Weathering and Soils• Weathering divided into 2 categories

- Physical (disintegration)

> breaks rocks and sediments into smaller pieces

- Chemical (decomposition)

> chemical reactions on the surface of minerals

• both are separate processes but usually relate

to each other

- surface area to volume changes as physical processesbreak sediments into smaller pieces

- chemical reactions on minerals changes dimensionsof mineral grains and can cause physicaldisintegration

Volume to surface area ratios

• Chemical weathering works on the surface. Smaller particle sizes (ina 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 manyof 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 chemicalreactions> water often dissociates into OH and H which then acts on

electrically imbalanced surfaces on mineral grains

- +

Ion exchange due to water

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

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

• Implication is that the reactions can achievesome initial maximum rate and extent and then

the rates drop off considerably

Gives a measure of how effective water is at weathering any given mineral- implies a steady state condition is reachedGives a measure of how effective water is at weathering any given mineral- implies a steady state condition is reached

Etching• related to chemical weathering acting and

pieces breaking off along cleavage surfaces- keeps rates from achieving some steady state

processes of chemical weathering• Oxidation/Reduction

- a function of available free oxygen and the Eh (redoxpotential)

- Oxygen strips electrons away from other atoms

• the process is reversible

- often related to water levels in streams andgroundwater systems

processes of chemical weathering• solution

- removal of atoms from mineral structures

- causes minerals to fall apart

• related to pH of the solvent

- typically the water

chemical processes• Hydrolysis

- 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

• 2KAlSi3O8 + 2H + 9H2 O H4Al2Si2O9 + 4H4SiO4 + 2K

kaoliniteorthoclase

feldspar

chemical processes• Ion exchange

- usually a function of a polarized moleculararrangement

> causes a propensity to try and balance the charges byreplacing ions with other ions through a process calledadsorption

> esp. Cations

- often measured as CEC (Cation Exchange Capacity)

> a measure of the adsorption ability of a surface

- particulary common in clay minerals

Other factors• Mobilty-how suceptiple atoms are to movementdue to normal chemical processes

• Leaching

- removal of elements from upper portions of theweathering profile to locations lower in the weatheringprofile

• Chelation- mobilization of relatively immobile

ions by encapsulating them in a complex molcular

structure

- common way to move things like Aluminum

- usually involves organic compounds that fix the metalinto a mobile compound

Oxidation• interaction bewteen substances and oxygen (andin some cases water)

• E.G., 4FeO + O2 2Fe2O3

The iron goes from the ferrous reducedstate (Fe2+) to the ferric (Fe3+) state

MgFeSiO4 + 2H2O Mg(OH)2 + H2SiO3 + FeOolivine magnesium

hydroxide

silicicacid

2FeS2 + 7H2O + 15O 2Fe(OH) + 4H2SO 4

pyrite sulfuricacid

Clay minerals• occur as layers or sheets arranged in a variety ofways

- layers consist of sheets of aluminum and silica thatalternate between discrete layers

> 1:1 - clay minerals include Kaolinite, Allophane, and Halloysite

> 2:1 - clay mionerals include Illite, Smectite, Montmorillonite, andGlauconite

• usually derived from the decomposition of silicate

minerals like feldspars, micas, and hornblende

• Can have unique properties that are important for

good construction> shrink-swell clays like smectite

Soil CharacteristicsWhat 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

Soil CharacteristicsWhat 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 developmentThey are the same things that influence

weathering1. Climate

• Temperature and moisture2. Parent Material

• Influences mineral content 3. Topography

• Includes elevation, slope and aspect4. Vegetation

• Dependent on temperature and climate5. Time

• Take time for mechanical and chemical processes to occur

Factors influencing soil developmentThey are the same things that influence

weathering1. Climate

• Temperature and moisture2. Parent Material

• Influences mineral content 3. Topography

• Includes elevation, slope and aspect4. Vegetation

• Dependent on temperature and climate5. Time

• Take time for mechanical and chemical processes to occur

Development with timeDevelopment with time• • most factors develop with timemost factors develop with time

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

Use of soilsUse of soils

• • Relative age toolRelative age tool- - Must isolate all variables to address the effects timeMust isolate all variables to address the effects time

• • Stratigraphic toolStratigraphic tool- - represents former land surfacerepresents former land surface

• • Paleoenvironmental toolPaleoenvironmental tool- - certain soil orders only occur in limited climatescertain soil orders only occur in limited climates

> > e.g., aridisol, vertisole.g., aridisol, vertisol

- - certain characteristics develop best in restrictedcertain characteristics develop best in restricted

climatesclimates

Soil ProfileThe 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

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

PedonThe 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.

PedonThe 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

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 horizonCan look like A-Horizon

•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 horizonCan look like A-Horizon

O

Ob-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

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

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A

2Bt1

3Bt2

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

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Master Horizons B-HorizonZone of IlluviationClays and chemicals collect in this horizonTypically marks the average depth of wetting front

Master Horizons B-HorizonZone of IlluviationClays and chemicals collect in this horizonTypically marks the average depth of wetting front

Bt-Horizon

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

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

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

“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).

“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.

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

Soil Description1- Clear off a face so that the color changes are easily viewable

•Natural light is best; colors can be very subtle2- 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 Description1- Clear off a face so that the color changes are easily viewable

•Natural light is best; colors can be very subtle2- 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

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 Characteristics- Color • • Described using 3 sets of numbersDescribed using 3 sets of numbers

- - Munsell Soil Color ChartMunsell Soil Color Chart

• • First tells the ratio of red to yellow (Hue)First tells the ratio of red to yellow (Hue)

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

- - these identify on what page you examine thethese identify on what page you examine thenext 2 numbersnext 2 numbers

• • Second tells light vs dark (Value)Second tells light vs dark (Value)

- - denoted as the numerator of a fractiondenoted as the numerator of a fraction

• • Third tells the strength of the colorThird tells the strength of the color

- - deviation from a neutral of the same colordeviation from a neutral of the same color

- - the denominator of the fractionthe denominator of the fraction

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PedsPeds

Other factorsOther factors

• • Calcium CarbonateCalcium Carbonate

• • Topography of the horizon boundaryTopography of the horizon boundary

- - smooth, wavy, irregularsmooth, wavy, irregular

• • Distinctiveness of the boundaryDistinctiveness of the boundary

- - diffuse, gradual, distinct, abruptdiffuse, gradual, distinct, abrupt

• • MottlesMottles

- - uniformity of color, "blotchiness"uniformity of color, "blotchiness"

• • Organic matterOrganic matter

• • and anything else you can see that doesn't falland anything else you can see that doesn't fall

into one of the above categoriesinto one of the above categories

Mottles in a soil

Lab determinationsBulk 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 determinationsBulk 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 availableCEC- 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

Lab determinations pH

•Take a small sample and mix with DI water. Insert the probe and take a reading

- other methods are availableCEC- 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

Paleosols

Soils no longer in the active soil forming environment

Paleosols

Soils no longer in the active soil forming environment

Multiple paleosols in loess