Soil classification is a foundational aspect of soil science, serving as the systematic categorization of soils based on various physical, chemical, and biological properties. This classification system provides scientists, engineers, and agriculturists with a structured framework for understanding soil diversity, behavior, and suitability for different purposes. By organizing soils into distinct groups, it facilitates effective communication, management, and decision-making in fields ranging from agriculture and environmental conservation to civil engineering and land-use planning. Through the delineation of soil types and their characteristics, soil classification plays a crucial role in guiding sustainable land management practices and ensuring the responsible utilization of Earth’s most essential natural resource.
Curtis Fletcher Marbut’s World Soil Classification, renowned in the field, highlights the significant interconnection between climate, vegetation, and soil within his classification system.
Marbut’s approach categorizes global soil into three main classes:
- Zonal soil group
- Azonal soil group
- Intrazonal soil
Zonal soils, within Marbut’s classification, represent well-developed soils that prominently demonstrate the impact of climate as the primary factor shaping their formation.
Intrazonal soils, on the other hand, are also well-developed but arise where a particular local factor holds dominance in their formation.
Azonal soils, in this classification system, are characterized as immature or poorly developed soils.
ZONAL SOIL
These soils are widespread across extensive geographical regions or zones and are primarily shaped by the climate and vegetation of the area, rather than by the type of underlying rock. Their maturity is a consequence of enduring stable conditions over a prolonged period.
Podzols (ash-soil):
- These soils, prevalent in cooler climates adjacent to the tundra, showcase distinctive bleached E horizons due to the cheluviation process.
- They may evolve into humus-enriched B horizons forming humus podzols or, in some cases, show a notable concentration of iron oxide, leading to iron podzols.
- Occasionally, an iron-pan may develop, obstructing drainage and creating gley podzols. Podzols are often associated with coniferous forests.
Brown Earth:
- Found in milder climates supporting deciduous forests, these soils exist south of the primary podzol region.
- While exhibiting leaching, it is less intense compared to podzols.
- The absence of free calcium in the upper part of the profile doesn’t lead to the downward movement of sesquioxides, giving rise to the overall brown color of the soil.
- Humus is evenly distributed and less acidic compared to podzols.
- These soils are widespread in Britain, except in the highland areas.
Tundra Soils:
- Soils in tundra areas vary due to complex ground ice patterns.
- Stable slope conditions lead to slow plant decomposition, resulting in a peaty layer at the surface.
- Areas with active slope movement typically have thin soils. In the most extreme conditions without plant growth, soils tend to be humic, as seen in the brown polar desert soils of Antarctica.
- Birch-forested tundra margins in the northern hemisphere possess Arctic brown forest soils, characterized by a thick dark organic A horizon.
Sierozems:
- Sierozems in desert and semi-desert areas can be seen as extreme versions of chestnut soils, with lime and gypsum closer to the surface due to upward capillary attraction.
- Organic matter in these soils is low as plant adaptation to arid conditions limits leaf fall. However, when irrigated, Sierozems can be highly fertile due to their high base status.
Chernozem Soils:
- Chernozems, found in steppe or prairie areas, undergo incomplete leaching due to light rainfall, leading to a calcium-rich horizon deep in the profile.
- These soils have a deep, dark layer of soil, with a surprisingly low humus content despite its dark color, associated with the base-rich mineral matrix.
- Chernozems typically have a well-developed crumb structure, often originating from loess parent material.
Chestnut soils:
- These soils occur in arid regions adjacent to the Chernozem belt under natural low grass-steppe vegetation.
- The illuvial carbonate layer is closer to the surface compared to chernozems, resulting in a lower organic content.
Prairie soils:
Occupying the transitional zone between chernozems and forest brown earth, prairie soils are found in areas of increasing wetness.
Grumusols:
- Dark clayey soils prevalent in savannas or grass-covered regions with a warm climate and distinct wet and dry seasons.
- These soils lack eluvial or illuvial horizons but are rich in bases, particularly calcium, resulting in their dark color.
- They are characterized by significant dry-season cracking.
Ferralsols:
- Often termed lateritic, though strictly speaking laterite is a weathering product rather than a soil type.
- Most tropical soils are rich in ferric oxide, collectively known as Ferralsols.
- Their red, brown, or yellow color stems from the abundance of iron and aluminum sesquioxides.
- These soils have a shallow A horizon, typically acidic with low humus content, while the clayey B horizon extends deeper but is notably low in fertility due to the absence of humus and bases.
INTRAZONAL SOIL
- These soils occur within other zonal soils.
- It is a well-developed soil reflecting the influence of some local factor of relief, parent material, or age rather than of climate and vegetation.
- Hydromorphic soils refer to those that have undergone gleying and are commonly linked with marshes, swamps, or poorly drained upland areas.
- These soils can be categorized into two main types based on the position of the water table in the profile: groundwater gleys, where the groundwater lies below the surface, and surface-water gleys.
Gleying, essentially, is the process involving waterlogging and reduction in soils. In waterlogged conditions where water displaces air in pores, microbes consume soil organic matter, rapidly depleting oxygen.
- Calcimorphic soils develop from calcareous parent materials. Rendzinas, rich in organics and dark in color, are associated with chalk rock in Britain. Another type, terra rossa, is predominantly mineral and commonly found in the Mediterranean region. The upper horizons of terra rossa are clay-rich and possess a reddish hue, creating a sharp contrast with the original parent material.
- Halomorphic (saline) soils, predominantly located in desert regions, encompass three common types:
- Solanchak (white alkali soils) develop in depressions and often display white salt crusts during dry periods.
