CH Soil Science & Analysis 2 — Questions and Answers
Question 1: Which soil property is most directly used to estimate saturated hydraulic conductivity (Ksat) in the field?
- Cation exchange capacity
- Soil texture and structure (Correct answer)
- Organic matter content
- pH level
Correct answer: Soil texture and structure
Soil texture and structure control pore size distribution, which is the primary determinant of saturated hydraulic conductivity.
Question 2: In a soil profile, a 'fragipan' horizon is best described as:
- A horizon enriched with illuviated clay
- A brittle, dense subsurface layer that restricts root penetration (Correct answer)
- A layer with accumulation of soluble salts
- An organic-rich horizon overlying mineral soil
Correct answer: A brittle, dense subsurface layer that restricts root penetration
A fragipan is a dense, brittle subsurface horizon that appears cemented when dry but slakes in water, restricting roots and water movement.
Question 3: Which of the following best describes the concept of 'field capacity' in soils?
- The moisture content when plants begin to wilt
- The maximum water a soil can hold before surface runoff begins
- The water content remaining after free drainage has ceased under gravity (Correct answer)
- The total porosity of the soil profile
Correct answer: The water content remaining after free drainage has ceased under gravity
Field capacity is the water content held in soil after excess water has drained away under gravity, typically measured 1–3 days after saturation.
Question 4: The 'permanent wilting point' of a soil is most commonly defined at a matric potential of approximately:
- -0.033 MPa
- -0.5 MPa
- -1.5 MPa (Correct answer)
- -15 MPa
Correct answer: -1.5 MPa
The permanent wilting point is conventionally defined at -1.5 MPa (–15 bars), the pressure at which most plants can no longer extract water.
Question 5: Which laboratory method is most commonly used to determine the particle-size distribution of fine-grained soils?
- Proctor compaction test
- Hydrometer analysis (Correct answer)
- Atterberg limits test
- Loss on ignition
Correct answer: Hydrometer analysis
Hydrometer analysis measures the rate at which particles settle in water to determine the proportion of silt and clay in fine-grained soils.
Question 6: In soil classification using the USDA system, which horizon suffix 'g' indicates?
- Strong gleying due to long-term saturation (Correct answer)
- Accumulation of gypsum
- A gravelly or coarse-fragment layer
- Gilgai microrelief features
Correct answer: Strong gleying due to long-term saturation
The suffix 'g' denotes strong gleying, characterized by gray or mottled colors indicating prolonged saturation and reducing conditions.
Question 7: Which process primarily drives the formation of 'plinthite' in tropical soils, and why is it hydrologically significant?
- Calcification, because it reduces permeability through carbonate cementation
- Ferralitization, because iron-rich material irreversibly hardens upon drying to form an impermeable layer (Correct answer)
- Podzolization, because it creates a spodic horizon that perches water
- Salinization, because salt crusts seal the soil surface
Correct answer: Ferralitization, because iron-rich material irreversibly hardens upon drying to form an impermeable layer
Plinthite forms by iron concentration under alternating wet-dry conditions and irreversibly hardens to laterite on drying, creating a nearly impermeable layer that redirects drainage.
CH Soil Science & Analysis 2
Which soil property is most directly used to estimate saturated hydraulic conductivity (Ksat) in the field?