December 1, 2007
Previous work in South Florida indicated that wintertime, bloom and postbloom foliar nutritional sprays of urea, K-phosphite, or NPK mixes resulted in increased yields of citrus on poorer-yielding flatwood soils. Whether similar sprays could enhance yields of better-performing Central Florida groves was addressed by tests for up to four consecutive years on the same trees that also all received standard soil-applied fertilizer. Wintertime urea sprays and bloom and/or postbloom NPK mixes in all combinations were applied to navel, 'Hamlin', or 'Valencia' oranges, or 'Flame' grapefruit. N rates from urea ranged from 8 to 16 lbs acre/spray with either 4 or 8 lbs of P and K. In the NPK tests, no consistent differences in yield were found and in seven of 16 cases, treatments that included winter urea were near the lowest yields. When all years were combined, the nonsprayed controls usually were equal to other treatments except for the 'Flame' grapefruit test. For P tests, K-phosphite or urea were sprayed during the winter and NK with or without PO4 or PO3 were sprayed at bloom and/or postbloom on 'Hamlin' and 'Valencia' trees in several locations in Central Florida. In these tests, 0, 2.4, or 8 lb of P were used with 16 lb N as urea and 8 or 9.8 lb of K. Again, there were no consistent treatment effects on yields. Overall, no consistent pattern emerged to suggest that foliar sprays of various combinations of foliar-applied NPK solutions using up to 44 or 60 lb additional N/acre increased yields on citrus blocks that were producing well using standard ground-applied fertilizer programs.