Soils 205-90

Lecture 14- Sulfur

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A. Soil-Plant Relations

1. Like N, S is a component of important biomolecules like amino acids, vitamins

2. Deficiency symptoms

· small and spindly

· yellowish leaves

· protein quality degraded

3. Recent deficiencies

· low S fertilizers

· clean air standards - lowered SO₂

· more S-removal by crops

4. Deficient areas

· high rainfall

Palouse region

5. Crop needs

· larger - legumes

· lesser - grasses and cereals

B. S-cycle

1. mineralization
2. immobilization
3. S oxidation
4. S reduction

 

like N - microbial redox processes control S cycling

C. Mineralization

1. Organic-S ¾® S^-2

2. S release - no oxidation state change

3. S^-2 is oxidized upon release - no accumulation

D. Immobilization

1. Organisms use SO₄^-2 for S requirement (like N)

2. Less S required by organisms

= less common than N-immobilization

 

E. S oxidation and reduction

1. Oxidation

(a) S^-2
sulfide

or ¾¾(O₂)¾® SO₄^-2 (sulfate)

S°
elemental S

(b) Autotrophic microorganisms

· energy obtained from S oxidation

(c) Aerobic process

· O₂ required

· rapid in well-aerated soils

(d) Acid producing process

· S° or S^-2 ¾® H₂SO₄

(e) SO₄^-2 is used by plants

2. Reduction

(a) SO₄^-2 ¾¾® S° ¾¾® S^-2

(b) Anaerobic process

· organisms use S in place of O₂ or N

as a terminal e- acceptor

(c) S^-2 can be toxic

· forms H₂S gas

· reacts with ions like Fe & Mn Þ insoluble compounds

(d) mostly, heterotrophic organisms

· require organic matter for energy source