Cation Exchange Capacity 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 m2·g-1 !!! Internal and external surfaces have electronegative or electropositive 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 soil solution and are replaced by other ions Ionic double layer: negatively charged micelle surrounded by a swarm of cations

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 atoms in each layer Eg. kaolinite, montmorillonite

Noncrystalline Silicate Clays Not organized into crystalline sheets Both + and – charges; can adsorb anions such as phosphate High water-holding capacity Malleable when wet, but not sticky Often form in volcanic soils (especially in Andisols) Eg. allophane and imogolite

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 and geothite), but often amorphous Low plasticity and stickiness

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

(Singer and Munns, 2002)

Phyllosilicates Tetrahedron: Two planes of O, with Si in between Basic building block is silicon atom, connected to 4 O atoms Octahedron: with Al or Mg in between is Al (or Mg), connected to six hydroxyl groups or O atoms There are many layers in each micelle

Trioctahedral Sheet Dioctahedral Sheet Isomorphous substitution 1 Al3+ atom, 1 Mg2+ atom Charge = -1 3 Mg2+ atoms Charge = 0 2 Al3+ atoms Charge = 0

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 Clay Each layer contains one tetrahedral and one octahedral sheet Eg. Kaolinite, halloysite, nacrite and dickite Sheets are held together because the apical oxygen in 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 foundations and ceramics (hardens irreversibly)

Each layer contains one octahedral sheet sandwiched 2:1 Silicate Clay Each layer contains one octahedral sheet sandwiched between two tetrahedral sheets O on both ends No attraction without cations

Expanding 2:1 Silicate Clays Smectite group: Interlayer expansion may occur as H2O fills spaces between layers in dry clay Montmorillonite is a very common smectite Smectites have a large amount of negative charge due 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

(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 tetrahedral Sheets Cation exchange capacity is higher in vermiculites than 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 tetrahedral oxygen 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 sheets Fe 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, plasticity and 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 Positive pH 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 edge to associate with O atom Less Negative to Positive Charge: As pH increases, more H+ ions bond to O atoms at the clay surface Protonation at very low pH (H+ ions attach to surface OH groups)

Less effective cation exchange More effective cation exchange

Cation exchange capacity is highest in soils with: High humus content High swelling capacity High pH

Humus A non-crystalline, organic substance Very large, organic molecules 50% 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 by microorganisms) (iii) alcoholic hydroxyl group

State of organic residues one year after 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: complex residual 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

1/5 to 1/3 of carbon remains in soil (i) live biomass (5%) 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 and lightest colour (most susceptible to microbes) (ii) Humic acid (intermediate) (iii) Humin: highest molecular weight, darkest, least soluble and most resistant to microbes

Humus: Amorphous and colloidal mixture of complex organic substances no longer identifiable as tissues Note: non-humic substances are biomolecules produced by microbes

Soil Acidity: Causes and Impacts

Soil Acidification Carbonic acid Carbon dioxide gas from soil air dissolves in water Root respiration and soil decomposition provide extra CO2 CO2 + H2O  H2CO3  HCO3- + H+ Acids from Biological Metabolism Microbes break down organic matter, producing organic acids such as citric acid, carboxylic acids and phenolic acids RCH2OH… + O2 + H2O  RCOOH  RCOO- + H+ 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

Oxidation of Nitrogen (Nitrification) Nitrogen enters soils as NH4+ Converted to nitric acid NH4+ + 2O2  H2O + H+ + H+ + NO3- Oxidation of Sulphur Acids in Precipitation H2SO4  SO42- + 2H+ HNO3  NO3- + H+ Plant Uptake of Cations Plants exude H+ ions or take up anions (eg. SO42-) to balance off cation uptake

Aluminium Toxicity H+ ions adsorbed onto clay surfaces may attack the mineral structure 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 H2SO4 2H+ + SO42- 4NO 4NO2 2N20 + O2 sun H2O  SO2 SO3 H2SO4 2H+ + SO42- O2 2O2 4NO 4NO2 2H2O 2N20 + O2 2HNO3 + 2HNO2 H+ + NO3-

Acidity of Precipitation in the United States

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 pH on cation exchange capacity Increase in CEC with pH due to: Binding and release of H+ ions on pH- dependent charge sites (ii) Hydrolysis reactions involving 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 more meaningful, because Ca, Mg, K and Na ions are not true bases!

Soils with high clay or organic content tend to Buffering Soils with high clay or organic content tend to have the highest buffering capacity Why? Importance of exchangeable and residual acidity Examples of Buffering: 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.

Protonation and deprotonation of organic matter H+ ions dissociate when a base is added, preventing pH from rising as much as expected. CEC increases as the H+ ions are removed, increasing the negative charges pH-dependent charge sites in clays Again, adding a base dissociates H+ ions from hydroxyl groups 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 a base is added, they are replaced by H+ ions or Al ions from exchange sites. (most effective when pH>6). Carbonate dissolution and precipitation (Eq. 9.18)

Liming Liming materials react with CO2 and H2O,     Soil Colloid + CaCO3               Soil Colloid-Ca++ + H2O + CO2 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

Effect of soil pH on nutrient content and soil microorganisms

pH Determination Color dyes Certain organic compounds change colour in response to pH Drops of dye solution can be placed on white spot plate (in contact with soil) Potentiometric method Difference between H+ ion activity in soil suspension and glass electrode gives pH

Soil sampling sites at Tambito, Cauca, Colombia Lower Montane Cloud Forest (LMCF)

Soil Organic Matter

N of Taber, Alberta on Highway 36 Organic matter decomposition rates are higher ET > P

SW of MacGrath Wind Farm, Alberta

8 metres of organic material Accumulation of organic Mer Bleue Bog (SE of Ottawa) 8 metres of organic material Accumulation of organic material has exceeded decay rates (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 acids water and some carbon dioxide Provides little energy for organisms involved, so byproducts contain more energy Rice paddies and natural wetlands release methane Concentrations on the rise

Effect of C/N ratio on Decomposition Rate

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