9-10 Interaction soil-liquid: soil solution

Interaction soil matrix - soil solution

Adsorption: Deposition (of ions, atoms, molecules) on surface (at interfaces)

Desorption: Redissolution and release into the mobile phase

Sorption: Besides surface adsorption, it also includes surface precipitation

Surfaces charges of soil component

The surface of almost all solid soil components is electrically charged, i.e. it attracts dissolved ions of opposite charge

The most important charge carriers: fine components with a high specific surface area:

• Clay minerals

• Organic substance

• Oxides and hydroxides (especially of iron, manganese, aluminium)

→ High environmental relevance

Specific surface area

Definition: the size of the surface area per unit mass of a solid substance, the unit is m2 g−1

Repetition of silicate

Flashcard – Main clay minerals (essential overview)

Kaolinite (1:1)

• Structure: 1 tetrahedral + 1 octahedral layer

• Interlayer: none (H-bonds)

• Charge: ~0 (very low substitution)

Illite (2:1)

• Structure: tetra–octa–tetra

• Interlayer: K⁺ fixed

• Charge: moderate (isomorphic substitution)

Smectite (2:1)

• Structure: tetra–octa–tetra

• Interlayer: exchangeable cations + H₂O

• Charge: high (isomorphic substitution)

Clay minerals

• Permanent negative charge
  → Caused by isomorphic replacement in the clay lattice (e.g. Si⁴⁺ → Al³⁺, Al³⁺ → Mg²⁺)
  → Typical of 2:1 clay minerals
  → Independent of pH

• Variable (pH-dependent) charge
  → From surface hydroxyl groups (≡Si–OH, ≡Al–OH)
  → Can be positive or negative depending on pH

• Cation binding
  → Cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) are held

  • on external surfaces

  • in interlayer spaces (especially in 2:1 clays)

• Key consequence
  → High cation exchange capacity (CEC) and strong control on soil fertility

Acidic → mostly positive charges

Alkaline → mostly negative charges

Oxide

• Surface groups: O⁻ and OH⁻ on oxide surfaces
  → Similar to edge sites of clay minerals

• Charge mechanism: protonation / deprotonation

  • Fe–OH + H⁺ ⇌ Fe–OH₂⁺ (positive)

  • Fe–OH ⇌ Fe–O⁻ + H⁺ (negative)

• pH effect:

  • ↑ pH (↓ H⁺) → less positive, more negative charge

  • ↓ pH → more positive charge

• Key property:
  → Only variable charge (no permanent charge)
  → Charge is fully pH-dependent

Organic matter

• Main functional groups:

  • Carboxyl (–COOH)

  • Phenolic & alcoholic –OH

• Charge mechanism:

  • Dissociation of H⁺:
    –COOH ⇌ –COO⁻ + H⁺

  • ↑ pH → ↑ dissociation → more negative charge

• Key property:
  → Only variable charge
  → Charge is pH-dependent (no permanent charge)

• Importance:
  → High contribution to CEC, nutrient retention, and aggregation

Point of zero charge

Definition: pH value at which a surface carries an equal amount of positive and negative charge, noted pHPZC

→ a net charge of zero

→pH below pHPZC means mostly positive charges

→pH above pHPZC means mostly negative charges

Variable charges (pH dependent charges)

• What it is: Charge on soil particles that changes with pH

• Mechanism: Protonation / deprotonation of surface –OH groups (oxides, OM, clay edges)

• pH effect:

  • Low pH → more positive charge

  • High pH → more negative charge

• Materials with variable charge:

  • Oxides (e.g. goethite)

  • Kaolinite (low permanent charge)

  • Organic matter

• Compared to 2:1 clays:

  • Smectite, illite → mainly permanent negative charge (less pH-dependent)

• In most soils: negative charges dominate

• Key concept: pHₚzc (point of zero charge) = pH where net charge = 0

surface charges of soil component (summary)

• Permanent charge:

  • Clay minerals (2:1 clays) → permanent negative charge from isomorphic replacement (independent of pH)

