WorksheetsSoil Chemistry Compre 6
Total questions: 83
Worksheet time: 42mins
Which component is considered the most chemically active portion of the soil due to its surface area and structure?
Sand fraction
Silt fraction
Soil colloids
Soil solution
Humus colloids are characterized by:
Crystalline structure with high plasticity
Convoluted carbon chains with net negative charge
High stickiness and cohesion
High bearing strength for construction
Why are Histosols unsuitable for building foundations?
They are too rich in iron oxides
They contain excessive crystalline clays
They lack bearing strength due to humus colloids
They are impermeable to water
Which type of colloid is typical of Andisols derived from volcanic ash?
Kaolinite
Allophane and imogolite
Illite
Montmorillonite
Iron and aluminum oxides in highly weathered soils usually:
Have net charges that are always negative
Contribute to high soil stickiness
Exhibit charges ranging from slightly negative to moderately positive
Prevent phosphate adsorption
The total surface area of colloids in the upper 15 cm of a hectare clay soil can reach approximately:
70 km² g⁻¹
7,000 km² g⁻¹
70,000 km² g⁻¹
700,000 km² g⁻¹
Why do colloids play a major role in cation exchange capacity (CEC)?
They are positively charged and repel anions
Their small size gives them high surface area and negative charges
They are impermeable to cations
They neutralize acids instantly
Which process describes attractive forces between similar molecules such as clay particles sticking together?
Cohesion
Adhesion
Flocculation
Dispersion
Which silicate clay group is associated with swelling and deep cracks during dry seasons?
Kaolinite
Smectite (montmorillonite)
Vermiculite
Illite
The reverse process of flocculation, where aggregates break into individual particles, is called:
Cohesion
Dispersion
Adhesion
Brownian movement
Brownian movement in soil colloids is caused by:
Earthworm activity in pores
Oscillation due to collision with water molecules
Shrinking and swelling of clays
Electrostatic repulsion of cations
Soil colloids are non-permeable because:
They swell excessively
They are larger than crystalloids and cannot pass through semi-permeable membranes
They are electrically neutral
They dissolve completely in soil solution
Which source of charge in soil colloids is pH-dependent?
Isomorphous substitution
Hydroxyl ionization on broken edges
Replacement of Al³⁺ by Si⁴⁺
Substitution of Mg²⁺ in octahedral sites
The permanent negative charge in clays arises primarily from:
Ionization of organic functional groups
Substitution of cations of different valence in crystal lattices
Adsorption of OH⁻ groups on edges
Root exudates
In the ionic double layer of clay micelles, the colloidal particle acts as:
A positively charged surface
A large anion attracting cations
A neutral adsorbent of both anions and cations
A site for hydrogen bonding only
Which statement best describes humus colloids?
They are cohesive and plastic
They exhibit high positive charges in acidic soils
They have net negative charge and very high CEC
They form crystalline lattices with silica
Which soil order is typically associated with humus-dominated colloids?
Ultisols
Andisols
Histosols
Vertisols
When colloidal charges are neutralized, particles settle as aggregates. This phenomenon is called:
Cohesion
Deflocculation
Flocculation
Brownian movement
The colloidal fraction of soil is considered the most reactive because of:
Its mineral parent material origin
Its large surface area and electric charges
Its coarse particle size
Its crystalline stability
Organic colloids differ from inorganic colloids mainly in that they:
Are crystalline and ordered
Are amorphous and rich in functional groups
Have no influence on soil fertility
Do not contribute to CEC
Which type of inorganic colloid is formed from volcanic ash and commonly found in Andisols?
Vermiculite
Allophane
Illite
Kaolinite
Non-crystalline minerals are prevented from forming stable crystals because:
They lack silica
Soluble silica and organic matter inhibit crystallization
They have very low CEC
They undergo isomorphous substitution
Crystalline silicate minerals occur primarily as:
Primary minerals in sand fraction
Weathering products in the clay fraction
Non-cohesive humus particles
Amorphous iron oxides
What is the basic structural unit of silicate clays?
Alumina-octahedron and silica-tetrahedron
Ferrihydrite crystal
Humus complex
Calcium carbonate crystal
In the silica-tetrahedron, the net charge is:
+4
–2
–4
0
The tetrahedral sheet in silicate clays is made up of:
Aluminum ions coordinated with six oxygens
Silica tetrahedra linked side by side
Iron oxides bound with hydroxyl groups
Chains of humic substances
In the alumina-octahedron, Al³⁺ is coordinated with:
Three silicon ions
Eight oxygens/hydroxyl groups
Four silicon tetrahedra
Six oxygens or hydroxyls
The 1:1 type clay such as kaolinite is characterized by:
Weak bonds between layers allowing expansion
H-bonding that firmly holds layers together
Net positive charge due to Fe substitution
High shrink-swell capacity
The expanding 2:1 clay (montmorillonite) differs from kaolinite because:
It contains K+ bridges holding layers tightly
Its layers expand and contract with water
It has zero internal surface area
It is non-plastic and amorphous
Which clay type has a non-expanding 2:1 structure due to K+ bridging layers?
