The Terrestrial Environmentwiki.colby.edu/download/attachments/1310810/CH217+Soils+2010.ppt.pdfCH217 1 The Terrestrial Environment The “lithosphere” = rocks and soils 29% of the

CH217 1

The Terrestrial Environment

The “lithosphere” = rocks and soils

29% of the Earth is exposed land mass

80% of the exposed land mass is soil(~15% is ice-covered; ~5% is exposed rock)

There are essentially two reasons to study soils:

1. Soils are the principal plant growth medium(sustain forests and agriculture)

2. Soils are affected (and affect) other “compartments” of the environment(global element cycles)

Soil notes from Prof. Jen Shosa

CH217 2

Soils are developed over geologic time

Human interaction with the soil has been recent(i.e. pesticides, waste disposal)

Soil Formation

Soil Properties

Soil Profiles

Environmental Properties of Soils

CH217 3

Soil Formation: The Chemistry of the Crust

Relative abundance of the elements in the crust (by weight)

O

Si

Al

Fe

Ca

Na

K

Mg

47.4

27.7

8.2

4.1

4.1

2.8

2.6

2.3

SiO2

Al2O3

Fe2O3

CaO

Na2O

K2O

MgO

58.2

15.4

7.2

5.1

3.8

3.1

3.5

CH217 4

Physical WeatheringFREEZE-THAW

HEATING

EXPANSION OF MINERALS IN CRACKS

TRANSPORT AND RELEASE OF OVERBURDEN

ABRASION

PLANTS

NET EFFECT: massive rocks broken down into smaller particles(with larger surface area) and soils are created

CH217 5

Chemical Weathering

Occurs simultaneously with physical weathering

Some reactions are biologically mediated; others are inorganic (or abiotic)

Hydrolysis Reactions (water is a reactant):

This set of reactions occurs to completion in tropical (humid) environments;the resulting soils are depleted with respect to silica and are called

laterites, latosols, and oxisols

2KAlSi3O8(s) + 2H3O+(aq) + 7H2O

feldsparAl2Si2O5(OH)4(s) + 4H4SiO4(aq) + 2K+

(aq)

kaolinite

kaoliniteAl2Si2O5(OH)4(s) + 5H2O 2Al(OH)3(s) + 2H4SiO4(aq)

gibbsite

desilicification

CH217 6

CH217 7

In General...

aluminosilicate + H3O+(aq) + H2O clay + H4SiO4(aq) + cations(aq)

agents of chemical weathering

Sources of H3O+:

atmosphere (PCO2~1x10-3.5)

respiration in root zone and decomposition of OM (PCO2 can increase to 1x10-1)

acid rainfall (sulfuric and nitric acids)

fertilizers (nitric and phosphoric acids)

Fe+2, Fe+3 and Al+3 have very low solubilities

The solubilities are enhanced by the presence of ligandsthat complex and chelate the cations and mobilize them

(90% of soluble Fe and Al is complexed with ligands)

Complexation Reactions and Chelation

CH217 8

Primary minerals with oxidizable elements in a low oxidation statewill oxidize when exposed to the atmosphere (e.g. Fe+2 to Fe+3)

Oxidation/Reduction Reactions

- oxidizable element is oxidized

- charge balance in mineral lattice is disrupted

- loss/gain of other elements to maintain neutrality

K2(Mg,Fe(II))6(AlSi3O10)2(OH)4

Mg0.84(Mg5.05,Fe(III)0.9)(Al1.26Si2.74O10)2(OH)4

K is lost

Mg moves to octahedral site

Si/Al ratio decreases

biotite

vermiculite

CH217 9

Under oxidizing conditions Fe(III) exists as:Fe2O3 (hematite)FeOOH (goethite, limonite)

These are highly insoluble but may dissolve under reducing conditionsand be reprecipitated under oxidizing conditions elsewhere

Iron is often the focus of investigations, but other elements also undergooxidation and reduction (Mn, As, Cr)

Also, elements that are co-precipitated with Fe(III) hydrous oxideswill be remobilized if the Fe(III) is reduced and the oxide dissolve

Ion Exchange Reactions

Changes in clay mineral chemistry due to cation exchange changesthe physical properties of the clay (reduced CEC and ability to expand)

CH217 10

Organic matter generally in soil ranges from <1% to 5% weight of soils(but its presence is disproportionately important to soil chemistry)

