Like many crops, fertilizer practices in blueberry (Vaccinium sp.) are routinely adjusted by comparing the results of leaf nutrient analysis at a standard time against the known optimal ranges of leaf nutrient concentrations. Effective fertilizer management, however, also requires a good understanding of plant nutritional demands both in terms of the nutrient amount (Santos, 2011) and the timing in which each nutrient is most needed (Mattson and van Iersel, 2011). Biomass determination through sequential plant excavation, coupled with nutrient analysis of each tissue type, is presently the most reliable way to obtain the amounts and seasonal patterns of plant nutrient uptake (Weinbaum et al., 2001). Nutrient analysis of entire plants at multiple times during annual cycles of growth and development is difficult and expensive, and only N has been examined in detail in highbush blueberry (Bañados et al., 2012; Hanson and Retamales, 1992; Retamales and Hanson, 1989; Throop and Hanson, 1997).
Nitrogen is the predominant nutrient applied to blueberry for successful commercial growth and production. Although the blueberry plant is relatively small and slow-growing compared with many temperate fruit tree crops, the amount of N fertilizer applied to the crop each year is comparable (Stiles and Reid, 1991). Typical rates in Oregon, for example, average 50 to 100 kg·ha−1 of N per year during planting establishment and 100 to 300 kg·ha−1 of N per year once the field matures. Other nutrients are also applied, largely based on soil tests and general recommendations from plant and soil testing laboratories. Hart et al. (2006) developed more stringent guidelines for nutrient management of blueberry based on leaf tissue and soil analysis. However, with the exception of N, the defined ranges of nutrient sufficiency were based on experience and not on controlled studies of each nutrient.
Nutrient requirements in perennial fruit crops such as blueberry depend on new biomass production in vegetative and reproductive tissues, the nutrients needed for production of the new tissue, and the amount of nutrients reallocated from existing plant tissues. Mature northern highbush blueberry plants produce most new shoots and leaves in the spring and early summer, before or during fruit development, and most new roots in early spring, before budbreak, and mid- to late summer after fruit harvest (Abbott and Gough, 1987). Most N is acquired during shoot and fruit development (Throop and Hanson, 1997), and therefore split applications of granular N fertilizer are recommended in the spring (April to June in the northern hemisphere) for blueberry (Hart et al., 2006). Reallocation of nutrients in woody plants occurs internally, especially in early spring from storage tissues such as the crown and woody stems and in the fall from senescing leaves (Mohadjer et al., 2001; Rempel et al., 2004; Strik et al., 2004) and externally from decomposition of plant tissues such as senesced leaves and roots and pruned wood (Strik et al., 2006).
We previously found that 50 kg·ha−1 N per year promoted more growth and yield than no N fertilizer during establishment of highbush blueberry, whereas rates 100 kg·ha−1 N or greater were excessive and resulted in salt stress and plant mortality in the young planting (Bañados et al., 2012). Through destructive harvests and the use of depleted 15N fertilizer, we estimated that unfertilized plants gained 1.6 g/plant of N from soil sources, whereas fertilized plants accumulated 2.3 g/plant of N, on average, 60% of which was from fertilizer N and 40% was from the soil. The objectives of the present study were to determine the requirements of N and other macro- and micronutrients in the young plants and to examine how the requirements were affected by the amount of N fertilizer applied. Plants were excavated, separated into relevant plant parts, and analyzed for nutrients on six key dates from leaf budbreak immediately after planting to the first fruit harvest the next year.
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