Pirzad, Ù., Yoosefi, M., Darvishzadeh, R., Raei, Y. (2013). 'Effect of Different Rates of Zeolite and Nitrogen Fertilizer on Yield and Harvest Index of Flower, Grain, Essential oil and Seed Oil of Calendula officinalis L.', JOURNAL OF AGRICULTURAL SCIENCE AND SUSTAINABLE PRODUCTION, 23(2), pp. 61-75.