Organic matter in soils comes from the decomposition of plant material and soil biomass

Breakdown of a plant by microbes:

75% water 25% dry matter

40% carbon, 40% oxygen, 10% hydrogen10% inorganic

Dry organic matter: 60% carbohydrate, 10% protein, 25% lignin, 5% lipid

mineralized orincorporated into

microbial structure

CO2 released duringrespiration

The net result of physical and chemical weathering is the development of an inorganic soil

Soil itself is not stable and continues to develop both physically and chemically

Also, soils are not entirely inorganic

CH217 11

In summary the development of a soil involves:

Geological Parent Material

Soil

physical weathering, chemical weatheringdecomposition of organic material

under given climatic conditions (temperature, availability of water)

MINERALOGY

CHEMISTRY

Three-phases: soil + water and air in the pores

CH217 12

Soil Properties

Physical Properties

Particle Size

Texture

Density

Structure

Permeability

Chemical Properties

Total elements

Available elements

Cation Exchange Capacity

Soils of Variable Charge

Soils pH

Soil Engineeringhttp://fbe.uwe.ac.uk/public/geocal/cwk/classification/

CH217 13

Physical PropertiesParticle Size: International Society of Soil Science Classification

Clay SiltSand

Fine Coarse

Gravel

2.0 mm

SOIL

2.0 µm 20 µm 200 µm

COLLOIDS (<10 µm)

Sand: high in quartz and feldspar

Clay: high in clay minerals, organic matter, hydrous Fe and Al oxides

http://www.fao.org/documents/show_cdr.asp?url_file=///docrep/003/y1899e/y1899e00.htm

CH217 14

Physical PropertiesTexture: based on triangular diagrams

0

100

0

0

100100

percent clay percent silt

percent sand

clay

silt

sandloam

CH217 15

Physical PropertiesDensity: reflects the composition

Particle density = density of individual particles<1 g/mL for organic matter

>5 g/mL for metal oxides~7g/mL for some metal sulfides

2.5-2.8 for quartz and aluminosilicates

Bulk density = density of the soilincludes the pore space

1.2-1.8 g/mL for sandy soils1.0-1.6 g/mL for clay-rich soils

Porosity (%) = 100 -bulk density

particle densityx 100

sands: 35-50%clay and organic matter rich soils: >60%

CH217 16

Physical PropertiesStructure: how individual particles are aggregated

Structure-lessGranular: crumb-like

Block-like: arranged around a pointPlate-like: arranged around a horizontal axisPrism-like: arranged around a vertical axis

Permeability: the ease with which water (and chemicals) flows through the soil

(not “also called hydraulic conductivity”)

typical vertical conductivities = 1-5 cm/hr

low = 0.5 cm/hr; high = 15 cm/hr

Basically a function of particle size, texture, and structure

Low permeability soils can become water-logged: reducing conditions

CH217 17

Chemical Properties

Total Elements: Organic

Temperate agricultural: 1-5%

Tropical agricultural: 0.1-2%

Forest soils: >10%

Peat soils: > 20%

Organic matter generally decreases with depth

Classified into humic and non-humic fractions

Humic: partially decomposed and resynthesized; relatively stablecontributes to soil structure and cation exchange capacity

Non-humic: ???

Available (Extractable) Elements

The fraction of elements that can take part in chemical and biological reactions

CH217 18

Chemical Properties

Cation Exchange Capacity

Some minerals (mostly clays and OM) have the capacity to adsorb cations

We can quantitatively assess this ability -- CEC (cation exchange capacity)

1. Extract the adsorbed cations with NH4Cl at pH=4.5

2. Measure the cation concentrations in extracted solution with AA spectroscopy

3. Calculate the milliequivalents/L present in solution for each cation

4. Sum of the meq/L = CEC

H3O+ is adsorbed in acid conditions; negligible in alkaline conditions

Base saturation =# exchange sites occupied by metals

# of exchange sites

CEC ranges from ~1 meq/L (sandy soils) to >100 meq/L (clay and organic rich soils)

CH217 19

Chemical Properties

Soils of variable charge

Clay minerals: fixed charge (function of neutrality of crystal lattice)

Hydrous oxides: variable charge (function of pH)characteristic pHo = zero point of charge

