Soil classification is complex, aiming to be meaningful to geographers while accurately reflecting soil types and gradations.
Two main types of classification are used:
Based on the assumed origins of the soil.
Based on the observable properties of the soil profile.
Soil Classification: Zonal, Intrazonal, Azonal
Zonal System
The zonal system was proposed by Russian pedologists (Dukuchaiev, Glinka) and highlights the relationship between climate, vegetation, and soil zones.
Three main classes of soils are recognized:
Zonal soils: Well-developed soils reflecting climate as the dominant factor.
Intrazonal soils: Well-developed soils formed due to dominant local factors.
Azonal soils: Immature or poorly developed soils.
Soil Classification on World Map
Zonal Types
Zonal soils occur in broad geographical areas and are influenced by climate and vegetation more than rock type.
Examples include:
Podzols (ash-soil): Characterized by a bleached E horizon due to cheluviation. Found in cool climates south of the tundra region, often associated with coniferous forests.
Brown Earth: Found in milder climates supporting deciduous forests. These soils exhibit less intense leaching compared to podzols and have a brown color due to dispersed sesquioxides.
Tundra Soils: Variations exist due to patterns of ground ice. Examples include brown polar desert soils and Arctic brown forest soils.
Sierozems: Found in desertic areas with low organic matter but high fertility when irrigated.
Chernozem Soils: Found in steppe or prairie regions. They have a calcium-rich horizon and a deep dark upper layer with high humus content.
Chestnut Soils: Located in the arid side of the chernozem belt, these soils have a lower organic content and shallow carbonate layers.
Prairie Soils: Found in the transition zone between chernozems and forest brown earth.
Grumusols: Dark clayey soils in savanna regions with seasonal wet and dry climates, characterized by dry-season cracking.
Ferralsols: Tropical soils rich in ferric oxide, with low fertility due to the lack of humus and bases.
Intrazonal Types
Intrazonal soils occur within zonal soil types and reflect the influence of local factors like relief, parent material, or age.
Examples include:
Hydromorphic Soils: Found in marshes and swamps, these soils undergo gleying due to waterlogging.
Calcimorphic Soils: Developed on calcareous parent material. Examples include rendzinas (rich in organic matter) and terra rossa (clay-rich and reddish in color).
Halomorphic (saline) Soils: Found in deserts with types such as solanchak, solonetz, and solodic soils.
Classification of Soils
Azonal Soils
Azonal soils are immature soils developed through deposition by agents of erosion and lack a well-developed profile.
Examples include:
Alluvial Soils: Found on active floodplains, these soils are frequently buried under new sediments.
Regosols: Composed of dry, loose dune sands or loess.
Lithosols: Found on steep slopes, consisting of imperfectly weathered rock fragments.
Criticisms of the zonal concept:
Zonal soils can exist outside their typical climate zones.
Azonal soils may be influenced by local factors, not just the lack of time for development.
Soil profiles may reflect past climates rather than current conditions.
USDA Soil Taxonomy
The US Department of Agriculture uses a soil classification system based on observed soil properties rather than genetic considerations.
Twelve orders of soils are identified based on their developmental properties and the presence or absence of diagnostic horizons.
Alfisols: Named for aluminum and iron content.
Andisols: Soils derived from volcanic ash.
Aridisols: Soils typical of arid climates.
Entisols: Recently formed soils lacking distinct horizons.
Gelisols: Found in regions with permafrost.
Histosols: Soils rich in organic matter.
Inceptisols: Young soils with weakly developed horizons.
Mollisols: Known as grassland soils with high fertility.
Oxisols: Found in tropical regions, rich in oxygen compounds.
Spodosols: Characterized by ashy horizons.
Ultisols: Leached soils with low nutrient bases.
Vertisols: Expansive soils formed from volcanic rock.
Soils Characterized by Maturity
Soil Orders Based on Climate and Maturity
Entisols: Mineral soils with no distinct horizons, recently deposited, found in equatorial to arctic zones.
Inceptisols: Weakly developed horizons, found in river floodplains and deltas, highly productive in warm, moist climates.
Soils Characterized by Climate
Mollisols: Grassland soils with high fertility and a thick mollic epipedon, found in the Great Plains, Pampas, and Eurasian steppes.
Aridisols:Desert soils with calcium carbonate layers, fertile with irrigation.
Gelisols: Permafrost soils churned by freeze/thaw action, rich in organic matter.
Soils Characterized by Parent Materials
Vertisols: Formed on volcanic rock, expansive, found in the Deccan Plateau of India.
Andisols: Derived from volcanic ash, typically fertile, supporting dense vegetation in moist climates.
Soils High in Organic Matter
Histosols: Known as peat or muck soils, formed in cool, poorly drained climates, rich in organic matter.
Specialized Soils
Alfisols: Found across diverse climates, from boreal forests to Mediterranean regions, characterized by clay-rich horizons formed through illuviation.
Spodosols: Acidic soils with light-colored albic horizons, commonly found under cold needle leaf forests.
Oxisols: Stable soils in equatorial regions, typically associated with rainforest ecosystems.
Ultisols: Vulnerable to soil erosion, found in regions like Southeast Asia, South America, and the United States.
General Classification of Soil
The most comprehensive basis for the classification of soils is the specific climate and vegetation under which the soil has grown and developed.
Accordingly, the soils of the world can be broadly classified into two main types:
Pedalfurs
These soils have grown in humid areas under rich vegetation cover.
They contain a greater proportion of aluminium and iron but lack essential plant nutrients such as potassium, calcium, and phosphorus.
Pedocols
These soils have developed under arid conditions and retain all the elements needed to make plant foods.
They are found in regions receiving less than 25 inches of rainfall annually and are generally lighter in color.
Pedocols do not suffer from leaching and are alkaline in nature.
These two main soil classes have further subdivisions or sub-varieties based on:
The type of vegetation cover
Temperature conditions
The amount of precipitation
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