Soil classification assigns a grade or name to soil types for specific applications. Different systems exist for engineering, agriculture, and geology. The Unified Soil Classification System (USCS) combines engineering and geological classifications. The USDA has its own system with over 20,000 soil types. The AASHO has a chart for building roads, while the FAO has a global system with three primary measures.
The soil classification process seeks to assign a grade or name to the different soil types of the Earth as they relate to specific applications. There are different types of soil, depending on whether the information requested is related to engineering, agriculture or geology. Different systems for measuring soil have evolved within certain fields of study. Additionally, some countries maintain their own soil comparison charts.
The actual soil classification process can vary from industry to industry. For some applications, appearance and other visible factors are the only factors considered for classification. In fields such as engineering, the soil’s suitability to support a structure is judged in addition to its appearance. Agricultural analysis often involves measuring the size of the grains in the soil or the pH levels present.
The engineering and geological classifications have been combined into a system known as the Unified Soil Classification System (USCS). This system uses two measurements to define the soil type in a given area. The first measure is the texture, which can be gravel, sand, silt, clay or organic. The second measure is grain size, which can range from poorly graded to well graded and from low plasticity to high plasticity. These two measurements are represented by a two-letter system where the combination describes the soil.
The United States Department of Agriculture (USDA) has established a much more intricate system of soil classification. According to USDA guidelines, there are more than 20,000 soil types in the United States. Each of these soils is given a name which usually reflects the area in which it was found. The various types of soil are classified in relation to other types according to dozens of properties, including the origin of its formation.
There are many derived and independent soil classification charts in use. The American Association of State Highway Officers (AASHO) has its own specific chart which is used to determine the feasibility of building roads or highways on different types of terrain. This chart shares some aspects with other systems, but is unique to the AASHO and primarily focuses on how the terrain will fare when used as the basis for a road.
Globally, the Food and Agriculture Organization of the United Nations (FAO) has established a world soil classification system. This system attempts to give broad definitions to the soils of a region. It has three primary measures. The first is the soil property, or phase, which can be thought of as a soil class like Cambisols. The second is the plot, such as course or ending. The third is the slope of the land on which the land is located.
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