The term soil is used in different ways in different disciplines. To engineers, soil is any loose material above solid bedrock, a usage equivalent to the term regolith. Soil scientists use the term for any material capable of growing plants.

To a geologist, soil is produced by weathering and is the residual product of the chemical, physical, and biological breakdown of rock, whether bedrock or material that has been transported. Climate, topography, the composition of the material, and the length of time the processes have been working determine the type of soil.

Rocks are broken down naturally by mechanical disintegration and by chemical decomposition. Freezing water in rock fractures breaks them apart, while daily temperature fluctuations also help to disintegrate rock. The result is an increase in surface area that provides more surfaces for chemical weathering to take place. Water and gases in the atmosphere attack minerals in the rocks, chemically producing new minerals that are stable or capable of being dissolved. Limestone, for example, is made up for the most part of the white mineral calcite. If limestone has a color other than white, it contains other minerals that give it its color. As the limestone is dissolved away, it leaves a small residue of non-calcitic minerals that becomes part of the developing soil.

Feldspars, components of granite, change to clay, while some of the darker minerals, fer-romangnesium silicates, also change to clay but include insoluble residues of iron oxides as well.

The mineral quartz, common in many rocks, is usually resistant to chemical attack and remains behind. Igneous rocks form from molten material; those that crystallize at high temperatures break down easily at the surface, while those that form at a lower temperature and pressure weather more slowly.

In general, the higher the temperatures and the greater the quantity of water, the greater the amount of weathering. Chemical and mechanical weathering go hand in hand, but climate determines which predominates. In cold climates mechanical weathering is dominant, while in tropical climates chemical weathering is. Biological weathering is another factor that plays an important role in the breakdown of minerals, by producing enzymes that are reactive with them.

As rocks break down the weathered product, soil forms on the surface. In newly exposed rock there is a rough succession of organisms that attack the rock. Lichens are the pioneers, succeeded by mosses and quickly by other plants that take the opportunity to grow in cracks and crevices. As the plants decay they aid in the soil-forming process. It turns out that soil is not a simple, homogenous mass but is divided vertically into distinct zones, the soil profile.

In a typical profile, the top zone, exposed to the surface, is designated the O-horizon, where newly fallen leaves and other plant parts accumulate. It is followed by the A-horizon, containing the most decayed rock and most of the organic remains. Rain percolating through the A-horizon, also called the zone of leaching, picks up any material that is soluble and transports it downward to the B-horizon, the zone of accumulation. The B-horizon rock material is not as decayed and contains some organic material. Below it is the C-horizon, which is composed of broken up bedrock that merges with the unaltered bedrock below. The boundaries between the zones may be sharp or gradational, and in well-developed profiles it is possible to subdivide the horizons. Not only do soil profiles contain subdivisions but, in addition, in some instances a horizon can be entirely missing, while others contain horizons that are transitional between A, B, and C. These variations are the result of the combination of different soil-forming factors, such as rate of formation and erosion, wind, amount of precipitation and running water, topography, and extent and kinds of human activity.

Soils were historically classified into two generalized groups based on climate: pedalfers forming in wetter climates, found in tall grass prairies, broadleaf deciduous, and needle leaf forests; and pedocals in dryer climates, such as desert shrub environments and short and medium grass steppes. Pedalfers are usually acid and subject to extensive leaching, which leaves behind oxides of aluminum and iron, and clay. The term pedalfer is a combination of ped (soil), al (aluminum), and fer (iron). A special type of pedocal is laterite soil, which often is bright red and forms in tropical climates with heavy rain and high temperature. Leaching takes place at a maximum rate, and these soils end up with insoluble aluminum and iron compounds. The iron gives laterites their red color.

Tropical rain forests have a very lush growth of vegetation, giving people the idea that the soil will make a productive farm. It comes as a great surprise that these farms do not live up to their potential. Although the forest itself contains considerable amounts of nutrients, there is little in the soil. The lushness comes from the continual accumulation and rapid decay of vegetation that has fallen to the surface, supplying new plants with nutrients. Clearing away the forest for crops clears away the nutrients.

In a matter of a few years, cleared land with laterite soil becomes increasingly unfarmable. Under natural conditions lateritic soil, while developing under the forest canopy, is shielded from the sun, and the roots keep the soil loose. Cleared of vegetation, however, the sun takes over and bakes the soil into a hard material (laterite comes from the Latin word for "brick") that doesn't allow much water to soak into it, or roots to find spaces to grow. Many buildings in tropical climates, such as the temples at Angkor Wat in Cambodia, are built of laterite. Even the application of fertilizer to such soil would not be beneficial. Farms are therefore soon abandoned, a new section of rain forest is cut down, and so on—thus the development of one farm can cause the destruction of a vast amount of rain forest. This process of deforestation also destroys the habitats of many animals and plants, ultimately causing some of them to become extinct.

Pedocal soils—ped for soil, cal for calcium carbonate—are found in drier climates; they leach less extensively, and soluble materials remain in the soil, especially in the B-horizon.

Plowing the rich prairie soil with a tractor, South Dakota, c. 1920 (From the collections of the Library of Congress)

Where calcium carbonate is present the soil is alkaline, and where it becomes dense it forms a tough cemented layer called caliche.

Objections to the above historical classification of soils centered on the notion that it did not take into account variations in bedrock composition, soil texture, and other characteristics. Many new attempts at classifying soils have been developed and adopted by different countries for their own needs, taking into account the various factors that produce soil: variations in composition and texture, and the bedrock whence it came. UNESCO uses 110 different types on its soil map of the world, while the U.S. Department of Agriculture has developed a classification that contains ten orders that are subdivided further into suborder, great group, subgroup, family, and series; there are about 12,000 soil series.

A brief look at this classification gives a good idea of the range of soil types, because it is based on appearance, nutrient status, organic content, color, and climate: entisols, soils with layering just forming and little structure; ver-tisols, containing clays that expand when wet and contract when dry, capable of mixing the upper layers; inceptisols, young soils with weakly developed horizons, especially the B;

aridosols, soils of deserts and semiarid regions, often saline or alkaline, with little organic matter; mollisols, grassland soils and forest soils, sometimes rich in calcium, with a thick layer of organic material; spodosols, arid soils with organic-rich A-horizon, and a B-hori-zon containing organic matter and iron leached from the A-horizon; alfisols, including most acid soils with clay-enriched B-horizon; ulti-sols, similar to alfisols but weathering is more advanced, including clay and some lateritic soils; oxisols, more weathered than ultisols, and includes most laterites; histosols, bogtype soils.

The formation of soils is complex, and they can vary over short distances even though the bedrock is similar. In a valley, for example, one valley wall may be warm and sunny and the other shady and moist, while the valley floor is wet. These conditions determine the type of vegetation that will grow, which ultimately plays an important role in determining the type of soil and the organisms within it.

—Sidney Horenstein

See also: Climatology; Deposition; Erosion; Geology, Geomorphology, and Geography; Topsoil Formation


Akin, Wallace. 1990. Global Patterns: Climate, Vegetation, and Soils. Norman: University of Oklahoma Press; Birkeland, Peter W. 1999. Soils and Geomorphology. New York: Oxford University Press; Brady, Nyle, C. 2001. The Nature and Properties of Soils. Upper Saddle River, NJ: Prentice Hall; Hunt, Charles B. 1972. Geology of Soils: Their Evolution, Classification, and Uses. San Francisco, W. H. Freeman and Company; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Soil Survey Staff. 1999. Keys to Soil Taxonomy 1999. Washington: U.S. Department of Agriculture.

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