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AGY 240: INTRODUCTORY SOIL SCIENCE
Lesson Title: Soils Origin and Development
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Colorado Agricultural Education Standards:
• Ag. II 10.9: The student will demonstrate an
understanding of soil fertility and its effect on
crop production.
Colorado Science Standards:
•4.1: Students know and understand the
composition of Earth, its history, and the natural
processes that shape it.
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Objectives
•As a result of this lesson topic, the student
will …
•Define a soil body
•List examples of the five soil-forming
factors
•Describe how soils develop
•Describe the horizons of the soil profile
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Pedology
•Study of soil formation
•Also known as soil genesis, and soil
classification and mapping
•Modern pedology dates to the
eighteenth and nineteenth centuries
in Germany, the United States, and
especially Russia
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The Soil Body
•Pedon
• Section of soil extending from the surface to the
depth of root penetration, but generally
examined to a depth of 5 feet.
• Generally, a pedon has dimensions of about 1
meter by 1 meter, and about 1.5 meters deep
(about 3 feet × 3 feet × 5 feet).
• Polypedon: collections of pedons that are much
the same
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© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.
Weathering
• Physical: the disintegration of rock by
temperature, water, wind, and other
factors. Frost wedging occurs when
water freezes and expands in rocks or
in cracks in the rock, causing it to break
apart.
• Chemical: dissolution, oxidationreduction, hydrolysis, and hydration
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Dissolution. Some minerals simply dissolve slowly in water, as in the dissolution
of gypsum
Hydrolysis, minerals react with the hydrogen that is in the water molecule,
splitting the water apart.
Hydration also involves water, but here the water molecule itself joins the
crystalline structure of the mineral, again creating a softer, more easily
weathered material
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• Oxidation-reduction and other reactions
are also important in chemical weathering.
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Plants also play an important role in rock
crumbling.
Roots can exert up to 150 pounds per square
inch of pressure when growing into a crack in
rock.
Root wedging from the pressure pries apart
stone.
Lichens growing on bare rock form mild acids
that slowly dissolve rock.
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Rocks and Minerals
• Original source of most soils is rock
• Unweathered material of the earth’s crust
• Solid rock breaks into smaller particles (i.e., parent materials)
• Rock types
• Igneous rock
• Sedimentary rock
• Metamorphic rock
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Rocks and Minerals (cont-)
• Igneous Rock
• The basic material of the earth’s crust is igneous rock, created by the
cooling and solidification of molten materials from deep in the earth.
Igneous rocks, such as granite, contain minerals that supply 13 of the
17 required plant nutrients
• Granite, a coarsely grained rock that weathers very slowly to sandier
soils.
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• Sedimentary Rock
• Igneous rock comprises only about one-quarter of the earth’s actual
surface, even if most of the crust is igneous.
• This is because sedimentary rock overlays about three-quarters of
the igneous crust.
• Sedimentary rock forms when loose materials such as mud or sand
are deposited by water, wind, or other agents, slowly cemented by
chemicals or pressure into rock.
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• Metamorphic Rock
• If igneous and sedimentary rocks are subjected to great heat and
pressure, they change to form metamorphic rock.
• For instance, limestone is a fairly soft, gritty rock. When subjected to
heat and pressure, it changes to marble, which is harder and can be
cut and polished.
• Sedimentary rocks tend to become much harder when
metamorphosed, becoming slower to weather.
• Sandstone, for example, changes to the far harder rock quartzite.
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Figure below shows gneiss, a form of metamorphosed granite.
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Parent Material
• Soil genesis
• This is the process of creating soil from parent material
• Residual soils which are formed in place from the residuum of broken-down
bedrock, are actually less common than soils of parent materials carried from
elsewhere by such pervasive agents of transport as wind, water, ice, or
gravity.
• Residual soils form slowly, as solid rock must be weathered first
• Transported soils develop from already weathered material, so they develop
more quickly.
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Major parent materials of soils of the United States.
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• Agents of transport and parent materials
• Glacial ice: glacial drift
• Glacial ice carried parent materials over the northern part of North America
during numerous glacial periods over the past 2 million years.
