Soil Formation
Main Objectives
1. Gain a general understanding of soil formation processes
2. Understand the importance of mineral weathering in soil
formation
3. Capable of explaining soil forming factors and their role in soil
genesis
Key Terms and Concepts
1.
2.
3.
4.
5.
6.
7.
Weathering
Soil forming factors
Soil forming processes
Alluvium
Colluvium
Loess
Glacial till
Statements:
Most soils are mineral soils formed by the weathering of
solid rock masses into unconsolidated materials, except
for organic soils that mostly develop from plant residues.
Soil formation consists of two inter-connected parts: (1)
the production/accumulation of unconsolidated materials
by weathering and subsequent movements; and (2)
horizon development, involves the changes occurring
within the loose material over time.
1. Weathering
The meaning of weathering in soil science is often broader
than just rock weathering.
Weathering processes
Physical weathering:
Temperature
Abrasion by water, ice and wind
Activities of plants roots and animals
Chemical weathering:
Hydration
Hydrolysis
Dissolution
Volatiles and weak acids
Oxidation-reduction
Complexation
Physical weathering - wind
Physical
weathering
water
Physical weathering - ice
Chemical weathering
of the mineral apatite
Ca5(PO4)3OH + 4H2CO3 Æ 5Ca2+ + 3HPO42- + 4HCO3- + H2O
Tree roots crack rocks
Figure 2.1 Two stone markers, photographed on the same day in the same cemetery, illustrate the effect of rock type on
weathering rates. The date and initials carved in the slate marker in 1798 are still sharp and clear, while the date and
figure of a lamb carved in the marble marker in 1875 have weathered almost beyond recognition. The slate rock consists
largely of resistant silicate clay minerals, while the marble consists mainly of calcite, which is much more easily attacked
by acids in rainwater. (Photo courtesy of R. Weil)
2. Soil forming factors
Parent material
Time
Topography
Climate
Biota
Putting all actions together = soil formation or genesis
Soil forming factors primarily
influence these four processes
of soil formation.
Figure 2.24
Soil forming factors: Parent material
3. Landforms and types of deposits (parent materials)
Landforms (plains, plateaus, mesas, playas, etc)
Deposits from water
(alluvium, alluvial fans, marine sediments, etc)
Deposits from wind
(eolian deposits, loess, up to 300 m in depth?)
Deposits from ice
(glacial till and moraines)
Deposits from gravity
(mass wasting-> colluvium, soil creep, mudflow, etc)
Figure 2.6 How various kinds of parent material are formed, transported, and deposited.
Figure 2.6
Figure 2.9 Characteristically shaped alluvial fans alongside a river valley in Alaska. Although the areas are
small and sloping, they can develop into well-drained soils. (Courtesy U.S. Geological Survey)
Figure 2.21 An interaction of topography and parent material as factors of soil formation.
The soils on the summit, toe-slope, and floodplain in this idealized landscape have formed
from residual, colluvial, and alluvial parent materials, respectively.
Figure 2.12 Illustration of how several glacial materials were deposited. (a) A glacier ice lobe moving to the
left, feeding water and sediments into a glacial lake and streams, and building up glacial till near its front. (b)
After the ice retreats, terminal, ground, and recessional moraines are uncovered along with cigar-shaped hills
(drumlins), the beds of rivers that flowed under the glacier (eskers), and lacustrine, delta, and outwash
deposits. (c) The stratified glacial outwash in the lower part of this soil profile in North Dakota is overlain by a
layer of glacial till containing a random assortment of particles, ranging in size from small boulders to clays.
Note the rounded edges of the rocks, evidence of the churning action within the glacier. Scale is marked every
10 cm. (Photo courtesy of R. Weil)
Areas in the United States covered by the continental ice sheet and the deposits either directly
from, or associated with, the glacial ice. (1) Till deposits of various kinds; (2) glacial-lacustrine
deposits; (3) the loessial blanket (note that the loess overlies great areas of till in the Midwest); (4)
an area, mostly in Wisconsin, that escaped glaciation and is partially loess covered.
Figure 2.10 Diagram showing sediments laid down in marine waters adjacent to coastal
residual igneous and metamorphic rocks. Note that the marine sediments are alternate
layers of fine clay, silts and coarse-textured sands and gravels. The photo shows such
layering on coastal marine sediment. This diagram and photo illustrate the relationship
between marine sediments and residual materials in the Coastal Plain of southeastern
United States. (Photo and diagram courtesy of R. Weil)
Figure 2.15
Figure 2.13 (a) Major eolian deposits of the world include the loess deposits in Argentina, eastern Europe,
and northern China, and the large areas of dune sands in north Africa and Australia. (b) Approximate
distribution of loess and dune sand in the United States. The soils that have developed from loess are
generally silt loams, often quite high in fine sands. Note especially the extension of the central loess deposit
down the eastern side of the Mississippi River and the smaller areas of loess in Washington, Oregon, and
Idaho. The most prominent areas of dune sands are the Sand Hills of Nebraska and the dunes along the
eastern shore of Lake Michigan.
