Soil Colloids and Soil Chemistry

Soil Chemical Properties

Section BSoil Fertility and Plant

Nutrition

Soil Texture The proportions of sand, silt, and clay

particles in soils:

Sand 2 to 0.05 mm effective diameterSilt 0.05 to 0.002 mmClay <0.002 mm

The most reactive fraction is ___________.

clay

Soil Colloids Soil particles <0.001 mm in

diameter Are the most reactive of soil

particles because of _________________ and _________________________.

Types of soil colloids:

surface areaelectrical charge

Inorganic: clay minerals, oxide mineralsOrganic: soil organic matter

Organic Colloids Mostly soil “humus”, the chemically

resistant organic matter in soils, that results from organic matter decomposition.

Characteristics: variably charged (usually -), high cation exchange capacity (CEC)

Humus

CarbonHydrogenOxygenNitrogen

Building Blocks of layer silicates Tetrahedral (Si+4 bonded to four O-

2) Octahedral (Al+3 bonded to six OH-) The long chains or layers of

tetrahedra and octahedra are bonded together to form layer silicates.

Mineral Colloids Layer silicate clays

1:1 clays (Kaolinite)

2:1 clays (Micas, Illite, Vermiculite, Montmorillonite)

2:1:1 clays (chlorite)

1:1 Clay Mineral

Layer

2:1 Clay Mineral

Interlayer

Layer Silicate Clays

Properties:

Surface Area

Charge

Expansion

Layer Silicate Clays

Have a charge because of: Isomorphous substitution

“Substitution of cations of equal or lesser charge within tetrahedrons or octahedrons. This can create a negative charge deficit on the clay particle”.

pH dependent chargeH+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

Layer Silicate Clays

Have a charge because of: Isomorphous substitution

“Substitution of cations of equal or lesser charge within tetrahedrons or octahedrons. This can create a negative charge deficit on the clay particle”.

pH dependent chargeH+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

Hematite Fe2O3

H+

H+

H+

Kaolinite

H+ On kaolinite, most pH-dependent charge occurson exposed octahedral Surfaces.

- H+

Increasing pH

H

HH2

+

H2+

H2+

H2+

H

H2+

H

-

--

H

H-

-

H

H

+ H+

- H+

+ H+

+

- pH

pzccharge

Na+Na+

Na+

H

HH2+

H2+

H2+H2+

H

H2+H

-

--

H

H-

-

H

HCl-Cl-

Cl-Cl-

Cl-Cl- Cl-Cl-Na+

Na+

Na+Na+ Na+Na+

Na+Na+

Soil Colloids

Other soil minerals may occur as colloidal particles: Fe, Al oxides - can have a pH

dependent charge Poorly crystalline clays such as

allophane - also have pH dependent charge

Sources of Charge on Common Soil Clays 2:1 clays (smectites, vermiculite,

etc.) Most charge is due to isomorphous

substitution (always negative) Little pH-dependent charge

1:1 clays (kaolinite) Little isomorphous substitution Most charge is due to pH-dependent

charge (positive or negative)

Cation Exchange Definition: The exchange of cations

adsorbed (attached) onto colloid surfaces with cations in solution.

Exchangeable cations are those attached to colloid surfaces.

Cations in solution and on colloid surfaces tend toward a state of _______________.

Exchangeable cations can be manipulated. e.g.:

equilibrium

Cation Exchange Capacity CEC is: The mass of exchangeable

cations that a given soil can retain per unit weight. Units are cmol(+)/kg soil or meq/100g.

Soils have CEC because of:

Soils have many more exchangeable cations than cations in solution (buffering capacity)

Definitions Atomic weight is weight in grams of 6 x

1023 atoms of a substance. One mole of substance is 6 x 1023 atoms, molecules etc. Thus, atom weight is grams/per mole.

Equivalent weight is the mass of substance that will react or displace 1 gram of H, which is 6 x 1023 charges (- or +).

Thus equivalent weight is atomic weight divided by valence.

CEC Is the quantity of negative charges per kg of soil Expressed in units of cmol(+)/kg (i.e meq/100g) 1 mole of (+) is 6.023 x 1023 (+) 1 cmol of (+) is 0.01 mol (+) 1 mol of Na+ is 23 g and contains 1 mol (+) 1 cmol of Na+ is 0.23 g and contains 1 cmol (+) 1 mol of Ca2+ is 40 g and contains 2 mol (+) 1 cmol of Ca2+ is 0.40 g and contains 2 cmol (+)

High CEC

2+

2+

2+

Low CEC

Strength of Adsorption Cations attraction to clays is a function

of charge and size. Strength of attraction:

Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

Clays and CEC Kaolinite 2-5 cmol(+)/kg Illite (fine mica) 15-40 cmol(+)/kg Vermiculite 100-180

cmol(+)/kg Montmorillonite 80-120

cmol(+)/kg

Humus 100-550 cmol(+)/kg

Clays and CEC What will be the CEC of a clay

loam soil with 30% kaolinite clay? 5 cmol(+)/kg clay x 30 kg clay/100 kg

soil = ____ cmol(+)/kg soil What will be the CEC of a clay

loam soil with 30% montmorollonite clay? 90 cmol(+)/kg clay x 30 kg clay/100

kg soil = ____ cmol(+)/kg soil

1.5

27.0

Brady and Weil, Figure 8.11

Measuring CEC

CEC is commonly measured in laboratories by:1. Saturating soil cation exchange sites with a cation (e.g. NH4

+)2. Extracting the soil with another cation to remove the NH4

+

3. Measure NH4+ extracted

Exchangeable Cations The exchangeable cations have very

important influences on soil properties: Ca2+ is the dominant exchangeable cation in

most soils. Soils become acidic when they contain

significant amounts of exchangeable _______ .

