听力原文：Today, I'd like to talk about some of the changes land can undergo, specifically
听力原文： Today, I'd like to talk about some of the changes land can undergo, specifically desertification, that's the process through which land becomes part of a desert. Now a desert is defined as a place that receives a certain maximum amount of rainfall. But you may not know that it usually takes more than just a lack of water to turn productive land into a desert. There are several specific human activities that when combined with a lack of rainfall encourage desertification. For example, over cultivation, growing more crops than soil can support. The soil loses its nutrients, so it needs either to be fertilized or to be left unused for at least a season. But if neither of these things happens, if these nutrients in the soil don't get replaced, the damaged soil stops producing. Another cause of desertification is overgrazing. That's when the grasses and trees and shrubs of an area are expected to feed more animals than they reasonably can. Too many animals eating in the same area will kill the vegetation. And because it's the roots of this vegetation that hold much of the soil together, when too much of the vegetation dies, the soil erodes. But maybe the most ironic example of human behavior. that can lead to desertification is irrigation. It may seem to run counter to common sense to say that introducing water into an area can cause it to become more like a desert. But there are plenty of bad irrigation practices that do just that. Bringing in too much salty water and then not providing adequate drainage for it will fill the soil with salt, and turn the area into a desert.
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A．Settlement patterns in desert.
B．Causes of desertification.
C．Plant adaptation to desert conditions:
D．Regions most affected by desertification.
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