- The effect of planting density and nitrogen (N) fertilization on growth, yield, and N partitioning in young and mature 'Bluecrop' blueberry plants was studied over a two year period. Depleted 15N-ammonium sulfate was applied at different rates and on different dates in a mature planting, and at different rates in a young, newly established planting during the first year of study (2002). Non-labeled fertilizer was applied the second year (2003). Three rates of N fertilizer (0, 100, and 200 kgha1 of N) in
combination with two in-row spacing treatments (0.45 m and 1.2 m) were studied in the mature planting. In addition, three different dates of application of labeled fertilizer at the same rate was also tested. In a young planting, four N fertilizer rates (0, 50, 100 and 150 kgha^-1 of N) were applied in the establishment year. In all studies, the N fertilizer was divided into three equal portions and applied from April through June. Plants were destructively harvested from the field and divided into parts on 6 to 11 dates from Feb. 2002 to Jan. 2004, depending on experiment. Plant parts were analyzed for dry weight (DW), N, and '5N concentration (%) and nitrogen derived from the fertilizer (NDFF) calculated. Shoots on mature plants were divided into small (S), medium (M), large (L) and extra large (XL) categories, based on length, and the effect of N and plant spacing on the number, DW, and flushes of growth characterized. The number of shoots per plant ranged from 249 to 298 with plants spaced at 1.2 m having more shoots than those at 0.45 m. Fifty percent of the shoots in the plant were S, whereas only 8% were XL. Nitrogen rate did not affect shoot number, but higher rates of N did increase shoot biomass and the proportion of XL and S shoots. One to four flushes of growth per shoot were recorded, with the number of flushes dependent upon shoot size; 60 to 80% of S shoots had only one flush of growth compared to 8 to 12% of XL shoots. Eighty percent of total shoot biomass was in the first flush of growth and 20% in the second or later flushes with no effect of in-row spacing or N rate. Yield per plant was 30 to 80% greater at 1.2 m than at 0.45 m. However, yield per hectare was 30 to 140% higher in plants at 0.45 m than those at 1.2 m. The roots and crown were the heaviest organs, whereas roots and leaves contained the most nitrogen. Percent biomass partitioning was affected by sampling date
for all plant parts, and by in-row spacing only for the crown and three-year-old wood. In the mature planting, total plant DW was affected by sampling date, in-row spacing, and N fertilization rate. Plants at the 1.2 m in-row spacing had 32% more DW over time than those at 0.45 m, but less DW per hectare. Nitrogen fertilization increased plant DW in the second year of study, affecting mainly the younger plant parts. Plants fertilized with 200 kgha^-1 of N had the greatest total N. Nitrogen concentration (%N) varied greatly with plant part and was affected by sampling date and N fertilization rate. Younger tissues had the highest %N in spring (3.5%) and flower buds in winter (2.4%). Total plant NDFF increased from Apr. 2002 to May 2003. The lowest NDFF per plant and per hectare was found in Apr. 2002, when almost 60% of the NDFF was in the new shoots. Nitrogen fertilization rate and in-row plant spacing had an impact on total NDFF accumulated per plant and per hectare. More total NDFF was found in plants fertilized with 200 kgha^-1 of N than with 100 kgha^-1 of N, independent of spacing. Fertilizer recovery was 17% for plants at 1.2 m and 23% for plants at 0.45 m, independent of N fertilization rate. Partitioning of 15N (mg per plant part) and percent of total 15N per part changed with sampling date. Nitrogen fertilization rate and spacing did affect the total amount of fertilizer-15N present in each part, but percent partitioning of 15N was only affected by plant part. Plants at 1.2 m had a higher percentage of 15N partitioned to the
crown and three-year and older wood, but reduced partitioning to large roots than plants at 0.45 m. Application date had a large effect on the total amount of NIDFF recovered in the plant at the end of the first season. Application of N fertilizer in either April or May resulted in five times more NDFF in the plant than fertilizer application in July. Percent partitioning of NDFF was also affected by application date. Late fertilization resulted in labeled N allocated mainly to small roots, leaves and shoots, whereas spring-applied fertilizer was allocated mainly to leaves and fruits. In the new planting, established using
two-year-old plants, N fertilization rate affected plant dry weight, total N content, percent NDFF, and fertilizer recovery. By October, plants fertilized with 50 kgha1 of N had the largest dry weight and N accumulation. Ammonium toxicity was observed in plants fertilized with 100 and 150 kgha1 of N. Percent NDFF was 60% and 67% for the 50
and 100 kgha^-1 of N, respectively. Fertilizer recovery reached a maximum of 10 to 17% in October, depending on N fertilization rate.