CFA Soil Science & Fertilizer Chemistry 3 — Questions and Answers
Question 1: Which form of phosphorus is most available for plant uptake in the soil solution?
- H3PO4
- H2PO4- (Correct answer)
- HPO42-
- PO43-
Correct answer: H2PO4-
H2PO4- (dihydrogen phosphate) is the dominant plant-available form in the slightly acid to neutral pH range (6.0–7.0) where P availability peaks.
Question 2: Sulfur deficiency symptoms in plants most closely resemble deficiency of which other nutrient?
- Phosphorus
- Potassium
- Nitrogen (Correct answer)
- Iron
Correct answer: Nitrogen
Both sulfur and nitrogen deficiencies cause yellowing (chlorosis) of leaves, but sulfur deficiency appears first on younger leaves while nitrogen deficiency starts on older leaves.
Question 3: The bulk density of a compacted soil is 1.6 g/cm³. Assuming particle density of 2.65 g/cm³, what is the total porosity?
- About 30%
- About 40% (Correct answer)
- About 50%
- About 60%
Correct answer: About 40%
Porosity = (1 - Db/Dp) × 100 = (1 - 1.6/2.65) × 100 ≈ 39.6%, approximately 40%.
Question 4: Which soil order is characterized by a spodic horizon containing illuviated organic matter, aluminum, and iron?
- Mollisols
- Spodosols (Correct answer)
- Ultisols
- Vertisols
Correct answer: Spodosols
Spodosols are defined by their spodic horizon (Bs or Bhs), where leached humus, Al, and Fe accumulate in a distinct subsurface layer.
Question 5: When diammonium phosphate (DAP, 18-46-0) dissolves near a seed, the pH of the fertilizer band temporarily:
- Drops sharply to below 4
- Rises to approximately 8 (Correct answer)
- Remains neutral at 7
- Fluctuates unpredictably
Correct answer: Rises to approximately 8
DAP dissolution releases NH3 gas initially, raising the localized pH to about 7.5–8.5 before nitrification proceeds.
Question 6: Which soil property is MOST directly measured by a standard lime requirement test?
- Total nitrogen content
- Buffering capacity against pH change (Correct answer)
- Available phosphorus level
- Soil moisture content
Correct answer: Buffering capacity against pH change
Lime requirement tests (such as the Shoemaker-McLean-Pratt buffer test) measure the soil's buffering capacity to determine how much lime is needed to raise pH to a target level.
Question 7: In waterlogged (anaerobic) soils, nitrate nitrogen is most likely to be lost through:
- Leaching to groundwater
- Volatilization as NH3
- Denitrification to N2 gas (Correct answer)
- Immobilization by microbes
Correct answer: Denitrification to N2 gas
Denitrification by anaerobic bacteria converts NO3- to N2O and N2 gases, which escape to the atmosphere in waterlogged, oxygen-depleted soils.
CFA Soil Science & Fertilizer Chemistry 3
Which form of phosphorus is most available for plant uptake in the soil solution?