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Practice MCQ

Unit 6 - Notes

SOL103

Unit 6: Soil classification

1. Soil Taxonomy

Soil taxonomy is a comprehensive system of soil classification developed by the United States Department of Agriculture (USDA). Unlike earlier systems based on soil genesis (formation), Soil Taxonomy is based on observable and measurable soil properties (morphology) as they exist today.

A. The Hierarchy of Soil Taxonomy

The system is hierarchical, consisting of six categories. As you move down the hierarchy, the information becomes more specific.

  1. Order (12): The highest category. Based on major soil forming processes and diagnostic horizons. (Suffix: -sol)
  2. Suborder: Differentiated by soil moisture, temperature regimes, and dominant chemical or physical properties.
  3. Great Group: Differentiated by the presence or absence of specific diagnostic horizons.
  4. Subgroup: Describes the central concept of the great group (Typic) or intergrades to other groups.
  5. Family: Based on properties important for plant growth (texture, mineralogy, temperature, pH).
  6. Series: The most specific unit; named after the location where the soil was first described.

B. Diagnostic Horizons

Soil taxonomy relies heavily on specific layers termed diagnostic horizons to classify soils.

1. Diagnostic Surface Horizons (Epipedons)

  • Mollic: Dark color, high organic matter (>0.6% organic C), thick (>25 cm), high base saturation (>50%). Common in grasslands.
  • Umbric: Similar to Mollic but with low base saturation (<50%). Acidic.
  • Ochric: Light-colored, low organic matter, or too thin to be Mollic or Umbric. Common in arid soils.
  • Histic: Very high organic matter content; peat or muck; formed in wet areas.
  • Melanic: Thick, black, high organic matter; associated with volcanic ash (allophane).
  • Plaggen: Man-made sod-like horizon created by years of manuring.

2. Diagnostic Subsurface Horizons (Endopedons)

  • Argillic: Accumulation of silicate clays (illuviation).
  • Natric: Argillic horizon with >15% exchangeable sodium and columnar structure.
  • Kandic: Accumulation of low-activity clays (Fe and Al oxides, kaolinite).
  • Spodic: Accumulation of illuvial humus and aluminum/iron oxides.
  • Cambic: Weakly developed horizon; physical alteration or chemical weathering but no major accumulation.
  • Oxic: Highly weathered; consists mostly of iron and aluminum oxides and kaolinite.
  • Calcic: Accumulation of calcium carbonate ().

2. Keys to Soil Orders (The 12 Orders)

The 12 soil orders are the broadest classification. They are identified by specific formative elements in their names.

Soil Order Formative Element Derivation Key Characteristics
Gelisoils el L. gelare (to freeze) Soils with permafrost within 100 cm of the surface. Cryoturbation (frost churning) is common.
Histosols ist Gr. histos (tissue) Organic soils (>20-30% OM). Peats and mucks. No permafrost.
Spodosols od Gr. spodos (wood ash) Acid soils with subsurface accumulation of humus and Al/Fe oxides. Common in coniferous forests.
Andisols and Jap. ando (black soil) Soils formed from volcanic ash. High in allophane and CEC. Low bulk density.
Oxisols ox Fr. oxide Highly weathered, tropical soils. Rich in Fe/Al oxides. Low fertility but good physical structure.
Vertisols ert L. verto (turn) Shrinking and swelling clays (>30% clay). Deep cracks when dry. Self-mulching.
Aridisols id L. aridus (dry) Dry soils (aridic moisture regime). Often have accumulation of salts, carbonates, or gypsum.
Ultisols ult L. ultimus (last) Strongly leached, acid forest soils with clay accumulation (Argillic). Low base saturation (<35%).
Mollisols oll L. mollis (soft) Grassland soils. Dark, thick surface horizon (Mollic). High base saturation. Very fertile.
Alfisols alf Aluminum/Iron Moderately leached forest soils with clay accumulation (Argillic). High base saturation (>35%).
Inceptisols ept L. inceptum (beginning) Soils with weak profile development (Cambic horizon). More developed than Entisols.
Entisols ent Recent Recent soils with little to no profile development (No B horizon). A over C horizon.

Simplified Key for Identification (Simplified Decision Logic)

  1. Frozen? Gelisols
  2. Organic? Histosols
  3. Volcanic Ash? Andisols
  4. Specific Horizons:
    • Spodic (Humus/Fe)? Spodosols
    • Oxic (Tropical/Red)? Oxisols
    • Swelling Clays (Cracks)? Vertisols
    • Dry/Salt? Aridisols
  5. Clay Accumulation:
    • Low Base Saturation (Acidic)? Ultisols
    • High Base Saturation (Fertile)? Alfisols
  6. Thick Dark Surface (Grassland)? Mollisols
  7. Weak B Horizon? Inceptisols
  8. No B Horizon? Entisols

3. Soils of India

Indian soils are traditionally classified based on the system developed by the Indian Council of Agricultural Research (ICAR), primarily influenced by geology and climate. Below is a detailed breakdown of Indian soil groups and their correlation with USDA taxonomy.

