20-3: Soils form as a result of parent material, climate, topography, organisms, and time
Soil Formation:
● Takes hundreds to thousands of years.
● Result of:
○ Physical and chemical weathering of rocks: Breaks down parent material from
below.
○ Gradual accumulation of detritus from the biosphere: Deposition of organic
matter from above.
● "Soil" is a mix of mineral and organic components.
● Young soil: Less organic matter and fewer nutrients.
● Mature soil: More developed, with more organic matter and nutrients.
● Very old soils: May be nutrient-poor due to plant removal and leaching.
Five Factors Determining Soil Properties:
1. Parent Material:
○ Underlying rock material from which inorganic components are derived.
○ Different parent materials lead to different soil types (e.g., quartz sand →
nutrient-poor soil; calcium carbonate → calcium-rich, high pH, potentially
productive soil).
2. Climate:
○ Long-term effects of temperature, humidity, and water.
○ Below freezing: Slow decomposition and water movement, leading to
undecomposed organic material (e.g., high latitudes).
○ Humid tropics: Accelerated weathering, leaching, and decomposition.
○ Indirect effect: Influences vegetation type, which affects detritus.
3. Topography:
○ Surface slope and arrangement of landscape.
○ Steep slopes: Erosion and mass movement (landslides) limit soil depth.
○ Bottoms of steep slopes: Accumulation of material, leading to deep soils.
4. Organisms:
○ Plants: Remove nutrients, excrete organic acids (speeding weathering).
○ Tunneling/burrowing animals (earthworms, gophers, etc.): Mix soil, distribute
organic and mineral matter.
○ Soil organisms (bacteria, fungi, etc.): Act as recyclers, break down organic matter,
release mineral nutrients.
, 5. Time:
○ Soils develop characteristics as they age.
○ Old grassland soils: Deep and fertile due to continuous organic matter input.
○ Other old soils: Can be infertile depending on vegetation and water movement.
Soil Horizons:
● Horizontal layers with distinct physical features (color, texture).
● Composition depends on climate, vegetation, and parent material.
○ O Horizon (Organic Horizon):
■ Surface layer composed of organic detritus (leaves, twigs, animal bodies)
in various stages of decomposition.
■
■ Most pronounced in forest soils and some grasslands.
■ Contains humus: Fully decomposed organic matter in the lowest part of
the O horizon, lacking recognizable plant or animal components.
○ A Horizon (Topsoil):
■ Frequently the top layer (or the top layer if mixed by nature or agriculture).
■ Zone of mixed organic material (including humus) and minerals.
○ E Horizon (Zone of Leaching or Eluviation):
■ Forms in some acidic soils under the O (or sometimes A) horizon.
■ Iron, aluminum, and dissolved organic acids are transported out of this
horizon.
○ B Horizon (Subsoil):
■ Below the E horizon (if present), or directly below the A horizon.
■ Primarily mineral material with very little organic matter (including
humus).
■ Nutrients leached from above may accumulate here.
○ C Horizon:
■ Least-weathered soil horizon, beneath the B horizon.
■ Similar to the parent material.
Key Terms:
, ● Parent Material: The underlying rock material from which the inorganic components of a
soil are derived.
● Climate: The long-term patterns of temperature, humidity, and water that affect soil
development.
● Topography: The surface slope and arrangement of a landscape.
● O Horizon: The organic horizon at the surface of many soils, composed of organic
detritus in various stages of decomposition.
● Humus: The most fully decomposed organic matter in the lowest section of the O
horizon.
● Horizon: A horizontal layer in a soil defined by distinctive physical features such as color
and texture.
● A Horizon (Topsoil): Frequently the top layer of soil, a zone of organic material and
minerals that have been mixed together.
● E Horizon: A zone of leaching, or eluviation, found in some acidic soils under the O
horizon or, less often, the A horizon.
● B Horizon (Subsoil): Commonly known as subsoil, a soil horizon is composed primarily
of mineral material with very little organic matter.
● C Horizon: The least-weathered soil horizon, which always occurs beneath the B horizon
and is similar to the parent material.
20-4: Soil erodes by wind and water, as well as increases in human activities
Soil Erosion:
● Occurs by wind and water.
● Increased by human activities.
● A major cause of soil degradation: Reduces the ability of soils to support plant growth.
Causes of Soil Erosion:
● Disturbance of topsoil: Example: Plowing a field.
