PART TREE

What is a soil colloid

An extremely small soil particle important for surface reactions and nutrient exchange.

Why are soil colloids chemically active

They have large specific surface area and may carry electrical surface charges.

What does specific surface area mean

The amount of exposed surface area per unit mass.

EXAM STYLE — List the four major types of soil colloids

Layer silicate clays, iron and aluminum oxides, allophane, and humus.

Which soil colloid category includes kaolinite and smectite

Layer silicate clays.

Which soil colloid category consists of relatively stable organic material

Humus.

Which soil colloid category includes iron and aluminum oxide minerals

Fe and Al oxides.

What is allophane

A poorly crystalline aluminosilicate soil material.

EXAM STYLE — Which statement is FALSE about soil colloids: A They have reactive surfaces; B They can participate in ion exchange; C All are dominated by positive charge; D Some contain clay minerals

C — Colloids are not all dominated by positive charge.

EXAM STYLE — Which old-exam answer described a common soil colloid size category

Particles smaller than approximately 2 micrometers in the class question — this is also the clay-size cutoff, not a universal strict colloid definition.

What is a cation

A positively charged ion.

What is an anion

A negatively charged ion.

Is Ca²⁺ a cation or anion

Cation with charge +2.

Is Mg²⁺ a cation or anion

Cation with charge +2.

Is K⁺ a cation or anion

Cation with charge +1.

Is Na⁺ a cation or anion

Cation with charge +1.

Is NO₃⁻ a cation or anion

Anion with charge -1.

EXAM STYLE — What do negatively charged soil colloids attract: A Cations; B Only anions; C Only air; D Only gravel

A — Cations.

What is adsorption

Attachment of a substance to a surface.

What is absorption

Movement of a substance into the interior of a material.

NEW PRACTICE — Calcium held on a clay surface is an example of adsorption or absorption

Adsorption.

What is a tetrahedral sheet

A clay mineral sheet based on tetrahedral units, mainly containing silicon and oxygen.

What is an octahedral sheet

A clay mineral sheet containing ions such as aluminum or magnesium in octahedral coordination.

What does T represent in a clay mineral diagram

Tetrahedral sheet.

What does O represent in a clay mineral diagram

Octahedral sheet.

EXAM STYLE — A 2:1 clay mineral has how many octahedral sheets

One.

EXAM STYLE — A 2:1 clay mineral has how many tetrahedral sheets

Two.

What is the structure of a 2:1 clay mineral

Tetrahedral, octahedral, tetrahedral or T-O-T.

What is the structure of a 1:1 clay mineral

One tetrahedral and one octahedral sheet or T-O.

Which mineral is a classic 1:1 clay

Kaolinite.

Which minerals are common 2:1 clays

Smectite, vermiculite, and illite.

What type of clay structure does chlorite have

A 2:1:1 structure with an additional hydroxide interlayer.

EXAM STYLE — Which clay is 1:1: A Kaolinite; B Vermiculite; C Montmorillonite; D Illite

A — Kaolinite.

EXAM STYLE — Which clay mineral has strong expansion: A Kaolinite; B Montmorillonite; C Illite; D Chlorite

B — Montmorillonite or smectite.

EXAM STYLE — Which combination has relatively limited expansion: A Kaolinite and vermiculite; B Kaolinite and montmorillonite; C Montmorillonite and smectite; D Humus and smectite

A — Kaolinite and vermiculite, as compared in the old exam.

Why does smectite or montmorillonite swell

Water can enter between silicate layers and increase their separation.

Why does illite have limited swelling

Potassium in the interlayer helps hold the layers together.

Why does kaolinite have limited swelling

Its 1:1 layers are tightly held together.

Why is vermiculite described as having more limited expansion than smectite

Its higher layer charge and interlayer interactions restrict expansion compared with smectite.

EXAM STYLE — What feature can allow expanding 2:1 clays to hold water and cations

Interlayer spaces between silicate layers.

True or False: All 2:1 clay minerals swell equally when wet

False.

True or False: Kaolinite is a strongly expanding 2:1 clay

False — kaolinite is a 1:1 mineral with little expansion.

True or False: Smectite is associated with swelling when wet and shrinking when dry

True.

What is isomorphic substitution

Replacement of one ion by another of similar ionic size within a mineral crystal.

EXAM STYLE — Isomorphic substitution occurs when ions have similar what

Ionic radii or ionic sizes.

What is ionic radius

A measure of the effective size of an ion.

EXAM STYLE — Can Mg²⁺ replace Al³⁺ in an octahedral sheet of montmorillonite

Yes.

EXAM STYLE — Can Al³⁺ replace Si⁴⁺ in a tetrahedral sheet of a silicate clay

Yes.

