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Soil Erosion Help

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Soil Erosion

Erosion converts soil into sediment. Chemical weathering produces clays on which vegetation can grow. A mixture of dead vegetation and clay creates soil which contains necessary minerals that plants need for growth.

Soil exists as a layer of broken, unconsolidated rock fragments created over hard, bedrock surfaces by weathering action.

Most geologists talk about soil as being part of three layers called soil horizons or soil zones . These three soil horizons are commonly recognized as “A,” “B,” and “C,” but it is important to remember that not all three horizons are found in all soils. Figure 15-4 illustrates the way soil horizons are stacked on top of each other.

Weathering and Topography Spheroidal Weathering

Fig. 15-4. Soil horizons form in a gradient with existing rock at the bottom.

Soil horizons are described from the top soil layer down to the lowest soil and bedrock level and are as follows:

  • “A” horizon includes the surface horizon, a zone of leaching and oxidation, where penetrating rain water dissolves minerals and carries the ions to deeper horizons. It also holds the greatest amount of organic matter.
  • “B” horizon describes the middle horizon, a zone of accumulation, where ions carried down by infiltrating rain water are reconnected to create new minerals. Blocky in texture it is made up of weathered rock mixed with clay, iron, and/or aluminum.
  • “C” horizon includes the bottom horizon, a zone of unconsolidated, weathered original rock.

Soil Types

Just as there are three different soil horizons, there are also several factors that determine which type of soil will form. These include structure, rainfall (lots or little), solubility, temperature (hot or cold), slope (gentle or steep), vegetation (types and amount), and weathering time (short or long.) Singly or in combination, soils form as a result of many different factors. A key factor in naming major soil types is rainfall amount. Everyone from toddlers making mud pies to petroleum geologists looking for oil can tell whether a soil is wet or dry, hard or soft.

Geologists have named three basic soil types based primarily on water content. These are the pedocal , pedalfer , and laterite .

The pedocal is found in dry or semi-arid climates with little organic matter, little to no leaching of minerals and is high in lime. Most nutrient ions are still present. In places where water evaporates and calcite precipitates in the “B” horizon, a hard layer called the caliche or hardpan , is formed. Pedocal soil also collects in areas of low temperature and rainfall and supports mostly prairie plant growth.

Pedalfer soil is found in wetter environments and contains greater amounts of organic matter and leaching. It is enriched with aluminum and iron after many other soluble nutrients are leached out. This type of soil is found in areas of high temperatures and humid climates with a lot of forest cover.

Laterite , the soggiest type of soil, is found in tropical and sub-tropical climates and is high in organic matter. Because of high equatorial rainfall in very wet climates, there is widespread leaching of silica and all soluble nutrients. Iron and aluminum hydroxides are left behind and cause well-drained laterite soils to be red in color. Besides iron and aluminum ores, laterites can also form manganese or nickel ores.

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