AGR405 SOIL FERTILITY MANAGEMENT
FINAL EXAMINATION
Questions and Answers Verified Solutions 2026/2027
1. What primary soil property determines the cation exchange
capacity (CEC) of a soil matrix?
A. Sand particle diameter
B. Amount and type of clay minerals and soil organic matter (humus)
C. Subsoil compaction bulk density
D. Proportion of macro-pores occupied by air
Rationale: Cation exchange capacity is driven by negative surface charges on clay
minerals (like montmorillonite) and decomposed organic matter (humus).
2. What chemical form of nitrogen is predominantly absorbed by plant
roots in well-aerated, agricultural soils?
A. Nitrite (NO2-)
B. Nitrate (NO3-)
C. Atmospheric nitrogen gas (N2)
D. Elemental nitrogen (N0)
Rationale: In aerobic soils, nitrifying bacteria convert ammonium into nitrate (NO3-),
which is highly mobile and readily absorbed by roots.
3. What process converts organic nitrogen compounds in soil crop
residues into inorganic ammonium (NH4+)?
A. Nitrification
B. Ammonification (Mineralization)
C. Denitrification
D. Volatilization
Rationale: Heterotrophic soil microbes break down complex organic proteins and amino
acids into inorganic ammonium ions through ammonification/mineralization.
4. Which environmental condition accelerates gaseous ammonia
volatilization from surface-applied urea fertilizer?
A. Cool soil temperatures below 5 °C and soil pH below 6.0
B. High soil pH (above 7.5), warm temperatures, surface crop residue, and moist soil
drying with wind
C. Saturated, anaerobic subsoil conditions
D. Deep injection of urea 6 inches below the soil surface
Rationale: Urea hydrolysis yields ammonium carbonate, raising local pH; in warm,
alkaline, surface-moist soils, ammonium rapidly converts to volatile ammonia gas (NH3).
,5. What bacterial enzyme catalyzes the reduction of nitrate (NO3-) to
gaseous nitrogen oxides and N2 under anaerobic, saturated soil
conditions?
A. Nitrogenase
B. Nitrate reductase and denitrification enzymes
C. Urease
D. Phosphatase
Rationale: Facultative anaerobic bacteria use nitrate as a terminal electron acceptor
when oxygen is depleted, reducing NO3- to N2O and N2 gas (denitrification).
6. Which primary plant nutrient is considered highly immobile in soil,
moving toward root surfaces predominantly via slow diffusion?
A. Nitrogen (as NO3-)
B. Phosphorus (as H2PO4- or HPO42-)
C. Sulfur (as SO42-)
D. Chlorine (as Cl-)
Rationale: Phosphate ions bind strongly to soil minerals (iron, aluminum, calcium),
resulting in low soil solution concentrations; root uptake relies on diffusion over short
distances.
7. At what soil pH range is orthophosphate phosphorus most
available for plant uptake in mineral soils?
A. Extremely acidic pH (4.0 to 5.0)
B. Slightly acidic to near neutral pH (6.0 to 7.0)
C. Strongly alkaline pH (8.2 to 9.0)
D. pH range does not impact phosphorus availability
Rationale: Phosphorus availability peaks between pH 6.0 and 7.0; below 6.0 it
precipitates with Fe/Al, and above 7.0 it precipitates with Ca.
8. What soil chemical reaction causes phosphorus fixation in strongly
acidic soils (pH < 5.5)?
A. Precipitation with soluble calcium ions to form hydroxyapatite
B. Adsorption and fixation by soluble aluminum (Al3+) and iron (Fe3+) oxides
C. Volatilization as phosphine gas
D. Leaching below the root zone as free phosphate
Rationale: Acidic soils contain soluble Al and Fe cations that react with orthophosphate
ions to form highly insoluble aluminum and iron phosphate minerals.
9. What essential nutrient is a key constituent of the amino acids
cysteine, methionine, and cystine?
