Soil Lecture 5, Soil Formation + Transportation

Class logistics and lab context

• The speaker introduces practical lab guidance: wear long pants and closed-toed shoes; no sandals. Bring your soil sample for labs.
• If not formally introduced, the coordinator for labs is Irene Paul; she is described as the most important person in the world for the course because she coordinates labs and handles issues that arise.
• For quizzes on Canvas, check the quiz title on Canvas to know exactly what material is covered (read the specified chapter or material). The instructor emphasizes there are no surprises about quiz content.
• This portion covers soil formation, modes of transport of sediment, soil horizons, and horizonation; it ties to earlier ideas about soil-forming factors and environment.

Overview of foreign material and modes of transport

• Foreign material (sediment) is deposited by different processes and environments; the material type depends on transport mechanism and energy of the environment.
• Major modes of transport discussed:
  • Colluvial (gravity): deposition down slope; common on hill slopes and at the base of slopes.
  • Alluvium (water): deposition by streams; includes floodplains and channels; sorting depends on distance from source and flow energy.
  • Glacial (ice): deposition by glacial processes; includes glacial till (unsorted, deposited directly by ice) and glacial outwash (sorted by meltwater).
  • Aeolian (wind): deposition by wind; includes dune sands and loess (finer particles carried farther); highly important in some arid and semi-arid regions.
• Key terms:
  • Sorting: the degree to which sediment grains are uniform in size; alluvium can be well-sorted in floodplains; till is unsorted due to ice transport; wind can deposit very fine material (loess) far from source.
  • Edges and rounding: colluvial materials often have sharp edges and are angular; glacial till tends to be unsorted with varied grain sizes; after glacial recession, smoother deposits may occur due to subsequent weathering and transport.

Colluvial material (gravity-driven)

• Deposited by gravity, not transported far from the source.
• Characteristics:
  • Found downslope from source areas, not at the summit.
  • Poorly sorted with a mix of large and small clasts; edges may be sharp; limited rounding due to mechanical breakage rather than abrasion.
• Significance: marks the start of deposition on hillslopes and contributes to mixed material on the slope.

Alluvium (water transport)

• Deposited by streams; includes a range of sediment types from coarse to fine.
• Sorting and stratification:
  • In floodplains or near stream edges, material becomes progressively finer from channel to floodplain due to energy changes.
  • Large/heaviest particles tend to settle first; finer particles travel farther downstream.
  • Downstream transport results in well-sorted materials in certain habitats (e.g., floodplains) and poorly sorted deposits where energy fluctuates.
• Relevance to soils:
  • Alluvial deposits are common in coastal plains and near rivers; can form distinct soil profiles depending on deposition history and subsequent soil formation.

Glacial deposition (ice-age processes)

• Ice age concept: large areas covered by ice, which erodes and transports material from the surface below the ice.
• Two main products:
  • Glacial till: material deposited directly by ice as it melts; characteristics are unsorted, with a range of grain sizes, and often angular clasts due to abrasion under ice.
  • Glacial outwash: meltwater streams carry and sort sediments after ice retreat; deposits are better sorted than till and can form braided streams, terraces, and layered sediments.
• Moraines: left behind when the ice edge pauses or retreats; contain depositional features indicating former ice extent.
• Post-glaciation dynamics:
  • After ice melt, meltwater deposits and soils begin to develop on new substrates; soil profiles may be buried by subsequent deposits if another cycle occurs.
  • If soil formation has time to proceed between glacial episodes, soils may show horizons; otherwise, buried soil horizons may be preserved under newer deposits.
• Example references:
  • Glacial outwash deposits can be seen forming behind retreating ice in various regions; till is typically more angular and unsorted than outwash sediments.

Aeolian deposition (wind-driven)

• Common in arid and semi-arid regions; less common in humid temperate zones like Alabama but present in drier landscapes.
• Material types and features:
  • Dune sands: wind-transported sands forming dunes; can shift over time.
  • Loess (very fine silt): wind-blown dust deposits that can accumulate far from the source and are highly fertile in many regions.
• Fertility and regional examples:
  • Loess soils are some of the most fertile in the United States (e.g., parts of Iowa) and worldwide (Israel, Gaza Strip, etc.).
  • Loess deposits can influence soil texture and horizon development due to fine particle dominance.
• Visual cues in landscapes:
  • Sand dunes and loess-rich plains show characteristic windborne sediment; loess tends to form thicker, more homogeneous soil horizons.

Coastal plain soils and marine deposition

• Marine or coastal deposition scenarios:
  • Soils may form on marine sediments that were laid down during sea-level fluctuations; soil formation occurs on top of marine sediments when land is emergent.
  • The “last deposited” material often forms the most recent surface soil, with earlier sediments buried beneath new deposition.
• Regional examples:
  • Coastal plains in the northern Southeast (Maryland area) with marine sediments underlying newer soils.
  • Higher elevations inland showing igneous, metamorphic (residual) materials or different rock types depending on local geology.
• Weathering and soil formation implications:
  • Marine sediments are often associated with higher water table and redox conditions that influence soil-forming processes (e.g., oxidation/reduction) and horizon development.
  • In areas with marine deposition, soils may show characteristic horizons influenced by reducing conditions and drainage patterns.
• Hard materials and variability:
  • Soils near uplands can show a mix of hard materials (igneous, metamorphic) and clastic marine sediments; composition varies with topography and proximity to source rocks.

