Sulfur

Sulfur is a primary and macro nutrient delivered to higher plans and, most, organisms in the soil as soluble ions.

Sulfer has volatile components that can be lost from the soil to the atmosphere.

A constituent of several amino acids and vitamins
Found in numerous oils responsible for flavors of vegetables such as cabbage and onions

Sulfur deficiency visual symptoms are chlorosis and spindly plant parts. Sulfur is not mobile like nitrogen.

Sulfur deficiency is becoming more common due to increased purity of fertilizers, cleaner air (less SO₂), and increased demand for croppings.

Sources

Mineralization of soil organic matter (SOM)
Desorption of sulfate (SO₄²¯) held by anion exchange sites (lower horizons of hightly weathered soil)
Dissolution of sulfate minerals such as gypsum (CaSO₄)
Adsorption of sulfur dioxide gas from the atmosphere (plant foliage and soils)

Hydrogen sulfide (H₂S) is the most reduced form of sulfur, elemental sulfur (S) is intermediate, and sulfate ion (SO₄²¯) is the most oxidized form. Elemental sulfur and sulfides (S₂) are oxidized to sulfate (SO₄²¯) by soil microorganisms under aerobic conditions. Under anaerobic conditions, sulfate is reduced to sulfide. The microbially catalyzed oxidation sequence for inorganic S compounds sequence is S²¯ → S⁰ → S₂O₃²¯ → SO₄²¯

The oxidation of reduced forms of S consume oxygen and produce sulfate (SO₄²¯) and H⁺ ions. Every mole of S oxidized results in 2 moles of acidity (H⁺ ions). If iron sulfides are oxidized the hydrolysis of the iron also contributes to acidity (Fe³⁺ + 3 H₂O → Fe(OH)₃ + 3 H⁺).

The principle form of S taken up by plants is the sulfate ion (SO₄²¯).