18.3 Soil and Its Formation
•Soil Formation
•Composition and Characteristics
•Interaction between Soil and Climate
Soil Formation
Weathering occurs by mechanical or chemical
means.
ex. mechanical—waves on sea cliff
chemical—acidic rain on limestone
Soil Formation
Soil formation occurs from the breakdown of
various levels of rock. These rock layers
are:
Bedrock—the igneous, metamorphic, and
sedimentary rock of the lithosphere the may
exist as mountains, cliffs, or low-lying
plains; provides the material needed to
build soil.
Soil Formation
Parent rock—an area of bedrock that is the
source for an area of soil
•the lithosphere is the most easily weathered
layer.
•Mechanical and chemical weathering
breakdown the lithosphere into smaller and
smaller mineral particles until soil is
formed.
Soil Formation
•This process may take 1,000s of years to
form only a few centimeters of soil.
• Soil= a mixture of mineral particles, air,
water, bedrock, and living and decaying
organisms.
Soil Formation
• Soil forms from the breakdown or
weathering of rock. This weathering can be
influenced by plants and animals.
– Ex. Earthworms breakdown organic material
(leaves) and excrete minerals into the soil.
Rove beetle
Pseudoscorpion
Flatworm
Centipede
Ant
Ground
beetle
Mite
Adult
fly
Roundworms
Fly
larvae
Beetle
Protozoa
Mites
Springtail
Millipede
Sowbug
Bacteria
Slug
Fungi
Actinomycetes
Snail
Mite
Earthworms
Organic debris
Fig. 10.13, p. 221
Soil Formation
• Organisms aerate the soil allowing water
and gases (like carbon dioxide and oxygen)
to reach deeper into the soil and contribute
to the weathering process.
• Soil profiles are formed as weather
progresses.
Soil Profiles
A horizon= topsoil
B horizon= subsoil
C horizon=partially
weathered
bedrock
R horizon=
bedrock
Oak tree
Fern
Word
sorrel
Lords and
ladies
Dog violet
Earthworm
Millipede
Mole
Honey
fungus
Grasses and
small shrubs
Organic debris
Builds up
Moss and
lichen
Rock
fragments
O horizon
Leaf litter
A horizon
Topsoil
Bedrock
B horizon
Subsoil
Immature soil
Regolith
Young soil
Pseudoscorpion
C horizon
Parent
material
Mite
Nematode
Actinomycetes
Root system
Mature soil
Fungus
Red earth
mite
Springtail
Bacteria
Fig. 10.12, p. 220
Composition and Characteristics
of Soil
Soil is separated into three major texture
categories, based upon particle size within
it: Sand, Clay, Silt.
•Sandy soil
•Clay soil
•Loam soil
Water
High permeability
Water
Low permeability
Fig. 10.17, p. 224
Sandy Soils
• Mineral grains vary in size from 0.05 mm to
2.0 mm in diameter
• Have good drainage and aeration, but does
not store water well; therefore, it does not
suit plants well.
Clay Soils
• Mineral grains are smaller than 0.002 mm
in diameter.
• Have poor drainage and no space for air, but
hold water very well; not suited for many
plants
Loam Soils
Silt—mineral grains 0.002 to 0.05 mm in
diameter
•Loam soils consist of roughly equal amounts
of sand, clay, and silt particles.
•Have excellent ability to hold water and air;
due to the even mix of particles, loam is
ideal for plant life.
Soils and Climate
• Because weather has the greatest effect on
weathering, climate drastically effects soil
formation.
• Ex. Desert and Tundra
Grasslands
Tropical regions
Deserts or Tundra
• Less water to breakdown bedrock, causing
soil particles to be large in size
• Strong winds and infrequent rainfall remove
lighter, smaller particles, as well.
• Soil profiles tend to be only a few mm or
cm thick.
Grasslands
• Weathering of bedrock may occur at a faster
rate due to more activity from burrowing
plants and animals.
• Soil profiles tend to be thicker due to more
precipitation.
Mosaic
of closely
packed
pebbles,
boulders
Alkaline,
dark,
and rich
in humus
Weak humusmineral mixture
Dry, brown to
reddish-brown
with variable
accumulations
of clay, calcium
carbonate, and
soluble salts
Desert Soil
(hot, dry climate)
Clay,
calcium
compounds
Grassland Soil
(semiarid climate)
Fig. 10.15a, p. 223
Tropical regions
• High amounts of rainfall quickly leaches
nutrients and small clay particles into the
subsoil.
• Subsoil is therefore does not have good
drainage or aeration.
• Thick soil profiles
• Nutrients in soil come from the rapid decay
of organic materials.
Forest litter
leaf mold
Acidic
lightcolored
humus
Humus-mineral
mixture
Light-colored
and acidic
Light, grayishbrown, silt loam
Iron and
aluminum
compounds
mixed with
clay
Tropical Rain Forest Soil
(humid, tropical climate)
Acid litter
and humus
Humus and
iron and
aluminum
compounds
Dark brown
Firm clay
Deciduous Forest Soil
(humid, mild climate)
Coniferous Forest Soil
(humid, cold climate)
Fig. 10.15b, p. 223