Soil Chemistry and Mineralogy Notes

Soil Chemistry and Mineralogy

Basic Concepts of Soil Chemistry

• Cation Exchange Capacity (CEC):
  • Definition: The ability of a soil to retain exchangeable cations (positively charged ions) relative to the mass of the sample.
  • Cations are primarily retained on negative charge sites located on the surfaces of soil clay and organic matter.
  • CEC can also be defined as the sum of exchangeable cations that a soil can adsorb (or sorb).
  • Units: centimole of positive charge per kg of soil (cmol/kg).
  • Conversion: 1 cmol/kg = 1 milliequivalent of cations per 100 g soil (meq/100g).
  • Calculation:
    • One cmol of a cation is the atomic or molecular weight in centigrams (cg) divided by the valence.
      • Example: 1 cmol Ca²⁺ = 40 cg/2 = 20 cg Ca²⁺
    • One milliequivalent of a cation is the atomic or molecular weight in mg divided by the cation valence.
      • Example: 1 meq Ca²⁺ = 40 mg/2 = 20 mg Ca²⁺
    • If two ions are in a soil reaction, one cmol of one ion will react with one cmol of the other. The weights of each ion will be different because each ion has a unique cmol weight.
• Anion Exchange Capacity (AEC):
  • Definition: The amount of exchangeable anions held by positive charges in a given weight of soil.
  • Anions are primarily retained on positively charged sites on surfaces of organic matter and some types of clay minerals.
  • AEC can also be defined as the sum of exchangeable anions that a soil can retain.
  • Units: cmol/kg.
  • Calculations are the same as for cation exchange capacity except that charges are negative.
  • Some anions are readily exchangeable if adsorbed (Cl⁻, NO₃⁻, SO₄²⁻), while others are more strongly adsorbed and readily form precipitates (H₂PO₄⁻, HPO₄²⁻).
• pH Dependent and Independent Charge:
  • pH Dependent Charge:
    • Portions of CEC and AEC that have charges that change with pH are termed pH dependent.
    • As pH increases, the CEC increases and AEC decreases and vice-versa.
    • Charges on soil particle surfaces that are pH dependent (variable charge) vary with the pH of soil solution due to hydrogen or hydroxide interactions with functional groups on the surfaces of organic matter, terminal hydroxyl groups on clay surfaces, or interaction with certain ions, such as phosphates, sorbed to the clays.
  • pH Independent Charge:
    • Other charges contributing to CEC and AEC do not change with pH and are termed pH independent.
    • Permanent charge results from isomorphous substitution of structural cations with cations of different valences within the crystal structure of the mineral (i.e. Mg²⁺ replacing Al³⁺ in an octahedral position of a dioctahedral smectite).
    • Permanent charge sites are not directly influenced by pH or other potential determining ions.
• Calculating pH from Hydrogen Ion Activity:
  • Definition of pH: The negative logarithm (base 10) of the hydrogen ion activity (M, molarity) in water.
  • Equation: $pH = -log10(H^+)$
  • The hydrogen ion activity can be approximated by the hydrogen ion concentration.
  • (H⁺) changes by an order of magnitude (a factor of 10) for every 1 unit change in pH.
• Measuring Soil pH (Field and Laboratory Methods):
  • Field pH Test Kits:
    • May be based on acid-base indicators, such as test strips.
    • The indicator methods are not as accurate as electrometric methods.
    • Colorimetric methods involve mixing solutions containing pH indicators with soil. The pH indicators change color as pH changes. Comparing the color of the soil mixture with a color chart is used to determine the pH. Often the soil is placed on a white porcelain plate, indicator added and color noted.
  • Electrometric pH Sensors:
    • Handheld and on-the-go electrometric pH sensors are available for field measurements that utilize battery powered pH meters or colorimetric methods.
    • On-the-go sensors are based on combination electrodes, which combine the reference and glass (hydrogen ion-selective) electrodes.
    • Electrometric methods require that the pH meter be calibrated relative to buffer solutions of known pH.
    • The pH meter measures the difference in the electrical potential between a reference electrode and a glass electrode. The reference electrode has a constant potential, while the glass electrode is made of a special glass that changes potential with pH.
    • The traditional approach is to use separate electrodes, not combination electrode, placing the reference electrode in the supernatant solution while the glass electrode is placed in the soil/water slurry. This is to minimize errors associated with electrical potentials that can occur when KCl moves from the reference electrode to the soil water mixture.
    • Each laboratory will have specific procedures that detail the amount of soil, the kind of solution or water and standing time.
