota (e.g., arthropods, Skinner et al. 2019), as well as the nutrient cycling processes they control (Bell et al. 2009; Nguyen et al. 2018; Siebielec et al. 2020). The interaction of soil microbes with specific soil moisture conditions experienced in HTs is an area with much to gain from future research.
In summary, there is a lack of information addressing the role of water management in HT systems. Because of its overarching role in nutrient cycling processes, soil fertility, biological activity, and crop growth, the need for clear, improved soil water management recommendations has been articulated by both producers and the Extension and research professionals contributing to this work.
Soilborne pathogen management
HT systems are characterized by microclimatic and management characteristics that may increase the risk of soilborne disease. Warm soil and air temperatures, zones of high and low soil moisture status, and periods of high relative humidity can result in favorable conditions for pathogen reproduction and crop disease incidence. Further, extended cropping seasons and limited crop rotation may worsen soilborne disease issues (e.g., Saha and Egel 2015). These research results are consistent with responses from grower interviews, in which many growers noted that continuous cropping in HTs can create an environment conducive to disease over time.
The producers interviewed observed increased incidence of soilborne pathogens that they attributed to lack of rotation, continuously elevated temperatures, and irrigation and nutrient management regimes that favor disease development. These include sclerotinia (Sclerotinia sclerotiorum and Sclerotinia minor), downy mildew (Peronospora spp., Plasmopara spp., Pseudoperonospora spp.), and plant parasitic nematodes in all regions. Southern blight (Sclerotium rolfsii) was noted as a problematic pathogen by producers in the Deep South region, and rhizoctonia (Rhizoctonia solani) and fusarium wilt (Fusarium oxysporum) were common and problematic pathogens in HTs in the northeast region.
Our literature review found preliminary work documenting the common and emerging pathogens in HTs. A survey of Ohio HT soils (Testen and Miller 2018) found pathogens such as anthracnose (Colletotrichum coccodes), corky root rot (Pyrenochaeta lycopersici), and verticillium wilt (Verticillium dahlia) were present in 90%, 47%, and 46% of HTs, respectively. Plant parasitic nematodes (PPNs) have been documented in HTs in areas where they are pathogens in the open field, infestation may be exacerbated in HTs due to warmer soil temperatures and lack of crop rotation with nonhost plants (e.g., Frey et al. 2020b). However, PPNs are also emerging pathogens in HTs in regions where they are uncommon pests in open field systems, particularly in temperate regions and on sites with sandy or silty soil types. Root knot nematodes (Meloidogyne spp.) were also detected in 38% of HTs across the state of Ohio. In Kentucky, 175 HTs were surveyed in 62 of 120 counties in the state (Bajek et al. 2023). Root-knot nematode was detected in 55% of the HTs surveyed. Southern root-knot nematode (Meloidogyne incognita) was the predominant species, but Meloidogyne hapla and Meloidogyne arenaria were also detected in Kentucky.
Producers we interviewed were adopting practices to manage pathogens in their HTs in ways novel from their management in open field systems. In the southern regions, soil solarization was a common practice among experienced producers for pathogen, particularly for sclerotinia and southern blight management, as well as weed management. In the northern regions, soil steaming was noted as an emerging practice primarily for weed management but also for diseases. Although producers reported these practices as being efficacious for pathogen management, producers had many questions regarding the effect of soil solarization and steaming on soil biology and nutrient cycling and indicated a need for technical assistance to guide the use of the practices from a holistic, systems perspective. These and other solutions present nonchemical alternatives to fumigation, which may be inappropriate in HT systems due to scale, cost, prohibition of chemical use in HT systems, and use of organic production practices. Anaerobic soil disinfestation (ASD, or biological soil disinfestation/disinfection) is also among this suite of practices and may be a promising practice for pathogen management (e.g., Shrestha et al. 2021b; Swilling et al. 2022). No published work exists to date that addresses how the soil chemical and biological processes that drive ASD treatment effects are altered by the unique soil biological, physical, and chemical properties of HT soils, or how the ASD process influences the properties of HT soils.
