Chapter 3. Geology and Soils

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Chapter 3. Geology and Soils

• Chapin et al., 2002

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Figure 3.1. The rock cycles as proposed by Hutton
in 1785. Rocks are weathered to form sediment,
which is then buried. After deep burial, the
rocks undergo metamorphosis or melting, or both.
Later, they are deformed and uplifted into
mountain chains, only to be weathered again and
recycled.
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Introduction

• What are soils?
• Natural bodies
• Soil as a medium for plant growth
• Vegetation acquires resources from atmosphere and
  lithosphere
• Soils are multiphasic systems
• Soils have a large impact on local and global
  ecosystems

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Controls over Soil Formation

• Clorpt soil f(climate, organisms, relief,
  parent material, and time) Dokuchaev (1883) and
  Jenny (1941)
• Jenny treated each of the factors as independent

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Parent Material

• Major control on soil properties and vegetation
  (e.g., serpentine soils)
• Organic and mineral (predominant)
• As soils age, influence of parent material on
  soil properties lessens

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Parent Material Classification

• Parent materials are made up of consolidated and
  unconsolidated material that has undergone some
  degree of weathering
• Parent materials can vary greatly over space
• Unconsolidated parent materials are usually
  classified according to size and uniformity of
  particle size as well as mode of origin (e.g.,
  alluvium, loess, till, marine clay, etc.)

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Parent Material Classification

• Consolidated parent materials can be classified
  in many ways
• based on rock type (igneous, sedimentary,
  metamorphic)
• acidity/mineral composition (Table 2.2)

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Intermediatequartz
Diorite
Andesite
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Climate and Weathering

• Generally thought of as the dominant factor of
  soil formation
• Directly affects weathering through influence of
  temperature (Q10) and moisture on the rate of
  physical and chemical processes
• Physical weathering
• Chemical weathering
• Biological weathering (chelation)
• Effect on carbon gain and loss

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Relief (topography)

• Influences soils through its effect on climate
  and differential transport of soil material
• Includes aspect, slope, and topographic position
• Catena - toposequence
• Climate
• Snow distribution
• Solar input (soil temperature, moisture, and ET
  rates) different effects for high lat. and wet
  climates compared to low lat. and dry climates)

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Figure 3.3. Relationship between hillslope
position, likelihood of erosion or deposition,
and soil organic carbon concentration (Birkeland
1999).
Summit
Shoulder
Backslope
Toeslope
Valley
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Table 3.1. Climatic and Topographic Effects on
Long-Term Erosion Ratesa.   Climate
zone Relief Erosion rate (mm
1,000y-1) Glacial gentle (ice
sheets) 50-200 Steep (valleys) 1,000-5,000
Polar montane Steep 10-1,000 Temperate
maritime Mostly gentle 5-100 Temperate
continental Gentle 10-100 Steep 100-200
Mediterranean -- 10-? Semi-arid Gentle 10
0-1,000 Arid -- 10-? Wet subtropics -- 10
-1,000? Wet tropics Gentle 10-100 Steep
100-1,000
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Time

• Genesis of a soil begins with a catastrophic
  event
• Time for a soil to develop (or reach equilibrium
  with environment) depends on the other factors
• Some features of soils change rapidly (e.g., C
  and N stores) others change slowly (e.g., loss
  of phosphorus and P limitation)

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Figure 3.4. The generalized effects of long-term
weathering and soil development on the
distribution and availability of phosphorus
(Walker and Syers 1976). Newly exposed geologic
substrate is relatively rich in weatherable
minerals, which release phosphorus. This release
leads to accumulation of both organic and readily
soluble forms (secondary phosphorus such as
calcium phosphate). As primary minerals disappear
and secondary minerals capable of sorbing
phosphorus accumulate, an increasing proportion
of the phosphorus remaining in the system is held
in unavailable (occluded) forms. Availability of
phosphorus to plants peaks relatively early in
this sequences and declines thereafter.
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Organisms (Biota)

• Impact of the biota on soil development can be
  large, but hard to tease apart from climate
  influences
• Vegetation alters microclimate
• Roots physical and chemical
• Mixing by soils fauna (e.g., earthworms, gophers,
  termites, etc.)
• Acid conifer litter promotes podzolization basic
  deciduous litter promotes faunal activity and
  mixing of O and A horizons
• Microbes intermediaries of chemical reactions
  also affect soil structure

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Humans the Sixth Factor of Soil Formation?

• Cultivation, nutrient additions, irrigation,
  change in vegetation, etc.

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Pedogenic Processes Operate Together at Different
Rates

• Mineral and organic material -
• (1) additions
• (2) transformations
• (3) translocations (transfers)
• (4) losses

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Figure 3.6. Processes leading to additions,
transformations, transfers, and losses of
materials from soils (Birkeland 1999).
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Additions

• Melinization OM addition to A horizon

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Transformations

• Primary minerals weather to secondary minerals
• Isomorphous substitution
• Soil forming minerals consist of 5 major groups
• Nonsilicates (oxides, hydroxides, hydrous oxides,
  salts)
• Ortho- and ring silicates (e.g., olivine, garnet)
• Chain silicates (pyroxene, amphiboles)
• Sheet silicates (mica, serpentine, clay minerals)
• Block silicates (feldspar, quartz)

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Figure 3.7. Diagram showing the molecular
structure of a simple clay layer (a) a
tetrahedral unit, (b) a tetrahedral sheet, (c) an
octahedral unit, and (d) an octahedral sheet
(Birkeland 1999).
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Clay Minerals

