soil colloids particles less than 1 or 2 m behave as soil colloids total surface area ranges from...

Soil Colloids

•Particles less than 1 or 2 m behave as soil colloids

•Total surface area ranges from 10-800 m2·g-1 !!!

•Internal and external surfaces have electronegative orelectropositive charges (electronegative charge dominant)

•Each micelle adsorbs thousands of hydrated Al3+, Ca2+, H+, K+, Mg2+ and Na+ ions (enclosed within several H2O molecules)

•Cation exchange occurs when ions break away into the soilsolution and are replaced by other ions

•Ionic double layer: negatively charged micelle surrounded bya swarm of cations

I. Crystalline Silicate Clays

• Dominant colloid in most soils (not andisols, oxisols or organic soils)

• Crystals layered as in a book

• 2-4 sheets of tightly-bonded O, Si and Al atomsin each layer

• Eg. kaolinite, montmorillonite

II. Noncrystalline Silicate Clays

• Not organized into crystalline sheets

• Both + and – charges; can adsorb anionssuch as phosphate

• High water-holding capacity

• Malleable when wet, but not sticky

• Often form in volcanic soils (especially in Andisols)

• Eg. allophane and imogolite

III. Iron and aluminium oxides

• Found in highly weathered soils of warm, humid regions (eg. oxisols)

• Consist of Fe and Al atoms connected to oxygen atoms or hydroxyl groups

• Some form crystalline sheets (eg. gibbsite andgeothite), but often amorphous

• Low plasticity and stickiness

IV. Humus

• Present in nearly all soils, especially A horizon

• Not mineral or crystalline

• Consist of chains of C atoms, bonded to H, O & N

• Very high water adsorption capacity

• Not plastic or sticky

• Negatively charged

Mica

HumicAcid

Kaolinite

(kandite)

Montmorillonite

(smectite)

©2002 Prentice Hall, Inc. Pearson EducationUpper Saddle River, New Jersey 07458

Soils: An Introduction, 5th Editionby Michael J. Singer and Donald N. Munns

Figure 2–11 Summary of aluminosilicate clay structures. (A) Building blocks: Oxygen, OH, or H2O—each 0.3 nm diameter—coordinate around smaller atoms of Si and Al, forming the two basic building blocks: the Si–O tetrahedron and the Al–O, OH octahedron. These units are represented in three ways: as polyhedra, as stick-and-ball drawings showing positions of atom centers and bonds, or as space-fill (sphere-packing) drawings indicating volumes filled by oxygen electron shells. (Parentheses—(Al), (Mg, Fe)—indicate possible isomorphoussubstitutions.) (B) Sheet structures: These are formed by Si–O tetrahedra, each sharing three of their oxygens, or by octahedra sharing all six of their OH or O. Sheets combine to form layers.

(Singer and Munns, 2002)

Phyllosilicates

Tetrahedron:•Two planes of O,with Si in between •Basic building blockis silicon atom,connected to 4 Oatoms

Octahedron:•Two planes of O,with Al or Mg in between•Basic building blockis Al (or Mg), connected to six hydroxyl groups or O atoms

There are many layers in each micelle

3 Mg2+ atomsCharge = 0

2 Al3+ atomsCharge = 0

TrioctahedralSheet

DioctahedralSheet

Isomorphoussubstitution

1 Al3+ atom,1 Mg2+ atomCharge = -1

Isomorphous substitution

•Each Mg2+ ion that substitutes for Al3+ causes a negative charge in a dioctahedral sheet

•Each Al3+ ion that substitutes for Si4+ causes a negative charge in a tetrahedral sheet

1:1 Silicate ClayEach layer contains one tetrahedral and one octahedral sheet

Eg. Kaolinite, halloysite, nacrite and dickite

•Sheets are held together because the apical oxygenin each tetrahedron also forms the bottom corner of one or more octahedra in the adjoining sheet

•Hydroxyl plane is exposed: removal or addition of hydrogen ions can produce positive or negative charges (hydroxylated surface also binds with anions)

•Hydroxyls of octahedral sheet are alongside Oxygens of the tetrahedral sheet: hydrogen bonding results, with no swelling in kaolinites!

