Which Soil Type Is The Most Common

The earth beneath our feet is far from uniform; it's a complex mosaic of different soil types, each with its own unique characteristics and distribution. Determining which soil type is the "most common" globally is a multifaceted question, dependent on the scale of analysis, the criteria used for classification, and the ever-changing influences of climate, geology, and human activity. However, by examining the major soil classification systems and their global distribution patterns, we can gain a clearer understanding of the prevalence of different soil types across the planet.

Understanding Soil Classification

Before diving into the specifics, it's essential to understand the framework used to classify soils. Several systems exist, each with its own hierarchy and criteria. Two of the most widely used are the USDA Soil Taxonomy and the World Reference Base for Soil Resources (WRB).

• USDA Soil Taxonomy: This system focuses on soil properties and horizons (layers) to categorize soils into a hierarchical system ranging from Orders (the broadest level) down to Series (the most specific). The major soil orders in USDA Soil Taxonomy include:

  • Entisols: Young soils with minimal horizon development.
  • Inceptisols: Soils with weak horizon development.
  • Andisols: Soils formed from volcanic ash.
  • Gelisols: Soils with permafrost within 2 meters of the surface.
  • Histosols: Organic soils, often found in wetlands.
  • Aridisols: Soils in arid or semi-arid environments.
  • Mollisols: Dark, fertile soils common in grasslands.
  • Alfisols: Moderately leached soils with a subsurface horizon of clay accumulation.
  • Spodosols: Acidic, sandy soils with a subsurface accumulation of organic matter and iron/aluminum oxides.
  • Ultisols: Highly weathered, acidic soils with a subsurface horizon of clay accumulation.
  • Oxisols: Highly weathered soils in tropical environments, rich in iron and aluminum oxides.

• World Reference Base for Soil Resources (WRB): This system emphasizes diagnostic horizons and properties to classify soils. Some key reference soil groups in WRB include:

  • Acrisols: Highly weathered, acidic soils with a subsurface accumulation of clay.
  • Albeluvisols: Soils with a bleached (albic) horizon overlying a clay-rich horizon.
  • Andosols: Soils formed from volcanic materials.
  • Arenosols: Sandy soils with weak development.
  • Calcisols: Soils with a significant accumulation of calcium carbonate.
  • Cambisols: Soils with moderate development, showing some alteration of parent material.
  • Chernozems: Dark, fertile soils rich in organic matter.
  • Ferralsols: Highly weathered soils rich in iron and aluminum oxides.
  • Gleysols: Soils with prolonged water saturation, leading to anaerobic conditions.
  • Histosols: Organic soils.
  • Leptosols: Very shallow soils over rock or gravel.
  • Lixisols: Soils with a clay-rich horizon and low activity clays.
  • Luvisols: Soils with a clay-rich horizon formed by illuviation.
  • Nitisols: Deeply weathered soils with shiny clay aggregates.
  • Phaeozems: Dark soils with a high base saturation.
  • Planosols: Soils with a slowly permeable, abruptly contrasting layer.
  • Podzols: Acidic soils with a subsurface accumulation of organic matter and iron/aluminum oxides.
  • Regosols: Very weakly developed soils in unconsolidated materials.
  • Solonchaks: Saline soils.
  • Solonetz: Sodic soils with a high concentration of sodium ions.
  • Vertisols: Clay-rich soils that shrink and swell significantly with changes in moisture content.

Determining the Most Common Soil Type

Identifying the single "most common" soil type is challenging due to the complexity of soil distribution and the variations in classification systems. However, based on global estimates and the extent of different soil orders/reference groups, we can identify several contenders for the title:

1. Entisols/Regosols: These are often considered among the most widespread due to their presence in diverse environments.
2. Inceptisols/Cambisols: Similar to Entisols, these soils are also found globally and are quite common.
3. Aridisols/Solonchaks: Due to the large area covered by arid and semi-arid regions, these soils are extensively distributed.

Let's analyze each of these contenders in more detail.

