CCA Soil Fertility and Analysis 3 — Questions and Answers
Question 1: A corn crop yielding 200 bu/ac removes approximately how many pounds of nitrogen per acre in the harvested grain?
- 60–80 lb
- 100–120 lb
- 150–170 lb (Correct answer)
- 200–220 lb
Correct answer: 150–170 lb
Corn grain contains roughly 0.75–0.85 lb N per bushel, so 200 bu/ac removes approximately 150–170 lb N/ac in grain alone.
Question 2: Sulfur deficiency symptoms in corn most commonly appear:
- On older leaves as interveinal chlorosis
- On younger leaves as uniform yellowing (Correct answer)
- As purple coloration on lower stems
- As necrotic leaf margins on all leaves
Correct answer: On younger leaves as uniform yellowing
Sulfur is relatively immobile in the plant, so deficiency symptoms appear first on young (upper) leaves as uniform yellowing.
Question 3: Which soil property most directly determines the lime requirement (LR) to raise pH to a target level?
- Total nitrogen
- Buffer pH (buffering capacity) (Correct answer)
- Available phosphorus
- Electrical conductivity
Correct answer: Buffer pH (buffering capacity)
Buffer pH measures the soil's resistance to pH change; soils with high buffering capacity (high CEC, organic matter) require more lime to achieve the same pH increase.
Question 4: Denitrification occurs most rapidly under which soil conditions?
- Dry, aerobic, low temperature
- Wet, anaerobic, warm temperature (Correct answer)
- Dry, acidic, high organic matter
- Wet, aerobic, neutral pH
Correct answer: Wet, anaerobic, warm temperature
Denitrifying bacteria convert NO3- to N2 gases under anaerobic (waterlogged) conditions, and the process accelerates as soil temperature rises.
Question 5: A soil has a CEC of 20 meq/100g and is 70% base-saturated. How many meq of bases per 100g of soil are present?
- 7
- 14 (Correct answer)
- 20
- 28
Correct answer: 14
Base saturation (%) × CEC = meq of bases, so 0.70 × 20 meq/100g = 14 meq/100g.
Question 6: When comparing urea (46-0-0) and anhydrous ammonia (82-0-0) as N fertilizer sources, which statement is accurate?
- Urea has a higher risk of volatilization loss when surface-applied without incorporation (Correct answer)
- Anhydrous ammonia is safer to handle and requires no specialized equipment
- Urea provides more N per ton but has lower agronomic efficiency on all soils
- Anhydrous ammonia acidifies soil less than urea per unit of N applied
Correct answer: Urea has a higher risk of volatilization loss when surface-applied without incorporation
Surface-applied urea can lose significant N as ammonia gas through urease-driven hydrolysis, especially at warm temperatures and high pH; incorporation or urease inhibitors reduce this risk.
Question 7: The Bray P-1 soil test is NOT recommended for use in soils with pH above 7.4 because:
- It overestimates P on calcareous soils
- It underestimates P by dissolving calcium phosphates poorly (Correct answer)
- The reagent destroys organic matter, giving false readings
- It fails to extract potassium simultaneously
Correct answer: It underestimates P by dissolving calcium phosphates poorly
Bray P-1's dilute acid is neutralized quickly by free carbonates in calcareous soils, preventing full extraction of available P and causing underestimation.
CCA Soil Fertility and Analysis 3
A corn crop yielding 200 bu/ac removes approximately how many pounds of nitrogen per acre in the harvested grain?