• Influence of Test Method on Interpretation of pH:
  • Soil test procedures can influence the pH of a soil.
  • Factors: Soil sample size, soil to solution ratio, water versus dilute salt solutions, drying, and the time of year soils are sampled all impact the soil pH measured.
  • Neutral Salt Solution vs. Distilled Water: The pH of a soil slurry in a neutral salt solution will generally be lower than the pH of the same soil slurry prepared using distilled water because the cations from the salt solution will displace exchangeable acidity from surfaces of soil particles.
  • Water to Soil Ratio: The ratio of water to soil (i.e. 2:1 or 1:1) used to make the slurry has only a minimal effect on pH because the soil solids buffer the slurry pH. As the ratio changes, this equilibrium can change. The effect is smaller with dilute salt solutions such as 0.01 M $CaCl<em>2$. Use of a dilute salt (e.g. 0.01M $CaCl</em>2$) will give a lower pH because salt brings exchangeable acidity into solution. Aluminum ions (Al³⁺) and Al hydrolysis products are exchanged from soil colloids. These Al species hydrolyze to produce H⁺.
  • Electrode Maintenance: Over time glass electrodes should be dipped in dilute HF to keep them from equilibrating too slowly.
  • Drying: Drying can change soil pH by several tenths of a unit. Drying also reduces the effect of soil carbon dioxide on soil pH because carbon dioxide is lost to the atmosphere during drying.
  • Time of Year: The time of the year samples are taken is also important because the antecedent conditions affect soil pH. Recent additions of fertilizer or organic matter, extremely wet or dry periods and tillage can impact soil pH for example.
  • Fertilizers: Fertilizers can raise or lower pH (e.g. hydrolysis of urea causes a pH increase while nitrification of $NH<em>4^+$ causes a pH decrease), while the addition of organic matter will tend to lower pH as carbonic acid forms from $CO</em>2$ during decomposition.
  • Wet Periods can cause soil pH to increase for acid soils or decrease for alkaline soils, particularly if soils are flooded.
  • Dry Periods may bring salts to the soil surface, which can increase soil solution H⁺ by ion exchange.
  • Tillage increases soil aeration and biological activity, which can, in turn, impact soil pH.
• Calculating Moles:
  • A mole is the atomic, molecular or formula weight of the substance expressed in grams.
  • Example: One mole of calcium ion (Ca²⁺) is the atomic weight (40) in grams or 40 grams. One mole of sulfate ($SO_4$) is the molecular weight (96) in grams or 96 grams.
• Factors Influencing Flocculation and Dispersion of Clays:
  • Flocculation:
    • Clays tend to flocculate in a suspension of high ionic strength when the solution is dominated by di- and tri-valent cations (calcium (Ca²⁺), magnesium (Mg²⁺), and aluminum (Al³⁺, AlOH²⁺, Al(OH)₂⁺)).
    • Solutions containing organic polymers that are capable of bridging between clay particles.
  • Dispersion:
    • Clays tend to disperse in suspensions of low ionic strength and when the solution is dominated by monovalent cations, especially Na⁺ and Li⁺.
  • Type of Clay:
    • Clays dominated by variable charge surfaces tend to flocculate when the solution pH is near the point of zero net charge and will disperse if the pH is above or below the point of zero net charge.
  • Soil Amendments:
    • If a soil is limed or amended with gypsum, calcium is added to the soil and flocculation is favored.
    • If a soil is amended with alum, aluminum is added to the soil, soil pH declines and flocculation is favored.
• Soil Solution Terms:
  • Electrical Conductivity: Conductivity of electricity through a solution such soil extract or a saturated or unsaturated porous medium such as a soil. The electrical conductivity is often used to estimate the soluble salt content in a solution or in a volume of soil; however, electrical conductivity is also influenced by the type and amount of clays or organic matter in the soil.
  • Ion Activity: The effective concentration of an ion that permits the application of the mass action law. Single ion activity is calculated by multiplying the total concentration of an ion by an activity coefficient. The activity of an ion in solution is generally less than the total concentration of the ion but approaches the total concentration at infinite dilution.
• Base Saturation:
  • Definition: the ratio of the quantity of exchangeable bases to the cation exchange capacity.
  • The exchangeable bases are calcium, magnesium, potassium, and sodium.
  • CEC can include exchangeable acidity as well.
  • Calculation: $$
  • Example: If a soil has a CEC of 12 cmol/kg and the total cmol/kg of base-forming cations is 8 cmol/kg, the % base saturation is (8/12)×100 or 67%.

