Chapter 13 Soils GEOL 215

Soils definition

Unconsolidated surficial deposits produced indirectly or directly by weathering processes and capable of supporting plant life

Residual soils

direct weathering of bedrock

Transported soils

erosion and redeposition

Humus

Partially decayed organic matter

Loam

soils that contain equal proportional parts of sand and silt and low amounts of clay

Soil formation factors

Geology (bedrock)

Climate (T and Moisture)

Organisms (topsoil, aeration, structure, nutrient)

Relief (controls thickness)

Time

Time

C horizon decreases as weathered away, organic matter increases, B horizon increases as C decreases

Weathering factors in soils

Jointing

Climate (precip, T, freeze-thaw)

Sun exposure

Vegetation (breakup)

Disaggregation

breaking of rock with no chemical alterations

Disintegration

decomposition of primary minerals comprising the rock forming secondary minerals more stable at surface

Pedon

Hexagonal column profile of soil from soil to bedrock. polypedon, multiple

O-horizon

Very top layer; darkly colored, humus, organic, plant material

A-horizon

2nd layer but technically first, top soil; Mineral material, zone of leaching where dissolution, cation exchange, and fluid percolation occurs that removes ions

E-horizon

3rd layer; Lightly colored, Zone of leaching, leaching of Fe, rich in resistant minerals like quartz, zone of eluviation

Leaching

Removal of dissolved substances

Eluviation

removal of both dissolved and suspended materials like clay and iron oxides and nutrients

illuviation

Where Salts, Fe, clay, minerals are percolated and deposited

B-horizon

4th layer; Reddish in color, Zone of accumulation, enriched from constituents leached from A-horizon, illuviation, carbon precipitation common in arid env

B-horizon precipitates

Carbon in arid env

Silcrete and gypsum common

C-horizon

5th layer; Soil mantle, bedrock, regolith, moderately, minimally weathered materials, not significantly enriched

Horizon development ideal conditions

presence of vegetation

precipitation

absence of erosion

No disturbances

Soil taxonomy classification properties

Soil horizons

Nutrient chemicals

Distribution of organic materials

Soil color

Overall climate

Oxisols

Where: S. America and S. Africa

Color: Reddish (iron) and Yellowish (Al Oxides)

Characteristics: Low CEC and Fertility b/c eluviation of material

Aridisols

Where: W. Americas and Australia. Arid

Characteristics: shallow horizons, lacks O, salinization high POTET, Calcification

Mollisols

Where: Middle U.S. (great plains) and Middle Eurasia ("fertile triangle" of Russia-ukraine region)

Characteristics: Grasslands, dark organic later, high CEC, high fertility

Alfisols

Where: Widespread, SE Australia, Europe

Color: Grayish brown to reddish

Characteristics: CEC adequate, fertile with enough moisture and T, mostly under forests and mixed vegetation

Engineering Grain size

DIFFERENT than Wentworth-Udden (WU) scale that we use, dependent on mechanical properties, are a bit larger than WU

Strong soils

Resistant to stress, excellent for building

Weak soils

subject to compression, collapse, flow when stressed

Soil sensitivity

measure of tendency of soils to change

Atterberg limits

measure of Effect of water. It's shrinkage limit, plastic limit, liquid limit. 4 classes

Shrinkage limit (SL)

boundary btw brittle solids and semi-solids. Below SL, soils don't shrink bc no moisture

Plastic Limit (PL)

Boundary that separates semi-solid and plastic soils. Plasticity is Measure of cohesiveness

Liquid Limit (LL)

Separates plastic soils from liquid soils. Water content makes soil lose shear strength, liquefaction

Plasticity Index (PI)

Range of water contents over which the soil behaves as a plastic substance, is the difference between LL and PL

Casagrande diagrams

Plasticity vs LL.

Silt-rich: low plasticity, low LL

illite/smectite: very high plasticity and LL

Compressibility

tendency of soils to consolidate and lose volume. Only a problem when differential compaction occurs

Paleosols

Old soils. May be altered during diagenesis or truncated by erosion. Alteration may remove O, oxidize Fe, Dehydrate limonite to hematite