Soil is composed of minerals (rock, clay, silt, and sand), air, water, and organic (plant and animal) material. The chemistry of soils and rocks is also affected by climate, and through the study of paleosoils (ancient soils), scientists can infer what past climate was like. In humid climates such as jungles, for instance, the soluble minerals are dissolved and washed out of the soil. Because only certain minerals do this, soils of humid climates are characteristically missing or deficient on these certain elements: potassium, calcium, sodium, magnesium, and silicon. Humid climate soils generally have characteristically large percentages of iron and aluminum because they are the least soluble and are the ones that tend to remain in place. Iron and aluminum form large deposits called oxides. Iron gives soil a red color, which is why a lot of soils in the Tropics have a rich reddish color.
In dry climates, water picks up small amounts of soluble minerals from the soil, carries them to low-lying flat areas, and deposits them. Then, under the hot desert sun and extremely low humidity, the water evaporates, leaving the soluble minerals behind. Over time, these minerals accumulate and form deposits called evaporites. Desert evaporites are generally composed of a mixture of sand, gypsum, various salts (sodium chloride and sodium nitrate), and borates. Evaporite mineral deposits are often found in playas and closed basins. The Great Basin desert of the United States is a major source of evaporite resources. These resources are considered mineral deposits and are mined commercially. Death Valley in California and the Bonneville Salt Flats in Utah are both well-known examples of evaporite deposits. Boron, obtained from borate evaporates, is mined from Death Valley and is a key ingredient in the manufacture of ceramics, glass, agricultural chemicals, water softeners, enamel, and pharmaceuticals. Salt and magnesium chloride are mined from the Bonneville Salt Flats area around the Great Salt Lake in Utah. The Bonneville Salt Flats are also home to the world-class car racing speedway.
Many types of soils exist on Earth, and each has its own unique properties, such as texture, color, structure, and mineral content. The depth of the soil also varies, partly determined by climate.
The soil-forming process is extremely slow. The parent material (rock) must first erode into small pieces in and on the ground. During this lengthy process, organic matter decays and mixes with the rock to slowly form soil. Developing soil is made of distinct, identifiable horizontal layers called horizons, which vary in characteristics. The deepest horizons (subsoil, regolith, bedrock) most closely resemble the parent materials, while the surface layers (humus, topsoil) contain the most organic nutrients, which is why they are fertile enough to support plant life.
Because there are several different soils that exist worldwide and climate has a large influence on their formation, pedologists (soil scientists) have classified them into 12 soil orders. Worldwide, there are various types of classifications. In the United States, soils are classified based on the concept of diagnostic horizons.
The breakdown below shows the typical layers of a soil profile. They are usually divided into the O, A, E, B, C, and R horizons with the following characteristics.
O: This horizon is on the surface and is the richest in nutrients, which is where most plant life and interaction of life-forms exist. It contains decomposed organic matter.
A: This horizon is the topsoil. This is where seeds germinate and plant roots thrive. It is composed of humus and mineral particles. E: This layer is lighter in color and is composed of mostly sand and silt, because most of the clay and minerals have been eluviated (leached) by water as it flows through the soil under the force of gravity. B: This layer is the subsoil and contains clay and mineral deposits, such as iron, aluminum oxides, and calcium carbonate that wash down into it from the layers above.
A soil profile consists of distinct horizons. The climate plays an important role in the type of soil that exists in an area.
C: This is the regolith and is made up of broken rock fragments. Organic matter is rarely deposited in this horizon.
R: This is the unweathered bedrock that the soil profile sits on.
Soils serve as a valuable diagnostic tool for climatologists, because different soil types develop in different climates. In forest biomes, the soils have a light gray upper horizon, a horizon rich in aluminum and iron, and form in humid regions that generally range from warm to cool, allowing conifers to grow. In grassland biomes, such as prairies, these rich soils have a dark surface layer and are typically rich in minerals. Today, these soils are common in the Earth's midlatitude regions. When ancient versions of these soils are found, it indicates what the climate was like when the soils formed.
In lowland regions, such as swamps and wetlands, the soils have a high water content and are poorly drained. The soils in these zones are dark-colored organic soils typically rich in organic matter. As illustrated earlier, desert soils are usually rich in calcium carbonate. In tropical zones, where it is humid and warm all the time, soils are a reddish color and extremely rich in iron oxide. They are depleted in nutrients. In fact, in the world's rain forests, most of the nutrients are contained within the forest vegetation, not the soils. In cold environments, such as the Arctic tundra, soils have a dark organic-rich upper layer and a mineral-rich layer that covers frozen ground.
There are 12 soil orders. By identifying the soil—or paleosoil—much can be deduced about the climate that once existed. The following table lists the 12 soil orders and their characteristics.
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