SAF Soil Science & Analysis 3 — Questions and Answers
Question 1: During a forest road construction project, a forester observes mottling (rust-colored and gray spots) in a soil profile at 45 cm depth. What does this indicate?
- High organic matter decomposition activity
- Seasonal or periodic water saturation causing anaerobic conditions (Correct answer)
- Recent volcanic ash deposition
- Deep freeze-thaw cycling
Correct answer: Seasonal or periodic water saturation causing anaerobic conditions
Mottling results from alternating oxidizing and reducing conditions caused by fluctuating water tables, indicating seasonally saturated soils.
Question 2: What is the significance of the C:N ratio in forest floor organic matter for nutrient cycling?
- A high C:N ratio (>30) promotes rapid nitrogen mineralization and release
- A low C:N ratio (<20) indicates nitrogen immobilization by decomposers
- A high C:N ratio indicates slow decomposition and potential nitrogen immobilization (Correct answer)
- C:N ratio has no practical bearing on nitrogen availability in forests
Correct answer: A high C:N ratio indicates slow decomposition and potential nitrogen immobilization
When C:N exceeds 30, microbial decomposers immobilize nitrogen for their own growth, making it temporarily unavailable to trees.
Question 3: Which method is most appropriate for estimating fine root biomass distribution in a forest soil profile?
- Shovel-pit visual assessment
- Soil core extraction and root washing with sequential sieving (Correct answer)
- Penetrometer resistance measurements
- Electromagnetic induction surveys
Correct answer: Soil core extraction and root washing with sequential sieving
Soil core extraction followed by washing and sieving allows quantitative separation and measurement of fine roots by diameter class across soil depths.
Question 4: A silviculturist notes that a harvested site has poor natural regeneration and compacted soils. Which soil remediation technique is most appropriate before replanting?
- Broadcast burning to increase soil temperature
- Subsoil ripping or deep tillage to fracture compacted layers (Correct answer)
- Heavy lime application to raise pH above 7.5
- Irrigation to saturate the soil profile
Correct answer: Subsoil ripping or deep tillage to fracture compacted layers
Subsoil ripping fractures compacted hardpans, restoring macroporosity and allowing root penetration and water infiltration.
Question 5: What role does soil organic matter play in water repellency (hydrophobicity) sometimes observed after wildfires?
- Organic matter always increases water infiltration regardless of fire
- Heat volatilizes organic compounds that recondense in cool soil layers, creating hydrophobic coatings (Correct answer)
- Fire-induced hydrophobicity is caused solely by ash deposition on the surface
- Organic matter combustion always eliminates hydrophobicity
Correct answer: Heat volatilizes organic compounds that recondense in cool soil layers, creating hydrophobic coatings
Fire volatilizes hydrophobic organic compounds from litter; these gases move downward and condense on cooler soil particles, forming water-repellent layers.
Question 6: Which soil order typically supports the most productive temperate broadleaf forests in the eastern United States?
- Spodosols
- Ultisols
- Alfisols (Correct answer)
- Aridisols
Correct answer: Alfisols
Alfisols have high base saturation and a clay-rich argillic horizon, providing good moisture and nutrient retention for productive hardwood forests.
Question 7: What does a field test using 10% HCl (hydrochloric acid) on a soil sample indicate if it fizzes vigorously?
- Presence of iron oxides
- High organic matter content
- Presence of carbonates such as calcium carbonate (Correct answer)
- High potassium concentration
Correct answer: Presence of carbonates such as calcium carbonate
Vigorous effervescence with dilute HCl indicates the presence of carbonates (CaCO₃), signaling alkaline soil conditions and possible calcium excess.
SAF Soil Science & Analysis 3
During a forest road construction project, a forester observes mottling (rust-colored and gray spots) in a soil profile at 45 cm depth.
What does this indicate?