Demand for local fruits and vegetables is growing steadily in the United States, and many consumers are willing to pay a premium for local food (Feldmann and Hamm, 2015). Unfortunately, local farmers are struggling to meet this growing demand due to gaps in technical knowledge about specialty crop production systems that are new or potentially unique to a given agroecoregion. Given the small-scale and adverse climatic conditions of midwestern United States specialty crop farms, novel production systems are often needed to make local fruit and vegetable production feasible.
Strawberries, the fifth most popular fresh fruit in the United States [U.S. Department of Agriculture (USDA), 2012], typically are produced in California, but open-field strawberry production systems used in California are not ideal for the midwestern United States. Two major challenges of open-field strawberry production in the midwestern United States are pest management and the relatively short growing season. However, both of these challenges can be at least partially addressed by growing strawberries in a hydroponic system within high tunnels. Hydroponic production, through the use of nutrient solution or soilless substrates as growing media, eliminates weed management and soil pathogen concerns that typically plague strawberry (LaMondia et al., 2002). High tunnels extend the growing season in the midwestern United States allowing farmers to achieve earlier and more abundant strawberry yields (Knewtson et al., 2010). Vertical, hydroponic production systems used within high tunnels may help maximize the productivity and profitability of this valuable growing space (Karakoudas et al., 1998). While promising, vertical, hydroponic strawberry production in high tunnels combines two relatively new production systems; thus, growers have many practical management questions about the best cultivars and cultural practices before adopting this new system.
Many strawberry cultivars are available to farmers, but the majority of these cultivars were developed under soil-based field conditions in traditional strawberry production regions (e.g., California and Florida). Anagnostou and Vasilakakis (1995) demonstrated significant variability in yield and quality between strawberry cultivars grown in a vertical, hydroponic system, yet there are few published reports comparing modern strawberry cultivars in hydroponic culture (Miranda et al., 2014). Moreover, when hydroponic systems are used in high tunnels, cultivar performance will be further influenced by local climatic conditions and the unique microclimatic properties of high tunnel environments (Wien, 2009). Thus, studies and cultivar recommendations are needed for individual agroecoregions where there is farmer interest in this production system.
Vertical, hydroponic systems require the use of a soilless growing media, but there are many types of media available and each has unique physical and chemical properties. Moreover, media can be mixed in various ratios to achieve multiple benefits. Caso et al. (2009) compared various mixtures of rice husks, pumice, and sand, but the greatest yields were observed in 100% rice husks. In contrast, other studies have found that a mixture of perlite (60% to 80%) and peat (20% to 40%) maximizes growth and yield of strawberry (Anagnostou and Vasilakakis, 1995; Linardakis and Manios, 1991). While these previous studies demonstrated the value of peat in a soilless mix with perlite, coconut coir may be a sustainable alternative to peat. Peat is harvested from wetland ecosystems, often at unsustainable rates, whereas coconut coir is derived from the mesocarp of coconut fruit (a renewable resource) and has many physical and chemical properties similar to peat (Meerow, 1994).
Many local fruit and vegetable growers are certified organic, and there is increasing interest in organically certified hydroponic systems. In response to this interest, the USDA National Organic Program (NOP) recently appointed a “Hydroponic and Aquaponic Task Force” to assess the compatibility of hydroponic and aquaponic practices with current USDA organic regulations (e-CFR, 2016). Despite consumer demand and farmer interest in organic hydroponic production (Atkin and Nichols, 2004), there has been limited research on the feasibility of using bio-based liquid nutrient solutions currently or potentially allowed under USDA organic regulations to grow vegetables or fruit, including strawberries, in hydroponic culture. Some of the potential challenges of growing organic hydroponic crops include low nitrate to ammonium ratio, alkaline pH, and low dissolved oxygen content of bio-based nutrient solutions (Jewell and Kubota, 2005). Moreover, bio-based nutrients must be mineralized via microbial communities, which can vary greatly among soilless media (Grunert et al., 2016).
Atkin and Nichols (2004) demonstrated the possibility of growing lettuce (Lactuca sativa) with organic nutrient solution in a nutrient film technique hydroponic system, but yield was between two and four times greater in conventional nutrient solution. Given the chemical limitations of organic nutrient solutions, Gül et al. (2007) tested the idea of “charging” soilless media with solid poultry manure to supplement regular fertigation with a solution of dissolved poultry manure. Compared with the inorganic nutrient solution, cucumber (Cucumis sativus) yield was reduced by only 11% when fertilized with a combination of solid and dissolved poultry manure (Gül et al., 2007). Thus, organic hydroponic production of strawberries may benefit from a combination of liquid and solid nutrient sources.
The objectives of this study were to 1) identify the best cultivars and growing media for vertical, hydroponic, and high tunnel production of strawberries in the midwestern United States and to 2) assess potential strategies for replacing synthetic fertilizer with organic nutrient sources in hydroponic strawberry production.
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