Soils are porous natural bodies composed of inorganic and organic matter. They form by interaction of the earth's crust with atmospheric and biological influences. They are dynamic bodies having properties that reflect the integrated effects of climate (atmosphere) and biotic activity (microorganisms, insects, worms, burrowing animals, plants, etc.) on the unconsolidated remnants of rock at the earth's surface (parent material). These effects are modified by the topography of the landscape and of course continue to take place with the passage of time. Soils formed in parent materials over decades, centuries, or millennia may be lost due to accelerated erosion over a period of years or a few decades.
The exposed surfaces of soils are a common sight on almost any landscape not dominated by rock. The surface of a soil reveals very little about the depth of the soil or its subsurface characteristics. A vertical cross-sectional view of a soil is called a soil profile. Each of the horizontal layers which can be seen in the vertical section is called a soil horizon. Horizons are formed because of the integrated effects of climate and biosphere change and generally become less pronounced with depth. The depth of soils, usually 0.6 to 1.8m, is determined by the depth to which the mantle material has been altered in a significant way. That part of the three-dimensional soil body in which the effects of climate and biological activity are most pronounced is the soil solum. In succeeding pages the nature and properties of soils, their management, and environmental public policy issues will be discussed.
Lesson 2. General Composition
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