Đọc hiểu
DESERTIFICATION
Desertification is the degradation of once-productive land into unproductive or poorly
productive land. Since the first great urban-agricultural centers in Mesopotamia nearly 6,000
years ago, human activity has had a destructive impact on soil quality, leading to gradual
desertification in virtually every area of the world.
It is a common misconception that desertification is caused by droughts. Although
drought does make land more vulnerable, well-managed land can survive droughts and
recover, even in arid regions. Another mistaken belief is that the process occurs only along
the edges of deserts. In fact, it may take place in any arid or semiarid region, especially where
poor land management is practiced. Most vulnerable, however, are the transitional zones
between deserts and arable land; wherever human activity leads to land abuse in these
fragile marginal areas, soil destruction is inevitable.
[1] Agriculture and overgrazing are the two major sources of desertification. [2] Large-
scale farming requires extensive irrigation, which ultimately destroys lands by depleting its
nutrients and leaching minerals into the topsoil. [3] Grazing is especially destructive to land
because, in addition to depleting cover vegetation, herds of grazing mammals also trample the
fine organic particles of the topsoil, leading to soil compaction and
erosion. [4] It takes about 500 years for the earth to build up 3 centimeters of topsoil. However,
cattle ranching and agriculture can deplete as much as 2 to 3 centimeters of topsoil every 25
years - 60 to 80 times faster than it can be replaced by nature.
Salination is a type of land degradation that involves an increase in the salt
content of the soil. This usually occurs as a result of improper irrigation practices. The greatest
Mesopotamian empires- Sumer, Akkad and Babylon- were built on the surplus of the
enormously productive soil of the ancient Tigris- Euphrates alluvial plain. After nearly a
thousand years of intensive cultivation, land quality was in evident decline. In response,
around 2800 BC the Sumerians began digging the huge Tigris-Euphrates canal system to irrigate
the exhausted soil. A temporary gain in crop yield was achieved in this way, but over-irrigation
was to have serious and unforeseen consequences. From as early as 2400 BC we find Sumerian
documents referring to salinization as a soil problem. It is believed that the fall of the Akkadian
Empire around 2150 BC may have been due to a catastrophic failure in land productivity; the
soil was literally turned into salt. Even today, four thousand years later, vast tracks of salinized
land between the Tigris and Euphrates rivers still resemble rock-hard fields of snow.
Soil erosion is another form of desertification. It is a self-reinforcing process;
once the cycle of degradation begins, conditions are set for continual deterioration. As the
vegetative cover begins to disappear, soil becomes more vulnerable to raindrop impact. Water
runs off instead of soaking in to provide moisture for plans. This further diminishes plan cover
by leaching away nutrients from the soil. As soil quality declines and runoff is increased,
floods become more frequent and more severe. Flooding washes away topsoil, the thin,