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Soil Formation:
Concepts, Factors, and
Lecture #1
Subject Outline
What is Soil
Why do we study Soils?
Composition of Soils
Soil Formation- Factors
Soil Formation- Processes
What is soil?
Concepts of soil differ greatly among
users of soil
• Clerk/Secretary: Soil is dirt; it may be in the
wrong place
• Mining engineer: Soil is material for mining ores
• Civil engineer: Soil is material that support
• Home owner: Soil is needed to set up flower
• Farmer: Soil is medium to grow crops to make a
Soil as defined by scientists
… natural product formed from
weathered rock by the action of
climate and living organisms modified
by topography over a period of time.
Why Study Soils?
Reason 1:
Soils are crucial to life
 Soil is an essential part, and some would
argue, the most important component of
the terrestrial ecosystem
 Soils perform vital functions
Functions of soils
Soil as medium for plant growth
Physical support –anchors root system so that plant
does not fall over.
Air –Plants depend on respiration to obtain energy.
Soils provide ventilation.
Water – soil through pores absorb water and hold it
for plants to use.
Temperature moderation – moderates temp
fluctuations for the roots.
Nutrient elements – soils supply mineral nutrients
(dissolved ions) to plants .
Protection –soils protect plants from phytotoxic
Soil as regulator of water
Soil regulates both the quality and quantity of
water in rivers, lakes, and underground
Quantity of water supplies
Some of the water may be stored in the soil and
used by trees and other plants
Quality of water supplies
Water is purified and cleansed as it soaks through
the upper layers of soil.
Soil as recycler of raw materials
Soils play a role in geochemical cycles
Assimilate organic waste
Turn it into beneficial humus
Convert the mineral nutrients into plant and
animal usable forms
Returning carbon to the atmosphere to be used
for photosynthesis again
Soil as Habitat for soil organisms
Handful of soil is home to billions of
organisms in thousands of species
How does this happen?
Micro-environment differences
Pore spaces
Organic matter
Soil as Engineering Medium
Soil is firm and solid
Good base to build structures
Soils differ in stability
Designs for structures are different for soils
Physical properties influence engineering uses
Why Study Soils?
Reason 2:
Soil is an environmental interface
Lithosphere -rocks
Atmosphere -air
Hydrosphere -water
Biosphere –living organisms
Soil Composition
Soil consists of four major components in
three major phases:
1. Soil Air (Gas phase)
2. Soil Water (Liquid phase)
3a. Soil Mineral Matter
3b. Soil Organic Matter
(Solid phase)
The relative proportions of these components
influence the behavior and productivity of soils
Relative Proportions of Soil
Components Determine Soil Behavior
and Productivity
Inorganic Minerals
(Soild Phase)
Inorganic minerals in soils (major solid framework of soil)
Soil minerals are either primary or secondary minerals
Primary minerals are the minerals that formed in the original rocks.
They range from large particle stones, gravels to small particles –sand,
silt, clay.
Secondary minerals are formed in soil by weathering of the primary
minerals (examples are Kaolinite, smectites, illites etc)
The secondary minerals normally are found in the clay fraction of the
soil which is the fraction of the soil solids which is less the 2 micron or
0.002 mm. Clay minerals are minerals which mainly occur in the clay
sized fraction of the soil.
Both inorganic and soil organic matter make up the solid fraction of
Organic Matter
(Solid Phase)
Organic matter which includes living, dead and
synthesized matter are continuously broken down
and incorporated.
OM ultimately decomposes to humus –last stage of
Humus is the product of the decay of organic residues
such as wood, leaves, and other biological materials.
OM is also constantly lost from soil as CO2 from
microbial respiration.
Soil water (or soil solution)
Soil Liquid Phase
Water is vital to the ecological functioning of
the soil.
Soil solution contains water, dissolved
ions, molecules and gases.
Soil water is different from free flowing water
in 2 ways:
Soil water is held by many types of forces within
the pores of the soil.
Soil water is never pure but contains hundreds of
dissolved organic and inorganic compounds.
Soil Air (or Soil Atmosphere)
Soil Gas Phase
contains similar gases as found in the
atmosphere above the soil
But often in very different proportions.
Usually higher in carbon dioxide and lower
in oxygen than the atmosphere.