• Glaciers that expanded out of Canada crushed and ground the earth; picked
up and transported clay, sand, rocks, and other materials; and deposited
them elsewhere to become the parent materials of new soil. These deposits
are termed glacial drift
• Glaciers deposited materials in many ways, so there are several kinds of
glacial drift. During the melting process, some debris simply dropped in place
to form deposits called glacial till.
• Some till dropped at the margins of the glacier, forming hills called moraines.
• Because there was no sorting action in the deposition, glacial till is extremely
variable, and so are the soils derived from it.
• Till soils often contain pebbles, stones, and even boulders.
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• Wind: eolian deposits are wind deposited soil material , mostly silt and
fine sand
• The wind-deposited silt (medium particles) are know as loess soils. —
They are important agricultural soils and are found in much of Iowa,
Illinois, and neighboring states. Loess often blankets other materials, so
often forms the upper parts of a soil.
• Water: alluvial soils are soils whose parent materials were carried and
deposited in moving freshwater to form sediments
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Water-and marine-deposited soils.
1.Floodplains form along rivers from materials deposited during flooding.
2.Alluvial fans are deposited at the base of slopes by running water.
3.Deltas form when smaller particles drop out as a river enters an ocean.
4.River terraces are old floodplains left above a new river level.
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Gravity: Colluvium are parent materials that move simply
by sliding or rolling down a slope
• An example of a colluvial material is a talus—sand
and rocks that collect at the foot of a slope
• Avalanches, mudslides, and landslides are other
examples.
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Volcanic
• The ash blown out of a volcano and deposited nearby or carried some
distance by wind forms a chemically distinct, dark, and lightweight parent
material.
• The Pacific Northwest, Hawaii, and Alaska are areas of the United
States where such deposits are common.
Organic
• Organic soils, containing 20 percent or more organic matter, form
underwater as aquatic plants die
• Organic soils are extensive in Minnesota, Wisconsin, Florida, Michigan,
and Alaska.
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Climate
• Extremely complex
• We are speaking primarily of temperature and precipitation
• Terms
• Arid, semiarid, and humid
• Climates which, in order, experience very low, low, and higher rainfall
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Organisms
• Actively affect soil formation
• Organisms that live in soil: plants, insects, and microbes
• Mineral soils having the highest organic matter content
• Form under grasslands
• Grassland vegetation, mostly herbaceous, forms a deep, dense mat of fibrous
roots
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Topography
• Slope
• Position
• Length
• Moisture retention
• Slope aspect – The direction the slope is facing
• Solar energy
• Wind exposure
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© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.
Time
• Important considerations
• Aging -Soils change over time
• Weathering -Weathering of the young soil continues, and many generations
of plants live and die, so the young soil becomes deeper and higher in
organic matter.
• Biological processes and Leaching -As soils age, biological processes tend to
increase the nitrogen content, while leaching tends to reduce phosphorus.
• Soil development is progressive when it becomes deeper and more complex
with more and better defined layers.
• But if the combination of soil-forming factors changes, soil can also regress,
becoming shallower and less complex.
• Very old soils may have undergone repeated cycles of progression and
regression.
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Humans
• Human activity modifies soil
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Land use
Pollutants
Fertilization
Waste disposal
Cultivation
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The Soil Profile
• Soil profile is the vertical section of a soil through all its horizons,
ending in the parent material
• Soils change over time in response to their environment, represented
by the soil-forming factors
• Soil forming factors
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Additions -Materials may be added to the soil
Losses -Materials may be lost from the soil
Translocations-Materials may be moved within the soil
Transformations -Materials may be altered in the soil
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The Soil Profile (cont’d.)
• Soil horizons
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O: organic layer
A: topsoil
E: eluviation
B: subsoil
C: parent material
R: bedrock
• The A, B, and C horizons are known as master horizons. They are part of a
system for naming soil horizons in which each layer is identified by a code: O,
A, E, B, C, and R.
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The Soil Profile (cont’d.)
• Subdivisions of the master horizons
• Between master horizons in position and properties
• Transitional layers
• Identified by the two master letters, with the dominant one written first
© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.
© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.
© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.
Summary
• This chapter reviewed several topics
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Soil body
Rocks and minerals
Parent material
Climate
Topography
Time
Soil-forming factors
Soil profiles
© 2013 Cengage Learning. All Rights Reserved. May not be scanned, copied
or duplicated, or posted to a publicly accessible website, in whole or in part.