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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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/
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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
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Physical PropertiesTexture: based on triangular diagrams
0
100
0
0
100100
percent clay percent silt
percent sand
clay
silt
sandloam
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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%
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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)
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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)
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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
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Environmental Properties of Soils
Soil erosionPhysical problems:
Too little or too much water
LeachingChemical problems:
AciditySaltTrace metals
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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
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“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:
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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
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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
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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)