Soils Up Close: The Soil Profile and Horizon Characteristics

I. The Soil Profile A. Profiles vs I. The Soil Profile A. Profiles vs. Pedons Soil Profile: a two-dimensional body, commonly studied in artificial cuts. Soil Pedon: a three-dimensional body, with the same thickness as a profile, but with a surface area that ranges between 1 and 100 square meters. From: Birkeland, 1999

I. The Soil Profile B. Gaining the 3rd Dimension

I. The Soil Profile B. Gaining the 3rd Dimension Roadcut/Construction Excavation

I. The Soil Profile B. Gaining the 3rd Dimension 2. Pit Excavation

I. The Soil Profile B. Gaining the 3rd Dimension 3. Soils Augering

The Soil Profile C. Major Components of Soil

The Soil Profile C. Major Components of Soil Mineral Matter 45% Organic Matter 05% Water 25% Air 25%

II. Soil Profile Horizons A. Master Horizons

I. The Soil Profile A. Master Horizons…… O A E B C R Solum Solum: Consists of the surface and subsoil layers that have undergone the same soil forming conditions. Epipedon: “over the soil"; the uppermost layer of soil that serves as the diagnostic horizon.

E R

E

E

“Zone of Leaching” Or “Zone of Eluviation”

Zone of illuviaton

Diagnostic properties (B horizon) Thickness Color Texture Structure

R

II. The Soil Profile C. Subhorizons

Subhorizons

Subhorizons

Subhorizons

Subhorizons

III. Soil Profile Characteristics

III. Soil Profile Characteristics A. Texture

III. Soil Profile Characteristics A. Texture Major categories Boulder Stones Cobble Gravel Sand Silt Clay 600 mm 250 mm 76 mm 2 mm 0.05 mm 0.002 mm

< 0.002 mm Texture 2.0 to 0.05 mm 0.062 to 0.05 mm

Texture < 0.002 mm Common in B Horizons Common in A Horizons 2.0 to 0.05 mm 0.05 to 0.002 mm Common in A Horizons over Granites

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work Reflects character of Parent Material

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work Reflects character of Parent Material Affects processes within the soil

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work Reflects character of Parent Material Affects processes within the soil Degree of horizon development (fine vs. coarse-grained soils) Drainage properties Engineering properties

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work Reflects character of Parent Material Affects processes within the soil Degree of horizon development Drainage properties Engineering properties Degree of chemical reactivity Water retention

III. Soil Profile Characteristics A. Texture 2. Importance of Texture in Soils Work Reflects character of Parent Material Affects processes within the soil Degree of horizon development Drainage properties Engineering properties Degree of chemical reactivity Water retention Erositivity Tilth

III. Soil Profile Characteristics A. Texture 3. Determining Texture

Qualitative Methods

Qualitative Methods

III. Soil Profile Characteristics A. Texture 3. Determining Texture

III. Soil Profile Characteristics A. Texture 3. Determining Texture

20 ml pipette

  Determine the settling velocity of a clay particle. Given: Clay = 0.002 mm Ρw = 1.00 gm/cm3 ρs = 2.70 gm/cm3 µ = 1.002 * 10-3 (kg/m*s) g = 981 cm/s2 Find V

III. Soil Profile Characteristics B. Color 1.Mechanics Munsell Color Chart Hue Value Chroma

III. Soil Profile Characteristics B. Color Hue: Actual color of the soil relative to The primary colors. 7.5 YR is 75% yellow, 25% red

III. Soil Profile Characteristics B. Color Value: Lightness or darkness of color. Value of 8 will be very light, whereas Value of 2 is dark.

III. Soil Profile Characteristics B. Color Chroma: Strength of color, e.g., the amount of pigment in the sample. A chroma of 7 reflects intense color, whereas a chroma of 2 exhibits a weak color.

Hue LIGHT/DARK (VALUE) PIGMENTATION (COLOR) Munsell Color Chart

2. The Application of Munsell Colors a. Good Drainage (Well Drained) Oxidizing Conditions Yellow-Brown-Red Colors Fe 2+ → Fe 3+

2. The Application of Munsell Colors a. Good Drainage (Well Drained) Oxidizing Conditions Yellow-Brown-Red Colors Fe 2+ → Fe 3+

2. The Application of Munsell Colors b. Poor Drainage (Reducing Conditions) Greyish-green-blue Colors “Gleyed Colors” Fe 3+ → Fe 2+ Hues > 5Y or “Gleyed Page” of Munsell Chart Low Chromas (~1) “Mottles or Redox Features”

“Mottles or Redox Features”

Soil Colloids

III. Soil Profile Characteristics C. Structure

III. Soil Profile Characteristics C. Structure In General: An aggregation of soil particles called peds

III. Soil Profile Characteristics C. Structure 2. Significance Critical in soil drainage (determines infiltration rates) Aeration, root penetration, nutrient retention, tilth Related to genesis and rock type

III. Soil Profile Characteristics C. Structure 3. Types/Categories

Common in A Horizon in VA Loose/Granular Sand A Sand and Organics E,Bx Km Bq,Bm Bs Soil Drainage Problems Bt Well drained, well developed soils in Virginia Bt Bn Related to alkaline (salty) soils, and/or expansive clays Bt

“Durability” Very Fine Fine Medium Coarse Very Coarse

Very Fine Fine Medium Coarse Very Coarse

III. Soil Profile Characteristics D. Organic Matter

III. Soil Profile Characteristics D. Organic Matter What is it, and where is it found? Underdecomposed plant and animal tissue Humus (“colloidal soil organic matter that decomposes slowly and colors soils brown or black”) from Birkeland, 1999

III. Soil Profile Characteristics D. Organic Matter …..where is it found?

III. Soil Profile Characteristics D. Organic Matter 2. Importance Increases water holding capacity and tilth Increases cation exchange capacity (nutrient holding capacity)

2. Importance Increases water holding capacity and tilth Increases cation exchange capacity (nutrient holding capacity) Produces chelating compounds (organic acids) that increase solubility of ions Releases high amounts of carbon dioxide, which accelerates chemical weathering Sources of CO2: --atmosphere (0.03% concentration) --soils (2 - 25% concentration)

III. Soil Profile Characteristics E III. Soil Profile Characteristics E. Bulk Density A measure of the weigh of soil per unit volume Important for studying soil moisture movement rates and rates of clay and carbon accumulation (pedogenesis)

IV. The Larger View The value of solid profile descriptions (qualitative and quantitative)

Harden Indices