Growth and Fruit Production of Highbush Blueberry Fertilized with Ammonium Sulfate and Urea Applied by Fertigation or as Granular Fertilizer

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the American Society for Horticultural Science and can be found at: http://hortsci.ashspublications.org/.Fertigation with liquid sources of nitrogen (N) fertilizers, including ammonium sulfate and urea, were compared with granular applications of the fertilizers in northern highbush blueberry (Vaccinium corymbosum L. ‘Bluecrop’) during the first 5 years of fruit production (2008–12). The planting was established in Apr. 2006 at a field site located in western Oregon. The plants were grown on raised beds and mulched every 2 years with sawdust. Liquid fertilizers were injected through a drip system in equal weekly applications from mid-April to early August. Granular fertilizers were applied on each side of the plants, in three split applications from mid-April to mid-June, and washed into the soil using microsprinklers. Each fertilizer was applied at three N rates, which were increased each year as the plants matured (63 to 93, 133 to 187, and 200 to 280 kg·ha⁻¹ N) and compared with non-fertilized treatments (0 kg·ha⁻¹ N). Canopy cover, which was measured in 2008 only, and fresh pruning weight were greater with fertigation than with granular fertilizer and often increased with N rate when the plants were fertigated but decreased at the highest rate when granular fertilizer was applied. Yield also increased with N fertilizer and was 12% to 40% greater with fertigation than with granular fertilizer each year as well as 17% greater with ammonium sulfate than with urea in 2011. The response of berry weight to the treatments was variable but decreased with higher N rates during the first 3 years of fruit production. Leaf N concentration was greater with fertigation in 4 of 5 years and averaged 1.68% with fertigation and 1.61% with granular fertilizer. Leaf N was also often greater with ammonium sulfate than with urea and increased as more N was applied. Soil pH declined with increasing N rates and was lower with granular fertilizer than with fertigation during the first 3 years of fruit production and lower with ammonium sulfate than with urea in every year but 2010. Soil electrical conductivity (EC) was less than 1 dS·m⁻¹ in each treatment but was an average of two to three times greater with granular fertilizer than with fertigation and 1.4 to 1.8 times greater with ammonium sulfate than with urea. Overall, total yield averaged 32 to 63 t·ha⁻¹ in each treatment over the first 5 years of fruit production and was greatest when plants were fertigated with ammonium sulfate or urea at rates of at least 63 to 93 kg·ha⁻¹ N per year