SAF Soil Science & Analysis 2 — Questions and Answers
Question 1: Which soil horizon is characterized by the accumulation of leached materials such as clay, iron oxides, and organic matter?
- A horizon
- B horizon (Correct answer)
- C horizon
- O horizon
Correct answer: B horizon
The B horizon (subsoil) is the illuvial zone where materials leached from upper horizons accumulate.
Question 2: What does a soil pH of 4.5 indicate about a forest soil's nutrient availability?
- Optimal nutrient availability for most tree species
- Likely deficiency of phosphorus and molybdenum with possible aluminum toxicity (Correct answer)
- Excess calcium and magnesium availability
- Neutral conditions favorable for nitrogen mineralization
Correct answer: Likely deficiency of phosphorus and molybdenum with possible aluminum toxicity
At pH 4.5, phosphorus and molybdenum become unavailable while aluminum and manganese reach toxic levels.
Question 3: In a forest soil with a bulk density of 1.6 g/cm³, which condition is most likely indicated?
- High organic matter content
- Compaction or lack of structure (Correct answer)
- High macroporosity
- Sandy loam texture
Correct answer: Compaction or lack of structure
Bulk density above 1.4-1.5 g/cm³ typically indicates compaction, which restricts root growth and water infiltration.
Question 4: What is the primary function of mycorrhizal fungi in forest soils?
- Decomposing woody debris to release nitrogen
- Fixing atmospheric nitrogen into plant-available forms
- Extending effective root surface area for water and nutrient uptake (Correct answer)
- Suppressing pathogenic soil organisms
Correct answer: Extending effective root surface area for water and nutrient uptake
Mycorrhizal fungi form symbiotic associations with tree roots, dramatically increasing the absorptive surface area for phosphorus, water, and other nutrients.
Question 5: Which soil texture class has the highest water-holding capacity per unit volume?
- Coarse sand
- Sandy loam
- Silty clay loam (Correct answer)
- Loamy sand
Correct answer: Silty clay loam
Fine-textured soils like silty clay loam have abundant small pores that retain water against gravity, giving them the highest water-holding capacity.
Question 6: What is cation exchange capacity (CEC) and why is it important for forest management?
- The rate at which soil releases carbon dioxide; important for respiration studies
- The soil's ability to hold positively charged nutrient ions; indicates nutrient retention potential (Correct answer)
- The capacity of soil to neutralize acid rain; used in liming recommendations only
- The measure of soil microorganism activity; used for organic matter decomposition rates
Correct answer: The soil's ability to hold positively charged nutrient ions; indicates nutrient retention potential
CEC measures how many cations a soil can hold and exchange with plant roots, directly indicating its fertility and resistance to nutrient leaching.
Question 7: Which diagnostic horizon defines a Spodosol in USDA soil taxonomy?
- Argillic horizon with clay accumulation
- Calcic horizon with calcium carbonate
- Spodic horizon with accumulated organic matter and aluminum/iron oxides (Correct answer)
- Mollic epipedon with dark, base-rich surface
Correct answer: Spodic horizon with accumulated organic matter and aluminum/iron oxides
Spodosols are defined by the presence of a spodic horizon — a subsurface zone of illuvial organic matter and sesquioxides, common under coniferous forests.
SAF Soil Science & Analysis 2
Which soil horizon is characterized by the accumulation of leached materials such as clay, iron oxides, and organic matter?