CPESC Soil Science 2 — Questions and Answers
Question 1: Which soil particle size range defines silt according to the USDA classification system?
- 0.002–0.05 mm (Correct answer)
- 0.05–2.0 mm
- 0.0001–0.002 mm
- 2.0–75 mm
Correct answer: 0.002–0.05 mm
USDA classifies silt as particles with diameters between 0.002 and 0.05 mm, finer than sand but coarser than clay.
Question 2: A soil with high shrink-swell potential is most likely dominated by which clay mineral?
- Montmorillonite (Correct answer)
- Kaolinite
- Illite
- Chlorite
Correct answer: Montmorillonite
Montmorillonite (smectite) has a 2:1 layer structure that allows water to enter between layers, causing significant shrink-swell behavior.
Question 3: What does the term 'cation exchange capacity' (CEC) measure in a soil?
- The ability of soil to hold and exchange positively charged ions (Correct answer)
- The total amount of water a soil can retain
- The rate at which water moves through soil pores
- The proportion of base cations to acid cations
Correct answer: The ability of soil to hold and exchange positively charged ions
CEC measures the total capacity of a soil to hold exchangeable cations, expressed in milliequivalents per 100 grams of soil.
Question 4: During a storm event, which soil structure type provides the greatest resistance to surface sealing and crusting?
- Granular or crumb structure (Correct answer)
- Platy structure
- Massive structure
- Columnar structure
Correct answer: Granular or crumb structure
Granular and crumb structures have numerous pores between aggregates that promote infiltration and resist surface sealing during rainfall.
Question 5: Which factor in the RUSLE equation accounts for the protective effect of plant cover on soil erosion?
- C factor (Cover-management factor) (Correct answer)
- K factor (Soil erodibility factor)
- P factor (Support practice factor)
- LS factor (Slope length and steepness)
Correct answer: C factor (Cover-management factor)
The C factor in RUSLE represents the ratio of soil loss from land with specified cover to that from tilled bare soil.
Question 6: A soil percolation rate of less than 0.5 inches per hour is generally classified as:
- Very slow (Correct answer)
- Slow
- Moderate
- Rapid
Correct answer: Very slow
Saturated hydraulic conductivity or percolation rates below 0.5 in/hr are classified as very slow, indicating high clay content or restrictive layers.
Question 7: Which soil horizon is most commonly associated with illuviation of clay, iron, and organic matter?
- B horizon (Correct answer)
- A horizon
- C horizon
- O horizon
Correct answer: B horizon
The B horizon is the zone of illuviation where materials leached from upper horizons accumulate, forming features like argillic or spodic horizons.
CPESC Soil Science 2
Which soil particle size range defines silt according to the USDA classification system?