CH Soil Science & Analysis 3 — Questions and Answers
Question 1: A soil survey report describes a unit as 'somewhat poorly drained.' Which water table depth range typically corresponds to this drainage class?
- Greater than 150 cm below surface
- Between 90 and 150 cm below surface
- Between 30 and 90 cm below surface (Correct answer)
- At or above the surface for extended periods
Correct answer: Between 30 and 90 cm below surface
Somewhat poorly drained soils typically have a seasonal high water table between 30 and 90 cm, causing intermittent saturation in the upper profile.
Question 2: Which soil order in USDA Soil Taxonomy is defined primarily by the presence of a spodic horizon?
- Ultisols
- Alfisols
- Spodosols (Correct answer)
- Oxisols
Correct answer: Spodosols
Spodosols are defined by the presence of a spodic horizon—an illuvial accumulation of amorphous organic-aluminum-iron complexes.
Question 3: The 'van Genuchten model' is used in hydrology to describe:
- Soil erodibility in the RUSLE equation
- The soil water retention curve relating matric potential to volumetric water content (Correct answer)
- Solute transport in vadose zone soils
- Thermal conductivity as a function of soil moisture
Correct answer: The soil water retention curve relating matric potential to volumetric water content
The van Genuchten model provides a mathematical equation fitting the relationship between soil matric potential and volumetric water content in the retention curve.
Question 4: In soil horizon designation, which master horizon letter indicates a zone of maximum biological activity and organic matter accumulation?
- B
- C
- E
- A (Correct answer)
Correct answer: A
The A horizon is the surface mineral horizon where organic matter accumulates and biological activity is highest.
Question 5: A hydrographer observes mottles with Munsell chroma ≤2 within 50 cm of the soil surface. This most likely indicates:
- High iron oxide content from well-drained conditions
- Seasonal saturation and reducing conditions causing iron reduction (Correct answer)
- Accumulation of manganese from deep groundwater
- Deposition of fine sediment from recent flooding
Correct answer: Seasonal saturation and reducing conditions causing iron reduction
Low chroma (≤2) mottles indicate redoximorphic features formed under seasonal anaerobic conditions where iron is reduced and mobilized.
Question 6: Which soil texture class has the highest available water holding capacity per unit depth?
- Coarse sand
- Silt loam (Correct answer)
- Clay
- Gravelly loam
Correct answer: Silt loam
Silt loam soils typically have the highest plant-available water capacity because they balance adequate pore size (for drainage) with sufficient small pores (for retention).
Question 7: The Darcy-Weisbach equation's application to porous media flow is modified to become Darcy's Law. In Darcy's Law, the proportionality constant (K) represents:
- Soil bulk density
- Hydraulic conductivity (Correct answer)
- Porosity of the medium
- Tortuosity factor
Correct answer: Hydraulic conductivity
In Darcy's Law, hydraulic conductivity (K) is the proportionality constant relating flux to hydraulic gradient, reflecting the medium's ability to transmit water.
CH Soil Science & Analysis 3
A soil survey report describes a unit as 'somewhat poorly drained.' Which water table depth range typically corresponds to this drainage class?