Soil databases

Worldwide Data

The Harmonized World Soil Database

• 30 arc-second raster database
• combines existing regional and national updates of soil information worldwide (SOTER, ESD, Soil Map of China, WISE) with the information contained within the 1:5 000 000 scale FAO-UNESCO Soil Map of the World.
• also available at FAO Soils Portal.
• Used by FAO GAEZ project (if I understand correctly)

FAO/UNESCO Soil Map of the World

• Used by Deininger and Minten (2002).

WISE (World Inventory of Soil Emission Potentials)

• Implemented by ISRIC, which appears to maintain various other soil databases.

International Soil Moisture Network

Finally, likely to be outdated, but from Atlas of the Biosphere, a few soil databases can be downloadable:

• SoilData(V.0): Soil pH
• SoilData(V.0): Soil Organic Carbon
• Terrestrial Water Budget Data Archive: Soil Moisture (Willmott and Matsuura 2001)

Continental Data

For Latin America, SOTERLAC version 2.0 is available and downloadable here. Used by Dell (2010).

For Africa, European Digital Archive of Soil Maps (EuDASM) has digitized paper soil maps to produce The Soil Maps of Africa.

National Data

See the list compiled by FAO Soils Portal. The same website also make available scanned soil maps (which you yourself need to georeference to use).