The sulfur cycle describes how sulfur moves through the Earth’s systems — the lithosphere, atmosphere, hydrosphere, and biosphere.
- Sulfur sources: Sulfur is stored in rocks (lithosphere) and released by weathering, volcanic eruptions, hot springs, and the decay of organisms in swamps and bogs.
- In the atmosphere: Gases like hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) are released naturally and by burning fossil fuels.
- Chemical changes: SO₂ reacts with oxygen to form sulfur trioxide (SO₃), which then combines with water to make sulfuric acid (H₂SO₄) or with ammonia to form sulfate salts.
- Precipitation: These compounds return to Earth through rainfall, adding sulfates to the soil.
- In the biosphere: Plants absorb sulfate from the soil to make proteins. Animals obtain sulfur by eating plants. When organisms die and decompose, sulfur is released back into the environment as hydrogen sulfide or sulfate.
Bacteria play key roles: Anaerobic bacteria convert hydrogen sulfide to sulfur gas, while aerobic bacteria change sulfur back into sulfate—completing the cycle.
In short, sulfur continually cycles between the earth, air, water, and living things—supporting life and maintaining balance in ecosystems.
📄 Free Worksheets
Check out our full list of free worksheets!
Teachers: Use these worksheets to support your classroom lessons.
Students: Use them to practice and learn something new.
