Lecture 13 Soil Formation and Chemistry - ?· Lecture 13 Soil Formation and Chemistry Please read Manahan…

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    GG425/625 Wk 7 L13, S2018

    Lecture 13

    Soil Formation and Chemistry

    Please read Manahan Chapter 14 AND 15 (for this week and next).


    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


    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/



    rich topsoil


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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.

    GG425/625 Wk 7 L13, S2018

    WeatheringThe breakdown of rock to form secondary deposits is controlled by



    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:


    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

    Earths 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


    GG425/625 Wk 7 L13, S2018

    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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    GG425/625 Wk 7 L13, S2018

    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

    GG425/625 Wk 7 L13, S2018

    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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    GG425/625 Wk 7 L13, S2018

    Alteration minerals:The structure and composition of

    these solids is important because

    they modify soil water and affect

    the availability of nutrients to


    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


    ion exchange with water is


    CECs are variable.

    GG425/625 Wk 7 L13, S2018

    Simple oxide/ hydroxide examples are goethite and gibbsite.

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    GG425/625 Wk 7 L13, S2018

    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


    2 Layer Clays - T-ORepeated units of 1 tetrahedral and 1 octahedral layer

    3 Layer = T-O-T)Repeated units of 2 tetrahedral and 1 octahedral layers

    GG425/625 Wk 7 L13, S2018

    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,


    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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    GG425/625 Wk 7 L13, S2018

    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).

    GG425/625 Wk 7 L13, S2018

    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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    GG425/625 Wk 7 L13, S2018

    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).

    GG425/625 Wk 7 L13, S2018

    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


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    GG425/625 Wk 7 L13, S2018

    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.

    GG425/625 Wk 7 L13, S2018

    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


    changes that

    occur during



    changes that

    occur during


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    GG425/625 Wk 7 L13, S2018

    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


    GG425/625 Wk 7 L13, S2018


    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


    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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    GG425/625 Wk 7 L13, S2018

    Al: Weathering-resistant. Conc. increases with depth. No significant DOC complexation.

    Ca: Soluble element. Conc. decreases with depth due to CaCO3precipation 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.

    GG425/625 Wk 7 L13, S2018

    Organic Solids in Soils.

    Organic solids typically make up

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    GG425/625 Wk 7 L13, S2018

    An example of the effect of soil DOC on silica dissolution rate,

    and a possible mechanism, are given below.

    GG425/625 Wk 7 L13, S2018

    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


    Lipids extractable by organic


    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



    Nitrogen bound to humus, amino

    acids, amino sugars, other


    Provide nitrogen for soil fertility



    Phosphate esters, inositol

    phosphates (phytic acids),


    Sources of plant phosphate

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    GG425/625 Wk 7 L13, S2018

    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.

    GG425/625 Wk 7 L13, S2018

    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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    GG425/625 Wk 7 L13, S2018

    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.

    GG425/625 Wk 7 L13, S2018

    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-


    Soil images from: http://soils.usda.gov/

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    GG425/625 Wk 7 L13, S2018

    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.

    GG425/625 Wk 7 L13, S2018

    In humid temperature

    regions relatively



    clay-rich zones commonly

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    GG425/625 Wk 7 L13, S2018

    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.


    Cool to temperate

    conditions and reducing

    conditions both slow

    hetereotrophic organic

    matter degradation.

    GG425/625 Wk 7 L13, S2018

    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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    GG425/625 Wk 7 L13, S2018

    Soils from very arid

    environments support

    limited plant growth.

    Precipitation of minerals

    from simple salts are


    calcium carbonate,


    These soils tend to have

    low organic content.

    GG425/625 Wk 7 L13, S2018

    Caliche a layer.

    This common at

    shallow levels in

    soils from arid

    regions. It is

    common in

    leeward Hawaii


    Caliche (CaCO3)is a precipitate mineral that forms near the base of the B-zone

    of many soils.

    Ca2+ and...


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