Certified organic blueberry (Vaccinium sp.) production has increased rapidly in the United States from an estimated 194 ha in 2003 to 1665 ha (40% of the total worldwide) in 2011 (Strik, unpublished data). The main challenges to such rapid expansion include greater production costs or input, limited options for disease or pest control, and reduced yields in organic plantings (Julian et al., 2011a). Organic crops usually command higher prices than those produced conventionally, but higher prices may not always outweigh added costs associated with more expensive agricultural inputs.
Most new blueberry fields, including those used for organic production, are planted on raised beds (Strik, 2007). Raised beds improve soil drainage and help protect susceptible cultivars from developing problems with phytophthora root rot (Bryla et al., 2008; Scherm and Krewer, 2008). Two cultivars of northern highbush blueberry, ‘Duke’ and ‘Liberty’, had more growth and fruit production on raised beds than on flat ground in an organic field in Oregon (Larco et al., 2013). Both of these cultivars are popular choices commercially and are commonly used for organic production in the northwestern United States (Julian et al., 2011a).
Like many crops, fertilizer practices in blueberry are routinely adjusted based on leaf tissue analysis. Nitrogen is the predominant nutrient applied to blueberry, and the best growth and yield is usually achieved with ≈25 to 100 kg·ha−1 N in conventional systems (Bañados et al., 2012; Chandler and Mason, 1942; Eck, 1988; Griggs and Rollins, 1947; Hanson, 2006; Hart et al., 2006). The most common N fertilizers applied to blueberry are ammonium sulfate and urea in conventional systems and Organic Materials Review Institute-approved fish emulsion or feather meal in organic systems. Most species of blueberry are adapted to low soil pH conditions in the range of 4.5 to 5.5 and require NH4-N over NO3-N for uptake. Nitrogen uptake increases throughout the growing season in young plants but primarily occurs during shoot and early fruit development in the spring in mature plants (Bañados et al., 2012; Retamales and Hanson, 1989; Throop and Hanson, 1997). The current recommendation is to apply N in split applications in the spring when using granular fertilizers (April to June in the northern hemisphere; Hart et al., 2006) or to inject it gradually through the irrigation system (“fertigation”) from spring to midsummer when using drip and liquid fertilizers (Bryla and Machado, 2011). Many organic blueberry farmers commonly apply liquid fish emulsion to the soil directly, however, especially in young plantings. Granular feather meal products are also applied directly to the in-row area. Many nutrients other than N are present in organic fertilizers and are thus applied to the planting whether required or not. The impact of using organic fertilizers on plant and soil nutrient levels is largely unknown in most crops, including blueberry.
Compost is also used by many organic blueberry growers to increase soil organic matter content and provide additional nutrients. Depending on the feedstock, compost supplies 3% to 10% of the total N during decomposition for several years after application (Gale et al., 2006; Sikora and Szmidt, 2001). Burkhard et al. (2009) found greater growth and yield in blueberry when using seafood- or manure-based composts. Larco et al. (2013) also reported better growth and yield in blueberry when using yard-debris compost covered with sawdust mulch than when using sawdust mulch alone.
Sawdust mulch is often used for weed control as well as other benefits in blueberry such as reducing soil water loss and increasing soil organic matter content (Burkhard et al., 2009; Clark and Moore, 1991; Karp et al., 2006; Kozinski, 2006; Krewer et al., 2009; Savage, 1942; Sciarappa et al., 2008; White, 2006). Sawdust is becoming expensive in many growing regions (Julian et al., 2011a, 2011b). It also has a very high carbon (C)-to-N ratio, and therefore, as it decomposes, it tends to immobilize N applied from fertilizers (White, 2006). Hart et al. (2006) recommend applying an additional 25 kg·ha−1 N for an 8-cm deep layer of fresh sawdust applied. Although the extra fertilizer is a relatively minor expense in conventional systems, it is costly and often more difficult to apply when using organic products.
Many organic growers are using weed mat as an alternative to sawdust. Weed mat is a woven, polyethylene landscape fabric or groundcover, which is usually black in color and permeable to water. Weed mat reduces weed management costs and results in a higher early yield than sawdust in blueberry (Julian et al., 2012; Krewer et al., 2009; Larco et al., 2013; Sciarappa et al., 2008). The material also likely affects availability and uptake of nutrients differently than sawdust and other organic mulches as a result of its numerous impacts on soil properties. For example, soil temperature is typically higher during the day with (black) weed mat, water infiltration is often lower, and blueberry root growth is reduced (Cox, 2009; Larco, 2010; Larco et al., 2013; Neilsen et al., 2003). Most new fields are irrigated by drip when using weed mat.
The objectives of the present study were to evaluate the effects of bed type, organic fertilizers, and different mulches on availability of soil nutrients and plant nutrient status in a new organic planting of highbush blueberry. The study included ‘Duke’ and ‘Liberty’ blueberry and was conducted during the first 2 years after establishment.
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