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Lecture 13
Soil Formation and Chemistry
Please read Manahan Chapter 14 AND 15 (for this week and next).
Today
1. Weathering – the context
2. Clay Minerals – the materials
3. Organic solids – the special sauce
4. Some soil examples
5. Inorganic reactions/transformation in soils
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Soils Intro…
Important substrate and growth medium
for terrestrial biosphere.
Susceptible to many anthropogenic
effects.
They contain many materials in a
gradient between
organic rich surface deposits
deeper inorganic deposits called saprolite.
Saprolite with original rock textures preservedhttp://www.nicholas.duke.edu/eos/geo41/
saprolite
organic
rich topsoil
http://www.teara.govt.nz/en/photograph/12319/organic-soil
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� On a gentle slope, rock is altered in place,
sometimes to form soil.
� On a steep slope, weathered solids are whisked
away by wind or water and deposited elsewhere, resulting in sediment accumulation elsewhere.
Soils Intro…
How do soils form? Initially, physical and chemical breakdown of surficial rocks (“weathering”) produces secondary materials.
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WeatheringThe breakdown of rock to form secondary deposits is controlled by
� Physical
� Chemical
� and biological processes
Chemical and biological weathering are almost always mediated by H2O.
During weathering new solid materials are formed and the composition of the
mediating H2O is modified.
The rates of alteration, and thus rates of soil (or sediment) accumulation and
maturation, are governed by climate:
� temperature,
� the availability of H2O
� biome factors (flora/fauna and the DOC they produce)
The formation of a soil is also dependent upon
� the bedrock type in the area
� physical factors (such as rock porosity and texture)
� mineralogic factors (solubility)
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Weathering Primary minerals can be weathered from the source rock intact (mineralogically) or
dissolved (recall congruent and incongruent dissolution)
Mineral dissolution susceptibility is related to stability at the P, T and pE conditions of
Earth’s surface.
The higher their T and P, or more reducing the pE of formation, the
more susceptible to weathering their minerals are.
Many crustal rocks were formed at elevated P and/or T, and lower pE, in the lower
crust or upper mantle. They were then "moved" to their present location at the surface
through the combined processes of tectonics and erosion.
The Bowen's reaction series (a
gross generalization of mineral
stability as a function of magma
temperature) can also be used to
understand weathering of many
silicate minerals, because high
temperature minerals are the first to
form from a crystallizing magma
and are more susceptible to
weathering.
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Sequence of events for
weathering common rock
forming minerals
The most soluble chemical
elements are transported in the
aqueous state to a new location
(eventually the sea)
The least soluble elements are
mostly left behind.
Elements of any solubility may be
dissolved during weathering,
redeposited by the aqueous
solution somewhere down its
flow path, and then re-dissolved
in a new, later episode of
weathering later.
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� Inorganic constituents: Minerals stable at high temperatures and pressures are broken down into hydrous sheet silicates (clays) and oxide minerals (Fe, Al and Mn oxides)
�Organic constituents: derived from flora, and soil microorganisms.
�Org.-Inorg. Proportion: Typical composition is 95% inorganic material and 5% organic matter -highly variable though.
Soil Composition Basics
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Three main forms:
�Very resistant Primary minerals
�Alteration minerals (incongruently formed clays/oxides)
�Precipitation minerals (mostly carbonates/hydroxides)
Inorganic Solids in Soils
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Alteration minerals:The structure and composition of
these solids is important because
they modify soil water and affect
the availability of nutrients to
plants.
The types of secondary minerals
formed from the weathering and
hydrolysis of common primary
minerals are given below.
The mineral names are not
important here, except to note that
� both clays and oxides can be
formed
� ion exchange with water is
involved
� CECs are variable.
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Simple oxide/ hydroxide examples are goethite and gibbsite.
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Clay minerals:
Clays are structurally more complex.
They are composed of layered matrices of Si, Al and Mg
bonded to O.
Of the 3 basic clay types (platy, fibrous and
amorphous), the most important in soils are the
��platy "phylosilicate" clays��
The layers are of two types:
“tetrahedral” and “octahedral”
These occur in clay minerals in “2 layer” and “3 layer”
varieties:
2 Layer Clays - “T-O”Repeated units of 1 tetrahedral and 1 octahedral layer
3 Layer = “T-O-T)Repeated units of 2 tetrahedral and 1 octahedral layers
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tetrahedral (Si surrounded by tetrahedrally-
arranged O)
SiO4 tetrahedra share 3 basal oxygens
with neighboring tetrahedra, forming a
sheet structure. The Si:O ratio = 1 to 1
lone O + 3 50% shared oxygens =
1: (1 + 3 x 0.5) = 1:2.5 = 2:5
octahedral (Al or Mg surrounded by
octahedrally-arranged O as hydroxyl groups).
Octahedral Mg clays are commonly formed
only during alteration of magnesian rocks.
Octahedral layers of pure Al and Mg occur in
the minerals gibbsite, Al(OH)3, and brucite,
Mg(OH)2.
each Al(OH)6 (or Mg(HO)6) octahedron
shares all of its oxygens with neighboring
octahedra. Al:O ratio of 3, as in gibbsite.
