Section 9: Soils & Groundwater

GES 108

Soil

The layer of weathered material and organic matter covering Earth’s surface where plants grow. It is essential for life, providing nutrients, water, and a foundation for ecosystems.

Pedogenesis

The process of soil formation, influenced by the weathering of rocks (parent material), biological activity, rainfall, topography, and time. Life and water are critical to this process.

Bedrock

The solid rock layer that is part of Earth’s crust and lies beneath soil and regolith. When exposed at the surface, it is called an outcrop.

Outcrop

A visible exposure of bedrock at Earth’s surface.

Regolith

The layer of loose, unconsolidated Earth material covering bedrock, including soil and broken rock fragments.

Soil Horizon

A distinct layer within the soil profile that differs in color, composition, and texture. Soil horizons are created by pedogenesis and typically follow the sequence O, A, E, B, C, and R from top to bottom.

Leaching

The process by which rainwater dissolves and carries nutrients or chemicals downward through the soil. Leaching helps form soil layers but can also transport pollutants.

Eluviation

The downward movement of fine soil particles (such as clay and organic material) from an upper layer, caused by percolating rainwater. This process contributes to the development of soil horizons.

Illuviation

The accumulation or deposition of materials, such as iron or aluminum, that have been transported downward by eluviation into lower soil layers.

Physical Weathering

The mechanical breakdown of rocks into smaller fragments without changing their chemical composition. Examples include freeze-thaw cycles and abrasion.

Chemical Weathering

The process by which minerals in rocks are chemically altered or dissolved by reactions with water, oxygen, or acids.

Parent Material

The original rock or sediment that is weathered to form soil. It influences the soil’s chemical composition, texture, and fertility. Harder rock forms thinner soil layers, while softer rock forms thicker soils.

Topography

The shape and slope of the land surface, which affects drainage and soil depth. Steeper slopes tend to have thinner, less developed soils.

Aspect

The compass direction that a slope faces, which affects sunlight exposure and soil moisture. In the Northern Hemisphere, south-facing slopes are typically warmer and drier, while north-facing slopes are cooler and moister.

Climate (as a Soil Factor)

The temperature and precipitation of a region, which influence soil development. Cold regions have thin soils; temperate regions have well-developed soils; and tropical regions have nutrient-poor soils due to heavy rainfall.

Anthropogenic Soil

Soil created or heavily modified by human activity, often over short time periods. These soils can form within weeks or months and require large amounts of energy, carbon, and organic material.

Time (in Soil Formation)

The duration over which soil-forming processes occur. Natural soils typically take 200 to 1,000 years to form, while anthropogenic soils form much faster.

Lava Flow/Igneous Bedrock

Newly formed igneous rock that has not had sufficient time to weather or develop soil. The younger the lava flow, the less developed the soil on top of it.

Soil Erosion

The removal and transport of soil by wind, water, or human activity. It can occur rapidly or slowly over time, often beginning with the loss of topsoil.

Topsoil

The uppermost and most fertile layer of soil, rich in organic material and essential for plant growth. It is the first to erode.

Soil Texture

The relative proportions of sand, silt, and clay in a soil sample, which determine its permeability and fertility.

Soil Degradation

The decline in soil quality due to erosion, nutrient loss, or contamination.

Loam

A soil composed of roughly equal parts sand, silt, and clay, considered ideal for agriculture due to its balanced texture and moisture retention.

Anthropogenic Erosion Solutions

Modern efforts to replace lost soil through human-made or managed soils to combat rapid erosion from agriculture and deforestation.

Soil Color

An indicator of soil composition and environmental conditions, such as organic content, mineral presence, and moisture.

Black or Dark Brown

High organic content (temperate grasslands).

Brown

Iron oxides and high organic matter (temperate deciduous forests).

Red or Orange

Strong chemical weathering of iron and aluminum (tropical rainforests).

Blue or Green-Gray

Persistently saturated soils (estuaries).

Gray

Heavy leaching of iron (boreal forests).

Soil Order

A broad classification of soils based on their properties and stage of development.

Andisol

Soil formed from volcanic ash; highly fertile but easily erodible.

Entisol

Poorly developed soil with little to no horizon formation, often found in areas of recent deposition.

Histosol

Soil formed in wetlands or areas with deep organic matter; rich in carbon but lacking horizon development.

Inceptisol

Young soil with weakly developed horizons, typically found in mountainous or recently disturbed regions.

Aridisol

Desert soil with little organic matter and rapid drainage.

Oxisol

Deeply weathered tropical soil rich in iron and aluminum but low in nutrients.

Ultisol

Old, clay-rich soil common in humid subtropical regions, such as the southeastern United States.

Vertisol

Clay-dominated soil that shrinks and cracks when dry; poor in organic matter.

Alfisol

Moderately fertile soil developed in humid climates, typically found under forests.

Gelisol

Soil containing permafrost or evidence of frost activity, found in very cold climates.

Mollisol

Fertile, humus-rich soil with a thick, dark A horizon, common in grasslands and highly productive for agriculture.

Spodosol

Acidic, nutrient-poor soil with a bleached E horizon, typically found under coniferous forests in cold, snowy regions.

Surface Water

Freshwater found on Earth’s surface in rivers, lakes, and streams, accessible for human use.

Porosity

The percentage of open space (pores) in soil, sediment, or rock that can hold air or water. Higher porosity means more storage capacity.

Permeability

The ease with which water can flow through soil or rock, determined by the size and connectivity of pores.

Infiltration

The process by which water on the ground surface enters the soil under the influence of gravity.

Infiltration Rate

The speed at which water enters the soil, influenced by porosity, permeability, and soil saturation.

Percolation

The movement of water through the soil, between particles, and through narrow channels, driven by cohesion and adhesion rather than gravity.

Aquifer

A body of permeable rock or sediment that stores and transmits groundwater. Aquifers are critical sources of water, especially in arid regions.