Soil begins to form when environmental forces break down rocks and similar materials that lie on or near the earth's surface. Pedologists call the resulting matter parent material. As soil develops through the centuries, organic material collects, and the soil resembles the parent material less and less. Glaciers, rivers, wind, and other environmental forces may move parent material and soil from one area to another.
Soils are constantly being formed and destroyed. Some processes, such as wind and water erosion, may quickly destroy soils that took thousands of years to form.
Soil formation differs according to the effects of various environmental factors. These factors include (1) kinds of parent material, (2) climate, (3) land surface features, (4) plants and animals, and (5) time.
Kinds of parent material. The type of parent material helps determine the kinds of mineral particles in a soil. A process called weathering breaks down parent material into mineral particles. There are two kinds of weathering, physical disintegration and chemical decomposition. Physical disintegration is caused by ice, rain, and other forces. They wear down rocks into smaller particles that have the same composition as the parent material. Sand and silt result from physical disintegration.
Chemical decomposition mainly affects rocks that are easily weathered. In this kind of weathering, the rock's chemical structure breaks down, as when water dissolves certain minerals in a rock. Chemical decomposition results in elements and in chemical compounds and elements that differ from the parent material. Some of these substances dissolve in the soil solution and become available as plant nutrients. Others recombine and form clay particles or other new minerals.
The mineral content of parent material also affects the kinds of plants that grow in a soil. For example, some plants, including azaleas and rhododendrons, grow best in acid soils that contain large amounts of iron.
Effects of climate. Climate affects the amount of biological and chemical activity in a soil, including the kinds and rates of weathering. For example, physical disintegration is the main form of weathering in cool, dry climates. Higher temperatures and humidity encourage chemical decomposition as well as disintegration. In addition, decaying and most other soil activities require warm, moist conditions. These activities slow down or even stop in cold weather. Therefore, soils in cool, dry climates tend to be shallower and less developed than those in warm, humid regions.
Effects of land surface features also influence the amount of soil development in an area. For example, water running off the land erodes the soil and exposes new rock to weathering. Also, soils on slopes erode more rapidly than those on flat areas. They generally have less time to form and therefore develop less than do soils on flat terrains.
Effects of plants and animals. Soil organisms and organic material help soil develop, and they also protect it from erosion. The death and decay of plants and animals add organic material to the soil. This organic material helps the soil support new organisms. Soils that have a cover of vegetation and contain large amounts of organic material are not easily eroded.
Effects of time. Soils that are exposed to intense soil formation processes for long periods of time become deep and well developed. Soils that erode quickly or have been protected from such processes for a long time are much less developed.