Optimizing Nitrogen Fertilizer Rates for Commercial Southern Highbush Blueberry Production during Establishment

Effects of different nitrogen fertilizer rates on southern highbush blueberry (SHB) growth and soil nitrate levels during the first year of plant establishment were studied. One-year-old ‘Emerald’ and ‘Farthing’ liners were planted in April 2015 in pine-bark amended soil and fertilized with 0, 42, 84, 168, or 336 kg N per hectare (0N, 42N, 84N, 168N, 336N, respectively) per year. Fertilizer (UAN-32) was applied through fertigation daily from April through November. Soil cores were collected four times per year for analyses of soil nitrate content at depths of 0.15, 0.3 and 0.76 m. Significantly higher soil nitrate levels were found at all three depths in plots treated with 336N compared to plots fertilized with lower N rates. Since both SHB cultivars are shallow-rooted, and their roots are mainly located in the top 0.4 m of the soil profile based on the observation, the greater soil nitrate concentration in plots with 336N at the depth of 0.76 could not be absorbed by plants and thus resulted in nitrogen loss. Based on the measurements throughout the year, nitrogen fertilizer rates did not affect plant height or leaf nitrogen concentration for either ‘Emerald’ or ‘Farthing’. According to the canopy area measured by an image-processing program ImageJ, both cultivars had significantly larger canopies when fertilized at 336N compared with the 0N, 42N, and 84N treatments. Additionally, higher N rates advanced bloom the next season. ‘Emerald’ plants fertilized with 336N bloomed one, two, or three weeks earlier than those fertilized with 168N, 84N, or 42N, respectively. There was no difference in time of bloom between the 0N and 42N treatments. Similarly, bloom on ‘Farthing’ plants fertilized with 336N or 168N was advanced by one week compared with the other treatments.