T and O layers combine by sharing the non-
basal O of the silica tetrahedra with the one of
the octahedral O atoms on each Al (or Mg).
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T-O clays:
we can think of each Al as having effectively lost one O atom to a
Si, and Al:O goes from 1:3 to 1:2 (octahedral O atoms are
actually in hydroxide form).
Kaolinite, the simplest T-O clay, has Si:O of 2:5,
Al:Si of 1:1 (or 2:2) and Al:OH of 1:2 (or 2:4).
This gives the formula Al2Si2O5(OH)4.
T-O-T clays:
Similar arguments can be made to show that T-O-T clays have
Si:Al of 2:1
The simplest chemical formula is Al2Si4O10(OH)2 (pyrophyllite).
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Solute-Solids interactionsStill and through flowing water interacts with solids to exchange compositional attributes:
3 mechanisms of compositional “exchange” with water operate,
as discussed last week.
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Ion Substitution:
Ion substitution for Si, Al and Mg gives clays exchangeable ion
sites that can exert a compositional control on aqueous solutions
contacting them. The degree of substitution depends on the
environment of their formation.
• Octahedral replacement is by ions such as Fe+3, Fe+2, Cr+3,
Zn+2, Li+.
• Tetrahedral Si replacement is less common and mostly limited
to Al-for-Si substitution.
Structural substitutions result in a charge imbalance on the clay
backbone that is balanced by addition of interlayer (non-
structural) ions and accounts for the CEC of clays (as discussed
last week).
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Charge on clay particles:In addition to cation exchange, clays and oxides take charges in
natural waters (discussed earlier this semester).
The sign of the charge
is a function of pH and
the density of charge is
a function of the
structure
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Al (OH)3
Na, Ca containing clay Clay mineral depleted
in Alkalis & Alkali
Earths: Al2Si2O5(OH)4
Increased water flow during weathering leads to increased leaching of cations…
which lowers CEC and charge on clays.
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The gain or loss of chemical constituents in saprolite records the progress of weathering/ soil formation …in the absence of significant DOC.
In practice, Al is the least soluble element during weathering followed by Ti and Fe.
Please note that % metal oxide
is a way of expressing bulk
composition of a rock. Many of
these oxides are not actually
present in the rock as oxides.
SOIL INORGANIC SOLIDS – saprolite development
mineralogical
changes that
occur during
weathering
elemental
changes that
occur during
weathering
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Elements removed during saprolite formation have high concentration in soil
and ground waters.
Si is removed slower than Ca and Na. Lower but still significant Si
concentrations remain at high % Al.
Fe and Ti continually increase with Al, suggesting that a totally weathered rock
would be mostly Al, Fe, Ti and Si (-the Si curve eventually flattens out as some
of the Al is found in Kaolinite, Al2Si2O5(OH)4).
Remember that Fe2+ is soluble and Fe3+ is not. The typical accumulation of
Fe in saprolites indicates that this process takes place at fairly high pe.
Most species decrease as % Al2O3 increases.
EXCEPTIONS: Ti and in some cases Fe.
The faster the rate of decrease, the more
mobile the element is.
Note that Ca and Na are removed very quickly
(at relatively low Al2O3) and then K and Mg are
removed.
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SOIL WATER
Depth profiles of elemental concentrations in soil water, provide
insight into geochemical processes during soil formation/
weathering and in bio-availability of some important nutrient
elements.
It is important to examine the total amount of ion present and the
relative proportions of “free” versus DOC-complexed ions.
� inorganic solubility during saprolite formation
� solubility in the presence of DOC/POC higher up in the
soil column.
The common rock-forming chemical elements are found in soil
waters as a function of:
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Al: Weathering-resistant. Conc. increases with depth. No significant DOC complexation.
Ca: Soluble element. Conc. decreases with depth due to CaCO3
precipation at higher pH. No significant DOC complexation
Mg: Relatively constant, somewhat more DOC-complexed at depth. Mg enters soil waters fairly easily; there are few reactions for its removal (e.g., incorporation in CaCO3), so it only diminishes slightly with depth.
Zn: Analogous to Ca. At high pe ZnCO3 (pH 8-9) or Zn(OH)2 (pH 9-12) forms. No significant DOC complexation.
Fe: Similar to Al except its peak concentration is somewhat higher up in the profile because of significant DOC complexation above the saprolites.
Cr: Generally soluble but even more so in the presence of DOC. It's profile looks similar to Ca except that in the upper layers, it is almost all DOC-complexed.
Cu: Moderately soluble but more so in the presence of DOC. Similar to Cr but found only in A zone.
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Organic Solids in Soils.
Organic solids typically make up <5% of a soil
yet they largely determine the soil’s
productivity.
Organic matter:
�sets the availability of nutrients
�supports soil biota
�binds some organic contaminants (i.e.,
pesticides)
�helps determine soil pH (through DOC)
�mediates mineral dissolution (through DOC)
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An example of the effect of soil DOC on silica dissolution rate,
and a possible mechanism, are given below.
