Theme 3

Question 1

What is the most abundant element in soil by weight?

Answer 1

Oxygen (O): ~47% by weight

Oxygen forms oxides and silicates.

Question 2

What is the second most abundant element in soil?

Answer 2

Silicon (Si): ~28%

Silicon is the backbone of silicate minerals.

Question 3

Which element is common in clay minerals?

Answer 3

Aluminum (Al): ~8%

Aluminum is a key component of clay minerals.

Question 4

What role does Iron (Fe) play in soil?

Answer 4

Important for redox reactions and soil color

Iron constitutes about 5% of soil composition.

Question 5

Which elements are essential nutrients in soil?

Answer 5

Calcium (Ca), Magnesium (Mg), Sodium (Na), Potassium (K)

These elements are vital for plant nutrition.

Question 6

How does the elemental composition of soil compare to the Earth’s crust?

Answer 6

Soil is similar but enriched in carbon and organic matter

The Earth’s crust is dominated by O, Si, Al, Fe, Ca, Na, K, Mg.

Question 7

What is the significance of Silicon (Si⁴⁺) in soil minerals?

Answer 7

Forms strong bonds with oxygen, creating stable silicate minerals

Examples include quartz and feldspar.

Question 8

What are the two forms of Oxygen relevant in soil systems?

Answer 8

Molecular Oxygen (O₂) and Oxide Ions (O²⁻)

O₂ is essential for respiration, while O²⁻ forms oxides.

Question 9

What role does molecular oxygen (O₂) play in soil?

Answer 9

Supports aerobic microbes and root respiration

O₂ is crucial for soil respiration and nutrient availability.

Question 10

What impact does photosynthesis have on soil oxygen levels?

Answer 10

Produces O₂, driving oxidation processes in soil

Plants and microbes use O₂ for respiration.

Question 11

What are primary minerals?

Answer 11

Formed from cooling magma (igneous rocks)

Examples include quartz, feldspar, and mica.

Question 12

What defines secondary minerals?

Answer 12

Formed by chemical weathering of primary minerals

Examples include clay minerals and iron oxides.

Question 13

What are the processes involved in weathering?

Answer 13

Physical, Chemical, Biological

Weathering breaks down rocks through various mechanisms.

Question 14

What is ‘neoformation’ in the context of soil minerals?

Answer 14

New minerals form from dissolved ions

An example is the formation of clay minerals.

Question 15

What distinguishes crystalline materials from amorphous materials?

Answer 15

Crystalline materials have an ordered atomic structure, while amorphous materials do not

Examples of amorphous materials include allophane and ferrihydrite.

Question 16

What is the role of amorphous materials in soil?

Answer 16

High surface area allows nutrient adsorption and pollutant removal

They stabilize organic matter and enhance carbon storage.

Question 17

What is the coordination number of Si⁴⁺?

Answer 17

4

Si⁴⁺ bonds with 4 oxygen ions to form a tetrahedral arrangement.

Question 18

What is a silicon tetrahedron?

Answer 18

The basic building block of silicate minerals

It consists of 1 Si⁴⁺ ion at the center and 4 O²⁻ ions at the corners.

Question 19

What types of silicate structures exist?

Answer 19

Nesosilicates, Inosilicates, Tectosilicates, Phyllosilicates

Each type has different bonding and weathering resistance properties.

Question 20

Which silicate group has the weakest resistance to weathering?

Answer 20

Nesosilicates (e.g., olivine)

They consist of isolated tetrahedra.

Question 21

What is the significance of di-octahedral and tri-octahedral sheets in clay minerals?

Answer 21

Di-octahedral sheets have two-thirds Al³⁺ occupancy, while tri-octahedral sheets have complete occupancy

This affects stability and charge balance.

Question 22

What is isomorphous substitution in clay minerals?

Answer 22

When ions of similar size but different charge substitute for Si⁴⁺ or Al³⁺

This increases the cation exchange capacity (CEC) and reactivity.

Question 23

What are 1:1 phyllosilicates?

Answer 23

Consist of one tetrahedral sheet bonded to one octahedral sheet

An example is kaolinite.

Question 24

What are 2:1 phyllosilicates?

Answer 24

Consist of two tetrahedral sheets sandwiching one octahedral sheet

Examples include smectite and vermiculite.

Question 25

How does temperature affect weathering?

Answer 25

Higher temperatures accelerate weathering ## Footnote Temperature is a key environmental factor influencing weathering rates.

Question 26

What are the main components of an octahedral aluminium complex?

Answer 26

Al³⁺ ion and 6 ligands (O²⁻ or OH⁻) ## Footnote This arrangement creates stable octahedral sheets in clay minerals.

