Soil Formation and Profiles

Soil Formation

• Soil is the loose layer of weathered rock particles and decaying organic matter (humus) on Earth's surface.
• It acts as a medium for plant growth.
• Soil formation begins with weathering breaking down solid bedrock into smaller pieces.
• This weathering process continues breaking down the smaller pieces further.

Soil Formation Processes

• Most soil volume comes from weathered parent material of a source rock.
• Residual soil develops from weathered bedrock.
• Transported soil forms from weathered rock material moved and deposited elsewhere.

Soil Profile

• A soil profile is a vertical sequence of soil layers.
• Undeveloped soils lack distinct layers.
• Mature soils have distinct layers, which can take thousands of years to develop.
• Distinct layers in a soil profile are called soil horizons.
• Mature soils typically have four major horizons: O, A, B, C, and R (bedrock).

Soil Horizons

• O Horizon: Top layer of organic material like humus and leaf litter.
• A Horizon: Weathered rock combined with organic material from the O horizon, commonly dark brown.
• B Horizon: Enriched with clay and minerals, often red or brown due to deposits from water above or below.
• C Horizon: Near the parent material, with little to no organic matter and made of broken-down bedrock.
• R Horizon: Below C horizon, this is the unweathered parent rock.

Factors of Soil Formation

• Climate: A major factor influencing soil development; temperature, wind, and rainfall influence the type of soil formed.
• Topography: Slope and orientation of the land, particularly in the northern hemisphere, influences the amount of sunlight and vegetation, affecting the type of soil.
• Parent Material: If soil is residual, it has same chemical composition as bedrock. If transported, minerals differ from bedrock.
• Biological Activity: Presence of organisms (fungi, bacteria, plants, animals) influence soil type and reduce erosion.
• Time: Lengthy periods of weathering and erosion change original soil composition.

Soil Texture

• Soil texture is determined by the size of materials in the soil.
• Soil texture affects a soil's capacity to retain moisture and support plant growth.

Soil Fertility

• Soil fertility measures a soil's ability to support plant growth.
• Factors affecting soil fertility include topography, availability of minerals/nutrients, amount of microorganisms, precipitation, and acidity.

Soil Color

• Soil color reflects minerals, organic matter, and moisture levels within a soil horizon.