Effect of Sulfur Biological Oxidation Trend on Release of Iron and Zinc

Due to the high levels of Ca++ in soil solutions and high pH values in calcareous and alkali soils that exist in large areas of Iran, the soil nutrients, whose absorptions depend on pH, will become less available. 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. A major requirement in utilizing elemental sulfur to lower soil pH is the presence and activity of sulfur oxidizing bacteria (especially the Thiobacilli) in the soils. This investigation involved the effects of sulfur rates, Thiobacillus inoculant on sulfur oxidation trend and the subsequent release of iron and zinc in calcareous soils. In this study, a completely randomized factorial experiment was conducted using three replications, eight levels of sulfur, two levels of Thiobacillus inoculant, in two different calcareous soils. During three months incubation period, care was taken to keep the soil moisture at F.C and temperature at 28-30oC. Soil samples were taken at time intervals 0, 15, 30, 60, and 90 days after incubation. Then, soil pH, EC and available sulfate, iron, and zinc were measured in the samples. The analysis of data by SAS software showed that during the experimental period the nutrients availability, such as sulfate and iron, increased in both calcareous soils but available zinc increased in one of the study and reached maximum content 60 days after incubation. Also, application of sulfur increased the concentrations of sulfate, iron, and EC, but decreased soil pH. Additionally, increasing sulfur application rates, increased nutrient concentrations and EC, but decreased soil pH as compared with the control.