Response of Blackberry Cultivars to Fertilizer Source in an Organic Fresh Market Production System

Blackberry (Rubus L. subgenus Rubus Watson) cultivars, three trailing types (‘Marion’, ‘Black Diamond’, and ‘Obsidian’) and one semi-erect type (‘Triple Crown’), were studied from 2011-2012 at a certified organic, grower site located in Jefferson, OR. Plants were fertilized with 56 kg·ha^–1 N each spring using three different sources: 1) a liquid fish and molasses blend (4–0–2); 2) pelletized soybean meal (8–1–2); and 3) pelletized, processed poultry litter (4–3–3). Plants were drip irrigated, and weeds were managed using a polypropylene, permeable landscape fabric. Marketable yield differed significantly between years, and total yield, berry weight, firmness, and percent soluble solids were affected significantly by year x cultivar. ‘Triple Crown’ produced the greatest yield in both years, whereas ‘Black Diamond’ and ‘Marion’ had the lowest yield in 2011 and 2012, respectively. ‘Triple Crown’ fruit had the highest percent soluble solids and were the least firm in 2011, while ‘Marion’ fruit were the least firm in 2012. The effect of fertilizer source on yield and fruit quality was relatively small and was inconsistent among cultivars and years. Soy meal produced the greatest total yield in ‘Triple Crown’ in 2011 and in ‘Obsidian’ in both years. In contrast, poultry and fish produced the greatest total yield in ‘Black Diamond’ in 2011, and fish produced the greatest yield in ‘Black Diamond’ and ‘Marion’ in 2012. Fruit weight differed more among the four cultivars, particularly when plants were fertilized with soy than with poultry or fish in 2011, but was not significantly different among the three fertilizers within each cultivar in either year. Most soil nutrient levels were within the recommended range for all fertilizer treatments, except for B, which declined to deficient levels in the second year. Fertilizer source had no effect on soil nutrient levels other than fertilization with fish increased soil K and Na. Primocane leaf tissue nutrient concentrations were above or within recommended standards for most nutrients, except for Mg, Ca, and B, which, depending on the cultivar, were below standards. Over the 2-year study, the cultivars responded similarly to the three sources of organic fertilizer. The cost per kg N, however, varied from $18.00 for the liquid fish and molasses blend, $11.80 for the soybean meal, and $5.60 for the poultry litter. Supplemental fertilization with B, Mg, and Ca would be required with each fertilizer studied to maintain recommended soil fertility levels.