In order to explore mechanisms of tea plant roots acidifying soils, a hydroponic experiment was carried out using the method of automatic potentiometric titration to investigate effects of different forms of nitrogen, i.e. NH₄⁺-N, NO₃^--N and ammonium nitrate, on proton release from tea plant roots in the solution of constant pH. Results show that tea plant roots released protons in the presence of ammonium at a higher rate than they did in the solution containing ammonium and nitrate with mole ratio of NH₄⁺/ NO₃^-=1:1; but they released hydroxyl in the presence of nitrate. The release of proton release was positively related the initial concentration of ammonium in the solution and with the uptake of ammonium by tea plant roots, too. Within the range of pH from 4.5 to 5.5, the maximum proton occurred in the treatment with initial pH being 5.0, and then in the treatment with initial pH being 5.5, and the least in the treatment with initial pH being 4.5. To culture tea plants with ammonium nitrate solution validated the plant’s ammoniaphilic feature. It was found that longer duration of incubation in ammonium nitrate solution increased the plant’s uptake of both ammonium and nitrate and its release of proton as well. However, at the end of the incubation, the plants were found to have absorbed more ammonium than nitrate. The findings suggest that tea plants preferred to absorb ammonium, which leads to release of more protons from their roots, thus causing acidification of the soil in tea gardens.