Soil Horizons: Prefixes, Master Horizons, and Sub Horizons (Notes from Transcript)

Prefix, Lithologic Discontinuities, and Horizon Concepts

  • Prefix of Horizon Designation

    • A prefix indicates a lithologic discontinuity, meaning a change in parent material between horizons. This can be due to different parent materials or mixing events (e.g., flooding) that result in a distinct horizon composition.

    • For the first parent material, no numerical prefix is used (typically omitted, but a dash (-) can be used if explicitly marking the absence of a discontinuity for that layer).

    • If a second distinct parent material input occurs across horizons, a numerical prefix is added to the subsequent horizons (e.g., 2A, 2Bt). This number (e.g., 2) signals the lithologic discontinuity.

    • If there is no second material input (i.e., no lithologic discontinuity within a sequence), a dash (-) serves as the prefix.

  • Lithologic Discontinuity

    • Illustrated by contrasts in parent material across horizons, such as visible changes in texture or grain size (e.g., a layer with smooth rocks vs. jagged rocks).

    • Caused by natural events like flooding, landslides, or eolian deposition (e.g., loess deposits).

    • It represents a shift in the dominant material or material source rather than just a change in soil structure. Variation in soil structure alone does not automatically necessitate a new prefix.

  • Practical Cues and Field Notes

    • Drastic differences in horizon characteristics (e.g., a sudden change from clay loam to sandier textures, or from organic-rich to mineral horizons) strongly suggest a second material input and the need for a prefix.

    • In situations where a second material input isn't obvious, default to using a dash (-) or simply omitting a prefix for horizons derived from the initial parent material.

    • When documenting, describe both the material change and the resulting horizon nomenclature.

Master Horizons (O, A, E, B, C, R)

  • Definition and Order

    • Master horizons are primary mineral and organic layers in soil profiles, categorized by dominant characteristics.

    • The traditional, general order when all are present is O, A, E, B, C, R.

    • Note: While R (bedrock) is listed in the sequence, it's not a soil mineral horizon in the same sense as O, A, E, B, or C. Its presence indicates unaltered rock beneath the soil.

  • O Horizon (Organic Layer)

    • Definition: Dominated by organic matter, such as leaf litter and decomposed plant material.

    • Function: Accumulates and stores organic matter.

    • Presence: Often absent in many soils (e.g., prairies) but common in forested areas.

    • Depth Rule: In some contexts (e.g., textbooks or competition rules), an O horizon should be described if it reaches approximately 8 cm8\,\text{cm} or more in thickness.

  • A Horizon (Topsoil)

    • Definition: The uppermost mineral horizon, typically darker than underlying horizons due to accumulated organic matter.

    • Characteristics: Darker color; generally less clay than lower horizons. May exhibit granular structure, especially in agricultural or tilled soils.

    • Relationship to O: If an O horizon is absent, the A horizon is the surficial mineral horizon. It can be described even if thinner than 8 cm8\,\text{cm}.

  • E Horizon (Eluviation Zone)

    • Definition: A zone of maximum leaching (eluviation), where materials like clay, iron, or aluminum oxides are removed.

    • Characteristics: Pale or gray colors (often bleached) due to the loss of coloring agents. Typically low in clay. It is a zone of depletion, not accumulation.

    • Presence: More common in wetter, forested regions; less common in grasslands or arid zones. If present, the E horizon is distinctly lighter than adjacent horizons.

  • B Horizon (Subsoil; Zone of Accumulation)

    • Definition: A mineral horizon where materials leached from above accumulate (illuviation). Accumulated products include clays, iron/aluminum oxides, and secondary weathering products.

    • Characteristics: Typically darker than the C horizon but often lighter than the A. Shows an increase in clay content due due to downward movement (e.g., denoted by Bt).

    • Differentiation from C: If it appears soil-like (not bedrock) and shows signs of clay accumulation or other pedogenic development, it is likely a B horizon.

    • Common Suffixes: Often carries suffixes indicating specific processes (e.g., Bt for clay illuviation, Bk for carbonate accumulation, Bg for gleying).

  • C Horizon (Parent Material; Relatively Unweathered)

    • Definition: Dominated by parent material with limited weathering or soil structure development. Resembles the original material from which the soil formed.

    • Characteristics: More intact and less developed than B horizons. May contain calcium carbonates.

    • Indicators: More rock-like than B horizons; may show features like cementation but little to no pedogenic clay illuviation.

  • R Horizon (Bedrock)

    • Definition: Solid, unweathered rock layer beneath the soil profile.

    • Practical Note: Difficult to sample using standard soil tools.

    • Variants: A cemented or partially cemented zone (sometimes labeled CR) might be encountered, which is harder than typical C horizons but not true bedrock.

  • Depth and Identification Rules (Summary)

    • O horizons: commonly described if 8 cm8\,\text{cm} or more thick.

    • A and E horizons: can be described even if thinner than 8 cm8\,\text{cm}.

    • B, C, and R horizons: typically described at greater depths.

    • Observed order: Always O $\rightarrow$ A $\rightarrow$ E $\rightarrow$ B $\rightarrow$ C $\rightarrow$ R when present, adjusting for absent horizons. For instance, if O is absent, the sequence starts with A.

    • Important: Always verify specific depth rules and conventions with current competition handbooks and field guides, as these can vary regionally.

Subhorizons (Suffixes) — Lowercase Designations

  • How to Write Subhorizons

    • Subhorizons are lowercase letters appended as suffixes to master horizon designations (e.g., Bt, Bg, Ak).

    • If a horizon has no applicable subhorizon properties, a dash (-) can be used to indicate absence (though often just a simple master horizon letter suffices, e.g., B).

    • Never capitalize subhorizon letters.

  • Specific Commonly Used Subhorizon Concepts

    • b: Buried Genetic Horizon

      • Meaning: An old, soil-formed horizon that has been buried by new material, disrupting the normal sequence (e.g., an Ab horizon indicates a buried A horizon).

      • Significance: Indicates past landscape instability or depositional events.

    • g: Gley

      • Meaning: Indicates gleying conditions, characteristic of poorly drained, waterlogged soils where iron is chemically reduced.

      • Appearance: Often results in blue-gray, greenish, or gray coloration with low chroma.

    • k: Calcium Carbonate (Calcic Horizon)

      • Meaning: Significant accumulation of calcium carbonate (CaCO3\text{CaCO}_3).

      • Appearance: Often visible as white streaks, nodules, or soft masses. Common in arid or semi-arid regions.

    • t: Clay Illuviation

      • Meaning: Accumulation of silicate clay that has been moved from an overlying horizon (e.g., an A or E horizon).

      • Appearance: Often visible as clay films (cutans) on ped faces or pore linings, giving a shinier or denser appearance.

Field Practice and Exam Preparation

  • Distinguishing Prefixes vs. Suffixes

    • Prefixes (e.g., 2A): Indicate a lithologic discontinuity – a change in the original parent material between horizons.

    • Suffixes (e.g., Bt, Bg): Describe specific processes or characteristics within a given horizon.

  • Practical Field Tips

    • Soil profiles vary widely: expect some, none, or many horizons, as their presence is influenced by regional climate and land use.

    • Use color, texture (e.g., rocky vs. soil-like), and the presence of clay accumulation to effectively differentiate between B and C horizons.

    • Always be prepared to justify your horizon designations based on observable properties.

  • Key Terminologies for the Exam

    • Lithologic Discontinuity: A significant change in parent material from one horizon to the next, often observed by differences in texture or grain size, and denoted by a numerical prefix.

    • Prefix (in horizon designation): A number (e.g., 2) that indicates a second distinct parent material input across horizons, signaling a lithologic discontinuity.

    • Master Horizons: The main diagnostic layers in a soil profile: O, A, E, B, C, R.

    • Subhorizons (Suffixes): Lowercase letters (e.g., b, g, k, t) appended to master horizons to describe specific features or processes within that horizon. A dash (-) indicates no distinct subhorizon.

    • Buried Genetic Horizon (b): A horizon formed by soil processes that was subsequently buried, placing it out of sequence with current surface horizons.

    • Gley (g): A condition in poorly drained soils characterized by chemical reduction, often resulting in distinct grayish or greenish colors.

    • Calcic/Carbonate Horizon (k): A horizon with evident accumulation of calcium carbonate.

    • Clay Illuviation (t): The process of clay accumulation, often identified by clay films in the Bt horizon.

  • Final Reminder:

    • Always align your notes and field observations with the latest version of your competition's field book guidance and rules. Regional variations in terminology