Laterite soil

Also called: Laterite · 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

Laterite soil is a deep, acidic soil left behind when heavy rain in hot places washes most minerals out of the rock. Its name comes from the Latin word later, meaning brick. It covers the high-rainfall parts of Peninsular India and the north-east. It also shows clearly how climate decides soil type. It matters for plantation crops (tea, coffee, cashew), for soil erosion, and as a building material.

Explanation

How it forms (cause → process → result)

• Climate it needs: tropical and subtropical areas with high rainfall and alternate wet and dry seasons [2].
• Cause: very heavy seasonal rain and high heat.
• Process: leaching.
• Rainwater sinks down through the soil.
• It dissolves minerals and carries them down and away. This is leaching.

• Silica and bases (calcium, magnesium, potassium) are washed out [2].

• Result:

• Iron and aluminium oxides stay behind.

• They give the soil its red to reddish-yellow colour [2].

• NCERT calls it the "intensively leached soil of the monsoon climate".

Properties

• Depth: deep to very deep.
• Acidity: acidic, with pH below 6.0. pH is a scale that shows how acidic or alkaline a soil is, and 7 is neutral.
• Nutrients: generally poor in plant nutrients, because leaching has removed them.
• Humus depends on plant cover. Humus is the dark organic matter that comes from rotted plants and animals.
• Under thick forest: rich in humus, because leaves keep falling and rotting.

• Under sparse plants and in semi-arid areas: poor in humus.

• Erosion: on slopes it washes away easily once the plant cover is removed.

• Hardens in air:
• Wet laterite is soft.
• In air and sun it sets hard, like brick.
• So it is cut into building blocks.

Place in the ICAR classification

• ICAR (Indian Council of Agricultural Research) puts Indian soils into eight major groups [2].
• Laterite is one of the four most important for farming. The other three are alluvial, black and red soils [2].
• Both laterite and red and yellow soil form on the old, hard rocks of the Peninsular plateau. The main difference is how much rain falls on that rock.

Making it productive

• Soil conservation keeps the soil on hill slopes:
• terracing (cutting steps into a slope)
• contour planting (planting along lines of equal height)

• mulching (covering the soil with leaves or straw)

• Farmers add manure and fertiliser to deal with the acidity and low nutrients and to raise output.

Where it occurs

• Main areas in India: the southern states, the Western Ghats of Maharashtra, Odisha, parts of West Bengal and the north-east.
• Other states with laterite: eastern Andhra Pradesh, Karnataka, Kerala, Madhya Pradesh, Odisha, Assam and parts of Maharashtra [2].
• Crop belts:
• Tea and coffee: laterite in the hills of Karnataka, Kerala and Tamil Nadu, where it is protected by soil conservation.

• Cashew: red laterite in Tamil Nadu, Andhra Pradesh and Kerala.

• Building blocks: laterite is cut into bricks in Malabar (Kerala), Goa and the Konkan coast.

• Geo-heritage site:
• The laterite near Angadipuram is a declared geo-heritage site / National Geological Monument [3].
• It lies in the PWD rest house premises in Malappuram district, Kerala.

• The Geological Survey of India (GSI) or the state government is responsible for protecting it [3].

• Swachhata (cleanliness) campaign: on 6 October 2023, GSI held this campaign at the Angadipuram laterite. It was one of 15 geo-heritage sites covered in the drive [4].

• World: laterite forms in tropical and subtropical lands that have high rainfall and alternate wet and dry seasons [2].

Don't confuse with

• Red and yellow soil: it forms in low-rainfall areas through mild weathering, and iron spreads through the soil. Laterite forms in high-rainfall areas through intense leaching. Red soil is also not especially acidic, while laterite has a pH below 6.0.
• Red colour vs laterite colour: red soil gets its colour from diffusion of iron (iron spreading through the soil), and turns yellow where the iron is hydrated (joined with water). Laterite's red to reddish-yellow colour comes from iron and aluminium oxides left behind after leaching [2].
• Leaching vs weathering: weathering is rock breaking down where it lies. This is the main process for red soil. Leaching is water carrying dissolved minerals down and out of the soil. This is the main process for laterite.
• Black soil: it forms from basalt (volcanic lava rock) in the Deccan trap, holds moisture well and suits cotton. Laterite forms by leaching, holds few nutrients and suits tea, coffee and cashew.

Prelims Hooks

• "Laterite" comes from the Latin later = brick. It hardens in air and is cut into blocks in Malabar, Goa and Konkan.
• Process–result pair: leaching removes silica and bases, and iron and aluminium oxides stay behind [2].
• Trap: laterite needs high rainfall with alternate wet and dry seasons. Red and yellow soil forms in low rainfall.
• Laterite is acidic (pH < 6.0). It is humus-rich under thick forest and humus-poor under sparse plants.
• Crop–soil pairs: cashew grows on red laterite (Tamil Nadu, Andhra Pradesh, Kerala). Tea/coffee grow on laterite hills (Karnataka, Kerala, Tamil Nadu).
• Angadipuram laterite (Malappuram, Kerala) is a GSI National Geological Monument [3].

Mains Points

• Climate decides soil type (GS-I): red soil and laterite form on the same old Peninsular crystalline rock.
• Where rain is low, weathering is mild and red soil forms.

• Where heavy monsoon rain falls, as in the Western Ghats, Kerala and the north-east, intense leaching produces laterite.

• Farming limits and solutions (GS-III): laterite is acidic and low in nutrients.

• It needs liming (adding lime to cut acidity), organic manure, balanced fertiliser and irrigation.

• Soil testing under the Soil Health Card scheme (2015) helps farmers give these soils the right fertiliser.

• Degradation and protection (GS-III): laterite on slopes erodes fast once forest is cleared.

• Plantations in Karnataka, Kerala and Tamil Nadu need terracing, contour bunds and cover crops to stay productive.
• Laterite blocks are cheap local building material, but quarrying damages the land. That is why sites like Angadipuram are protected as geo-heritage [3][4].

Related concepts

• Red and yellow soil

Read more

• Red and yellow soils, and laterite soils