Question

Prismatic soil structure is found in:
(A). Subsurface of arid and semi-arid soils
(B). Grassland Soil
(C). Poorly drained soil
(D). Soils with swelling clay
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (A), (C) and (D).
• D. (A), (B) and (D) only.

Hint:

Prism-like structures (prismatic and columnar) are typical of subsurface (B) horizons, especially where clay accumulation and drying cycles occur.
Granular structures are associated with surface organic-rich horizons like those in grasslands.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil structure refers to the grouping of individual soil particles into secondary units called peds.
These structures are classified based on shape into platy, prism-like, blocky, and spheroidal (granular/crumb).
Prism-like structures are characterized by vertical axes that are significantly longer than the horizontal axes.
These are further divided into prismatic (flat tops) and columnar (rounded tops).

Step 2: Detailed Explanation:
Prismatic soil structure is formed primarily due to vertical cracking caused by swelling and shrinking processes during wetting and drying cycles.
In the subsurface (B horizon) of arid and semi-arid soils, clay accumulates and undergoes intense drying cycles.
This promotes the development of vertical cracks, resulting in prismatic peds. Thus, (A) is correct.
Poorly drained soils also frequently develop prismatic structures in their lower horizons due to alternating high and low water tables, which cause localized shrink-swell patterns. Thus, (C) is correct.
Soils with swelling clays (such as Vertisols or vertic subgroups) are dominated by expanding 2:1 clay minerals like montmorillonite.
These mineralogical properties cause severe vertical cracking during dry periods, creating prominent prismatic structures. Thus, (D) is correct.
In contrast, grassland soils (such as Mollisols) are rich in organic matter.
These soils are dominated by granular or crumb structures in the surface A horizon, which are stabilized by organic glues and root systems.
Therefore, prismatic structures are not typically characteristic of grassland topsoils.
Thus, prismatic soil structure is found in (A), (C), and (D).

Step 3: Final Answer:
The correct combination is (A), (C), and (D).