• pH-dependent (variable) charges:

  • Clay edges & fracture surfaces → gain or lose H⁺ depending on pH

  • Soil organic matter (SOM) → mainly negative charge from dissociation of carboxyl (–COOH) and phenolic –OH groups (↑ with pH)

  • Fe and Al oxides → can be positive or negative, controlled by pH (surface –OH groups)

• Key idea:

  • Total soil charge = permanent clay charge + variable pH-dependent charge

  • In most soils, negative charges dominate

Absorption mechanism

Absorption Isotherms

Describe adsorption of substances in soils: Relationship between adsorbed quantity and equilibrium concentration in the solution at constant temperature

Ion Exchange between Soil Particles, Soil Solution & Plant Roots

Nutrients are found in two elements in the soil:

• soil particles → where most of them are found

• soil solution → dissolved ions, few in number, it’s only a means of transport

→ Plants can only take up nutrients from the soil solution, not directly from solid particles.

Particles have more nutrient because:

• Clay minerals and organic matter have negative charges

• Some oxides can have positive charges

• Opposite charges attract → ions are adsorbed on surfaces

→ not permanent

Ion exchange = swapping ions without changing total charge

Example (cation exchange):

• A soil particle holds Ca²⁺

• The soil solution brings 2 NH₄⁺ or 2 H⁺

• They swap places because charges must stay balanced

So:

• Ca²⁺ is released into soil water

• NH₄⁺ (or H⁺) takes its place on the surface

→ Nothing is destroyed, nothing is created — only exchanged

Plant roots

They:

• Release H⁺ (protons) into the soil

• This acidifies the root zone slightly

What happens then:

• H⁺ competes for negatively charged sites

• Nutrient cations (Ca²⁺, K⁺, Mg²⁺, NH₄⁺) are pushed off the soil particles

• These nutrients enter the soil solution

• Roots absorb them immediately

→ Roots “unlock” nutrients from soil particles

Important because:

1. Continuous nutrient supply

Even if soil water contains little nutrient, exchange keeps replenishing it.

2. Protection against leaching

• If all nutrients were dissolved → rain would wash them away

• Adsorption acts like a buffer

3. Nutrient storage

Soil works like a warehouse, not a pipeline.

Cation exchange capacity (CEC) of main soil component

Cation exchange capacity (CEC) = maximum adsorbable cation quantity per mass of soil (cmolc/kg)

CEC of soils

The CEC of soils fluctuates in a wide range, depending on:

• Texture

• Type of clay minerals

• SOM content

Common values are between 5 and 100 cmolc/kg

Effective CEC (CECeff) at the current pH value of the soil

Potential CEC (CECpot) with a reference pH of 7

CEC and pH

Core idea:
Cation Exchange Capacity (CEC) increases with pH, mainly due to pH-dependent (variable) charges.

Key points:

• Permanent charge (from clay minerals) is constant, independent of pH.

• Variable charge (from organic matter and oxide/clay edges) increases as pH rises (deprotonation).

• Therefore:

  • Acidic soils: CEC_eff<CEC_pot → many charges not yet expressed

  • Neutral soils: CEC_eff = CEC_pot

Main contributor to pH effect:

• Organic matter contributes more to pH-dependent CEC than the mineral fraction, especially in topsoils.

Bottom line:
Higher pH → more negative charges → higher effective CEC → better nutrient retention.

Base saturation

Definition:
Base saturation (BS) is the percentage of the CEC occupied by base cations.

Base cations:
Ca²⁺, Mg²⁺, K⁺, Na⁺
(H⁺ and Al³⁺ are acid cations)

Interpretation:

• High BS → many nutrient cations, higher fertility, higher pH

• Low BS → dominance of H⁺/Al³⁺, acidic soil

Key idea:
BS links CEC, soil acidity, and nutrient availability.

Cationic coating of a soil

As soil pH decreases, the effective CEC (CECₑff) decreases and soil surfaces become increasingly coated with acid cations.

