How Many Soil Orders Are There?
There are 12 soil orders in the United States Department of Agriculture (USDA) soil taxonomy, the most widely used system for classifying soils globally. These orders represent the broadest level of soil classification, grouping soils with similar dominant characteristics and processes.
Understanding Soil Orders: The Foundation of Soil Science
Soil is far more than just “dirt.” It’s a complex, dynamic, and living ecosystem that supports nearly all terrestrial life. To understand and manage this vital resource effectively, scientists have developed systems for classifying soils, and at the highest level of these systems are soil orders. Understanding the characteristics that define each soil order is crucial for agriculture, environmental management, and construction. Soil orders are determined primarily by identifiable pedogenic processes, or processes of soil formation, which leave distinct morphological, chemical, and physical traces.
The 12 Soil Orders: A Detailed Overview
Each soil order reflects specific environmental conditions and dominant processes. Let’s examine each in detail:
- Gelisols: These are permafrost-dominated soils found in very cold climates. They contain a permafrost layer within 2 meters of the surface. Slow decomposition rates result in high organic matter accumulation. They are primarily found in high-latitude regions and at high elevations.
- Histosols: These are organic soils formed in areas with poor drainage. They are characterized by a high percentage of organic matter (more than 20% by weight). Histosols are often found in bogs, swamps, and marshes.
- Spodosols: These soils form in acidic, sandy soils under coniferous forests in cool, humid climates. They are characterized by a distinct spodic horizon, a layer of accumulated humus and iron and aluminum oxides. This gives them a characteristic bleached appearance near the surface.
- Andisols: These are volcanic soils formed from volcanic ash and cinders. They are highly fertile due to their excellent water-holding capacity and nutrient content. Andisols are often dark in color and have a fluffy texture.
- Oxisols: These are highly weathered soils found in tropical and subtropical regions. They are characterized by a high content of iron and aluminum oxides, giving them a reddish or yellowish color. Oxisols have low nutrient content and are often acidic.
- Vertisols: These are clay-rich soils that swell and shrink dramatically with changes in moisture content. This shrinking and swelling creates cracks in the soil, leading to self-plowing. Vertisols are often found in areas with distinct wet and dry seasons.
- Aridisols: These are soils of arid and semi-arid regions. They are characterized by low organic matter content and accumulation of salts, carbonates, and gypsum. Water scarcity is the defining feature.
- Ultisols: These are highly weathered soils found in humid subtropical and tropical regions. They are similar to Oxisols but less severely weathered. They have a clay-rich subsoil and low nutrient content.
- Mollisols: These are grassland soils characterized by a thick, dark, organic-rich surface horizon called a mollic epipedon. They are highly fertile and productive soils. Mollisols are found in temperate grasslands around the world.
- Alfisols: These are moderately weathered soils found in temperate humid and subhumid regions. They have a clay-enriched subsoil and a base saturation of 35% or more. Alfisols are generally fertile soils.
- Inceptisols: These are young soils with weakly developed horizons. They are transitional soils that are more developed than Entisols but lack the diagnostic horizons of other soil orders. Inceptisols are found in a wide range of environments.
- Entisols: These are the least developed soils. They lack distinct horizons and are often found in areas with recent deposition or erosion. Entisols are common on floodplains, sand dunes, and steep slopes.
FAQs: Deepening Your Understanding of Soil Orders
Here are some frequently asked questions to further clarify the complexities of soil orders and their significance:
1. What is the purpose of classifying soils into orders?
The classification of soils into orders provides a hierarchical system for understanding soil properties and their relationship to environmental factors. It allows for better communication among soil scientists, improved soil mapping, and more effective land management practices.
2. What are the key factors used to distinguish between soil orders?
The key factors include climate, vegetation, parent material, topography, and time. These factors influence the pedogenic processes that shape soil characteristics and determine the order to which a soil belongs. Diagnostic horizons are also very important.
3. How do soil orders impact agricultural practices?
Understanding soil orders is crucial for selecting appropriate crops, irrigation techniques, and fertilization strategies. For example, Mollisols are ideal for grain production, while Oxisols may require extensive amendments to support agriculture. The order defines the properties of the soil, which in turn defines the best management practices.
4. Can a soil belong to more than one order?
No, a soil can only belong to one soil order. However, soils are often transitional and exhibit characteristics of multiple orders. These are then categorized within the specific order that most closely aligns with their dominant features.
5. What are the subdivisions within each soil order?
Each soil order is further subdivided into suborders, great groups, subgroups, families, and series. This hierarchical system provides increasing detail about soil properties and characteristics. Suborders refine the characteristics further by considering features like moisture regimes.
6. Where can I find information about the soil order in my area?
The USDA Natural Resources Conservation Service (NRCS) provides detailed soil surveys and maps that identify the soil orders and other soil classifications in specific geographic areas. Web Soil Survey is a great place to start.
7. How does soil order relate to climate?
Climate is a major factor in determining soil order. Temperature and precipitation patterns influence the types of weathering and biological processes that occur in the soil. For example, Gelisols are found in cold climates, while Oxisols are found in warm, humid climates.
8. What are the environmental implications of different soil orders?
Different soil orders have different capacities for carbon sequestration, water infiltration, and nutrient cycling. Understanding these implications is essential for managing soil resources sustainably and mitigating climate change. Vertisols, for instance, can store significant amounts of carbon, while Aridisols are more susceptible to desertification.
9. How do human activities impact soil orders?
Human activities such as deforestation, agriculture, and urbanization can alter soil properties and even lead to the degradation of soils. Intensive agriculture can lead to soil erosion and nutrient depletion, while urbanization can seal soil surfaces and disrupt natural drainage patterns. Poor management can transform an Alfisol into an Ultisol, which is less fertile.
10. What is the difference between soil order and soil texture?
Soil order is a broad classification based on soil-forming processes, while soil texture refers to the proportion of sand, silt, and clay in the soil. Soil texture is a physical property of the soil that influences water-holding capacity, drainage, and aeration. Soil texture can affect the specific properties within a soil order, but the order remains the primary classification.
11. Are the 12 soil orders universally recognized and used?
While the USDA soil taxonomy is widely used, other soil classification systems exist, such as the World Reference Base for Soil Resources (WRB). The WRB attempts to correlate with and bridge across national systems, but the USDA system remains dominant in the United States.
12. How does understanding soil orders help in land use planning?
Knowing the soil order present in an area is essential for effective land use planning. It allows planners to assess the suitability of the land for different purposes, such as agriculture, construction, or conservation. This information helps to minimize environmental impacts and ensure sustainable land management. Knowing if you’re building on a Vertisol will require special engineering to avoid damage from the shrinking and swelling.
Conclusion
The 12 soil orders represent a fundamental framework for understanding the diverse characteristics and functions of soils around the world. By understanding the processes that shape these orders, we can better manage and protect this vital resource for future generations. This knowledge is critical for addressing global challenges related to food security, climate change, and environmental sustainability.