Soils are vital, fragile, finite natural resources that are essential for the sustained production of food and fiber. Soils, however, are subject to degradation and erosion when mismanaged. Between 1950 and 1993, grain area per person worldwide decreased from 0.58 to 0.33 acres (0.23 to 0.13 hectares). As human populations increase, soil resources are used more intensively, with increasing probability that many practices will lead to deterioration of the resource. Competition between agricultural uses and non-agricultural uses of land, such as support of structures, disposal of wastes, and growing plants for recreational and aesthetic purposes will increase.
In ecosystems, soils, water, air, plants, animals and people have interdependent relationships. Soils are dynamic, living systems whose productivity, through management that often includes additions of nutrients, organic materials and water, can be sustained indefinitely. Soils exhibit unique physical and chemical sorptive qualities and dynamics reflective of their inorganic and organic composition. Cycling of carbon, nitrogen and other nutrient elements in nature involves transformations in soils.
Great diversity occurs among soils, sometimes in very small geographical areas, such as building lots in urban areas. The rise and fall of civilizations sometimes has been related to the wise use and misuse of natural resources including soil and water.
Definitions of soil vary, but one view is that the unconsolidated material at the earth's surface becomes soil when biological activity results in a noticeable accumulation of organic matter as revealed by a dark surface color. Soils that form in loose, fine-grained material weathered from the rock immediately below them are called residual soils. More frequently, soils are formed in materials that have been transported away from the source rock. Examples of such materials are alluvial materials, which have been deposited from running water as in flood plains or deltas; lacustrine material which is deposited in lakes; glacial material which has been moved by ice, and aeolian material which has been transported and deposited by wind.
Many of the intensively used soils in the world are formed in transported materials. Their usefulness often is associated with topography, or with physical and chemical properties inherited directly from the transported material.
Lesson 1. Soil Formation
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