Entisols and Regosols: The Immature and Ubiquitous

Entisols (USDA Soil Taxonomy) and Regosols (WRB) are young soils with minimal profile development. They lack distinct horizons due to their recent formation from parent material or because they are constantly disturbed by erosion, deposition, or human activity.

• Global Distribution: Entisols and Regosols are found on every continent and in a wide range of climates. They are particularly common in:

  • Areas with active erosion: Steep slopes, floodplains, and coastal regions.
  • Recently deposited sediments: Alluvial fans, sand dunes, and volcanic deposits.
  • Areas with limited weathering: Cold regions and deserts.

• Characteristics: Entisols and Regosols exhibit a wide range of properties depending on their parent material. They can be sandy, silty, or clayey, and their fertility varies greatly. Because they are young, they often have low organic matter content and limited nutrient availability.

• Why They Are Common: The prevalence of Entisols and Regosols is due to their ability to form in diverse environments and their resilience to disturbance. They represent the starting point for soil development and are constantly being created and recreated across the globe.

• Limitations: Due to their low fertility and limited water-holding capacity, Entisols and Regosols often pose challenges for agriculture. However, they can support specialized ecosystems and provide valuable ecosystem services in certain environments.

Inceptisols and Cambisols: The Developing World

Inceptisols (USDA Soil Taxonomy) and Cambisols (WRB) represent a step up in soil development from Entisols/Regosols. They exhibit weak horizon development, indicating that they have undergone some weathering and alteration but are still relatively young.

• Global Distribution: Inceptisols and Cambisols are also widely distributed, though often in areas with slightly more stable conditions than those where Entisols dominate. They are common in:

  • Mountainous regions: Where erosion and deposition are active, but weathering is also occurring.
  • Glacial landscapes: Where soils are developing on recently exposed sediments.
  • Humid temperate regions: Where weathering rates are moderate.

• Characteristics: Inceptisols and Cambisols typically have a thin A horizon (topsoil) and a weakly developed B horizon (subsoil). Their properties vary depending on climate, parent material, and vegetation. They generally have higher organic matter content and nutrient availability than Entisols.

• Why They Are Common: The widespread occurrence of Inceptisols and Cambisols reflects the prevalence of landscapes undergoing moderate weathering and soil formation. They represent a transitional stage between young, undeveloped soils and more mature, highly differentiated soils.

• Limitations: While generally more fertile than Entisols, Inceptisols and Cambisols can still have limitations for agriculture. They may be shallow, rocky, or poorly drained, depending on the specific environment in which they form.

Aridisols and Solonchaks: The Dry Land Soils

Aridisols (USDA Soil Taxonomy) and Solonchaks (WRB) are soils that develop in arid and semi-arid environments, where water is scarce.

• Global Distribution: Aridisols and Solonchaks are dominant in:

  • Deserts: Such as the Sahara, the Arabian Desert, and the Sonoran Desert.
  • Semi-arid regions: Including the steppes of Central Asia and the grasslands of the American Southwest.
  • Coastal areas: Where salt accumulation is high due to evaporation.

• Characteristics: Aridisols and Solonchaks are characterized by:

  • Low organic matter content: Due to limited plant growth.
  • Accumulation of salts: Due to high evaporation rates and limited leaching.
  • Calcium carbonate horizons: Formed by the precipitation of dissolved minerals.
  • Weakly developed horizons: Reflecting the slow rate of soil formation in arid environments.

• Why They Are Common: The extensive distribution of Aridisols and Solonchaks is a direct consequence of the vast areas of the world that experience arid and semi-arid climates. These soils are well-adapted to the harsh conditions of these environments and support specialized ecosystems.

• Limitations: Aridisols and Solonchaks pose significant challenges for agriculture due to their low fertility, high salt content, and limited water availability. Irrigation can improve productivity, but it can also exacerbate salt accumulation if not managed carefully.

