USDA Soil Taxonomy
Overview
The USDA Soil Taxonomy is a comprehensive classification system developed by the United States Department of Agriculture (USDA) to categorize and describe soils based on their physical and chemical properties. This system is widely used in the United States and internationally for soil management, research, and education. It provides a framework for understanding soil characteristics and their implications for land use, agriculture, and environmental management.
History and Development
The development of the USDA Soil Taxonomy began in the mid-20th century, with the first edition published in 1975. The system was designed to address the limitations of previous classification methods and to provide a standardized approach that could be used globally. The taxonomy has undergone several revisions, with the most recent edition reflecting advances in soil science and changes in land use practices.
Structure of the Soil Taxonomy
The USDA Soil Taxonomy is organized into a hierarchical structure consisting of six categories: Order, Suborder, Great Group, Subgroup, Family, and Series. Each category provides increasingly detailed information about the soil's characteristics.
Soil Orders
There are 12 soil orders in the USDA Soil Taxonomy, each representing a major grouping of soils with similar properties. These orders are:
1. **Alfisols** - Soils with a subsurface horizon of clay accumulation and high base saturation. 2. **Andisols** - Soils formed in volcanic ash, characterized by high water-holding capacity. 3. **Aridisols** - Soils found in arid regions, with limited leaching and accumulation of salts. 4. **Entisols** - Soils with little to no horizon development, often found in areas of recent deposition. 5. **Gelisols** - Soils with permafrost within two meters of the surface, common in polar regions. 6. **Histosols** - Organic-rich soils, often found in wetlands. 7. **Inceptisols** - Soils with weakly developed horizons, found in a variety of environments. 8. **Mollisols** - Soils with a thick, dark, organic-rich surface horizon, typical of grasslands. 9. **Oxisols** - Highly weathered soils, rich in iron and aluminum oxides, found in tropical regions. 10. **Spodosols** - Acidic soils with a subsurface accumulation of organic matter and aluminum, common in coniferous forest regions. 11. **Ultisols** - Soils with a subsurface horizon of clay accumulation and low base saturation. 12. **Vertisols** - Soils with high clay content that swell and shrink with moisture changes.
Suborders
Suborders further divide soil orders based on specific characteristics such as moisture regime, temperature regime, or dominant soil-forming processes. For example, within the Aridisols order, suborders may include Argids (soils with clay horizons) and Salids (soils with salt accumulation).
Great Groups
Great groups provide additional differentiation based on features such as horizon sequence, soil temperature, and soil moisture. These groups offer more detailed classification, allowing for more precise soil management practices.
Subgroups
Subgroups refine the classification by indicating variations within great groups. They describe soils that have properties indicating a transition to another soil order or that possess unique characteristics not captured by higher categories.
Families
Soil families are defined by physical and chemical properties that affect soil behavior and use, such as particle size, mineralogy, and temperature. This level of classification is particularly useful for agricultural and engineering applications.
Series
The soil series is the most specific category in the USDA Soil Taxonomy. It describes soils with similar horizons, color, texture, structure, and other properties. Soil series are often named after geographic locations where they were first identified.
Diagnostic Horizons and Properties
The USDA Soil Taxonomy uses diagnostic horizons and properties to classify soils. These diagnostic features are observable characteristics that provide information about soil formation and processes.
Diagnostic Horizons
Diagnostic horizons are layers within the soil profile that have specific physical or chemical properties. Examples include:
- **Mollic Horizon**: A thick, dark surface horizon rich in organic matter and nutrients. - **Argillic Horizon**: A subsurface horizon with clay accumulation. - **Spodic Horizon**: A subsurface horizon with organic matter and aluminum accumulation.
Diagnostic Properties
Diagnostic properties are specific soil characteristics used to define and differentiate soils. These include:
- **Base Saturation**: The proportion of soil bases (calcium, magnesium, potassium, and sodium) relative to total cation exchange capacity. - **Soil Temperature Regime**: The average soil temperature and its seasonal variation. - **Soil Moisture Regime**: The presence and movement of water within the soil profile.
Applications and Uses
The USDA Soil Taxonomy is used in various fields, including agriculture, forestry, environmental science, and land use planning. It provides a standardized language for describing soils, which is essential for communication among scientists, land managers, and policymakers.
Agriculture
In agriculture, the taxonomy helps farmers and agronomists understand soil properties that affect crop production, such as nutrient availability, drainage, and erosion potential. This information guides decisions on crop selection, fertilization, and irrigation.
Environmental Management
The taxonomy is also used in environmental management to assess soil health, monitor changes in soil properties, and develop conservation strategies. It aids in identifying soils susceptible to degradation and in designing sustainable land management practices.
Land Use Planning
In land use planning, the USDA Soil Taxonomy provides critical information for evaluating land suitability for various uses, such as construction, recreation, and wildlife habitat. It helps planners assess soil limitations and potential impacts on infrastructure and ecosystems.
Challenges and Limitations
While the USDA Soil Taxonomy is a valuable tool, it has limitations. The system is complex and requires detailed soil analysis, which can be time-consuming and costly. Additionally, the taxonomy may not fully capture the diversity of soils in certain regions, leading to challenges in classification and interpretation.
Future Directions
Ongoing research in soil science continues to refine and expand the USDA Soil Taxonomy. Advances in technology, such as remote sensing and soil spectroscopy, offer new opportunities for soil classification and mapping. These developments may lead to more efficient and accurate soil assessments, enhancing the taxonomy's utility in addressing global challenges such as climate change, food security, and sustainable development.