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Tonight. Module 1 exam Mineralogy Soil colloids. Sand. May be rounded or irregular Not sticky, even when wet Water holding capacity is low Quite inactive chemically. Silt. Irregularly fragmental Diverse in shape Seldom smooth or flat Really is microsand Has an adhering film of clay.

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Tonight l.jpg
Tonight

  • Module 1 exam

  • Mineralogy

  • Soil colloids


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Sand

  • May be rounded or irregular

  • Not sticky, even when wet

  • Water holding capacity is low

  • Quite inactive chemically


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Silt

  • Irregularly fragmental

  • Diverse in shape

  • Seldom smooth or flat

  • Really is microsand

  • Has an adhering film of clay


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Physical properties of silt

  • By itself, acts like sand

  • Quite inactive chemically

  • Coated with clay:

    • some plasticity

    • some cohesion

    • some adsorption


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Soil Colloids

  • Phyllosilicate clays

  • Hydrous oxides of Al and Fe

  • Amorphous minerals

  • Organic soil colloids


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Importance of Soil Colloids

  • Shrinking

  • Swelling

  • Plasticity (Stickiness)

  • Cohesion

  • Dispersion

  • Flocculation


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Properties of Soil Colloids

  • Small size

  • High specific surface area

  • Surface charge


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What is clay?

  • Size of <2 m

  • Layered structure, thin plates

  • Tetrahedral and octahedral layers

  • Covalent bonding


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Clay

  • High surface area

  • High adsorption power

  • Highly plastic when wet

  • High heat of wetting

  • Chemically active


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Permanent charge

  • Due to charge imbalance in the crystal structure

  • Charge imbalance due to isomorphous substitution

  • Charge due to broken edges of crystals


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Cation exchange capacity

  • A cation has a positive charge

  • Once cation can be exchanged for another

  • CEC = cation exchange capacity

  • CEC is pH dependant


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Cation exchange

  • Cations attracted to the negatively charged clay

  • Cations concentrated next to the clay surface

  • Less concentration of cations as move away from the clay surface


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Adsorbed cations

  • H+, Al3+ and Ca2+

  • Mg2+

  • K+ and Na+


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Lyotrophic Series

  • How close a cation can get to the surface of a clay depends on the hydrated size and charge of the cation

  • Al3+>Ca2+>Mg2+>NH4+>K+>Na+


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1:1 and 2:1 layers

  • 1:1 One sheet of tetrahedral and one of octahedral

  • 1:1 (open faced sandwich)

  • 2:1 two tetrahedral sheets (bread) with an octahedral sheet in the middle (filling)

  • (closed sandwich)


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Swelling clay

  • Swelling in the interlayer space is due to water.

  • Not all clays swell

  • Some have their layers bonded too tightly to swell


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Surface area of clay

  • Clays have a high surface area

  • The area depends on their structure

  • 2:1 clays have more surface area than 1:1 clays

  • More area = more opportunity for reactions


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Effect of surface area

  • Adsorbing power

  • Swelling

  • Plasticity and cohesion

  • Heat of wetting

    Brady 1974

SAND

SILT

CLAY

COLLOIDAL CLAY


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Iron and aluminum oxides

  • pH dependant charge

  • Al or Fe bonded to OH2+ at low pH

  • Al or Fe bonded to OH at neutral pH

  • Al or Fe bonded to O- at high pH


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Organic matter

  • Starts off as litter

  • Decomposed to humus

  • Humus has colloidal properties


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Organic molecules

  • Small permanent negative charge

  • large pH dependant charge

  • R - COOH at low pH

  • R - COO- at high pH


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Charge on clay and OM

  • Varies with the type of clay

  • Varies with the amount of clay

  • Varies with the amount of OM

  • Varies with the state of decomposition of the OM


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Fixed and exchangeable ions

  • Some ions are part of the crystal structure and cannot be exchanged after formation

  • Some cations are absorbed onto the crystal structure and can be exchanged with ions in the soil solution


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CEC

  • Displace all the cations from the soil

  • Replace them with something else

  • Control the pH, make it neutral

  • express CEC as cmol(+)/kg soil

  • Note: old unit was milliequivalents/100g


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CEC example

  • CEC = sum of all the cations

  • Add up all the ions with a positive charge

  • = CEC = 4.96 me/100g

  • %BS = 100 x sum of all the basic cations

  • sum of all the cations


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Basic and Acidic cations

  • Basic cations are Ca2+, Mg2+, K+, Na+

  • Acidic cations are H+, Al3+, Fe3+

  • Easier to remember if it’s not H+, Al3+ or Fe3+

    it’s BASIC


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Base Saturation

  • Base saturation = BS

  • = all the basic cations/CEC

  • %BS = BS X 100

  • “good cations”/”all the cations”


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Base Saturation Calculation

  • Data set: me/100g

  • Ca2+ = 2.57

  • Na+ = 0.03

  • K+ = 0.14

  • Mg2+ = 1.11

  • H+/Al3+/Fe3+ = 1.11

  • pH = 7.0


Base saturation example l.jpg
%Base Saturation example

%BS = 100 x sum of all the basic cations

sum of all the cations

%BS = 100 x 4.02/4.96 = 81.05%


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Next week

  • Soil reaction

  • Read chapter 7

  • Soil Water

  • Read chapter 8


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