As citrus occupies the greatest acreage among all fruit species grown in Egypt, the factors controlling its growth and productivity had received considerable attention during the last few years. Nevertheless, the yield of many citrus groves is still unsatisfactory, despite favorable climate and other environmental factors. Citrus trees require large quantities of mineral nutrients to attain adequate growth and yield, and the requirements for some of the nutrients vary with soil fertility and type. Although the mineral nutrition of citrus trees has been studied intensively, additional information is published frequently, especially after the introduction of new fertigation technologies and of other manipulations. Among the important elements, potassium plays a major role, second only to nitrogen, and is considered as a key element in fruit production and quality world wide. Washington Navel orange cultivar has an important role in the economy of many countries, where the trees are grown in clay soil. The fertilization programs in most orange orchards play a big role in improving fruit quality, specially potassium fertilization. So the production of such trees become very good.