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Soils
quiz 3
36
Agriculture
Undergraduate 3
10/27/2011
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Term
General Properties
 Definition
 Size - < 2 m
 Clay fraction
 Surface area - area per unit mass
 High surface area
○ 1 g clay 1000 X greater than 1 g sand
○ Expressed as m2 g-1
 Internal vs. external
Term
General Properties
 Definition
 Primarily composed of Al, Si, Fe, and Mg,
and O
 5 out of the 8 most common elements in the
earth’s crust
 Cations coordinated to an anion (O)
 Coordination will depend on the size of the
cation
Term
Types of soil colloids
 Definition
 Layer silicate clays
 1:1, 2:1, 2:1:1
 Fe and Al oxides
 Fe-oxides: red colors in many SW VA soils
 Allophane and associated materials
 Short-range order or amorphous
 Humus
 Dark brown to black colors
Term
Layer silicate clays
 Definition
 Phyllosilicates - Greek meaning leaf
 Dominant inorganic colloid in most soils
 Dominant cations Si, Al, Mg, Fe
coordinated to oxygen
 Generate negative charge -
 Isomorphic substitution gives rise to the
cation exchange capacity (CEC) of soils
Term
Layer silicate clays
 Definition
 Basic building blocks
 Silica and Al tetrahedron (Si4+/Al3+
coordinated to 4 O )
 Al and Mg octahedron (Mg2+ or Al3+
coordinated to 6 O )
 These tetrahedron can polymerize into
tetrahedral and octahedral sheets
 Arrangement and composition of these
tetrahedral and octahedral sheets give rise
to different layer silicates
Term
Layer silicate clays
 Definition
 Tetrahedral and octahedral layers are held
together by shared oxygen atoms
 1:1 clay minerals
 1 tetrahedral and 1 octahedral layer
 2:1 clay minerals
 2 tetrahedral and 1 octahedral layer
 2:1:1
 2 tetrahedral and “2” octahedral layers
Term
Layer silicate clays
 Definition
 Isomorphic substitution - substitution of
one element for another of equal size
 Occurs during mineral formation
 Occurs in both tetrahedral/octahedral sheets
 Al3+ subst for Si4+ in tetrahedral layer
 Fe2+, Mg2+, Ni2+, Zn2+, Cu2+ for Al3+ in
octahedral layer
 KEY -- element of lower positive charge
substituting for one of higher charge
Term
Layer silicate clays
 Definition
 1:1 minerals
 Tetrahedral and octahedral sheets held
together by shared oxygen atoms
 Layers or platelets bound to adjacent layers
via hydrogen bonding
○ No internal surface area
○ No shrink swell
 Small amount of isomorphic substitution
○ Low CEC
Term
Layer silicate clays
 Definition
 2:1 clay minerals
 Octahedral sheet sandwiched between 2
tetrahedral sheets
 Sheets held together by shared oxygens
 Plates held together weakly
○ Expanding vs. semi-expanding
 Both internal and external surface area
 High amount of isomorphic substitution
○ High CEC
Term
Layer silicate clays
 Definition
 Expanding 2:1 minerals (e.g. smectites)
 Caused by swelling as water enters the
interlayer space and forces plates apart
○ Problem from a construction standpoint
 High specific surface area due to internal
surface
 Isomorphic substitution primarily in
octahedral sheet (Mg2+ for Al3+)
Term
Layer silicate clays
 Definition
 Semi-expanding 2:1 minerals (e.g.
vermiculite)
 Internal surface area containing cations and
water
 Platelets held together more strongly than
smectites hence only semi-expansible or
limited expansion
 Lower specific surface area compared to
smectites
 IS primarily Al3+ for Si4+ in tetrahedral layers
Term
Shrink/Swell Soils
 Definition
 Clay fraction dominated by smectites
 Used in landfill liner construction
 Used to seal groundwater monitoring wells
 Detrimental from a construction standpoint
○ Homes
○ Highways
Term
Layer silicate clays
 Definition
 2:1 nonexpanding minerals (mica)
 Large amount of Al3+ for Si4+ isomorphic
substitution in tetrahedral layer
 Excess negative charge satisfied by K+
cations in the interlayer
 K+ right size to fit in hexagonal hole created
by Si-tetrahedra
 K+ acts as a binding agent preventing
expansion of interlayers
Term
Layer silicate clays
 Definition
 2:1 with hydroxide sheet or 2:1:1 or 2:2
minerals (chlorite)
 Basic 2:1 structure and isomorphic
substitution as mica (high net negative
charge)
 Negative charge compensated by a
positively charged hydroxide sheet in the
interlayer
 Nonexpanding and no internal surface area
Term
Fe and Al oxides
 Definition
 Fe oxides (goethite and hemitite)
 Al oxides (gibbsite)
 Weather from Fe and Al bearing primary
minerals
 Very reactive surfaces
 Common in SE soils
Term
What the heck is “red mud’
 Definition
 When bauxite is mined (primarily Al(OH)3, it
is essentially highly weathered Oxisol
subhorizon (Bo and Cv) material that also
contains significant Fe-oxides like Fe2O3
(hematite).
 The Fe-oxide is removed via washing the
bauxite with NaOH which drives the pH
above 10 and solubilizes the Fe-oxides and
and any other entrained or sorbed metals.
Term
What the heck is “red mud’
 Definition
 So, basically this is really just high pH and Na rich
Fe-oxides and is not particularly toxic (other than
high pH/caustic) unless the bauxite was also high
in Cd or some other heavy metal.
