Soil Science (Collids)

Collids

What is a soil colloid?

A very small soil particle with a large surface area relative to its mass

What is the approximate size of a soil colloid?

Generally less than about 1 µm; the clay-size cutoff is 2 µm.

Why are soil colloids important?

They influence nutrient retention, ion exchange, soil structure and water interactions.

Why does small particle size matter?

Smaller particles have a greater surface area relative to their mass

What are the four main classes of soil colloids?

Crystalline silicate clays, non-crystalline silicate clays, hydrous Fe/Al oxides and humus.

What is a crystalline clay mineral?

A clay mineral with an ordered, repeating atomic structure.

Name two crystalline silicate clay minerals.

Kaolinite and montmorillonite. Illite is another example

What are non-crystalline silicate clays?

Silicate clays without a regular, repeating long-range crystal structure.

What are hydrous iron and aluminium oxides?

Iron- and aluminium-rich soil colloids containing oxygen and hydroxyl groups.

What is humus?

Decomposed, relatively stable organic matter in soil.

Is humus an organic or mineral colloid?

Organic.

What is the ionic double layer?

A charged colloid surface surrounded by oppositely charged ions

What is the inner part of the ionic double layer?

The electrically charged colloid surface.

What is the outer part of the ionic double layer?

A surrounding region of counterions attracted to the surface.

Why do negatively charged colloids attract cations?

Opposite electrical charges attract.

What is a cation?

A positively charged ion, such as K⁺ or Ca²⁺.

What is an anion?

A negatively charged ion, such as NO₃⁻.

What is flocculation?

Small soil particles joining together into larger clusters called flocs.

What is dispersion?

Clay particles separating rather than remaining aggregated.

Which cation commonly encourages flocculation?

Calcium (Ca²⁺).

Which cation commonly encourages dispersion?

Sodium (Na⁺).

Why can sodium damage soil structure?

It promotes clay dispersion, weakening aggregation and potentially reducing infiltration.

What is soil aggregation?

Soil particles binding together into larger structural units.

How can clay and organic matter help aggregation?

They bind particles through surface interactions and bridging mechanisms.

What are metal-ion bridges?

Metal ions linking charged surfaces of clay and organic matter.

What are water bridges?

Water-mediated attractions helping soil particles or surfaces adhere.

What is soil stickiness?

The tendency of moist soil to adhere to objects touching it.

What is soil plasticity?

The ability of moist soil to be moulded and retain its new shape.

Does sticky soil necessarily have poor structure?

No. Stickiness and soil structural quality are different properties.

What is shrink–swell behaviour?

Clay expanding when wet and contracting when dry.

Why do shrink–swell soils crack during dry weather?

Water leaves the clay, causing contraction and opening cracks.

What are Vertisols known for?

Strong shrink–swell behaviour and deep cracks during dry periods.

Why can shrink–swell clays damage buildings?

Repeated expansion and contraction can move the ground beneath foundations.

What does CEC stand for?

Cation Exchange Capacity.

Define CEC

The capacity of soil to retain and exchange positively charged ions.

Where are exchangeable cations held?

On negatively charged exchange sites of clay and organic matter.

Why is high CEC useful?

It provides greater capacity to retain nutrient cations against leaching.

How do exchangeable nutrients reach plant roots?

They enter the soil solution, from which roots can absorb them.

Does high CEC guarantee high fertility

No. Fertility also depends on which cations occupy exchange sites.

How does CEC contribute to pH buffering?

Exchange sites help resist rapid changes in soil chemistry and pH.

Name four important nutrient cations.

K⁺, Ca²⁺, Mg²⁺ and NH₄⁺.

Name four micronutrient cations

Fe²⁺/Fe³⁺, Mn²⁺, Zn²⁺ and Cu²⁺.

Name the four main non-acid cations.

Ca²⁺, Mg²⁺, K⁺ and Na⁺.

Name the two main acid cations.

H⁺ and Al³⁺.

Which cations commonly dominate acidic exchange complexes?

H⁺ and Al³⁺.

Why can acidic soil have poor nutrient availability?

Acid cations occupy exchange sites and low pH can reduce nutrient availability.

Is sodium a non-acid cation?

Yes, although excessive exchangeable sodium can damage soil structure.

What are phyllosilicate clay minerals?

Silicate minerals built from sheets of tetrahedral and octahedral units.

What forms a tetrahedral sheet?

Silicon–oxygen tetrahedra.

What does T represent in clay structures?

