Two field experiments were conducted at the experimental farm of the Faculty of Agriculture at Fayoum, Egypt, to study the effect of soil and foliar applications of potassium in the form of potassium sulphate. The treated cotton plants with potassium soil application and with foliar spray improved all the studied growth traits, that is, number of leaf per plant, the total leaf area per plant (dm-2) and the dry weight of leaf per plant (g). The best results were found when cotton plants were treated with soil application plus K foliar spray twice, each at early boll formation and peak boll formation stage as compared to the control treatment. Moreover, this treatment also produced significantly higher yield with improved growth and fiber traits. The interaction between varieties and potassium fertilizer treatments had a significant effect on growth, cotton yield and fiber quality, whereas the Giza-90 variety x soil and foliar application (2 times) of potassium out of the interactions proved to have the best results.
Key words: Cotton varieties, foliar potassium application, fiber quality, Nitrogen, Phosphorous, and Potassium (NPK).
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