HOW DOES SOIL FORM?
GLOBE NY Metro, 2008
White, 2014
Soil is:
the naturally occurring, unconsolidated or loose
covering of broken rock particles and decaying
organic matter on Earth’s surface, capable
of supporting life.
Soil has three components: solid, liquid, and gas.
The solid phase is a mixture of typically
loosely-packed mineral and organic matter particles
forming a soil structure filled with voids.
The void space contains water (liquid) and air (gas).
Soil is also known as earth: it is the substance from
which our planet takes its name.
Soil is a portion of the regolith, the
mantle of loose, heterogeneous
material that rests on solid bedrock.
Regolith includes saprolite
or weathered bedrock typically
considered to be the lowermost
portion of a soil profile.
Why is soil important?
Soil formation, or pedogenesis, is
controlled by five variables in nature,
the so-called state factors:
1. parent material
2. climate
3. topography
4. biota
5. time
6. humans
Parent material: residual -v- transported
Parent material: Chemical changes during soil formation depend on
what minerals and rocks are present. Example: Calcium-rich soils
generally form from calcium-rich rocks (like limestone) but not from
calcium-poor rocks like granite.
Climate: temperature and precipitation
Climate: heat and water accelerate chemical changes (so
moist, temperate areas like NYC have different soils than
arid, tropical, or polar areas).
Topography:
elevation,
slope,
aspect
Topography: Loose soil stays in place in flat areas, allowing more
thorough physical and chemical alteration of its grains. On steep
slopes, the soil moves downhill before complete alteration can
occur.
Biota:
organisms
Living things: Plant roots physically break rocks into
small pieces; lichen dissolves rock; burrowing animals
mix the soil and help aeration
Biota:
ecosystem
Time
TIME: When bedrock is exposed at the surface, chemical, biologic,
and physical processes combine to produce a thin soil layer. Over
time, the processes extend vertically downward, developing soil
horizons whose position and thickness change over time.
State factor interaction:
e.g. topography/parent material
Soils are characterized in the field in natural
exposures, dug soil pits, or using augers to bore
holes and obtain samples from the subsurface.
White, 2014
5 FACTORS CONTROL THE TYPE OF SOIL
4 factors
control soilforming
processes
Living things
Climate
Topography
Parent Material
Photo courtesy,
Ray Weil, PhD
TIME is the 5th factor
Soil forms by the
interaction of the first
four factors. It changes
to create soil profiles
unique for the
conditions and elapsed
time. A soil profile
consist of layers
called “soil horizons”
If all five factors are the same in two geographic regions,
the soil will be the same in both. Some basic examples of
different soil types include:
Temperate deciduous soil
Coniferous forest soil
Tropical rain forest soil
Grassland soil
Desert soil
Soils are very different, depending on how they form
United States Department of Agriculture
Why do we study soil? Because It’s A(n)
Medium of crop
production
Great integrator
Producer and
absorber of
gases (CO2
and others)
Waste decomposer
Medium for
plant
growth
Medium of heat
and
water storage
Home to organisms
(plants, animals and
others)
Snapshot of
geologic,
climatic,
biological, and
human history
Source material
for construction,
medicine, art, etc.
Essential natural resource
Filter of
water and
wastes
Soil is an excellent place to study interactions in the
Earth System, including contributions from the
Atmosphere
• Dew (moisture from the air) begins chemical alteration of parent rock
• Rain erodes loose soil, preventing further alteration
• temperature controls rate and extent of chemical processes
Hydrosphere
• Water seeps into the ground, dissolving and redistributing elements
• evaporation dries soil, changing its physical characteristics
Biosphere
• plants add and remove chemicals
• plant roots anchor soil in place, enabling chemical reactions to be completed
• animals mix soil; transport seeds, etc.
Geosphere
• solid rock and unconsolidated sediment are the parent material for soil
• geologic processes (surface and internal) expose and bury rock, etc.
Hydrologic Cycle and the Soil
Soil Properties related to the hydrologic cycle.
Soil moisture
Color
Temperature
Structure
pH
Texture
Horizon
Depths
Bulk Density
Review: ROLES OF THE FIVE FACTORS OF SOIL FORMATION
Living things: Plant roots physically break rocks into small pieces;
lichen dissolves rock; burrowing animals mix the soil and help aeration
Climate: heat and water accelerate chemical changes (so moist,
temperate areas like NYC have different soils than arid, tropical, or
polar areas).
Topography: Loose soil stays in place in flat areas, allowing more
thorough physical and chemical alteration of its grains. On steep
slopes, the soil moves downhill before complete alteration can occur.
Parent material: Chemical changes during soil formation depend on
what minerals and rocks are present. Example: Calcium-rich soils
generally form from calcium-rich rocks (like limestone) but not from
calcium-poor rocks like granite.
TIME: When bedrock is exposed at the surface, chemical, biologic,
and physical processes combine to produce a thin soil layer. Over
time, the processes extend vertically downward, developing soil
horizons whose position and thickness change over time.