🚦Soil Taxonomy: The 12 Soil Orders and Their Classification

Walk from the Indo-Gangetic floodplain in Uttar Pradesh to the Deccan plateau in Maharashtra, and then to the laterite hills of Kerala. In each region, the soil looks, feels, and behaves differently — light sandy alluvium in UP, dark sticky black soil in Maharashtra, and red crumbly laterite in Kerala. To make sense of this diversity, scientists developed Soil Taxonomy — a universal system that classifies all soils on Earth into just 12 orders based on measurable properties.

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What is Soil Taxonomy?

Soil Taxonomy is the US Comprehensive Soil Classification System based on the 7th Approximation, developed by the USDA (United States Department of Agriculture).

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Six Hierarchical Categories

From broadest to most specific:

• The smallest unit of classification is the soil series
• The smallest volume of soil (1-10 m²) is called a pedon — the three-dimensional unit large enough to represent all horizons
• Many pedons within defined limits make up a soil series

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Epipedons (Diagnostic Surface Horizons)

Epipedons are the uppermost soil horizons darkened by organic matter. They are not synonymous with the A-horizon. ^{ICAR JRF 2020}

There are 9 epipedons:

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The 12 Soil Orders

Mnemonic: “A VGHAMI HOUSE” — Aridisols, Vertisols, Gelisols, Histosols, Alfisols, Mollisols, Inceptisols, Histosols… The full set of 12 is easier to learn from the table below.

Overview Table

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1. Inceptisols — The Most Widespread Soil in India

Latin inceptum = beginning. Young soils showing the beginning of horizon formation.

Farm example: The alluvial soils of the middle Gangetic plain (Bihar, eastern UP) are mostly Inceptisols — young soils with moderate development, suitable for rice-wheat cropping.

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2. Entisols — Recently Formed Soils

From “recent.” Soils with virtually no horizon development.

Farm example: Fresh alluvial deposits in the Brahmaputra floodplain (Assam) are Entisols — constantly receiving new sediment prevents horizon development.

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3. Alfisols — Forest Soils with Clay-Enriched Subsoil

Farm example: Red soils of the Chotanagpur plateau (Jharkhand) are Alfisols — their clay-enriched subsoil helps retain moisture for upland rice.

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4. Vertisols — The Self-Ploughing Black Soils

Latin verto = to turn. Dark or black swelling clays.

Farm example: The black cotton soils of Maharashtra, MP, and Gujarat are classic Vertisols. ICRISAT developed the Broad Bed and Furrow (BBF) system specifically for managing these difficult soils.

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5. Aridisols — Desert Soils

Farm example: The Thar desert soils of western Rajasthan are Aridisols. With irrigation from the Indira Gandhi Canal, these soils can produce crops if salinity is managed.

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6. Ultisols — Highly Weathered Acidic Soils

Farm example: The deeply weathered laterite soils of the Western Ghats (Kerala, Karnataka) are Ultisols. Heavy rainfall has leached away bases, making them acidic and nutrient-poor.

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7. Mollisols — The Most Productive Soils on Earth

Farm example: The foothill soils of the Shivalik range (Uttarakhand) have Mollisol patches — deep, dark, fertile soils formed under grassland vegetation.

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8. Oxisols — Extremely Weathered Tropical Soils

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9. Histosols — Organic/Peat Soils

Farm example: The Kuttanad (below-sea-level farming) region of Kerala has Histosol-like soils where paddy is cultivated in peaty, waterlogged conditions.

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10. Spodosols — Soils with Subsurface Accumulation

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11. Andisols — Volcanic Ash Soils

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12. Gelisols — Permanently Frozen Soils

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Special Soil Features

Cat Clays

Wet clay soils high in reduced forms of sulphur (iron sulphides like pyrite, FeS₂). When drained, pyrite oxidizes to produce sulphuric acid, dropping pH below 3.5. The name comes from the clay-sulphate surface that shines like a cat’s eye.

Farm example: Acid sulphate soils (cat clays) in coastal Kerala become extremely acidic when drained, making them hostile for most crops except paddy under submerged conditions.

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Catena

A sequence of soils of similar age, parent material, and climate, but with different characteristics due to variation in relief and drainage. Illustrates how topography alone creates different soils on the same landscape — from well-drained soils on ridges to waterlogged soils in valley bottoms.

Farm example: In a single watershed of the Deccan plateau, the hilltop has thin, gravelly red soil (Alfisol), the midslope has deeper red soil, and the valley bottom has black soil (Vertisol) — a classic catena.

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Caliche

A hardened layer near the surface, cemented by secondary carbonates of Ca and Mg precipitated from soil solution. Forms in arid and semi-arid regions where evaporation exceeds precipitation. When thick, caliche impedes root penetration and water infiltration.

Farm example: In parts of western Rajasthan and Gujarat, caliche (kankar) layers restrict root growth of fruit trees, requiring deep chiseling before planting.

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Exam Tips and Mnemonics

• 12 soil orders mnemonic: “I Eat All Vegetables And Understand My Old Home Smells Awful Good” — Inceptisols, Entisols, Alfisols, Vertisols, Aridisols, Ultisols, Mollisols, Oxisols, Histosols, Spodosols, Andisols, Gelisols
• India’s top 5 by area: “I EAT Very Adequate food” — Inceptisols (39.74%) > Entisols (28.08%) > Alfisols (13.55%) > Vertisols (8.52%) > Aridisols (4.28%)
• Not found in India: Gelisols, Andisols, Spodosols — “GAS not in India”
• Most productive soil: Mollisols (grassland soils)
• Self-ploughing soil: Vertisols (montmorillonite clays, deep cracks)
• 9 epipedons: Mollic (important), Ochric (most common in India), Umbric (important)
• Pedon = smallest 3D soil unit; Series = basic unit of classification
• Cat clays = acid sulphate soils; pH < 3.5 when drained
• Catena = soil sequence due to topography changes
• Caliche = carbonate-cemented layer in arid soils

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Summary Table

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Summary Cheat Sheet