•Kaolinite useful for roadbeds, building foundationsand ceramics (hardens irreversibly)

2:1 Silicate ClayEach layer contains one octahedral sheet sandwichedbetween two tetrahedral sheets

O on both endsNo attraction without cations

Expanding 2:1 Silicate Clays

Smectite group: Interlayer expansion may occur asH2O fills spaces between layers in dry clay

•Montmorillonite is a very common smectite

•Smectites have a large amount of negative chargedue to isomorphous substitution•Mg2+ often replaces Al3+ in the octahedral sheet

•Al3+ sometimes replaces Si4+ in the tetrahedral sheet

•Weak O:cation linkages between layers leads to plasticity, stickiness, swelling and a very high specific surface area



Figure 2–11 Continued. (C) Layer structures: The two basic types, 1:1 and 2:1, are shown. Each is represented (left to right) as polyhedral, stick-and-ball, and space-fill drawings, each depicting a side view of two unit layers and the interlayer space between them.

(Singer and Munns, 2002)

Vermiculite Group (2:1 Expanding Silicate Clay)

•Very high negative charge, due to frequent substitution of of Si4+ ions with Al3+ in the tetrahedralSheets

•Cation exchange capacity is higher in vermiculitesthan in any other clay

Swelling occurs, but less than in smectites due to strongly adsorbed H2O molecules, Al-hydroxy ions and cations, which act more as bridges than wedges.

Non-Expanding 2:1 Silicate Minerals

Mica Group (illite and glauconite)•Al3+ substituded for 20% of Si4+ in tetrahedral sheets•K+ fits tightly into hexagonal holes between tetrahedraloxygen groups: virtually eliminates swelling

Chlorites are also non-expansive:

Mg-dominated trioctahedral hydroxide sheet fits between 2:1 layers (2:1:1). H-bonded to O atoms between sheetsFe or Mg occupy most octahedral sites

Iron and Aluminium Oxides

•Modified octahedral sheets with either Fe2+ or Al3+ in the cation positions

•No tetrahedral sheets and no silicon

•Lack of isomorphous substitution (little negative charge)

•Small charge (+ or -) due to removal or addition of hydrogen ions from surface hydroxyl groups

•Non-expansive and relatively little stickiness, plasticityand cation absorption

Variable Charge (pH-dependent)

• Hydrous oxides whether crystalline or amorphous get their charge from surface protonation and deprotonation

• >AlO- + H+ >AlOH + H+ AlOH2+

Negative Neutral PositivepH decreasing

• Layer aluminosilicates have a small amount of variable charge because of OH at the edges

• All the negative charge on humus is variable

• Hydrous oxides are positively charged in some very acid soils and help retain anions

Negative charge:•Dissociation of H+ ions, lack of Al & Si at edgeto associate with O atom

Less Negative to Positive Charge:•As pH increases, more H+ ions bond toO atoms at the clay surface•Protonation at very low pH (H+ ions attachto surface OH groups)



Box 2-3 Fixed and Variable Charge

More effectivecation exchange

Less effectivecation exchange

Cation exchange capacityis highest in soils with:

•High humus content•High swelling capacity•High pH

Humus

•A non-crystalline, organic substance•Very large, organic molecules50% C, 40% O, 5% H, 3% N and sometimes S•Structure highly variable

•Very large negative charge due to three types of -OH groups (H+ ions gained or lost)

(i) carboxyl group COOH(ii) phenolic hydroxyl group (due to

partial decomposition of lignin bymicroorganisms)

(iii) alcoholic hydroxyl group

State of organicresidues one yearafter incorporation into a soil

Humic Substances

• Microbes break down complex components• Simpler compounds created; CO2 is released• Synthesize new biomolecules, using C not respired,

as well as N, S & O

• Lignin not completely broken down: complexresidual molecules often retain lignin characteristics

• Microbes polymerize new, simpler molecules with one another and with residual molecules

• This creates long, complex chains, resistant to further decomposition

• Chains interact with amino compounds• Polymerization process is stimulated by colloidal

clays

After one year:

• 1/5 to 1/3 of carbon remains in soil(i) live biomass (5%)(ii) humic fraction (20%)(iii) nonhumic fraction (5%)

Humic substances include:

(i) Fulvic acids: lowest molecular weight andlightest colour (most susceptible to microbes)

(ii) Humic acid (intermediate)(iii) Humin: highest molecular weight, darkest,

least soluble and most resistant to microbes

Humus: Amorphous andcolloidal mixtureof complex organicsubstances nolonger identifiableas tissues

Note: non-humic substances are biomolecules produced by microbes

Soil Acidification

1. Carbonic acidCarbon dioxide gas from soil air dissolves in waterRoot respiration and soil decomposition provide extra CO2

CO2 + H2O H2CO3 HCO3- + H+

2. Acids from Biological MetabolismMicrobes break down organic matter, producing organicacids such as citric acid, carboxylic acids and phenolic acids

