The effects of foliar application of different potassium forms on yield, tree development and pomological properties in orange ('Washington Navel')

This study investigated the effects of foliar application of different potassium sources on yield, tree development (tree height, canopy diameter and canopy volume) and pomological properties (fruit-fruit pulp and rind weight, rind thickness, fruit width, height and shape index (width/height), brix, vitamin C) in the 'Washington Navel' orange cultivar (Citrus sinensis, L.) at the productive age (23-24 years) grafted onto Trifoliata citrange rootstock. For this purpose, an experiment was carried out through the years of 2016-2017 and 2018 in the district of Gumuldur (Izmir, Turkey) where a Mediterranean climate is dominant with 5 replications according to the random blocks experiment design. In addition to the control group that received no foliar substance application, in the foliar applications, potassium nitrate (KNO3, 46% K2O; 300 g 100 L-1 da(-1)), potassium thiosulfate (25% K2O; 400 mL 100 L-1 da(-1)) and potassium phosphite (25% K2O; 500 mL 100 L-1 da(-1)) were used as K sources. In all examined parameters, while the lowest values were obtained in the control group, the highest values were obtained with the potassium phosphite application, and the potassium phosphite application was found to create a significant difference in all examined parameters. As a result of the potassium phosphite application, the 3-year average values of yield, fruit weight, fruit shape index (width/height), rind thickness, brix, vitamin C and tree volume were, respectively, 136 kg tree(-1), 247 g fruit(-1), 1.1, 4 mm, 13.61 mg 100 mL(-1) juice and 156 m(3). Consequently, it may be stated that the most effective fertilizer among the potassium sources was potassium phosphite.