The biggest cause is human mismanagement. The second biggest cause is natural process. When the natural processes are exacerbated by human mismanagement you get the loss of these great agricultural sites(deforestation -> erosion, over farming -> salinization and infertility). Its not a quincidence that the great historically fertile areas that gave birth to great civilizations have been reduced over the years and some are/have been turned into deserts.
Heck look at the situation in the American Midwest. I live in Kansas, here there's the problem of wet agriculture in the dry western half of the state. Instead of the traditional dry farming, people drain the fossil water supplies at an unreplenishable rate. This has/will turn western Kansas into a dryer area due to the lowering of hte water table. Look at the dust bowl because people left the fields fallow without any plants to bind the top soil together. The American Midwest has only been the subject of intense western farming techniques for about 200 years and despite still being the most productive land on the planet, its fertility has been greatly impacted.
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