Regolith is composed of rock and mineral fragments produced by weathering. If this material is transported by wind, water, or ice it is termed sediment. Soil is that portion of the regolith that supports plant life and includes organic material, water and air.
Weathering
rate controls soil thickness. Bare rock surfaces will take
thousands of years to develop even thin soils in cold climates (e.g.
Alaska). Soil is absent on bare rock surfaces in northern Canada that were
scrapped clean by glaciers thousands of years ago Soils are two to three
feet thick in the temperate Midwest where soil formation occurs at rates
of 1 inch per 250 years. The thickest soils may approach 100 feet in
thickness in tropical regions with warm temperatures and plentiful
rainfall. Unfortunately, soil erosion in much of the U.S. outpaces soil
formation.
Soil can be divided into a series of distinct horizontal layers (soil horizons) that collectively are termed a soil profile. Each horizon is designated by a letter. Beginning at the top the horizons are;
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horizon: organic debris,
dead leaves and other plant and animal remains.
A horizon: topsoil, dark organic material mixed with mineral grains. Soluble ions and fine particles are carried downward (leached) from the A horizon to the B horizon. B horizon: dissolved ions precipitated in B horizon. Little organic material present. The accumulation of iron oxide will give the soil a red color (see image), forming a pedalfer soil. Accumulation of calcium carbonate will form a white layer creating a pedocal soil. C horizon: soil parent material. Composed of either weathered bedrock or unconsolidated sediment. |
Soil Profile Willamete Valley Oregon. Image courtesy of Dr. Annabelle Foos. |
Soil scientists have identified thousands of soil types. Pedalfers and pedocals are simple designations given to soils in the wetter (pedalfer) and dryer (pedocal) regions of the U.S. The relatively abundant precipitation in the eastern U.S. results in iron minerals being leached from the topsoil and precipitated in the B horizon. In contrast, in dry regions calcium ions (Ca2+) are leached and combine with carbonate ions (CO32-) to form calcium carbonate (limestone) in the B horizon.
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Distribution of pedalfer and
pedocal soils in the U.S. Areas to the east of the blue line
receive more than 30 inches of rain per year; areas to the west
(except in the northwest and mountainous regions) receive less.
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