Organic blackberry (Rubus L. subgenus Rubus, Watson) production is becoming an important niche market in Oregon, where almost 50% of the U.S. hectarage (organic or conventional) is located [U.S. Department of Agriculture (USDA), 2014]. Oregon primarily grows trailing types used for the processed market, and nearly all of the fruit is harvested by machine (Strik and Finn, 2012; USDA, 2014). Although there is an increasing body of knowledge about organic blackberry production (Fernandez-Salvador et al., 2015a, 2015b; Harkins et al., 2013, 2014; Kuepper et al., 2003), some gaps still remain, such as how cultivars other than Marion, which was used to develop the Oregon caneberry nutrient management guide (Hart et al., 2006), use and allocate nutrients.
Floricane-fruiting blackberry plants have perennial crowns and roots, with biennial aboveground canes. In the spring, vegetative primocanes emerge and grow throughout the summer until the autumn, when they go dormant and overwinter. The following spring they are called floricanes and produce lateral shoots (fruiting laterals), which flower and produce fruit. Later in the summer, the floricanes senesce and are removed from the plant. In an annual or every-year production system (Strik and Finn, 2012), primocanes are growing simultaneously with the floricanes and during fruit production. Because of this unique growth habit, the nutrient status of the vegetative and reproductive plant parts may be quite different as plants could be allocating resources independently to the different cane types (Malik et al., 1991; Mohadjer et al., 2001; Naraguma et al., 1999; Whitney, 1982). For example, primocanes primarily acquire nutrients from the soil, so adequate fertilization during primocane growth is important (Malik et al., 1991; Mohadjer et al., 2001; Naraguma et al., 1999; Whitney, 1982). Floricanes, on the other hand, rely on stored nutrients during early fruiting lateral growth and fruit production (Malik et al., 1991; Mohadjer et al., 2001; Naraguma et al., 1999; Whitney, 1982).
Primocanes are typically trained onto a wire trellis to facilitate management and harvest, usually in late summer or late winter (Strik and Finn, 2012). Summer training of the primocanes has been shown to increase light exposure and flower bud initiation and therefore yield in some cases (Bell et al., 1995). However, in other cases, training time did not affect yield or even decreased yield when primocane training in August was followed by a particularly cold winter (Bell et al., 1992; Dixon et al., 2015).
Weed control is one of the most difficult management problems in organic production because there are few Organic Materials Review Institute (OMRI)-listed products and labor for hand-weeding can be expensive. Some blackberry growers allow weeds to grow in the row (B.C. Strik, personal observation), although this has been shown to be detrimental to blackberry plant growth and yield (Dixon et al., 2015; Harkins et al., 2013; Meyers et al., 2014). Weed mat, or porous landscape fabric, has been used successfully to manage weeds in various production systems (Dixon et al., 2015; Harkins et al., 2013; Makus, 2011; Meyers et al., 2014). It is particularly well suited to trailing blackberry because, unlike many other types of caneberry, trailing types only produce canes from the crown of the plant, so only a small hole in the weed mat is needed for the plant.
Withholding irrigation after harvest has been shown to have little effect on blackberry plant growth and fruit production in Oregon, which has a Mediterranean climate with relatively dry summers and continuous summer irrigation is typical (Dixon et al., 2015; U.S. Department of the Interior, 2013). Blackberry can be irrigated by a variety of methods, from drip to overhead sprinklers, depending on the desired market (Strik and Finn, 2012). However, drip irrigation is usually used in organic production systems because it has the potential to decrease weeds outside of the drip zone, reduce disease presence in the canopy, and can be used to apply fertilizers (fertigation). Fertigation has been used effectively with OMRI-listed materials in blackberry (Dixon et al., 2015; Fernandez-Salvador et al., 2015a; Harkins et al., 2013).
Soil in the Willamette Valley, where most Oregon blackberries are grown, tends to be sufficient in P, but N, K, and B frequently need to be applied to sustain good growth (Hart et al., 2006). Organic fertilizers are often applied for a certain N rate [55 to 80 kg·ha−1 for mature blackberry (Hart et al., 2006)], but unlike many conventional fertilizers, they contain varying levels of other macro- and micronutrients. The effect of organic fertilizers on blackberry growth and soil properties was studied by Harkins et al. (2013) during establishment and by Fernandez-Salvador et al. (2015a, 2015b), although effects over a longer time period and combined with other management practices are still unknown.
The objective of this study was to evaluate several production practices (cultivar, weed management, training time, and irrigation) for their effect on the nutrient status of primocane and fruiting lateral leaves, and fruit as well as on soil pH, organic matter, and nutrients in a mature, organic planting of trailing blackberry. The planting was machine harvested for the processed market. Marion and Black Diamond were the cultivars used, along with nonweeded, hand-weeded, and weed mat management strategies, August and February primocane training times, and two irrigation strategies (continuous summer irrigation and no irrigation after fruit harvest).
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