CFA Soil Science and Testing 3 — Questions and Answers
Question 1: A grower's soil test shows a base saturation of 65% calcium, 12% magnesium, and 4% potassium. What does this suggest?
- Potassium and magnesium levels are both excessive
- The calcium-to-magnesium ratio is within an acceptable range
- The soil needs immediate calcium application
- Potassium is deficient and magnesium may be high (Correct answer)
Correct answer: Potassium is deficient and magnesium may be high
Optimal potassium base saturation is typically 2–5%, but 4% combined with 12% Mg suggests magnesium may be competing with potassium uptake, and potassium should be monitored.
Question 2: Which method is used to determine the soil texture class in the field without lab equipment?
- Hydrometer method
- Sieve analysis
- Ribbon and feel method (Correct answer)
- Loss-on-ignition test
Correct answer: Ribbon and feel method
The ribbon and feel method involves moistening a soil sample and working it between fingers to assess the length of ribbon formed, estimating sand, silt, and clay content.
Question 3: Soil bulk density is used to assess:
- Nutrient concentration per acre
- Degree of soil compaction (Correct answer)
- Cation exchange capacity
- Organic matter decomposition rate
Correct answer: Degree of soil compaction
Bulk density measures the mass of dry soil per unit volume, with higher values indicating greater compaction and reduced pore space.
Question 4: When sulfur is applied to a high-pH soil, the expected outcome is:
- Increased soil pH
- Decreased soil pH over time (Correct answer)
- No change in soil pH
- Immediate neutralization of all alkalinity
Correct answer: Decreased soil pH over time
Elemental sulfur is oxidized by soil bacteria to form sulfuric acid, which acidifies the soil and lowers pH over time.
Question 5: The term 'soil profile' refers to:
- The nutrient content of surface soil only
- A vertical cross-section of soil showing distinct horizontal layers (Correct answer)
- The slope and topography of a field
- The total organic carbon of a soil sample
Correct answer: A vertical cross-section of soil showing distinct horizontal layers
A soil profile is a vertical section from the surface to the parent material, showing distinct horizons (O, A, B, C) with different properties.
Question 6: Which of the following best describes a 'perched water table'?
- The normal water table near sea level
- A localized saturated zone above an impermeable layer (Correct answer)
- Water held in surface depressions
- The upper boundary of the C horizon
Correct answer: A localized saturated zone above an impermeable layer
A perched water table forms when water accumulates above an impermeable layer (such as a clay pan or hardpan) before reaching the regional water table.
Question 7: To convert a soil test phosphorus result from ppm to lbs/acre for a 6-inch sampling depth, you multiply by approximately:
- 1.0
- 2.0 (Correct answer)
- 4.0
- 0.5
Correct answer: 2.0
For a standard 6-inch sampling depth, soil weight per acre is approximately 2 million lbs, so ppm × 2 converts to lbs/acre.
CFA Soil Science and Testing 3
A grower's soil test shows a base saturation of 65% calcium, 12% magnesium, and 4% potassium.
What does this suggest?