CFA Soil Science & Fertilizer Chemistry 2 — Questions and Answers
Question 1: Which soil textural class has the highest water-holding capacity per unit volume?
- Sandy loam
- Silt loam
- Clay (Correct answer)
- Loamy sand
Correct answer: Clay
Clay soils have the greatest surface area per gram, giving them the highest water-holding capacity.
Question 2: The process by which ammonium ions are converted to nitrate in the soil is called:
- Denitrification
- Volatilization
- Nitrification (Correct answer)
- Mineralization
Correct answer: Nitrification
Nitrification is the two-step microbial oxidation of NH4+ to NO2- and then to NO3- by Nitrosomonas and Nitrobacter bacteria.
Question 3: Which fertilizer material has the highest potential acidity when applied to soil?
- Calcium nitrate
- Ammonium sulfate (Correct answer)
- Urea
- Potassium nitrate
Correct answer: Ammonium sulfate
Ammonium sulfate has the highest potential acidity (110 lb CaCO3 equivalent per 100 lb) because it supplies two acid-forming NH4+ ions plus the acidifying sulfate anion.
Question 4: What is the primary function of molybdenum (Mo) in plant nutrition?
- Chlorophyll synthesis
- Cell wall formation
- Nitrate reductase activation (Correct answer)
- Water transport regulation
Correct answer: Nitrate reductase activation
Molybdenum is a cofactor for nitrate reductase, the enzyme that reduces NO3- to NH4+ within plant tissue.
Question 5: A soil with a CEC of 20 meq/100g and 60% base saturation contains how many meq of base cations per 100g?
- 10 meq
- 12 meq (Correct answer)
- 15 meq
- 18 meq
Correct answer: 12 meq
Base saturation percentage × CEC = 0.60 × 20 meq/100g = 12 meq/100g of base cations.
Question 6: Which nitrogen fertilizer is most susceptible to ammonia volatilization when surface-applied to high-pH soils?
- Ammonium nitrate
- Urea (Correct answer)
- Ammonium sulfate
- Calcium nitrate
Correct answer: Urea
Urea undergoes rapid hydrolysis to ammonium carbonate at the soil surface, which at high pH promotes ammonia gas loss.
Question 7: Soil organic matter contributes to CEC primarily because of:
- Positive charges from protonated amino groups
- Negative charges from dissociated carboxyl and phenolic groups (Correct answer)
- Neutral Van der Waals forces
- Covalent bonding with mineral particles
Correct answer: Negative charges from dissociated carboxyl and phenolic groups
Organic matter carries pH-dependent negative charges from deprotonated carboxyl (-COOH) and phenolic (-OH) functional groups that attract and hold cations.
CFA Soil Science & Fertilizer Chemistry 2
Which soil textural class has the highest water-holding capacity per unit volume?