The Disappearing Soil:

The total land area on the globe is 13 billion hectares, 11% is arable suitable for sloughing and crop production) land and 24%  is potentially arable. Each year about 11 million hectares are lost through erosion, becoming desert, becoming toxic, and cropland conversion to non-agricultural uses. We stand to lose 18% of the world's arable land by the year 2000. Between 1945 and 1975 about 30 million hectares of land in the U.S. were lost under concrete and asphalt - half was arable land.