Chapter 2
Soil Origin and
Development
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”
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.
Types of Physical Weathering
root growth
exfoliation
ice wedging
gravity
Types of Chemical Weathering
Dissolution - usually happens to limestone or marble
because they contain calcite
H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3water + 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
Soil Profile
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.
Topsoil upper or A
horizon
Subsoil middle or B
horizon
Parent
Material lower or C
horizon
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).
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.
Parent Material - the material from which a soil forms
Agents of transport:
talus
• Water
• Glacial Ice
• Wind
• Gravity
• Volcanic Deposits
alluvial fans
glacial till
eolian deposits
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.
Topography - the lay of the land…includes slope, aspect, and
elevation
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.
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
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.
l
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.
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.
Importance of Rock Types
Different rock types
contain different
minerals which in turn
contain the nutrients
essential for plant
growth and development
igneous
metamorphic
sedimentary
granite
marble
sandstone
K
Ca+2
Ca
+
basalt
Mg+2
Fe+2
+2
gneiss
+
Ca+2
limestone
K
Ca+2
Mg+2
Fe+2
Ca+2
Mg+2
schist
K+
+
K
Mg+2
Fe+2
shale