A study was conducted in western Oregon to assess the impact of cultivar and weed management strategy on accumulation and loss of plant biomass and nutrients during the first 3 years of establishment when using organic fertilizer. The study was conducted in trailing blackberry (Rubus L. subgenus Rubus Watson) planted in May 2010 and certified organic in May 2012. Treatments included two cultivars, Marion and Black Diamond, each with either no weed control after the first year after planting or with weeds managed by hand-weeding or the use of weed mat. Each treatment was amended with organically approved fertilizers at pre-plant and was drip-fertigated with fish emulsion each spring. Most primocane leaf nutrient concentrations were within the range recommended for blackberry. However, leaf nitrogen (N) was low in ‘Black Diamond’, especially when grown without weed control, whereas leaf boron (B) was low in all treatments. In many cases, leaf nutrient concentrations were affected by cultivar and weed management in both the primocanes and the floricanes. The concentration of several nutrients in the fruit differed between cultivars, including calcium (Ca), magnesium (Mg), sulfur (S), B, and zinc (Zn), but only fruit Ca was affected by weed management and only in ‘Marion’. In this case, fruit Ca was higher when the cultivar was grown with weed mat than with hand-weeding or no weeding. Total biomass production of primocanes increased from an average of 0.3 t·ha−1 dry weight (DW) during the first year after planting to 2.0 t·ha−1 DW the next year. Plants were first cropped the third year after planting and gained an additional 3.3 t·ha−1 DW in total aboveground biomass (primocanes, floricanes, and fruit) by the end of the third season. Fruit DW averaged 1.4 t·ha−1 in non-weeded plots, 1.9 t·ha−1 in hand-weeded plots, and 2.3 t·ha−1 in weed mat plots. Biomass of senesced floricanes (removed after harvest) averaged 3.2 t·ha−1 DW and was similar between cultivars and among the weed management treatments. ‘Marion’ primocanes accumulated a higher content of N, phosphorus (P), potassium (K), Mg, S, iron (Fe), B, copper (Cu), and aluminum (Al) than in ‘Black Diamond’. Weeds, however, reduced nutrient accumulation in the primocanes in both cultivars, and accumulation of nutrients was greater in the floricanes than in the previous year’s primocanes. Total nutrient content declined from June to August in the floricanes, primarily through fruit removal at harvest and senescence of the floricanes after harvest. Depending on the cultivar and weed management strategy, nutrient loss from the fruit and floricanes averaged 34 to 79 kg·ha−1 of N, 5 to 12 kg·ha−1 of P, 36 to 84 kg·ha−1 of K, 23 to 61 kg·ha−1 of Ca, 5 to 15 kg·ha−1 of Mg, 2 to 5 kg·ha−1 of S, 380 to 810 g·ha−1 of Fe, 70 to 300 g·ha−1 of B, 15 to 36 g·ha−1 of Cu, 610 to 1350 g·ha−1 of manganese (Mn), 10 to 260 g·ha−1 of Zn, and 410 to 950 g·ha−1 of Al. Overall, plants generally accumulated (and lost) the most biomass and nutrients with weed mat and the least with no weed control.
Nitrogen and Phosphorus Resorption in Planted Forests Worldwide