1. The “Dirt” on Dirt
Biology 20 : Soil

2. 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.

3. Soil
Profil
e

4. Soil Formation
 Soil is formed from the weathering of rocks
and minerals.
 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
rocks

5. Soil Formation & Plants
 Plants help the development of the soil.
 How?
 The plants attract animals, and when the
animals die, their bodies decay.
 Decaying matter makes the soil thick and
rich.
 This process continues until the soil is fully
formed.

8. Soil Types
 Sand, silt, and clay are the basic types of soil.
 Most soils are made up of a combination of
the three.
 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.

9. Soil Composition
 Soils are a mixture of different things; rocks,
minerals, and dead, decaying plants and
animals.
 Soil can be very different from one location
to another.
 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.

10. 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
composition
 The amount of water in the soil also changes
the amount of oxygen in the soil
 Example:
 Very wet soil like you would find in a wetland probably
has very little oxygen.

11. 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).

12. Soil Horizons
 O Horizon - The top,
organic layer of soil, made
up mostly of leaf litter and
humus (decomposed organic
matter).
 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

13. Soil Horizons
 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
this layer.
 R Horizon - The
unweathered rock (bedrock)
layer that is beneath all the
other layers.