GAP Soil Science & Analysis 3 — Questions and Answers
Question 1: Which soil amendment is most appropriate to raise the pH of an overly acidic field soil?
- Sulfur
- Agricultural lime (calcium carbonate) (Correct answer)
- Ammonium sulfate
- Iron chelate
Correct answer: Agricultural lime (calcium carbonate)
Agricultural lime (calcium carbonate) neutralizes soil acidity and raises pH by releasing calcium ions that displace hydrogen ions from exchange sites.
Question 2: What is the primary purpose of a soil percolation test in a GAP context?
- Determining nitrogen content
- Assessing how quickly water drains through the soil profile (Correct answer)
- Measuring organic matter percentage
- Identifying compaction depth
Correct answer: Assessing how quickly water drains through the soil profile
A percolation test measures water drainage rate, which is critical for assessing flood risk, irrigation scheduling, and pathogen runoff potential on produce farms.
Question 3: Soil organic matter content directly improves all of the following EXCEPT:
- Water-holding capacity
- Microbial activity
- Soil pH stability
- Soil particle size (texture) (Correct answer)
Correct answer: Soil particle size (texture)
Soil texture (the proportion of sand, silt, and clay particles) is determined by geology and cannot be changed by adding organic matter.
Question 4: A soil bulk density reading of 1.7 g/cm³ in a crop field most likely indicates:
- Excellent aeration and root penetration conditions
- Significant compaction limiting root growth and water infiltration (Correct answer)
- High organic matter content
- Sandy texture with ideal drainage
Correct answer: Significant compaction limiting root growth and water infiltration
Bulk density above 1.6 g/cm³ in most agricultural soils signals compaction that restricts root penetration, reduces pore space, and limits water infiltration.
Question 5: When should a GAP producer collect soil samples for the most accurate fertility recommendations?
- Immediately after heavy rain
- At least 2–4 weeks after fertilizer or lime application (Correct answer)
- During active crop growth at peak season
- Only in summer when soils are dry
Correct answer: At least 2–4 weeks after fertilizer or lime application
Sampling too soon after fertilizer or lime application gives skewed results; waiting 2–4 weeks allows amendments to equilibrate with the soil.
Question 6: Which practice best describes how cover crops contribute to soil health in a GAP program?
- They directly apply synthetic nutrients to the soil
- They add organic matter and protect against erosion while improving soil biology (Correct answer)
- They eliminate the need for any additional soil testing
- They sterilize the soil to prevent pathogen growth
Correct answer: They add organic matter and protect against erosion while improving soil biology
Cover crops build organic matter, protect bare soil from erosion and compaction, suppress weeds, and support beneficial soil microorganism populations.
Question 7: In soil science, what does the term 'horizon' refer to?
- The geographic boundary of a farm field
- A distinct layer in the soil profile with characteristic properties (Correct answer)
- The depth limit for soil sampling
- The pH gradient across a field
Correct answer: A distinct layer in the soil profile with characteristic properties
A soil horizon is a distinct layer within the soil profile, such as the O (organic), A (topsoil), B (subsoil), and C (parent material) horizons, each with unique properties.
GAP Soil Science & Analysis 3
Which soil amendment is most appropriate to raise the pH of an overly acidic field soil?