Red and yellow soil

Also called: Red soil · Topic: Soils of India: Types, Degradation and Conservation · NCERT: Class 7, Ch 1 "The Story of Indian Farming"; Class 10, Ch 1 "Resources and Development"

Meaning

Red and yellow soil forms where old crystalline rock breaks down slowly under low rainfall. Iron spread through the soil makes it red. Where that iron is hydrated (joined with water), the soil looks yellow.

It is one of the two main soils of the Peninsular plateau, along with laterite. It covers large areas of Tamil Nadu, Karnataka, Telangana and Jharkhand, and it supports dry-land crops such as ragi, groundnut and millets.

Explanation

How it forms (cause → process → result)

• Parent rock: it forms on old, hard rock of two kinds.
• Crystalline igneous rocks, such as granite. These rocks formed when molten magma cooled.

• Metamorphic rocks, such as gneiss. These are rocks changed by heat and pressure.

• Another source traces the soil to weathered metamorphic rocks [2].

• Cause: low rainfall.
• Process: low rain means only mild weathering (the slow breaking down of rock where it lies). Iron from the rock spreads through the soil. This is called diffusion of iron.
• Result: the soil turns red.

Why it is red or yellow

• Red colour: iron is spread through the crystalline and metamorphic rocks (diffusion of iron).
• Yellow colour: the same soil looks yellow where the iron is hydrated (joined with water).
• Key point: the colour depends on the state of the iron. Red and yellow patches do not come from different parent rocks.

Properties

• Texture: light and porous (full of tiny air spaces, so water drains fast). It is also friable (it crumbles easily) [2].
• Nutrients: poor in nitrogen, phosphorus and humus (dark organic matter from rotted plants and animals). It is also low in lime [2].
• pH: not especially acidic, unlike laterite.
• Fertility depends on grain size:
• Fine-grained red soils become fertile when fertiliser and irrigation are added.
• Coarse, upland red soils stay poor.

Crops and place in soil classification

• Crops: ragi, groundnut, millets, pulses and tobacco.
• ICAR (Indian Council of Agricultural Research) groups Indian soils into eight major groups. Four of these matter most for farming: alluvial, black, red and lateritic [2].

Where it occurs

• Core area: the low-rainfall parts of the eastern and southern Deccan plateau.
• Other areas:
• Odisha and Chhattisgarh
• the southern parts of the middle Ganga plain

• the piedmont zone of the Western Ghats (piedmont: the strip of land at the foot of a mountain range)

• States with large areas: Tamil Nadu, Karnataka, Telangana and Jharkhand.

• Another list of states: Tamil Nadu, Karnataka, north-east Andhra Pradesh, eastern Madhya Pradesh, Bihar, West Bengal and Rajasthan [2].

Don't confuse with

• Laterite soil: it forms under high rainfall with alternate wet and dry seasons. Heavy rain causes intense leaching (rainwater carries dissolved minerals down and away). Red and yellow soil forms under low rainfall with only mild weathering.
• Red laterite: this is a kind of laterite, not red soil. Cashew grows on red laterite in Tamil Nadu, Andhra Pradesh and Kerala. Red soil is linked with ragi and groundnut.
• Yellow soil as a "separate soil type": yellow is the same soil with hydrated iron. It does not come from a different parent rock.
• Black soil: it forms on basalt (dark rock made from cooled lava) of the Deccan trap. It is clayey and holds moisture well. Red soil forms on granite and gneiss, and it is light and porous, so water drains away fast.

Prelims Hooks

• Red colour = diffusion of iron; yellow colour = hydrated iron.
• Red and yellow soil forms under low rainfall. Laterite forms under high rainfall with alternate wet and dry seasons. This is a common trap.
• Parent rock: crystalline igneous rock (granite) and metamorphic rock (gneiss), not basalt.
• Red soil is found in the piedmont zone of the Western Ghats and the southern middle Ganga plain, not only on the Deccan.
• Soil–crop pair: red soil – ragi, groundnut, millets, pulses, tobacco. It is poor in N, P, humus and lime [2].
• ICAR has 8 soil groups. Red is one of the 4 most important for farming, along with alluvial, black and laterite [2].

Mains Points

• Climate decides soil type on the same rock (GS-I):
• Red soil and laterite both form on the old crystalline rocks of the Peninsula.
• Low rain gives mild weathering and red soil. This happens on the eastern and southern Deccan, Odisha and Chhattisgarh.

• Heavy monsoon rain gives intense leaching and laterite, as in the Western Ghats and Kerala.

• Farming limits and fixes (GS-III):

• The soil is porous, so water drains fast. It also lacks N, P, humus and lime [2].
• So it needs liming, organic manure, balanced fertiliser and irrigation.

• Soil testing under the Soil Health Card scheme (2015) helps match fertiliser to these soils.

• Dry-land food security (GS-III):

• Fine-grained red soils become fertile with fertiliser and irrigation. Coarse upland red soils stay poor.
• Rain-fed areas on this soil depend on hardy crops such as ragi, millets, pulses and groundnut.
• These crops make the soil important for food and income in semi-dry parts of the Deccan.

Related concepts

• Laterite soil

Read more

• Red and yellow soils, and laterite soils