Comparison of liquid, granulated, and controlled release fertilizer in blueberry on fruit quality and growth

Southern highbush blueberries (SHB; Vaccinium corymbosum L. hybrids) are acid-soil-loving plants that thrive at pH levels of 4.0-5.5. Under the acidic soil conditions, ammoniacal nitrogen (NH₄-N) fertilization is applied for plant nitrogen. Most fertilizer formulations contain some level of nitrate (NO₃^-); however, for blueberry, NO₃^- in the fertilizer formulation is suggested not to exceed 25%. Fertigation and controlled release fertilization (CRF) are being adopted by growers for convenience. However, some liquid and CRF formulations can contain levels of nitrate above 25%. In 2014 and 2015 at the University of Georgia blueberry farm in Alapaha, GA, USA, five treatments with three replications per treatment of SHB 'Star' were fertilized at the rate of 112 kg ha^-1 nitrogen (N) with 1) Granular 10-10-10 (GF; 95% NH₄-N, 5% NO₃^-), 2) Liquid 10-5-5 (LF; 63.5% NH₄-N, 36.5% NO₃^-), CRF 18-6-12 (100.0% NH₄-N, 0.0% NO₃^-) as 3) (CRF3, 3 month), 4) (CRF4, 6 month), and CRF 15-8-11 (66% NH₄-N, 34% NO₃^-) 5) (CRF12, 12 month) controlled release. Leaf tissue N (LTN) and NO₃^- concentration, plant growth (shoot length and number), and fruit firmness, color, total acidity, and sugars were assessed. In 2014, significant differences were seen in shoot length and firmness but not in nitrogen, shoot number, or other fruit quality measurements. The CRF12 had 29% longer shoots with 10% firmer fruit than GF. In 2015, differences were noted but with different treatments. The CRF6 measured 22% longer shoots but had 31% fewer shoots than the LF. In both years, the NO₃^- leaf tissue level was ≤0.02% for all treatments. This work shows that in SHB 'Star' applying either CRF at 66% NH₄-N or LF at 63.5% NH₄-N will produce adequate plant growth and fruit quality.