What Is Residual Soil?
Residual soil constitutes a confusing concept to those beyond the walls of soil science and engineering, not least of all because different sources provide different definitions for the phrase. Soils deemed residual appear in numerous locations throughout the world at varying depths in the Earth's surface. Residual soils bear some similarities to volcanic and sedimentary soils, with some scientists going so far as to classify them together, though significant differences exist among these types. Does this Spark an idea?
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Definitions
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Many definitions of residual soil exist, some with similar elements and others completely different from one another. According to Princeton University online dictionary WordNET, residual soil constitutes soil left over after all soluble elements dissolve. According to the authors of the book "Tropical Residual Soils Engineering," two separate scientists and the Public Works Institute of Malaysia define residual soil as soil that forms and stays in one place, and from which all original rock material dissolves. A third definition, from South Africa, describes residual soil as all soil below the country's ancient erosion, or pebble, line. The second definition constitutes the most commonly accepted one. Residual soils generally exhibit claylike consistency.
Where Residual Soils Occur
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As per their most commonly accepted definition, residual soils can occur anywhere in which rock forms into soil before the initial rock material dissolves. Formation occurs commonly in tropical regions, where soil formation occurs naturally in the soil and tropical weather elements, in particular heavy, persistent rains, erode soluble material --- that is, the original rock material --- in the soil. The humidity of tropical regions also precipitates the formation of residual soil. Despite high instances in tropical locations, residual soil occurs throughout the world, in locations as disparate as Germany and India, Nigeria and China, New Zealand and the United States.
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Characteristics of Residual Soil
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Regardless of the definition ascribed to residual soil, soils of this type always exhibit certain common characteristics. Residual soils always form in a thick mantle, or layer. The exact thickness of a mantle depends upon the age of the soil and the weather conditions. Residual soils always show compositional traces of rocks native to an area, because they form from these rocks and by most definitions always remain in place. Distinct horizons also constitute typical characteristics of residual soil. This means that clear lines of demarcation between residual soil and surrounding soil types exist.
Additional Information
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In many instances, engineers and farmers must take special measures when working in areas with residual soil in order to balance the needs of a project with the consistency and qualities of layers of residual soil present on a farm or project site. Residual soils evolved along with the climate system of the Earth and depend upon climactic elements when forming. These soils comprise part of the Earth's geological cycle, and as such extend back hundreds of millions of years.
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