“Only one comprehensive assessment has been done on a global scale to date: the Global Assessment of Soil Degradation (GLASOD) by Oldeman et al. (1991). (A 1992 study by Dregne and Chou was global in extent but limited to dry areas.) Based on the judgment of over 250 experts around the world, GLASOD estimated that nearly 2 billion hectares of land (15 percent of total global land area of 13 billion hectares, or 23 percent of the 8.7 billion hectares used by humans for crops, pasture, and forest and woodlands) had been degraded as a result of human activity since World War II.” (pg 15).
“GLASOD estimated that 38 percent of the world’s cropland had been degraded to some extent since 1945. Degradation had affected 65 percent of cropland in Africa, 51 percent of cropland in Latin America, 38 percent of cropland in Asia, and 25 percent of cropland in North America, Europe, and Oceania. GLASOD identified erosion (primarily due to water) as the principal cause of cropland degradation, affecting 1.6 billion hectares (mostly in Asia and Africa). Loss of soil nutrients was the primary cause of degradation on 136 million hectares (mostly in South America and Africa); salinization affected 77 million hectares (mostly in Asia); compaction, sealing, or crusting affected 68 million hectares (mostly in Europe); and other physical and chemical processes affected 42 million hectares.” (pg 15)
Wiebe, Keith, Linking Land Quality Agricultural Productivity, and Food Security, (Resource Economics Division, Economic Research Service, U.S. Department of Agriculture, Agricultural Economic Report 823, 2003), 16, http://www.ers.usda.gov/media/522187/aer823_1_.pdf