An Introduction to Soils
June 21, 2016
A nation that destroys its soils, destroys itself.
– President Franklin D. Roosevelt, Feb. 26, 1937.
Soil Profiles
Sample Soil Profile
A Horizon
B Horizon
C Horizon
Leached Soils
• In areas of high precipitation
• Downward movement of water
• Dissolves chemical nutrients and carries them away (down)
making them unavailable to plants
• Results in a poor, often thin topsoil layer
Calcification
• In dry climate areas
• Upward movement of water through soil (capillary action)
• Water evaporates leaving behind the minerals that were
dissolved in it
• Results in a thick topsoil layer rich in mineral deposits that is
poisonous to plants
Soil Creation Continued
Physical weathering breaks rocks into small mineral
particles.
Soil Creation Continued
Chemical weathering dissolves and changes minerals at
the Earth’s surface.
Soil Creation Continued
Decomposing organic material from plants and animals
mixes with accumulated soil minerals.
Different soil types develop in
different climates.
Image: T. Loynachan
Prairie soils have a dark surface layer (horizon), are
rich in minerals, and form in grasslands widespread
across Earth’s middle latitudes.
Images: NRCS
Soils-4-2
Forest soils have a light gray upper horizon, a
horizon rich in aluminum and/or iron, and form in
warm to cool humid regions where coniferous
forests grow.
Image: Soil Classifiers of Michigan
Image: Bruce Molnia
Soils-4-3
Tropical soils are reddish and iron-oxide rich,
depleted in nutrients, and form in humid and warm
regions.
Images: National Cooperative Soil Survey, University of Nebraska
Soils-4-4
Organic soils are dark colored, rich in decomposed
organic matter, and form in poorly drained lowlands
such as swamps and wetlands.
Images: Bruce Molnia, Soil Classifiers of Michigan
Soils-4-5
Desert soils form in arid settings and are commonly
rich in calcium carbonate.
Images: Martin Miller, NRCS
Soils-4-6
Tundra soils form in Arctic environments, have a
dark organic-rich upper layer, and a mineral rich
layer over frozen ground.
Images: Travis Hudson, Alaska/Yukon Society of Professional Soil Scientists
Soils-4-7
Different soil types develop in different climates.