Illite
Montmorillonite
Vermiculite
Kaolinite
Vermiculite is considered a 2:1 clay with limited expansion because:
It has K+ bridges holding sheets tightly
Most interlayer K+ is removed, leaving space for hydration
It is a crystalline oxide mineral
It is dominated by organic functional groups
Isomorphous substitution occurs when:
An ion of higher valence replaces one of lower valence
An ion of lower valence replaces one of similar size but higher valence
Hydrogen ions neutralize hydroxyl groups
Organic matter contributes to functional group ionization
Which substitution commonly causes permanent charge in clays?
Si⁴⁺ replaced by Al³⁺ in tetrahedral sites
Ca²⁺ replaced by Mg²⁺ in solution
K⁺ replaced by Na⁺ in interlayer space
OH⁻ replaced by Cl⁻ in edge sites
Which source of charge is pH-dependent?
Isomorphous substitution
Ionization of phenolic and carboxylic radicals in OM
Substitution of Mg²⁺ for Al³⁺
Replacement of Si by Al in illite
According to the lyotropic series, which cation is most strongly held by colloids?
Na⁺
K⁺
Ca²⁺
Al³⁺
Which factor strengthens the bond between cations and clay surfaces?
Lower valence and larger hydrated size
Higher valence and smaller hydrated size
Neutral charge of organic matter
Adsorption of anions instead of cations
Cation exchange in soils primarily occurs because:
Soil colloids are positively charged
Soil colloids are negatively charged
Anions are strongly adsorbed on clays
Roots excrete organic acids only
Which property best defines Cation Exchange Capacity (CEC)?
The number of hydrogen ions in solution
The total amount of adsorbed cations per unit soil
The proportion of base cations to acidic cations
The number of negatively charged hydroxyl ions
Which is the ideal range of CEC for fertile soils?
1–5 cmol(+) kg⁻¹
10–18 cmol(+) kg⁻¹
25–40 cmol(+) kg⁻¹
50–60 cmol(+) kg⁻¹
Which type of nutrient is more prone to leaching losses due to repulsion by colloids?
Calcium
Nitrate
Potassium
Magnesium
What is the correct stoichiometric relationship for cation exchange?
1 Na⁺ replaces 2 Ca²⁺
2 Na⁺ replace 1 Ca²⁺
1 Ca²⁺ replaces 1 Na⁺
3 H⁺ replace 1 Al³⁺
Cation exchange reactions are generally:
Very slow and irreversible
Rapid, reversible, and stoichiometric
Permanent and pH-independent
Limited only to calcium and potassium
Which factors strongly influence the CEC of a soil?
Sand content and bulk density
Clay mineralogy and organic matter content
Rainfall and soil temperature
Microbial activity and soil aeration
The CEC of 2:1 clays such as montmorillonite is largely:
pH-dependent
Permanent (non-pH dependent)
Caused by organic matter ionization
Determined by leaching intensity
In contrast, the CEC of soil organic matter is:
Independent of pH
Caused by isomorphous substitution
Strongly pH-dependent
Absent in acidic soils
Calculate the CEC of a soil with exchangeable cations: Ca²⁺ = 8.0, Mg²⁺ = 4.0, K⁺ = 0.5, Na⁺ = 1.2, H⁺ = 4.0 cmol(+) kg⁻¹.
12.5
13.7
17.7
19.5
Which statement best describes the role of CEC in plant nutrition?
High CEC soils retain more cations, enhancing fertility
High CEC soils repel nutrient cations, lowering fertility
Low CEC soils always produce better yields
CEC only affects nitrogen availability
Liming increases soil fertility partly by:
Adding Na⁺ ions to soil solution
Replacing Ca²⁺ with H⁺ ions in the colloid
Displacing H⁺ and Al³⁺ with Ca²⁺ from lime
Lowering the proportion of exchangeable bases
Sodic soils can be improved when:
Calcium ions displace Na⁺ on colloids
Hydrogen ions accumulate on colloids
Potassium is replaced by sodium
Magnesium ions are replaced by Na⁺
Which statement correctly compares anions and cations in soils?
Cations are prone to leaching, while anions are tightly held
Anions like nitrate are prone to leaching, while cations are retained
Both cations and anions are strongly retained by colloids
Neither cations nor anions interact with colloids
The sum of all adsorbed cations on soil colloids is expressed in:
cmol(–) kg⁻¹
cmol(+) kg⁻¹
mg L⁻¹
ppm
Which factor is least likely to influence soil CEC?
Organic matter content
Clay content
Soil pH
Air humidity
When a soil has higher organic matter, its CEC generally:
Decreases significantly
Remains unchanged
Increases significantly
Depends only on rainfall
A soil with low CEC is best described as:
Highly fertile, nutrient-rich soil
Prone to nutrient leaching and low fertility
Resistant to acidification
Always sandy with no organic matter
Which cations are classified as basic cations in soils?
