Phosphorus (P) fertilizer use efficiency is only 15 % in calcareous alkaline soils as in Pakistan. Sulfur oxidizing bacteria (SOB) especially Thiobacillus spp. solubilize the unavailable P in soil by synthesizing sulfuric acid. This study was performed in three steps: (i) screening of SOB from different microbial ecologies, (ii) use of different SOB with S levels for enhancing bio-available P, and (iii) effect of SOB along with P fertilizer and S on plant growth and yield. Sulfur oxidizing bacteria were isolated, screened, identified and the most efficient SOB were found as the genus Thiobacillus in sulfur based ecologies such as industrial wastewater, sewerage water and sulfur mud. These SOB isolates were IW1, SW2, SS1, IW13, IW14, IW16 and SM1 which reduced the pH of different media (thiosulphate, tricalcium phosphate and rock phosphate) in 07 to 40 days after inoculation. Four Thiobacillus isolates viz., IW16, SW2, IW1 and IW14 were inoculated in soil with three S levels 50, 37.5 and 25 mg kg-1. Thiobacillus spp. IW16 and SW2 reduced soil pH with 50 mg S kg-1 from 7.90 to 7.12 and 7.28 respectively. Similarly, Thiobacillus strain IW16 in combination with 50 mg S kg-1 reduced CaCO3 contents from 7.14 to 6.93 % and solubilized Ca8-P and Ca10- P. Increase in the concentration of sparingly soluble Ca2-P (20.33 mg kg-1) and xvii bioavailable P contents (net increase of 22.26 mg kg-1) were also recorded as a result of P solubilization phenomenon. Lastly field experiments were conducted on two permanent lay outs (plot size 3 m × 3 m) at two different places growing for rice-wheat and maize-maize crops. The best Thiobacillus isolates (IW16 and SW2) along with 100 kg S ha-1 were inoculated in combination with two doses of P fertilizer viz., 45 and 90 kg P2O5 ha . Significant increase in the concentration of bio-available P in soil was recorded through bacterial S oxidation in both experiments. Growth and yield parameters of the tested crops (rice, wheat and maize) exhibited positive significant correlation with P solubilization through S oxidation by Thiobacilli with the highest values by strain IW16 along with 100 kg S ha-1. Interaction between Thiobacillus spp. and S was highly significant in enhancing the growth and yield of crops. Treatment of soil with Thiobacilli and S was the best practice for enhancing bioavailability of P already present as fixed P in huge quantity.