Fertilizer practices in blackberry (Rubus L. subgenus Rubus Watson) are routinely adjusted based on leaf tissue analysis (Hart et al., 2006). N is the predominant nutrient applied to trailing blackberry, and the best growth and yield are usually achieved with ≈28 to 56 kg·ha−1 N during the establishment year and ≈45 to 84 kg·ha−1 N during the subsequent years in conventional and organic systems (Bushway et al., 2008; Hart et al., 2006; Kuepper et al., 2003). The most common N fertilizers applied to blackberry are calcium nitrate, urea, and ammonium sulfate in conventional systems and OMRI-listed (Organic Materials Review Institute) fish emulsion, pelletized chicken litter, soybean meal, or feather meal in organic systems (Fernandez-Salvador, 2014; Harkins et al., 2013). In blackberry, N must be available in early spring for sufficient primocane growth (Malik et al., 1991; Mohadjer et al., 2001; Naraguma and Clark, 1998). The current recommendation is to divide the total recommended rate of N into equal, “split” applications, generally from April to June in the northern hemisphere (Hart et al., 2006). Fertilizing through the irrigation system (fertigation) is an important tool because it targets the planting row directly and maximizes water and nutrient uptake while minimizing leaching of important nutrients (Gärdenäs et al., 2005). Organic blackberry has been established successfully when fertigating using fish emulsion from spring to midsummer (Harkins et al., 2013).
Blackberry plant nutrient status and fruit quality were affected by N fertilization when using granular, inorganic sources of N (Archbold et al., 1989; Naraguma and Clark, 1998; Nelson and Martin, 1986). In ‘Arapaho’, fertilizer N rate had little effect on yield, berry weight, or primocane number, but leaf N, P, K, Ca, sulfur (S), and manganese (Mn) levels were influenced by N application rate and timing (Naraguma and Clark, 1998). Spiers and Braswell (2002) found that varying rates of N, P, K and magnesium (Mg) influenced leaf macronutrient concentration, where an increased rate of N led to higher leaf N and P and reduced leaf K and Mg. In red raspberry (Rubus idaeus L.), applications of fertilizer N increased leaf N and fruit size but had no effect on yield (Chaplin and Martin, 1980). In organic production, Fernandez-Salvador et al. (2015) found relatively little effect of fertilizer source on yield and quality of hand-harvested blackberry when the fertilizer was applied to the in-row area. Little is known, however, about the influence of fertilizer sources of N that can be injected and fertigated on fruit yield and quality in organic blackberry.
In organic production systems, many factors must be taken into account in nutrient management programs, including availability and release rate of N from the fertilizer, ease of application, and cost. In addition, many nutrients other than N are present in organic fertilizers and are thus applied to the planting whether required or not (Harkins et al., 2014; Larco et al., 2013). Common organic sources of N range from cover crops to green manure for on-farm sources and fish byproducts, vegetable hydrolysate (e.g., corn steep liquor), molasses, animal manures (or manure-derived products), plant and animal byproducts (e.g., plant-based meals such as soybean meal and animal-based meals such as feather, bone, and meat), or mineralized materials (e.g., sodium nitrate or bat and bird guano) for off-farm sources (Gaskell and Smith, 2007; Kuepper, 2003; Mikkelsen and Hartz, 2008; OMRI, 2013; Sideman, 2007). Manure, for example, is a relatively inexpensive and abundant source of N; however, the U.S. Department of Agriculture (USDA) organic regulations only permit manure use with a restriction on preharvest interval (90 d for blackberry; OMRI, 2013). USDA National Organic-Program (NOP) compliant, OMRI-listed liquid fertilizer sources are limited and are believed to clog emitters, reducing fertilizer and water application efficiency (Schwankl and McGourty, 1992).
Liquid fertilizer sources allowed in organic production include fish emulsions and hydrolysates, microbial digestions of vegetable byproducts (e.g., corn steep liquor), molasses, soy-derived amino acids, manure slurries and soluble guanos, and, in some cases, soluble mined minerals such as sodium nitrate and combinations of all of these (OMRI, 2013). Corn steep liquor has been studied as a N source in fertigation studies in tomato (Solanum lycopersicum L.) and muskmelon (Cucumis melo L.) (Nakano et al., 2001, 2003; Nakano and Uehara, 2003), whereas fish emulsion has widely been studied in various vegetables and fruits (Fernandez-Salvador, 2014; Fonte et al., 2009; Harkins et al., 2013; Larco et al., 2013; Strik et al., 2012; Young et al., 2005; Zhao et al., 2007). Corn steep liquor uses a high-volume byproduct of the corn syrup and starch industries and has been reviewed by the U.S. National Organic Standards Board (NOSB) for its compliance and use in organic production (NOSB Crops Committee, 2011). Corn steep liquor is comprised of the solubilized protein components of the corn endosperm and is relatively high in N and other minerals necessary for adequate plant growth (Keller and Heckman, 2006). Fish emulsion is also a readily available N source and is a byproduct of the fishing industry, primarily in Alaska and Mexico (OMRI, 2013). For example, over 1,000,000 t of fish waste is produced in Alaska every year and used for fertilizer or animal feed (Zhang et al., 2007). Schwankl and McGourty (1992) found that spray-dried fish protein and poultry protein performed well in fertigation systems with minimal clogging. Corn steep liquor has not been specifically studied for its performance in drip fertigation systems; however, it is known to be soluble in water (Keller and Heckman, 2006). Corn steep liquor and fish emulsion fertilizers are valued in organic agriculture as rapid-release N sources with no required preharvest application period restriction, which is mandatory for uncomposted or unprocessed animal manures under the USDA NOP.
Management of fertigation through the drip system is important. Less optimal fertigation management can lead to uneven distribution of nutrients and water, a reduction in the lifespan of a fertigation system and/or increase in maintenance costs, a reduction in nutrient availability, and may reduce yield (Hanson et al., 2006). It is thus important for organic growers to have information regarding fertilizer options suited for use in fertigation systems.
The objectives of this study were to 1) determine the impact of two organically approved liquid fertilizer sources applied through fertigation on plant growth, yield, fruit quality, and soil and plant tissue nutrient status of ‘Marion’ and ‘Black Diamond’ blackberry grown in an organic production system; and 2) assess the impact of fertilizer source on the performance of the drip irrigation/fertigation system. ‘Marion’ and ‘Black Diamond’ are predominantly harvested by machine for high-value processed markets and together accounted for greater than 75% of the 2914 ha of blackberries produced in Oregon in 2012 (National Agricultural Statistical Service, 2013). Like all trailing types, the plant crowns and roots are perennial, but the shoots are biennial, producing primocanes the first year, which then become floricanes with flowers and fruit the next year and senesce after harvest. Mature plants will have both primocanes and floricanes in the same year in a typical annual or every-year production system (Julian et al., 2009; Strik and Finn, 2012).
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