Overall, producers noted their HT management required adaptation over time due to general increases in pathogen pressures. Experienced producers were using adequate interventions via cultural and chemical practices but noted uncertainty and a need for improved understanding of the effects of practices such as solarization, ASD, and steaming on soil nutrient availability and beneficial biota. Further, producers indicated the importance of peer-to-peer learning for effective pathogen management strategies and how to fit them effectively into HT systems.
Economic considerations
High tunnels are catalysts for expansion of seasonal market access, growth and product diversification in existing markets, and expansion into new market sectors (Ernst et al. 2020). In a survey of HT producers in Kentucky and neighboring states, more than 90% of respondents said their early- and late-season market activity increased after installing their tunnel. Sixty-five percent of respondents also said that HTs diversified their product offerings. This increased market activity, early- and late-season price premiums, and increased product diversification represent reduced marketing risk for producers. Previous research suggests HTs are comparable to other production risk management tools such as crop insurance, due to the ability of HTs to increase yield while reducing weather-related yield variability (Belasco et al. 2013).
Clearly, HTs can reduce production and marketing risk in the short run, and potentially the long run as well. However, these benefits may be reduced or disappear if declining soil conditions resulting in decreased yields are not managed. Farmers’ near-term HT production decisions have long-standing implications for future soil health, productivity, and system resilience. Nearly all the growers we interviewed experienced yield declines due to disease and fertility issues, which differed from the problems these seasoned producers experienced in the open field. However, the efficacy of new inputs or management tools to avoid or mitigate problems of declining soil quality in HTs, and the economic feasibility of adopting these management tools, including the potential impacts on costs and revenues are largely unknown. There is limited work in this area, although a recent study of the economic trade-offs of incorporating cover crops into HT rotations (DiGiacomo et al. 2023) found short-term financial benefits from cover crop adoption were insufficient to offset the cost of the cover crop practice. Further, the authors noted the need for economic valuation of the long-term, indirect cover crop benefits to sustained soil fertility and production capacity of such soil-conserving interventions.
Conclusion and recommendations
Producers interviewed indicated a lack of clarity and in-depth resources specific to HT systems, particularly concerning nutrient, irrigation, and disease management. Our research literature review explained this lack of resources as, in part, due to a paucity of relevant research in HT systems. Many of our assumptions of soil processes in HTs—be they nutrient cycling, host-disease complexes, or soil water dynamics—are based on research conducted in open field systems. However, assuming that these foundational principles will hold true in HT environments has proven insufficient to explain the behavior of soil processes and properties observed in HTs and represents a poor mechanistic understanding of soil functioning in HT systems. Unpacking the complex interactions between management practices, climatic drivers, and the cultural and economic factors that influence producer decision-making is needed to inform improved management recommendations. Furthermore, applied research to develop nutrient management tools (e.g., fertilizer recommendations) and soil test interpretation specifically for HT crops appear to be lacking and are an articulated need across all study regions. Further research into HT design modifications such as user-friendly covering and uncovering, moving, or overhead irrigation could support these needs to maintain soil productivity. Finally, when problems arise, producers use novel mitigation strategies that have been unstudied in HT environments, beyond cursory work that establishes efficacy. Research into the effects of select interventions, their effects on soil processes, and the economic viability of these practices should be a priority to support sustained production in these growing systems that need support. Our findings clearly suggest that sustaining HT crop production relies on sustaining HT soil productivity. We suggest bringing this perspective to the forefront of HT management from the time structures are constructed. This is a necessary shift in perspective that will require capacity building among technical service providers and applied research to improve management guidance.
The following recommendations synthesize stakeholder input, research findings, and Extension resources. Recommendations are designed to be constructive rather than prescriptive, providing general guidance to the applied research and technical assistance community working to help producers sustain their HT soil quality.
