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Volume to surface area ratios. •. Chemical weathering works on the surface. Smaller particle sizes (in. a given volume) have greater surface area for chemical weathering. Decomposition (chemical weathering). Geochemical weathering. •. -. inorganic chemical reactions. -.

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Volume to surface area ratios

Chemical weathering works on the surface. Smaller particle sizes (in

a given volume) have greater surface area for chemical weathering.


Decomposition (chemical weathering)

Geochemical weathering


inorganic chemical reactions


causes rock to get "rotten" while still preserving many

of the original structures created during formation of

the rock

Creates a bedrock residuum called a Saprolite

Water is most important element


facilitates transfer of chemical elements and chemical





water often dissociates into OH and H which then acts on

electrically imbalanced surfaces on mineral grains


Abrasion pH

change in pH of water (or other fluids) by

powdering a mineral and placing in water


related to ion exchange, and reaches a steady state

Gives a measure of how effective water is at weathering any

given mineral- implies a steady state condition is reached

H+ is depleted, OH- is enriched relative to H+

Implication is that the reactions can achieve

some initial maximum rate and extent and then

the rates drop off considerably



related to chemical weathering acting and

pieces breaking off along cleavage surfaces


keeps rates from achieving some steady state


processes of chemical weathering



a function of available free oxygen and the Eh (redox



Oxygen strips electrons away from other atoms

the process is reversible


often related to water levels in streams and

groundwater systems


chemical processes



reactions between a salt and water


produces Acids and Bases


H ions replace metallic elements (K in the example below)


reaction continues until all H atoms have been used up



+ 2H + 9H





+ 4H


+ 2K














Soil Characteristics

  • What is a soil?
  • naturally occurring 4 dimensional feature (Up/Down; Left/Right; Front/Back; Time)
  • Consists of layers or horizons that parallel the earth’s surface
  • Layers have distinctive mineralogical, chemical, particle size composition that are a function of pedogenesis acting on the parent material
  • Consists of unconsolidated materials*
    • * some horizons act like lithified materials though
  • Some include capable of supporting plant life

Factors influencing soil development

    • They are the same things that influence weathering
  • Climate
    • Temperature and moisture
  • Parent Material
    • Influences mineral content
  • Topography
    • Includes elevation, slope and aspect
  • Vegetation
    • Dependent on temperature and climate
  • Time
    • Take time for mechanical and chemical processes to occur

Development with time

most factors develop with time


not a steady process though- rates of change vary with time.


Use of soils

Relative age tool


Must isolate all variables to address the effects time

Stratigraphic tool


represents former land surface

Paleoenvironmental tool


certain soil orders only occur in limited climates


e.g., aridisol, vertisol


certain characteristics develop best in restricted



Soil Profile

    • The vertical arrangement of the soil from the surface to the parent material
      • Soils take isotropic materials like basalt or as or quartzite and make them anisotropic at the surface

The soil profile follows the landscape, and when conditions warrant, cuts across existing strata



  • The 3 dimension version of a soil profile. It typically includes an areal dimension lacking in the soil profile
    • This is needed to describe the lateral variability in soils within a given region
    • Pedon is the fundamental mappable unit in soil science
    • The pedon is not able to be photographed, and is commonly poorly described by researches due to time and monetary constraints.

The Weathering Profile

In areas with extensive moisture and or extensive permeability, weathering extends very deep

primarily in the form oxidation and reduction

The soil profile is the uppermost part of the weathering profile

The weathering profile may be only as thick as the soil profile in many circumstances


Master Horizons - O (organic)

  • Common in areas with high biological productivity
  • Typically black or dark brown
  • Usually thin, but can be quite thick
  • Can indicate continued deposition if very thick
    • cumulic O horizon
  • Can look like A-Horizon




  • Commonly, a lab analysis is needed to distinguish between A and O horizons
  • O is more than 30% organics
  • A is less than 30%

Master Horizons - A

  • Dark in color
    • often black
    • Doesn’t have to be
  • Mineral content dominant
  • Can be quite thick
  • Zone of eluviation
  • Often difficult to differentiate between A and O horizons





Master Horizons E-Horizon

Zone of extreme leaching of minerals- eluviation

Bleached- a gray color below the A/O horizons but above the B

Primary minerals provide color- rest has been washed away

Common under conifer forests and grasslands


Master Horizons B-Horizon

Zone of Illuviation

Clays and chemicals collect in this horizon

Typically marks the average depth of wetting front


Master horizons K-Horizon

Occurs at the base of the B-horizon

Common in arid environments

Depth of wetting front determines location

Caliche creates an impermeable zone

like concrete

K horizon


Master Horizons C-horizon

Slightly altered parent materials

Typically oxidized only

Recognizable as parent material


Master Horizons - subordinate designations (1-3)

“b” is the most common one of these 5 designations

They appear immediately after the Master Horizon designation and are always lower case


Master Horizons - subordinate designations (2-3)

“k” is the most common of these designations

“ss” commonly occurs in areas with shrink-swell clays like smektite (e.g., Texas Gulf Coast).


Master Horizons - subordinate designations (3-3)

“x” commonly designates a fragipan, a dense brittle horizon that slakes in water, but is impermeable otherwise.


Soil Description

  • 1- Clear off a face so that the color changes are easily viewable
    • Natural light is best; colors can be very subtle
  • 2- Mark any locations where the color changes
    • Golf tees work great (expensive), as do plastic toothpicks (cheap)
  • 3- Use your dirt knife and start poking the soil gently moving down from the top
    • Feel for changes in the resistance to the “poke” and mark where it changes (except where the change coincides with already marked locations
  • 4- Stretch a tape measure and record the intervals between the markers (from the top to the bottom)
    • e.g.,0-10 cm; 10-26 cm, etc…

Soil description (continued)

  • 5- Determine the horizon name for the first layer by describing the following criteria
    • Color * see specific slide
    • Structure * see specific slide
    • Texture * see specific slide
  • 6- Describe the character of the bottom contact with underlying layers
  • 7- Look at samples using a hand lens to determine if there are clay films or silt coats
    • Look in pore spaces and on ped faces
  • 8- Test with hydrochloric acid solution for reactivity

Characteristics- Color

Described using 3 sets of numbers


Munsell Soil Color Chart

First tells the ratio of red to yellow (Hue)


e.g., 7.5 YR, 5R, 10YR, 2.5YR, 7.5R


these identify on what page you examine the

next 2numbers

Second tells light vs dark (Value)


denoted as the numerator of a fraction

Third tells the strength of the color


deviation from a neutral of the same color


the denominator of the fraction


Other factors

Calcium Carbonate

Topography of the horizon boundary


smooth, wavy, irregular

Distinctiveness of the boundary


diffuse, gradual, distinct, abrupt



uniformity of color, "blotchiness"

Organic matter

and anything else you can see that doesn't fall

into one of the above categories


Lab determinations

  • Bulk density-
  • the weight of the soil per unit volume
    • A measure of how dense the soil is (grams/cm3)
      • Increases as clay content increases
      • Coat peds in parafin wax and measure weight in water and weight without wax.

Lab determinations

  • pH
    • Take a small sample and mix with DI water. Insert the probe and take a reading
      • - other methods are available
  • CEC- Cation Exchange Capacity
    • Determines the total negative electrical charge on clay surfaces in meq/100 g of soil (milliequivalents)
  • Calcium Carbonate content
    • Measured using a chittack apparatus
  • Particle size
    • Hydrometer; Pipette; Laser
  • Carbon
    • Loss on ignition


Soils no longer in the active soil forming environment