Understanding Soil Formation and Importance

Mollisol

Soil formed in sandstone - SE Minnesota

Soil

The "living" skin of the Earth, a natural body consisting of solids, liquids, gases, distinguishable from parent material due to pedogenesis.

Parent material

The original material from which soil is formed.

Pedogenic

Processes involved in soil formation.

Importance of Soils

Soils hold more carbon than the atmosphere and organisms combined.

Reservoir Gigatons C

Soils: 1,500-2,400; Oceans: 38,000; Atmosphere: 730; Land Organisms: 500-700; Marine organisms: 3; Rocks: 60,000,000; Permafrost Soils: ~1,700.

Soil Morphology

The physical properties of soil, described by texture, structure, color, and others.

Soil texture

The percentage of sand, silt, and clay in the soil.

Sand

Soil texture with grain size of 2-0.050 mm (2000-50 μm), described as gritty.

Silt

Soil texture with grain size of 0.050-0.002 mm (50-2 μm), described as slippery.

Clay

Soil texture with grain size of <0.002 mm (<2 μm), described as sticky.

Soil structure

Aggregates of individual soil particles formed by clumping from electrostatic attraction and binding by organic materials.

Granular structure

Soil structure formed from burrowing animals and organic matter.

Blocky structure

Soil structure resulting from clay particle buildup.

Prismatic structure

Soil structure formed from repeated wetting and drying.

Soil color

Varies considerably within the same soil, influenced by organic matter, types of minerals, and moisture content.

Soil horizons

Roughly horizontal layers formed as a result of pedogenesis, characterized by uniform texture, structure, and color.

Master horizons

Layers in soil: O (organic layer), A (organic matter + mineral grains), E (eluviated horizon), B (illuvial horizon), C (parent material).

O Horizon

Organic layer consisting of plant litter.

A Horizon

Layer rich in organic matter mixed with mineral grains.

E Horizon

Eluviated horizon characterized by losses of iron and aluminum.

B Horizon

Illuvial horizon where iron and aluminum are added back into the soil.

A soil layer where materials like clay, iron, and aluminum from upper layers are deposited, making it denser and often red or brown in color.

C Horizon

Parent material layer in soil.

Translocation

Physical movement of clay and other materials by water in soil.

Soil Formation Factors

Soils are the result of five factors: Climate, Organisms, Relief (Topography), Parent material, Time.

Residual Soil

Parent material is the bedrock.

Transported Soil

Parent material has been carried from elsewhere and deposited.

Climate

The most influential control of soil formation, affecting temperature and precipitation.

Weathering

Influences the rate and depth of soil weathering.

Leaching

Amount of precipitation influences the rate materials are leached from the soil.

Tropical vs. Temperate Soils

Different climates lead to different soil types.

Organisms

Plants and animals, including microorganisms, influence the soil's physical and chemical properties.

Humus

Decaying plant and animal debris that forms organic acids and furnishes organic matter to soil.

Soil Moisture Retention

Organisms help retain soil moisture.

Soil Temperature

Orientation of the slope influences soil temperature.

Soil Degradation

Activities which lower the capacity of the soil to support life.

Causes of Soil Degradation

Includes agriculture, overgrazing, industrialization, and deforestation.

Vegetation's Role

Vegetation reduces degradation by protecting soil from raindrop impacts.

Soil Erosion

Occurs following logging and fire.

Soil Taxonomy

Grouping soils based on their properties in a hierarchical system.

Alfisol

Characterized by a thin A horizon, light colored E horizon, clay accumulation in B horizon, found in deciduous forest soils.

Mollisols

Thick, dark A horizons found in grasslands, the most naturally fertile soils in the world.

Mollisol Formation

Dry climate favors grasses
(fire helps)
• Roots add OM belowground
• Abundant burrowing animals
also mix OM down
• Dry, cool climate leads to slow
OM decay

Results in Thick, dark, Om-rich, a horizon.

Thin and Rocky Soil

Most likely found on a steep slope with a high erosion rate, indicating dominance of relief.

Thick, Dark A Horizon

Indicates a soil formed under grassland, characteristic of Mollisol.