Laterite Soil and Red Soil in India: Formation, Difference and Crops

Laterite Soil and Red Soil: Meaning and Key Difference

Red and Laterite Soils Among India's Six Soil Types

India's soils are classified by the factors that form them and by colour, thickness, texture, age and chemical and physical properties. The six major soil types of India are alluvial, black, red and yellow, laterite, arid and forest soils. Of these six, red and laterite soils are the pair most often confused.

The word laterite was coined in 1807 by Francis Buchanan-Hamilton, from rock he saw near Angadipuram in Kerala that could be cut into blocks for building. That double identity, soil for farming and rock for building, runs through everything about laterite.

Difference Between Red Soil and Laterite Soil: Rainfall and Leaching

Rainfall decides which of the two soils forms. Beyond that, red soil and laterite soil differ in what weathering leaves behind, in their acidity, in the crops they carry and in their uses.

Red and Laterite Soils in World Soil Classifications

The six types form a descriptive scheme, and red soils do not map onto a single order of the United States Department of Agriculture's Soil Taxonomy. Worldwide, soils called red include ultisols, alfisols and oxisols that share a reddish colour from iron.

The world scheme of the Food and Agriculture Organization (FAO) handles laterite differently. It calls the iron-rich, hardening layer plinthite and groups the soils that carry it as Plinthosols, keeping the word laterite out of the classification because it had been used in too many senses. The intensely weathered red soils of the humid tropics, the Ferralsols, are chemically poor. They are low in nitrogen, potassium and calcium and hold phosphorus so tightly that crops cannot use it, so settled farming on them needs liming and full fertilisation.

Laterite Soil vs Black Soil and Red Soil vs Black Soil

Black soil, the other great soil of the Peninsula, is the natural contrast to both. It is typical of the Deccan trap (basalt) region of the north-west Deccan plateau, where it has formed from the weathering of lava flows; it covers the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh and extends south-east along the Godavari and Krishna valleys. It is also called regur; the name black cotton soil comes from its suitability for growing cotton.

The sharpest difference is water. Black soil is well known for holding moisture; it develops deep cracks in hot weather, which aerate the soil, and turns sticky when wet. Red soils hold far less water, and both red and laterite soils are poor in plant nutrients, so they need manure, fertiliser or tolerant crops.

Red Soil and Laterite Soil in India: States and Regions

Map of Red and Laterite Soil Areas by State

Red soil covers the eastern and southern Deccan Plateau, including large tracts of Tamil Nadu, Karnataka, southern Maharashtra, Telangana, Andhra Pradesh, Chhattisgarh, Odisha and the Chota Nagpur plateau of Jharkhand. Red and yellow soils also occur along the piedmont zone of the Western Ghats and in the southern parts of the middle Ganga plain.

Laterite occurs mostly in the southern States, the Western Ghats region of Maharashtra, Odisha, some parts of West Bengal and the North-east, and its tea and coffee lands lie in the hills of Karnataka, Kerala and Tamil Nadu. It also caps the plateau in Madhya Pradesh, where the layer is 30 m thick.

Red Soil in India: Formation, Colour and Characteristics

How Is Red Soil Formed? Crystalline Rock and Low Rainfall

Red soil develops on crystalline igneous rocks in areas of low rainfall. In India these are the ancient crystalline and metamorphic rocks of the Pre-Cambrian shield, chiefly granites, gneisses and quartzites, weathering under low to moderate rain.

The colour comes from a wide diffusion of iron through the crystalline and metamorphic rocks. Where the iron occurs in a hydrated form, the same soil looks yellow, which is why red and yellow soils are treated as one class. Worldwide, red soils come in shades from reddish brown to reddish yellow because of their high iron content.

Characteristics of Red Soil and Red Soil Crops

Red soil is not uniformly poor, and its value for farming depends on where it lies and how it is managed. Its main characteristics are these:

• Fertility: Usually low in plant nutrients and humus, and often deficient in nitrogen.
• Acidity: Red soils are generally acidic, which often leaves them short of nutrients.
• Water retention: Many red soils hold little water, and in drier regions they are prone to drought.
• Depth: In the uplands they are thin, gravelly, sandy or stony; on lower plains and valleys they are deep, dark and fertile loams.

What grows on red soil follows its depth. On the thin, porous soils of the uplands only highly tolerant food crops such as bajra can be grown. On the deep loams of the lower plains and valleys, with irrigation, red soil produces excellent crops:

• Fibre and cash crops: Cotton and tobacco.
• Cereals and millets: Wheat, jowar and millet.
• Pulses and oilseeds: Pulses and linseed.
• Others: Potatoes and fruits.

Laterite Soil in India: Laterization, Characteristics and Uses

Laterite Soil Formation: Laterization, Sesquioxides and Plinthite

Laterite soil develops in areas of high temperature and high rainfall, under a tropical or subtropical climate with alternating wet and dry seasons. The repetition of wet and dry seasons is essential. In the wet season, percolating rain leaches silica and bases out of the weathering rock; in the dry season, the iron left behind turns to ferric oxide and the layer begins to harden. The process is called laterization.

