Soybean plants can grow by solely depending on fixed N2 through their root nodules, a symbiotic organ with rhizobia. In this study, when nodulated soybeans were cultivated using hydroponics and an N-free culture solution, the pH rapidly decreased to 4.0, which may be harmful for root growth and nutrient absorption. Therefore, we prepared a new N-free culture solution characterized by less pH fluctuation due to the addition of potassium bicarbonate. A total of 1–2 mM sodium bicarbonate optimized the pH between 6 and 7. However, the solution pH increased to 8–9 during soybean cultivation when 5–20 mM of sodium bicarbonate was applied. The addition of potassium bicarbonate did not affect the dry weight of each organ. The evapotranspiration rate of the plants with bicarbonate on the 37th day after planting was higher than that of the control without bicarbonate. When the K2SO4 was replaced by KHCO3, the pH just after preparation ranged between 6.3 and 6.5, while that after cultivation for two days ranged frp, about 6.1 to 6.5. It was found that more than half of the bicarbonate remained in the culture solution after 3 days of cultivation. The optimum P concentrations for the modified culture solution were found to be 50 and 100 μM, while the P concentrations over 150 μM reduced the plant growth and led to yellowing in the lower leaves.