Potassium (K) is an essential nutrient element for plant growth. However, in most soils, K is low in both concentration and availability. Chemical K supplemented through fertilization is rapidly adsorbed by soil minerals or transformed by soil microbial organisms, resulting in low K use efficiency, as low as 20%~35%. Studies have found that Azotobacter sp., which can fix nitrogen from the atmosphere to supplement plants with nitrogen nutrient, also has the ability to mobilize soil K for plant use. The finding has aroused much attention the world over. However, little information is available to illustrate the mechanisms of Azotobacter mobilizing K in the soil. Five strains of Azotobacter sp., which were isolated from a gray brown purple soil in Chongqing, South China and coded as N01, N02, N03, N04 and N05, separately, were cultured a liquid medium in an experiment to study their K mobilization capacities. The medium contained 1L of H₂O, 10 g of mannitol, 0.176 g of NaH₂PO₄, 0.2 g of MgSO₄•7H₂O, 0.2 g of NaCl, 0.2 g of CaSO₄•7H₂O and 5.0 g of CaCO₃. Compared to the non- Azotobacter control, all the five Azotobacter treatments were found to be 40 times higher in concentration of hydrogen ions in the liquid media, as a consequence, pH dropped significantly after 7 days of incubation. The Azotobacterstrains all exude organic acids, including formic acid, acetic acid, oxalic acid, succinic acid, citric acid, malic acid and lactic acid, but vary in capacity and excretion of acids. Oxalic acid and malic acid was the most common ones all the five strains could exude. K concentrations in the liquid media were significantly higher than in the control, whilst the contents of soil mineral structure K were significantly lower in the Azotobacter treatments than in the non- Azotobacter treatment. Taking into account the soil being the sole K source, it is quite obvious that Azotobacter could promote dissolution of mineral K in the soil. Correlation analysis shows that content of soil mineral structure K is negatively related to the total organic acids Azotobacter exude (r= -0.845*,n = 6), but positively related to pH of the liquid medium (r = 0.702*, n = 6), which indicates that both the organic acids Azotobacters exude and hydrogen ions could dissolve soil K. Oxalic acid is the major organic acid Azotobacters exude in high volume and the most competent in complexing calcium, magnesium, iron and aluminum, and moreover, it is positively correlated with total organic acid (r= 0.990**, n = 6), which suggests that oxalic acid exuded by Azotobacter might contribute directly to the mobilization of soil K. Meanwhile, soil mineral K was significantly reduced in all the Azotobacter treatments, to a varying extent, which depended on which strain of Azotobacter was used in this study, because the strains of Azotobacters vary sharply in exudation, in terms of amount and type of organic acids they exude. The findings demonstrate that inoculation of Azotobactersor intercropping with legumes is a good alternative to improve plant K nutrition, besides, Azotobactersand legumes can also help supplement the soil with nitrogen by fixing it from the atmosphere,. Therefore, more efforts should be devoted to researches on the capability of Azotobacters of mobilzing soil K, and their associated nitrogen and K benefits to their host plants and neighbouring plants.