Soil Orders Quiz: Soil Taxonomy

Soil Taxonomy is the official soil classification system of the United States, developed by the USDA Natural Resources Conservation Service. It classifies soils into six hierarchical levels with soil orders at the broadest level. The system is based on measurable, observable soil properties rather than assumed formation processes, making it widely applicable for agricultural and environmental management worldwide.

Soil Taxonomy recognizes 12 soil orders, the broadest and most general level of soil classification. Each order groups soils sharing major defining characteristics such as climate influence, degree of weathering, organic matter content, or dominant mineral type. The 12 orders encompass all soils found on Earth and provide the starting point for more detailed classification at lower hierarchical levels.

Entisols are characterized by minimal soil development. They form on recently deposited sediments, steep eroding slopes, or very resistant parent materials where pedogenic processes have not had time to develop distinct horizons. They are found worldwide in environments ranging from sand dunes to river floodplains and are identified by the absence of diagnostic horizons that define more developed soil orders.

Gelisols are defined by the presence of permafrost within two meters of the soil surface, or evidence of freeze-thaw mixing called cryoturbation within one meter. They are found in arctic and subarctic tundra regions and at high altitudes. Gelisols cover large portions of Alaska, northern Canada, and Siberia and are sensitive to climate warming because thawing permafrost releases large amounts of stored carbon.

Histosols are defined by thick accumulation of organic material, typically exceeding 40 centimeters, where organic carbon makes up more than 20 to 30 percent of the soil mass. Waterlogged conditions suppress aerobic decomposition, allowing organic matter from dead plants and mosses to accumulate over centuries. Histosols store enormous amounts of carbon and are found in peatlands, bogs, and wetlands worldwide.

Aridisols develop in arid and semi-arid climates where evapotranspiration exceeds precipitation. Limited organic matter input produces a pale ochric epipedon. Subsurface accumulations of calcium carbonate and soluble salts form as water moves through the profile. Thick dark organic-rich surface horizons are characteristic of Mollisols, not Aridisols, making that option incorrect.

Mollisols are defined by a mollic epipedon, a thick dark surface horizon rich in organic matter and base cations such as calcium and magnesium. They form under grassland vegetation in temperate climates where organic matter accumulates over millennia. The fertile plains of the American Midwest, the Pampas of South America, and the Ukrainian steppes are classic Mollisol landscapes supporting much of the world's grain production.

Spodosols are defined by a spodic B horizon enriched with illuviated iron, aluminum, and humus leached from the overlying bleached E horizon. They form through podzolization under cool, humid conditions with acid-generating vegetation such as conifer forests and heathlands. The strongly bleached E horizon and reddish-brown spodic B are their most visually distinctive features in field descriptions.

Oxisols are among the most weathered soils on Earth, formed under humid tropical conditions over millions of years. Intense leaching removes soluble bases and silica, leaving behind highly stable iron and aluminum oxides such as goethite, hematite, and gibbsite. Despite their deep red color, Oxisols are nutrient-poor because most plant-available nutrients have been leached away, requiring careful management for sustainable agriculture.

Mollisols are among the world's most productive soils due to deep, base-rich, organic-rich surface horizons. Alfisols have moderate to high base saturation and argillic B horizons retaining nutrients, supporting productive forests and crops. Vertisols are challenging to manage due to cracking but are naturally base-rich and productive in many regions. Oxisols are nutrient-poor and require heavy inputs to support agriculture, making the statement about them the incorrect option.

Andisols form from volcanic ash and pyroclastic materials. They are defined by the dominance of short-range-order minerals such as allophane, imogolite, and ferrihydrite, which give them unusual properties including very high water retention, low bulk density, and high phosphorus fixation. Andisols are found in volcanically active regions including the Pacific Ring of Fire, Japan, Indonesia, and Central America.

Vertisols are defined by high content of swelling clay minerals, primarily smectite, that cause the soil to crack deeply when dry and swell when rewetted. Material falls into the cracks during dry periods and when the soil swells it cannot re-accommodate it, causing a churning process called pedoturbation. This self-mixing disrupts normal horizon development and is the defining diagnostic feature of the Vertisol order.

Both Ultisols and Alfisols have argillic B horizons enriched with illuviated clay, but they differ in base saturation. Alfisols retain moderate to high base saturation because leaching has not removed all exchangeable cations. Ultisols have been leached more intensively, producing low base saturation below 35 percent in the argillic horizon. Ultisols typically form in warmer, more humid climates with longer weathering histories than Alfisols.

Inceptisols show moderate development with some horizon differentiation but lack the diagnostic features of mature orders such as argillic, spodic, or mollic horizons. They are found across a wide range of climates and environments where soil formation has begun but not fully advanced. The cambic B horizon showing color or structural alteration without significant clay accumulation is a common feature. Inceptisols are not restricted to any single climate type.

The 12 officially recognized soil orders in Soil Taxonomy are Entisols, Inceptisols, Alfisols, Ultisols, Oxisols, Mollisols, Aridisols, Spodosols, Vertisols, Histosols, Andisols, and Gelisols. Each order name ends in the suffix sol from the Latin word for soil. The other options contain invented names such as Loamosols, Permafrosols, and Peatosols that are not recognized orders in the official system.