The response of mature Washington Navel orange trees to Mg SO4, soil (0.5, 1.0 and 1.5 Kgs / tree / year) or foliar (1.0, 1.5 and 2.0 % / tree / year) applications, each added at 1, 2 and 3 times of applica‌tion was investigated in two successive seasons (97/1998 and 98/ 1999), at Biyala citrus orchard, where the soil is classified as slightly al‌kaline clay soil, exchangeable Mg 22 mg/100g soil and mean leaves Mg concentration (D.W) was ❑.136%. Yield as Kgs and fruit number was increased by all Mg SO4 rates, methods and numbers of application, but the highest significant increase was obtained by 1.5% (spray) or 1.0 Kg (soil application) Mg SO4 / tree / year added 3 times annually for foliar and soil application, respectively. Moreover, the highest average fruit weight, TSS, vitamin C, juice volume and lowest acidity were obtained by Mg SO4 foliar spray 3 times, while peel thickness was unaffected. Magnesium treatments markedly increased leaf area, shoot length and numbers, total chlorophyll and Mg %, but slightly increased F', Fe, Mn, and Zn. leaf Ca and K content were decreased, whereas, N was unaf‌fected. Generally, spraying Mg SO4 gave the highest values for yield, fruit quality vegetative growth, chlorophyll and total carbohydrate per‌cent. The best treatment is spraying 1.5% or adding 1.0 Kg soil appli‌cation Mg SO4/tree/year three times annually, but the superior treat‌ment is spraying trees with 1.5% Mg 804.