What Is Soil?
Soil is not just dirt. It is the evolving layer of decomposing rock that covers parts of the Earth not covered by water or sand. Soil covers just 10 percent of the Earth's surface but produces all of the world's crops. The biology of soil and its physical properties are the subject of study for agronomists, soil and materials scientists as well as engineers. Does this Spark an idea?
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Identification
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Soil is rock broken up by natural processes like earthquakes and volcanism and cultivated by rain, decaying plant and animal matter. The soils that cover the earth are actually self-perpetuating systems of mineral and organic materials.
Features
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Soil texture is classified as "sand, "silt" or "clay," depending on the composition of minerals, organic compounds, its age and the physical processes that have acted upon it. Soil consists of three major layers: humus-rich topsoil, less fertile subsoil and horizon. The degree of organic matter decreases as the depth increases until the gravel and broken rock lacking any organic material that defines the horizon is reached just above bedrock of the area. The color of soil may be any shade of brown or gray to dark black, depending on the physical environment and biological history of the area.
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History
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As plants and animals developed during the Pleistocene period, the rocky crust of the planet began to break down. Physical movements of tectonic plates, volcanism and the movement of water all began to wear down rock into gravel, then sand. In areas where plant and animal life flourished, organic matter worked into the soil, leaving layers of humus and peat and creating silt to be washed down the rivers to the new oceans, creating river deltas, peninsulas and alluvial plains. Throughout history, the movement of the Earth, tides and the water cycle broke down layers of rock and mineral and washed over land, creating unique layers of soil, compressing them into clay and capturing images of species as they reached extinction.
Function
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Soil absorbs decaying matter and provides minerals and other nutrients for plants. It functions as a part of the nitrogen cycle, processing this element into compounds that can be used by plants. Earth's soils are aquifers, extracting bacteria and filtering out impurities as rain and floodwater settles through the layers into caverns of bedrock. Just like all living things, however, soil can degrade and die, losing the biological properties that sustain life, due to pollution, acidification or the absence of humus, the top coat of the topsoil layer that renews fertility.
Considerations
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The life cycle of soil is determined by its ability to sustain life and function as an effective aquifer. A soil that contains too much or too little organic matter cannot support plant life, and too much clay will not allow water to pass through to recharge groundwater. We may use "soilless" mixtures of sand, peat and vermiculite for our flower pots in the summer, but it's just dirt--without fertilizer and water, the potted plants will die. The soil in our garden, provided it has been maintained, is a living organism that will nourish plants and retain water without any human intervention.
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Resources
- Photo Credit USDA, DRW & Associates, Inc