Solid Phase

• Clay Mineral Structures:

  • 1:1 phyllosilicate: with a tetrahedral silicate sheet and an octahedral aluminum sheet, i.e. kaolinite, nacrite, dickite are examples.
  • 2:1 phyllosilicate: with two silicate tetrahedral sheets around one aluminum octahedral sheet, i.e. examples are montmorillonite, beidellite, illite.
  • In nature, clay minerals are stacked many unit cells thick. Each unit cell of the clay mineral structure is a combination of tetrahedral and octahedral layers or sheets. These layers are three-dimensional, crystalline, and either one tetrahedron or octahedron thick.
  • The properties of a clay mineral depend upon the arrangement of these layers and the central ion in the tetrahedron or octahedron. When a central ion of larger positive charge is replaced by a central ion of smaller positive charge, isomorphic substitution has occurred and the clay mineral exhibits an overall negative charge. This negative charge contributes to the CEC of the clay mineral.

• Differences in Clay Minerals:

  • Kaolinite:
    • 1:1 phyllosilicate mineral.
    • Low surface area.
    • Low CEC.
    • Exchange sites are dominantly variable charge located on the lateral edges of the particle.
    • May have a small anion exchange capacity at low pH.
    • Not much shrink-swell potential.
  • Mica (illite):
    • 2:1 phyllosilicate mineral with most interlayers collapsed around potassium.
    • Permanent charge sites are on external surfaces and exposed surfaces along frayed edges.
    • Variable charge sites are along exposed lateral edges of individual layers.
    • Surface area and CEC higher than kaolinite but generally lower than most other smectites and vermiculites.
    • Minor shrink-swell potential.
  • Smectite (montmorillonite):
    • Smectite is a group name for 2:1 expanding phyllosilicates that have high CEC and high surface areas.
    • Dominated by permanent charge surfaces but may have a small amount of variable charge on lateral edges of the layers.
    • The major shrink-swell minerals in soils as they are capable of adsorbing large amounts of water both between layers within a quasicrystal and between quasicrystals.
  • Vermiculite:
    • 2:1 clay mineral with extensive polymeric aluminum hydroxy interlayer material, which restricts cation exchange in the interlayer, reducing pH-independent charge.
    • Aluminum polymers increase variable charge sites.
    • CEC and surface area are higher than illite but lower than montmorillonite.
    • Some shrink-swell potential if interlayers not completely filled with polymeric aluminum.
  • Chlorite:
    • 2:1:1 clay or 2:1 with continuous hydroxy aluminum interlayer.
    • Little shrink-swell potential.
  • Oxides of iron, aluminum, and manganese:
    • High variable-charge CEC but little, if any, pH-independent CEC.
    • Intermediate surface area.
    • Overall CEC low.
    • Anion exchange capacity at low pH.
    • No shrink-swell potential.
  • Amorphous minerals (poorly crystalline minerals):
    • Minerals with short-length or barely repeating crystal structure that is not detected by X-ray diffraction analysis.
    • High surface area and high variable-charge CEC, with low amount of pH-independent CEC.
    • Substantial anion exchange capacity at low pH.
    • Little shrink-swell.

• CEC, Expandability and Surface Area of Clay Minerals Table:

  Clay Mineral	CEC, cmol/kg	Surface Area, m²/kg	Expandability
  Kaolinite	3 to 15	7 to 30	none
  Mica (Illite)	10 to 40	60 to 100	none
  Smectite	80 to 150	600 to 800	high
  (Montmorillonite)
  Vermiculite	10 to 200	50 to 800	medium to low
  Chlorite	10 to 40	25 to 150	none
  Oxides of iron, aluminum and manganese	small	50 to 500	none
  Amorphous minerals	5 to 350	100 to 800	none

• CEC in relation to Soil Organic Matter Content, Clay Content, and Mineralogy

  • Cation exchange capacity may be influenced by pH-dependent charges on organic matter and certain clays (i.e. kaolinite, and hydroxy aluminum interlayer material).
  • Smectite clays have high permanent charges that are available for exchange, whereas illite minerals have much internal charge blocked by fixed potassium.

• Approximate CEC Table:

  Material	CEC (cmol/kg)
  Humus at pH 7	200
  Smectite	100
  Illite	30
  Kaolinite	5

• Estimating CEC from Components:

  • Multiply the decimal fraction of each component by its CEC and total the CECs of all the components.
  • Example:
    • A soil contains 20% smectite, 5% kaolinite and 3% humus.
    • CEC = 0.2 x 100 cmol/kg + 0.05 × 5 cmol/kg + 0.03 × 200 cmol/kg
    • CEC = 26.2 cmol/kg

• Definitions:

  • Rock: consolidated solid earth material.
  • Mineral: