IV

Epipedon

• Definition: Identify the type of epipedon at the surface.

• Types:

  • Mollic: Thick, dark, high organic matter; high base saturation ($\, \geq 50\%$).

  • Umbric: Similar to Mollic but has low base saturation ($\, <50\%$).

  • Ochric: Too thin, too light in color, or too low in organic matter to be classified as Mollic or Umbric.

Tips for Epipedon Identification

• Check thickness (usually $\, \geq 25\%$ cm for Mollic/Umbric).

• Examine color (dark value and chroma).

• Utilize pit card for base saturation and organic carbon content.

Subsurface Horizons

• Definition: Identify special horizons located below the surface.

• Types:

  • Argillic (Bt): Characterized by clay increase ($\, \geq 1.2\times$ more clay than above), clay films, blocky/prismatic structure.

  • Cambic (Bw): Weakly developed, some color/structure change, but insufficient for Argillic.

  • Fragipan (Bx): Dense, brittle, root-restricting, often possessing fragic properties.

  • Spodic (Bh/Bs): Accumulation of organic matter and/or iron/aluminum, typically found in sandy soils.

Tips for Subsurface Horizon Identification

• Look for horizon symbols (Bt, Bw, Bx, Bh, Bs, etc.).

• Check for clay increase, structural alterations, and color shifts.

Other Characteristics

• Assign the broadest category based on diagnostic horizons and properties.

1. Buried Horizon

• Definition: A soil horizon (often an old surface, like an A or O) that has become covered by a younger deposit (like alluvium or colluvium).

• Clues: A dark, organic-rich layer beneath the current surface, typically with a clear dividing line and sometimes a color/texture contrast.

• Tip: Search for a buried A horizon beneath a lighter or sandier layer.

2. Fe/Mn Concretions

• Definition: Hard, rounded, or irregular nodules of iron (Fe) or manganese (Mn) within the soil.

• Clues: Rusty, black, or dark brown spots that are considerably harder than surrounding soil.

• Tip: Concretions are distinct from soft redox concentrations—they are hard and can often be removed from the soil.

3. Fragipan

• Definition: A dense, brittle subsurface horizon that restricts root and water movement.

• Clues: Prismatic or blocky structure, brittleness when moist, roots and water have difficulty penetrating, often presenting a "platy" or "prism" appearance.

• Tip: Break a clod—if it snaps with a brittle fracture and is challenging to dig, it may be a fragipan.

4. Fragic Properties

• Definition: A horizon with some, but not all, fragipan characteristics (e.g., brittle, firm, but not as dense or thick as an actual fragipan).

• Clues: May be less continuous or thinner than a fragipan yet still restrict roots and water more than layers above or below.

• Tip: If a horizon is somewhat brittle and hinders root growth, but does not conform strictly to fragipan criteria, label as "fragic properties."

5. Lithologic Discontinuity

• Definition: A noticeable change in parent material within the profile (e.g., abrupt transition from loamy to sandy material, or from alluvium to residuum).

• Clues: Sudden change in texture, color, or rock fragment content, often highlighted by a prefix numeral in horizon designation (e.g., 2Bt).

• Tip: Identify rapid shifts in soil properties that indicate new deposits or materials.

6. Lithic Contact

• Definition: Hard, unweathered bedrock (R horizon) found within the judging depth (typically within 150 cm).

• Clues: Cannot be excavated with a spade or auger; roots cannot penetrate; very abrupt boundary.

• Tip: If solid rock is encountered that is impossible to break through, note as "lithic contact."

7. Paralithic Contact

• Definition: Weathered or soft bedrock (Cr horizon) within the judging depth.

• Clues: Softer than lithic contact and may occasionally be excavated with some effort. Roots may enter cracks but still limit root growth.

• Tip: Mark "paralithic contact" if soft rock that restricts root growth is reached but is not as hard as true bedrock.

8. Slickensides

• Definition: Shiny, grooved surfaces in soil resulting from soil movement (typically found in clay-rich soils that swell and shrink).

