The way the farmers misused it and how does it cause desertification
They cut down trees close to desert areas, widening the desert and also they over farm the land, reducing grass cover. Wind travels over the land from areas of high pressure (the deserts) to areas of low pressure(the equator), absorbing moisture from the land and beconing heavier with water droplets. This wind precipitates over the equator and then travels back to the desert areas. If there are no trees/vegetation cover, then that air will absorb moisture in greater amounts than if there was trees to stop the water from rbeing absorbed. Result : the desert spreads since the land will become drier, faster. Fantastic question by the way.
As safeya mentions, deforestation and poor agricultural practices (and often the combination of the two) are the principal causes of desertification.
More specifically, we can discuss what is physically changing the structure of the soil. Grazing is a huge problem–not only because forests are often replaced by rangeland, but because overgrazing a grassland never gives vegetation a chance to really recover. Soil is exposed to the elements that safeya cites–wind and water–and erodes away when it is no longer permeable. Constant impacting from animal hooves hastens this process.
Salinization is often associated with poor irrigation practices. When we add water to the land, some of it is lost to evapotranspiration (particularly in the arid and semiarid regions we are discussing). But while water may evaporate from the surface, it leaves behind dissolved salts that accumulate in the soil and greatly diminish its fertility.
Source: Christopherson, R. W. 2007. Elemental Geosystems. 5th edition. Prentice Hall: Upper Saddle River, NJ.
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