CLARB Soils and Geology 1 — Questions and Answers
Question 1: What USDA soil texture classification contains a balanced mixture of sand, silt, and clay in roughly equal proportions?
- Clay loam
- Loam (Correct answer)
- Sandy loam
- Silty clay
Correct answer: Loam
Loam is the USDA texture class that contains approximately 25-50% sand, 25-50% silt, and 10-25% clay, making it ideal for most plant growth.
Question 2: Which soil horizon is commonly referred to as 'topsoil' and contains the highest concentration of organic matter?
- O horizon
- A horizon (Correct answer)
- B horizon
- C horizon
Correct answer: A horizon
The A horizon (topsoil) is the uppermost mineral soil layer and contains the greatest accumulation of organic matter and biological activity.
Question 3: What term describes the maximum amount of water a soil can hold after excess gravitational water has drained away?
- Permanent wilting point
- Saturated capacity
- Field capacity (Correct answer)
- Available water capacity
Correct answer: Field capacity
Field capacity is the soil moisture content remaining after gravitational drainage has ceased, typically reached 24-48 hours after saturation.
Question 4: Which pH range is considered slightly acidic and generally optimal for most landscape plants?
- 4.0–5.0
- 5.5–6.5 (Correct answer)
- 7.0–8.0
- 8.5–9.5
Correct answer: 5.5–6.5
A pH of 5.5–6.5 allows maximum nutrient availability for most plants, as key nutrients like nitrogen, phosphorus, and potassium are most soluble in this range.
Question 5: Which soil property describes the soil's ability to resist pH changes when acids or bases are added?
- Cation exchange capacity
- Buffer capacity (Correct answer)
- Soil reactivity
- Hydraulic conductivity
Correct answer: Buffer capacity
Buffer capacity measures how resistant soil is to pH change; soils high in organic matter or clay tend to have greater buffering ability.
Question 6: The Unified Soil Classification System (USCS) is primarily used by which professional group for soil classification?
- Agricultural scientists
- Geotechnical engineers (Correct answer)
- Ecologists
- Hydrologists
Correct answer: Geotechnical engineers
The USCS, standardized under ASTM D2487, is the primary classification system used by geotechnical engineers to describe engineering behavior of soils.
Question 7: What is cation exchange capacity (CEC) in soils, and why is it important in landscape design?
- The rate at which water moves through soil
- The soil's ability to hold and exchange positively charged nutrient ions (Correct answer)
- The percentage of air-filled pore spaces in soil
- The soil's resistance to compaction
Correct answer: The soil's ability to hold and exchange positively charged nutrient ions
CEC measures how many cations (like Ca²⁺, Mg²⁺, K⁺) a soil can hold; high CEC soils retain more nutrients and are generally more fertile and resilient.
CLARB Soils and Geology 1
What USDA soil texture classification contains a balanced mixture of sand, silt, and clay in roughly equal proportions?