Soil forming factors: Climate
• Precipitation: enhances weathering,
leaching of elements (e.g. plant nutrients),
biological processes (plant growth,
microbial activity)
• Temperature: enhances chemical
weathering and biological processes
Soil forming factors: Climate
Soil forming factors: Topography
• Topography: influences soils through its
effect on hydrologic pathways and transport
of soil material (e.g. erosion)
• Catena: hillslope complex. Characteristics
such as soil depth, texture, and mineral
content vary with hillslope position
Soil forming factors: Topography
Catena
Soil forming factors: Biota
• Plants: produce different forms and
amounts of litter entering the soil, affecting
decomposition
• Animals: such as earthworms and termites
can significantly affect soils (e.g.
decomposition)
Figure 2.16 Natural vegetation influences the type
of soil eventually formed from a given parent
material (calcareous glacial till, in this example).
The grassland vegetation is likely to occur in a
somewhat less humid climate than the deciduous
forest. The amount, and especially the vertical
distribution, of organic matter that accumulates in
the upper part of the profile differs markedly
between the vegetation types. The forested soil
exhibits surface layers (O horizons) of leaves and
twigs in various stages of decomposition, along
with a thin, mineral A horizon, into which some of
the surface litter has been mixed. In contrast, most
of the organic matter in the grassland is added as
fine roots distributed throughout the upper 1 m or
so, creating a thick, mineral A horizon. Also note
that calcium carbonate has been solubilized and
has moved down to the lower horizons (Ck) in the
grassland soils, while it has been completely
removed from the profile in the more acidic,
leached forested soil. Under both types of
vegetation, clay and iron oxides move downward
from the A horizon and accumulate in the B horizon,
encouraging the formation of characteristic soil
structure. In the forested soil, the zone above the B
horizon usually becomes a distinctly bleached E
horizon, partly because most of the organic matter
is restricted to the near-surface layers, and partly
because decomposition of the forest litter
generates organic acids that remove the brownish
iron oxide coatings. Compare these mature profiles
to the changes over time discussed in Sections 2.7
and 2.8. (Diagrams courtesy of R. Weil)
Soil forming factors: Biota
• Human activity:
– Directly: through agricultural practices
(irrigation, fertilization, plowing)
– Indirectly: through CO2 emissions affecting the
global climate, nitrogen emissions affecting
nitrogen deposition, and introduction of exotic
species
Soil forming factors: CO2 emmisions
Soil forming factors: Time
• Time: Many soil-forming processes occur
slowly, so the time over which soils develop
influences their properties
• Young soils: rich in primary minerals, often
low nitrogen content
• Old soils: highly weathered, often low
phosphorous content
Soil forming factors: Time
Young
Old
Figure 2.22 Progressive stages of soil profile development over time for a residual igneous rock,
in a warm, humid climate that is conducive to forest vegetation. The time scale increases
logarithmically from left to right, covering more than 100,000 years. Note that the mature profile
(right side of this figure) expresses the full influence of the forest vegetation as illustrated in
Figure 2.16. This mature soil might be classified as an Ultisol (see Section 3.14).
4. Relating soil formation to soil degradation and sustainability
Please consider the fact that the rate of soil erosion under the
influence of human activities is often much higher than the natural
rate of soil formation.
Can/should we practice soil conservation based on soil forming
factors?
Is climate change capable of modifying the natural rate of soil
formation? Why and how?
Can/should we artificially accelerate the rate of soil formation
within our current economic framework?
Tennessee Valley, Marin county, CA
From: Helmsath et al. 1997. Nature 388:358-361
Soil Profile
Figure 2.25
Minnesota Spotosol
Minnesota Mollisol
Figure 2.26 Generalized profile of the Miami silt loam, one of the Alfisols of the eastern United States, before and after land
is plowed and cultivated. The surface layers (O, A, and E) are mixed by tillage and are termed the Ap (plowed) horizon. If
erosion occurs, they may disappear, at least in part, and some of the B horizon will be included in the furrow slice.
Main Objectives
1. Gain a general understanding of soil formation processes
2. Understand the importance of mineral weathering in soil
formation
3. Capable of explaining soil forming factors and their role in soil
genesis
Key Terms and Concepts
1.
2.
3.
4.
5.
6.
7.
Weathering
Soil forming factors
Soil forming processes
Alluvium
Colluvium
Loess
Glacial till