Mechanism:

• Lower pH → fewer negative variable charges on clays, oxides, and organic matter

• → CECₑff decreases (potential charge not fully expressed)

Affinity of cations to negative charges

What controls affinity?

1. Cation charge

• Higher charge → stronger attraction (Al³⁺ > Mg²⁺ > Na⁺)

2. Hydration shell

• Smaller hydrated radius → stronger binding

• Cations must shed part of their hydration shell to bind

• Strongly hydrated ions bind less easily (e.g. Na⁺)

3. Solution concentration

• Higher external concentration → higher adsorption

Affinity series (strong → weak):
Al³⁺ > Ca²⁺ > Mg²⁺ > NH₄⁺ ≈ K⁺ > Na⁺

Key idea:
Valence + hydration control how tightly cations bind to negatively charged soil surfaces.

Cation distribution between soil solution and ion coating

Anion exchange

Important anions in soil

• Cl-

• NO3 -

• SO4 2-

• PO4 3-

• Organic anions and dissolved organic matter (DOM, mostly acids)

Factors influencing anions binding

• Type and charge of anion

• Concentration of the anion in the soil solution

• Composition of the adsorbents

• pH value (more acidic, more AEC)

→ Similar to CEC but in reverse!

1) Different anions can compete with each other. Example: PO4 3- and AsO4 3-

2) Adsorbed amount increases with increasing concentration in the solution (like cations)

3) Adsorbents: clay minerals and especially Al, Fe hydroxides and oxides, allophane; variable charge only

4) Strong pH influence: Increase of adsorption with decreasing pH (variable charge)

Adsorption mechanism: outer sphere

• What it is: Weak, purely electrostatic binding (and/or H-bonds) between ions and charged soil surfaces

• No direct chemical bond to the surface → no ligand exchange

• Hydration shell remains intact (water layer stays between ion and surface)

• Occurs in the electric double layer

• Reversible ion exchange (e.g. Cl⁻ ⇄ NO₃⁻)

• Also called physisorption

• Strength depends strongly on ionic strength of the soil solution (generally weak)

→ Typical for non-specific adsorption of ions on oxide and clay surfaces

Adsorption mechanism: inner sphere

• True chemical bonding via ligand exchange with (hydr)oxides

• Hydration shell partly/fully lost

• Strong and specific

• Little dependence on ionic strength

• Typical ions: PO₄³⁻, AsO₄³⁻, Cu²⁺, Zn²⁺

• Can be mono-/bidentate, mono-/binuclear

• = Chemisorption

Main differences between outer and inner

Outer-sphere = weak & electrostatic
Inner-sphere = strong & chemical

pH of the soil

pH values reflects:

• Soil development and resulting soil chemical properties

• Behaviour of nutrients and pollutants

• Suitability of soil for plant growth, habitat for soil MO, filtering pollutants

→Most important and most meaningful soil parameter that can easily be measured (together with texture and OM content)

Soil become more acidic with development in humid climate

• More H+ added (rain, soil processes) than can be neutralised by the soil

• Soluble product of chemical reaction with H+ are soluble and washed out: Loss of buffer function

Important to understand what is going on in the soil

Soil acidity

based on soil content:

1. Exchangeable/dissociable H+ ions

2. Exchangeable Al3+ ions

Al3+ is present in the soil solution in hydrated form and can dissociate H+. Simplified equation:

Al ions only occur in the soil solution from pH < 5

Soil pH typically ranges ~3–8 (most soils 5–6.5).

Lower pH → higher Al³⁺ in soil solution → Al toxicity to roots.

pH is measured in H₂O (actual pH) or CaCl₂/KCl (more reproducible); usually pH(CaCl₂) ≤ pH(H₂O).

