Previous Year Question

The red colour of red soil is primarily caused by ferric oxides, that is, oxidised compounds of iron, present on the soil particles.

During weathering, iron-bearing minerals in rocks come into contact with oxygen. Ironis oxidised into compounds containing iron in the ferric state

( Fe³⁺ ). A thin coating of iron oxide develops over the soil particles, giving the soil a red or reddish-brown appearance.

Relationship between red and yellow soils

1. Anhydrous ferric oxide, particularly hematite (Fe₂O₃), imparts a red colour to the soil.

2. When ferric oxide occurs in a hydrated form, the soil may appear yellow.

3. Red and yellow soils are therefore frequently placed within the same broad soil group.

Formation and distribution

Red soil generally develops through the weathering of ancient crystalline igneous and metamorphic rocks, particularly granite and gneiss. It is mainly found in:

• the eastern and southern parts of the Deccan Plateau;

• Tamil Nadu, Karnataka, Telangana and Andhra Pradesh;

• Odisha and Chhattisgarh;

• the Chota Nagpur Plateau; and

• the southern parts of the middle Ganga plain and the piedmont zones of the Western Ghats.

General characteristics

• In upland areas, it is often light, sandy, porous and shallow.

• In lowlands, it may become finer, deeper and comparatively more fertile.

• It is generally deficient in nitrogen, humus, phosphorus and lime.

• With proper irrigation and fertiliser management, it can support millets, pulses, groundnut, cotton and various coarse grains.

Why are the other options incorrect?

• (a) Abundance of magnesium: Magnesium is not the principal pigment responsible for the red colour of soil.

• (b) Accumulated humus: High humus content generally gives soil a dark brown or black colour.

• (d) Abundance of phosphates: Phosphates do not impart a red colour; red soils are generally deficient in available phosphorus.