● Removal of vegetation/clearing forests: Eliminates the root system that holds soil
particles together.
Erosion by Water:
● Lack of plant roots in disturbed or cleared areas leads to rapid removal of soil during
rainstorms.
● Examples:
○ Construction sites.
○ Farm fields after floods.
○ Logged steep slopes: Dead tree roots no longer hold soil, gravity assists erosion
leading to landslides (massive soil movement down slopes).
● Landslides can cause significant economic damage and bury homes.
, ● Topsoil loss can be rapid (single growing season) but takes centuries to replace.
Erosion by Wind:
● Similar to water, wind has a large impact on soils with removed natural vegetation.
● Example: Conversion of native grasslands to wheat fields in the US Midwest
(1920s-1930s).
○ Wheat stems are removed during harvest, exposing bare soil to wind more than
native grasslands.
○ Increased hectares of land susceptible to wind erosion.
● Severe and prolonged drought (early 1930s) led to crop failures.
○ Few remaining plants to hold soil.
○ Massive dust storms as wind picked up dry topsoil.
○ "Black Sunday" (April 14, 1935): Severe dust storm blocking the sun in the
southern Great Plains.
○ Topsoil traveled as far as Washington, D.C.
○ Southern Great Plains nicknamed "the Dust Bowl."
● Dust storms caused large-scale human migration and impacted the national economy.
● Improved farming practices today have reduced the susceptibility of soil to wind
erosion, making major dust storms less likely.
Key Concepts:
● Soil Degradation: Reduction in the ability of soil to support plant growth.
● Soil Erosion: The wearing away of topsoil by natural forces (wind, water) or human
activities.
● Landslide: A mass movement of soil down a slope, often exacerbated by logging and
heavy rain.
● Dust Storm: A severe weather condition where strong winds lift and carry large
amounts of dry topsoil.
● Dust Bowl: The nickname given to the southern Great Plains during the severe dust
storms of the 1930s.
20-5: Soils have different physical, chemical, and biological properties that affect productivity
Soil Functions for Humans:
● Different soil properties suit different human uses.
○ Some soils are good for growing crops (high productivity).
○ Others are better for building (housing developments).
● Soil productivity varies globally due to differences in:
○ Water-holding capacity.
○ Nutrient availability.
Soil Formation:
● Takes hundreds to thousands of years.
● Result of:
○ Physical and chemical weathering of rocks: Breaks down parent material from
below.
○ Gradual accumulation of detritus from the biosphere: Deposition of organic
matter from above.
● "Soil" is a mix of mineral and organic components.
● Young soil: Less organic matter and fewer nutrients.
● Mature soil: More developed, with more organic matter and nutrients.
● Very old soils: May be nutrient-poor due to plant removal and leaching.
Five Factors Determining Soil Properties:
1. Parent Material:
○ Underlying rock material from which inorganic components are derived.
○ Different parent materials lead to different soil types (e.g., quartz sand →
nutrient-poor soil; calcium carbonate → calcium-rich, high pH, potentially
productive soil).
2. Climate:
○ Long-term effects of temperature, humidity, and water.
○ Below freezing: Slow decomposition and water movement, leading to
undecomposed organic material (e.g., high latitudes).
○ Humid tropics: Accelerated weathering, leaching, and decomposition.
○ Indirect effect: Influences vegetation type, which affects detritus.
3. Topography:
○ Surface slope and arrangement of landscape.
○ Steep slopes: Erosion and mass movement (landslides) limit soil depth.
○ Bottoms of steep slopes: Accumulation of material, leading to deep soils.
4. Organisms:
○ Plants: Remove nutrients, excrete organic acids (speeding weathering).
○ Tunneling/burrowing animals (earthworms, gophers, etc.): Mix soil, distribute
organic and mineral matter.
○ Soil organisms (bacteria, fungi, etc.): Act as recyclers, break down organic matter,
release mineral nutrients.
, 5. Time:
○ Soils develop characteristics as they age.
○ Old grassland soils: Deep and fertile due to continuous organic matter input.
○ Other old soils: Can be infertile depending on vegetation and water movement.
Soil Horizons:
● Horizontal layers with distinct physical features (color, texture).
● Composition depends on climate, vegetation, and parent material.
○ O Horizon (Organic Horizon):
■ Surface layer composed of organic detritus (leaves, twigs, animal bodies)
in various stages of decomposition.
■
■ Most pronounced in forest soils and some grasslands.
