soil colloids and cation exchange capacity
DESCRIPTION
Soil Colloids and Cation Exchange Capacity. Soil Colloids Particles less than 1 or 2 m behave as soil colloids Total surface area ranges from 10-800 m 2 · g -1 !!! Internal and external surfaces have electronegative or electropositive charges (electronegative charge dominant) - PowerPoint PPT PresentationTRANSCRIPT
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
©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 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)
©2002 Prentice Hall, Inc. Pearson EducationUpper Saddle River, New Jersey 07458
Soils: An Introduction, 5th Editionby Michael J. Singer and Donald N. Munns
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
Lower Montane Cloud Forest
Upper Montane Cloud Forest
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
5cm: 31 ppm 25cm: 4 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
Lowest due to sandy texture ?
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)
5cm: 4.26% of soil dry weight 25cm: 0.49% of soil dry weight total: 9.45 t·ha-1
-
Higher than TMCF Intermediate: possibly stressed due to low transpiration rates
Lowest: possibly stressed due to low transpiration rates
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
0-15cm: 1.74 - 3.41% 5cm: Varies up to 70..8% 0-20cm: 9.8-19.5%
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
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
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)