CPESC Soil Science 5 — Questions and Answers
Question 1: What is the primary difference between 'interrill erosion' and 'rill erosion' in terms of erosive agent?
- Interrill erosion is caused by raindrop impact and thin sheet flow; rill erosion is caused by concentrated channelized flow (Correct answer)
- Interrill erosion occurs on steeper slopes; rill erosion occurs on flatter terrain
- Interrill erosion moves larger particles; rill erosion moves finer particles
- Interrill erosion is seasonal; rill erosion occurs year-round
Correct answer: Interrill erosion is caused by raindrop impact and thin sheet flow; rill erosion is caused by concentrated channelized flow
Interrill erosion is driven by raindrop splash detachment and shallow sheet flow between rills, while rill erosion results from concentrated flow with greater shear stress.
Question 2: On a construction site, the soil texture is classified as 'CH' under the Unified Soil Classification System (USCS). What does this indicate?
- High-plasticity clay with significant shrink-swell potential (Correct answer)
- Poorly graded clean sand with low plasticity
- Well-graded gravel with fine material
- Low-plasticity silt with moderate compressibility
Correct answer: High-plasticity clay with significant shrink-swell potential
In USCS, 'C' denotes clay and 'H' denotes high plasticity (liquid limit ≥50%), indicating a soil with significant shrink-swell potential and erodibility when disturbed.
Question 3: The Sodium Adsorption Ratio (SAR) of a soil is used to assess:
- The risk of soil structure breakdown due to excess sodium relative to calcium and magnesium (Correct answer)
- The amount of sodium available for plant nutrition
- The soil's capacity to buffer against pH changes
- The rate of sodium leaching from irrigation water
Correct answer: The risk of soil structure breakdown due to excess sodium relative to calcium and magnesium
High SAR values indicate sodium dominates the cation exchange sites, causing clay dispersion, structure degradation, reduced infiltration, and increased erosion risk.
Question 4: Which physical soil property best predicts the susceptibility of a slope to shallow landslides following prolonged rainfall?
- Low cohesion combined with high saturated hydraulic conductivity in the upper profile over a restrictive layer (Correct answer)
- High clay content throughout the entire soil profile
- Low bulk density indicating loose, uncompacted material
- High organic matter content in the surface horizon only
Correct answer: Low cohesion combined with high saturated hydraulic conductivity in the upper profile over a restrictive layer
Shallow landslides typically occur when rapid pore pressure buildup over an impermeable layer reduces effective stress to zero in low-cohesion materials, triggering failure.
Question 5: In RUSLE modeling for a construction site, what is the typical C factor value assigned to bare, freshly graded soil?
- 1.0 (Correct answer)
- 0.1
- 0.5
- 0.01
Correct answer: 1.0
A C factor of 1.0 represents the baseline condition of bare, fallow, tilled soil against which all other cover conditions are compared in RUSLE.
Question 6: What soil amendment is most commonly used to reclaim sodic soils and improve their structure for revegetation?
- Gypsum (calcium sulfate) (Correct answer)
- Lime (calcium carbonate)
- Elemental sulfur
- Ammonium nitrate
Correct answer: Gypsum (calcium sulfate)
Gypsum provides soluble calcium that replaces sodium on exchange sites without significantly altering pH, allowing sodium to be leached and soil structure to recover.
Question 7: A soil has a liquid limit of 55 and a plastic limit of 30. What is its Plasticity Index (PI), and what does it indicate?
- PI = 25, indicating high plasticity with significant clay content (Correct answer)
- PI = 85, indicating very low plasticity
- PI = 25, indicating the soil is non-plastic
- PI = 55, indicating a silty soil with moderate erodibility
Correct answer: PI = 25, indicating high plasticity with significant clay content
PI = LL − PL = 55 − 30 = 25; a PI above 20 indicates high plasticity and substantial active clay content, implying shrink-swell potential and cohesive erosion behavior.
CPESC Soil Science 5
What is the primary difference between 'interrill erosion' and 'rill erosion' in terms of erosive agent?