4.2 Soil Formation & Erosion

Soil

A mixture of geological (rock) material and organic components including sand, silt, clay, humus, nutrients, air, water, and living organisms; supports plant growth and filters water.

Humus

The main organic part of soil made from decomposed biomass such as leaves, dead animals, and waste; improves soil fertility and water retention.

Nutrients

Essential chemical elements in soil such as ammonium, phosphates, and nitrates that support plant growth.

Plants (Role of Soil)

Soil anchors plant roots and provides water, shelter, and essential nutrients like nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg).

Water (Role in Soil)

Soil filters rainwater and runoff by trapping pollutants in pore spaces and roots, allowing cleaner water to enter groundwater and aquifers.

Nutrient Cycling

The process where decomposers break down dead organic matter and return nutrients back into the soil for plant use.

Weathering

The breakdown of rocks into smaller particles that eventually form soil; can be physical, biological, or chemical.

Physical Weathering

Breakdown of rock by wind, rain, temperature changes, or freezing/thawing of ice.

Biological Weathering

Breakdown of rock by plant roots, burrowing organisms, or other biological activity.

Chemical Weathering

Breakdown of rock through chemical reactions such as acid rain or acids from moss and lichens.

Erosion

Movement and transport of weathered rock fragments by wind, water, or gravity, followed by deposition in new areas.

Soil Formation (From Below)

Weathering of parent rock material creates smaller fragments that make up the inorganic portion of soil.

Soil Formation (From Above)

Breakdown of organic matter adds humus while erosion deposits additional soil particles from surrounding areas.

Parent Material (Effect)

The original rock influences soil pH, mineral content, and nutrient availability.

Topography (Effect)

Steep slopes experience more erosion, while flatter ground allows deposition and deeper soil formation.

Climate (Effect)

Warm temperatures speed up organic breakdown and high precipitation increases weathering and soil formation.

O Horizon

The top layer of soil made of surface organic matter such as leaves, plant roots, and animal waste; provides nutrients and reduces water loss.

A Horizon (Topsoil)

A layer of humus mixed with minerals from parent material; contains most biological activity like earthworms and microbes that release nutrients.

B Horizon (Subsoil)

A lighter-colored layer with mostly minerals and little organic matter; contains some nutrients leached down from above.

C Horizon

The least weathered layer made from partially broken-down parent material, sitting above solid bedrock.

Soil Degradation

A decline in soil's ability to support plant growth due to erosion, nutrient loss, or compaction.

Loss of Topsoil

Removal of nutrient-rich surface soil caused by erosion, often triggered by loss of vegetation; leads to nutrient loss and soil drying.

Compaction

Compression of soil by machinery or livestock that reduces pore space, limits water infiltration, and increases erosion.

Nutrient Depletion

Repeatedly growing crops on the same land without replenishing nutrients, reducing soil fertility over time.