AGR405 SOIL FERTILITY MANAGEMENT
FINAL EXAMINATION
Questions and Answers Verified Solutions 2026/2027
Course: AGR405 — Soil Fertility Management
Total Questions: 100 Multiple-Choice Questions
Instructions: Select the correct response for each question. Correct answers and rationales
are highlighted in green.
1. Which essential plant nutrient is absorbed primarily as nitrate
(NO3-) or ammonium (NH4+) and plays a crucial role in protein
synthesis?
A. Phosphorus
B. Nitrogen
C. Potassium
D. Calcium
Rationale: Nitrogen is taken up by plant roots as nitrate or ammonium ions and is a key
structural building block for amino acids, proteins, and nucleic acids.
2. What soil physical property measures the relative proportions of
sand, silt, and clay particles in a soil horizon?
A. Soil structure
B. Soil texture
C. Bulk density
D. Cation exchange capacity
Rationale: Soil texture is defined by the relative percentages of sand (2.0-0.05 mm), silt (0.05-
0.002 mm), and clay (<0.002 mm) mineral particles.
3. What chemical property quantifies a soil's total capacity to hold and
exchange exchangeable cations?
A. Base saturation percentage
B. Cation Exchange Capacity (CEC)
C. Anion retention index
D. Electrical conductivity
Rationale: CEC measures the total negative charge sites on clay minerals and organic matter
capable of binding positively charged ions (cations).
,4. Which soil pH range maximizes the plant availability of primary
macronutrients (N, P, K, S, Ca, Mg)?
A. 4.5 to 5.5
B. 6.0 to 7.0
C. 7.5 to 8.5
D. 8.5 to 9.5
Rationale: A slightly acidic to neutral pH range (6.0-7.0) provides optimal solubility and balance
for primary plant nutrient availability.
5. What bacterial process converts ammonium ions (NH4+) into nitrite
(NO2-) and subsequently nitrate (NO3-) under aerobic conditions?
A. Nitrogen fixation
B. Nitrification
C. Denitrification
D. Ammonification
Rationale: Nitrification is a biological two-step oxidation performed by Nitrosomonas and
Nitrobacter bacteria in aerobic soils.
6. What secondary macronutrient forms the structural core of the
chlorophyll molecule?
A. Calcium
B. Magnesium
C. Sulfur
D. Iron
Rationale: Magnesium occupies the central position in the porphyrin ring structure of
chlorophyll molecules, making it essential for light capture.
7. Which mechanism converts insoluble organic phosphorus into
plant-available inorganic orthophosphate forms?
A. Immobilization
B. Mineralization
C. Fixation
D. Volatilization
Rationale: Biological mineralization breaks down organic phosphorus compounds into inorganic
H2PO4- or HPO42- ions via microbial enzymatic activity.
8. What soil condition promotes gaseous nitrogen losses via
denitrification?
, A. Well-aerated sandy soil
B. Waterlogged, anaerobic soil with high organic matter
C. Dry alkaline soil
D. Frozen soil horizons
Rationale: Anaerobic conditions in saturated soils cause facultative bacteria to utilize nitrate as
an electron acceptor, reducing it to N2 and N2O gases.
9. Which liming material contains both calcium carbonate and
magnesium carbonate?
A. Calcitic limestone
B. Dolomitic limestone
C. Quicklime (CaO)
D. Hydrated lime
Rationale: Dolomitic limestone contains significant proportions of magnesium carbonate
[CaMg(CO3)2] alongside calcium carbonate.
10. What symptom is characteristic of mobile nitrogen deficiency in
crops?
A. Interveinal chlorosis on young upper leaves
B. V-shaped chlorosis progressing along the midrib of lower older leaves
C. Purpling of new terminal leaf margins
D. Marginal leaf necrosis on upper leaves
Rationale: Because nitrogen is mobile inside plant tissue, plants translocate it from mature
foliage to new growth, causing chlorosis on lower leaves first.
11. What form of phosphorus is predominantly absorbed by plant
roots in strongly acidic soils (pH < 5.5)?
A. HPO4(2-)
B. H2PO4(-)
C. PO4(3-)
D. Elemental P
Rationale: Dihydrogen phosphate (H2PO4-) is the primary orthophosphate species present and
absorbed in acidic soil solutions.
12. What primary micronutrient becomes highly toxic to plant roots in
soils with pH levels below 5.0?
