Soils Up Close: The Soil Profile and Horizon Characteristics.

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


1.Roadcut/Construction Excavation

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

I. The Soil Profile


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.

“Zone of Leaching”Or

“Zone of Eluviation”

Zone of illuviaton

Diagnostic properties (B horizon)• Thickness

• Color

• Texture

• Structure

II. The Soil Profile

C. Subhorizons

Subhorizons

III. Soil Profile Characteristics


A. Texture


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


A. Texture

2. Importance of Texture in Soils Work


A. Texture


• Reflects character of Parent Material


A. Texture



• Affects processes within the soil


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


A. Texture



• Affects processes within the soil– Degree of horizon development– Drainage properties– Engineering properties– Degree of chemical reactivity– Water retention


A. Texture



• Affects processes within the soil– Degree of horizon development– Drainage properties– Engineering properties– Degree of chemical reactivity– Water retention– Erositivity– Tilth


A. Texture

3. Determining Texture

Qualitative Methods


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


B. Color 1.Mechanics


B. Color

Hue: Actual color of the soil relative to The primary colors.

7.5 YR is 75% yellow, 25% red


B. Color

Value: Lightness or darkness of color.Value of 8 will be very light, whereasValue of 2 is dark.


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+


a. Good Drainage (Well Drained)

•Oxidizing Conditions

Yellow-Brown-Red Colors

Fe 2+ → Fe 3+


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”


C. Structure


C. Structure

1.In General:

•An aggregation of soil particles called peds


C. Structure

2. Significance

•Critical in soil drainage (determines infiltration rates)

•Aeration, root penetration, nutrient retention, tilth

•Related to genesis and rock type


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


D. Organic Matter


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


D. Organic Matter

1.…..where is it found?


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)


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

Soils Up Close: The Soil Profile and Horizon Characteristics.

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