ssc 100 week 3

soil colliods

small and highly reactive soil particles with electrically charged surfaces (negative charge, but some positive) bc of small shape and size, they give soil large surface area

crystalline soil colloids

Layer silicate clays: phyllosilicates , Fe, Al, Mn Oxides: Hematite, Gibbsite, Goethite

non crystalline soil colloids

Short-range ordered minerals:

Allophane, imogolite

organic colloids formed through

decomposition of organic matter

mineral colloids are formed through

the process of weathering primary materials

hydrolysis

feldspar + carbonic acis + H20 = kalonitie (clay) + dissolved K+ (potassium) ion

+ dissolved bicarbonate ion

+ dissolved silica

k ends up oxidized

1. Iron pyroxene dissolves to release silica and ferrous iron to solution 2. Ferrous iron is oxidized by oxygen molecules to form ferric iron 3. Ferric iron combines with water to precipitate a solid, iron oxide, from solution

property of soil colloids

size, 2. surface area, 3. surface area charges (Adsorption of

water and Adsorption of cations and anions)

colloids size

Very small (less than 1 micron, μm or 0.001mm)*, highly reactive particles

colloids surface area

Small-sized particles result in a larger specific surface area in the soil, also have internal surface

colloids surface charges

External and internal surfaces are charged, negative charges, but positive charges predominate in some cases (very acid soils). Charges result in adsorption or adhesion of molecules to the colloid surfaces

colloid surface charge for water molecules

Water molecules are polar. Colloids attract water molecules. Water forms films around the colloids and it is not available for plants (hygroscopic water

Capillary Movement of Water in Soil

attraction of water in soil particles surronding the pores (adhesion) and surface tension of water (cohesion). smaller pores like in clay soils have more capillary movement

Colloids are responsible for the

retention of ions in the soil, charges attract or repluse other charged eleements in the soil

cations are not strongly absorbed to colloid surface

Continuous exchange of cations from the colloid surface and the soil solution, important exchangeable cations in soil are H+, Al3+, K+, Na+, Ca2+, Mg2+

Factors Determining Binding Force of Cations

1. Cation charge (valence) 2. Size of cation (size of the hydrated radius) 3. Concentration of charge

cation charge

strength of a cation to exchange surface (energy absoprtion) as a function of valence (charge), trivalent cations Al³+ and divalent cations Ca2+, Mg2+are more strongly attracted than monovalent cations (K+, Na+)

Size of the hydrated radius

Charges attract water molecules forming hydrated ions (cations and anions) and hydrated ions are larger

When the valences of the ions are equal the ion with the smallest hydrated radius is most strongly adsorbed

K+ more strongly adsorbed than Na+ because it has a smaller hydrated radius (0.53 nm) than Na+ (0.79 nm), Ca2+ more strongly adsorbed than Mg2+ because it has a smaller hydrated radius (0.96 nm) than Mg2+ (1.08 nm)

Lyotropic Series

order of strength of adsorption for common cations) Higher charge, stronger attraction Larger hydrated radius, weaker attraction Easier to replace (weaker adsorption) to the right

Concentration of charge

if the concentration of a cation in soil solution is high theres an increased tendency for that cation to be absorbed

importance of cation exchange

resevior of available plants nutrients. partocularly ca, mg, k

cation exchange leads to

reduction in nutrient losses from soil

Cation Exchange Capacity (CEC)

quantifies the capacity of colloids to adsorb cations. Expressed as the number of centimoles of (positive) charge that is adsorbed in the colloid per unit mass

What is a cmolc?