RCH2OH… + O2 + H2O RCOOH RCOO- + H+

3. Accumulation of Organic Matter(i) Loss of cations by leaching due to soluble humic complexes combining with non-acid nutrient cations (eg. Ca2+)(ii) Organic matter is a source of H+ ions

4. Oxidation of Nitrogen (Nitrification)Nitrogen enters soils as NH4

+

Converted to nitric acid

NH4+ + 2O2 H2O + H+ + H+ + NO3

-

5. Oxidation of Sulphur

6. Acids in Precipitation

H2SO4 SO42- + 2H+

HNO3 NO3- + H+

7. Plant Uptake of CationsPlants exude H+ ions or take up anions (eg. SO4

2-) tobalance off cation uptake

Aluminium Toxicity

H+ ions adsorbed onto clay surfaces may attack the mineralstructure and release Al3+ ions in the process

Aluminium is highly toxic to most plants

Al promotes hydrolysis of H2O (see Fig. 9.12)

Al combines with OH-, leaving H+ ions in the soil solution

Tolerant plants secrete organic acids into the soil around the root. Organic acids such as (eg. malate or citrate) are able to chelate the Al that is in the soil solution near the root tip. Al that is bound to organic acids cannot enter the plant root.

•Acids are neutralized in soils with available bases•Canadian Shield severely affected in central and eastern Canada

H+ + HSO3-

SO2 SO3

sun

O2

H2SO4 2H+ + SO4

2-

H+ + NO3-

2N20 + O24NO

H2O

4NO2 2HNO3 + 2HNO2

2O2 2H2O

Acidity of Rainfall in New Hampshire

Susceptibility to Acidification

• Weathering of non-acid cations from minerals An example is the weathering of calcium from silicates

Ca-silicate + 2H+ H4SiO4 + Ca2+

• Soil maintains its alkalinity if the release of cations from weathering minerals exceeds leaching losses

• Acid soils therefore form:

(i) in a high rainfall environment(ii) where parent materials are low in Ca, Mg, K

and Na(iii) where there is a high degree of biological

activity, resulting in H2CO3 formation

Effect of soil pHon cation exchangecapacity

Increase in CEC with pHdue to:

(i) Binding and release ofH+ ions on pH-dependent charge sites

(ii) Hydrolysis reactionsinvolving Al

Percent “base” saturation

= cmol of exchangeable Ca2+ + Mg2+ + K+ + Na+

cmolc of CEC

= 100 – percent acid saturation

Note: Percent acid saturation, though less often cited, is determined by cmolc Al3+ & H+ ions divided by cmolc of CEC. This is actually moremeaningful, because Ca, Mg, K and Na ions are not true bases!

Buffering

• Soils with high clay or organic content tend tohave the highest buffering capacity

• Why? Importance of exchangeable and residualacidity

Examples of Buffering:

(i) Aluminium hydrolysis (in very acid soils)

Al(OH)2+ + H2O Al(OH)3 + H+

Adding more H+ ions will drive the reaction to the left.

(ii) Protonation and deprotonation of organic matter

H+ ions dissociate when a base is added, preventingpH from rising as much as expected.

CEC increases as the H+ ions are removed, increasingthe negative charges

(iii)pH-dependent charge sites in clays

Again, adding a base dissociates H+ ions from hydroxylgroups and oxygen atoms

(iv) Cation exchange

As H+ ions are added, most end up attracted to negative charge sites so that pH changes less than expected. If abase is added, they are replaced by H+ ions or Al ionsfrom exchange sites. (most effective when pH>6).

(v) Carbonate dissolution and precipitation (Eq. 9.18)

Liming

•Liming materials react with CO2 and H2O,to produce bicarbonate (HCO3

-)

Example:CO3

- + 2H+ CO2 + H2O

CaMg(CO3)2 + 2H2O + 2CO2 Ca2+ + 2HCO3- + Mg2+ + 2HCO3

-

•Bicarbonate is reactive with exchangeable and residual soil acidity

•Ca2+ and Mg2+ replace H+ and Al3+ on clay colloids

H+

Soil Colloid + CaCO3 Soil Colloid-Ca++ + H2O + CO2

H+

Effect of soil pHon nutrient contentand soil microorganisms

pH Determination

(i) Color dyesCertain organic compounds change colour inresponse to pHDrops of dye solution can be placed on white spot plate (in contact with soil)

(ii) Potentiometric methodDifference between H+ ion activity in soil suspension andglass electrode gives pH

Soil sampling sites at Tambito, Cauca, Colombia

Lower Montane Cloud Forest(LMCF)