When pH<pHo, surface is protonated with + charge; has anion exchange capacity

When pH>pHo, surface has - charge; has cation exchange capacity

Soil pH

Depends on the nature and history of the soil

Carbonate-rich soils tend to be alkaline; humic-rich soils tend to be acidic

CH217 20

Soil Properties

Physical Properties

Particle Size

Texture

Density

Structure

Permeability

Chemical Properties

Total elements

Available elements

Cation Exchange Capacity

Soils of Variable Charge

Soils pH

CH217 21

Soil is LayeredSoil is layered into sections called "horizons". Figure 1 shows atypical soil profile developed on granite bedrock in a temperateregion. The top horizon is composed of humus and contains mostof the organic matter. This layer is often the darkest. The "A"horizon consists of tiny particles of decayed leaves, twigs andanimal remains. The minerals in the A-horizon are mostly claysand other insoluble minerals. Minerals that dissolve in water arefound at greater depths. The "B" horizon has relatively littleorganic material, but contains the soluble materials that areleached downwards from above. The "C" horizon is slightlybroken-up bedrock, typically found 1-10 meters below thesurface. While this is a typical soil profile, many other types exist,depending on climate, local rock conditions and the community oforganisms living nearby. The U.S. Department of Agriculture hasclassified 10 orders and 47 suborders of soils. If you includeother subsets, there are over 60,000 types of soil.

http://www.globalchange.umich.edu/globalchange1/current/lectures/soils/soils.html

CH217 22

Soil ProfilesSoils consist of a number of horizons

The assemblage of horizons is called a “soil profile”

A-horizon

B-horizon

C-horizon

High OM content, insoluble minerals (quartz)soluble minerals absent

Relatively little OM, soluble minerals andoxides/hydroxides are present

Slightly altered bedrock, broken and decayed,mixed with clay

Bedrock

CH217 23

1. Tropical rainforest (equatorial regions including South America, Africa,Indonesia, southeast Asia): chemical weathering rates would be rapid as theseregions have both high temperatures and plenty of rainfall.2. Hot desert (subtropical regions including North Africa [Sahara], southwestSouth America [Atacama], southwest Africa [Namib], Asia [Gobi], southwesternU.S., central Australia): plenty of heat but insufficient water to cause significantphysical and/or chemical weathering.3. Temperate mountains (Rocky Mountains, Sierra Nevada Mountains, Alps,Andes Mountains): insufficient temperatures for rapid chemical weathering butelevations contribute to freeze-thaw cycles necessary for ice wedging.4. Polar Regions (Alaska, Antarctica, Siberia): too much cold weather to permitthawing. Water in solid form (ice) unable to react with rock.

http://lists.uakron.edu/geology/natscigeo/Lectures/weath/weath.htm#physical

CH217 24

Soil ProfilesRainfall and Temperature affect Soil Formation

higher T, faster reactions

more water, more alteration

Tropical soils: thick and devoid of unstable mineralsArid soils: thin and rich in unstable minerals

Pedalfers:High-rainfall soils

Rich in insolubles: quartz, clays, iron oxides/hydroxidesCalcite absent

High in Al and FePedocals:

Arid soils: soil water evaporates rather than infiltratesCalcite present

A and B horizons are leached

Laterites:Deep red soil of the tropics

Silicates completely altered to Fe and Al hydroxides

CH217 25

Soil Profiles

Aridosols: dry, high evaporation, may have a calcite/salt horizon

Entisols: new soils with no horizons

Inceptisols: young soils formed by parent alteration; no horizons

Histosols: organic soils; OM>~30% to 0.4m

Mollisols: nearly black surface horizons; high CEC

Alfisols: usually moist, subsurface clay accumulation; medium to high CEC

Oxisols: highly weathered; high in Al/Fe oxides/hydroxides

Spodosols: mineral soils; accumulations of OM and Al/Fe oxides/hydroxides

Ultisols: moist; clay accumulation with low base saturation (clays are protonated)

Vertisols: soils with high content of swelling clays

CH217 26

Spodosol (Podzol)Typical of humid, temperate regions usually under forest cover

Formed on relatively acidic parent material

O1O2E (“eluvial”, loamy sand, OM=5%, Fe+Al=0%, CEC=5cmol/kg)

Bhs1 (sandy loam, OM=4.5%, Fe+Al=3.5%, CEC=100cmol/kg)

