ESC101 Soils

Soil Texture and Composition

• The inorganic portion of soil consists of different-sized particles in varying proportions.
• Soil properties are determined by the combination of particle sizes and their proportions.
• Permeable soil allows water to flow through easily because of large, well-connected spaces between inorganic particles.
• Sandy or silty soils are permeable, water-draining 'light' soils because of their spaces.
• Soils with small spaces retain water.
• Clay-rich soils are heavier, hold together more tightly, and retain water.
• Loam is a soil containing a mixture of grain sizes.
• Soil scientists measure percentages of sand, silt, and clay to determine soil type precisely.
• Soil type is determined by plotting the percentages of sand, silt, and clay on a triangular diagram.
• Clay is at the top of the triangular diagram.
• Sand is at the left corner of the triangular diagram.
• Silt is at the right corner of the triangular diagram.
• Loams have less than 50% clay and are in the lower middle of the triangular diagram.

Soil Horizons and Profiles

• Residual soil forms over years via mechanical and chemical weathering of solid rock.
• Bedrock fractures through weathering from ice wedging or other physical processes.
• Water, oxygen, and carbon dioxide seep into cracks, causing chemical weathering.
• Plants like lichens or grasses establish and contribute to biological weathering.
• Weathered material accumulates until soil forms.
• Soil horizons develop as each layer undergoes progressive alteration, with the most alteration in the top layer.
• Each successive lower layer is altered gradually less due to reduced water and air contact.
• Soil horizons are the different layers of soil visible on a hillside.
• Soil profile refers to all the soil horizons put together.
• Simplest soils are composed of topsoil (A horizon), subsoil (B horizon), and C horizon.
• Soil horizons are distinctly visible.

Topsoil (A Horizon)

• The A horizon is commonly refered to as topsoil.
• Topsoil is usually the darkest layer due to highest organic material proportion.
• Topsoil is the region of intense biological activity, including insects, worms, and roots.
• Plant roots help hold topsoil in place.
• Minerals in topsoil dissolve in water and transfer to lower layers.
• Small particles like clay can be carried to lower layers by seeping water.

Subsoil (B Horizon)

• The B horizon is commonly referred to as subsoil.
• Subsoil is where soluble minerals and clays accumulate.
• Subsoil is lighter brown, retains more water than topsoil because of iron and clay minerals.
• Subsoil has less organic material than topsoil.

C Horizon

• The C horizon is a layer of partially altered bedrock.
• The C horizon contains evidence of weathering and pieces of original rock that can be identified.
• The amount of soil horizons varies by climate region.
• The amount of soil horizons varies by region.
• Some areas develop five or six distinct layers.
• Some areas develop thin soils or absent soils.

Types of Soils

• Soil scientists recognize thousands of soil types, each with specific characteristics.
• Pedalfer, pedocal, and laterite are three basic soil types.

Pedalfer

• Pedalfer is a dark, fertile soil that forms in forested regions.
• Deciduous trees require at least 65 cm of rain per year.
• Pedalfers are common in the temperate, eastern part of the United States.
• The word pedalfer comes from aluminum (Al) and iron (Fe).
• Pedalfers are very fertile, dark brown or black soils.
• Pedalfers are rich in aluminum clays and iron oxides.
• Soluble minerals are carried away, leaving less soluble clays and iron oxides due to high rainfall.

Pedocal

• Pedocal is an alkaline soil that forms in grassland regions.
• Pedocal soils form in drier, temperate areas with grasslands and brush.
• Climates forming pedocals have less than 65 cm of rainfall annually.
• Pedocals experience less chemical weathering and have less water.
• Pedocals contains more soluble minerals and fewer clay minerals compared to pedalfers.
• Pedocal soils have lower amounts of organic material and are less fertile.
• Pedocal is named for the calcite-enriched layer called caliche.
• Caliche is where soluble minerals concentrate due to the evaporation of water.

Laterite

• Laterite is a thick, nutrient-poor soil found in rainforests.
• Laterite soils form in hot, wet, tropical regions with intense chemical weathering.
• Tropical rainforests contains laterite because it rains everyday.
• Little to no humus and all soluble minerals and plant nutrients are removed.
• Laterite is commonly composed of least soluble materials, such as aluminum and iron oxides.
• Laterite soils are often red from the iron oxides and bake hard like brick when exposed to the sun.
• Each climate produces a unique soil type.
• Thinner soils and soluble minerals forms where there is less weathering.
• Thick, nutrient-poor soils form where there is intense weathering.
• Soil development is a slow process typically taking hundreds or thousands of years for fertile topsoil to form.
• Soil can form at an estimated rate of about 1mm/year under the best conditions.
• Poor conditions may require thousands of years for soil formation.