Lowland Rainforest

Lower Montane Cloud Forest

Upper Montane Cloud Forest

Soil Type Oxisols and Ultisols Oxisols and Ultisols Oxisols and Ultisols

Soil N* 5cm: 0.49-0.56% 25cm: 0.095 -0.405%

5cm: 1.25-1.85% 25cm: 0.40-0.49%

-

Soil P (Bray) in Panamá*

5cm: <0.5 ppm 25cm: <0.5 ppm

5cm: 31 ppm 25cm: 4 ppm

-

Soil P (Bray) in Borneo**

0-15cm: 1.18-1.56 ppm

0-15cm: 0.84-2.70 ppm

0-15cm: 0.80-20.93 ppm

Soil K* 5cm: 0.95 meq/100g 25cm: 0.39 meq/100g

5 cm: 0.86-0.90 meq/100g 25cm: 0.15-0.33 meq/100g

-

Soil Ca* 5cm: 29 meq/100g 25cm: 12 meq/100g

5cm: 5 - 8 meq/100g 25cm: 1-2.9 meq/100g

Lowest ?

Soil B Higher due to greater clay fraction and reduced wetness

5cm: 9.1 ppm 25 cm: 0.6 ppm

Lowest due to sandy texture ?

Soil Al 5cm: 0.1 meq/100g 25cm: 0.2 meq/100g

5cm: 2.5 meq/100g 25cm: 1.8 meq/100g

-

Root Biomass Higher root biomass in Amazon (but also greater above-ground biomass)

5cm: 4.26% of soil dry weight 25cm: 0.49% of soil dry weight total: 9.45 t·ha-1

-

Nutient Uptake Rate

Higher than TMCF Intermediate: possibly stressed due to low transpiration rates

Lowest: possibly stressed due to low transpiration rates

Soil Texture Sandy clay loams; more clay and less sand than TMCF 31-69% clay 14-26% silt 17-49% sand

Organic at surface then sandy loams or sandy clay loams 5-22% clay 17-24% silt 57-78% sand

Organic at surface, then sandy loams

Soil Organic Matter Content

0-15cm: 1.74 - 3.41% 5cm: Varies up to 70..8% 0-20cm: 9.8-19.5%

Highest

Bulk Density 5cm: 0.74 gcm-3 10-25cm: 0.73 gcm-3

5cm: 0.26 gcm-3 10-25cm: 0.49 gcm-3

Lowest

Soil Acidity 5cm: 15 meq/100g 25cm: 18 meq/100g pH: 5.4-6.5

5cm: 68 meq/100g 25cm: 26 meq/100g pH: 3.7-5.1

Acid

Cation Exchange Capacity

5cm: 54 meq/100g 25cm: 37 meq/100g

5cm: 65 meq/100g 25cm: 26 meq/100g

High

Base Saturation 5cm: 67 meq/100g 25cm: 42 meq/100g

5cm: 12 meq/100g 25cm: 4 meq/100g

Low

Soil Wetness High, except dry season

Wetter due to cloud interception year-round and cooler

Wettest, due to cloud interception year-round; little

Lowland Rainforest



Oxisols and Ultisols Oxisols and Ultisols Oxisols and Ultisols

5cm: 0.49-0.56% 25cm: 0.095 -0.405%

5cm: 1.25-1.85% 25cm: 0.40-0.49%

-

5cm: <0.5 ppm 25cm: <0.5 ppm


-

0-15cm: 1.18-1.56 ppm

0-15cm: 0.84-2.70 ppm

0-15cm: 0.80-20.93 ppm

5cm: 0.95 meq/100g 25cm: 0.39 meq/100g

5 cm: 0.86-0.90 meq/100g 25cm: 0.15-0.33 meq/100g

-

5cm: 29 meq/100g 25cm: 12 meq/100g

5cm: 5 - 8 meq/100g 25cm: 1-2.9 meq/100g

Lowest ?

Higher due to greater clay fraction and reduced wetness

5cm: 9.1 ppm 25 cm: 0.6 ppm


5cm: 0.1 meq/100g 25cm: 0.2 meq/100g

5cm: 2.5 meq/100g 25cm: 1.8 meq/100g

-

Higher root biomass in Amazon (but also greater above-ground biomass)


-



Sandy clay loams; more clay and less sand than TMCF 31-69% clay 14-26% silt 17-49% sand




