Protein content is an important quality parameter and depends on the amount of assimilated N by the plants (Selim et al., 2009a). Leonardite used in the study contained 1.8% N (Table 1). Leonardite also serves to increase available N, P and other plant nutrients, such as K; in the soil which in turn could be taken up by plants (David et al., 1994). The lowest rates for protein and vitamin C contents were found in control (Table 4). For both traits, the best results were obtained from 400 and 600 kg ha-1 leonardite applications. It was also noted that cultivars differed from each other for protein and vitamin C contents; such as, while Van Gogh had the highest level of protein, the same cultivar had the lowest level of vitamin C content (Table 4). Differences observed among cultivars for protein and Vitamin C contents may depend on genetic background of the cultivars. Potassium fertilization and humic acid applications affect concentration of vitamin C and amount of antioxidative compounds in solanaceous crops (Ünlü et al., 2010; Wuzhong, 2002).