Soils have poor structure when they contain significant amounts of exchangeable _____ .

Al3+

Na+

Weathering and Soil Minerals

Soil mineralogy depends on: Parent material Weathering

Soils that are not highly weathered will tend to contain smectite and illite (mica) colloids in the clay fraction.

Soils that are highly weathered will tend to contain kaolinite and oxide colloids in the clay fraction

How does this affect soil CEC?

Buffering Capacity Definition:

The soil solids control or “buffer” the composition of the soil solution. Caused by dissolution of minerals,

adsorption/desorption of exchangeable cations.

The resistance of the soil solution to a change in composition.

“

Titration Curve—Weak Acid

pH

Base added

“Buffering”

Acid

Alkaline

Buffering in Solutions Acetic Acid in water:

HC2H3O2 H+ + C2H3O2-

Keq ≈ 10-5

Add a base:NaOH + H+ Na+ + H2O

Buffering

SoilMinerals Soil Organic

MatterAvailableNutrientPool

MineralWeathering

Mineralization

Fertilization, AtmosphericInputs, N fixation

PlantUptake

Leaching, Erosion, Gaseous losses

Exch.cations Desorption

Buffering Capacity

10 gallon fuel tank

30 gallon fuel tank

Highly bufferedWhat about fertilization?Poorly buffered soils:1. Store limited amounts

of available nutrients2. Should be fertilized

more often3. Should be fertilized

with lesser amounts

Poorly buffered

Buffering Capacity

The amount of buffering capacity is: Proportional to minerals present (e.g. soils

high in K-feldspars will be highly buffered with respect to K).

Proportional to amount of exchangeable cations (e.g. soils high in exchangeable Ca will be highly buffered with respect to Ca)

Typically, highly-weathered soils are less well-buffered with respect to nutrients than are lightly-weathered soils (more CEC, more primary minerals)

Buffering Capacity

Solution Concentration

Am

t. O

f exc

h. O

r min

eral

nut

rient

Highly Buffered

Poorly Buffered

Affects how frequently some soil amendments, fertilizers need to be added, and how much.

{

∆x1

∆y

∆x2

{ {{∆y

Potassium Buffering Capacity

K in soil solution mmol/L

Exch

ange

able

K m

mol

/kg

From Barber, 1984 p.37

Oxidation-Reduction (Redox) Involves exchange of electrons between

chemical species. In soils, redox reactions often are

catalyzed by ____________________. Oxidation is _______________________. Reduction is _______________________. Oxidation and reduction always occur

together.

microorganismsa loss of electronsa gain of electrons

Redox Reaction2FeO + 2H2O 2FeOOH + 2H+ + 2e-

(oxidation )½ O2 + 2H+ + 2e- H2O

(reduction)_________________________________2FeO + 1/2O2 + H2O 2FeOOH

(oxidation-reduction [redox])Represents Fe oxidation in an aerobic soil environment

Redox Reactions (1) Typical redox reaction in an aerobic

soil:

CH2O + ½ O2 CO2 + H2O Represents the decomposition of organic matter in soils.

C in CH2O is oxidized in the reaction, O in O2 is reduced in the reaction. The O2 is called the “electron acceptor”.

Redox Reactions (2) If a soil becomes anaerobic because of

waterlogging, O2 is not present, so another electron acceptor is needed:

3 CH2O + 2 NO3- 3 CO2 + N2 + 2 H2O +2H+

Represents the decomposition of organic matter in an anaerobic soil. C in CH2O is oxidized in the reaction, N in NO3

- is reduced in the reaction. The NO3- is called

the “electron acceptor”.

Redox Organisms gain energy by

oxidizing compounds (e- donors). They have to dispose of the electrons using other compounds (e- acceptors).

Common e- donors in soils: Organic matter, NH4

+ , S, Fe2+

Common e- acceptors in soils: O2, NO3

-, Fe3+, SO42-, Mn4+

Oxidation State The oxidation state is the

difference between the charge of an atom in its current state and the charge of the neutral atom. Is equal to the number of electrons gained or lost.

In redox reactions, electron gain and loss must be balanced.

Redox Redox reactions have very

important effects on many nutrients in soils:

Oxidized ReducedNO3

- NH4+, N2

Fe3+ Fe2+

Mn3+ Mn2+

SO42- H2S

Soil Redox Potential Aerobic soils have sufficient supplies of

O2, which is the primary e- acceptor. Inorganic N, Mn, Fe, and S tend to be present in their oxidized forms.

Anaerobic soils have little or no O2. An anaerobic condition may be caused by _________________. In this case, N, Mn, Fe, and S may be used as e- acceptors.

N and S availability to plants decrease when reduced, availability of Fe and Mn increase when reduced.

flooded soil