CCA Soil Fertility and Analysis 2 — Questions and Answers
Question 1: A soil test reports 25 ppm of available phosphorus using the Mehlich-3 extraction method. This value is best described as being in which fertility category for most Midwestern crops?
- Very low
- Low
- Medium to high (Correct answer)
- Excessive
Correct answer: Medium to high
Mehlich-3 P values around 25 ppm generally fall in the medium to high range adequate for most crops, so additional P applications may provide little economic response.
Question 2: Cation exchange capacity (CEC) is primarily determined by which two soil components?
- Sand and silt
- Clay and organic matter (Correct answer)
- Calcium and magnesium
- pH and texture
Correct answer: Clay and organic matter
Clay minerals and organic matter carry negative surface charges that attract and hold cations, making them the primary determinants of CEC.
Question 3: Which of the following nitrogen forms is most susceptible to leaching losses in sandy, well-drained soils?
- Ammonium (NH4+)
- Nitrate (NO3-) (Correct answer)
- Urea-N (before hydrolysis)
- Organic N in crop residue
Correct answer: Nitrate (NO3-)
Nitrate carries a negative charge and moves freely with soil water, making it highly susceptible to leaching in coarse-textured soils.
Question 4: A producer applies 200 lb/ac of 0-46-0 (triple superphosphate). How many pounds of P2O5 are applied per acre?
- 46
- 92 (Correct answer)
- 200
- 460
Correct answer: 92
The second number in the fertilizer grade represents % P2O5, so 200 lb × 0.46 = 92 lb P2O5/ac.
Question 5: Soil pH of 5.5 is likely to cause reduced availability of which nutrient due to increased fixation by iron and aluminum?
- Potassium
- Phosphorus (Correct answer)
- Sulfur
- Boron
Correct answer: Phosphorus
At pH below 6.0, iron and aluminum ions react with phosphate ions to form insoluble compounds, significantly reducing P availability.
Question 6: The base saturation percentage for calcium in a productive agricultural soil should ideally range from:
- 5–15%
- 20–35%
- 60–80% (Correct answer)
- 90–100%
Correct answer: 60–80%
Optimal calcium base saturation for most crops is 60–80% of CEC, ensuring adequate Ca supply without displacing other cations.
Question 7: Which buffered soil test extractant is commonly used in the southeastern United States to assess available potassium and phosphorus?
- Bray P-1
- Olsen
- Mehlich-3 (Correct answer)
- Morgan
Correct answer: Mehlich-3
Mehlich-3 is a multi-element extractant widely adopted in the southeastern US because it effectively extracts P, K, Ca, Mg, and micronutrients in a single solution.
