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Background: The utilization of nutrition management, has recently been developed as a means of improving the growth and production of phytochemical compounds in herbs. The present study aimed to improve the growth, physiological, and phytochemical characteristics of lemon balm (Melissa officinalis L.) using different NH 4 + (ammonium) to NO 3 ̄ (nitrate) ratios (0:100, 25:75, 50:50, 75:25 and 100:0) under floating culture system (FCS)., Results: The treatment containing 0:100 - NH 4 + :NO 3 ̄ ratio showed the most remarkable values for the growth and morpho-physiological characteristics of M. officinalis. The results demonstrated that maximum biomass (105.57 g) earned by using the ratio of 0:100 and minimum at 75:25 ratio of NH 4 + : NO 3 ̄. The plants treated with high nitrate ratio (0:100 - NH 4 + ̄ ratio caused the highest dry matter (DM), N and K macronutrients in the leaves. The highest antioxidant activity by both DPPH (37.39 µg AAE/mL) and FRAP (69.55 mM Fe 3 ̄) showed the greatest concentration of total phenolics (60.40 mg GAE/g DW), chlorophyll a (31.32 mg/100 g DW), flavonoids (12.97 mg QUE/g DW), and carotenoids (83.06 mg/100 g DW). Using the 75:25 - NH 4 + :NO 3 ̄ ratio caused the highest dry matter (DM), N and K macronutrients in the leaves. The highest antioxidant activity by both DPPH (37.39 µg AAE/mL) and FRAP (69.55 mM Fe ++ /g DW) methods was obtained in 75:25 - NH 4 + :NO 3 ̄ treatment. The p-coumaric acid as a main abundant phenolic composition, was detected by HPLC analysis as the highest content in samples grown under 0:100 - NH 4 + :NO 3 ̄ treatment. Also, the major compounds in M. officinalis essential oil were identified as geranial, neral, geranyl acetate and geraniol by GC analysis. With increasing NO 3 ̄ application, geraniol and geranyl acetate contents were decreased., Conclusions: The findings of present study suggest that the management of NH4 + to NO 3 ̄ ratios in nutrient solutions could contribute to improving growth, physiological and phytochemical properties of M. officinalis. The plants treated with high nitrate ratio (especially 0:100 - NH 4 + :NO 3 ̄) showed the greatest effects on improving the growth and production of morpho-physiological and phytochemical compounds. By comprehensively understanding the intricate dynamics among nitrogen sources, plants, and their surroundings, researchers and practitioners can devise inventive approaches to optimize nitrogen management practices and foster sustainable agricultural frameworks., (© 2024. The Author(s).)