Response of Highbush Blueberry to Nitrogen Fertilizer during Field Establishment—II. Plant Nutrient Requirements in Relation to Nitrogen Fertilizer Supply

A study was done to determine the macroand micronutrient requirements of young northern highbush blueberry plants (Vaccinium corymbosum L. ‘Bluecrop’) during the first 2 years of establishment and to examine how these requirements were affected by the amount of nitrogen (N) fertilizer applied. The plants were spaced 1.2 · 3.0 m apart and fertilized with 0, 50, or 100 kg ha of N, 35 kg ha of phosphorus (P), and 66 kg ha of potassium (K) each spring. A light fruit crop was harvested during the second year after planting. Plants were excavated and parts sampled for complete nutrient analysis at six key stages of development, from leaf budbreak after planting to fruit harvest the next year. The concentration of several nutrients in the leaves, including N, P, calcium (Ca), sulfur (S), and manganese (Mn), increased with N fertilizer application, whereas leaf boron (B) concentration decreased. In most cases, the concentration of nutrients was within or above the range considered normal for mature blueberry plants, although leaf N was below normal in plants grown without fertilizer in Year 1, and leaf B was below normal in plants fertilized with 50 or 100 kg ha N in Year 2. Plants fertilized with 50 kg ha N were largest, producing 22% to 32% more dry weight (DW) the first season and 78% to 90% more DW the second season than unfertilized plants or plants fertilized with 100 kg ha N. Most DW accumulated in new shoots, leaves, and roots in both years as well as in fruit the second year. New shoot and leaf DW was much greater each year when plants were fertilized with 50 or 100 kg ha N, whereas root DW was only greater at fruit harvest and only when 50 kg ha N was applied. Application of 50 kg ha N also increased DW of woody stems by fruit harvest, but neither 50 nor 100 kg ha N had a significant effect on crown, flower, or fruit DW. Depending on treatment, plants lost 16% to 29% of total biomass at leaf abscission, 3% to 16% when pruned in winter, and 13% to 32% at fruit harvest. The content of most nutrients in the plant followed the same patterns of accumulation and loss as plant DW. However, unlike DW, magnesium (Mg), iron (Fe), and zinc (Zn) content in new shoots and leaves was similar among N treatments the first year, and N fertilizer increased N and S content in woody stems much earlier than it increased biomass of the stems. Likewise, N, P, S, and Zn content in the crown were greater at times when N fertilizer was applied, whereas K and Ca content were sometimes lower. Overall, plants fertilized with 50 kg ha N produced the most growth and, from planting to first fruit harvest, required 34.8 kg ha N, 2.3 kg ha P, 12.5 kg ha K, 8.4 kg ha Ca, 3.8 kg ha Mg, 5.9 kg ha S, 295 g ha Fe, 40 g ha B, 23 g ha copper (Cu), 1273 g ha Mn, and 65 g ha Zn. Thus, of the total amount of fertilizer applied over 2 years, only 21% of the N, 3% of the P, and 9% of the K were used by plants during establishment. 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 of N per year during planting establishment and 100 to 300 kg ha 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 midto 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 N per year promoted more growth and yield than no N fertilizer during establishment of highbush blueberry, whereas rates 100 kg ha N or greater were excessive and resulted in salt stress and plant mortality in the young planting (Bañados et al., 2012). Through destructive HORTSCIENCE VOL. 47(7) JULY 2012 917 harvests and the use of depleted N 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 macroand 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. Materials and Methods Two-year-old ‘Bluecrop’ blueberry plants were obtained from a commercial nursery (Fall Creek Farm & Nursery, Lowell, OR) and transplanted on 27 Mar. 2002 to a field located at the North Willamette Research and Extension Center in Aurora, OR. The plants were delivered from the nursery in 4-L pots filled with a mix of peat, bark, and pumice. Both roots and the soil mix were placed in the planting hole and packed firmly in place with surrounding soil. Soil at the site was a Quantama fine loam (mixed, mesic Aquatic Haploxeralfs) with a pH 5.0 and 3.0% organic matter content. The soil was amended before planting with a 10-cm deep layer of Douglas fir (Pseudotsuga menziesii Franco) sawdust and 66 kg ha of N [21N–0P–0K (NH4)2SO4]. The sawdust and fertilizer was incorporated 0.2 m deep by cultivation in 1-m wide strips to form five 50-m-long rows of planting beds. Plants were spaced 1.2 m apart within rows and 3.0 m apart between rows (equal to a density of 2778 plants/ha). Grass alleyways (1.1 m wide) were planted and maintained between the rows. The field was irrigated as needed ( 25 to 38 mm/week) from May to September each year using overhead sprinklers. Three N fertilizer treatments were applied to plants during the first 2 years after planting. The treatments were arranged in a randomized complete block design with three replicates per treatment. Each treatment plot consisted of one row of 12 plants. The N fertilizer used was granular ammonium sulfate (21N–0P–0K). Three equal applications of the fertilizer were sprinkled around the base of the plants each spring at a total rate equivalent to 0 (no additional fertilizer N), 50, or 100 kg ha N per year. A fourth set of plants was fertilized at a rate of 150 kg ha N but was not used in the present study as a result of severe problems in the treatment with salt injury and plant death (Bañados et al., 2012). The fertilizer was applied on 11 Apr., 20 May, and 27 June in 2002 (Year 1) and on 24 Mar., 8 Apr., and 23 June in 2003 (Year 2). Each plant was also fertilized with 35 kg ha P and 66 kg ha K each spring. Plants were winter-pruned in Feb. 2003 and lightly cropped the second year after planting. One randomly selected plant from each treatment plot was excavated and sampled Fig. 1. Leaf (A) nitrogen (N), (B) phosphorus (P), (C) calcium (Ca), (D) magnesium (Mg), (E) sulfur (S), (F) manganese (Mn), (G) boron (B), and (H) iron (Fe) concentrations in response to N fertilizer rate in ‘Bluecrop’ blueberry during the first and the second year after planting. The plants were fertilized each spring with 0, 50, or 100 kg ha N and leaves were collected 24 July 2002 (Year 1) and 25 July 2003 (Year 2). Results from individual and combined analysis of variance are inset in each graph: NS, *, ** = nonsignificant and significant at P # 0.05 and 0.01, respectively. Each symbol represents the mean of three replicates and error bars represent ± 1 SE. Received for publication 27 Feb. 2012. Accepted for publication 14 May 2012. We thank G. Buller, H. Rempel, and M. Resendes for technical support and acknowledge financial support from the Oregon Blueberry Commission and the Northwest Center for Small Fruits Re-