In 2014, the total value for strawberries reached nearly $2.8 billion in the United States, ≈$2.6 billion of which came from fresh-market berries [U.S. Department of Agriculture (USDA), 2015]. Similar to other crops, there has been a growing demand for organic strawberries with a reported farm gate price premium ≈55% for organic strawberries between 2007 and 2012 (Carroll et al., 2012). Although there are a variety of studies examining sustainable and organic practices for strawberry production (Reganold et al., 2010; Werner et al., 1990), much of this research is focused in California and may not be applicable to the challenging environmental conditions in North Carolina (NC) or the southeastern United States (SE). North Carolina is one of the top fresh-market strawberry-producing states (North Carolina Department of Agriculture Agricultural Statistics, 2012) in the SE where strawberries are usually produced on small- to midsized family farms (Sydorovych et al., 2006). The focus on direct sales and pick-your-own operations in NC has resulted in strawberry production systems located in the same fields year after year (Poling, 1993). Moreover, the warm climate, high humidity, poor fertility, and acidic soils in NC exacerbate soil-borne pathogen and weed problems, especially when strawberry production is not rotated between fields. The combination of market forces, lack of rotation, and inherent pest pressures has resulted in strawberry production systems that depended on previous methyl bromide fumigation and now other fumigation strategies to control pests in NC. Furthermore, these chemically based controls are not applicable to organic production. There is a critical need for research on soil and pest management strategies relevant for organic producers, those interested in transitioning to organic certification, and those seeking alternatives to fumigation practices that improve the long-term viability of strawberry production systems in NC and the SE.
Biologically based soil management practices, such as cover crop rotations, additions of compost and vermicompost are important practices in organic systems, but may also serve as important transitions from fumigation in conventional strawberry systems. These soil management practices can increase organic matter additions, soil fertility, and enhance beneficial soil organisms, potentially reducing the amount of synthetic fertilizer inputs in conventional systems. As fumigation and pesticide restrictions increase, researches on practices that are cost-effective and reduce environmental impacts are critical for the future of strawberry production in the SE and beyond.
Although cover crop rotations are common cultural practices in organic crop production that provide numerous benefits, including increased nitrogen fertility, retention of nutrients, reduced erosion and runoff, improved soil physical properties, building soil organic matter, suppressions of pest populations, and weed control (Creamer and Baldwin, 2000; Snapp et al., 2005), they are underused in strawberry production. Moreover in NC, growers often replant strawberry crops on the same site, leaving inadequate time for an intervening cash crop, and while summer cover crops are recommended (Poling et al., 2005), they are not commonly implemented. Although planting a summer cover crop between the final harvest in mid-June and soil preparation starting early September is feasible in NC, this may not provide adequate time to maximize cover crop biomass and subsequent yield benefits to strawberry plants. Garland et al. (2011) suggested summer cover crop growth in NC strawberry production may be enhanced through increased seeding rate and compost additions when cover crops are planted; yet, research is lacking to provide growers any practical recommendations.
Although compost and cover crops may improve soil quality in strawberry agroecosystems, adding vermicompost provides an opportunity to augment the soil ecosystem with an array of beneficial organisms that are often eliminated through chemical fumigation (Werner et al., 1990) and even biofumigation practices (Owen et al., 2010). Vermicompost is the stabilized product of the interaction between earthworms and soil microorganisms in a nonthermophilic process, resulting in a material with high porosity, aeration, drainage, water-holding capacity, and microbial activity (Arancon et al., 2006; Singh et al., 2008). Vermicompost applications to strawberry crops can enhance microbial populations, including bacteria, fungi, and actinomycetes (Arancon et al., 2004). Several studies have demonstrated vermicompost can improve strawberry growth, including leaf area, shoot biomass, number of flowers and runners (Arancon et al., 2004), and yield (Arancon et al., 2004; Singh et al., 2008), although the mechanisms for this are unclear. Moreover, vermicompost applications to strawberry have increased microbial biomass N (Arancon et al., 2006) and protected fruit marketability through reduction in physiological disorders and fruit disease, such as botrytis rot [Botrytis cinerea (Singh et al., 2008)]. Well-designed cover crop rotations that increase soil organic matter along with vermicompost applications may help reduce fertilizer applications while also improving the below-ground environment for the soil microbial community; yet, little is understood concerning the potential interactions of these practices in strawberry production.
The primary objective of this study was to examine the effects of six summer cover crop treatments including pearl millet, soybean, cowpea, pearl millet/soybean combination, pearl millet/cowpea combination, and a no cover crop control, vermicompost additions, and their interactions on strawberry growth and yields in a 2-year field study in NC. The cover crop species and combinations examined in this study were previously found to be easily integrated into NC strawberry production (Garland et al., 2011). Compost was additionally added at cover crop seeding to augment cover crop growth and reduce preplant fertility. An additional objective of this study examined the effects of these treatments on available soil N throughout the strawberry season. We hypothesize that cover crops (with composts) and vermicompost would increase strawberry growth and yields and cover crop treatments, primarily those with legumes, would increase soil N during strawberry production. This study has implications for organic strawberry production, those in transition to organic production, as well as practices for conventionally managed strawberry production in NC and in the SE regions with similar soil, climatic, and pest conditions.
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