Is highly variable in space
Has high relative humidity
Interaction of the
The components interact to determine the
nature of a soil e.g.,
soil moisture controls air and nutrient supply
mineral particles control water movement
Organic matter controls arrangement of
minerals which influence pores that determine
water and air relationships
Soil Formation
a) Five factors of soil formation
b) Processes of soil formation
Soil Profile
Soils reflect their
Montana Sri Lanka Brazil
Five factors of soil
s = f(cl, o, r, p, t…)
s = any soil property
cl = climate (rainfall & temperature)
o = organisms (biota)
p = parent material
r = relief (slope aspect and position)
t = time (relative age of soil formation)
1. Parent Material
Soil parent material is the material that soil develops from,
and may be rock that has decomposed in place, or material
that has been deposited by wind, water, or ice.
The character and composition of the parent material plays
an important role in determining soil properties, especially
during the early stages of development. E.g.,
The texture of sandy soils is determined by parent material
Movement of water is controlled by texture of the parent material
Parent material influences the chemistry of the soil
Parent material influences the type of clay minerals present in soil.
Rock and Mineral Weathering to
form parent material
(… and on to form soils)
What is rock and mineral weathering?
.. the modification or breakdown and destruction
of the physical and chemical characteristics of
rocks and minerals and carrying away the
soluble products.
..the nature of the breakdown (weathering)
depends on the type of material (soft or hard?)
Effect of rock type on weathering rates
Slate Rock
Marble Rock
Two stone markers, photographed on the same day in the same cemetery (Photos
courtesy of R. Weil). The slate rock (metamorphosed shale) consists largely of resistant
silicate clay minerals, while the marble consists mainly of calcite (metamorphosed
limestone), which is much more easily attacked by acids in rainwater.
Types of Rocks
Igneous rocks
Form from molten magma
Granite and diorite
Sedimentary rocks
Compacted or cemented weathering products from
preexisting rocks
Sandstone and shale
Metamorphic rocks
Formed by change in the form of other rocks
 Gneiss,
marble, and slate
Two main types of
a) Physical (Mechanical) weathering
.. Causes rocks to disintegrate into smaller
pieces without affecting their composition
Types/Factors of Physical weathering:
Exfoliation by temperature
Abrasion by water, ice and wind
Disintegration by plants and animals
b) Chemical weathering
.. Degradation of rocks and minerals by the
chemical activities of water, oxygen, and
microbial action
Types/Factors of Chemical weathering:
1. Hydration
Process of binding of water molecules to a mineral
5Fe2O3  9H 2O  Fe10O15.9H 2O
2. Hydrolysis
Splitting of water molecules into its components. Split
components in turn attack the minerals.
H 2O  H   OH 
KAlSi3O8  H 2O  HAlSi3O8  K   OH 
3. Dissolution
Process of hydrating of ions until they become dissociated
CaSO4 .2 H 2O  2 H 2O  Ca 2  SO4
 4 H 2O
4. Carbonation
Carbon dioxide dissolves in water to form carbonic acid which
accelerates chemical breakdown of materials
CO2  H 2O  H 2CO3
H 2CO3  CaCO3
 Ca 2  2HCO3
5. Oxidation-Reduction
Minerals that contain Fe, Mn,or sulfur are susceptible to this reaction
when exposed to environments different from the ones in which
they formed. This destabilizes the mineral.
4FeO  O2  2H 2O  4FeOOH
Fe(II) oxide
Fe(III) oxyhydroxide –Goethite
6. Complexation
Biological processes produce organic acids that can form complexes
with elements within the structure of a mineral thereby pooling
the element from the mineral and destabilizing it.
The list on the right hand side are
the products of weathering of rocks
Formation of Parent Materials
Parent materials could be derived from the
1. Organic Deposits – residues of plants
2. Rock that weathered in place
3. Rock that was deposited from elsewhere
Parent materials are commonly classified by
their mode of deposition at their current
Types of Parent materials
Types of Parent Material
1. Gravity
2. Ice transport
glacial till, moraine, outwash
3. Wind transport
Eolian (dune sand, loess, dust)
4. Water transport
Lakes -lacustrine
Streams –alluvium (floodplain, alluvial fans, delta)
Oceans –marine
5. Volcanic ash
2. Climate
May be the most influential of the four factors
acting on the parent material
• Determines the nature and intensity of weathering
(precipitation and temperature)
 Both affect the physical, chemical and biological processes
Climate also exerts influence indirectly through a
second soil forming factor, the living organisms
(natural vegetation).
