Chapter II. Soil formation and its factors

Regolith

Unconsolidated, weathered debris that overlies bedrock; its upper portion can develop into soil.

Parent Material

The upper regolith providing minerals and texture for soil development, directly influenced by soil-forming factors.

Residual (Sedentary) Parent Material

Soil parent material formed in place from the weathering of underlying bedrock.

Transported Parent Material

Unconsolidated material moved from its source by water, gravity, wind, ice, or waves before soil formation.

Alluvial Deposit

Parent material transported and laid down by rivers or streams.

Colluvial Deposit

Coarse gravel or rock fragments moved downslope by gravity.

Eolian Deposit

Fine clay, silt, or sand carried and deposited by wind.

Lacustrine Deposit

Sediments deposited in lake bottoms that later become parent material.

Marine Deposit

Materials laid down on sea or ocean floors and later uplifted to form soils.

Glacial Deposit

Parent material transported and dropped by moving or melting ice.

Climate (Active Soil-Forming Factor)

Combination of temperature and precipitation that strongly controls weathering, organic matter production, and soil development.

Temperature Effect

Increased heat speeds expansion–contraction cracking and boosts organic matter production where moisture is adequate.

Precipitation Effect

Higher rainfall accelerates chemical weathering, leaching, and plant growth—provided temperatures permit vegetation.

Relief / Topography

Landscape position; slopes lose water and soil by runoff/erosion, while lower areas receive water, favoring deeper soils.

Run-Off

Water flowing off slopes that removes soil and slows profile development.

Biotic Factor

Influence of vegetation, microorganisms, and animals that produce acids, mix soil, and enhance infiltration.

Earthworm Activity

Burrowing by worms increases aeration and water entry, hastening soil formation.

Time Factor

Duration over which other factors act; older soils show more weathering, clay accumulation, and mineral alteration.

Embryonic Soil

Initial stage consisting mainly of fresh parent material (C horizon) with minimal profile development.

Young Soil

Profile displaying an A horizon over C horizon; clay formation has just begun.

Old Soil

Weathered profile rich in clay, dominated by 1:1 clays and sesquioxides; most easily weathered minerals removed.

Very Old Soil

Highly weathered profile where clay (<0.002 mm) dominates and soil-forming processes have nearly ceased.

Sesquioxides

Oxides of iron and aluminum that accumulate in old, intensely weathered soils.

1:1 Clay Minerals

Clays such as kaolinite with one silica sheet per alumina sheet, common in highly weathered soils.

2:1 Clay Minerals

Clays like smectite and illite with two silica sheets per alumina sheet, more common in less-weathered soils.

Quartz Fertility Status

SiO₂ mineral contributes virtually no plant nutrients to soil fertility.

Biotite Contribution

K(Mg,Fe)₃AlSi₃O₁₀(OH)₂ mica supplying potassium, magnesium, and iron to soil.

Sandstone Weathering Rate

Weakly cemented sandstone can disintegrate 1 cm in roughly 10 years under humid conditions.

Quartzite Weathering Rate

Metamorphosed sandstone weathers so slowly that products are usually removed before soil can form.

Limestone Residue Formation

In humid regions it may take about 100,000 years to produce 30 cm (1 ft) of weathered residue.

Soil Formation

The set of natural processes responsible for converting unconsolidated parent material into true soil.

CLORPT

Mnemonic for the five soil-forming factors—Climate, Living organisms, Relief/topography, Parent material, and Time—expressed as Soil = f(clorpt).

Parent Material

The unconsolidated mineral or organic matter from which soil develops.

Two Broad Processes of Soil Formation

(1) Formation of parent material, and (2) Evolution of soil layers (horizons).

Soil Development

Changes in the soil profile caused by leaching, translocation of colloids, accumulation of organic matter, and continued weathering.

Soil Profile

A vertical exposure of soil showing its sequence of horizons.

Soil Horizon

A distinct layer within the soil profile that runs parallel to the land surface.

Horizonation

The process by which soil horizons are formed.

Soil-Forming Processes

Collective term for gains/additions, losses, transformations, and translocations occurring within soil.

Gains (Additions)

Inputs to soil such as organic matter, precipitation water, atmospheric gases (O₂, CO₂), nutrients (N, Cl, S), sediments, and solar energy.

Losses

Removal of soil constituents through processes like leaching of soluble salts, evaporation, denitrification, oxidation of organic matter, erosion, radiation, and runoff.

Transformations

Chemical or physical changes within soil, including rock weathering, mineral alteration from primary to secondary forms, particle size reduction, and clay–organic matter reactions.

Translocation

The movement of mineral and organic particles, nutrients, soluble salts, or entire soil masses from one horizon to another, often downward by water or upward via plant circulation and animals.

Leaching

Downward movement of soluble materials through the soil, often classified under losses and translocation.

Colloid Translocation

Movement of fine clay and organic particles from topsoil to subsoil, contributing to horizon differentiation.