Soil is actually very difficult to define. Organic material
Inorganic material: The size of the pieces varies. It may appear as pebbles, gravel, or as small as particles of sand or clay.
The “Dirt” on Dirt
Biology 20 : Soil
What is soil?
Soil is the top layer of the earth’s surface,
composed of rocky material and also mineral
particles mixed with organic matter.
Mineral particles come from weathered “parent
material” (rock material)
Organic matter is decayed plant and animal material
Soil is formed over a long period of time
Soil Formation takes place when many things
interact, such as oxygen (air), water, plant
life, animal life, rocks, and chemicals.
Soil is formed from the weathering of rocks
The surface rocks break down into smaller
pieces through a process of weathering and
is then mixed with moss and organic matter.
Over time this creates a thin layer of soil.
Weathering – is the process of breaking down
Soil Formation & Plants
Plants help the development of the soil.
The plants attract animals, and when the
animals die, their bodies decay.
Decaying matter makes the soil thick and
This process continues until the soil is fully
Sand, silt, and clay are the basic types of soil.
Most soils are made up of a combination of
The type of soil varies from place to place on
our planet and can even vary from one place
to another in your own backyard.
Soils are a mixture of different things; rocks,
minerals, and dead, decaying plants and
Soil can be very different from one location
The inorganic materials are the rocks that
have been broken down into smaller pieces.
The organic material is decaying living
matter. This could be plants or animals that
have died and decay until they become part
of the soil.
Soil Composition & H2O
The amount of water in the soil is closely
linked with the climate and other
characteristics of the region.
The amount of water in the soil changes its
The amount of water in the soil also changes
the amount of oxygen in the soil
Very wet soil like you would ﬁnd in a wetland probably
has very little oxygen.
Soil Horizons (layers)
Soil is made up of distinct horizontal layers;
these layers are called horizons.
They range from rich, organic upper layers
(humus and topsoil) to underlying rocky
layers ( subsoil, regolith and bedrock).
O Horizon - The top,
organic layer of soil, made
up mostly of leaf litter and
humus (decomposed organic
A Horizon - The layer called
topsoil; it is found below the
O horizon and above the E
horizon. Seeds germinate
and plant roots grow in this
dark-colored layer. It is
made up of humus
(decomposed organic matter)
mixed with mineral particles.
E Horizon - This eluviation
(leaching) layer is light in
color; this layer is beneath
the A Horizon and above the
B Horizon. It is made up
B Horizon - Also called the
subsoil - this layer is
beneath the E Horizon and
above the C Horizon. It
contains clay and mineral
deposits (like iron, aluminum
oxides, and calcium
carbonate) that it receives
from layers above it when
mineralized water drips from
the soil above.
C Horizon - Also called
regolith: the layer beneath
the B Horizon and above the
R Horizon. It consists of
slightly broken-up bedrock.
Plant roots do not penetrate
into this layer; very little
organic material is found in
R Horizon - The
unweathered rock (bedrock)
layer that is beneath all the
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