Climate is so important in soil formation that
certain evidence of climatic change could be found
in the soil
1. Water is essential for all the major chemical
weathering reactions.
2. The deeper water penetrates the parent
material, the more effective it is in soil
weathering and development.
 Water percolating through the soil profile transports
soluble and suspended material from the upper to
the lower layers.
 Thus percolating water stimulates weathering
reactions and helps differentiate soil horizons.
1. Every 10 deg C, the rate of chemical reaction
 If warm temperatures and abundant water are
present in soil at the same time, the processes of
weathering, leaching, and plant growth will be
maximum and lead to deep soil profiles.
 Compare this to very modest soil profile
development processes that are obtained in cold
3. Organisms (Biota)
Soil organisms, both the animals (fauna) and
the plants (flora) physically churn the soil
and help stabilize the soil structure
a) Role of natural vegetation
Organic matter accumulation
Cation cycling by trees
• E.g., Grassland vs. Forest
• E.g., Heterogeneous rangelands
Heterogeneous Rangelands
(Plants & soil formation)
b) Role of animals
Animals such as gophers, moles, prairie dogs bore
into lower soil horizons and bring materials to the
surface –tunnels.
Earthworms and termites
• Bring about considerable soil mixing
Human influence
destruction of natural vegetation
Soil tillage for crop production
Fertilizer application
4. Topography
Relates to the configuration of the land surface
It is described in terms of differences in elevation,
slope and landscape position
Steep slopes encourage soil loss by erosion and
allow less rainfall to enter the soil; thus prevents
formation of soils, and helps destruction of soils.
In the depressions where runoff tends to
concentrate, the soil is usually more deep
Role of Topography in Soil Formation
5. Time
Time that materials have been subjected to
weathering is important because soil forming
processes take time to show their effects.
Clock of soil formation starts when e.g.
Landslide exposes a new rock
Flooding river deposits sediment on floodplain
Glacier melts and dumps load of debris
Bulldozer cuts and fills landscape, etc.
Rates of weathering and soil development
This is a function of the interaction of time and the other
factors of soil formation.
Age of soils (Rate of Soil Devpt)
Both soils are developed soils, but soil on the left is more
mature than soil on the right in terms of soil horizon
Role of Time in Soil Formation
Processes of Soil
Processes of Soil
Processes that are involved in
soil formation can be
placed in four main groups
2. Transformations
3. Transfers or Translocations
4. Losses
1. Additions
Additions entail the inputs of materials
to the developing soil profile from
outside sources. E.g.
Addition of organic matter from Plant
leaves and sloughed-off roots
Addition of water by precipitation
Addition of dust particles that fall on the
soil surface
Addition of salts and silica that is dissolved
in groundwater and deposited near or at soil
2. Transformations
Transformations entail disintegration and/or
altering of composition and form of organic
and inorganic components of soil
Physical weathering of lager particles to
smaller particles
Decomposition of organic residues
Recombination of decomposition products to
form new minerals such as silicate clays and
oxides, organic acids, humus and other
Aggregation of mineral particles
3. Transfers or Translocations
Translocations involve the movement of
organic and inorganic materials laterally
within a horizon or vertically from one
horizon to the another.
Movement of water
Movement of dispersed fine clay particles
Movement of dissolved organic substances
The most common translocation agents are
water and soil organisms.
4. Losses
Materials are lost from the soil by the
Drainage and leaching to groundwater
Erosion of surface materials
Plant uptake
Microbial decomposition
Animals and humans
Combination of Processes
Combination of The Soil Forming
Processes in Action
(Soil Horizon Development)
A-Horizon development
Accumulation of organic matter
Clumping of individual soil particles
Distinct from parent material and other layers
B and C horizon development
Carbonic and organic acids are carried by water into soil
where dissolve various minerals (transformations)
Soluble materials (ions –Ca2+, CO32-, SO42-, etc) are
carried by water and precipitate in the soil from upper to
lower horizons (translocation)
Weathering of primary minerals into secondary minerals
Wetting and drying cracks soils and makes structures.
Soil Horizon Development
Result of the soil forming
processes in action