Soil
 1.
 2.
 3.
 4.
 5.
What is soil?
How does it form?
What are soil profiles?
What are the major soil types?
How is a soil’s texture classified?
1. Soil is:
 A loose covering of weathered rock and
humus (decaying organic matter).
2. Soils forms when:
 Rocks are weathered into smaller pieces
mechanically
 Root pry, frost wedging, gravity, etc.
 Plant roots and lichen release acids that
chemically weather rock
 Bacteria, fungi and insects break down dead
organic material
 This humus adds important nutrients to the soil
 Humus also increases the soil’s ability to hold
water
3. Soil profiles contain
horizons or distinct layers
 The A horizon
(topsoil) contains
lots of humus
 Rich soils are grey to
black in color
The B horizon (subsoil) has
less developed soil
 This may be rich in
clay that makes it
red or brown and
very impermeable
 Minerals may seep
through the ground
and build up here
The C horizon is directly above
the solid bedrock
 This contains
weathered rock
 This soil near Red Wing,
MN. has a C horizon of
loess. The loess has
changed very little and
looks very similar to the
way it looked when it
was deposited by the
wind over 20,000 years
ago

http://www.soils.umn.edu/academics/classes/
soil2125/doc/s3chap2.htm
Soil types are mainly
determined by climate
 Other factors affecting soil formation
are:
 Parent bedrock type
 Types of plants and animals living in the
soil
 The topography of the area (ups and
downs)
 Length of time the soil has had to develop
4 main types of soil are:
 Polar
 Shallow, no distinct horizons, permafrost
 Temperate
 Abundant humus under grasslands, less deep and
fertile under forests
 Desert
 Little or no humus, thin A horizon abundant
nutrients
 Tropical
 Very weathered and infertile
5. Soil texture
Is based on the
percent of sand, silt
and clay
Sand is the largest
particle and clay is
the smallest.
Other important soil
characteristics
Color
Dark usually indicates organic matter
Red or yellow may mean iron oxidation has occurred
Gray-blue means poorly drained, wet, low oxygen
soil
Fertility is affected by:
Available nutrients - NPK, Ca, Fe and others
Microbes (N fixing bacteria)
Topography
pH