A. Alluvial Soils

  • Distribution: Largest soil group in India (approx. 43-46% of total area). Found in the Indo-Gangetic plains (Punjab to West Bengal), Brahmaputra valley, and coastal deltas.
  • Formation: Deposition of sediments (alluvium) brought by rivers.
  • Characteristics:
    • Texture: Varies from sandy loam to clay.
    • Color: Light grey to ash grey.
    • Chemical: Rich in Potash (K) and Lime; Poor in Nitrogen (N), Phosphorus (P), and Humus.
    • Types: Khadar (New alluvium, sandy, light) and Bhangar (Old alluvium, clayey, darker, contains 'Kankar' or lime nodules).
  • USDA Taxonomy: Mostly Entisols (recent deposits) and Inceptisols (slightly developed). Some are Alfisols.
  • Crops: Rice, wheat, sugarcane, jute.

B. Black Soils (Regur Soils)

  • Distribution: Deccan Plateau (Maharashtra, Madhya Pradesh, Gujarat, parts of Karnataka).
  • Formation: Weathering of basaltic lava flow (volcanic rocks).
  • Characteristics:
    • Texture: Clayey (Montmorillonite clay mineral).
    • Color: Deep black to light black (due to Titaniferous magnetite and humus).
    • Properties: High water-holding capacity. Swells and becomes sticky when wet; shrinks and develops deep cracks when dry (Self-ploughing).
    • Chemical: Rich in Lime, Iron, Magnesia, Potash; Poor in N, P, and Organic Matter.
  • USDA Taxonomy: Vertisols.
  • Crops: Cotton (Cotton Soil), soybean, sorghum, citrus.

C. Red Soils

  • Distribution: Tamil Nadu, Karnataka, Andhra Pradesh, Odisha, Jharkhand (surrounding the black soil belt).
  • Formation: Weathering of ancient crystalline and metamorphic rocks (granite and gneiss).
  • Characteristics:
    • Color: Red due to diffusion of Iron Oxides (). Yellow when hydrated.
    • Texture: Sandy to loamy. Porous and friable.
    • Chemical: Poor in N, P, K, and Humus. Generally acidic to neutral.
  • USDA Taxonomy: Alfisols, Ultisols, and Inceptisols.
  • Crops: Millets, pulses, oilseeds, tobacco.

D. Laterite Soils

  • Distribution: Summits of Western Ghats (Kerala, Karnataka), Eastern Ghats, and parts of North-East India. Areas with high temperature and high rainfall.
  • Formation: Formed by Laterization (intense leaching). Silica is leached away, leaving behind Iron and Aluminum oxides.
  • Characteristics:
    • Nature: Hardens like a brick upon drying.
    • Chemical: Rich in Fe and Al oxides. Very poor in N, P, K, Lime, and Organic Matter. Highly acidic.
  • USDA Taxonomy: Ultisols and Oxisols.
  • Crops: Plantation crops (Tea, Coffee, Rubber, Cashew).

E. Arid and Desert Soils

  • Distribution: Rajasthan, North Gujarat, Southern Punjab.
  • Characteristics:
    • Texture: Sandy.
    • Chemical: High in soluble salts and phosphates; Low in Nitrogen and Organic Matter.
    • Presence of "Kankar" (calcium carbonate) layer limits water infiltration.
  • USDA Taxonomy: Aridisols and Entisols.
  • Crops: Drought-resistant crops like Bajra, Guar (with irrigation).

F. Forest and Mountain Soils

  • Distribution: Himalayan region and Western Ghats.
  • Characteristics: High organic matter but often acidic due to leaching. The character changes with altitude.
  • USDA Taxonomy: Undifferentiated. Includes Inceptisols, Spodosols (in conifers), and Mollisols.

G. Saline and Alkaline Soils

  • Local Names: Usara, Reh, Kallar.
  • Characteristics:
    • Saline: High soluble salts (EC > 4 dS/m), pH < 8.5. White encrustation.
    • Alkaline (Sodic): High Exchangeable Sodium Percentage (ESP > 15), pH > 8.5. Poor structure.
  • USDA Taxonomy: Aridisols or salt-affected phases of other orders.

H. Peaty and Marshy Soils

  • Distribution: High rainfall/humid areas. Kerala (Kari soils), Coastal Odisha, Sunderbans.
  • Characteristics: Black, heavy, highly acidic, high accumulation of organic matter (sometimes bluish-green due to ferrous iron).
  • USDA Taxonomy: Histosols.