UNEP provides access to a number of global environmental datasets via the GEO data portal. For example, the Global Assessment of Human Induced Soil Degradation data base was created in 1990 by ISRIC. The database contains information on soil degradation within map units reported by a survey of soil experts from around the world. It includes the type, degree, extent, cause and rate of soil degradation.
The main causes of degradation are (in order) overgrazing, deforestation, and agricultural mismanagement. Each of these is responsible for a bit less than a third of total degraded area. Other causes of degradation make up less than 8% of the total degraded area. Deforestation dominates in Asia, while overgrazing is the main driver in Africa and Australia.
For details of assessment see:

Oldeman LR, Hakkeling RTA and Sombroek WG 1991. World Map of the Status of Human-Induced Soil Degradation: An explanatory Note (rev. ed.), UNEP and ISRIC, Wageningen pdf