11/19/2021 - By Dr. Tom Reed

Conclusion:

The study investigated the effects of three planting densities and two Harrell’s controlled release fertilizer (CRF) blends on bearing age (4 to 6-year-old) ‘Ray Ruby’ grapefruit on Kuharske citrange in a split-plot design. Fertilizer blends were applied three times per year (February, July, October). First fertilizer blend was a 16-3-20 (81% CRN; 50% CRK) with Fe as chelate and other micronutrients as sulfates. The second blend was a 12-3-9 (100% CRN; 95% CRK) with Fe as chelate and other micronutrients sulphur coated. Target application rate was 161 lb N/ac (180 kg N/ha). The use of the 12-3-9 blend with higher amounts of micronutrients along with lower K increased canopy volume in both seasons and resulted in 24% and 29% reduction in fruit yield per ac and yield of solids, respectively, in 2017–18. In 2018-19, there were no differences in yields between blends, however the site and yield were affected by Hurricane Irma. Balanced nutrient application can mitigate Huanglongbing (HLB) disease, high-density planting optimizes yield under HLB-endemic conditions, and increasing micronutrient levels may not increase yield or quality in HLB-affected grapefruit particularly without adequate K applied.

Figure 1: Nutrient rates for Harrell’s CRF blends in study.