1. Leaching: Heavy rain removes silica and bases from the soil.
2. Enrichment: Most minerals wash away, leaving largely iron and aluminium, which react with air and water and give the reddish colour.
3. Loss of humus: High temperature speeds the decomposition of organic matter, so humus does not build up.
4. Hardening: Laterite is soft while wet; exposed to air it gradually hardens, as moisture evaporates and iron salts lock into a rigid structure.

Laterite is described as rich in sesquioxides, oxides with three atoms of oxygen to two of a metal, the oxides of iron and aluminium. They accumulate because the silica and bases around them are removed by deep chemical weathering, while they themselves resist it.

Iron behaves in a particular way. While the soil is saturated, much of the iron is in the ferrous form and mobile. When conditions turn drier it precipitates as ferric oxide, and it will not, or only partly, dissolve again when the soil is wet once more. That is why the iron-rich layer shows red mottles, and why nearly all laterites are rusty red from their high iron oxide content.

Soil scientists call this iron-rich layer plinthite. It hardens to ironstone on repeated drying and wetting, and many tropical plateaus are protected by an ironstone cap. Most laterite is old. A period of active laterization ran from about the mid-Tertiary to the mid-Quaternary, between 35 and 1.5 million years ago. Laterite covers are thick on old, stable land surfaces, where they can form an ironstone crust.

Characteristics of Laterite Soil: Rich in Iron, Poor in Nutrients

The leaching that forms laterite also decides its character:

• Rich in: Iron oxide and aluminium compounds, which leaching leaves behind.
• Poor in: Plant nutrients generally, since the heavy rain that forms laterite washes them out.
• Acidity and depth: Mostly deep to very deep and acidic, with a pH below 6.0.
• Humus varies with cover: Rich in humus under deciduous and evergreen forest, poor under sparse vegetation in semi-arid areas.
• Erosion: Prone to erosion and degradation because of its position on the landscape.

For these reasons laterite is poor farmland as it stands. Manures and fertilisers have to be applied to make it fertile, and in the hills soil conservation is needed first.

Laterite Soil Uses: Cashew, Tea, Coffee, Bricks and Bauxite

With care, laterite supports valuable tree and plantation crops. After soil conservation, the laterite of the hilly areas of Karnataka, Kerala and Tamil Nadu is very useful for tea and coffee, and red laterite in Tamil Nadu, Andhra Pradesh and Kerala suits tree crops such as cashew nut.

• Cashew: A hardy crop introduced by the Portuguese about five centuries ago, first in Goa; well-drained red, sandy and laterite soils are ideal for it.
• Tea and coffee: Grown on conserved laterite of the southern hills.
• Tapioca: Cassava, called tapioca in Indian English and kappa in Malayalam, grows productively on poor soil and is a staple of Kerala.

Laterite is widely cut into bricks for house construction. It is quarried while below the water table, when it is wet and soft enough to cut with a spade; it then hardens in the air and resists the weather. The laterite around Angadipuram in Malappuram district, Kerala, where the rock was first named, was declared a National Geological Monument by the Geological Survey of India in 1979. It is still quarried for building, and the quarrying now threatens the deposit.

Laterites are associated with deposits of bauxite, iron and nickel, and lateritic bauxites form by laterization of silicate rocks such as granite, gneiss and basalt. In India bauxite occurs mainly in Tertiary deposits with laterite on the plateaus and hill ranges of the Peninsula and in the coastal tracts. Odisha is the largest producer, led by Kalahandi and Sambalpur, with Balangir and Koraput rising, and the patlands of Lohardaga in Jharkhand have rich deposits. India's reserves are among the ten largest in the world, and most lie on tribal land.

Managing Red and Laterite Soils for Farming

Improving Red and Laterite Soils: Liming, Manure and Conservation

Both soils lose nutrients to leaching, so management starts by putting them back. The main methods are these:

• Liming: Raises the pH of acidic red soils so that crops intolerant of acidity can grow; used too heavily, it can speed microbial breakdown of organic matter and harm soil structure.
• Manures and fertilisers: Needed on laterite before it can be cultivated, and on red soils to replace the nitrogen lost to leaching and erosion, and the phosphorus and potassium taken by repeated harvests.
• Soil conservation: On hill slopes of Karnataka, Kerala and Tamil Nadu, conservation comes first; only then does laterite carry tea and coffee.
• Choosing tolerant crops: Bajra on thin red upland soil, cashew and tapioca on poor laterite.

Sources

• NCERT: Exploring Society, India and Beyond (Grade 7, second book), Chapter 1, The Story of Indian Farming
• NCERT: Curiosity, Science (Grade 6), Chapter 11, Nature's Treasures
• NCERT: India, People and Economy (Class XII), Chapter 5, Mineral and Energy Resources
• NCERT: Contemporary India II (Class X), Chapter 1, Resources and Development
• FAO: Lecture Notes on the Major Soils of the World, Plinthosols and Ferralsols
• PIB: Geo-Heritage Sites (Geological Survey of India national geological monuments)
• Directorate of Cashewnut and Cocoa Development: Crop history and production technology of cashew
• Wikipedia: Laterite
• Wikipedia: Red soil
• Wikipedia: Angadipuram Laterite
• Wikipedia: Bauxite
• Wikipedia: Cassava
• UPSC: Civil Services Examination question papers