A. Phosphorus
B. Sulfur
C. Potassium
, D. Magnesium
Rationale: Sulfur is a structural component of sulfur-containing amino acids required for
plant protein synthesis.
10. Which soil Amendment material is standard for raising soil pH and
neutralizing soil acidity?
A. Gypsum (CaSO4·2H2O)
B. Agricultural Limestone (Calcium Carbonate, CaCO3)
C. Elemental Sulfur (S0)
D. Ammonium Sulfate ((NH4)2SO4)
Rationale: Agricultural limestone neutralizes soil acidity as carbonate ions (CO32-) react
with H+ ions, forming water and carbon dioxide.
11. How does dolomitic limestone differ from calcitic limestone?
A. Dolomitic lime contains pure calcium oxide without carbonate
B. Dolomitic limestone contains significant magnesium carbonate (CaMg(CO3)2)
alongside calcium carbonate
C. Dolomitic lime lowers soil pH rather than raising it
D. Calcitic lime contains sulfur additives
Rationale: Dolomitic limestone provides both magnesium (Mg) and calcium (Ca),
whereas calcitic lime supplies predominantly calcium carbonate.
12. What primary symptom identifies severe potassium (K) deficiency
in corn leaves?
A. Interveinal chlorosis on the youngest leaves
B. Yellowing and necrosis along the outer leaf margins of older, lower leaves
C. Dark purple discoloration across the leaf midrib
D. Whitening of shoot growing points
Rationale: Potassium is mobile in plants; deficiency causes translocation from older
leaves, leading to marginal leaf scorch on lower foliage.
13. What micronutrient deficiency causes "rosetting" and stunted
terminal leaf growth in fruit trees and corn ("white bud")?
A. Manganese
B. Zinc
C. Molybdenum
D. Iron
Rationale: Zinc is essential for auxin synthesis (indole-3-acetic acid); deficiency
suppresses stem internode elongation, resulting in rosetted leaves.
14. Which secondary nutrient is a key component of middle lamella
cell walls as calcium pectate?
A. Calcium
B. Potassium
FINAL EXAMINATION
Questions and Answers Verified Solutions 2026/2027
1. What primary soil property determines the cation exchange
capacity (CEC) of a soil matrix?
A. Sand particle diameter
B. Amount and type of clay minerals and soil organic matter (humus)
C. Subsoil compaction bulk density
D. Proportion of macro-pores occupied by air
Rationale: Cation exchange capacity is driven by negative surface charges on clay
minerals (like montmorillonite) and decomposed organic matter (humus).
2. What chemical form of nitrogen is predominantly absorbed by plant
roots in well-aerated, agricultural soils?
A. Nitrite (NO2-)
B. Nitrate (NO3-)
C. Atmospheric nitrogen gas (N2)
D. Elemental nitrogen (N0)
Rationale: In aerobic soils, nitrifying bacteria convert ammonium into nitrate (NO3-),
which is highly mobile and readily absorbed by roots.
3. What process converts organic nitrogen compounds in soil crop
residues into inorganic ammonium (NH4+)?
A. Nitrification
B. Ammonification (Mineralization)
C. Denitrification
D. Volatilization
Rationale: Heterotrophic soil microbes break down complex organic proteins and amino
acids into inorganic ammonium ions through ammonification/mineralization.
4. Which environmental condition accelerates gaseous ammonia
volatilization from surface-applied urea fertilizer?
A. Cool soil temperatures below 5 °C and soil pH below 6.0
B. High soil pH (above 7.5), warm temperatures, surface crop residue, and moist soil
drying with wind
C. Saturated, anaerobic subsoil conditions
D. Deep injection of urea 6 inches below the soil surface
Rationale: Urea hydrolysis yields ammonium carbonate, raising local pH; in warm,
alkaline, surface-moist soils, ammonium rapidly converts to volatile ammonia gas (NH3).
,5. What bacterial enzyme catalyzes the reduction of nitrate (NO3-) to
gaseous nitrogen oxides and N2 under anaerobic, saturated soil
conditions?
