In order to develop bioremediation technology for acidified soils, effect of NO3-/NH4+ molar ratio on release of hydroxyl from tomato roots under acidic conditions was investigated with solution culture experiments and automatic potentiometric titrations. When the initial pH was 4.0 and NO3-/NH4+ molar ratios were 15:1, 5:1, 1:1 and 1:5, tomato took up more nitrate from culture solution and its roots released hydroxyl into culture solutions, and thus led to the increase of solution pH. The pH of culture solutions with NO3-/NH4+ molar ratios of 15:1, 5:1, 1:1 and 1:5 was increased by 1.60, 1.15, 0.57 and 0.29 units after 7 days of tomato cultivation. The change of solution pH was consistent with the amounts of nitrate absorbed by tomato and hydroxyl released by its roots. When tomato grew in the culture solution with initial pH of 5.0, its roots released hydroxyl in the solutions with NO3-/NH4+ molar ration of 15:1 and 5:1, while they released proton in the solutions with NO3-/NH4+ molar ratios of 1:1 and 1:5. Tomato absorbed more nitrate and its roots released more amount of hydroxyl in the solutions of initial pH 4.0 than in the solutions of initial pH5.0. Therefore, tomato can be used to remediate acidified soils due to its preferential absorption for nitrate and the release of hydroxyl from its roots.