Recommendation 1: Develop integrated soil nutrient management strategies that address the complexities of HT production
In addressing the need for HT-specific fertilizer recommendations, attuned to the soils and crops grown in a region, recommendations must balance crop nutrient demand and soil organic matter management to minimize nutrient imbalances and salt accumulation. Integrated nutrient management that incorporates soil organic matter management strategies such as compost and cover crop use were found lacking in the available literature and were identified as a high priority for producers. Equally important is the development of nutrient recommendations targeted to the unique fertility needs of HT-grown crops and the unique properties of HT soils. Applied research that incorporates treatments that reflect the interactions of composts, cover crops, and fertilizers and assesses the impacts on fertility, salinity, and organic matter are needed. Further, such applied research must be conducted and interpreted in the context of how HT soils change over time due to management practices and abiotic factors driven by microclimate. Developing Extension resources for HT producers that include HT-specific soil testing packages and guidance for interpreting soil test results, incorporating the use of foliar analyses and in-season fertility management, and selecting amendments suited to HT soils (e.g., fertilizers and composts with low salt indexes) could create a framework for resources that could be updated as emerging research informs improved management recommendations.
Recommendation 2: Develop mitigation strategies for soil-based factors that limit yield and/or result in environmental degradation, including pathogen, salinity, and nutrient management
Currently producers experiencing critical problems in these areas rely largely on general guidance and word-of-mouth from experienced growers. These are a starting point, but research is needed on the effects of soil biological, chemical, and physical properties from common interventions. Specifically, further research on solarization, ASD, soil steaming, and irrigation and fertility strategies to reduce salinity and over application of phosphorus are needed. Accompanying analyses of the economic feasibility of adopting these practices at the farm level are paramount.
Recommendation 3: Improve understanding of the role of water management on soil processes
A clear research need is improved understanding of the spatial variability of the HT environment, both within and between crop rows, and how such variability affects microbial processes critical for nutrient management. This is especially important in organic HT systems that rely on microbes for nutrient availability. Anecdotal information from producers indicates that there are HT production zones that limit moisture to the degree that decomposition of added organic inputs, such as pelleted manure, does not occur. Irrigation spatial variability creates significant horizontal and vertical wet and dry zones specific to HT systems. Further research on the depth and frequency of watering is needed to balance retention of nutrients in the root zone with leaching of salts for sustained productivity. Further, the ability for producers to use sensor-based irrigation strategies more thoroughly in HT environments is needed, as well as a cost–benefit analysis of these technological approaches. Despite lack of clear research guidance, producers should limit extreme dry-down events to facilitate an active microbial population to regulate nutrient cycling.
Recommendation 4: Develop collaborative technical assistance networks to support HT producers at all stages of experience
More can be done to create robust technical assistance networks that integrate Extension, Natural Resource Conservation Service, and nongovernmental entities working with HT producers throughout the life cycle of their tunnels. We recommend a systematic plan to provide follow-up support to HT growers during the first three growing seasons. This assistance must begin with the tunnel installation process and continue for at least the first 3 years of HT production because this is the time period in which our producers and specialists observe that HT soils begin to transition from “field soils” to “HT soils” (e.g., Sideman et al. 2019). Ideally, such support would extend for longer periods, with further assistance in seeking new resources and conducting research to understand long-term soil quality trends. For example, for HTs, a “starting toolkit” that connects funders or manufacturers to Extension and other technical assistance providers is recommended. As new HT modifications and designs are released, cooperation between funders or manufacturers and Extension becomes paramount to ensure producers receive the most up-to-date technical assistance. Furthermore, in-depth and specific applied research and management recommendations for nutrient and pathogen control, as well as peer-to-peer farmer activities, are also recommended.
Sustaining HT production requires positioning soils as the center of sustainable HT system management. A number of production-oriented resources exist for beginning HT producers. However, these tend to focus on HT construction and production basics while soil management recommendations are lacking, beyond general strategies such as soil testing. As HTs mature, adapting to changing (often declining) soil conditions requires a cycle of increasing inputs and management complexity. Both producers interviewed in this study and Extension personnel on our team have articulated that growers lack downstream support in the transition from early to advanced HT production stages, when experienced producers begin navigating soil-based management problems that arise as a function of edaphic and management factors.
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