SOIL: - Loose material on earth’s surface

•  One of most valuable resources
•  From weathering of rock fragments on earth’s surface
•  Supports all plants & animals on earth
•  Available soil controls number of people earth can hold

•  Different things to different people

Engineer: Loose material for constructing structures (houses, bridges etc.) without first blasting

Geologist: Weathered material (rock and mineral grains)

Factors in Soil Development

•  Several, 5 most important:

•  Structure of surface:

 d. PARENT MATERIAL:

•  Nature of rock, minerals and/or plants that formed soil

• Since soil development
•  Short-term = immature soil
•  Long-term = mature soil
•  (quality of soil)

•  Process by which rocks & minerals break down near earth surface
•  Different from EROSION
•  Removal and transportation of weathered/unweathered materials by gravity, wind, water

•  Physical disintegration
•  Chemical decomposition

1. Temperature:
•  Rocks adjust to new temp. by breaking up
• Thermal Expansion/Contraction
2. Water: Freezes & expands => Frost Wedging
3.  Pressure: From expansion of H2O OR due to loading (weight)
4.  Organic Activity:
•  Human beings
•  Insects, worms, burrowing animals
•  Roots of plants

• From chemical reactions to produce new minerals

Oxidation: Rocks + Oxygen

 Solution: Rocks + Carbon dioxide (+H2O)

H2CO3 can wear off cementing materials of rocks

(Figure here)

Solution forms/causes large openings in rocks
e.g., Mammoth Caves in Kentucky, Carlsbad Caverns in New Mexico

Rate of Weathering

•  Controlled by :
1.  Factors in the Environment:
•  Environment must change
•  More intense change = faster weathering
2. Surface Area

(Figure here)
3.  Climate: (T, P, Wind)
•  More intense changes in climatic factors = faster weathering
•  More physical weathering in dry areas
4. Nature of Materials:
•  Minerals in parent materials are formed under different conditions of T, P
•  Stability of rock depends on parent material

*Note that weathering occurs at surface i.e., low T & P
•  Content of parent materials also determines rate of weathering

Quartz (most stable, crystallizes at lowest I)
Orthoclase feldspar
Amphibole
Pyroxene
Olivine (least stable, crystallizes at highest T)
*Opposite of crystallization series of magmas

 SOILS

•  Formed from sufficient weathering to support plant life
•  Can form from different rock types: e,g.,

Granites - coarse graine
Diorites
Ryolites - fine grained
• When young (immature soils), reflect composition of parent rock

• When enough time (mature soils), reflect climate rather than parent material

•  Best described using profiles
•  Profile has recognizable horizons
•  Product of soil forming processes

•  Based on GENETIC ORIGIN OR ZONAL LOCATION OR PHYSICAL CHARACTERISTICS

•  Residual -> in place on parent rock
•  Transported -> moved from origin

e.g., Laterite soils in Tropics (Res) Loess in Central US (Trans)

 Based on 4 main zones where found:

1.  Laterites (tropics & subtropics)
2.  Pedocals (arid regions)
3.  Pedalfer (temperate regions)
4.  Tundra soils (Polar regions)

The US Comprehensive Soil Classification

 SOIL PROBLEMS
A. EROSION

•  Lakes 1000s and millions of years to form
• Yet lost continuously through erosion - by winds, running water, etc.
•  Soil is removed by different types of erosion

(Figure here)

 Gully Erosion:

•  Soil is saturated by melting snow
•  Soil then flow away in gullies

•  Strong winds cause “Dust storm”
• Soil erosion

 Mitigation: Soil conservation method are used: e.g., crop rotation, contour plowing, planting of grass during fallow seasons
 

•  Certain soils have clay minerals that allow H2O in their structures
•  On expansion, soils exert pressure on foundations of houses, bridges, etc., causing failure

 Mitigation:

•  Remove soils
•  Mix with non expansive soils
•  Reduce soil moisture, e.g., by pumping H2O
•  Reinforce foundations

•  Weak soils may settle
•  Settlement may be uniform OR uneven  Mitigation: Using different approaches