Other Notable Soil Types and Their Distribution

While Entisols/Regosols, Inceptisols/Cambisols and Aridisols/Solonchaks are strong contenders for the title of "most common," other soil types also play significant roles in the global soil landscape:

• Mollisols/Chernozems: These dark, fertile soils are characteristic of grasslands and are found in regions like the Great Plains of North America, the steppes of Eastern Europe and Asia, and the pampas of South America. They are highly productive agricultural soils.
• Alfisols/Luvisols: Moderately leached soils with a subsurface horizon of clay accumulation, Alfisols and Luvisols are found in temperate deciduous forests and mixed conifer-deciduous forests. They are common in the eastern United States, Europe, and parts of Asia.
• Ultisols/Acrisols: Highly weathered, acidic soils with a subsurface horizon of clay accumulation, Ultisols and Acrisols are common in warm, humid regions like the southeastern United States, Southeast Asia, and South America. They are often infertile due to nutrient leaching.
• Oxisols/Ferralsols: Highly weathered soils rich in iron and aluminum oxides, Oxisols and Ferralsols are found in tropical rainforests and savannas. They are common in the Amazon Basin, Central Africa, and Southeast Asia. While often deep, they can be infertile due to low nutrient retention capacity.
• Histosols: Organic soils found in wetlands, Histosols occur in areas like bogs, swamps, and marshes. They are common in Canada, Russia, and Scandinavia. They play a critical role in carbon storage and water regulation.
• Andosols: Soils formed from volcanic ash, Andosols are found near active and dormant volcanoes. They are common in Japan, Indonesia, the Pacific Northwest of the United States, and parts of South America. They are often fertile due to the release of nutrients from volcanic materials.
• Gelisols: Soils with permafrost, Gelisols are found in the Arctic and subarctic regions of the world. They are common in Siberia, Alaska, and Canada. They are sensitive to climate change, and thawing permafrost can release large amounts of greenhouse gases.

The Human Impact on Soil Distribution

It's crucial to recognize that human activities have a profound impact on soil distribution and properties. Deforestation, agriculture, urbanization, and industrial pollution can all alter soil characteristics and lead to soil degradation.

• Deforestation: Removes protective vegetation cover, leading to increased erosion and loss of topsoil.
• Agriculture: Intensive farming practices can deplete soil nutrients, compact the soil, and increase the risk of erosion.
• Urbanization: Seals soil surfaces with impervious materials, preventing water infiltration and disrupting natural soil processes.
• Industrial pollution: Contaminates soils with heavy metals, chemicals, and other pollutants, rendering them unproductive and potentially harmful to human health.

Sustainable land management practices are essential to mitigate the negative impacts of human activities on soils. These practices include:

• Conservation tillage: Minimizing soil disturbance to reduce erosion and improve soil health.
• Crop rotation: Alternating crops to improve soil fertility and reduce pest and disease pressure.
• Cover cropping: Planting crops to protect the soil surface and improve soil structure.
• Integrated nutrient management: Using a combination of organic and inorganic fertilizers to optimize nutrient availability.
• Agroforestry: Integrating trees into agricultural systems to improve soil fertility, reduce erosion, and provide shade and habitat.

Conclusion: A Mosaic of Soil Types

While pinpointing the single "most common" soil type remains elusive due to the complexities of soil classification and distribution, it is clear that Entisols/Regosols, Inceptisols/Cambisols, and Aridisols/Solonchaks are among the most widespread soil types globally. Their prevalence reflects the diversity of landscapes and climates across the planet, as well as the dynamic processes of soil formation and degradation.

Understanding the distribution and properties of different soil types is crucial for sustainable land management and environmental conservation. By recognizing the unique characteristics of each soil type and the challenges they present, we can develop strategies to protect and enhance soil resources for future generations.

The soil beneath our feet is a vital natural resource that supports life on Earth. It is a complex and dynamic system that requires careful management and stewardship. By promoting sustainable land use practices and investing in soil research and education, we can ensure that soils continue to provide essential ecosystem services for years to come.