Question 27

What forms the structural arrangement of magnesium octahedra?

Answer 27

Mg²⁺ ion surrounded by 6 ligands (O²⁻ or OH⁻) ## Footnote This forms stable octahedral layers in clay minerals.

Question 28

What are 1:1 minerals?

Answer 28

No interlayer space (e.g., kaolinite) ## Footnote 1:1 minerals consist of one tetrahedral layer and one octahedral layer.

Question 29

What are 2:1 minerals?

Answer 29

Interlayer space can contain water, cations, or organic molecules ## Footnote 2:1 minerals consist of two tetrahedral layers and one octahedral layer.

Question 30

What causes swelling in clay minerals?

Answer 30

Occurs in smectite due to water entering the interlayer ## Footnote Swelling can affect soil structure and water retention.

Question 31

What is Cation Exchange Capacity (CEC)?

Answer 31

Determined by the ability of interlayer cations to exchange with soil solution ## Footnote CEC is crucial for nutrient availability in soils.

Question 32

What characterizes Di-Octahedral clay minerals?

Answer 32

Two-thirds of the octahedral sites are occupied by Al³⁺ (e.g., kaolinite, smectite) ## Footnote Di-octahedral minerals typically have a lower charge density.

Question 33

What characterizes Tri-Octahedral clay minerals?

Answer 33

All octahedral sites are occupied by Mg²⁺ (e.g., talc, chlorite) ## Footnote Tri-octahedral minerals generally have higher charge density.

Question 34

How do clay minerals impact soil texture?

Answer 34

Contribute to fine soil texture, improving water and nutrient retention ## Footnote Fine texture is essential for agricultural productivity.

Question 35

Which clay minerals have high water retention?

Answer 35

High in smectite and vermiculite due to swelling and interlayer water ## Footnote These minerals can significantly influence irrigation needs.

Question 36

How does clay mineral CEC influence soil fertility?

Answer 36

High CEC in 2:1 minerals enhances nutrient availability ## Footnote Nutrient availability is crucial for plant growth.

Question 37

What role do clay minerals play in soil aggregation?

Answer 37

Bind soil particles into aggregates, improving structure and stability ## Footnote Stable aggregates reduce erosion and enhance root penetration.

Question 38

How do clay minerals interact with organic matter?

Answer 38

Stabilize organic matter, enhancing carbon sequestration ## Footnote This is vital for soil health and climate regulation.

Question 39

What is the effect of weathering on clay mineral distribution?

Answer 39

Low weathering: Primary minerals dominate; Moderate weathering: 2:1 clay minerals form; High weathering: 1:1 clay minerals dominate ## Footnote The type of clay can indicate soil health and fertility.

Question 40

What types of clay minerals dominate arid and semi-arid environments?

Answer 40

Smectite and illite dominate due to limited leaching ## Footnote These minerals are adapted to drier conditions.

Question 41

What types of clay minerals dominate in humid environments?

Answer 41

Kaolinite dominates due to intense leaching and weathering ## Footnote This reflects the high nutrient loss in such conditions.

Question 42

What is soil surface charge?

Answer 42

Electrical charges on the surfaces of soil particles, both organic and inorganic ## Footnote Surface charge is crucial for nutrient retention.

Question 43

What causes inorganic soil components to carry charge?

Answer 43

Develop charge due to isomorphous substitution and protonation/deprotonation reactions ## Footnote This affects the soil's ability to retain nutrients.

Question 44

What is the difference between permanent and variable charge in soils?

Answer 44

Permanent charge is constant and arises from isomorphous substitution; Variable charge changes with pH due to ionization of functional groups ## Footnote Understanding this is key to managing soil fertility.

Question 45

How does low pH affect soil surface charge?

Answer 45

Surfaces are protonated, leading to positive charge ## Footnote This can affect nutrient availability.

Question 46

How does high pH affect soil surface charge?

Answer 46

Surfaces deprotonate, leading to negative charge ## Footnote This change can influence soil chemistry.

Question 47

What characterizes highly weathered soils?

Answer 47

Contain more variable charge minerals, low CEC due to loss of permanent charge minerals ## Footnote Common in tropical and subtropical regions.

Question 48

What are the characteristics of soils in Highveld, South Africa?

Answer 48

Moderately weathered soils with mixed mineralogy, moderate permanent and variable charge ## Footnote Soil management practices must consider these characteristics.

Question 49

What are the characteristics of soils in Pretoria, South Africa?

Answer 49

Highly weathered soils with kaolinite and iron oxides, dominated by variable charge ## Footnote These soils require specific management strategies.