The units of CEC are expressed as cmolc/kg: the number of

cmol of cation charges per kg (cmolc/kg), or meq charge

per 100 g (meq/100 g).

cmol conversion

1 mol (mole)= 6.02 x 1023 atoms = 1 molC= 6.02 x 1023 charges = 1 cmolc= 6.02 x 1021 charges

Soil Cation Exchange Capacity

Normal range in soils: 0-180 cmolc/ kg Depends on the total amount of negative sites or negative charges: the relative abundance of different

colloids, the CEC of those colloids, and Soil pH

What is the CEC (cmolc/kg) of a soil containing 5% humus, 5%

illite, 10% montmorillonite, 15% vermiculite, and 5% kaolinite?

Humus : 0.05 x 200 cmolc/kg = 10 cmolc/kg,

Principles governing CEC reactions

Reversibility (cations are not permanently bound), Concentration of charge (Ratio law): chemical equilibrium with the soil solution, cation selectivity , Principle of equivalence

Principle of equivalence

Exchange is chemically equivalent, it takes place on a charge-for-charge basis

the charge of a cation is expressed in

cmol and depends on the valence; 1 cmol of K+= 1 cmolc, 1 cmol of Ca2+ = 2 cmolc, 1 cmol of Al3+= 3 cmolc

How many cmol of Ca2+ do we need to neutralize all

the charge in this exchange complex?

• 1 cmol Ca2+: 2 cmolc

• 5

There are 3 different types of parent materials

igneous, metamorphic, sedimentary rocks

main agents of physical weathering

temperature, gravity, abrasion, plants and animals

temperature on physical weathering

differential thermal expansion/contraction of minerals, freeze-thaw cycles and ice

abrasion effects on mechanical weathering

sediment trapped in water, ice or wind can grind down rocks

plants and animals effect on physical weathering

minimal effect- roots and burrowing

chemical weathering

Chemical transformation of the soil minerals through the action of

geological agents and organisms

geological agents on chemical weathering

oxygen, water, geological weathering

organisms effect on chemical weathering

microorganisms, plant roots, biogeochemical weathering

soil minerals according to degree of weathering

weathering is essential in forming soils and releasing nutrients from parent materials that are essential for life

weathering on mafic rock

less resistant to weathering, made out of heavy elements, mg, fe

weathering on felsic rock

more resistant to weathering made out of light elements (Al, Si)

mineral weather ability also depends on grain size

Granite and basalt are formed from molten magma and composed of minerals that haven’t been altered chemically since they formed. Molten lava solidified

The minerals in granite weather differently:

Quartz (SiO2) is resistant to weathering and contains no plant nutrients

Feldspars and muscovite are more easily weathered but contain few nutrients. Soils derived from granitic rock are coarse-textured (sandy) and have low fertility

Basalt

Dominated by small-grained, easily weathered, nutrient-rich minerals, Hornblende (Ca, Mg, Fe), Augite (Ca, Mg, Fe), Biotite (K, Mg, Fe), Parent materials and soils derived from basaltic rock are fine-textured and fertile

weathering of sedimentary

Compacted or cemented particles weathered from any type of preexisting rocks,

Resistance to weathering depends on dominant minerals and cementing agent

1. Silica-high resistance

2. Rocks that contain the mineral calcite (limestone, calcite- cemented sandstone, marble) are the least resistant to chemical weathering

Sandstone

Sand-sized particles translocated and deposited by water. Cemented by pressure and CaCO3 or SiO2. Coarse-textured soils with relatively low

fertility.

Shale

Clay particles are translocated and deposited by water and compacted. Shale rock is susceptible to physical weathering—fine-textured, relatively fertile soils.

Limestone

Mainly formed in shallow oceans where CaCO3 from skeletal fragments of marine organisms such as coral or microscopic foraminifera accumulate and are cemented, Easily weathered, Soil developed from limestone

are fine- textured, have a neutral to basic pH, and high fertility

Metamorphism (change)

Resistance to chemical weathering depends on mineralogy. Usually more resistant to physical weathering.