A tetrahedral sheet.

What does O represent in clay structures?

An octahedral sheet.

What does 1:1 clay mean?

One tetrahedral sheet combined with one octahedral sheet.

What does 2:1 clay mean

Two tetrahedral sheets surrounding one octahedral sheet.

What is the sheet order of a 2:1 clay?

T–O–T: tetrahedral–octahedral–tetrahedral.

Which clay mineral has a 1:1 structure?

Kaolinite.

Which two studied clays have 2:1 structures?

Montmorillonite and illite.

What holds adjacent kaolinite layers together?

Hydrogen bonds.

Does kaolinite swell strongly between layers?

No. Hydrogen bonding restricts interlayer expansion.

Why is kaolinite useful in pottery?

Its plasticity allows moist clay to be shaped and retain its form.

Which clay is strongly associated with swelling

Montmorillonite, a smectite clay mineral.

Why does montmorillonite swell when wet?

Water enters between its 2:1 layers, increasing interlayer spacing.

Why does montmorillonite shrink when dry?

Interlayer water leaves and the layers move closer together.

Which clay is associated with potassium fixation?

Illite.

How does illite fix potassium?

K⁺ becomes strongly held between its 2:1 layers.

Why can potassium fixation reduce plant availability?

Fixed K⁺ is less readily released into the soil solution.

Do all 2:1 clay minerals swell strongly?

No. Montmorillonite swells, but illite's interlayer K⁺ restricts swelling.

What specific surface area is given for smectite?

Approximately 725 m²/g.

What is isomorphous substitution?

An ion replacing another similar-sized ion within a mineral crystal structure.

Which substitution example was in the lecture?

Mg²⁺ replacing Al³⁺ in a clay mineral structure.

Why can Mg²⁺ occupy an Al³⁺ position?

Their sizes and coordination are sufficiently compatible under suitable formation conditions.

Does clay substitute Mg²⁺ because it wants negative charge?

No. Charge is a consequence of chemically permitted substitution.

When does isomorphous substitution commonly occur?

During mineral formation, as ions are incorporated into the growing crystal.

Why does Mg²⁺ replacing Al³⁺ create negative charge?

Mg²⁺ contributes one less positive charge, leaving a relative −1 charge.

Why is substitution charge permanent?

It is built into the crystal structure and does not depend on soil pH.

What balances permanent negative charge on clay?

Nearby positively charged counterions, including exchangeable cations.

What is pH-dependent charge?

Surface charge that changes as chemical groups gain or lose H⁺.

Where does pH-dependent charge occur?

At clay mineral edges and on humus.

What happens to –OH when it loses H⁺?

It becomes –O⁻, developing negative charge.

What generally happens to negative charge as pH rises?

More surface groups lose H⁺, increasing pH-dependent negative charge.

What generally happens to pH-dependent charge as pH falls?

More groups retain H⁺, reducing negative charge.

Does increasing pH turn variable charge into permanent charge?

No. It remains pH-dependent even when negative charge increases.

How can low pH reduce CEC?

Protonation reduces negative charge at pH-dependent exchange sites.

How do H⁺ and Al³⁺ affect exchange-site occupancy?

They can occupy sites that might otherwise hold nutrient cations.

Does H⁺ occupying a site automatically destroy that site?

No. Occupancy and loss of pH-dependent negative charge are different processes.

How does humus contribute to CEC?

Its functional groups develop negative charges that retain exchangeable cations.

Which charge type comes from isomorphous substitution?

Permanent, pH-independent charge.

Which charge type comes from H⁺ gain or loss?

pH-dependent or variable charge.

Compare kaolinite, montmorillonite and illite.

Kaolinite: 1:1, H-bonded. Montmorillonite: 2:1, swells. Illite: 2:1, fixes K⁺.

Why are clay and humus central to soil fertility

Their charged surfaces retain and exchange nutrient cations.

Why does high surface area enhance colloid activity?

It provides more surface for ion exchange and particle interactions.

What is the key difference between flocculation and dispersion?

Flocculation joins particles; dispersion separates them.

What is the key difference between permanent and variable charge?

Permanent charge is pH-independent; variable charge changes with pH.

Why does soil structure matter for plant growth?

It affects pore space, aeration, water movement and root penetration.

How can organic matter improve soil structure?

It helps bind particles into aggregates and contributes charged surfaces.

What is the overall role of the soil colloidal system?

It controls important interactions affecting structure, nutrient retention and exchange