What happens to structural charge when Mg²⁺ replaces Al³⁺

A one-unit negative charge difference develops.

What happens to structural charge when Al³⁺ replaces Si⁴⁺

A one-unit negative charge difference develops.

NEW PRACTICE — An ion with charge +2 replaces one with charge +3. What is the charge difference

-1.

NEW PRACTICE — An ion with charge +3 replaces one with charge +4. What is the charge difference

-1.

What is permanent structural charge

Charge created within the crystal structure, often by isomorphic substitution.

What is variable charge

Surface charge that changes with conditions such as pH.

What happens to many variable-charge surfaces as soil pH increases

They become more negatively charged.

Why can increasing pH produce more negative surface charge

Surface groups lose positively charged hydrogen ions.

EXAM STYLE — Which crystal-form soil colloid has largely pH-dependent negative surface charge among kaolinite, smectite, Fe/Al oxides, and humus

Kaolinite — the expected clay-mineral comparison in the old exam.

EXAM STYLE — True or False: Permanent structural charge of vermiculite must increase whenever soil pH increases

False — permanent charge is approximately pH-independent, although variable edge charge can change.

EXAM STYLE — True or False: Variable negative edge charge may increase as soil pH increases

True.

What does soil pH describe

How acidic or alkaline the soil solution is.

What does a soil pH below 7 indicate

Acidic conditions.

What does a soil pH above 7 indicate

Alkaline conditions.

What does a soil pH of 7 indicate

Neutral conditions.

NEW PRACTICE — Soil pH increases from 5 to 6. Does the soil become more or less acidic

Less acidic.

NEW PRACTICE — Soil pH decreases from 7 to 5. Does the soil become more or less acidic

More acidic.

True or False: Increasing pH always makes soil more acidic

False.

What is humus

Relatively stable transformed organic material in soil.

Why can humus contribute a large amount of CEC

It contains many functional groups capable of carrying negative charge.

EXAM STYLE — Name three functional groups that affect humus negative charge

Carboxyl, phenolic, and enolic groups.

What does CPE stand for in the humus memory trick

Carboxyl, phenolic, enolic.

What is the chemical notation for a carboxyl group

-COOH.

What is the chemical notation for a hydroxyl group

-OH.

What happens when a carboxyl group loses H⁺

-COOH becomes -COO⁻, producing a negative site.

True or False: Humus can develop more negative charge when its acidic functional groups lose H⁺

True.

What does CEC stand for

Cation Exchange Capacity.

What does CEC measure

The capacity of soil to hold exchangeable positively charged ions.

Why is CEC important for crop nutrition

It helps soil retain and exchange plant nutrient cations.

True or False: High CEC automatically guarantees that a soil contains large quantities of all essential nutrients

False — CEC is capacity, not actual nutrient concentration.

Which generally has the highest CEC among humus, kaolinite, illite, and quartz

Humus.

Which mineral generally has higher CEC, vermiculite or kaolinite

Vermiculite.

Which mineral generally has higher CEC, smectite or kaolinite

Smectite.

What is a useful approximate CEC ranking from highest to lowest

Humus, vermiculite, smectite, illite, kaolinite.

NEW PRACTICE — Which soil material generally has the lowest CEC among vermiculite, smectite, illite, and kaolinite

Kaolinite.

What is a cation exchange reaction

Replacement of one positively charged ion at an exchange site by another cation.

What is one common unit for CEC

cmol(+) per kg of soil.

What is an equivalent older unit for CEC

meq per 100 g of soil.

How many meq per 100 g equal 1 cmol(+) per kg

1.

Why does one Ca²⁺ ion balance twice as much negative charge as one K⁺ ion

Ca²⁺ carries two positive charges while K⁺ carries one.

NEW PRACTICE — How many Ca²⁺ ions balance the same negative charge as 12 K⁺ ions

6 Ca²⁺ ions.

What is base saturation

The percentage of total exchange capacity occupied by exchangeable base cations.

Which cations are commonly counted in base saturation

Ca²⁺, Mg²⁺, K⁺, and Na⁺.

What is the formula for base saturation

Total exchangeable base-cation charge divided by CEC times 100.

NEW PRACTICE — CEC is 20 and base cations total 15 in the same charge units. What is base saturation

75 percent.

NEW PRACTICE — CEC is 25 and base cations total 10 in the same charge units. What is base saturation

40 percent.

True or False: Positively charged nutrient ions may replace each other on negatively charged exchange sites

True.

True or False: Adsorption and absorption mean the same thing

False — adsorption is on a surface, while absorption involves entering the material.

EXAM STYLE — An old short-answer question described inorganic substances as having charges and excluding carbon. What distinction should you remember

That was simplified exam wording; inorganic substances can also contain carbon, as in calcium carbonate.