SOIL222 - Topic 6: Soil Profile Formation - Chemical Processes

Soil Horizons

• Definition: Roughly horizontal layers in soil reflecting soil formation processes.

• Layers: O, A1, A2 (topsoil), B (subsoil), C (parent material), R (bedrock).

• O and A horizons are influenced by organic matter and were noted as rare in Australia.

Organic Matter Inputs

• Sources include plant residues, animal remains, and microorganisms.

• Key for soil quality and productivity.

Plant Residue Composition

• Composition: Mostly C, H, O (90%), remaining 8-10% includes N, S, cations, P, etc.

• Key components: Carbohydrates, proteins (rapid decomposition), cellulose (slow), and lipids (recalcitrant).

Decomposition Rates

• Fast pool: 1 year (<10%), slow pool: 10-100 years (40-80%), passive pool: 1000+ years (10-50%).

• Controlled by chemistry, palatability, environmental conditions (moisture, temperature, O2).

Benefits of Soil Organic Matter (SOM)

• Enhances chemical and physical fertility.

• Contains nutrients essential for plant growth and promotes soil structure.

Carbon Storage in Soils

• Distribution influenced by climate, soil type, and vegetation.

• Higher organic matter found in cold, moist climates and in clay-rich soils.

Leaching Processes

• Removal of soluble materials (e.g., salts, nutrients) from the soil.

• Degree of leaching influenced by climate, soil permeability, and slope position.

Texture Contrast Soils

• Characterized by abrupt changes in clay content between horizons.

• Formation theories include clay movement and bioturbation/erosion by soil organisms.

Waterlogging Effects

• Excessive water leads to poor aeration and mottling in soils.

Salinity Issues in Australia

• Salt accumulation due to limited leaching; currently affecting significant land area.

• Related to land clearing practices leading to increased salinity risk.