Standard Soil Classification

Soil Orders

• The American system of soil classification has 12 main soil orders. Each soil order classifies the types of soils that form in definite ecological regions. For example, soil in the tundra has a certain classification that is different than a soil found in a tropical rain forest or in a prairie. Each soil order has several main divisions to more specifically classify different soils. The 12 soil orders are: Gelisols, which are soils with permafrost; Histosols, almost pure organic matter soils; Spodosols, forest soils; Andisols, volcanic ash soils; Oxisols, weathered tropical soils; Vertisols, lots of shrink swell clays; Aridisols, arid land soil with lots of calcium carbonate; Ultisols, very old leached soils; Mollisols, fertile grassland soils; Alfisols, forest soils with leaching; Inceptisols, poorly developed soils; and Entisols, which are the newest soils.

Texture

• The texture of a soil is the percentage of sand, silt and clay that it contains. Texture refers to the fine earth fraction of the soil, starting with coarse sand and going down in size to microscopic clay particles. Rocks and gravel are part of the classification of a soil but not in the aspect of texture of the fine earth portion. Based on its texture, soil may be classified as sandy, silty or clayey, depending on which sized particles dominate. If the soil has roughly equal parts of sand, silt and clay, it is considered a loamy soil.

Organic Matter

• Organic matter plays an important part in soil classification because it indicates what kind of climate a soil is in and what kind of organisms are exposed to the soil. In an area with plenty of water and temperature variations throughout the year, a diverse set of organisms live, die and become incorporated into the soil. With increased organic matter, a soil becomes a more fertile place for increased amounts of plants and animals to flourish. The decayed organic matter in the soil alters its color, which is a primary measure used in soil classification. Organic matter also changes the amount of water and nutrients a soil can hold. Both of these factors are considered in soil classification.

Parent Material

• The parent material of soil can be bedrock, a sandy prehistoric ocean bed or any material that the overriding soil has subsequently formed on top of, such as glacial till or windblown loess. This means that the parent material of a soil may have been blown to a new area or deposited there by a flood. Ancient lake beds are another parent material that affects the type of soil that forms. The type of parent material influences the color of the soil as well as the kinds of organisms likely to be in contact with the soil. Parent material is a principle factors in understanding other evidence that leads to how a soil is classified.

Depth and Structure

• Depth of a soil is measured by how far it is from the soil surface to the underlying parent material. The deeper a soil, the older it is. Some soil orders, by definition, must reach a certain depth. Other soils are identified because of how shallow they are. Related to depth are the structural features of a soil. The soil structure relates to how the small clods and aggregates of soil particles appear when they are removed from the soil at certain depths. A well-developed soil structure typically depends on the amount of organic matter in the soil, as well as clay content. Clay is an indicator of certain chemical processes at work in the soil that help to classify it.