Sulfur Dynamics in Soils

1. Sulfur Dynamics in Soils Introduction Sources of Sulfur Sulfur Cycle Behavior of Sulfur Compounds Sulfur Management Conclusion

2. 1. Introduction • Sulfur is very important in many reactions in living cells • It is also at the center of many air, water and soil pollution • Acid precipitation • Acid sulfate soils • Forest decline, etc.

3. Sulfur in Plants and Animals • Constituent of amino acids –protein • Constituent of vitamins –B1 • It is an important element for plants. • Deficiencies of Sulfur • Spindly and tiny stems • Slow growth delayed maturity • Chlorotic light yellowish green appearance

4. Sulfur Deficiency in Plants

5. 2. Sources of Sulfur • Organic matter • 90-98% S is in the organic form • Microorganisms break this down to sulfate • Soil minerals • Minerals of sulfur are sulfates and sulfites • Sulfate minerals are common in regions of low rainfall • These are very soluble and sulfate ion is easily easily assimilated by plants

6. The atmosphere Atmosphere contains H2S, SO2 and other sulfur gases, and dust particles. • The gases come from volcanic eruptions, volitilization from soils, industrial plants, metal smelters, and biomass fires. • Some of the materials oxidize in the atmosphere to sulfate, forming H2SO4 and CaSO4 and MgSO4 • The solids return as dry particles and gases -drydeposition • Solids return to earth with precipitation –wet deposition

7. 3. The Sulfur Cycle

8. The Sulfur Cycle • Major transformations of sulfur • Four forms of sulfur • sulfides • sulfates • organic sulfur • elemental sulfur • Relationship between these major forms • Major sources of sulfur and how it’s lost in air

9. Sulfur Vs Nitrogen • Like Nitrogen, atmosphere is an important source of sulfur • Like N, S is largely in soil organic matter • Like N, S is subject to microbial oxidation and reduction • Both enter and leave soils in gaseous form • Both are subject to leaching in anionic form

10. 4. Behavior of Sulfur Compounds in Soils • Mineralization • To be usable by plants, the organic forms of S must be mineralized by soil organisms • The rate of mineralization depends on: • Moisture • Aeration • Temperature • pH

11. Immobilization • Occurs when organic matter is low in Sulfur –C/S ratio > 400:1 • Microorganisms need the organic matter to grow and in that process assimilate the sulfate into microbial tissue • During microbial breakdown of organic materials many sulfur containing gases are formed –H2S, CS2 especially in anaerobic soils that are either adsorbed by colloids or released to the atmosphere.

12. Oxidation Reduction of Sulfur • This reaction shows how the oxidation of sulfur is a soil acidifying process • Thus elemental sulfur can be applied to extremely alkaline and sodic soils of arid regions to reduce pH of the soils

13. 5. Sulfur Management

14. Sulfur Management • The objective of sulfur management is to maintain adequate quantities of sulfur for mineral nutrition of plants • This can be achieved through • Sulfur fertilizer application • Using crop residues and farm yard manures to replenish sulfur from crop removal

15. Conclusion • Sulfur and N have a lot in common in their global cycles • Both are held as part of soil organic matter • Their release to inorganic ions (SO42- and NO3-) is accomplished by microorganisms • Anaerobic conditions change both elements into gaseous forms, released to the atmosphere and deposited to plants, soils and other materials through wet and dry deposition • Some microorganiams have the ability to fix N, but no microorganism is known to fix sulfur.