CFA Soil Properties and Testing 3 — Questions and Answers
Question 1: Which soil horizon is typically where the most weathering, leaching, and organic matter accumulation occurs?
- C horizon
- B horizon
- A horizon (Correct answer)
- R horizon
Correct answer: A horizon
The A horizon (topsoil) is the zone of maximum organic matter accumulation and biological activity.
Question 2: Soil with a high clay content tends to have what effect on fertilizer application timing?
- Nutrients leach rapidly, requiring split applications
- Nutrients are held longer, allowing single large applications (Correct answer)
- Nutrients become unavailable due to high pH
- No effect; timing is determined only by crop stage
Correct answer: Nutrients are held longer, allowing single large applications
Clay soils have high CEC and hold cations longer, reducing leaching and allowing less-frequent nutrient applications.
Question 3: A grower applies lime to raise soil pH. Which property of lime determines how quickly it reacts in soil?
- Color (whiteness)
- Fineness (particle size) (Correct answer)
- Calcium-to-magnesium ratio
- Moisture content at time of application
Correct answer: Fineness (particle size)
Finer lime particles have greater surface area and react faster with soil acidity than coarser particles.
Question 4: The term 'cation exchange capacity' (CEC) describes:
- The ability of soil to conduct electrical current
- The total negative charge sites available to hold positively charged ions (Correct answer)
- The ratio of calcium to magnesium in the soil
- The capacity of soil to hold water against gravity
Correct answer: The total negative charge sites available to hold positively charged ions
CEC measures the quantity of cations a soil can adsorb and retain on its negatively charged surfaces.
Question 5: Which soil property most directly determines the buffering capacity against pH change?
- Soil color
- Sand percentage
- CEC and organic matter content (Correct answer)
- Bulk density
Correct answer: CEC and organic matter content
Soils with high CEC and organic matter resist pH changes because they can adsorb or release H⁺ ions.
Question 6: A producer notices iron chlorosis in soybean on a field with pH 7.8. The most probable cause is:
- Iron toxicity from over-fertilization
- Iron becoming unavailable due to high pH converting it to insoluble forms (Correct answer)
- Excessive soil moisture dissolving iron
- Manganese competition blocking iron uptake
Correct answer: Iron becoming unavailable due to high pH converting it to insoluble forms
At high pH, iron oxidizes to Fe³⁺ hydroxides that are largely insoluble and unavailable to plants.
Question 7: Soil respiration measurements are used in soil testing to assess:
- Nitrogen mineralization potential and microbial activity (Correct answer)
- Phosphorus sorption capacity
- Water infiltration rate
- Heavy metal contamination
Correct answer: Nitrogen mineralization potential and microbial activity
Soil respiration (CO₂ release) reflects microbial activity, which drives organic matter decomposition and nitrogen mineralization.
CFA Soil Properties and Testing 3
Which soil horizon is typically where the most weathering, leaching, and organic matter accumulation occurs?