托福托福【模拟测试-3】-2017服务一体化阅读Look at the four squares [[■]] that indicate where the following sentence could be added to the pa答案解析【小站备考】

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 fragile marginal areas, soil destruction is inevitable.

[■]Agriculture and overgrazing are the two major sources of desertification [■] Large-scale farming requires extensive irrigation, which ultimately destroys land by depleting its nutrients and leaching minerals into the topsoil. [■]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 compactionand erosion. [■] 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 degradationthat involves an increase in the salt content of the soil. This usually occurs as a result of improper irrigation practices. The great 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 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 tracts 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 plants. This further diminishes plant 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, rich, uppermost layer of the earth's soil, and leaves finer underlying particles more vulnerable to wind erosion. Topsoil contains the earth's greatest concentration of organic matter and microorganisms, and is where most of the earth's land-based biological activity occurs. Without this fragile coat of nutrient-laden material, plant life cannot exist. An extreme case of its erosion is found in the Sahel, a transitional zone between the Sahara Desert and the tropical African rain forests; home to some 56 million people. Overpopulation and overgrazing have opened the hyperarid land to wind erosion, which is stripping away the protective margin of the Sahel, and causing the desert to grow at an alarming rate. Between 1950 and 1975, the Sahara Desert spread 100 kilometers southward through the Sahel.

One-third of the earth's land area of 150 million square kilometers is already classified as semiarid, arid or hyperarid. It is estimated that is increasing by 215,000 square kilometers of land rendered unproductive by desertification every year. In America, salinization affects over 160,000 square kilometers of land in western and midwestern states, and is worsening annually. Africa loses 300 to 400 million tons of topsoil a year with the spreading of the desert. As a billion people are added to the world's population every decade, there is less and less arable land to provide food for them. At the present rate of soil deterioration, ecologist warn, another quarter of the earth's productive soil will disappear within the next century.

托福【模拟测试-3】-2017服务一体化 Look at the four squares [[■]] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [[■]] to add the sentence to the passage.

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