Proton sources in the soil → carbonic acid

Source: Root and microbial respiration
Key idea: CO₂ produced in soil forms carbonic acid → releases H⁺
Importance: Main natural acidification process in soils (pH ~7–5), continuous/inexhaustible

Proton sources in the soil → Organic acids

Source: Roots and organic matter decomposition
Key idea: Dissociation of organic acids releases H⁺
Role: Mobilises nutrients (e.g. P, metals) and contributes to soil acidity

Proton sources in the soil → cation uptake by plants

Source: Plant nutrient uptake
Key idea: Uptake of base cations (K⁺, Ca²⁺, Mg²⁺) is balanced by H⁺ release
Effect: Rhizosphere becomes more acidic
Note: Acid balance restored only if biomass is not harvested

Proton sources in the soil → Nitrogen transformations (nitrification)

Source: Microbial N cycling
Key idea: Conversion of NH₄⁺ to NO₃⁻ releases H⁺
Importance: Major acidification process in fertilised and forest soils

Proton sources in the soil → Oxidation of Fe²⁺ and sulfides

Source: Oxidation of Fe²⁺, Mn²⁺, and sulfide minerals
Key idea: Oxidation reactions release H⁺ (or sulfuric acid)
Effect: Strong acidification, typical in acid sulfate soils

Proton sources in the soil → acid rains

Source: Atmospheric SO₂ and NOₓ deposition
Key idea: Acidic inputs from the atmosphere add H⁺ to soils
Impact: Contributes to long-term soil acidification, especially in sensitive soils

Proton sources in the soil → Loss of Acid Neutralization Capacity (ANC)

Source: Combined biological and chemical processes
Key idea: Removal of base cations and proton inputs reduce buffering capacity
Result: Soil becomes more vulnerable to acidification over time

Soil changes due to acidification

1. Decline in cation exchange capacity

2. Decline in nutrient availability

3. Reduction in biological activity

4. Increase in heavy metal mobility

5. Increase of Al concentration in soil solution and water bodies and thus Al toxicity to plants

Tropical and ancient soils are very acid

Also places where you have lot's of industrial production really acid (USA…)

Soil as a buffer

Buffering: reversible or irreversible binding of H+ ions

• Stabilization of the pH value (= buffering)

• pH value changes only when buffer substance is used up

• pH ranges, in which substances buffer, overlap

→ Various buffersystems are active in the soil

Carbonate buffer

Where: Carbonate-rich soils (CaCO₃ present)
Key idea: Carbonates neutralize incoming acidity by consuming H⁺
Effect on pH: Soil pH is strongly buffered and stays ≥ ~7
Buffer range: ~6.5 – 8
Limit: Dissolved Ca and bicarbonate can be leached → buffer weakens over time

Silicate buffer

What it is:
pH buffering by silicate minerals (especially clays) via surface silanol groups (Si–OH).

How it works:

• H⁺ is consumed by silicate surfaces

• Adsorbed K⁺ and Al³⁺ are released into solution

Effective pH range:
~4.2 – 6

Key consequence:

• Continued acidification → silicate breakdown

• Released Al³⁺ can occupy clay interlayers or exchange sites

Role:
Important buffer after carbonates are depleted, but weaker than carbonate buffer.

Al & Fe Oxide Buffer

Mechanism:
Protonation and dissolution of Al/Fe (hydr)oxides consume H⁺.

pH range:

• Al hydroxides: ~pH 3–4.8

• Fe oxides: < 3

Key effect:
Strong buffering at very low pH, but releases Al³⁺ / Fe³⁺ → toxicity risk.

Variable charges buffer

Where:
Organic matter, clay minerals, Fe/Al (hydr)oxides

Mechanism:
H⁺ binds to variable surface charges → base cations (Ca²⁺, Mg²⁺, K⁺) are released.

pH range:
~ 3–10 (material-dependent)

Key effect:
Buffers acidity but promotes base cation leaching → soil impoverishment over time.

