While there have been tremendous increases in productivity through the use of modern agricultural practices, these practices have had a significant impact on some other natural resources. Soil is one of the most overlooked and misunderstood resources. Most people think of soil as an inert medium from which plants grow. In reality, topsoil that upper 15 to 25 centimeters (6 to 10 inches) of the earth's terrestrial surface in which nearly all plants grow is a complex mixture of weathered mineral materials from rocks, partially decomposed organic molecules, and a large number of living organisms.
The process of soil formation is very slow. Under ideal conditions, enough topsoil can form in one year to produce a layer of about 1 millimeter (0.04 inch) thick when spread over 1 hectare (2.5 acres). With proper management, topsoil can be kept fertile and productive indefinitely. Unfortunately, many agricultural techniques lead to the removal of trees and shrubs, which provide windbreaks, or to the depletion of soil fertility, which reduces the plant cover over the field. These practices expose the soil to increased erosion from wind and moving water. As a result, as much as one-third of the world's current croplands are losing topsoil faster than it can be replaced.
See also: Modern Agriculture, Agricultural Diversity
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