• Clues: Polished, wavy, or grooved surfaces on peds or clods, frequently observed at depth.

• Tip: Slickensides are common in Vertisols or soils with a high content of shrink-swell clay.

Soil Orders

1. Alfisols

   • Argillic horizon (Bt): Characterized by subsurface clay increase ($\, \geq 1.2\times$ more clay than above), clay films, blocky/prismatic structure.

   • Base saturation: $\, \geq 35\%$ (consult pit card data).

   • Color: Surface may be lighter than Mollisol.

   • Diagnosis: Presence of a Bt horizon and high base saturation signifies an Alfisol.

2. Entisols

   • Absence of diagnostic subsurface horizon; only consist of A and C horizons (no Bw, Bt, etc.).

   • Typically young soils with minimal development.

   • Diagnosis: If only A and C horizons are observed, classify as Entisol.

3. Inceptisols

   • Cambic horizon (Bw): Weakly developed with minor changes in color/structure without meeting Argillic requirements.

   • No argillic, spodic, or mollic epipedon present.

   • Diagnosis: If there is a Bw horizon without a Bt and the surface is not thick/dark enough for Mollisol, classify as Inceptisol.

4. Mollisols

   • Mollic epipedon: Thick ($\, \geq 25\%$ cm), dark, high organic matter content, and high base saturation ($\, \geq 50\%$).

   • Color: Value $\, \leq 3$, chroma $\, \leq 3$ in moist conditions.

   • Common in grassland regions; fertile soils.

   • Diagnosis: Likely a Mollisol if both thickness and darkness are confirmed, along with high base saturation as indicated by the pit card.

5. Ultisols

   • Argillic horizon (Bt): Similar to Alfisol but with base saturation $\, <35\%$.

   • Often more weathered and acidic. Diagnosis: Presence of a Bt horizon accompanied by low base saturation is indicative of an Ultisol (check pit card).

6. Vertisols

   • High content of shrink-swell clay (typically $>30\%$ clay, often smectite).

   • Characteristic Features:

     • Slickensides: Shiny, grooved surfaces on peds due to soil movement.

     • Wedge-shaped peds: Soil blocks with a wedge shape due to shrinking and swelling.

     • Deep cracks: Form when dry and close when wet.

Suborder Classification

• Definition: The second level of soil classification, often based on soil moisture regimes or other special features.

• Suborders

  • (Alb) Albic: Leached E horizon. Prominent, light colored E horizon.

  • (Aqu) Aquic: Soils that are saturated with water for significant periods.

  • (Fluv) Fluvial: Floodplain soils. Alluvial parent material, floodplain landform.

  • (Orth) Typical: No special moisture, color, or sandiness.

  • (Psamm) Psammentic: Sandy texture throughout, no cleyey horizons.

  • (Ud) Udic: Soils that are moist throughout the year.

Great Group Classification

• Definition: The third level of soil classification, based on key diagnostic features, the degree of horizon development, or parent material characteristics.

• Fragi: Fragipan present. Dense, brittle, root restricting horizon.

• Argi: Argillic horizon present. Subsurface clay increase, Bt horizon.

• Gloss: Glossic horizon present. Bleached tongues into argillic horizon.

• Dystr: Low base saturation. Base saturation less than 35% (check pit card)

• Hapl: “Typical”/minimal development. No special redox features, most common.

• Endo: Water table at/near surface. Redox depletions at or near surface.

• Pale: Old, thick argillic horizon. Very thick, well developed Bt horizon.

• Epi: Water table just below surface. Redox reactions just below surface.

• Psamm: Sandy throughout. Sandy texture, no clayey horizons.

• Eutr: High base saturation. Base saturation greater than 50% (check pit card)

• Quartzi: Quarts rich sandy soils. Sandy, very low silt/ clay, quarts grains.

• Fluv: Floodplain/alluvial soils. Alluvial parent material, floodplain landform.

• Udi: Udic moisture regime. Well-drained, humid climate.