Environmental Engineering Reference
In-Depth Information
•
Time:
It takes time for weathering and erosion to occur, so the amount of time an area has been exposed
to various elements affects the makeup of the soil and size of the soil particles.
Weathering
Parent material is broken down by water, wind, temperature fluctuations, and living organisms through a pro-
cess called
weathering.
Three types of weathering have been identified, and each breaks down rock in a differ-
ent way:
•
Biological weathering
occurs from the daily activities of organisms moving through and over soil.
•
Physical (mechanical) weathering
occurs when rock material is broken down without any chemical
change taking place, usually through wind, water, and other forces.
•
Chemical weathering
occurs when chemical reactions occur from water and atmospheric gases reacting
with parent material.
Soil Profile
Soil types vary from place to place in both composition and characteristics. Layers of soil develop over time,
and the cross-section of these layers is called the
soil profile.
Each layer has specific characteristics based on
its location in the soil profile.
Starting with the outermost layer and ending with the deepest, the following list of items describes a cross-sec-
tion of a soil profile:
•
O Horizon (organic litter):
The O horizon is composed of organic matter, including living organisms, as
well as decaying organic matter and waste.
•
A Horizon (topsoil):
The A horizon is a mix of organic matter with inorganic materials. This includes
weathered parent material. Topsoil is an important factor in plant growth and productivity.
•
E Horizon (eluviated):
The E horizon is mainly composed of mineral material. E horizon soil is leached
from this layer and transported with water as it percolates downward through the soil.
•
B Horizon (subsoil):
The B horizon contains many components leached from the soil layers above it, in-
cluding nutrients, organic matter, and minerals.
•
C Horizon (minimally weathered):
The C horizon is weathered from parent material, but the weather-
ing and erosion it has experienced is minimal, so it contains mainly larger fragments.
•
R Horizon (bedrock):
The R horizon is the parent material and is called bedrock.
The O and A horizons are where most soil organisms live, while the A horizon contains the most nutrients for
plant growth. The concentrations of organic matter and the extensiveness of weathering decrease with depth.
Minerals are transported through each horizon by
leaching.
