Decreasing the application of micro nutrients bearing fertilizers through pilote production of Thiobacillus bacteria and its use in agriculture.

Sulfur is an essential element for plant growth and development, which is used to decreasing soil pH and controlling some soil born disease. The chemical sulfur oxidation is very slow, while sulfur oxidizing microorganisms application would accelerate the oxidation process and soil pH declane. The Thiobacillus bacteria are the most important sulfur oxidizing microorganisms. In order to collect and identify Thiobacillus bacteria in some soils and waters of Iran and to determine their effect on lowering the soil pH, 72 agricultural soils from the provinces of Mazandaran and Ardabil, Damavand region, the farms around Tehran refinery, and sulfur springs of the province of Boushehr, Ramsar and Ardabil were sampled. 245 isolates were obtained and their morphological characteristics and reactions to Gram stain were studied. Out of those strains which were fastest in lowering solution pH, 8 strains which did not oxidize iron and did not reduce nitrate were selected and identified and named according to the Bergey' classification system, using tests such as growth characteristics in different temperatures and growth in different concentrations of sodium chloride, heterotrophic growth, autotrophic growth, mixotrophic growth, growth at different pH levels, and their elemental sulfur as well as thiocyanate oxidizing abilities. The effects of these bacteria on two different soils that had different calcium carbonate contents were studied. These soils were either sterilized or left unsterilzied and treated with sulfur at three levels of 0, 0.5 and 1%. The test results from the Thiobacillus selection and identification showed that 73% of the agricultural soils contained neutrophile and 27% contained acidophile Thiobacilli. Of the eight selected bacteria, five were identical with Bergey' system Thiobacilli and three had characteristics which distinguished them from those classified by the system mentioned. Studies on the effect of Thiobacillus-inoculated, sterilized soils on lowering soil pH showed that the pH of the soil containing 4% calcium carbonate decreased by 0.8 after seven weeks, and the pH of the soil containing 12% calcium carbonate decreased by 0.76 after 5 weeks. The same results were obtained with unsterilized soils with regard to the pH changes. The lowering of pH values ranged from 0.2 to 0.4 in a sulfur-fertilizer treated uninoculated soil after seven weeks. This study showed that adding sulfur to soils during the growing season increases the number of sulfur oxidizing microorganisms which could possibly lower the pH of calcareous soils during the following growing season. Furthermore, lowering pH levels in the soils containing 12% calcium carbonate shows that inoculating calcareous soils which are devoid of sulfur oxidizing bacteria, with Thiobacilli (and adding sulfur) is a very effective way to lower the soil pH.