Soil Classification Methods and Importance

Soil Classification

• Creating a system to classify objects helps humans order the world, thus making it easier to understand.
• This supports better communication, remembering, and knowledge development.

How Soils are Classified

• A way to organize information regarding soils makes soil classification much more useful.
• Because there are so many different soils, each having unique biological, chemical, and physical traits, a hierarchical system is frequently employed when classifying soils.

Soil Classification Objects

• When talking about soil classification, it is important to know exactly what is being classified.
• A soil mantle, as defined by AFES in 1998, constitutes a three-dimensional soil body.
• A pedon is classified under the USDA Soil Taxonomy (Soil Survey Staff, 2003).
• WRB (2007) defines any material within 2 m of the Earth's surface with a horizontal area ranging from 1–10 m2.

Lecture Outcomes

• Understanding the reasons for soil classification.
• Naming the soil orders associated with Soil Taxonomy.
• Identifying the general traits which characterize the soil orders.
• Elaborating how soil orders are distributed at the global level.

Soil Taxonomy

• A system of soil classification known as soil taxonomy places emphasis on the physical and chemical characteristics of the soil profile.

Diagnostic Horizons

• Soil horizons define soil types and taxonomic units.
• Diagnostic horizons are features which are quantitatively defined and are used to differentiate between taxa.
• Horizon symbols point to the likely direction of pedogenesis.
• Diagnostic horizons show the extent of that expression.

Epipedons

• An epipedon is a horizon that develops close to the surface where most of the rock structure has been broken down.
• They are distinguished by the amount of plant nutrients in addition to their color, texture, and structure.

Endopedons

• Soil layers known as endopedons are located below the epipedon; their texture, structure, and color are among their distinguishing characteristics.

Examples of Illuvial Horizons

• Spodic: pH ≤ 5.9 and SOC ≥ 0.6%, accumulation of organic carbon and aluminum with or without iron.
  • Appears redder or blacker than the overlying horizon or both colors are present.
• Argillic: Illuvial accumulation of clay, the amount of clay increase depends upon texture, minimum 3% or 1.2 x clay of the overlying layer
• Cambic: Evidence of soil development in the form of changes in color or structure derived from parent material.
  • Does not meet the requirements for other types of endopedons.

Soil Orders

• Alfisols: These soils commonly form in forests and have moderate weathering, are typically fertile and temperature and moisture affect productivity.
• Andisols: Associated with areas of volcanic activity, especially those associated with the Pacific Rim. Weathering is important, notably the presence of volcanic glass, high CEC, and they are generally fertile.
• Aridisols: Containing significant amounts of salt, these desert soils make up around 19% of the land on Earth. Salinization occurs where the potential evapotranspiration is greater than actual evapotranspiration.
• Entisols: Exhibit limited development and have properties derived from parent material and lack of a B layer, irrespective of climate.
• Gelisols: These soils are associated with high latitudes and have permafrost within 100cm of the surface.
• Histosols: Organic soils that occur in areas that are wet
• Inceptisols: Display weak development and only a few diagnostic features, and are common in humid regions
• Mollisols: These soils are extremely fertile grassland soils. The largest single soil order in the US, they remain soft even when left to dry.
• Oxisols: Containing deeply-developed Fe/Al oxides, these tropical soils are known for their moisture.
• Spodosols: The formation of soil occurs within sandy parent materials in forested portions of the humid continental mild-summer climates which are seen across N. America and Europe.
• Ultisols: Often found in subtropical forest environments, with highly weathered forest soils. The Southeast United States are a great example of where they are commonplace.
• Vertisols: These contain more than 30% swelling clays that swell significantly when they absorb water. The black color of vertisols is because of specific mineral content, rather than organics.

Soil Classification

• Review the location of the soil profile
• Describe the soil profile/pedon
• Discover which diagnostic features are present
• Determine soil moisture and temperature patterns
• Decide on the soil class

Soil Survey and Mapping

• Soil characteristics are crucial to make sure utilization and management are appropriate.
• Assessing location and scope of different types of soil for land use, development, and planning is important
• Examples such as: construction, irrigation, and cropping projects
• The inventory is called a soil survey.

Soil Spatial Variability

• Using the influence of these five conditions: climate, parent material, organisms, topography, and time, is useful in understanding spatial variations in soils.
• Understanding the variables in soil formations is helpful in creating a mental model which predicts which soils are more prone to form certain areas of the landscape.

Small Scale Variability

• Within a single soil, properties of soil are subject to abrupt and noticeable change across short distances within a farm field.
• Variations most often occur in small changes in topography and thickness of parent material.
• Measuring small-scale variability and casual observation can be difficult
• Soil fertility variability typically reflects previous methods in soil profile management

Creating Soil Maps

• A book of soil data that describes and categories certain uses, is often used with soil maps
• Soil maps help to pinpoint the spatial variability of soil at that specific point in measurement
• Following the survey of a soil, it is then grouped in its correct classifications
• Each soil helps evaluate reasonable uses and environmental management

Other Processes

• A soil scientist must choose a representative location for study
• A pit should be dug to expose the soil profile for observation
• Soil horizons should be differentiated and labeled based on color, texture, and structure variation
• Horizon boundaries are recorded for their distinctness on a vertical face.
• Different soil samples are gathered and studied in the laboratory for analysis
• Determine and categorize soil type based on development and material
• Record the amount of soil depth and patterns of different horizons
• Any essential characteristics like root channels should also be recorded

Soil Boundaries

• To determine existing soil boundaries, soil scientists can dig pits within the landscape.
• Small holes are to bring up soil samples for analysis.
• The texture, color, and attributes are compared with established soil regions
• Soil scientist use bore holes that aren't entirely random.