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What is a Soil Order?

A soil order represents the broadest level of classification in soil taxonomy, grouping soils with similar dominant physical, chemical, and biological properties that reflect major differences in their genesis and morphology. Understanding soil orders provides a foundational framework for comprehending soil behavior, distribution, and suitability for various land uses.

Decoding the Foundation: Understanding Soil Orders

Soil is far from a uniform substance. It’s a dynamic, living ecosystem sculpted over time by climate, organisms, topography, parent material, and time itself (the five soil-forming factors). These forces interact to create a dazzling array of soil types, each with unique characteristics. To manage and understand this complexity, scientists developed a hierarchical system of soil classification, culminating in the soil order, the broadest taxonomic level.

A soil order groups soils that share similar dominant characteristics, reflecting broad similarities in their formative environments and processes. These characteristics are often visible in the soil profile – a vertical cross-section revealing distinct layers or horizons. The presence, absence, and properties of these horizons are key to identifying the soil order.

Think of soil orders as the “families” of the soil world. Just as a family shares common traits and lineage, soil orders share overarching characteristics dictated by the prevailing climate, vegetation, and parent material.

The 12 Soil Orders: A Global Overview

The United States Department of Agriculture (USDA) Soil Taxonomy recognizes 12 soil orders, each representing a distinct set of environmental conditions and soil-forming processes. Here’s a brief overview:

Alfisols: Moderately weathered soils with a subsurface horizon of clay accumulation (argillic horizon) and a high base saturation. They typically form under deciduous forests in humid and subhumid climates.
Andisols: Young soils formed in volcanic ash, characterized by high water-holding capacity and unusual mineralogy.
Aridisols: Soils of arid climates, characterized by low organic matter content and often containing accumulations of calcium carbonate (calcium carbonate horizon), gypsum, or soluble salts.
Entisols: Young soils with little or no profile development. They can be found in a wide range of environments, from floodplains to deserts.
Gelisols: Soils of permafrost regions, characterized by the presence of permanently frozen ground (permafrost) within 2 meters of the surface.
Histosols: Organic soils formed in wetlands, characterized by a high percentage of organic matter (greater than 20% by weight).
Inceptisols: Soils with weakly developed horizons, representing an intermediate stage of soil development.
Mollisols: Dark, fertile soils with a thick, organic-rich surface horizon (mollic epipedon) and high base saturation. They typically form under grasslands in subhumid climates.
Oxisols: Highly weathered soils of tropical regions, characterized by low fertility and high content of iron and aluminum oxides.
Spodosols: Acidic soils with a subsurface horizon of accumulated humus and iron and aluminum oxides (spodic horizon). They typically form under coniferous forests in cool, humid climates.
Ultisols: Highly weathered soils with a subsurface horizon of clay accumulation (argillic horizon) and a low base saturation. They typically form in warm, humid climates.
Vertisols: Soils with a high clay content that shrink and swell dramatically with changes in moisture content, resulting in deep cracks in dry periods.

Frequently Asked Questions (FAQs) About Soil Orders

H3: What is the difference between a soil order and a soil series?

A soil order is a broad classification based on major, widespread characteristics related to climate and soil-forming processes. A soil series is a much more specific classification within a soil order, based on detailed characteristics like texture, structure, drainage, and parent material. Think of it like this: soil order is the family, and soil series are the individual members.

H3: Why is soil classification important?

Soil classification allows us to predict soil behavior, assess its suitability for different uses (agriculture, construction, etc.), and transfer knowledge gained in one area to similar soils in other areas. Without classification, managing and utilizing soil resources would be much more difficult.

H3: How are soil orders determined?

Soil orders are determined by examining the soil profile and identifying the presence or absence of diagnostic horizons and other key characteristics, such as organic matter content, base saturation, and mineralogy. Field observations are combined with laboratory analysis to accurately classify a soil.

H3: Which soil order is considered the most fertile for agriculture?

Mollisols are generally considered the most fertile soil order due to their thick, dark, organic-rich surface horizons and high base saturation. They are prevalent in the world’s major grain-producing regions.

H3: What is the significance of “base saturation” in soil classification?

Base saturation refers to the proportion of exchangeable bases (calcium, magnesium, potassium, and sodium) relative to the total cation exchange capacity of the soil. High base saturation indicates a soil with good nutrient availability and buffering capacity, making it more suitable for plant growth.

H3: Where are Oxisols typically found, and why are they problematic for agriculture?

Oxisols are typically found in tropical regions with high rainfall and temperatures. They are problematic for agriculture due to their low inherent fertility, high acidity, and susceptibility to erosion. The intense weathering in these climates leaches away essential nutrients and leaves behind oxides of iron and aluminum.

H3: What is permafrost, and how does it affect Gelisols?

Permafrost is permanently frozen ground that remains at or below 0°C for at least two consecutive years. It significantly influences Gelisols by limiting drainage, restricting root growth, and influencing decomposition rates. Thawing permafrost can also release greenhouse gases and destabilize the ground surface.

H3: How do Vertisols crack during dry periods?

Vertisols are characterized by a high content of smectite clay minerals, which expand significantly when wet and shrink significantly when dry. This shrinking and swelling causes deep cracks to form in the soil during dry periods, which can damage infrastructure and make it difficult to cultivate.

H3: Can a soil belong to more than one soil order?

No, a soil can only belong to one soil order. The classification system is hierarchical, with each soil falling into a single category at each level. However, soils may exhibit characteristics that are transitional between two orders.

H3: What are the limitations of using soil orders for detailed land management decisions?

While soil orders provide a valuable overview of soil characteristics, they are not detailed enough for specific land management decisions. For that, you need to consult lower levels of the classification system, such as soil subgroups, families, and series, which provide more detailed information about soil properties and behavior.

H3: How do human activities affect soil orders?

Human activities can significantly alter soil properties and, in some cases, even change the soil order over long periods. For example, intensive agriculture can lead to soil degradation and erosion, potentially transforming a Mollisol into a less productive soil type. Deforestation can alter soil moisture regimes and nutrient cycles, impacting soil formation processes.

H3: Where can I find more information about soil orders in my local area?

You can find more information about soil orders in your local area by consulting soil surveys published by the USDA Natural Resources Conservation Service (NRCS). These surveys provide detailed maps and descriptions of the soils in your area, including their classification, properties, and suitability for various uses. You can also contact your local NRCS office or land grant university extension service for assistance.