3.3 Soil classification systems and mapping

Soil classification systems and mapping

Soil classification systems organize soils into categories based on measurable properties and formation processes. Without them, soil scientists in different regions would have no common language for describing what they observe, and land managers would struggle to predict how a soil will behave. This section covers the principles behind classification, the two major systems (USDA Soil Taxonomy and FAO World Reference Base), how hierarchical categories work, and how soil maps translate classification into practical land management tools.

Soil classification systems

Purpose and principles

Classification systems give scientists and land managers a standardized way to describe, compare, and predict the behavior of soils. Rather than relying on informal descriptions ("dark, sticky soil"), these systems use measurable, quantitative properties so that two people examining the same soil profile reach the same classification.

The systems are hierarchical: broad categories sit at the top, and increasingly specific categories narrow down the soil type. At every level, the criteria come from diagnostic horizons (specific layers with defined characteristics) and measurable soil properties rather than subjective judgment.

Key measurable properties used in classification include:

• Soil pH (acidity or alkalinity)
• Organic matter content (percent by weight)
• Clay content (affects water-holding capacity and shrink-swell behavior)
• Cation exchange capacity (CEC) (the soil's ability to hold and supply nutrients to plants)

A good classification system also needs to be comprehensive enough to cover all known soil types while remaining flexible enough to incorporate new discoveries.

Development and application

Building a classification starts with systematic field and lab work. Here's the general process:

1. Examine the soil profile in the field, identifying each horizon (O, A, E, B, C, R) and noting thickness, boundaries, and structure.
2. Determine soil color using a Munsell color chart, which standardizes color into hue, value, and chroma. Color reveals a lot: dark colors suggest high organic matter, gray or bluish tones indicate poor drainage, and reddish hues point to iron oxide accumulation.
3. Analyze texture by determining the proportions of sand, silt, and clay (either by feel in the field or by particle-size analysis in the lab).
4. Run chemical tests for properties like base saturation, organic carbon content, and the presence of specific minerals (calcite, gypsum).
5. Identify diagnostic features that slot the soil into the classification. For example, an argillic horizon (a B horizon with significant clay accumulation from illuviation) is a key diagnostic feature in several soil orders.

All of this requires a solid understanding of soil-forming processes: weathering breaks down parent material, leaching moves soluble compounds downward, illuviation deposits material in lower horizons, and organic matter accumulation darkens surface layers.

USDA vs FAO systems

System characteristics and structure

The two dominant classification systems serve different geographic scopes and use different architectures.

USDA Soil Taxonomy is the primary system used in the United States. It has six hierarchical levels, from broadest to most specific:

1. Order (12 total, e.g., Mollisols, Alfisols)
2. Suborder (defined by moisture or temperature regime)
3. Great Group (defined by additional diagnostic horizons or properties)
4. Subgroup (typical vs. transitional forms)
5. Family (particle size, mineralogy, temperature class)
6. Series (the most specific level, named after a geographic location, e.g., "Miami series")

FAO World Reference Base (WRB) is designed for international use. Its structure is simpler, with just two levels:

1. Reference Soil Groups (32 total, e.g., Chernozems, Luvisols)
2. Qualifiers (prefixes and suffixes that refine the description)

Classification criteria and nomenclature

The USDA system places heavy emphasis on soil temperature and moisture regimes as classification criteria. The WRB focuses more broadly on soil-forming factors and processes, making it more adaptable across diverse global environments.

Their naming conventions also differ. USDA terms are primarily Latin- and Greek-derived and follow strict formative element rules (e.g., "Moll" in Mollisols comes from the Latin mollis, meaning soft). WRB names draw more from common or traditional language across multiple countries (e.g., "Chernozem" comes from Russian for "black earth").

Both systems rely on diagnostic horizons and properties, but they may define the same feature differently or weight it differently in the hierarchy. For instance, both recognize clay-enriched subsurface horizons, but the exact thresholds and naming conventions differ.

Soil orders, suborders, and great groups

Soil order classification

Soil orders are the broadest category in USDA Soil Taxonomy. Each of the 12 orders reflects a dominant soil-forming process or a defining set of properties. Three examples illustrate the range:

• Alfisols: Develop under forests in moderately weathered settings. Their defining feature is an argillic (clay-rich) B horizon with moderate to high base saturation (≥35%).
• Aridisols: Form in arid climates where evaporation exceeds precipitation. They often accumulate salts, carbonates, or gypsum near the surface because there isn't enough water to leach these materials downward.
• Histosols: Composed primarily of organic material (peat or muck). They form in waterlogged environments where decomposition is slow, so organic matter builds up rather than breaking down.

Suborder and great group differentiation

Within each order, suborders are distinguished mainly by soil moisture regimes and temperature regimes:

• Moisture regimes include Aquic (saturated/waterlogged), Udic (moist, humid climates), and Ustic (dry with intermittent moisture).
• Temperature regimes include Frigid (cold), Mesic (moderate), and Thermic (warm).

Great groups add further specificity by incorporating additional diagnostic horizons, chemical properties (like base saturation or pH), and physical characteristics (like depth or texture).

Here's how the hierarchy works in practice:

• Alfisols → Udalfs (Alfisols in moist/udic environments) → Hapludalfs ("Hapl-" means typical, so these are the most common, straightforward moist Alfisols)
• Aridisols → Argids (Aridisols with a clay-enriched argillic horizon) → Haplargids (typical Argids without unusual additional features)

The formative elements in these names aren't random. Once you learn the root meanings ("Ud-" = humid, "Arg-" = clay, "Hapl-" = simple/typical), you can decode unfamiliar soil names on sight.

Soil maps for land management

Map components and interpretation

Soil maps show the spatial distribution of soil types across a landscape. Each map unit is a color-coded polygon representing a single soil series or, more commonly, a complex of closely associated soil types that occur together.

A soil map's legend provides the classification, characteristics, and sometimes management ratings for each unit. Scale matters a great deal:

• Detailed maps (1:12,000 or larger) are used for site-specific decisions like building foundations or designing drainage for a single farm field.
• General maps (1:250,000 or smaller) support regional planning and broad land-use assessments.

Reading a soil map well requires connecting topography to soil distribution. Soils on ridgetops, slopes, and valley bottoms often differ predictably because of differences in drainage, erosion, and deposition. Contour lines on the map help you see these relationships.

Applications and technological advancements

Soil maps are practical tools across many fields:

• Agriculture: Choosing appropriate crops, planning irrigation, and managing fertilizer application based on soil drainage and nutrient-holding capacity.
• Construction: Assessing foundation stability, shrink-swell potential, and drainage design.
• Environmental management: Delineating wetlands, identifying erosion-prone areas, and planning conservation practices.

Two technologies have transformed how soil maps are created and used:

Geographic Information Systems (GIS) allow soil data to be stored digitally, layered with other spatial data (topography, land cover, hydrology), and updated as new surveys are completed. Instead of a static printed map, GIS produces interactive, queryable maps.

Remote sensing extends mapping capability beyond what field surveys alone can achieve. Satellite imagery identifies broad soil patterns over large areas, while drone-mounted sensors (multispectral, thermal) can map specific soil properties like moisture content or organic matter at high resolution across individual fields.