Nowadays, most of medicinal plants are harvested from natural fields, which if harvesting continued with the trade aims would lead to lack of these species in their own habitats and the lack of their raw materials for dependent industries. Based on medicinal attributes of St. Johns wort, an investigation was carried out in the factorial experiment (3×3) with three replicates based on completely randomized block design. Treatments consisted of potassium sulfate (Kx) at three concentrations (0, 60 and 100 kg.h-1) and humic acid (Hx) also at three concentrations (0, 20 and 40 l.ha-1).Studied parameters consisted of essential oil content, antioxidant activity determined by two methods (FRAP and DPPH), macro elements such as N, P and K. Results showed that essential oil content achieved the highest amount in treatments H20×K100 andH40×K100and were significantly different with other treatments. EC50 was used to express the antioxidant activity of this plant, so that the lower EC50 value expressed the higher antioxidant activity. Plants treated with the highest levels of potassium sulfate and humic acid possessed the highest antioxidant activity. These finding was as the same in two antioxidant activity evaluation methods. Macronutrient uptake analysis also showed that the levels of phosphorus and nitrogen in the plant were not significantly affected by potassium sulfate, while the potassium content affected significantly. Regarding the humic acid, phosphorus was not affected by this treatment; while nitrogen and potassium showed significant difference. The interaction effect of these two fertilizers was not significantly different for phosphorus content but significantly affected the nitrogen and potassium content. However, application of organic humic acid and chemical potassium sulfate fertilizers could improve the phytochemical and nutritive attributes of this medicinal plant [1, 2].

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