- Solonetz (black alkali soils) result from intense alkalinization and are typified by the presence of sodium carbonate.
- Solodic soils emerge from leaching processes in the presence of excess sodium, leading to the loss of clays and sesquioxides, resulting in a bleached, eluviated horizon resembling a podzol.
AZONAL SOIL
- Immature soils are formed through the process of deposition by erosion agents, consisting of fine rocky particles transported from distant regions.
- These soils lack well-developed profiles and are considered immature due to various factors.
- This lack of development might arise from insufficient time for the soil to fully form, the terrain’s nature, or characteristics of the parent material.
These immature soils often occur in areas where there’s ongoing deposition of fresh parent material or on steep slopes that hinder profile development.
For instance, alluvial soils in active floodplains show minimal profile development due to frequent burial under new sediments. Regosols are made up of dry, loose dune sands or loess. Lithosols consist of accumulations of imperfectly weathered rock fragments on steep slopes, where erosion rates often remove soil almost as quickly as it forms.
USDA SOIL TAXONOMY
- In recent years, the US Department of Agriculture has implemented a soil classification system based on observed soil properties rather than genetic considerations.
- This classification comprises twelve distinct orders of soils, primarily distinguished by properties that reflect a significant course of development, giving considerable importance to the presence or absence of notable diagnostic horizons.
The twelve soil orders are as follows:
- Alfisols – Named for the presence of aluminum (“al”) and iron (“f”), two prominent elements in these soils.
- Andisols – Soils high in volcanic ash, formed from a type of magma in Andes Mountains volcanoes.
- Aridisols – Soils characterized by dry conditions.
- Entisols – Recently formed soils, often in early stages of development.
- Gelisols – Soils found in areas affected by permafrost.
- Histosols – Soils containing predominantly organic matter.
- Inceptisols – Young soils at the initial stage of their development.
- Mollisols – Soils known for their soft consistency.
- Oxisols – Soils rich in oxygen-containing compounds.
- Spodosols – Soils with ashy characteristics.
- Ultisols – Soils that have undergone significant leaching, resulting in the loss of nutrient bases.
- Vertisols – Soils in which material from upper horizons falls through surface cracks, ending up in deeper horizons.
Soil classification based on climate is a useful way to understand the characteristics of soils in different environmental conditions.
Here’s a breakdown of soil types according to climate:
- Arid Soils: These soils are prevalent in arid or desert climates with minimal precipitation and high evaporation rates. They typically have a sandy texture and contain limited organic matter.
- Semi-arid Soils: Found in semi-arid regions with moderate precipitation and evaporation rates, these soils exhibit variable textures and may contain more clay and organic matter compared to arid soils.
- Subtropical Soils: Subtropical regions, characterized by warm and humid climates, host these soils. They experience significant weathering and may display distinct horizons. Texture and fertility can vary within this category.
- Tropical Rainforest Soils: In regions with high rainfall and elevated temperatures, tropical rainforest soils undergo leaching, resulting in nutrient-poor characteristics with a thin organic layer.
- Temperate Soils: Occurring in temperate climates featuring moderate precipitation and temperatures, these soils are typically well-drained and fertile, making them suitable for agriculture.
- Boreal Soils: These soils are found in boreal or taiga regions, characterized by cold temperatures and moderate to low precipitation. They are often acidic and may contain a high amount of organic matter.
- Polar Soils: Located in polar regions with extremely cold temperatures and minimal precipitation, polar soils tend to be shallow, frozen, and contain limited organic matter.
(FAQs) related to the different soil classifications
1. What is the significance of Curtis Fletcher Marbut’s World Soil Classification?
Curtis Fletcher Marbut’s soil classification system underscores the substantial connection between climate, vegetation, and soil. It categorizes global soils into zonal, azonal, and intrazonal classes based on various factors influencing their development.
2. How are Zonal Soils different from Azonal and Intrazonal Soils?
Zonal soils represent well-developed soils primarily shaped by the climate and vegetation of an area. In contrast, Azonal and Intrazonal soils are less mature and influenced by local factors or specific relief rather than climate and vegetation.
3. What are the distinctive features of Zonal Soil Types?
Zonal soils such as Podzols, Brown Earth, Tundra Soils, Sierozems, Chernozem, Chestnut soils, Prairie soils, Grumusols, Ferralsols are examples with specific characteristics related to climate, organic content, and profile development.
4. How do Immature Soils differ from Mature Soils?
Immature soils, formed through deposition by erosion agents, lack well-developed profiles due to insufficient time for development, terrain characteristics, or ongoing deposition of fresh parent material. Mature soils, like Zonal soils, reflect stable conditions over a prolonged period.
5. What is the USDA Soil Taxonomy, and how does it differ from Marbut’s Classification?
The USDA Soil Taxonomy is a system based on observed soil properties, distinguishing twelve distinct orders of soils mainly by diagnostic horizons. Unlike Marbut’s approach, which emphasizes climate and vegetation, the USDA system focuses on observable soil properties.
6. How do Soil Types vary based on Climate?
Soil types vary concerning climate. For instance, Arid, Semi-arid, Subtropical, Tropical Rainforest, Temperate, Boreal, and Polar soils are each adapted to specific environmental conditions such as moisture levels, temperature, and vegetation, influencing their texture and fertility.
7. Can you explain the characteristics of Halomorphic (saline) soils?
Halomorphic soils, found predominantly in desert regions, encompass Solanchak (white alkali soils), Solonetz (black alkali soils), and Solodic soils. Each type exhibits distinct characteristics due to factors like the presence of sodium carbonate, leaching, or the abundance of certain elements.
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