■ Contains humus: Fully decomposed organic matter in the lowest part of
the O horizon, lacking recognizable plant or animal components.
○ A Horizon (Topsoil):
■ Frequently the top layer (or the top layer if mixed by nature or agriculture).
■ Zone of mixed organic material (including humus) and minerals.
○ E Horizon (Zone of Leaching or Eluviation):
■ Forms in some acidic soils under the O (or sometimes A) horizon.
■ Iron, aluminum, and dissolved organic acids are transported out of this
horizon.
○ B Horizon (Subsoil):
■ Below the E horizon (if present), or directly below the A horizon.
■ Primarily mineral material with very little organic matter (including
humus).
■ Nutrients leached from above may accumulate here.
○ C Horizon:
■ Least-weathered soil horizon, beneath the B horizon.
■ Similar to the parent material.
Key Terms:
, ● Parent Material: The underlying rock material from which the inorganic components of a
soil are derived.
● Climate: The long-term patterns of temperature, humidity, and water that affect soil
development.
● Topography: The surface slope and arrangement of a landscape.
● O Horizon: The organic horizon at the surface of many soils, composed of organic
detritus in various stages of decomposition.
● Humus: The most fully decomposed organic matter in the lowest section of the O
horizon.
● Horizon: A horizontal layer in a soil defined by distinctive physical features such as color
and texture.
● A Horizon (Topsoil): Frequently the top layer of soil, a zone of organic material and
minerals that have been mixed together.
● E Horizon: A zone of leaching, or eluviation, found in some acidic soils under the O
horizon or, less often, the A horizon.
● B Horizon (Subsoil): Commonly known as subsoil, a soil horizon is composed primarily
of mineral material with very little organic matter.
● C Horizon: The least-weathered soil horizon, which always occurs beneath the B horizon
and is similar to the parent material.
20-4: Soil erodes by wind and water, as well as increases in human activities
Soil Erosion:
● Occurs by wind and water.
● Increased by human activities.
● A major cause of soil degradation: Reduces the ability of soils to support plant growth.
Causes of Soil Erosion:
● Disturbance of topsoil: Example: Plowing a field.
● Removal of vegetation/clearing forests: Eliminates the root system that holds soil
particles together.
Erosion by Water:
● Lack of plant roots in disturbed or cleared areas leads to rapid removal of soil during
rainstorms.
● Examples:
○ Construction sites.
○ Farm fields after floods.
○ Logged steep slopes: Dead tree roots no longer hold soil, gravity assists erosion
leading to landslides (massive soil movement down slopes).
● Landslides can cause significant economic damage and bury homes.
, ● Topsoil loss can be rapid (single growing season) but takes centuries to replace.
Erosion by Wind:
● Similar to water, wind has a large impact on soils with removed natural vegetation.
● Example: Conversion of native grasslands to wheat fields in the US Midwest
(1920s-1930s).
○ Wheat stems are removed during harvest, exposing bare soil to wind more than
native grasslands.
○ Increased hectares of land susceptible to wind erosion.
● Severe and prolonged drought (early 1930s) led to crop failures.
○ Few remaining plants to hold soil.
○ Massive dust storms as wind picked up dry topsoil.
○ "Black Sunday" (April 14, 1935): Severe dust storm blocking the sun in the
southern Great Plains.
○ Topsoil traveled as far as Washington, D.C.
○ Southern Great Plains nicknamed "the Dust Bowl."
● Dust storms caused large-scale human migration and impacted the national economy.
● Improved farming practices today have reduced the susceptibility of soil to wind
erosion, making major dust storms less likely.
Key Concepts:
● Soil Degradation: Reduction in the ability of soil to support plant growth.
● Soil Erosion: The wearing away of topsoil by natural forces (wind, water) or human
activities.
● Landslide: A mass movement of soil down a slope, often exacerbated by logging and
heavy rain.
● Dust Storm: A severe weather condition where strong winds lift and carry large
amounts of dry topsoil.
● Dust Bowl: The nickname given to the southern Great Plains during the severe dust
storms of the 1930s.
20-5: Soils have different physical, chemical, and biological properties that affect productivity
Soil Functions for Humans:
● Different soil properties suit different human uses.
○ Some soils are good for growing crops (high productivity).
○ Others are better for building (housing developments).
● Soil productivity varies globally due to differences in:
○ Water-holding capacity.
○ Nutrient availability.
