Soil is a mixture of organic matter and inorganic material that develops at the earth's surface and can support plant life. An inch of soil is the result of thousands of years of interactions between geology, topography, climate and vegetation.
The primary material for soil formation can be bedrock, an old soil surface, or deposits from water, wind, glaciers, volcanoes or material moving down a slope. This parent material breaks down over time by environmental forces such as rain, snow, sunshine, heat, and wind. Soil development is faster in warm, wet climates and the soil is deeper. In most of northern North America the soils are very young and thin, compared to other parts of the world.
Plants and animals, including microorganisms and humans, have an impact on soil formation. Animals living in the soil and the way they move around in and on the soil has an effect on soil decomposition. Biological processes transport soil material within the profile; for example the substances drawn by the roots of plants eventually end up on the surface. Organic matter left by plants and animals is incorporated into the soil with the help of bacteria.
Landforms also determine how soil will be formed. Topography affects mineral accumulations, type of plants and nutrients, rate of vegetation growth, erosion, and water drainage. Slopes that face the sun dry out quickly; soil at the bottom of a hill receives more water than upland slopes. Soil is replenished regularly in alluvial valleys, where flood waters deposit silt in the floodplains. Many early civilizations became established in alluvial valleys such as the Indus, Nile, and Ganges.
In Soil Time
Soil formation is a continuous process. A thin soil in the northern hemisphere may have taken 15,000 years to develop. A mature soil in a warmer climate may be millions of years in the making but can now replenish much more quickly. In human terms, most soil is ancient.
- NRCS: Soil Formation and Classification
- Fundamentals of Physical Geography; Briggs, Smithson, Ball, Johnson, Kershaw, Lewkowicz; 1993
- Photo Credit Jupiterimages/Photos.com/Getty Images
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