1. Soil Origin and Development
Chapter 2
Intro to Soil Science

2. Pedology Study of soil formation
Also known as soil genesis, and soil classification and mapping
Modern pedology dates to the eighteenth and nineteenth centuries in Germany, the United States, and especially Russia
Early researched developed concepts of soil as evolving body arising from weathered rocks of crust under a variety of influences

3. The Soil Body Pedon Weathering Section of soil
Polypedon: collections of pedons
Weathering
Physical: frost wedging, temperature, water, and wind
Chemical: dissolution, oxidation- reduction, hydrolysis, and hydration
Biological: root wedging and lichens
Soil is collection of natural bodies of the earth’s surface containing living matter that is able to support the growth of plants
Pedon generally examined at a depth of 5 feet (3ft x 3ft x 5ft usually)
Pedons created through weathering
Frost Wedging:
Occurs when water freezes and expands in rocks or cracks in the rock causing it to break apart
Temperature:
Heating and cooling cycles that stress the fabric of the rock → causes fracture or outer layers to peel
Rain, running water and wind-blown dust can also wear away at surfaces
Dissolution:
Dissolve slowly in water (gypsum CaSO4 + H2O → CA+2 + SO-4 + H2O)
Hydrolysis
Minerals react with hydrogen in water molecule, splitting the water apart (creates softer mineral) i.e. feldspar
Hydration
Water molecule itself joins the crystal-line structure of the mineral
Creates a softer, more easily weathered material i.e. hematite to ferrihydrite

4. Rocks and Minerals Original source of most soils is rock Rock types
Unweathered material of the earth’s crust
Solid rock breaks into smaller particles (i.e., parent materials)
Rock types
Igneous rock
Sedimentary rock
Metamorphic rock
Igneous rock
Created by cooling and solidification of molten materials from deep in earth
Contain minerals that supply 13 of 17 required nutrients
Sedimentary
Forms when loose materials such as mud or sand are deposited by water, wind or other agents, slowly cemented by chemicals or pressure into rock
American soils derive from sandstone and limestone
Metamorphic
Igneous and sedimentary rocks subjected to heat and pressure
Limestone changes to marble
Sandstone to quartzite

5. Mode of Formation of Rock Type Description of rock-types
Class of Rock
Mode of Formation of Rock
Type
Description of rock-types
Principle minerals of soil-forming rocks
Igneous Rocks
Formed by cooling of molten magma (lava)
Granite
Usually light in color, coarse to medium grain
Quartz, feldspar, mica, amphibole, iron oxides
Diorite
Gray to dark in color, coarse to medium grain
Feldspar, amphibole, iron oxide, biotite
Basalt
Dark to black in color, dense to fine grain
Feldspar, pyroxene, iron oxide, biotite

6. Mode of Formation of Rock Type Description of rock-types
Class of Rock
Mode of Formation of Rock
Type
Description of rock-types
Principle minerals of soil-forming rocks
Sedimentary Rocks
Formed by deposition of weathered minerals which are derived from igneous rocks
Shale
Light to dark in color, thinly laminated
Clay minerals, quartz
Sandstone
Light to red in color, granular and porous
Quartz, clay minerals, iron oxides, calcium carbonate
Limestone
Light gray, red, brown, or black in color, fine grained and compact
Calcite, dolomite, iron oxide, clay minerals

7. Mode of Formation of Rock Type Description of rock-types
Class of Rock
Mode of Formation of Rock
Type
Description of rock-types
Principle minerals of soil-forming rocks
Metamorphic Rocks
Formed by change of pre-existing rock through heat and pressure
Gneiss
Light and dark bands
Granite
Schist
Foliated structure
Basalt or shale
Slate
Gray to black in color, compact and uniform texture
Composition similar to shale
Quartzite
Light to brown in color, compact and uniform texture
Composition similar to sandstone
Marble
Light red, green or black in color, compact, fine to coarse texture
Calcite, dolomite