Bhs2 (loamy sand, OM=2.2%, Fe+Al=4.8%, CEC=50cmol/kg)

B (sandy loam, OM=0.3%, Fe+Al=1.0%, CEC=30cmol/kg)

D (sandy loam, OM=0.1%, Fe+Al=0.2%, CEC=25 cmol/kg)

Also LFH (litter fermentation horizon)

~0.5 m

release of Horgand CO2

increased acidityFe+Al depleted

pH risesOM decreases

Fe+Al enriched

unaltered

less affected

(h=humic materials; s=sesquioxides)

CH217 27

AlfisolRed soils

Deep, clayey, moderately acidic

B21t (sandy clay, OM=0.46%, Fe=3.34%, CEC=8cmol/kg)

B22t (sandy clay, OM=0.5%, Fe=3.65%, CEC=8.7 cmol/kg)

B23t (clay, OM=0.49%, Fe=3.95%, CEC=9.1 cmol/kg)

B24t (clay, OM=0.36%, Fe=4.22%, CEC=9.4cmol/kg)

Ap (sandy loam, OM=0.5%, Fe=1.95%, CEC=2.9cmol/kg)

~1.5 m

pH~6.8low OM

Si depleted

First digit: 1=transitional A-B; 2=B; 3=transitional B-C

Second digit: morphological differences

t represents presence of transported material (kaolinite)

CH217 28

VertisolHot summers, mild-dry wintersDeep, clayey, moderately acidic

A12 (clay, OM=2.4%, Fe=4.2%, CEC=65cmol/kg)

A13 (clay, OM=2.2%, Fe=3.4%, CEC=68cmol/kg)

B (clay, OM=1.9%, Fe=3.2%, CEC=49cmol/kg)

C (sandy loam, OM=0.3%; Fe=3.9%, CEC=43cmol/kg)

A11 (clay, OM=3.2%, Fe=4.2%, CEC=66cmol/kg)

~1.5 m

heavy black soils

Distinction is made based on structure and root content

Dominant clay is montmorillonite: high CEC and shrink-swell

CH217 29

Environmental Properties of Soils

Soil erosionPhysical problems:

Too little or too much water

LeachingChemical problems:

AciditySaltTrace metals

CH217 30

Leaching

Chemical problems:

The amount of leaching is a function of rainfall,soil texture, plant cover, and CEC

NO3-

PO4-3

Nitrate flushes through system to groundwatercontributes to eutrophication of lakes

Phosphate is strongly and specificallybound to soil minerals: leaching is negligible

CH217 31

“Natural” pH:

Mineral composition is the principal determinant of in pH of soilsCarbonate soils: pH 7.5-8.0Soils with Al and Fe tend to be acidic

CO2 and organic acids reduce pH of soil solution

“Anthropogenic” pH:

Acid rainFertilizers

Neutralizing Acidity:

Geochemical reactionsBiological processes

Acidity

Chemical problems:

CH217 32

Geochemical reactions that neutralize acidity

Carbonate dissolution

CaCO3 + H2SO4 = 2Ca+2 + 2HCO3- + SO4

-2

Acidity reduced; Ca+2, bicarbonate, andsulfate in solution increase

Acid rain: input of sulfur dioxide (+ H2O = H2SO4)

Cation exchange (exchanging adsorbed K+, Na+, orCa+2 for H+ in solution)

Cations in solution increase

Base saturation of exchanging clays decreases

Adsorption of H+ on negatively charged surfaces of oxide-hydroxides

CH217 33

Biological reactions that neutralize acidity

Fixation of nitrogen

Nitrogen is fixed; nitrate in solution reduced

Bicarbonate in solution increases

Root: CO32- +2NO3

- + H3O+ = Root: (NO3)2 + HCO3- + H2O

Soils unlikely to neutralize acidity:

No carbonates

Small CEC values

Low content of Al or Fe oxides

No vegetation

CH217 34

The Chemistry of Solid Wastes

Mine tailings (rejected rock) are main solid waste

Mining and Metal Production

Benign tailings:basically host-rock and are not readily altered

Tailings from sulfide ore (and coals):internal generation of sulfuric acid (from pyrite)pH can reach 1.0 -- acid mine drainage

Red Mud (from alumina processing)

Bayer process: bauxite + sodium hydroxide = “red mud” (Fe+Al+Ti+Si and highly alkaline)