CPESC Soil Science 4 — Questions and Answers
Question 1: Which soil characteristic most directly determines whether a soil will be classified as 'hydric' under wetland regulations?
- Presence of redoximorphic features indicating seasonal saturation (Correct answer)
- High clay content with low permeability
- Organic carbon content exceeding 12% by weight
- Depth to bedrock less than 20 inches
Correct answer: Presence of redoximorphic features indicating seasonal saturation
Hydric soils are identified by redoximorphic features such as mottles, iron depletions, and gleyed matrices that form under conditions of prolonged seasonal saturation.
Question 2: When evaluating a cut slope for erosion potential, a geotechnical investigation reveals a fragipan at 24 inches depth. This layer would:
- Promote lateral water movement and perched water tables increasing instability (Correct answer)
- Increase infiltration rates and reduce runoff
- Improve root penetration and vegetation establishment
- Reduce erodibility by providing a stable base layer
Correct answer: Promote lateral water movement and perched water tables increasing instability
A fragipan is a brittle, cemented layer with very low permeability that causes perched water tables and lateral flow, potentially destabilizing slopes above it.
Question 3: The 'dispersivity' of a soil refers to its tendency to:
- Break apart and disperse in water without mechanical agitation (Correct answer)
- Compact under repeated wheel loads
- Release nutrients during rainfall events
- Form stable aggregates when wet
Correct answer: Break apart and disperse in water without mechanical agitation
Dispersive soils deflocculate in the presence of water due to high exchangeable sodium, causing particles to separate and making these soils extremely erodible.
Question 4: In which situation would you expect soil erodibility (K factor) to be HIGHEST?
- Fine sandy loam with low organic matter content (Correct answer)
- Clay soil with high shrink-swell potential
- Sandy soil with coarse, single-grain structure
- Well-aggregated silty clay loam with high organic matter
Correct answer: Fine sandy loam with low organic matter content
Fine sandy loams with low organic matter have moderate-to-high silt content combined with weak aggregation, making them highly susceptible to raindrop detachment and transport.
Question 5: What does a soil's 'field capacity' represent in the context of stormwater management?
- The water content held after gravitational drainage ceases (approximately -1/3 bar) (Correct answer)
- The maximum water content before surface runoff begins
- The minimum water content at which plants can survive
- The rate of water infiltration during a 1-hour rainfall
Correct answer: The water content held after gravitational drainage ceases (approximately -1/3 bar)
Field capacity is the water content remaining in soil 2–3 days after saturation when gravitational drainage has essentially stopped, representing the upper limit of plant-available water storage.
Question 6: Which parent material type typically produces soils with the highest natural erodibility due to its fine particle size and limited cohesion?
- Loess deposits (Correct answer)
- Granite residuum
- Limestone residuum
- Alluvial gravels
Correct answer: Loess deposits
Loess-derived soils are predominantly silt-sized particles with low cohesion, making them among the most erodible parent materials when disturbed.
Question 7: A soil profile shows a horizon designated 'Bt.' What does the lowercase 't' signify?
- Illuvial accumulation of silicate clay (Correct answer)
- Evidence of plowing or tillage disturbance
- Accumulation of secondary carbonates
- High content of iron-manganese nodules
Correct answer: Illuvial accumulation of silicate clay
The lowercase 't' is a subordinate horizon suffix indicating illuvial accumulation of silicate clay, designating an argillic horizon within the B horizon.
CPESC Soil Science 4
Which soil characteristic most directly determines whether a soil will be classified as 'hydric' under wetland regulations?