Enhancing Bio-Available Phosphorous in Soil Through Sulfur Oxidation by Thiobacilli

Aims: The objectives were to evaluate the phosphate solubilization efficiency of different Thiobacilli strains and to find out the best combination of sulfur and Thiobacilli for enhancing bio-available P in soil.
Study Design: An experimental study.
Place and Duration of Study: Microbiology and Soil Fertility Labs, Department of Soil Science and Soil and Water Conservation, Pir Mehr Ali Shah Arid Agriculture University, Rawalpindi, Pakistan and Microbiology and Soil Chemistry Labs, Auriga Research Center, Lahore, Pakistan, between May 2011 and November 2012.
Methodology: Fifty Thiobacilli strains were isolated from ten different ecologies. Then an incubation study of soil was performed wherein the most efficient four Thiobacilli strains were inoculated in combination with three different levels of elemental sulfur to determine pH, water soluble sulfur, sequential P fractions and bio-available phosphorous contents in the incubated soil.
Results: All the four Thiobacillus strains (IW16, SW2, IW1 and IW14) dropped pH of the incubated soil along with three doses of S° (50, 75 and 100 kg ha-1). However, Thiobacillus strains IW16 and SW2 reduced soil pH quite sharply from 7.90 to 7.12 (net reduction of 0.78 points) and 7.28 (net reduction of 0.62 points) respectively where inoculated with S° @ 100 kg ha-1. The best P solubilizer was Thiobacillus strain IW16 and the best dose of S° was @ 100 kg ha-1 and their combination enhanced maximum quantity of P (22.26 mg kg-1) in the soil by solubilizing already present insoluble calcium bounded P fractions like octa-calcium phosphate (Ca8-P) and apatite (Ca10-P).
Conclusion: The present study suggests the use of Thiobacilli along with elemental sulfur for the dissolution and enhancement of bio-available P in alkaline and calcareous soils.