This experiment was designed to evaluate the effects of organic (liquid humic acid) and inorganic (potassium sulphate) on phytochemical and morphological attributes of St. John’s Wort (Hypericum perforatum). Thus, a research was conducted in a factorial experiment (3×3) based on completely randomized design with three replications. Treatments consisted of potassium sulphate (Kx) at three concentrations (0, 60 and 100 Kg.h1) which were treated before flowering and humic acid (Hx) at three concentrations (0, 20 and 40 L.h-1) which were fertigated four times of 15- days intervals. Results showed that the plant stem height, number of flowering stems and number of flowers were significantly affected by simple effect of each fertilizers (p<0.01), while their interaction effect was not significant for the plants height. The highest contents of fresh and dry weight were achieved under the highest amounts of fertilizers (K100 and H40). The highest stem height, number of flowers and number of flowering stems also belonged to these treatments. Increment of applied fertilizers led to increase of obtained essential oils, so that application of these fertilizers simultaneously increased the essential oil content up to 6-fold. Regarding the antioxidant activity, applied fertilizers at their high levels showed significant effects on decrease of EC50, which means the increment of antioxidant activity of H. perforatum.

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