Landscape interpretation and quick exercise (mapping foreign materials)

• A classroom exercise maps foreign material to imagined landscapes using a color-coded diagram (red, green, blue, orange):
  • Red areas: residual material near the summit (untransported material from weathering; high likelihood of residual, fine material early in soil development).
  • Green areas: alluvial/fluvial (water-transported material; channel and floodplain deposits).
  • Blue areas: aeolian (wind-transported material; loess or dune sands).
  • Orange areas: (not explicitly named in the transcript, but part of the exercise with various landscapes).
• Expected assignments (based on the discussion):
  • From red areas (mountain tops): residual fine material (weathered from parent rock in place).
  • From green areas (slopes and valleys): alluvial/collected by streams; wind-blown or other inputs less dominant here.
  • From blue areas: eolian deposits (dunes or loess).
• Takeaway: with time and weathering, soils develop more horizons and show progressively more developed soil profiles; the exercise emphasizes recognizing transport and deposition histories from landscape context.

Soil horizons and horizonation: core concepts

• Horizonation is the process of forming distinct soil layers (horizons) from the parent material through soil-forming processes.
• Soil genesis is the overall process: the formation of soil from current material through the action of soil-forming factors and processes.
• Master horizons (the major, recognized layers):
  • O: organic horizon, dominated by organic matter; accumulates from plant litter and organic debris.
  • A: topsoil; mineral soil mixed with some organic matter; zone of weathering and beginning of horizon differentiation.
  • E: eluviated horizon; zone of leaching/ELUVIATION where clay, organic matter, iron oxides, etc., are leached downward; typically pale in color due to loss of materials.
  • B: illuvial horizon; zone of accumulation and redistribution of materials leached from above (often clay, oxides, humus).
  • C: parent material; relatively little alteration from the original material; saprolite can be discussed as weathered bedrock material here.
  • R: bedrock; unweathered rock or consolidated rock chemical structure.
• These master horizons usually appear in this order: O ightarrow A ightarrow E ightarrow B ightarrow C ightarrow R
  • Note: not every soil has all six master horizons; some may be missing or appear in different orders due to boundary conditions (e.g., young soils may lack a well-developed B horizon).
  • If newer material overlies older horizons, you may see transition horizons (e.g., AB, BC, or BE) where properties shift gradually rather than abruptly.

Subhorizons and horizon transitions

• Subhorizons: subdivisions within the master horizons are labeled with lowercase letters (e.g., OA, OE, Oi) to indicate finer layers or specific characteristics within a master horizon.
• Transition horizons:
  • The space between two master horizons, where properties blend from one to the other, is often described as a transition horizon (e.g., AB, BE, BC).
  • If a single horizon contains properties of both parent horizons, it may be labeled with mixed indicators (e.g., AB, BA variants).
• Subhorizons examples and common notations:
  • O horizon can be split into Oi (slightly decomposed), OA (beginning humification), and OE (intermediate organic layer) depending on decomposition and organic content.
  • When there are multiple similar horizons of the same type layered, they may be denoted as B1, B2, etc., to indicate sequential development within the same horizon category.
• Important note on order:
  • In natural soil development, horizons generally form in the same order (O → A → E → B → C → R). You will not typically see a B horizon forming above an A horizon unless a new soil forms atop older soil; new C horizons can overlie upper horizons in some cases.

Soil-forming processes (the four key categories)

• Addition: materials added to the soil from above or below (e.g., organic matter from plant litter; dust with soluble nutrients from distant sources).
  • Example: Plant litterfall adds organic matter to the surface; dust can bring solutes (e.g., sulfur compounds transported long distances).
• Transformation: chemical or physical changes to materials within the soil; includes decomposition of organic matter and mineral weathering.
  • Example: Decomposition of organic matter; mineral weathering changing primary minerals to secondary minerals.
• Translocation: movement of materials within the soil profile; vertically or horizontally redistributed by water movement, roots, organisms, or diffusion.
  • This includes eluviation (leaching of materials from upper horizons) and illuviation (accumulation in lower horizons).
• Loss (or export): materials lost from the soil profile to other sinks (groundwater, surface runoff, atmosphere) via leaching, erosion, or runoff.
• Important definitions within translocation:
  • Eluviation: removal of material from one horizon moving to another (e.g., leaching of clays, Fe, Al, or organic matter).
  • The particles most likely to move downward are the fine ones; clays (and fine silt) tend to accumulate in the B horizon due to their small size and ease of movement through pores.
• Relative movement tendency:
  • Clay tends to move less than silt/ sand; however, in eluviation, clays tend to be removed from an E horizon and accumulate in the B horizon (illuvial process).
• Master horizons and transitions:
  • Common subhorizons (as subdivisions of master horizons) are used to describe more precise layers within the main horizons, especially when transitions are gradual or mixed.