A. Nitrogenase
B. Nitrate reductase and denitrification enzymes
C. Urease
D. Phosphatase
Rationale: Facultative anaerobic bacteria use nitrate as a terminal electron acceptor
when oxygen is depleted, reducing NO3- to N2O and N2 gas (denitrification).
6. Which primary plant nutrient is considered highly immobile in soil,
moving toward root surfaces predominantly via slow diffusion?
A. Nitrogen (as NO3-)
B. Phosphorus (as H2PO4- or HPO42-)
C. Sulfur (as SO42-)
D. Chlorine (as Cl-)
Rationale: Phosphate ions bind strongly to soil minerals (iron, aluminum, calcium),
resulting in low soil solution concentrations; root uptake relies on diffusion over short
distances.
7. At what soil pH range is orthophosphate phosphorus most
available for plant uptake in mineral soils?
A. Extremely acidic pH (4.0 to 5.0)
B. Slightly acidic to near neutral pH (6.0 to 7.0)
C. Strongly alkaline pH (8.2 to 9.0)
D. pH range does not impact phosphorus availability
Rationale: Phosphorus availability peaks between pH 6.0 and 7.0; below 6.0 it
precipitates with Fe/Al, and above 7.0 it precipitates with Ca.
8. What soil chemical reaction causes phosphorus fixation in strongly
acidic soils (pH < 5.5)?
A. Precipitation with soluble calcium ions to form hydroxyapatite
B. Adsorption and fixation by soluble aluminum (Al3+) and iron (Fe3+) oxides
C. Volatilization as phosphine gas
D. Leaching below the root zone as free phosphate
Rationale: Acidic soils contain soluble Al and Fe cations that react with orthophosphate
ions to form highly insoluble aluminum and iron phosphate minerals.
9. What essential nutrient is a key constituent of the amino acids
cysteine, methionine, and cystine?
A. Phosphorus
B. Sulfur
C. Potassium
, D. Magnesium
Rationale: Sulfur is a structural component of sulfur-containing amino acids required for
plant protein synthesis.
10. Which soil Amendment material is standard for raising soil pH and
neutralizing soil acidity?
A. Gypsum (CaSO4·2H2O)
B. Agricultural Limestone (Calcium Carbonate, CaCO3)
C. Elemental Sulfur (S0)
D. Ammonium Sulfate ((NH4)2SO4)
Rationale: Agricultural limestone neutralizes soil acidity as carbonate ions (CO32-) react
with H+ ions, forming water and carbon dioxide.
11. How does dolomitic limestone differ from calcitic limestone?
A. Dolomitic lime contains pure calcium oxide without carbonate
B. Dolomitic limestone contains significant magnesium carbonate (CaMg(CO3)2)
alongside calcium carbonate
C. Dolomitic lime lowers soil pH rather than raising it
D. Calcitic lime contains sulfur additives
Rationale: Dolomitic limestone provides both magnesium (Mg) and calcium (Ca),
whereas calcitic lime supplies predominantly calcium carbonate.
12. What primary symptom identifies severe potassium (K) deficiency
in corn leaves?
A. Interveinal chlorosis on the youngest leaves
B. Yellowing and necrosis along the outer leaf margins of older, lower leaves
C. Dark purple discoloration across the leaf midrib
D. Whitening of shoot growing points
Rationale: Potassium is mobile in plants; deficiency causes translocation from older
leaves, leading to marginal leaf scorch on lower foliage.
13. What micronutrient deficiency causes "rosetting" and stunted
terminal leaf growth in fruit trees and corn ("white bud")?
A. Manganese
B. Zinc
C. Molybdenum
D. Iron
Rationale: Zinc is essential for auxin synthesis (indole-3-acetic acid); deficiency
suppresses stem internode elongation, resulting in rosetted leaves.
14. Which secondary nutrient is a key component of middle lamella
cell walls as calcium pectate?
A. Calcium
B. Potassium
