Soil Formation Parent Material Time Climate Topography Biota
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. Weathering 2. Soil forming factors 3. Soil forming processes 4. Alluvium 5. Colluvium 6. Loess 7. 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 Ca 5 (PO 4 ) 3 OH + 4H 2 CO 3 5Ca 2+ + 3HPO 4 2- + 4HCO 3 - + H 2 O
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 CO 2 emissions affecting the global climate, nitrogen emissions affecting nitrogen deposition, and introduction of exotic species
Soil forming factors: CO 2 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. Weathering 2. Soil forming factors 3. Soil forming processes 4. Alluvium 5. Colluvium 6. Loess 7. Glacial till