Highest

5cm: 0.74 gcm-3 10-25cm: 0.73 gcm-3

5cm: 0.26 gcm-3 10-25cm: 0.49 gcm-3

Lowest

5cm: 15 meq/100g 25cm: 18 meq/100g pH: 5.4-6.5

5cm: 68 meq/100g 25cm: 26 meq/100g pH: 3.7-5.1

Acid

5cm: 54 meq/100g 25cm: 37 meq/100g

5cm: 65 meq/100g 25cm: 26 meq/100g

High

5cm: 67 meq/100g 25cm: 42 meq/100g

5cm: 12 meq/100g 25cm: 4 meq/100g

Low

High, except dry season

Lowland Rainforest



Soil Type Oxisols and Ultisols Oxisols and Ultisols Oxisols and Ultisols

Soil N* 5cm: 0.49-0.56% 25cm: 0.095 -0.405%

5cm: 1.25-1.85% 25cm: 0.40-0.49%

-

Soil P (Bray) in Panamá*

5cm: <0.5 ppm 25cm: <0.5 ppm


-

Soil P (Bray) in Borneo**

0-15cm: 1.18-1.56 ppm

0-15cm: 0.84-2.70 ppm

0-15cm: 0.80-20.93 ppm

Soil K* 5cm: 0.95 meq/100g 25cm: 0.39 meq/100g

5 cm: 0.86-0.90 meq/100g 25cm: 0.15-0.33 meq/100g

-

Soil Ca* 5cm: 29 meq/100g 25cm: 12 meq/100g

5cm: 5 - 8 meq/100g 25cm: 1-2.9 meq/100g

Lowest ?

Soil B Higher due to greater clay fraction and reduced wetness

5cm: 9.1 ppm 25 cm: 0.6 ppm


Soil Al 5cm: 0.1 meq/100g 25cm: 0.2 meq/100g

5cm: 2.5 meq/100g 25cm: 1.8 meq/100g

-

Root Biomass Higher root biomass in Amazon (but also greater above-ground biomass)


-

Nutient Uptake Rate



Soil Texture Sandy clay loams; more clay and less sand than TMCF 31-69% clay 14-26% silt 17-49% sand



Soil Organic Matter Content


Highest

Bulk Density 5cm: 0.74 gcm-3 10-25cm: 0.73 gcm-3

5cm: 0.26 gcm-3 10-25cm: 0.49 gcm-3

Lowest

Soil Acidity 5cm: 15 meq/100g 25cm: 18 meq/100g pH: 5.4-6.5

5cm: 68 meq/100g 25cm: 26 meq/100g pH: 3.7-5.1

Acid

Cation Exchange Capacity

5cm: 54 meq/100g 25cm: 37 meq/100g

5cm: 65 meq/100g 25cm: 26 meq/100g

High

Base Saturation 5cm: 67 meq/100g 25cm: 42 meq/100g

5cm: 12 meq/100g 25cm: 4 meq/100g

Low

Soil Wetness High, except dry season

N of Taber, Alberta on Highway 36

•Organic matterdecomposition rates are higher

•ET > P

SW of MacGrath Wind Farm, Alberta

Mer Bleue Bog (SE of Ottawa)

8 metres of organic material

Accumulation of organic material has exceeded decayrates (anaerobic) for 12,000 years

P >> ET

Photo: P Lafleur, Trent University

Composition of Green Plant Materials

Decomposition Rates of Organic Materials

Rapid

Very slow

Sugars and Starches

Proteins

Hemicellulose

Cellulose

Fats, Waxes and Oils

Lignins and phenolic compounds

Review: Oxidation products are CO2, H2O and energy (478 kJ/mol C)

Decomposition in Anaerobic Soils

•Very slow•Release of methane gas, alcohols, organic acidswater and some carbon dioxide•Provides little energy for organisms involved, sobyproducts contain more energy

Rice paddiesand naturalwetlands release methane

Concentrationson the rise

Effect of C/N ratio on Decomposition Rate

0 40 80

0 .0

0 .4

0 .8

1 .2

0 2 0 4 0 6 0

0

50 0 0

1 00 0 0

1 50 0 0

0 2 0 4 0 6 0

1 0

3 0

0

2 0

4 0

m.eq. p.p.m. p.p.m.

POTASSIUM NITROGEN PHOSPHORUS

0 20 40 60

0

4

8CALCIUM

0 20 40 60

0

4

8

12BORON ALUMINIUM

0 1 0 2 0 3 0 4 0 5 00

1

2

3

4

5

m.eq. p.p.m. m.eq.

Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm)

1st/late 2nd.Early 2nd.Deforested

Figure 5.14 Average soil potassium, nitrogen, phosphorus, calcium, boron and aluminium levels at Centro de Estudios Ambientales Tambito. Note the higher nutrient concentrations in primary/late secondary forest.

Soil Nutrient Concentrations vs. Successional Stage (Tambito, Cauca, Colombia)

soil colloids particles less than 1 or 2 m behave as soil colloids total surface area ranges from...

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