 In fact, a commercial product called Virosoil
is made out of seawater neutralized (and
rinsed) red mud and has been used widely to
renovate soils and remediate heavy metal
contaminated sites due to the strong abilities
of Fe-oxides to specifically adsorb Pb, Cu, Ni
and other nasties from soils!
Term
Allophane and Imogolite
 Definition
 Commonly associated with volcanic
material
 Poorly crystalline
 Very reactive
 Small quantities present in SE soils.
Abundant in soils weathered from
volcanic ash
Term
Humus
 Definition
 Breakdown and alteration of plant residues
 Dark in color
 Contain common organic functional groups
 Carboxyl (COOH)
 Phenolic
 Deprotonation of these functional groups
gives rise to pH-dependent CEC
 Important for good soil structure
Term
Introduction
 Definition
 pH and Eh (redox potential) two master
variables in soils.
 pH controls chemical, physical and
biological properties of soil.
 Nutrient availability
 Contaminant fate and mobility
 Aggregate stability and water and air
movement
○ Influence microorganisms
Term
Basic Concepts
 Definition
 Pure water ionizes slightly (autoprotolysis)
 H2O = H+ + OH-
 Equilibrium constant Kw = [H+][OH-] = 10-14
 Pure water has pH of 7
 Above equation can solve for proton or
hydroxyl concentration given the pH, [H+] X
[OH-] must always equal 10-14 (see Box 9.1 in
Brady)
Term
Basic Concepts
 Definition
 pH = -log [H+]
 Logarithm base 10
 pH 3 vs. 4
 0.001 M vs. 0.0001 M
 10 fold increase/decrease in concentration
of proton and hydroxyl
 Base vs. acid cations (Al3+, H+ vs. Ca2+,
Mg2+, K+, and Na+)
Term
Basic Concepts
 Definition
 Range in soil pH values
 Sodic Soils (pH 8.5-11)
 Calcareous soils (pH 7-8.2)
 Humid region soils (pH 5.0 -5.5)
 Forest Soils (pH 3.5-5.5)
 Acid sulfate soils (pH 2-3.8)
Term
Acid Generating Cations
 Definition
 Cations responsible for soils acidity are
H+ and Al3+ (acid cations)
 Al3+ readily hydrolyzes as pH of soil
solution is raised (Al is the major
culprit)
Al3+ + H2O = Al(OH)2+ + H+
Al(OH)2+ + H2O = Al(OH)2
+ + H+
Al(OH)2
+ + H2O = Al(OH)3
0 + H+
Term
Sources of H+ and OH-
 Definition
 Organic acids and functional groups that
release acidity by dissociation
 R----H = R- + H+
 Al3+ - organic complexes that release
acidity by hydrolysis
 R----Al3+----R = R----AlOH2+----R + H+
Term
pH and the Nature of the Exchange
Complex
 Definition
 Balance between prod/consumption of H+
 Low pH (3 - 4.5)
○ Al3+ and H+
 Moderately acid (4.5 - 6. 5)
○ Al(OH)x and base cations
 Neutral to alkaline pH (6.5 - 8.0)
○ Base cations
Term
Organic vs. mineral Colloid
 Definition
 pH-dependent vs. permanent charge
 OM binds Al3+ tightly rendering it nonexchangeable
 Potential implications?
Term
Classification of Acidity
 Definition
 Active Acidity - all titratable acidity in soil
associate with the soil solution. Usually
very small and best calculated by pH
 Exchangeable Acidity - bound Al3+ and
H+ displaced into soln by conc. neutral
salt (1 M KCl)
 organically complexed, exchange sites
 exchangeable H+ in OM, Al3+ in mineral
Term
Classification of Acidity
 Definition
 Non-exchangeable acidity or residual -
bound Al/H not displaced or very slowly
displaced
 strongly complexed by OM
 Al-hydroxy cations (Al(OH)2+ or Al(OH)2
+)
 Reserve acidity - all titratable acidity
associated with solid phase (NEA + EA).
Represents buffer capacity of soil
Term
Colloid Control of Soil Reaction
 Definition
 Active acidity only small part of soils
total acidity
 Nature of exchange complex dominates
soil pH
 % base saturation
 Nature of the solids
 Kinds of base forming cations
Term
Buffering of a Soil
 Definition
 Soils tend to resist change in solution pH
 Equilibrium between various forms of acidity
Term
Buffering of a Soil
 Definition
 Clay soils with exchangeable and residual
acidity associated much more buffered
than sandy soils
 Lime added to soil active acidity is
neutralized
 Some exchangeable acidity will also be
neutralized
Term
Buffering of a Soil
 Definition
 Soils low base saturation resist changes
in pH due to high quantity exchangeable
Al
 Similar buffer reactions involve
carbonates, bicarbonates, and carbonic
acid (occur higher pH values)
 All things being equal higher CEC
greater buffer capacity
Term
Variability in soil pH
 Definition
 Natural changes
 Weathering primary minerals consumes
protons and releases base forming cations
(Mg2+ and Ca2+)
 Microbial activity produces carbonic acid and
organic acids as they degrade organic matter
 Redox
○ Soil becomes flooded and reduction reactions
consume protons conversely oxidation reactions
release protons
Term
Anthropogenic Changes
 Definition
 Chemical fertilizers
 (NH)4SO4 + 4O2 ---> 2HNO3 + H2SO4 + 2H2O
 Tillage practices (OM oxidation)
 Acid Deposition (dry and wet)
 SO2 + H2O --> H2SO4
 NO2 + H2O --> HNO3
 Irrigation practices (sodic soils)
 Pyrite oxidation (Coal and marshes)
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