• Secondary alumino-silicate minerals of great
  importance to soils
• 6 major types
• Hydrous mica
• Vermiculite
• Montmorillonite (smectite)
• Chlorite
• Kaolinite
• Allophane (amorphous hydrous aluminum silicate
  volcanic origin mainly)

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Hydrous Mica

• Hydrated particles of clay size fewer K
  linkages than primary mineral mica, but still
  non-expanding upon hydration

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Chlorite

• Primary and secondary forms (22 or 211)
• Primary 21 layer silicate with Mg-OH sheet
  between the interlayer micelles instead of K like
  mica
• Secondary has Al-OH interlayer instead

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Table 3.2. Stability of Common Minerals under
Weathering Conditions at Earths
Surface.   Most stable Fe3 oxides Secondary
mineral Al3 oxides Secondary
mineral Quartz Primary mineral Clay
minerals Secondary mineral K
feldspar Primary mineral Na
feldspar Primary mineral Ca2
feldspar Primary mineral Least
stable Olivine Primary mineral
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Translocations

• Decalcification removal of CaCO3
• Lessivage downward migration of clay-sized
  particles leading to an argillic horizon
• Desilication (ferrilization)
• Leading to plinthite or laterite (indurated)
• Pedoturbation biological or physical (e.g., pit
  and mound surfaces)

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Figure 3.8. Hypothetical depth of leaching
related to the texture of the original parent
material (Birkeland 1999).
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Losses

• Liquid and gaseous

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Soil Horizons
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Figure 3.9. A generic soil profile, showing the
major horizons that are formed during soil
development. Density of dots reflects
concentration of soil organic matter.
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Soil Classification

• Soil Taxonomy (1975)
• Hierarchical system that uses soil properties
• Orders (12), suborders (63), great groups (319),
  subgroups (2,484), families (8,000), series
  (19,000 in US), (phases)

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Table 3.3. Names of the Soil Orders in the United
States Soil Taxonomy and Their Characteristics
and Typical Locations.
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Soil Properties and Ecosystem Function

• Soil texture relative proportion of individual
  soil separates lt 2 mm
• Chemical composition related somewhat to size
• Parent material has strong control
• USDA Textural classes

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Figure 3.11. Percentages of sand, silt, and clay
in the major soil textural classes (Birkeland
1999).
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Soil Properties and Ecosystem Function

• Soil structure arrangement of primary soil
  particles into groupings called aggregates or
  peds
• type, size (lt1 mm to gt 10 cm), and grade
• Influences water movement, heat transfer,
  aeration

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Soil Properties and Ecosystem Function

• Soil bulk density mass of dry soil (lt2 mm) per
  unit volume (g/cm3 or Mg/m3)
• Varies with texture and organic matter content
• Mineral soils (1-2 Mg/m3), organic soils
  (0.05-0.4 Mg/m3)
• Increases as soils are compacted

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Soil Properties and Ecosystem Function

• Soil water ideally occupies half of the
  porosity
• Saturated (driven by gravity) vs. unsaturated
  flow (driven by matric potential gradient)
• Field capacity
• Permanent wilting point
• Plant available water storage capacity (mm)
• Difference between FC and PWP

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Soil Properties and Ecosystem Function

• Soil aeration ideally occupies half of the
  porosity
• Alters redox potential (electrical)
• LEO the lion says GER
• Can be denoted by soil color (gley soils and
  mottled soils)
• Effects ion mobility (e.g., Fe2 vs., Fe3 and
  Mn2 vs. Mn4)
• Bacteria able to utilize alternate (other than
  oxygen) electron acceptors facultative anaerobes)

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Table 3.4. Sequence of H-Consuming Redox
Reactions that Occur with Progressive Declines in
Redox Potential (assumes coupling to CH2O H2O
--gt CO2 4H 4e- and equilibrium)
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Soil Properties and Ecosystem Function

• Soil organic matter affects weathering rates
  and soil development, water storage and
  availability, soil structure, nutrient retention
  and availability, and provides energy for
  heterotrophic organisms
• Varies from undecomposed tissues to resynthesized
  (and unrecognizable) amorphous humus
• Interacts with mineral components

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Soil Properties and Ecosystem Function

• Soil pH - -log (H)
• Strongly affects nutrient availability through
  its affect on soil organisms, cation exchange,
  and solubility of minerals (especially P, Fe, Zn,
  Cu, Mn)

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Soil Properties and Ecosystem Function

• Cation exchange capacity capacity of the soil
  to hold positively charged ions on surfaces of
  soil minerals and organic matter
• Values range from 1 to 200 cmol () / kg clay
• Organic matter values high but depend on pH
• Base saturation is the degree that Ca2, Mg2,
  K, and Na (base cations) occupy CE sites
• Ion affinity H(Al3) gt Ca2 gt Mg2 gt K NH4
  gt Na

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Soil Properties and Ecosystem Function

• Anion exchange capacity iron and aluminum
  oxides and allophane/imogolite at low pH become
  positively charged
• Anion affinity PO43- gt SO42- gtCl- gt NO3-
• Divalent anions specifically (covalent bonds)
  adsorbed

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Soil Properties and Ecosystem Function

• Buffering capacity ability of a soil to resist
  a change in pH
• High CEC and base saturation results in high
  buffering capacity
• As pH decreases (below 5), get increasing loss of
  base cations and release of Al3 (highly toxic to
  plant roots)

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Summary

• Soils, in part, a product of geological origin,
  but the other factors modify soil development
  greatly to generate a wide range in soil types
• Soil properties have a large effect on the
  structure and function of ecosystems