Abstract: To study the utilization of different forms of phosphorous by acid-tolerant Sinorhizobium meliloti, rhizobia were cultured under three pH conditions; pH of 4.1, 7.0 and 9.0.Solid and liquid culture were used to investigate the growth of rhizobia in different phosphorus media [phosphorus free control (CK), organic phosphorus with yeast extract (CK_YE-P), inorganic phosphorus with K₂HPO₄ (CKP), phytin (T_Phy-P), calcium phosphate (T_Ca-P), aluminum phosphate (T_Al-P) and iron phosphate (T_Fe-P)]. The colony morphology, diameter on solid media, absorbance and available phosphorous and pH in liquid medium were measured. Under all pH conditions, the rhizobia growth in CK was the poorest. Both the colony diameter of CK_YE-P and CKP in solid media and the absorbance in liquid media were significantly higher than that in CK, suggesting that phosphorous was the key factor influencing the growth of rhizobia in this experiment. The colony diameter of rhizobiain thephytin treatment (T_Phy-P) was largest in all media and pH levels; the aluminum phosphate treatment (T_Al-P) and the iron phosphate treatment (T_Fe-P) produced the smalled colonies. After liquid culturing, the available phosphorus and rhizobia absorbance of T_Phy-P as well as T_Ca-P in neutral and acid medium were much higher than those in alkaline medium. The available phosphorus and rhizobia absorbance of T_Al-P and T_Fe-P were low in all media. The growth of rhizobia led to decreased pH in all media. When compared to the non-inoculated control, available phosphorus enhancement in the inoculated treatment was variable. The results shows that alfalfa rhizobia could effectively utilize phosphorous in the form of phytin and calcium orthophosphate, but that the availability of phosphorous under alkaline conditions declined dramatically. The phosphorous inaluminum phosphate and ironic phosphate was unavailable to alfalfa rhizobia. Media pH had a big influence on the availability of phytin and calcium orthophosphate but had little effect on the availability of aluminum phosphate and iron phosphate. Rhizobia growth could result in decreased media pH and increased available phosphate, but this was affected by phosphorus form.