Effects of Nitrogen Source and Rate and Method of Fertilizer Application On Yield and Fruit Size In ‘Bluecrop' Highbush Blueberry

A study was done to determine the effects of N source and rate and two common methods of fertilizer application on yield and fruit size in a maturing field of highbush blueberry (Vaccinium corimbosum L. ‘Bluecrop’). Plants were fertilized by drip fertigation (weekly from mid April to mid August) or with a triple-split application of granular fertilizer (April, May, and June) using urea or ammonium sulfate applied at a rate of 0, 67, 133, and 200 kg·ha^-1 N in 2008 (year 3), 0, 75, 150, and 225 kg·ha^-1 N in 2009 (year 4), and 0, 83, 167, and 250 kg·ha^-1 N in 2010 (year 5). In 2008, which was the first year of fruit production, fertigation increased yield compared to granular fertilizer, producing 3.8 and 2.7 t·ha^-1 of fruit, respectively, while N source had no effect on yield in fertigated plants but was higher on average with granular urea than with granular ammonium sulfate. Average individual berry weight was also higher with fertigation than granular fertilizer and, regardless of application method, was greater when plants were fertilized with 67 kg·ha^-1 N than with 0, 133, or 200 kg·ha^-1 N. The following year in 2009, yield averaged 8.2 t·ha^-1 with fertigation and was similar among plants fertigated with 75-225 kg·ha^-1 N and averaged 7.2 t·ha^-1 with granular fertilizer and, in this case, was similar with 75 and 150 kg·ha^-1 N but lower with 225 kg·ha^-1 N. Berry weight was not affected by N source or fertilizer method in 2009 but was affected by N rate, decreasing in average weight from 1.6 g/berry without N to 1.5 g/berry with 75 and 150 kg·ha^-1 N and 1.4 g/berry with 225 kg·ha^-1 N. Finally, in 2010, yield was again higher with fertigation than with granular fertilizer, averaging 14.4 and 12.1 t·ha^-1, respectively, but was not affected by N source or N rate when N fertilizer was applied. The average size of the berries, however, was smaller with fertigation than with granular fertilizer in 2010, averaging 1.4 and 1.5 g/berry, respectively. Berry weight also decreased that year from 1.5 g/berry with 0 or 83 kg·ha^-1 N to 1.4 g/berry with 167 or 250 kg·ha^-1 N. Overall, regardless of N source, yield was higher each year with fertigation than with granular fertilizer, and 67-83 kg·ha^-1 N was adequate to maximize yield and fruit size during the first 3 years of production.