quiz soils today 2

What are soil colloids

Very small soil particles that participate in exchange reactions.

Why are soil colloids important

They have surface charges that attract and hold ions, including plant nutrients.

What are the two general types of soil colloids

Inorganic and organic.

What is the approximate size of soil colloids in the lecture

Less than 2 micrometers in diameter.

Why does small particle size matter

It produces a large surface area.

What is the external surface area shown in the lecture

10 m²/g.

What is the internal surface area shown in the lecture

800 m²/g.

What types of charges occur on soil colloids

Positive and negative charges.

Which charge dominates most soil colloids

Negative charge.

What do negative charges attract

Positively charged ions called cations.

What do positive charges attract

Negatively charged ions called anions.

What is electroneutrality

Balance between positive and negative electrical charges.

What is the ionic double layer

A layer of ions attracted to the charged surface of a soil colloid.

What does the ionic double layer also contain

Water.

What are the four major soil colloid types

Layer silicate clays, iron and aluminum oxides, amorphous clay/allophane, and humus.

What are layer silicate clays also called

Phyllosilicates.

What are phyllosilicates made of

Crystalline layers stacked together.

What elements occur in phyllosilicate minerals

Si, Al, Mg, Fe, O and H.

What are the two basic sheets of phyllosilicates

Tetrahedral and octahedral sheets.

Which element is central to a typical silica tetrahedron

Silicon.

What is the shape of a silica tetrahedron

Four oxygen atoms arranged around silicon.

Which metals commonly occupy octahedral positions

Aluminum and magnesium.

What does 1:1 clay mean

One tetrahedral sheet and one octahedral sheet.

What are examples of 1:1 clays

Kaolinite and halloysite.

What holds kaolinite layers together

Hydrogen bonding.

Does kaolinite expand significantly

No.

Does kaolinite have much isomorphic substitution

No, very little.

What is the specific surface area of kaolinite

10–30 m²/g.

What does 2:1 clay mean

One octahedral sheet between two tetrahedral sheets.

What are three important 2:1 clay minerals

Illite, smectite and vermiculite.

What is isomorphic substitution

Replacement of one ion by another of similar size during crystal formation.

Why can ions undergo isomorphic substitution

They have similar ionic radii.

Which ion can replace Al³⁺ in an octahedral sheet

Mg²⁺.

Which ion can replace Si⁴⁺ in a tetrahedral sheet

Al³⁺.

What type of charge is produced by isomorphic substitution

Permanent negative charge.

Is permanent charge strongly dependent on soil pH

No.

Is illite expanding or non-expanding

Non-expanding.

What holds illite sheets together

Potassium ions (K⁺).

Where is much of the isomorphic charge of illite located

In tetrahedral sheets.

What is the relationship between smectite and montmorillonite

Montmorillonite is a mineral in the smectite group.

Does smectite expand

Yes.

Which clay has very high shrink-swell capacity

Smectite.

Why can smectite swell

Water and exchangeable cations enter spaces between layers.

What ions may occur between smectite sheets

K⁺ and Ca²⁺, along with water.

What is the specific surface area of smectite in the lecture

700–800 m²/g.

Is smectite cohesive

Yes, very cohesive.

Does vermiculite expand as freely as smectite

No, expansion is more limited.

What ions can occur between vermiculite sheets

Mg²⁺, K⁺ and Ca²⁺.

Which substitution is important in vermiculite tetrahedral sheets

Al³⁺ replacing Si⁴⁺.

Where are iron and aluminum oxides commonly found

Highly weathered soils, including Ultisols and Oxisols.

What is an example of an aluminum oxide mineral

Gibbsite.

What is the formula for gibbsite shown in the notes

Al₂O₃·3H₂O.

Is the charge of iron and aluminum oxides constant

No, it depends on pH.

What is allophane associated with

Volcanic soils.

Is allophane a crystalline layer silicate

No, it is poorly crystalline or amorphous.

What is humus

The organic colloidal fraction of soil.

What elements are major components of humus

Carbon, hydrogen and oxygen.

Does humus charge change with pH

Yes.

What are the two major sources of negative colloid charge

Isomorphic substitution and pH-dependent charge.

Where does pH-dependent charge occur

Clay edges, iron and aluminum oxides, and organic matter.

What happens to positive surface charge when pH increases

It decreases.

What happens to negative surface charge when pH increases

It increases.

What happens to positive surface charge when pH decreases

It increases.

What happens to negative surface charge when pH decreases

It decreases.

How can an OH group develop negative charge

By releasing H⁺ and becoming O⁻.

Which organic functional groups can develop negative charge

Carboxyl and phenolic hydroxyl groups.

Why are exchange sites important for plant nutrition

They act as nutrient reservoirs.

What is the most important function of colloids in these lecture notes

Providing sites for ion exchange reactions.