Soil Formation
‣ Soils are a complex mixture of
unconsolidated weathered
rock and organic material.
‣ Soils are essential to terrestrial
life as plants require soil and
the microbial populations,
responsible for recycling
organic wastes, live in the soil
and contribute to its fertility.
‣ They are named and classified
on the basis of physical and
chemical properties in their
horizons (layers). Soils have
four basic horizons O, A, B and
C.
Photo: USDA
Soil Formation
‣ The O horizon is the organic
O
layer, rich in humus. This is
where you find leaf litter, dead
plants and animals that are just
beginning to decompose
‣ The A horizon is the topsoil,
B
which is rich in organic matter.
The dark color of the topsoil is
due to the carbon in the
decomposing organic material.
‣ The B horizon is a subsoil
C
containing clay and soluble
minerals.
‣ The C horizon is made up of
weathered parent material and
rock fragments.
Soil Formation
Stage 1
Stage 2
Stage 3
Stage 4
O horizon
Disintegrating
parent rock
Weathered
parent rock
(C horizon)
A horizon
B horizon
Bedrock
C horizon
Bedrock
C horizon
Bedrock
Bedrock
Soil Formation
‣ The parent rock is broken
Disintegrating parent rock
down by weathering to form
a regolith , which overlies
the solid bedrock.
‣ The soil that forms is part of
the regolith.
Bedrock
Soil Formation
Layer of organic
matter or O
horizon
‣ Plants begin to establish
and organic material builds
up on the surface.
‣ The organic material aids
Weathered parent rock
(C horizon)
the disintegration of the
parent material.
Bedrock
Soil Formation
Mineral fragments and organic matter
O horizon
Humus
Topsoil
‣ Horizons form as the mineral
and organic content mix.
C horizon
‣ Humus rich layers are at the
surface and mineral rich
layers are at the base.
Bedrock
Soil Formation
O horizon
‣ Horizons are well
developed in mature soils.
‣ The final characteristics of
the soil are determined by
the regional conditions and
the rock type.
Subsoil
B horizon
C horizon
Bedrock
Soil
Development
‣ The character and
composition of the parent
material is important in
determining the properties
of a soil.
‣ Parent materials include
volcanic deposits, and
sediments deposited by wind,
water, or glaciers.
Soil
Development
‣ The occurrence of freezethaw and wet-dry cycles,
as well as average
temperature and moisture
levels are important in the
development of soils.
‣ Climate also affects
vegetation, influencing
soil development.
Soil
Development
‣ Plants, animals, fungi, and
bacteria help to create a soil
both through their activities
and by adding to the soil's
organic matter when they
die.
‣ Moist soils with a high
organic content tend to be
higher in biological activity.
Soil
Development
‣ The topography of the land
influences soil development by
affecting soil moisture and
tendency towards erosion.
‣ Soils in steep regions are more
prone to loss of the topsoil and
erosion of the subsoil.
Climatic Influence on Soil
ARID
Desert soils
These are alkaline mineral
soils with variable amounts
of clay, low levels of
organic matter, and poorly
developed vertical profiles.
Horizon A
Horizon A
Horizon B
Horizon B
HUMID
Photo: USDA
Tropical soils
Leaching and
chemical weathering
make these soils
acidic.
Aluminum and iron
oxides accumulate in
the deep B horizon.
Photo: USDA
Climatic Influence on Soil
MID-LATITUDE
Grassland soils
Mature, alkaline, deep,
well drained soils. They
are typically nutrient-rich
and productive with a
high organic content.
Horizon A
Horizon A
Horizon B
Horizon B
POLAR
Very low
temperatures slow
the decomposition of
organic matter and
maintain the
permafrost layer in
these frozen soils.
Photo: USDA
Photo: USDA
Climatic Influence on Soil
Horizon A
Horizon B
TEMPERATE
Weathered forest soils
Well developed soils
with a deep organic
layer and accumulated
clay at lower levels.
Horizon A
Horizon B
SEASONALLY WET
Swelling soils
Marked seasonal rainfall
results in deep cracks as
the soil alternately swells
and shrinks.
Photo: USDA
Photo: USDA
‣
‣
Photo:Siim Sepp, Creative Commons share alike 3.0
Particle Size
Soils are made of many different sized
mineral particles and other material, but
the grains in them can be placed into
categories based on size.
The combination of sizes gives the soil
its texture.
Clay: particle diameter < 0.002mm
Photo: Infrogmation, Creative Commons share alike 2.5
Photo: Bobannye
Sand: particle diameter 0.05mm > 2mm
Silt: particle diameter 0.05mm > 0.002mm
Soil Texture
‣ Soil texture depends upon the amount of sand, silt and clay present.
90
10
80
20
70
30
Read silt in
this direction
Clay
The optimal soil texture
is called a loam and is
around 40% sand, 40%
silt and 20% clay.
60
50
40
30
Other loam exist but are
named after their various
components.
40
50
Sandy
clay
Silty
clay
Silty clay
loam
Clay loam
Sandy clay
loam
60
70
20
Read clay in
this direction
80
Loam
10
Silt
loam
Sandy loam
90
Silt
Sand
90
80
Loamy sand
70
60
50
40
Percentage sand
30
20
10
Read sand in this direction