The red colour of red soils is mainly caused by:

Red soils get their red colour from the wide diffusion of iron compounds, mainly ferric oxide (iron oxide), in the soil. When these iron oxides are in a hydrated form, the soil looks yellow, which is why these soils are often called "red and yellow soils".

How red soils are formed:

• They develop on old crystalline igneous and metamorphic rocks, such as granite and gneiss, in areas of low to moderate rainfall.
• Weathering of these rocks releases iron, which is oxidised in the presence of air and spread through the soil, giving it a red colour.

Distribution in India:

• Red soils are the second largest soil group in India (together with yellow soils), covering about 18 per cent of the land.
• They are found mainly in the eastern and southern parts of the Deccan plateau: Tamil Nadu, Karnataka, Andhra Pradesh, Telangana, Odisha, Chhattisgarh and Jharkhand (Chota Nagpur plateau).
• They are also found in parts of Maharashtra, the Aravallis, Meghalaya and the north-east.

Characteristics of red soils:

• They are generally sandy to loamy in texture in the uplands and more clayey in the lowlands.
• They are porous and friable (crumbly).
• They are poor in nitrogen, phosphorus, lime and humus, and fairly rich in potash.
• They are generally acidic to neutral.
• Uplands have thin, gravelly, less fertile soils, while lowlands and valleys have deeper, darker and more fertile soils.

Crops grown:

With irrigation and fertilisers, red soils can support rice, ragi, millets, groundnut, pulses, tobacco, cotton, potatoes and fruits such as mangoes and oranges.

Why not the other options?

• ✗Aluminium compounds are important in laterite and bauxite, but they are generally whitish and do not cause the red colour.
• ✗Humus gives soils a dark brown or black colour, not red.
• ✗Calcium carbonate gives a whitish colour and forms kankar nodules.