PSS 1411 — Exam 3
Actual Questions with Answers| Latest Update| Pass Guaranteed
1. What is a soil profile?
Answer: A vertical cross-section of soil from the surface down through the
underlying layers (horizons) to the parent material.
Rationale: The soil profile reveals how a soil formed over time and shows the distinct
horizons that differ in color, texture, structure, and composition.
2. List the master soil horizons in a typical soil profile, from the surface downward.
Answer: O, A, E, B, C, and R horizons.
Rationale: O is organic surface litter; A is topsoil rich in humus; E is a light-colored
eluviated (leached) layer; B is subsoil where leached materials accumulate; C is
weathered parent material; R is unweathered bedrock.
3. What characterizes the A horizon (topsoil)?
Answer: It is typically dark-colored, rich in organic matter and biological activity, and
is the zone where most plant roots and nutrient uptake occur.
Rationale: The A horizon forms from the accumulation of decomposed organic
matter mixed with mineral material, making it the most agriculturally important
layer.
4. What is the E horizon and how does it differ from the A horizon?
Answer: The E horizon is a light-colored, leached (eluviated) layer beneath the A
horizon that has lost clay, iron, and organic matter to lower horizons.
Rationale: Eluviation removes fine particles and soluble materials, leaving behind a
pale, sandy-textured layer distinct from the darker, organic-rich A horizon above it.
5. What accumulates in the B horizon (subsoil) and why?
Answer: Clay, iron oxides, aluminum, and other materials leached from the horizons
above accumulate in the B horizon through illuviation.
Rationale: Water moving downward carries dissolved and suspended materials out
of the A and E horizons; these materials are deposited in the B horizon, often giving it
a denser, more clay-rich texture.
6. What is soil texture?
, Answer: The relative proportions of sand, silt, and clay particles in a soil.
Rationale: Texture is a fundamental, largely permanent soil property that strongly
influences water-holding capacity, drainage, aeration, and nutrient retention.
7. Rank sand, silt, and clay particles from largest to smallest.
Answer: Sand (largest), silt (intermediate), clay (smallest).
Rationale: By the USDA classification, sand ranges from 0.05–2.0 mm, silt from
0.002–0.05 mm, and clay is less than 0.002 mm in diameter.
8. What soil textural class contains roughly equal properties of sand, silt, and clay and is
generally considered ideal for plant growth?
Answer: Loam.
Rationale: Loam balances the drainage/aeration benefits of sand with the water-
and nutrient-holding capacity of silt and clay, making it well-suited for most crops.
9. What tool is used to determine soil textural class from percentages of sand, silt, and
clay?
Answer: The USDA soil texture triangle.
Rationale: The texture triangle allows a soil scientist to plot the percentages of sand,
silt, and clay and read off the corresponding textural class name.
10. Why does clay have a much higher water- and nutrient-holding capacity than sand?
Answer: Clay particles are extremely small with a very high surface area to volume
ratio and carry negative surface charges that attract and hold water and cations.
Rationale: The large specific surface area and electrostatic charge of clay minerals
allow them to adsorb water molecules and nutrient cations far more effectively than
the comparatively inert, low-surface-area sand particles.
11. Define soil structure.
Answer: The arrangement of soil particles (sand, silt, clay) into aggregates or clumps
called peds.
Rationale: Unlike texture, which is based on particle size, structure describes how
those particles are grouped together, affecting pore space, aeration, and root
penetration.
12. Name the main types of soil structure.
Answer: Granular, blocky, prismatic, columnar, platy, and structureless (single-
grained or massive).
Actual Questions with Answers| Latest Update| Pass Guaranteed
1. What is a soil profile?
Answer: A vertical cross-section of soil from the surface down through the
underlying layers (horizons) to the parent material.
Rationale: The soil profile reveals how a soil formed over time and shows the distinct
horizons that differ in color, texture, structure, and composition.
2. List the master soil horizons in a typical soil profile, from the surface downward.
Answer: O, A, E, B, C, and R horizons.
Rationale: O is organic surface litter; A is topsoil rich in humus; E is a light-colored
eluviated (leached) layer; B is subsoil where leached materials accumulate; C is
weathered parent material; R is unweathered bedrock.
3. What characterizes the A horizon (topsoil)?
Answer: It is typically dark-colored, rich in organic matter and biological activity, and
is the zone where most plant roots and nutrient uptake occur.
Rationale: The A horizon forms from the accumulation of decomposed organic
matter mixed with mineral material, making it the most agriculturally important
layer.
4. What is the E horizon and how does it differ from the A horizon?
Answer: The E horizon is a light-colored, leached (eluviated) layer beneath the A
horizon that has lost clay, iron, and organic matter to lower horizons.
Rationale: Eluviation removes fine particles and soluble materials, leaving behind a
pale, sandy-textured layer distinct from the darker, organic-rich A horizon above it.
5. What accumulates in the B horizon (subsoil) and why?
Answer: Clay, iron oxides, aluminum, and other materials leached from the horizons
above accumulate in the B horizon through illuviation.
Rationale: Water moving downward carries dissolved and suspended materials out
of the A and E horizons; these materials are deposited in the B horizon, often giving it
a denser, more clay-rich texture.
6. What is soil texture?
, Answer: The relative proportions of sand, silt, and clay particles in a soil.
Rationale: Texture is a fundamental, largely permanent soil property that strongly
influences water-holding capacity, drainage, aeration, and nutrient retention.
7. Rank sand, silt, and clay particles from largest to smallest.
Answer: Sand (largest), silt (intermediate), clay (smallest).
Rationale: By the USDA classification, sand ranges from 0.05–2.0 mm, silt from
0.002–0.05 mm, and clay is less than 0.002 mm in diameter.
8. What soil textural class contains roughly equal properties of sand, silt, and clay and is
generally considered ideal for plant growth?
Answer: Loam.
Rationale: Loam balances the drainage/aeration benefits of sand with the water-
and nutrient-holding capacity of silt and clay, making it well-suited for most crops.
9. What tool is used to determine soil textural class from percentages of sand, silt, and
clay?
Answer: The USDA soil texture triangle.
Rationale: The texture triangle allows a soil scientist to plot the percentages of sand,
silt, and clay and read off the corresponding textural class name.
10. Why does clay have a much higher water- and nutrient-holding capacity than sand?
Answer: Clay particles are extremely small with a very high surface area to volume
ratio and carry negative surface charges that attract and hold water and cations.
Rationale: The large specific surface area and electrostatic charge of clay minerals
allow them to adsorb water molecules and nutrient cations far more effectively than
the comparatively inert, low-surface-area sand particles.
11. Define soil structure.
Answer: The arrangement of soil particles (sand, silt, clay) into aggregates or clumps
called peds.
Rationale: Unlike texture, which is based on particle size, structure describes how
those particles are grouped together, affecting pore space, aeration, and root
penetration.
12. Name the main types of soil structure.
Answer: Granular, blocky, prismatic, columnar, platy, and structureless (single-
grained or massive).