Time, climate, and soil aging (interactions among factors)

• Time and climate are central in determining how developed a soil is; soils in hotter, more humid environments weather more rapidly and can develop more progressively linked horizons (e.g., ultisols in strongly weathered tropical-climate zones).
• Organisms contribute to soil formation by adding organic matter, mixing, and accelerating weathering processes.
• Topography influences soil development by controlling drainage and erosion:
  • On hilltops: residual (less-weathered) parent materials persist longer; on backslope or footslopes, colluvial material accumulates.
  • The slope aspect and gradient influence water movement and the rate of horizon formation.
• Time scales for soil development vary; some soils can show significant horizon development in relatively short geological times if climate and weathering are favorable.
• Hawaiian sequence as a canonical example:
  • Islands formed at different times present soils under similar climates and parent materials; studying these soils demonstrates how time drives horizon development despite similar environments.
• Conceptual takeaway: with more time, weathering and soil-forming factors have more opportunity to act, leading to more complex horizons and deeper soil profiles. Time interacts with climate, organisms, topography, and parent material to shape the final soil.

Relationships among factors and soil-forming environments

• Interactions among time, climate, and parent material lead to diverse soils across landscapes (e.g., ultisols in hot, humid climates; coastal plain soils on marine sediments; loess soils in wind-borne deposits; glacial soils in temperate regions).
• Interactions with topography modify the deposition history and soil development across hill slopes, valleys, and plains.
• Practical implication: when interpreting a soil profile, consider how the landscape, transport history, and time have influenced horizon development and material distribution.

Common coastal plain soil example (Delaware region) and related features

• Coastal plain soils can show buried soils beneath younger marine sediments; a soil profile may begin with an A horizon overlain by C horizons, with older horizon development preserved beneath younger deposits.
• An example from a coastal plain in the northern Southeast (Maryland) shows marine sediments underlying current soils; this demonstrates how landforms are formed by repeated deposition and erosion and how soils record these cycles.
• Forest clearing and burning in historic times left charcoal layers in soils; a dark, char-rich layer can be found in some coastal plain soils, indicating a historical land-use impact on erosion and deposition.

Summary: how to think about soil horizons in practice

• Soil horizons form in a recognizable order (O → A → E → B → C → R) where present, with transitions and subhorizons indicating mixing and gradual changes.
• The presence or absence of particular horizons depends on time, climate, topography, parent material, and biotic activity.
• The nature of sediment (colluvial, alluvial, glacial, aeolian, marine) leaves a lasting imprint on the soil's horizon development and texture:
  • Colluvial: unsorted, angular, mixed materials along slopes.
  • Alluvial: sorted by energy; finer materials in distal floodplain deposits.
  • Glacial: till (unsorted, mixed sizes; angular) vs outwash (sorted, layered, finer downstream).
  • Aeolian: fine sands and loess; strong potential for deep, well-developed horizons in some regions.
  • Marine/Coastal: soils on marine sediments with potential redox features, buried horizons, and horizon development limited by drainage and sedimentation history.

Quick recap of key definitions and terms

• Master horizons: O, A, E, B, C, R
• Horizonation: formation of soil horizons
• Soil genesis: process of soil formation from parent material through soil-forming factors and processes
• Addition, Transformation, Translocation, Loss: the four main soil-forming processes
• Eluviation: removal of material from an upper horizon and its movement to another horizon
• Illuviation: accumulation of leached material in a lower horizon (often B)
• Transition horizons: blends between master horizons (e.g., AB, BC)
• Subhorizons: subdivisions within a master horizon (e.g., Oi, OA, OE)
• Common textural implications: loess (fine silt), dune sands (coarse to medium sands), till (mixed, unsorted), outwash (sorted by meltwater)
• Soil depth measurement note: depth is measured from the top of the A horizon downward; the portion above A is considered overlying material that is not part of the standard soil depth measurement.

Practical field tips mentioned

• When measuring soil depth, start at the top of the A horizon and move downward; note any transitions or missing horizons.
• Be aware that slides or figures in lectures may show additional detail (e.g., notation like AB, BC) not always perfectly aligned with spoken descriptions; consult the actual lab notes for precise nomenclature.
• Expect variation across landscapes due to topography and deposition history; use the landscape context to infer likely horizonation and sediment type.

Optional cross-checks and study prompts

• If given a landscape, identify the likely primary deposition process and predict the horizon sequence you would expect (e.g., a floodplain alluvium surface vs. a morainal glacial surface vs. a loess-covered plain).
• Explain the difference between eluviation and illuviation and give examples of horizon types where each is most likely to dominate.
• Describe how time and climate influence soil development using the Hawaiian sequence as a model.
• Identify the six master horizons and explain what material or processes typically characterize each one.
• Outline the four soil-forming processes and give an example for each from the lecture.

End-of-notes note

• The content above reflects the lecture materials discussed, including practical lab guidance, modes of sediment transport, horizonation concepts, and field interpretation strategies. It integrates the examples and regional references mentioned by the instructor to illustrate the diversity of soil formation across landscapes.