Soils Up Close:The Soil Profile and Horizon Characteristics

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

1.Roadcut/Construction Excavation

I. The Soil ProfileB. Gaining the 3rd Dimension2. Pit Excavation

I. The Soil Profile

B. Gaining the 3rd Dimension

3. Soils Augering

I. The Soil Profile

C. Major Components of Soil

I. 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 ProfileA. Master Horizons……

OAEBCR 

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.

R

E

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

1.Major categories

Boulder

Stones

Cobble

Gravel

Sand

Silt

Clay

600 mm

250 mm

76 mm

2 mm

0.05 mm

0.002 mm

Texture< 0.002 mm

2.0 to 0.05 mm 0.062 to 0.05 mm

Texture< 0.002 mm

2.0 to 0.05 mm 0.05 to 0.002 mm

Common in B Horizons

Common in A Horizons

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 mmΡw = 1.00 gm/cm3

ρs = 2.70 gm/cm3

µ = 1.002 * 10-3 (kg/m*s) g = 981 cm/s2

Find V

Munsell Color Chart

HueValueChroma

III. Soil Profile Characteristics

B. Color 1.Mechanics

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, whereasValue of 2 is dark.

III. Soil Profile Characteristics

B. Color

Chroma: Strength of color, e.g., theamount of pigment in the sample.A chroma of 7 reflects intense color,whereas a chroma of 2 exhibits aweak color.

Munsell Color Chart

LIGHT/DARK(VALUE)

PIGMENTATION(COLOR)

Hue

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”

III. Soil Profile Characteristics

C. Structure

III. Soil Profile Characteristics

C. Structure

1.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

Sand and Organics

Sand

Soil Drainage Problems

Well drained, well developedsoils in Virginia

Related to alkaline(salty) soils, and/orexpansive clays

Common in A Horizon in VA

A

E,BxKmBq,BmBs

Bt

Bt

Bn

Horizon

Bt

Loose/Granular

Very FineFine

MediumCoarse

Very Coarse

“Durability”

Very FineFine

MediumCoarse

Very Coarse

III. Soil Profile Characteristics

D. Organic Matter

III. Soil Profile Characteristics

D. Organic Matter

1.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

1.…..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. 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

A.The value of solid profile descriptions (qualitative and quantitative)

Harden Indices