1. Soil Origin and Development
Chapter 2
Soil Origin and Development

2. Pedon - a small section or body of soil for studying soil characteristics... typically 3’ x 3’ x 5’
Over time, soils form distinct layers known as “horizons”

3. Weathering - the breakdown of rock by physical and chemical means
Physical weathering involves the breakdown of rock by temperature, water, wind and plants. Particle size is reduced without changing the chemical composition.
Chemical weathering involves alteration of the chemical
composition of rocks and minerals.

4. Types of Physical Weathering
root growth
exfoliation
ice wedging
gravity

5. Types of Chemical Weathering
Dissolution - usually happens to limestone or marble because they contain calcite
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-
water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion
Oxidation - oxygen that is dissolved in water reacts with minerals that contain iron
4Fe+2 +3O2 --> 2Fe2O3
ferrous iron + oxygen combine to form ferric iron oxide (hematite)
Hydrolysis - usually happens in silicate minerals, turning them into clay minerals
2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH)
potassium feldspar in acidic water hydrolyses to kaolinite + quartz + potassium hydroxide

6. Soil Profile Topsoil - upper or A horizon
A soil profile is a vertical cross-section
of a soil. It is divided into a
number of distinct layers, referred to
as horizons.
The horizons are normally
designated by symbols and letters.
Subsoil - middle or B horizon
Parent Material - lower or C horizon

7. Soil Horizons O Horizon uppermost, organic layer of soil
made up mostly of leaf litter and humus (decomposed O.M.)
A Horizon
topsoil…seeds germinate and plant roots grow here
dark-colored, made up of humus mixed with minerals
E Horizon
eluviation (leaching) layer…light in color
made up mostly of sand and silt
may not always be present
B Horizon
subsoil…known as “zone of accumulation” (illuviation)
contains clay and mineral deposits (iron, aluminum oxides,
and calcium carbonate)
C Horizon
regolith…consists of slightly broken-up bedrock
R Horizon
unweathered rock (bedrock).

8. Topography (landforms) Organisms: vegetation, animals, microbes, etc.
Soil Forming Factors
Soil characteristics develop as a result of their origin and environment.
Soil begins as a particular rock material and develops slowly over many years. Each soil is a product of a combination of the following factors:
Parent Material
Time
Climate
Soil Forming Factors
Topography (landforms)
Organisms: vegetation, animals, microbes, etc.

9. Parent Material - the material from which a soil forms
Agents of transport:
Water
Glacial Ice
Wind
Gravity
Volcanic Deposits
talus
alluvial fans
glacial till
eolian deposits

10. Organisms - plants, microbes, soil animals, and humans
Microbes such as bacteria and fungi assist in the decomposition of plant litter. This litter is mixed into the soil by soil animals such as earthworms and beetles.
Roots contribute dead tissue to the soil, bind soil particles together and redistribute and compress soil.

11. Topography - the lay of the land…includes slope, aspect, and elevation

12. Climate - determines the nature (physical, chemical or biological) and rate of weathering that acts on parent material to form soil. The most important elements of climate for soil formation are precipitation and temperature
Climate governs the rate and type of soil formation and is also the main determinant of vegetation
distribution.
Both moisture and temperature influence evaporation, which can affect leaching.
Temperature determines the rate of
chemical and biological decay.

13. Time - the length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect

14. Soil Processes are responsible for the changes to soils over time
Soils are complex and dynamic systems in which many processes are taking place:
Additions - plant material and animal byproducts, nutrient-rich dust, air
pollutants, etc.
Losses - leaching, gases (N2) escaping into the atmosphere, etc.
Translocations - movement of materials within the soil layers due to capillary action, earthworms, etc.
Transformations - the altering of soil materials by physical or chemical weathering, organic matter decay, etc. l

15. Rocks and Minerals - the parent material for most soils
igneous
formed when magma (molten rock deep within the earth) cools and hardens.
metamorphic
formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure.
sedimentary
formed from particles of sand, shells, pebbles, and other fragments of material that gradually accumulate and harden into layers over a long period of time.

16. Importance of Rock Types
Different rock types contain different minerals which in turn contain the nutrients essential for plant growth and development