Abstract
Experiments were conducted during the 1995/96 and 1996/97 seasons in blocks of mature grapefruit located in the Indian River area to determine the effects of post bloom and summer foliar potassium sprays on grapefruit size. The trees were either left untreated or had foliar applications of potassium nitrate (KNO[sub3]) or monopotassium phosphate (MPK) in 1995/96 and MKP or dipotassium phosphate (DKP) in 1996/97. Foliar sprays were made at different rates (11.5, 23, 34.5 lb K[sub2]O acre[sup-]) in April, May, and August/September in the 1995/96 season. In the 1996/ 97 season, applications of MKP and DKP were made at 5 lb K[sub2]O acre[sup-] in April, April plus May, or April plus May plus July. In the 1995/96 experiment, fruit in each plot were tagged and the fruit diameters were measured at subsequent dates to determine effectiveness of foliar applications. All treatments except the 11.5 K[sub2]O lb acre[sup- app.[sup-] MKP treatment were effective in increasing fruit diameter 2-5 mm with respect to the non-treated control. Higher rates of KNO[sub3] did not enhance the enlargement over lower rates. The high MKP rate (34.5 lb) was no different than the medium MKP (23 lb) K[sub2]O rate. In the 1996/97 season, fruit from plots were measured in August ('Star Ruby') and September ('Marsh'). Fruit in both experiments receiving MKP or DKP were 4-5 mm larger than fruit on control trees. Trees that received additional applications of MKP or DKP in May and July had fruit that were larger than control treatment fruit, but of similar size to trees that had applications only in April.