8. Parent Material Soil genesis Agents of transport and parent materials
Creating soil from parent material
Agents of transport and parent materials
Glacial ice: glacial drift
Wind: eolian deposits
Water: alluvial soils
Gravity: colluvium
Volcanic
Organic
Residual soils- formed in place from the residuum of broken-down bedrock
Transported soils- develop from already weathered material
Glacial Ice:
Carried parent materials over northern part of N. America
Pick up and transport materials and deposit elsewhere (glacial drift)
Glacial till- unsorted deposits of glacial drift deposited by ice
Glacial outwash- drift deposited in water flowing away from melted glacier, sorted by flowing water, usually coarse texture
Wind:
Eolian deposits are parent materials carried by wind
Many left by glaciers- wind picked up silt-sized particles and deposited them elsewhere
Loess soils- wind deposited silt important for agriculture
Water
Alluvial soils- parent materials carried and deposited in moving freshwater to form sediments
Alluvial fans- form below hills and mountain ranges where streams deposit material
Levees and floodplains
Gravity
Colluviam- parent material moved by sliding or rolling down a slope
Example- talus- sand and rocks that collect at foot of slope
Other examples: avalanches, mudslides, landslides

9. Climate Extremely complex Terms
We are speaking primarily of temperature and precipitation
Terms
Arid, semiarid, and humid
Climates which, in order, experience very low, low, and higher rainfall
Affects soils by causing physical and chemical weathering
Temperature affects speed of chemical reactions
Effect organic matter- warmth promotes greater vegetation so more OM is added to soil
Warm temps also increase speed of decay and loss of OM → soils in warm climate have less OM
Water is crucial factor
Cold regions- water needed for freeze wedging
Soils in moist climates much different than drier climates

10. Organisms Actively affect soil formation
Organisms that live in soil: plants, insects, and microbes
Mineral soils having the highest organic matter content
Form under grasslands
Grassland vegetation, mostly herbaceous, forms a deep, dense mat of fibrous roots

11. Topography Slope Slope aspect Position Length Moisture retention
Solar energy
Wind exposure
Slope position influences soil moisture
Slopes loss soil from erosion, depressions tend to receive soil

12. Time Important considerations Aging Weathering Leaching
Biological processes
Erosion
Progression and regression cycles
Soils change over time
Biological processes tend to increase nitrogen content as soil ages while leaching tend to reduce phosphorus

13. Humans Human activity modifies soil Land use Pollutants Fertilization
Waste disposal
Cultivation

14. The Soil Profile Soil forming factors Additions Losses Translocations
Transformations

15. Additions
Materials added
Fallen leaves, alluvium, human-made materials (compost and air pollution)
Deposition of nutrient-rich dust is major contributor to fertility
Losses
Materials lost
Deep leaching, erosion from surface, gases filtering out of soil
Translocations
Materials moved within
Leaching deeper into the soil, carried upward with evaporating water, moved by animals (ants or earthworms)
Transformations
Materials altered in soil
Organic matter decay, weathering of minerals to smaller particles, chemical reactions

16. The Soil Profile (Continued)
Soil horizons
O: organic layer
A: topsoil
E: eluviation
B: subsoil
C: parent material
R: bedrock

17. O
Organic layer made of wholly or partially decayed plant and animal debris
Generally occurs in undisturbed soil because plowing mixes organic layer into soil A
Topsoil- surface mineral layer where organic matter accumulates
Darker than other horizons
Layer loses clay, iron, and other materials in downward-moving water
Eluviation E
Zone of greatest eluviation
Very depleted in clay, chemicals, and organic matter
Very light colored
Many soils have no E horizon B
Subsoil
Zone of accumulation or illuviation
Lower organic matter content
Enriched in clay deposited by illuviation C
Lacks properties of A and B
Layer little touched by soil-forming processes
Usually parent material of the soil
May include very soft, weathered bedrock that roots can penetrate R
Underlying hard bedrock (limestone, sandstone, granite)
**A, E, B, and O horizons together make up the solum**
Actively participated in soil-forming processes

18. The Soil Profile (Continued)
Subdivisions of the master horizons
Between master horizons in position and properties
Transitional layers
Identified by the two master letters, with the dominant one written first

20. Summary This chapter reviewed several topics Soil body
Rocks and minerals
Parent material
Climate
Topography
Time
Soil-forming factors
Soil profiles