5.2 - Soil Orders
The most general level of classification in the USDA system of Soil Taxonomy is the Soil Order. All of the soils in the world can be assigned to one of just 12 orders (Table 5.1). Soil orders are frequently defined by a single dominant characteristic affecting soils in that location, e.g., the prevalent vegetation (Alfisols, Mollisols), the type of parent material (Andisols, Vertisols), or the climate variables such as lack of precipitation (Aridisols) or the presence of permafrost (Gelisols). Also significant in several soil orders is the amount of physical and chemical weathering present (Oxisols, Ultisols), and/or the relative amount of Soil Profile Development that has taken place (Entisols).
This lesson will examine each of these 12 soil orders in turn: Entisols, Inceptisols, Andisols, Mollisols, Alfisols, Spodosols, Ultisols, Oxisols, Gelisols, Histosols, Aridisols, and Vertisols. To get the most out of this lesson, the student should carefully study each soil order, including the supplementary material provided by the embedded links.
|Table 5.1 - Soil Orders and General Descriptions|
|Entisols||Little, if any horizon development||Inceptisols||Beginning of horizon development|
|Aridisols||Soils located in arid climates||Mollisols||Soft, grassland soils|
|Alfisols||Deciduous forest soils||Acidic, coniferous forest soils|
|Ultisols||Extensively weathered soils||Oxisols||Extremely weathered, tropical soils|
|Gelisols||Soils containing permafrost||Histosols||Soils formed in organic material|
|Andisols||Soil formed in volcanic material||Vertisols||Shrinking and swelling clay soils|