Question

Prismatic soil structure is a distinct feature in

• A. Red soils
• B. Black soils
• C. Alluvial soils
• D. Sodic soils

Hint:

Think about which problem soil has high exchangeable sodium that hardens into vertical columns.

Answer 1

The Correct Option is D

This question checks if you know which problem soil is linked with prismatic (columnar) structure, one of the basic soil structure types along with granular, crumb, blocky, platy and single grained.

1. Red soils: These soils form from weathered granite and gneiss rocks. They stay open and porous, so they show a granular or crumb structure, not a columnar one.
2. Black soils: Rich in swelling clay, these soils expand when wet and shrink when dry, cracking into angular blocky peds. The structure here is blocky, not prismatic.
3. Alluvial soils: Deposited layer by layer by rivers, these soils are young and mostly loose, with a single grain or weak granular arrangement.
4. Sodic soils: Excess exchangeable sodium disperses the clay on wetting, and on drying the soil hardens into tall, flat topped columns. This columnar or prismatic build up is the classic field sign of sodic soils.

The correct option is sodic soils, since prismatic structure develops directly because of the sodium induced dispersion and hardening typical of these soils.

Let's summarize:

• Prismatic or columnar structure means tall, vertical peds with rounded or flat tops.
• High exchangeable sodium is what causes this structure to form in sodic soils.

So the answer is sodic soils, option 4.