A. Zinc
B. Aluminum (Al3+)
FINAL EXAMINATION
Questions and Answers Verified Solutions 2026/2027
Course: AGR405 — Soil Fertility Management
Total Questions: 100 Multiple-Choice Questions
Instructions: Select the correct response for each question. Correct answers and rationales
are highlighted in green.
1. Which essential plant nutrient is absorbed primarily as nitrate
(NO3-) or ammonium (NH4+) and plays a crucial role in protein
synthesis?
A. Phosphorus
B. Nitrogen
C. Potassium
D. Calcium
Rationale: Nitrogen is taken up by plant roots as nitrate or ammonium ions and is a key
structural building block for amino acids, proteins, and nucleic acids.
2. What soil physical property measures the relative proportions of
sand, silt, and clay particles in a soil horizon?
A. Soil structure
B. Soil texture
C. Bulk density
D. Cation exchange capacity
Rationale: Soil texture is defined by the relative percentages of sand (2.0-0.05 mm), silt (0.05-
0.002 mm), and clay (<0.002 mm) mineral particles.
3. What chemical property quantifies a soil's total capacity to hold and
exchange exchangeable cations?
A. Base saturation percentage
B. Cation Exchange Capacity (CEC)
C. Anion retention index
D. Electrical conductivity
Rationale: CEC measures the total negative charge sites on clay minerals and organic matter
capable of binding positively charged ions (cations).
,4. Which soil pH range maximizes the plant availability of primary
macronutrients (N, P, K, S, Ca, Mg)?
A. 4.5 to 5.5
B. 6.0 to 7.0
C. 7.5 to 8.5
D. 8.5 to 9.5
Rationale: A slightly acidic to neutral pH range (6.0-7.0) provides optimal solubility and balance
for primary plant nutrient availability.
5. What bacterial process converts ammonium ions (NH4+) into nitrite
(NO2-) and subsequently nitrate (NO3-) under aerobic conditions?
A. Nitrogen fixation
B. Nitrification
C. Denitrification
D. Ammonification
Rationale: Nitrification is a biological two-step oxidation performed by Nitrosomonas and
Nitrobacter bacteria in aerobic soils.
6. What secondary macronutrient forms the structural core of the
chlorophyll molecule?
A. Calcium
B. Magnesium
C. Sulfur
D. Iron
Rationale: Magnesium occupies the central position in the porphyrin ring structure of
chlorophyll molecules, making it essential for light capture.
7. Which mechanism converts insoluble organic phosphorus into
plant-available inorganic orthophosphate forms?
A. Immobilization
B. Mineralization
C. Fixation
D. Volatilization
Rationale: Biological mineralization breaks down organic phosphorus compounds into inorganic
H2PO4- or HPO42- ions via microbial enzymatic activity.
8. What soil condition promotes gaseous nitrogen losses via
denitrification?
, A. Well-aerated sandy soil
B. Waterlogged, anaerobic soil with high organic matter
C. Dry alkaline soil
D. Frozen soil horizons
Rationale: Anaerobic conditions in saturated soils cause facultative bacteria to utilize nitrate as
an electron acceptor, reducing it to N2 and N2O gases.
9. Which liming material contains both calcium carbonate and
magnesium carbonate?
A. Calcitic limestone
B. Dolomitic limestone
C. Quicklime (CaO)
D. Hydrated lime
Rationale: Dolomitic limestone contains significant proportions of magnesium carbonate
[CaMg(CO3)2] alongside calcium carbonate.
10. What symptom is characteristic of mobile nitrogen deficiency in
crops?
A. Interveinal chlorosis on young upper leaves
B. V-shaped chlorosis progressing along the midrib of lower older leaves
C. Purpling of new terminal leaf margins
D. Marginal leaf necrosis on upper leaves
Rationale: Because nitrogen is mobile inside plant tissue, plants translocate it from mature
foliage to new growth, causing chlorosis on lower leaves first.
11. What form of phosphorus is predominantly absorbed by plant
roots in strongly acidic soils (pH < 5.5)?
A. HPO4(2-)
B. H2PO4(-)
C. PO4(3-)
D. Elemental P
Rationale: Dihydrogen phosphate (H2PO4-) is the primary orthophosphate species present and
absorbed in acidic soil solutions.
12. What primary micronutrient becomes highly toxic to plant roots in
soils with pH levels below 5.0?
A. Zinc
B. Aluminum (Al3+)