H⁺ and Al³⁺
Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺
NO₃⁻ and SO₄²⁻
Fe²⁺ and Mn²⁺
Acidic cations in soils are primarily:
Ca²⁺ and Mg²⁺
Na⁺ and K⁺
H⁺ and Al³⁺
Fe³⁺ and Zn²⁺
The formula for calculating % base saturation is:
(Acidic cations ÷ total CEC) × 100
(Basic cations ÷ total CEC) × 100
(CEC ÷ total cations) × 100
(Na⁺ ÷ total cations) × 100
A soil with % base saturation ≥ 80% is generally considered:
Highly acidic
Very fertile
Nutrient-poor
Sodic
If %BS ≤ 50%, the soil is considered:
Fertile
Non-fertile
Saline
Well-buffered
Increasing % base saturation generally:
Lowers pH and fertility
Increases pH and fertility
Decreases Ca²⁺ and Mg²⁺
Increases Al³⁺ toxicity
Which effect is associated with higher %BS?
Higher Al³⁺ toxicity
Lower nutrient buffering
Easier release of cations to plants
Stronger phosphorus fixation
Relationship of % base saturation to pH is best described as:
More H⁺ and Al³⁺ = lower pH
More Ca²⁺ and Mg²⁺ = lower pH
More Na⁺ = neutral pH
More acidic cations = neutral soil
ESP is an indicator of:
Soil acidity
Sodicity and sodium dispersion
Total anion concentration
Organic matter decomposition
The formula for ESP is:
(Exchangeable Na⁺ ÷ total CEC) × 100
(Exchangeable Ca²⁺ ÷ CEC) × 100
(Exchangeable bases ÷ acidic cations) × 100
(CEC ÷ Na⁺) × 100
A soil with ESP > 15% is classified as:
Non-sodic
Slightly sodic
Highly sodic
Fertile
What happens when ESP is high?
Soil flocculates and improves structure
Soil disperses and develops poor physical properties
Soil develops neutral pH and high CEC
Soil retains nutrients efficiently
Soils with high ESP tend to:
Form hard clods and crusts when dry
Remain loose and granular
Have high permeability
Be ideal for root growth
Which rating corresponds to ESP < 6?
Slightly sodic
Non-sodic
Moderately sodic
Highly sodic
A soil with ESP = 12% is best classified as:
Non-sodic
Slightly sodic
Moderately sodic
Highly sodic
A soil with ESP = 20% will likely show:
Good aggregation
High sodicity and dispersion in <30 minutes
Very fertile conditions
Strongly acidic conditions
Which management concern is most associated with high ESP soils?
Poor drainage and sodicity
Excess phosphorus fixation
Strong leaching of nitrates
Enhanced microbial activity
A farmer finds his soil disperses easily, with ESP > 15%. The best classification is:
Saline soil
Sodic soil
Saline-sodic soil
Neutral soil
Soil reaction or pH is defined as:
The concentration of Al³⁺ ions in soil
The acidity or alkalinity of soil solution
The number of exchangeable bases
The buffering capacity of soil
Which equation correctly represents pH?
pH = log [H⁺]
pH = –log [H⁺]
pH = log [OH⁻]
pH = –log [OH⁻]
A soil with pH 7.0 is classified as:
Acidic
Neutral
Alkaline
Buffering
Which of the following contributes to active acidity?
Exchangeable H⁺ and Al³⁺
Concentration of H⁺ in soil solution
Hydrolysis of aluminum
Carbon dioxide in soil air
Potential acidity is mainly due to:
Only H⁺ in solution
H⁺ and exchangeable Al³⁺ in colloids
Neutral salts in soil
Base saturation > 80%
Most crop plants prefer to grow in soil pH range:
3.0–4.0
4.5–5.0
5.5–7.0
8.0–9.0
Nutrient disorders occur in soils with pH below 5.0 mainly because of:
Deficiency of N and S
Toxicities of Al, Fe, Mn and deficiencies of Ca, Mg, P, Mo
Excess base saturation
Neutralization of organic acids
In soils with pH > 7.5, crops commonly suffer from:
Toxicity of Fe and Mn
Deficiency of P, K, Fe, Zn
Excess Ca and Mg
Deficiency of Al and Mn
Which parent materials commonly lead to naturally acidic soils?
Basalts and ultramafic rocks
Granites, sandstones, and shales low in bases
Limestone-rich deposits
Calcareous alluvium
Root respiration contributes to soil acidity by:
Exuding NH₄⁺ ions into soil
Releasing CO₂, which forms carbonic acid
Absorbing Al³⁺ ions
Fixing nitrogen from the atmosphere
Which farming practice most contributes to acidification over time?
Continuous irrigation
Application of acid-forming nitrogen fertilizers
Use of phosphate rock
Liming with CaCO₃
