Abstract
Experiments were conducted over a period of three years in blocks of mature grapefruit located in the Indian River area to determine the effects of fall foliar potassium sprays on grapefruit size. The trees were either left untreated or had foliar applications of potassium nitrate (KNO[sub3], 13-0-46) or monopotassium phosphate (MPK, 0-52-36) in replicated field experiments. Foliar sprays (15-25 lb material per acre) were made in September and October each year. Fruit in each plot were tagged and the fruit diameters were measured to determine effectiveness of foliar applications. In the 1994/95 season, 'Marsh' grapefruit trees receiving the foliar KNO[sub3] applications increased 11.4% in diameter from 10 September to 23 November compared to 8.0% for fruit on the control trees and 45% of the fruit on trees receiving KNO[sub3] increased 2 or more sizes while only 14% of the non-sprayed control fruit increased a similar amount. In the 1995/96 season, fruit diameters were not significantly increased for 'Marsh' (KNO[sub3] applied) or 'Flame' (MKP applied), but MKP applications to 'Star Ruby' resulted in a 2.1% increase in growth rate 1 month after application. Late applications (October and November) of KNO[sub3] were not effective on increasing fruit size during the 1996/97 season on 'Marsh' grapefruit. Although average fruit diameter growth was only increased 0.6 to 2.4 mm for treated fruit as compared to control fruit, the greater growth in the smaller fruit sizes is likely to be economically significant in many years.