Soil organic matter (SOM)

Mixture of all plant, animal and microbial organic components of the soil at a given time

SOM depends on inputs and outputs

inputs: photosynthesis and animal materials

outputs: decomposition (Aerobic and aerobic), erosion

crystalline clays ; phyllosilicates or silicate clays

Dominant source of charge in most soils, Layered structure of mainly Si, Al and O atoms, Differ widely in particle shape, intensity of negative charge, stickiness, plasticity, and swelling

Building blocks for Silicate crystalline clays

Silica tetrahedron and Alumina octahedron, Phyllosilicates consist of two kinds of sheets built from tetrahedral and octahedral

blocks form sheets

sheets form layers

1:1 clays

layers made of 1 tetrahedral sheet + 1 octahedral sheet Layers are stacked on top of each other

2:1 clays:

layers made of 2 tetrahedral sheet + 1 octahedral sheet in the middle Layers are stacked on top of each other

The interlayer space

it’s a very important clay property because it is charged negatively (due to

the O2-) and can hold cations and water

Depending on the size, the interlayer space can hold large cations and water molecules

makes the clays expansible. Water is a very large molecule that expands the interlayer space

Sources of (permanent) charges in crystalline clays

In silica tetrahedra and alumina octahedra, during crystallization, the positive and negative charges balance in the mineral structure.

Isomorphous substitution during crystal formation and weathering

often results in a net negative charge on mineral structures, a primary source of cation exchange capacity.

Micas: Isomorphous substitution in tetrahedral sheets

Al3+ for Si4+ in every fourth tetrahedral site, Mod-low CEC 10-40 cmolc/kg

clay, Intermediate surface area 70-100 m2/g, K+ resides in interlayer spaces

strongly bonding adjoining sheets, Relatively stable: Low shrink- swell potential

Chlorite

Isomorphous substitution in the octahedral sheet, Charge is balanced by a

charged (Al, Mg, Fe) hydroxide sheet, Mod-low CEC 15-40 cmolc/kg clay Surface area: 25-150 m2/g, Stable: No shrink-swell

Vermiculites

Formed from weathering of mica replacement of K+ in interlayer, Isomorphous substitution in the tetrahedral sheet Al3+ for Si4+, High CEC: 100-150 cmolc/kg clay, Surface area: 600-800 m2/g, Stability: Low shrink-swell, Potential issues: K fixation

Smectites

A group of expandable clay minerals, Charge is in the octahedral layer

Mg2+ for Al3+. Moderate CEC: 60-100 cmolc/kg clay. Surface area 600-800 m2/g, Stability: High shrink-swell activity

Kaolinite

1 tetrahedral sheet & 1 octahedral sheet, very low isomorphic

substitution, low CEC: 4-10 cmolc/kg clay, surface area: 7-30 m2/g, no shrink swell, layers are held together by hydrogen bonding

Kaolinite and soil fertility

not optimal because it has low charge and cec but pretty stable for construction

crystalline fe and al oxides

gibbsite and geothite made up of fe, al and o (not si), low and variable charge, due to presence or absence of H+ on surface O-/OH- groups, colloids found in highly weathered soils of humid reagions

amorphous or non crystalline si clays

imogolite-allophane mixture common in soils weathered from volcanic ash, positive and negative charge from OH- groups, fixed anions such as PO4^-2 and SO4^-2 (not plant available)

humus

organic matter

decomposed humus made up of

large molecular C chains and ring 40-60% C, 1-5% N, non crystalline organic w large organic molecules of variable composition

humus importance

upper part of soil profile, has great surface area and hydrophilic groups w high water adsorption, does not expand or contract

humus charges

large negative charges from O- to OH-, some positive charges if the O- and OH- groups accept H+

Anion adsorption

when OH- negative sites in the clay minerals and humus become protonated (gain one H+) and surface charge becomes positive

anion exchange

capacity is higher in acid soils and contrivutes to retnetion of importat plant nutrients (PO4-, NO3-)

anion exchange process

Al → OH2 + H2PO4 → Al → OH2 H2PO4 + H2O