Formation of Soil and Soil
Profiles

What is soil?
• Soil is a mixture of weathered rock and
decayed plant and animal matter that
formed over a long time.
• Soil is just as important as water, air, and
sunlight.
– Without soil, there would be no plants.
– Without plants, there would be no food or
shelter for humans or animals.

What have you used today that
depended on soil?
-Clothing (cotton grows
in soil)
-Food (every plant and
animal depends on soil, even
wheat and flour for bread)
-Shelter (wood comes from
trees that grow in soil, -School Supplies
materials in drywall and
concrete come from soil or
depend on soil somewhere)

Why does it take soil so long to
form?
1. When rock is exposed at the surface, it
begins to erode.
2. Mosses and other types of vegetation
begin to grow.
3. Mosses and vegetation die and decay and
become part of the soil.

1. Insects, worms, bacteria, and fungi start
to live and grow and when they die they
decay and form a material called “humus”.
2. Seeds of plants blow in and take root.
3. As more organisms die, the soil becomes
more fertile. Animal droppings also add to
the fertileness.

Soil Profiles
• Soil forms in
layers.
• The set of layers is
called a profile.

Soil Profiles
• Sometimes the
layers are called
horizons.

Soil Layers
• The first layer or “O Horizon” is the
organic layer.
– This is a layer of dead leaves and organisms
that are decaying into humus.
• The second layer or “A Horizon” is the
topsoil layer.
– This is where most plant roots are.

Soil Layers
• The third layer or the
“B Horizon” is the
subsoil layer.
– This contains few plant
roots and many minerals
that it gets from water
seeping through the
two top layers.
• The fourth layer or
the “C Horizon” is the
bedrock layer.
– This is mostly rocks and
pebbles and this layer
is just beginning the
cycle.

Differences in Soils
• The types of soil that form depend on many
factors.
– Climate
– Type of bedrock
– Shape of the land
– Amount of humus
– Amount of time
– Amount of water
• For example, the soil in the desert is
different from the soil in a rainforest. Or
clay is different from farming soil.

Review
• What is soil?
• Why is soil important?
• How is soil formed?
• What are the layers of soil?
• What factors effect the type of soil?