Salinity is one of the major issues that limits field crop productivity in an arid and semiarid environment. Therefore, two field trials were carried out over two seasons of 2018 and 2019 to investigate the enhancement of different methods of potassium application (i.e., recommended soil amendment (control; K2O), seed soaking (SS) and foliar spray (FS) in the form of potassium sulfate (K2SO4, 6 mM)) on antioxidant protection, physio-biochemical, yield and quality traits of soybean (cv. Giza 22) grown in normal (electrical conductivity; EC = 2.68 dS m−1) and saline soil (EC = 7.46 dS m−1). Physio-biochemical attributes (total chlorophyll, carotenoids, K+ and K+/Na+ ratios, performance index and catalase (CAT) activity), growth traits (i.e., shoot length, number and area of leaves plant−1 and shoot dry weight), yield and its components and seed quality (number of pods plant−1, 100-seed weight, seed yield ha−1 and seed protein and oil contents) were significantly decreased when soybean plants were grown in saline soil compared with those grown in normal soil. In contrast, activity of enzymatic antioxidants (i.e., superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione peroxidase (GPX)), contents of non-enzymatic antioxidants and osmoprotectants (i.e., total soluble sugars, free proline, ascorbic acid and α-tocopherol), Na+, Cl−, H2O2 and malondialdehyde (MDA) were increased in soybean plants grown in saline soil compared with normal soil. However, under salt-stressed conditions, potassium applied through SS or FS significantly enhanced all soybean growth, photosynthetic efficiency, K+ content, ratio of K+/Na+ and activity of CAT, SOD, APX and GPX as well as improved yield and quality traits, while potassium application did not affect the contents of non-enzymatic antioxidants and osmoprotectants. For instance, foliar potassium application (FS) increased seed yield ha−1 by 92.31% and protein content by 63.19% compared with the control under the salt stress condition. In addition, both applications of potassium significantly reduced Na+, Cl−, H2O2 and MDA contents in soybean plants compared with those obtained from control treatments. Exogenous application of K2SO4 was more effective than SS at improving soybean physio-biochemical attributes, yield and seed quality traits under soil-salinity stress.
