The Study of Occurrence of Sulfur Oxidizing Microorganisms and Sulfur Oxidation Potential in Agricultural Soils of Four Provinces of Iran.

In calcareous and alkali soils, which have expanded in large areas of Iran, Due to the high levels of Ca++ in soil solutions and high pH values, the soil nutrients, whose absorptions depend on pH, will become less available and thus limit the growth of crop plants. It has been a common practice in many regions of the world to use elemental sulfur as an acidifying source to lower pH and to improve the availability of some soil nutrients. In addition, the supply of sulfur used for the reclamation of sodium affected soils serves as a source of plant nutrient and more importantly acts as a disease controlling agent. Application of sulfur is one way to increase the availability of insoluble nutrients in calcareous and alkaline soils. Effectiveness of elemental sulfur depends on the activity of sulfur oxidizing bacteria especially, Thiobacillus genus. Sulfur application in agricultural practices deserves more consideration especially, since a large quantity of this material is produced in the country, where a great percentage of the farmlands are calcareous. A major requirement in utilizing elemental sulfur is the ability of the soils to oxidizing applied sulfur. Determination of sulfur oxidation potential of a soil the ability of the soil to oxidizing applied sulfur would be obtained. So that in soils with low oxidation potential by using appropriate management, sulfur oxidation would increased. In this research from Kerman, Mazandaran, Azarbaijane- sharqhi and Khorasane-razavi. agricultural soils 50 soil samples were taken from 0-30 cm. Then some chemical and physical properties of soils as well as sulfur oxidizing microorganisms were measured. In the second part of the research, sulfur oxidation potential of the soils were evaluated as completely randomized experiment with 4 replicatins, using two levels of sulfur( 0 and 0.5%). After 2 months incubation at 28oC, some soil chemical properties such as pH. EC, available P, Fe, Zn, S and oxidized sulfur were measured for evaluating the effect of soil properties on sulfur oxidation rates. The results indicated that in many soil samples the rate of sulfur oxidation and consequently increasing EC, P, Fe Zn and SO4 2- and decreasing soil pH were noticeable and significant. Among different soil properties, soil organic matter and clay content had the highest effect on sufur oxidation rates, so the maximum oxidation values observed in soils with higher clay and organic matter. The results of sulfur oxidizing microorganisms enumeration indicated that only in soil samples which sulfur was applied previously, the number of sulfur oxidizing microorganisms were detectable while in other soil samples were negligible or zero. The highest and least number of Thiobacillus bacteria were enumerated in corn and wheat cultivated soils of Kerman and soils of Khorasane-razavi province respectively.