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Table 16.1. Major Classes of Organic Compounds in Soil from Manahan Ch16
Compound Type
Composition Significance
Humus Degredation-resistent residue from
plant decay, largely C, H, and O
Most abundant organic component, improves soil
physical properties, exchanges nutrients, reservoir of
fixed N
Fats, resins, and
waxes
Lipids extractable by organic
solvents
Generally, only several percent of soil organic matter may
adversely affect soil properties by repelling water,
perhaps phytotoxic
Saccharides Cellulose, starches, hemi-
cellulose, gums
Major food source for soil microorganisms, help to
stabilize soil aggregates
N-containing
organics
Nitrogen bound to humus, amino
acids, amino sugars, other
compounds
Provide nitrogen for soil fertility
Phosphorous
compounds
Phosphate esters, inositol
phosphates (phytic acids),
phospholipids
Sources of plant phosphate
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A Soil Primer:
Soils are the combined products of rock
breakdown and biological processes.
Soils are basically a stratified gradient
between mostly organic, biological and
resistive inorganic materials on the top
and rock weathering products below.
Ground water flow through soils is
mostly vertical (top down), leading
to distinctive layering.
Soil horizons generally build from the
bottom up; the further down one goes
toward bedrock, the more similar the
material gets to bedrock composition.
Notice the relationship between the
zones and tree roots.
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Soil zone nomenclature derives from physical and chemical
properties that occur more or less in stratified horizons in the
soil column:
� The A-zone is the least like the rock from which it was
originally produced.
� The C-zone is the most like the rock from which it was
originally produced.
� The B-zone is intermediate. It contains solid residues of
sparingly soluble materials mobilized and redeposited
from the A-zone.
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y Organic matter and porosity generally decrease with depth in a soil.
y Mineral grains in the very upper reaches of a soil are very resistive to weathering.
y Saprolite occurs at the base of the soil zone, so far removed from the organic zones of
the soil that DOC plays little role in its formation.
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Many soils, such as this one,
show a classic “topsoil”
horizon but this is not always
the case.
“O” Horizon - decomposing
organic matter
“A1” Horizon - brown humic-rich,
some mineral matter.
“A2” Horizon - light grey,
intensely leached; including loss
of Fe & Al; mostly residual SiO2.
“B” horizon -brown horizon,
accumulation of clays & Fe-
oxides
Soil images from: http://soils.usda.gov/
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Soils of tropical and
subtropical regions tend to
be deeply weathered.
They are mixtures of
quartz, kaolin, free oxides,
and some organic matter.
For the most part they lack
well defined soil horizons.
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In humid temperature
regions relatively
organic-rich
and
clay-rich zones commonly
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Organic matter dominated
soils tend to form in wet
“boggy” areas.
Wet conditions favor plant
growth and thus greater
organic matter production.
Water logged soils quickly
become very reducing.
Why?
Cool to temperate
conditions and reducing
conditions both slow
hetereotrophic organic
matter degradation.
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Very Organic or “peat” soils
(>25%) are wet throughout
the year. Plant debris
decomposes slowly and
thus builds up.
In this profile there is a 1m
thick layer of organic matter
over the B-zone.
Cultivation of these soils
often require draining first to
lower the water table.
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Soils from very arid
environments support
limited plant growth.
Precipitation of minerals
from simple salts are
characteristic:
calcium carbonate,
gypsum.
These soils tend to have
low organic content.
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Caliche a layer.
This common at
shallow levels in
soils from arid
regions. It is
common in
leeward Hawaii
locales.
Caliche (CaCO3)is a precipitate mineral that forms near the base of the B-zone
of many soils.
Ca2+ and CO32- dissolved from the A and B zones precipitate at
deeper levels as soil water reaches solution saturation, largely
controlled by changes in pH
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The caliche layer is essentially a CaCO3 "solubility front“.
The amount of caliche formed depends on how much Ca there is initially in the bedrock and on pH.
The depth to the caliche layer deepens with increasing surface rainfall. More water pushes CaCO3 precipitation to lower in the soil column
Rainfall also correlates with DOC/POC, so the enhanced CaCO3 solubility in part reflects changes to pH with depth in the soil.
Caliche (CaCO3)
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The Processes of Soil Formation are (Wild, 1993):
Always occur
1. Weathering of parent material
2. Addition and partial decomposition of organic matter
3. Formation of structural units
Depend on Environmental Conditions
4. Leaching and acidification
5. clay eluviation (washing of clay from upper horizons; deposition below)
6. Podzolization (transport of DOC complexed Fe and Al from upper
horizons; deposition below in sharp horizons)
7. Desilication (Leaching of Si relative to Fe and Al)
8. Reduction (i.e., Fe3+ Fe2+)
9. Salinization (accumulation of sulphate and chloride salts) and
Alkaization (accumulation of Na on cation exchange sites)
10. Erosion and deposition of eroded soil.
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Soil Evolution Reflects Time and Climate Factors: download larger versions from course webpage
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Soils can also be classified based upon the size fraction of mineral grains they contain.
Grainsize controls
porosity, drainage
(permeability), wetting
and rooting
characteristics.
