Soil Components and Weathering: Texture

Vocabulary flashcards covering core concepts of soil components, weathering, texture, and related properties.

Soil is a 3-phase system

Soil consists of solids, liquids, and gases; in ideal soil this is ~50% solids, ~25% liquids, and ~25% gases; the pore space is the volume that holds air and water.

Pore space

The portion of soil’s volume that contains air and water.

Mineral matter

Inorganic particles derived from rocks and minerals; varies in size and mineralogical composition; the solid fraction also includes living-origin material (organic matter).

Organic matter (SOM)

Material of living origin (and its remnants) that is part of the soil’s solid phase.

Solid phase in soils

The solid constituents of soil, including mineral matter and organic matter, which together form the soil’s structure.

Three core soil properties

Texture, mineralogy, and soil organic matter; these properties determine soil’s ability to provide ecosystem services and SOM can be rapidly manipulated.

Texture

The relative proportion of sand, silt, and clay in a soil; a very stable property that does not change much in a human lifetime.

Mineralogy

The mineral composition of the soil, i.e., which minerals are present and in what proportions.

Soil Organic Matter (SOM) management

Management of SOM is central to sustainable soil management because SOM responds quickly to changes and can be manipulated.

Rock vs mineral

A mineral is a solid crystalline material with defined chemical/physical properties; a rock is a natural mixture of two or more minerals.

Mechanical weathering

Breaking rocks into smaller pieces without changing their chemical composition; accelerates in very cold or very dry environments.

Biogeochemical weathering

Weathering that changes the chemical composition of rocks/minerals; most intense where climate is wet and hot.

Hydrolysis

A biogeochemical weathering process where minerals’ chemical composition and size are altered by water, often forming clay minerals.

Dissolution

Minerals dissolve in water, releasing ions; acids accelerate the process (e.g., limestone dissolving with rainwater, pH ~5.5).

Oxidation

Chemical reactions involving free oxygen; e.g., rust formation from iron reacting with air.

Limestone dissolution by rainwater

A specific dissolution example where limestone dissolves due to rainwater (acidic), releasing ions into solution.

Frost wedging

Mechanical weathering by freeze-thaw cycles that break rocks apart.

Root fracturing

Mechanical weathering where growing plant roots crack and split rock.

Abrasion by water, wind, and gravity

Mechanical weathering where particles wear away rock surfaces through physical contact and movement.

Life accelerates weathering

Biological activity speeds up both mechanical and chemical weathering; without life, chemical weathering would be much slower.

Weathering and rock type

The type of rock/mineral (e.g., slate vs marble) influences how it weathers and the soil properties that form from it.

Soil Particle Size

Continuous weathering produces smaller pieces; size is a stable property and strongly influences soil function.

Soil separates

Each distinct particle size class used to categorize soil particles; particles >2.0 mm are not considered soil.

Sand (particle size)

Soil particles sized 0.05–2.0 mm.

Silt (particle size)

Soil particles sized 0.002–0.05 mm.

Clay (particle size)

Soil particles smaller than 0.002 mm.

Soil texture

The relative proportion of sand, silt, and clay in soil; a stable property used to describe texture classes.

Texture naming rule

Textures are named based on the dominant particle size; exceptions include loam when no single size dominates.

Loam

A texture class used when neither sand, silt, nor clay dominates soil characteristics.

Examples of texture names

Names such as clay, silty clay, fine sand; loam is used when domination is not clear.

Determining soil texture (methods)

Hand texturing (field/home) and sedimentation (lab) to determine proportions of sand, silt, and clay.

Importance of soil texture

Influences water holding capacity, permeability, workability, and vegetation growth/distribution.

Texture and climate vulnerability examples

Clays → prone to compaction and flooding; sands → more erosion and drought susceptibility.

Specific Surface Area (SSA)

Surface area per unit mass of particles; higher for smaller particles; units in m2/kg or m2/g; clay has high SSA, sand has low SSA.

SSA and soil functions

SSA affects water films, nutrient retention, cation exchange capacity, weathering, aggregation, and microbial colonization.

Cation Exchange Capacity (CEC)

Ability of soil to hold and exchange cations (positively charged ions) on particle surfaces; important for nutrient availability.