Improved Nitrogen Fertilizer Practices for Highbush Blueberry

More than 77,000 ha of cultivated blueberry are grown worldwide, and production will increase another 46% over the next five years. Nearly 90% of the crop is produced in North America and South America. Annual total global production is predicted to reach 635,000 t by 2015. Blueberry is a long-lived perennial crop (30+ years), adapted to acidic soil conditions. Blueberries primarily acquire their N in the form of NH₄ and tolerate relatively low levels of P, K, Ca, and Mg in the soil and high concentrations of Mn and Al. Nutrient management guidelines for highbush blueberry are currently based on granular fertilizers. However, most new plantings are fertigated through drip systems, which based on 4R principles, may have very different nutrient requirements. Three field trials were initiated in Oregon to evaluate new N fertilizer practices for establishment of highbush blueberry.

The first trial compared the effects of N fertigation to conventional granular fertilizer. Fertigation produced more growth and less salt injury in young plants than granular fertilizers, but fertigation required more N to reach maximum plant growth potential. More N was needed by fertigation because NH₄-N, is immobile in soil. Unlike granular fertilizer, which was applied by hand around the base of the plants, much of the injected fertilizer wound up between the young plants and unavailable for plant uptake. In the second trial the effects of fertigation using three different methods of water placement in six cultivars was studied. Use of two irrigation laterals of drip (now a common practice) was worse than a single irrigation line. Drip emitters with two lines placed the NH₄-N fertilizer too far from the roots of the young plants. The third trial was planted to identify practices that increase N uptake and plant productivity during establishment, including organic acids, controlled-release fertilizers, and small amounts of granular fertilizer applied shortly after planting. After one year, importance of the dripper placement was confirmed. Fertigation with organic acid fertilizer or urea sulfuric acid also produced more growth than liquid urea alone. It is now recommended to use two lines of drip per row, but locating the lines near the base of the plants during few years after planting, and then later moving the lines away from the plant. Fertigation is also now recommended over the use of granular N fertilizers when possible, and using urea sulfuric acid or organic acids in high pH soils. Fertigation with liquid ammonium sulfate is also well suited to blueberry.