Buffer system

Importance of redox reactions

• Colour

• Mobility of metals

• Toxicity of trace elements

• C, N, S, Fe, Mn cycles

• Nutrient availability (e.g. P)

• pH value

  • Oxidation processes produce acid (H+)

  • Reduction processes are acid buffers

• Organisms are involved in redox reactions

Redox reactions

• Oxidation = loss of electrons

• Reduction = gain of electrons

• Electron donor = reducing agent (is oxidized)

• Electron acceptor = oxidizing agent (is reduced)

• In soils: organic matter is the main electron donor

• Most soil redox reactions are microbially driven (e.g. oxidation of organic C coupled to O₂ reduction)

Redox potential

• Redox potential (E or E°) measures how strongly a substance tends to gain or lose electrons (expressed in V or mV).

• More positive E° → substance easily gains electrons → strong oxidizing agent → undergoes reduction.

• More negative E° → substance easily loses electrons → strong reducing agent → undergoes oxidation.

• Direction of electron transfer: electrons flow from lower E° to higher E°.

• A redox reaction always couples two half-reactions (one oxidation, one reduction).

• Overall reaction potential: E°_cell = E°_reduction - E°_oxidation

• In soils, redox potential reflects oxidation–reduction conditions (e.g. well-aerated = high E, waterlogged = low E).

pH dependence of redox potential

• In soils, redox reactions often involve H⁺, so they are pH-dependent.

• Reduction reactions usually consume protons (H⁺) → tend to increase pH.

• Oxidation reactions usually release protons (H⁺) → cause acidification.

• Therefore, redox state and pH are tightly coupled in soils.

Chemical weathering: oxidation

• Oxidation of Fe²⁺, Mn²⁺ and sulfides (S²⁻) during weathering.

• Produces Fe³⁺, Mn³⁺/Mn⁴⁺ and sulfate (SO₄²⁻).

• Releases H⁺ → acidification of soils and waters.

• Key example: pyrite oxidation, responsible for acid mine drainage.

Redox reactions in soil

Core idea:
Soil redox conditions depend on oxygen availability and control which electron acceptors microbes use.

Key points:

• Organic matter = main electron donor

• Aerobic soils: O₂ is used → highest redox potential (Eh)

• Flooded / waterlogged soils: O₂ is consumed → soil becomes anaerobic → Eh decreases

• Microbes then use electron acceptors in a fixed sequence as Eh drops:
  O₂ → NO₃⁻ → Mn oxides → Fe oxides → SO₄²⁻ → CO₂

• As long as one acceptor is present, it buffers Eh; Eh drops further only when it is depleted

Consequences:

• Appearance of Mn²⁺, Fe²⁺, sulfide, CH₄ at low Eh

• Strong effects on nutrient availability, metal mobility, and toxicity

Key point:
When oxygen is depleted, soils become anaerobic and microbes switch step-by-step to weaker electron acceptors, causing a progressive decrease in redox potential (Eh).

Morphological characteristics in soil profile

Reducing conditions (waterlogged, low O₂):

• Fe and Mn oxides are reduced and dissolve

• Fe²⁺ and Mn²⁺ become mobile

• Soil colors: grey / blue-green, bleaching

• Typical of hydromorphic (gley) soils

Oxidizing conditions (well-aerated):

• Fe²⁺ and Mn²⁺ are oxidized and precipitate

• Formation of Fe(III) and Mn(IV) oxides

• Soil colors: rusty red (Fe), black concretions (Mn)

Key idea:
Water saturation controls redox state → reduction = dissolution & bleaching, oxidation = precipitation & staining.

redox summary

• Redox reactions = oxidation + reduction (electron transfer)

• They affect soil pH:

  • Oxidation → acid production

  • Reduction → acid buffering

• As redox potential drops, electron acceptors are used sequentially (O₂ → NO₃⁻ → Mn → Fe → SO₄²⁻ → CO₂)

• Energy yield decreases along this sequence

• Involve key element cycles: C, N, S, Fe, Mn

• Visible in soil profiles by characteristic color patterns (red, black, grey/blue-green)

Key idea: Redox reactions control soil chemistry, pH, energy use, and soil colors.